internal combustion engine

The airbox is a component in the intake system that collects and stabilizes air and is often connected to the air filter or intake trumpets. It influences pressure loss, flow uniformity, acoustics and engine response. In motorsport and high-performance engines, the airbox is also important for ram-air effects and uniform cylinder filling. A poorly designed airbox can cause local maldistribution, high losses or unstable intake conditions.

The air-fuel ratio describes the mass ratio of air to fuel in the mixture. It is a central quantity for combustion, efficiency, emissions, exhaust temperature and ignition stability. A high air-fuel ratio indicates a lean mixture, while a low ratio indicates a rich mixture. In engine development, it is often used together with lambda or equivalence ratio because these quantities make it easier to compare different fuels.

Atomization describes the breakup of a liquid fuel jet into small droplets. Fine droplets have a large surface area and evaporate faster, which improves mixture formation. Atomization depends on injection pressure, nozzle design, fuel viscosity, surface tension and ambient conditions. Poor atomization can lead to wall wetting, particulate emissions, incomplete combustion and poor repeatability.

Autoignition describes the ignition of a mixture without an external ignition source due to temperature, pressure and chemical reaction kinetics. In diesel engines, autoignition is the intended basic principle of the combustion process. In spark-ignition or hydrogen engines, however, autoignition can be undesired and lead to knock or pre-ignition. Autoignition tendency strongly depends on fuel, compression, temperature, residual gas, wall temperatures and operating condition.

Autoignition is often used synonymously with self-ignition, but in engineering it often emphasizes unintended ignition of a mixture. It occurs when local temperature, pressure and reaction time are sufficient for the chemical reaction to start without a spark. In hydrogen engines, autoignition is especially critical because hydrogen is highly ignitable and has high flame speeds. Undesired autoignition can trigger pre-ignition, super knock, component damage and severe pressure rise.

Autoignition means that a mixture ignites without an external ignition source due to pressure, temperature and chemical kinetics. Spark ignition means that combustion is initiated by an ignition source such as a spark plug. In diesel engines, autoignition is the intended basic principle of the combustion process. In spark-ignition, gas or hydrogen engines, uncontrolled autoignition is usually undesired. Spark ignition allows direct control of ignition timing and therefore combustion phasing. However, it requires an ignitable mixture at the spark location and stable flame kernel formation. Autoignition strongly depends on temperature, compression, fuel, residual gas and residence time. This comparison is important because it explains the difference between diesel combustion, spark-ignition combustion and abnormal combustion.

Backfire describes combustion that propagates back into the intake system or starts there. It is especially critical with premixed gaseous fuels and hydrogen because ignitable mixture may be present upstream of the intake valve. Causes can include hot spots, insufficient flow velocity, valve overlap, misfire or unfavourable mixture distribution. Backfire can damage air filters, intake manifolds, throttle valves and sensors and must be avoided through design and operating strategy.

Boost pressure is the pressure of the compressed intake air after the compressor or within the intake system. It increases the air mass in the cylinder and therefore the possible torque and power. Excessive boost pressure can cause knock, high exhaust temperatures, mechanical loading and thermal problems. The optimum boost strategy depends on engine concept, fuel, compression ratio, charge air cooling and emission requirements.

Boost pressure control regulates the desired boost pressure using wastegate, VGT, bypass valves, electric assistance or compressor operation. The goal is fast, stable and safe boost build-up over the entire operating range. Control must consider turbocharger speed, surge limit, knock limit, exhaust temperature, torque demand and component protection. In modern engines, it is closely linked to engine calibration, air path modelling and transient engine simulation.

Brake efficiency describes what fraction of fuel energy is available as usable mechanical power at the crankshaft. In addition to the thermodynamic process, it includes mechanical losses from friction, gas exchange and auxiliary components. It is particularly relevant for customers and vehicle applications because it describes the usable real engine efficiency. The difference between indicated and brake efficiency shows the magnitude of mechanical and gas dynamic losses in the engine.

Brake mean effective pressure describes the usable work delivered at the crankshaft relative to engine displacement. Unlike indicated mean effective pressure, it already includes mechanical losses of the engine. BMEP is a central metric for engine power density. High BMEP values mean high specific load and place increased demands on combustion, boosting, cooling, mechanics and durability.

Burning velocity generally describes the rate at which a combustible mixture is consumed by the flame. The term is often used in connection with laminar or turbulent flame speed, but it is not always defined in exactly the same way. In internal combustion engines, burning velocity is important because it influences pressure rise, burn duration and optimum spark timing. Fuel, lambda, temperature, pressure, residual gas and turbulence significantly affect burning velocity.

The camshaft controls the time history of valve lift and valve opening through its cam profile. It determines when and how far intake and exhaust valves open. Cam profile, phasing and variable adjustment influence torque, power, gas exchange, residual gas fraction and emissions. In turbocharged engines and gas engines, camshaft design is closely coupled with boosting, scavenging and combustion stability.

Carbon monoxide emission occurs when carbon in the fuel is not fully oxidized to CO2. Causes include rich mixtures, low temperatures, incomplete combustion, near-wall regions or short reaction times. CO is especially relevant in spark-ignition engines, cold start and transient operation. In hydrogen engines, the fuel itself produces no CO, although traces may come from oil combustion or environmental influences.

A catalytic converter accelerates chemical reactions in the exhaust gas without being consumed itself. It is used to convert pollutants such as CO, HC and NOx into less harmful components. Its effectiveness strongly depends on temperature, lambda, exhaust composition, ageing and coating. For low emissions, rapid catalyst light-off after engine start is especially important.

A charge air cooler reduces the temperature of the compressed intake air downstream of the compressor. This increases charge air density, improving cylinder filling and potentially reducing knock tendency. At the same time, the charge air cooler affects pressure loss, packaging, thermal inertia and transient engine operation. Its design is a compromise between cooling performance, pressure loss, packaging and cost.

The choke line describes the region in a compressor or turbine map where mass flow barely increases despite a further rise in pressure difference. This happens because the flow locally reaches very high velocities and the effective flow capacity becomes limited. In a compressor, the choke line limits the maximum possible air mass flow and therefore power at high engine speeds. In turbocharger design, it is important to avoid operating the charger outside its useful range at high load.

Combustion is the chemical reaction of a fuel with oxygen, releasing heat. In an engine, this heat release is converted into pressure rise and mechanical work. Combustion is governed by mixture formation, ignition, turbulence, combustion chamber geometry, fuel properties and operating condition. The goal of engine development is fast, stable, efficient and low-emission combustion over the relevant operating range.

Combustion analysis evaluates the combustion process using cylinder pressure, volume history, thermodynamics and, where available, measurement data. It provides quantities such as heat release rate, start of combustion, MFB50, burn duration, mean effective pressure, pressure rise rate and peak cylinder pressure. This allows efficiency, knock tendency, combustion stability and model quality to be assessed. It is a central link between test bench work, 1D engine simulation and 3D CFD combustion modelling.

The combustion chamber is the region where air, fuel and possibly residual gas are mixed, compressed and burned. Its geometry is shaped by the cylinder head, piston, valves, spark plug and injectors. The combustion chamber influences charge motion, mixture formation, flame propagation, wall heat losses, knock tendency, emissions and efficiency. In development, it is one of the most important levers for modern spark-ignition, diesel, gas and hydrogen engines.

The combustion chamber roof is the upper part of the combustion chamber within the cylinder head. It typically includes valve seats, spark plug location, injector position and surrounding wall surfaces. Its shape influences compression, charge motion, flame propagation, heat transfer and local thermal loading. Especially in compact combustion chambers, high power density or hydrogen operation, the combustion chamber roof is relevant for knock and pre-ignition risks.

Combustion efficiency describes what fraction of the chemical fuel energy is actually converted by combustion. Incomplete combustion, misfire, hydrocarbon emissions, CO formation or unburned fuel reduce this efficiency. With hydrogen, no hydrocarbon or CO emissions are produced from the fuel, but incomplete conversion and misfire can still be relevant. High combustion efficiency is a prerequisite for good fuel consumption and low emissions.

The combustion profile describes how combustion progresses over crank angle or time. It shows when combustion starts, how fast it proceeds and when the main portion of the fuel has been converted. Important quantities include start of combustion, burn duration and MFB50, the point where 50 percent of the fuel mass has burned. The combustion profile affects efficiency, pressure rise, knock tendency, exhaust temperature and emissions.

The combustion profile describes how far combustion has progressed over crank angle or time. The heat release rate describes how quickly heat is released during combustion. The combustion profile is often represented by mass fraction burned, for example MFB10, MFB50 and MFB90. The heat release rate is a rate and shows peaks, phases and intensity of heat release. Both quantities are often calculated from the cylinder pressure trace. The combustion profile helps understand when combustion starts, where combustion phasing lies and how long combustion lasts. The heat release rate shows in more detail whether heat release is fast, delayed, multi-stage or unstable. Both quantities are important for engine simulation, combustion analysis and calibration, but they answer different questions.

A compression-ignition engine is an internal combustion engine in which fuel is injected into hot, highly compressed air and ignites by itself. Power is typically controlled by the injected fuel quantity rather than by conventional throttling of the air path. Important development parameters include injection pressure, injection strategy, mixture formation, ignition delay, soot formation, NOx emissions and exhaust aftertreatment. Diesel engines often achieve high efficiency, but place high demands on combustion, injection system and emission control.

The compression ratio describes the ratio between maximum and minimum cylinder volume during an engine cycle. A higher compression ratio can improve thermal efficiency, but increases knock tendency in spark-ignition engines. In diesel engines, a sufficiently high compression ratio is important for autoignition and cold-start behaviour. In hydrogen engines, compression ratio must be matched with knock, pre-ignition, NOx formation and power density.

The compression ratio is typically much higher in diesel engines than in gasoline engines. The diesel engine requires high compression so that the air temperature is sufficient for autoignition of the injected fuel. The gasoline engine uses spark ignition and is limited by knock if the compression ratio becomes too high. Higher compression generally improves thermal efficiency, but increases pressure, temperature and component loading. In gasoline engines, compression ratio must therefore be matched with fuel knock resistance, boost pressure, combustion chamber cooling and spark strategy. In diesel engines, it affects cold start, ignition delay, noise, NOx and mechanical loading. Modern concepts also use effective compression through variable valve timing, Miller or Atkinson cycles. The comparison shows that compression ratio should not be evaluated in isolation, but always in relation to the combustion concept.

Compressor efficiency describes how efficiently the compressor converts mechanical shaft power into an increase in intake air pressure. High compressor efficiency means that less power is required for the same boost pressure and that the charge air is heated less. This improves cylinder filling, knock behaviour, charge air cooling demand and overall engine efficiency. In the compressor map, efficiency islands are shown and used deliberately for turbo matching and operating point selection.

The compressor map describes the operating behaviour of a turbocharger compressor in terms of mass flow, pressure ratio, speed and efficiency. It also shows limits such as surge line and choke line. In engine development, the compressor map is used to select a turbocharger that fits the engine operating range and to perform turbo matching. A well-selected map avoids unstable operating regions and enables high efficiency across the relevant load points.

The compressor map describes the behaviour of the compressor in the turbocharger. It shows mass flow, pressure ratio, speed, efficiency and limits such as surge line and choke line. The turbine map describes the behaviour of the turbine on the exhaust side. It shows how exhaust mass flow, pressure ratio and efficiency are converted into shaft power. For turbo matching, both maps must be considered together. Good compressor operation is of limited value if the turbine creates excessive back pressure or does not provide enough power. Conversely, a well-matched turbine can still drive the compressor into unfavourable regions. The matching determines boost build-up, efficiency, exhaust gas temperature, surge margin and transient behaviour.

The connecting rod links the piston and crankshaft and transfers forces from combustion and mass acceleration. It is exposed to alternating tensile and compressive loads and must provide high strength at low weight. Its length, stiffness and mass influence kinematics, friction, piston side forces and vibration behaviour. In engine development, the connecting rod is an important component for durability and mechanical efficiency.

The crankcase houses the crankshaft, bearings, oil volume and often other engine components. It is a central structural component of the engine and must carry loads from combustion, mass acceleration and bearing forces. In addition to strength and stiffness, oil management, ventilation, friction, NVH behaviour and manufacturing are important. In modern engines, the crankcase also affects packaging, thermal management and system integration.

The crankshaft converts the linear motion of the pistons into rotary motion through the connecting rods. It transfers engine torque to the drivetrain, generator or driven machine. Important development aspects include strength, torsional vibrations, bearing loads, lubrication, mass balancing and friction. In high-performance and heavy-duty engines, the crankshaft is especially loaded by high cylinder pressures and continuous operation.

The cylinder is the working space in which the piston moves and the combustion process takes place. Together with the cylinder head, piston, valves and, where used, cylinder liner, it forms the combustion chamber and guides the piston. Cylinder geometry, surface, cooling and sealing influence friction, oil consumption, heat transfer and durability. In simulation, the cylinder is a central region for charge motion, combustion and thermal loading.

The cylinder head closes the combustion chamber at the top and usually contains intake ports, exhaust ports, valves, spark plug or injectors. It influences charge motion, mixture formation, heat transfer, knock tendency, combustion speed and component temperatures. Especially in hydrogen engines and high-performance engines, cylinder head design is critical for safe combustion and cooling. Changes to the cylinder head always affect flow, combustion, mechanics and thermal management.

The cylinder liner is the cylindrical running surface in which the piston and piston rings move. It can be designed as a separate component or as a machined surface in the engine block. The cylinder liner affects friction, wear, oil consumption, sealing and heat transfer to the cooling system. Material, coating, surface texture and thermal integration are decisive for durability and efficiency.

Cylinder pressure is the pressure in the combustion chamber during the engine cycle. It results from compression, combustion, expansion and gas exchange. The time history of cylinder pressure is decisive for torque, efficiency, component loading, knock tendency and combustion analysis. Modern engine development uses high-resolution cylinder pressure measurement to correlate combustion concepts and simulation models.

The cylinder wall is the surface along which the piston and piston rings move. It influences friction, wear, oil film behaviour, sealing, heat transfer and blow-by. Surface structure, material, coating and lubrication are decisive for efficiency and durability. From a thermal perspective, the cylinder wall is an important interface between the hot combustion chamber, piston ring pack and engine cooling.

A diesel particulate filter removes soot and particulate emissions from diesel exhaust. The particles accumulate in the filter and must be oxidized regularly through regeneration. The filter influences exhaust back pressure, fuel consumption, thermal management and exhaust aftertreatment. Good design considers loading, regeneration strategy, temperature control and durability.

Diffusion combustion means that fuel and oxidizer come together through mixing during the combustion process. The reaction is strongly governed by injection, jet preparation, turbulence and local mixing. Classical diesel combustion contains important diffusion-controlled portions. Diffusion combustion can enable high power density, but places high demands on soot, NOx, mixing quality and injection strategy.

Direct injection means that fuel is injected directly into the combustion chamber. This allows targeted influence on mixture formation, charge cooling, knock limit and load control. In gasoline engines, direct injection enables high power density and good efficiency, but can increase particulate emissions and wall wetting. In hydrogen engines, direct injection is especially interesting because it can avoid backfire in the intake system and allow high air charge.

With direct injection, fuel is introduced directly into the combustion chamber. With port fuel injection, fuel is introduced upstream of the intake valve into the intake system. Direct injection allows targeted control of mixture formation, charge cooling, stratification and knock limit. However, it is more demanding in terms of spray targeting, wall wetting, particulate formation and injector loading. Port fuel injection provides more time for evaporation and mixing and can generate very homogeneous mixtures. With gaseous fuels, however, it can displace intake air and reduce cylinder filling. In hydrogen engines, direct injection is better at avoiding backfire in the intake system, while port injection can be simpler and less expensive. The right choice depends on fuel, power target, emissions, packaging, cost and combustion strategy.

Downsizing means reducing engine displacement while maintaining or increasing engine power. This is usually achieved through boosting, direct injection, higher mean effective pressures and optimized combustion. The goal is lower fuel consumption in real operation, especially when the engine operates more often in efficient load regions. Challenges include knock, thermal loading, exhaust temperature, transient boost build-up and durability.

The droplet size distribution describes which droplet diameters occur in a spray and how frequently they appear. It is important because small droplets evaporate faster, while large droplets penetrate further or can hit walls. The distribution influences mixture formation, ignition, emissions and combustion stability. In CFD, it is often represented using statistical spray models and characteristic diameters such as the Sauter mean diameter.

A dual-fuel engine uses two fuels or two different combustion contributions within one operating concept. Often, a gaseous main fuel is ignited by a small amount of diesel or another pilot fuel. The goal is to combine advantages such as high efficiency, alternative fuels or lower CO₂ emissions with reliable ignition. Development is complex because mixture formation, ignition behaviour, load control, emissions and fuel shares must be matched across the operating range.

Early ignition describes combustion that starts earlier than desired for optimum engine operation. It can result from advanced spark timing, pre-ignition or local autoignition. A moderate advance of spark timing can improve efficiency, but excessively early combustion causes high negative work, knock and component loading. The term should therefore be distinguished between controlled advanced spark timing and uncontrolled pre-ignition.

The effective compression ratio considers not only the geometric compression ratio, but also the actual cylinder filling and intake valve closing timing. It is therefore especially relevant for variable valve timing, Miller cycle and Atkinson cycle operation. Late or early intake valve closing can reduce effective compression compared with geometric compression. For knock tendency, efficiency, load control and combustion stability, the effective compression ratio is often more meaningful than the purely geometric value.

E-fuels are synthetic carbon-containing fuels that can be produced using renewable energy. Hydrogen is a carbon-free energy carrier that can be used directly in an engine or in a fuel cell. The major advantage of e-fuels is their potential use in existing fuel infrastructure and in many existing engines. Their disadvantage is that combustion still produces CO2, even if the carbon ideally comes from a renewable source. Hydrogen produces no fuel-derived CO2 when used in an engine, but places high demands on storage, safety, refuelling infrastructure and mixture formation. In the overall energy chain, hydrogen is often more direct, while e-fuels require additional synthesis steps. E-fuels can be attractive for existing fleets, aviation, motorsport or special applications. Hydrogen is especially relevant where local carbon-free operation, fast refuelling and high continuous power are important.

An electrically assisted turbocharger combines a conventional exhaust gas turbocharger with an electric machine on the turbocharger shaft. The electric machine can accelerate the turbocharger to improve response or recover energy under certain conditions. This can improve turbo lag, boost build-up and operating strategy. Compared with a separate electric compressor, integration is more closely coupled with turbine, compressor, shaft and thermal loading.

e-ATL is the abbreviation for an electric-assisted exhaust gas turbocharger. The term is often used for turbochargers in which an electric machine supports acceleration of the turbocharger. e-ATL systems can help build boost pressure quickly, especially at low engine speed. They are technically demanding because high rotational speeds, exhaust gas temperatures, electrical power and control must be combined in a compact system.

An electric compressor compresses intake air using an electric motor. It can operate independently of exhaust gas flow and therefore improves transient response, especially at low engine speed. It is often used to reduce turbo lag or support multi-stage boosting systems. Its design must consider electrical power, battery system, compressor map, thermal loading and control strategy.

An electric compressor is a separate electrically driven compressor in the intake system. An electrically assisted turbocharger has an electric machine integrated on the turbocharger shaft. The electric compressor can provide boost support immediately and independently of exhaust gas flow. It is especially useful for bridging turbo lag at low engine speed. The electrically assisted turbocharger directly supports turbocharger acceleration and, depending on the concept, can also recover energy. It is therefore more closely integrated with turbine, compressor, bearings, speed limits and exhaust gas temperatures. The electric compressor is often more flexible as an add-on component, while the e-turbo can be more systemically integrated into turbocharger operation. The decision depends on packaging, electrical power, cost, control strategy, target operating map and thermal loading.

Engine development includes the technical design, optimization and validation of an engine. It covers mechanics, thermodynamics, gas exchange, combustion, boosting, cooling, friction, emissions and control. Modern engine development combines test bench work, 1D engine simulation, 3D CFD, measurement data analysis and component simulation. The goal is an engine that achieves the required operating conditions reliably, efficiently and with low emissions.

Engine knock is an undesired abnormal combustion phenomenon in which unburned end gas autoignites and causes strong pressure oscillations. It occurs mainly in spark-ignition and hydrogen engines under high load, high temperature or high compression. Knock can limit efficiency and power and, when severe, damage components. Important influencing factors include fuel, spark timing, compression ratio, boost pressure, mixture temperature, residual gas and combustion chamber geometry.

The equivalence ratio describes the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio. It is the inverse of lambda and is often used in combustion research and chemical kinetics. An equivalence ratio greater than 1 indicates a rich mixture, while a value below 1 indicates a lean mixture. The quantity is especially useful when comparing different fuels such as gasoline, methane or hydrogen.

Evaporation describes the phase transition of a liquid fuel into the gas phase. It is decisive for forming an ignitable air fuel mixture. Evaporation depends on droplet size, temperature, pressure, fuel properties, flow field and wall contact. Incomplete evaporation can cause wall films, particulate emissions, ignition problems and non-uniform combustion.

The excess air ratio lambda describes the ratio between the actual air amount and the stoichiometrically required air amount. Lambda 1 means stoichiometric operation, lambda greater than 1 means a lean mixture and lambda below 1 means a rich mixture. Lambda is a central quantity for combustion, efficiency, exhaust temperature and emissions. Especially in gas engines and hydrogen engines, lean mixtures are often used to improve efficiency and thermal behaviour.

Exhaust aftertreatment includes all systems that chemically or physically reduce emissions after the engine. These include catalysts, particulate filters, SCR systems, oxidation catalysts and sensors. It is closely linked to engine operation, exhaust gas temperature, lambda, NOx, particulates and cold-start behaviour. Good engine development does not treat aftertreatment in isolation, but as part of the complete powertrain system.

Exhaust back pressure is the pressure against which the engine must push the exhaust gases during the exhaust stroke. It is caused by the exhaust manifold, turbocharger, catalysts, filters, silencers and pipes. High exhaust back pressure increases gas exchange work and can negatively affect power, efficiency and residual gas fraction. At the same time, a certain pressure and energy level may be required for the turbocharger and exhaust aftertreatment.

Exhaust enthalpy describes the thermal and flow-related energy contained in the exhaust gas. It is especially important for turbochargers, turbocompounding, exhaust heat recovery and thermal activation of exhaust aftertreatment. High exhaust enthalpy can drive the turbocharger, but it also represents energy leaving the engine as a loss. In engine development, exhaust enthalpy is therefore optimized between efficiency, boosting, emissions and component protection.

Exhaust gas recirculation feeds part of the exhaust gas back into the intake system or directly into the combustion chamber. This reduces oxygen concentration and combustion temperature, which can reduce NOx in particular. EGR also affects knock tendency, pumping losses, combustion stability and efficiency. The correct EGR rate strongly depends on operating point, fuel, boosting, mixture formation and exhaust aftertreatment.

Exhaust gas temperature describes the temperature of the combustion gases in the exhaust system. It influences turbocharger performance, catalyst activity, component temperatures, exhaust aftertreatment and thermal durability. Excessive exhaust gas temperatures can damage the turbine, manifold, valves or catalysts. Temperatures that are too low can impair aftertreatment warm-up and therefore worsen cold-start emissions.

An exhaust gas turbocharger is a turbocharger driven by exhaust gas energy. It mainly consists of turbine, compressor, shaft, bearings and housings. Unlike a mechanical supercharger, it uses exhaust enthalpy that would otherwise be partly lost. Exhaust gas turbochargers are central to modern downsized engines, heavy-duty engines, diesel engines and many hydrogen engine concepts.

The exhaust manifold collects exhaust gases from multiple cylinders and guides them to the turbine or exhaust system. Its geometry influences pressure pulsations, exhaust back pressure, turbocharger excitation, temperature distribution and cylinder interactions. In turbocharged engines, the exhaust manifold is decisive for turbocharger response and efficiency. Thermal loading, packaging, material selection and pulse separation are central design topics.

The exhaust system guides combustion gases from the cylinder to exhaust aftertreatment, turbocharger and ultimately the environment. It includes exhaust ports, exhaust manifold, turbine, catalysts, filters, silencers and pipes. The exhaust system affects back pressure, turbocharger performance, emissions, acoustics and component temperatures. Good design reduces losses and uses exhaust enthalpy effectively for boosting or aftertreatment.

The exhaust valve controls the discharge of combustion gases from the cylinder. It is exposed to particularly high thermal loads because it is in direct contact with hot exhaust gas. Exhaust valve timing influences residual gas fraction, gas exchange work, exhaust temperature and the energy available for the turbocharger. Good design is important for power, emissions, component temperatures and durability.

The expansion ratio describes how strongly the burned gas expands during the power stroke. A high expansion ratio can extract more useful work from the hot gas and improve efficiency. In conventional engines, it is often geometrically linked to the compression ratio. Concepts such as the Atkinson or Miller cycle aim to deliberately decouple effective compression and expansion.

The flame front is the thin zone where unburned mixture transitions into burned gases. Heat release and chemical reactions are especially intense in this zone. The shape and speed of the flame front are influenced by turbulence, combustion chamber geometry, mixture distribution and wall proximity. In spark-ignition engines, gas engines and hydrogen engines, the flame front is decisive for burn duration, pressure rise and efficiency.

The flame kernel is the first small burning zone directly after ignition. It typically forms at the spark plug and must grow stably for a full flame to develop. Its development depends on ignition energy, local mixture composition, turbulence, pressure, temperature and the electrode environment. An unstable flame kernel can lead to misfire, slow combustion or cycle to cycle variations.

Flame propagation describes how the reaction zone moves through the combustible mixture after ignition. It depends on laminar flame speed, turbulence, mixture composition, pressure, temperature and in-cylinder flow. Fast and controlled flame propagation improves efficiency and reduces the duration of high wall heat losses. Slow or non-uniform flame propagation can lead to unstable combustion, knock or increased emissions.

Flame quenching describes the extinction of a flame when the reaction can no longer maintain enough heat and active radicals. Causes can include excessive heat losses, overly lean or rich mixtures, strong flame stretch, dilution or unfavourable flow. In engines, flame quenching is relevant near walls, in crevices, at cold surfaces, for emissions and incomplete combustion. Especially in lean combustion and hydrogen operation, quenching tendency must be considered.

The flammability limit describes the concentration range in which a fuel-air mixture is generally combustible. Below the lower flammability limit the mixture is too lean, and above the upper flammability limit it is too rich. Hydrogen has particularly wide flammability limits, enabling very lean combustion but also increasing safety and backfire risks. In engines, flammability limits are important for ignition stability, lean operation and misfire tendency.

A four-stroke engine operates with the four strokes intake, compression, power and exhaust. The crankshaft requires two revolutions for one complete working cycle. This concept is widely used in passenger car, commercial vehicle, motorcycle, industrial and racing engines. The clear separation of strokes enables good control of gas exchange, combustion and emissions.

Friction mean effective pressure describes the mechanical losses of an engine relative to displacement. It includes friction in piston rings, bearings, valvetrain, auxiliaries and other moving components. FMEP is important for understanding the difference between indicated and brake efficiency. Reducing friction mean effective pressure improves fuel consumption, part-load efficiency and the engine’s CO2 balance.

Fuel injection refers to the controlled introduction of fuel into the intake system or directly into the combustion chamber. It defines fuel quantity, timing, pressure, spray pattern and fuel distribution. Injection influences mixture formation, combustion, emissions, efficiency, wall wetting and component temperatures. Modern engines use complex injection strategies to optimize load, engine speed, emissions and combustion stability.

A fuel injector meters liquid or gaseous fuel into the intake system or combustion chamber. It is a central component of the injection system and affects spray pattern, atomization, mass flow and repeatability. For liquid fuels, droplet formation, evaporation and wall wetting are especially important. For gaseous fuels, flow capacity, sealing, jet propagation and fast actuation are more central.

The fuel rail is a distribution pipe that supplies fuel to the injectors at a defined pressure. It acts as a pressure accumulator and supports uniform supply to the individual cylinders. Pressure fluctuations in the rail can affect injection quantity, injection dynamics and cylinder to cylinder distribution. In modern engines, the rail is closely linked to pressure control, sensors, pump and injector strategy.

A gas engine is an internal combustion engine operated with gaseous fuels such as natural gas, biogas, hydrogen or synthesis gas. Depending on the concept, it can be designed as a spark-ignition engine or as a dual-fuel engine. Important topics include mixture formation, methane number, knock resistance, ignition energy, lean operation and exhaust emissions. Gas engines are often used in combined heat and power plants, industrial systems, commercial vehicles and stationary applications.

Gas exchange describes the replacement of exhaust gas in the cylinder by fresh air or fresh mixture. It includes intake, exhaust, scavenging, residual gas behaviour and the interaction with valve timing and pressure waves. Good gas exchange improves cylinder filling, efficiency, combustion stability and emissions. In development, gas exchange is often analysed using 1D engine simulation and locally supplemented by 3D CFD.

A gasoline engine is usually a spark-ignition engine operated with gasoline. A diesel engine uses diesel fuel and ignites it through high compression. Gasoline engines often operate stoichiometrically so that a three-way catalyst can reduce CO, HC and NOx simultaneously. Diesel engines usually operate with excess air, which can provide high efficiency but makes NOx and particulate control more important. A gasoline engine is strongly influenced by knock limit, spark strategy and evaporation behaviour. A diesel engine is strongly influenced by injection pressure, spray behaviour, ignition delay and diffusion-controlled combustion. In passenger car applications, both concepts also differ in exhaust aftertreatment, drivability, noise and cost. The technical comparison therefore involves fuel, ignition principle, mixture formation, emissions and efficiency.

A gasoline particulate filter reduces particulate emissions in spark-ignition engines, especially with direct injection. It operates similarly to a diesel particulate filter, but is adapted to the particle and temperature conditions of gasoline engines. Causes of particulates include wall wetting, local rich mixtures and incomplete evaporation. The filter must reduce particulates effectively without unnecessarily worsening back pressure, fuel consumption or packaging.

The heat release rate describes the rate of heat release during combustion over crank angle or time. It is often calculated from measured cylinder pressure and shows when and how strongly chemical energy is released. Heat release rate is not identical to the combustion profile, but it is closely related to it. In engine development, it is used to evaluate combustion phasing, burn duration, ignition delay and combustion modelling.

A heavy-duty engine is an engine for vehicles such as trucks, buses, construction machines, agricultural machines or special vehicles. It is designed for high continuous power, high torque, robustness, efficiency and long service life. Important topics include exhaust aftertreatment, boosting, thermal management, fuel consumption, operating strategy and real-world emissions. For alternative fuels such as hydrogen or methane, packaging, range, tank concept and durability also play a major role.

A high-performance engine is designed for high specific power, high engine speeds, high mean effective pressures or demanding dynamic requirements. This significantly increases the demands on combustion, boosting, cooling, lubrication, strength and control. Typical development targets are high power density, rapid load response, thermal stability and reproducible performance. High-performance engines are used in motorsport, performance vehicles, aviation applications and special industrial drives.

High-pressure EGR takes exhaust gas upstream of the turbine and returns it to the air path downstream of the compressor or upstream of the intake. It can act quickly and is especially controllable during transient operation. The disadvantage is that it can remove exhaust energy from the turbocharger and affect boost build-up. High-pressure EGR must therefore be matched with turbocharger, boost control, exhaust back pressure and thermal management.

A homogeneous mixture has a largely uniform distribution of fuel and air. This reduces local rich or lean zones. Homogeneous mixtures are typical of many spark-ignition engines with port fuel injection or sufficient mixing time. In practice, completely homogeneous mixing is difficult to achieve, so local deviations remain important for emissions, knock and combustion stability.

A homogeneous mixture has a fuel-air distribution that is as uniform as possible throughout the combustion chamber. A stratified mixture deliberately has different local mixture compositions. With a homogeneous mixture, ignition and combustion are usually more stable and predictable. With a stratified mixture, for example, the region around the spark plug can be ignitable while the rest of the chamber remains leaner. This can enable fuel consumption benefits, especially at part load. At the same time, the requirements for injection timing, spray, charge motion and spark plug position increase. Stratified mixtures can promote local rich zones, particulates, NOx or unstable combustion. In modern engines, mixture strategy is therefore closely adapted to operating condition, emission concept and fuel.

A hybrid powertrain combines at least two energy converters or energy storage systems, usually an internal combustion engine and an electric motor with a battery. This allows the combustion engine to operate at more favourable points, while the electric motor enables recuperation, boosting and electric driving. For engine development, this changes load profiles, start-stop frequency, warm-up behaviour, exhaust aftertreatment and thermal management. Hybridization can reduce fuel consumption and emissions, but significantly increases system complexity.

Hydrocarbon emissions consist of unburned or partially burned fuel components. They can result from wall quenching, crevices, wall films, misfire, incomplete evaporation or poor mixture formation. HC emissions are especially relevant during cold start and in spark-ignition engines with liquid fuels. In hydrogen operation, there are no fuel-derived hydrocarbon emissions because hydrogen is not a hydrocarbon compound.

Hydrogen combustion is the reaction of hydrogen with oxygen to form water while releasing heat. Hydrogen has a high flame speed, wide flammability limits and very low minimum ignition energy. This enables lean and efficient combustion concepts, but also increases the risks of backfire, pre-ignition and knock. Since hydrogen contains no carbon, no CO2 emissions are produced from the fuel, although NOx can still form at high temperatures.

A hydrogen combustion engine is an internal combustion engine that uses hydrogen as fuel. Hydrogen burns very quickly, has wide flammability limits and requires only low ignition energy. This enables lean mixtures and high efficiencies, but also increases the demands on mixture formation, backfire, pre-ignition, knock and NOx prevention. In development, combustion chamber, injection strategy, charge motion, ignition and boosting are strongly coupled.

A hydrogen combustion engine uses hydrogen as fuel and is usually designed as a spark-ignition or dual-fuel combustion engine. A diesel engine uses diesel fuel and operates with compression ignition. The hydrogen engine produces no fuel-derived CO2, CO, hydrocarbon emissions or soot. In diesel engines, soot, NOx, particulates and complex exhaust aftertreatment are central development topics. In hydrogen engines, NOx remains relevant because of high combustion temperatures, especially if operation is not sufficiently lean. In addition, backfire, pre-ignition, knock, high flame speed and hydrogen storage are critical. The diesel engine offers high efficiency, high robustness and established infrastructure. The hydrogen engine is attractive when carbon-free fuel, fast refuelling and reuse of engine expertise are priorities.

A hydrogen combustion engine is a heat engine that burns hydrogen and produces mechanical work. An electric motor converts electrical energy directly into torque. During operation, the electric motor has no local combustion, no exhaust emissions and very direct response. The hydrogen engine requires air path, injection, ignition, cooling, exhaust aftertreatment and often a gearbox or generator connection. The advantage of the hydrogen engine lies in high power density, fast refuelling and reuse of known engine platforms. The electric motor is usually superior in efficiency, controllability, noise and maintenance effort. The system decision depends on energy storage, range, duty cycle, infrastructure, cost and application. For heavy-duty vehicles, stationary applications or special machinery, the hydrogen engine can be technically meaningful, while passenger car applications often favour battery-electric drives.

A hydrogen combustion engine burns hydrogen in an internal combustion engine and produces mechanical work through pistons, cranktrain and possibly boosting. A fuel cell converts hydrogen electrochemically into electrical energy and drives an electric motor. The hydrogen engine uses many known components and development processes from engine technology. The fuel cell has no local combustion and can operate very efficiently and quietly under suitable conditions. In hydrogen engines, NOx, backfire, pre-ignition, knock, mixture formation and boosting are key topics. In fuel cells, membrane behaviour, water management, air supply, cooling, hydrogen purity and system cost are decisive. The hydrogen engine can be attractive for high power density, robust continuous operation and existing industrial infrastructure. The fuel cell is especially relevant when electric vehicle architecture, high part-load efficiency and local zero-emission operation are the main priorities.

H2ICE stands for hydrogen internal combustion engine, while FCEV stands for fuel cell electric vehicle. In an H2ICE, hydrogen is burned and mechanical work is generated by the engine. In an FCEV, the fuel cell produces electrical energy that drives an electric motor. The H2ICE can reuse existing engine platforms, manufacturing know-how and parts of the supplier chain. The FCEV follows a more electric powertrain architecture and requires fuel cell, battery, power electronics and electric motor. In H2ICE development, NOx, mixture formation, backfire, pre-ignition, knock and efficiency are central challenges. In FCEVs, system cost, hydrogen purity, lifetime, water and thermal management and infrastructure are decisive. Both concepts use hydrogen, but they differ fundamentally in energy conversion, system architecture, development risks and application profile.

A hydrogen combustion engine and a gasoline engine can both be designed as spark-ignition combustion engines. The central difference lies in the fuel and its combustion properties. Hydrogen has very high flame speed, very wide flammability limits and very low minimum ignition energy. This enables very lean and unthrottled combustion concepts, but also makes backfire, pre-ignition and knock more critical. Gasoline has much higher volumetric energy density and is easier to store and handle. In gasoline engines, CO2, CO, HC, particulates and knock are key topics. In hydrogen engines, the fuel produces no CO2, CO, HC or soot, but NOx remains relevant. For development, the two concepts differ especially in injection, mixture formation, ignition system, materials, safety and boosting.

A hydrogen dual-fuel engine combines hydrogen with a second fuel or ignition concept. Hydrogen often acts as the main energy carrier, while a small amount of diesel or another fuel supports ignition. The concept can be interesting for converting existing engine platforms, but places high demands on control, safety, mixture formation and emission behaviour. Critical topics include ignition stability, NOx formation, pre-ignition and clean separation of the fuel paths.

The hydrogen fuel rail distributes hydrogen at a defined pressure to the hydrogen injectors. Because of hydrogen’s low density and high diffusivity, flow areas, pressure stability, sealing and safety are particularly important. Pressure pulsations in the rail can affect injection quantities and therefore mixture distribution. For hydrogen engines, the rail is a safety-critical and functionally important component of the fuel system.

A hydrogen injector is an injector that meters gaseous or, in special cases, cryogenic hydrogen. Because of hydrogen’s low density, high flow capacity, short opening times and suitable injection pressures are especially important. At the same time, sealing, material compatibility, backfire safety and precise mixture formation must be controlled. In H2ICE concepts, the hydrogen injector directly affects power, efficiency, NOx formation and abnormal combustion risks.

H₂ICE is the common English abbreviation for hydrogen internal combustion engine. It refers to a hydrogen combustion engine, not a fuel cell system. The term is frequently used in research, vehicle development and industrial decarbonization projects. For technical assessment, efficiency, NOx emissions, power density, hydrogen storage, mixture formation and operating strategy are key factors.

A hydrogen internal combustion engine is an internal combustion engine that operates with hydrogen and produces mechanical work through internal combustion. The term is often used to distinguish it from a hydrogen fuel cell. Technically relevant topics include direct injection or port fuel injection, lean combustion, high flame speed, NOx formation and abnormal combustion phenomena. Hydrogen internal combustion engines are especially interesting when existing engine and manufacturing expertise is to be reused.

Hydrogen is a carbon-free energy carrier, while hydrocarbons consist of hydrogen and carbon. Combustion of hydrocarbons generally produces CO2 and, depending on combustion quality, CO, HC, soot or particulates. Hydrogen combustion mainly produces water from the fuel, so no fuel-derived CO2 or soot is formed. Hydrogen has very high gravimetric energy density, but very low volumetric energy density. This makes storage, tank volume and pressure level much more challenging for hydrogen. Hydrocarbons are easier to store, compatible with existing infrastructure and have high energy density per volume. Hydrogen has wide flammability limits, high flame speed and low minimum ignition energy, enabling lean combustion but making ignition safety more demanding. In engines, this difference leads to completely different requirements for injection, safety, mixture formation and emission strategy.

Ignition describes the start of combustion in a fuel-air mixture. It can be initiated deliberately by a spark plug, by autoignition due to high temperature and compression, or unintentionally at hot spots. Ignition quality affects flame kernel formation, burn duration, cycle-to-cycle variation and misfire tendency. In engines, ignition is closely coupled with mixture formation, turbulence, pressure, temperature and ignition energy.

Ignition delay is the time or crank angle between injection or spark trigger and the measurable start of combustion. In diesel engines, it describes the delay between start of injection and autoignition. In spark-ignition engines, the term can describe the early phase until stable flame development. Ignition delay affects pressure rise, combustion noise, emissions and the phasing of the combustion profile.

Ignition energy is the energy introduced into the mixture by the ignition system. It must be sufficient to form a stable flame kernel that continues to grow despite flow, heat losses and mixture inhomogeneity. Higher ignition energy can improve lean operation, EGR tolerance and ignition stability. However, excessive or poorly applied energy does not automatically improve combustion because mixture, turbulence and electrode environment are also decisive.

The ignition limit describes the range in which a mixture can be ignited by an ignition source under specific conditions. It is closely related to the flammability limit, but more strongly considers the actual ignition conditions such as energy, temperature, pressure and flow. In an engine, a mixture may be theoretically flammable but practically difficult to ignite. Ignition limits are therefore important for lean operation, cold start, hydrogen engines and ignition system design.

In-cylinder turbulent kinetic energy describes the energy of turbulent velocity fluctuations inside the cylinder. It is important for mixture formation, flame propagation and combustion speed. A higher turbulence level can accelerate combustion, but can also influence wall heat losses and flow losses. In development, this quantity is often evaluated using 3D CFD to tune intake ports, valve lift, swirl, tumble and combustion chamber geometry.

Indicated efficiency describes how efficiently the fuel energy released in the cylinder is converted into indicated work. It is calculated from the cylinder pressure trace and evaluates the thermodynamic engine process before mechanical losses. This makes it useful for assessing combustion concept, combustion phasing, heat release and wall heat losses. Indicated efficiency is higher than brake efficiency because friction and auxiliary losses are not yet subtracted.

Indicated mean effective pressure is a comparison quantity calculated from cylinder pressure for the work generated inside the cylinder. It describes the theoretical work produced in the combustion chamber relative to displacement. IMEP is useful for comparison independent of engine size and is used to evaluate combustion, load and cylinder-to-cylinder variation. It does not yet include mechanical losses in the cranktrain or auxiliaries.

Injection duration describes how long an injector remains open during an injection event. Together with injection pressure, injector geometry and fuel properties, it determines the injected fuel quantity. A long injection duration may be necessary at high load, but can reduce the available mixing time. Especially in direct injection, injection duration is closely coupled with piston position, valve timing, wall contact and ignition timing.

Injection pressure is the pressure at which fuel is introduced through the injector into the intake system or combustion chamber. It affects jet velocity, penetration depth, atomization, evaporation and mixture formation. High injection pressures can promote fine droplets and fast mixing, but increase the demands on pump, rail, injector and sealing. For gaseous fuels such as hydrogen, injection pressure also affects mass flow capability and jet propagation.

Injection timing describes when fuel is injected within the engine cycle. It is usually specified in crank angle degrees and has a strong influence on mixture formation, evaporation, wall wetting, ignition and combustion. Early injection provides more mixing time, but can promote wall contact or reverse flow effects. Late injection enables stratification and load control, but increases the demands on spray targeting and rapid mixing.

An injector is the component that meters and introduces fuel into the engine. Through nozzle design, opening duration, pressure and actuation, it determines fuel quantity and distribution. Injectors are decisive for mixture formation, emissions, power density and combustion stability. In modern engines, they must operate precisely, quickly and reproducibly over many load points.

The intake manifold distributes intake air or air-fuel mixture to the individual cylinders. It influences pressure loss, cylinder filling, distribution uniformity, pulsations and torque curve. Length, cross-section, plenum volume and runner geometry can be optimized for resonance effects and gas exchange. In port fuel injection or gas engines, the intake manifold is also important for mixture formation and cylinder-to-cylinder distribution.

The intake system guides air or air-fuel mixture to the engine. Depending on the concept, it includes air filter, airbox, throttle valve, charge air cooler, intake manifold, intake ports and sensors. Its geometry influences pressure loss, volumetric efficiency, flow distribution, acoustics and in-cylinder charge motion. In engine development, the intake system is often designed using both 1D simulation and 3D CFD.

The intake valve controls the entry of fresh air, air-fuel mixture or premixed gas into the cylinder. Its size, position, opening duration and lift curve influence cylinder filling and charge motion. Swirl, tumble and in-cylinder turbulent kinetic energy strongly depend on the intake port and intake valve. In engine development, the intake valve is therefore a key lever for efficiency, combustion speed and emissions.

Internal combustion engine development is the development of engines in which energy conversion takes place through internal combustion. It considers not only individual components, but also the interaction of combustion chamber, air path, injection, ignition, boosting, cooling and exhaust aftertreatment. A change in intake port, boost pressure or combustion chamber geometry can affect mixture formation, turbulence, combustion duration, knock tendency and efficiency at the same time. This makes the combination of simulation, testing and system understanding especially important.

Knock and pre-ignition are both abnormal combustion phenomena, but they occur at different times and through different mechanisms. Knock typically occurs after regular ignition when unburned end gas autoignites. Pre-ignition means that the mixture ignites before the intended spark timing. Knock is strongly linked to end-gas temperature, pressure, spark timing, fuel and combustion chamber geometry. Pre-ignition is often triggered by hot spots, deposits, oil droplets, overheated components or local reaction centres. Knock can often be reduced by retarding spark timing, reducing boost pressure or improving cooling. Pre-ignition is more difficult to control because it occurs before controlled ignition. Especially in highly turbocharged spark-ignition engines and hydrogen engines, a clear distinction is crucial for component protection and calibration.

Knock is undesired autoignition of the end gas that causes pressure oscillations in the combustion chamber. Super knock is an extremely severe form of abnormal combustion with very high pressure rise rates and peak cylinder pressures. Normal knock often occurs repeatably near certain operating limits and can be influenced by spark retard. Super knock is often rarer, more stochastic and much more severe. It can be linked to pre-ignition, oil droplets, deposits or local hot spots. The mechanical loading during super knock is much higher and can damage components very quickly. While knock is a classical limitation for power and efficiency, super knock is an acute durability risk. Modern highly boosted spark-ignition engines and hydrogen engines therefore require very conservative protection against super knock.

Laminar flame speed describes how fast a planar flame propagates through a quiescent, homogeneous mixture without turbulence. It is a fundamental fuel property and depends on pressure, temperature, mixture composition and residual gas fraction. Hydrogen has a very high laminar flame speed compared with many hydrocarbons. In engine simulation, it is often used as a base quantity for turbulent combustion models and flame propagation models.

Lean combustion occurs with excess air, meaning lambda greater than 1. It often lowers combustion temperature and can improve efficiency and NOx behaviour. At the same time, overly lean combustion can become slow, unstable or difficult to ignite. In gas engines and hydrogen engines, lean combustion is a central approach, but must be carefully matched with turbulence, ignition, boosting and exhaust aftertreatment.

Lean combustion takes place with excess air, meaning lambda greater than 1. Rich combustion takes place with excess fuel, meaning lambda below 1. Lean combustion can increase efficiency and reduce combustion temperatures. However, it can also lead to slow flame propagation, misfire or higher local NOx peaks under certain conditions. With hydrocarbons, rich combustion is often used for full-load enrichment, component protection or stable ignition. It usually causes higher fuel consumption and more CO, HC or soot emissions. In hydrogen engines, no carbon-based emissions are produced from the fuel, but mixture strength strongly affects NOx, knock and pre-ignition. The comparison is therefore central to combustion development, calibration and exhaust aftertreatment.

A lean mixture contains more air or less fuel compared with the stoichiometric composition. It can reduce combustion temperature and improve efficiency, especially in gas engines and hydrogen engines. At the same time, an overly lean mixture can lead to slow combustion, misfire or unstable combustion. The limit strongly depends on fuel, turbulence, ignition system, combustion chamber and operating condition.

A lean mixture contains more air or less fuel than required for stoichiometric combustion. A rich mixture contains more fuel or less air than required stoichiometrically. Lean operation can improve efficiency and thermal behaviour because lower throttling losses and lower combustion temperatures are possible. However, overly lean mixtures can burn slowly, become unstable or cause misfire. Rich mixtures can improve ignition stability and component cooling, but with hydrocarbons they worsen fuel consumption and emissions. In spark-ignition engines with a three-way catalyst, lambda 1 is often required for optimum aftertreatment performance. In hydrogen engines, lean operation is especially attractive because hydrogen has very wide flammability limits. The choice between lean and rich is therefore always a trade-off between efficiency, emissions, stability and component protection.

Low-pressure EGR takes exhaust gas downstream of the turbine and aftertreatment and returns it upstream of the compressor. This can allow higher EGR mass flow with less direct impact on turbine energy. At the same time, it increases demands on compressor compatibility, condensation, contamination, control and cooling. Low-pressure EGR is often used when high EGR rates and good NOx reduction are required under acceptable boost conditions.

Low-speed pre-ignition, or LSPI, is an abnormal combustion phenomenon that typically occurs at low engine speed and high load. It is especially known from highly turbocharged direct-injection spark-ignition engines. LSPI can trigger individual severe combustion events with high pressure peaks and component damage. Influencing factors include oil, fuel, deposits, injection strategy, boost pressure, temperature and combustion chamber geometry.

LSPI stands for low-speed pre-ignition and describes premature ignition at low engine speed and high load. Classical knock, by contrast, usually occurs after regular ignition through autoignition of the end gas. LSPI often occurs stochastically and can cause individual very severe pressure events. Knock is usually more closely linked to spark timing, end-gas temperature and operating limits. LSPI is strongly influenced by oil, fuel, deposits, direct injection, boost pressure and combustion chamber condition. Simple spark retard is not always sufficient against LSPI because the event begins before the intended ignition. Knock often limits optimum efficiency, while LSPI mainly threatens component protection and durability. Highly boosted downsizing engines therefore require specific LSPI avoidance strategies.

Main injection introduces the largest share of fuel energy into the combustion chamber. It largely determines power, torque, combustion profile and pressure rise. In diesel engines, it is central to the balance between premixed and diffusion-controlled combustion. In spark-ignition or hydrogen engines, its effect strongly depends on injection timing, spray targeting, charge motion and ignition strategy.

Minimum ignition energy is the smallest amount of energy required to reliably ignite an ignitable mixture. It depends on fuel, mixture composition, pressure, temperature, turbulence and electrode geometry. Hydrogen has a very low minimum ignition energy, making it easy to ignite but also more sensitive to unintended ignition sources. In engine development, minimum ignition energy influences the ignition system, backfire risk, pre-ignition and safety concept.

A misfire occurs when an engine cycle does not burn or burns only incompletely. Causes can include an overly lean mixture, insufficient ignition energy, poor mixture formation, high residual gas fraction, low temperature or technical faults. Misfires increase emissions, reduce smoothness and can stress exhaust aftertreatment or turbochargers. In development, misfire limits are especially important for lean operation, cold start, gas engines and hydrogen engines.

Mixture formation describes how fuel and air are mixed in the combustion chamber or intake system. It is influenced by injection strategy, fuel properties, charge motion, evaporation, wall contact, turbulence and combustion chamber geometry. Good mixture formation is a prerequisite for stable combustion, high efficiency and low emissions. In hydrogen engines, it is especially critical because local rich zones, backfire or NOx formation must be avoided.

Multiple injection describes splitting the total fuel quantity into several individual injection events within one engine cycle. This allows targeted influence on mixture formation, combustion noise, pressure rise, emissions and component temperatures. In diesel engines, pilot, main and post injections are commonly used. In spark-ignition and hydrogen engines, multiple injections can also help improve local mixture distribution and combustion stability.

Multi-stage turbocharging uses several compression stages to cover a wide operating range with high boost pressure and good efficiency. Typical combinations include small and large turbochargers, supercharger and turbocharger, or electrically assisted systems. The goal is good response at low engine speed and sufficient air mass at high load. Calibration is complex because switching points, bypass valves, boost control and thermal loading must work together.

A naturally aspirated engine fills its cylinders without a turbocharger or mechanical supercharger. Air enters the combustion chamber due to the intake stroke vacuum and ambient pressure. Naturally aspirated engines often provide direct response and lower air path complexity. However, their power density is limited because the maximum air mass per cycle is determined by ambient conditions and gas exchange behaviour.

A naturally aspirated engine fills its cylinders without additional compression of the intake air by a turbocharger or supercharger. A turbocharged engine uses exhaust gas energy to compress the intake air and increase the air mass in the cylinder. As a result, a turbocharged engine can produce significantly more torque and power at the same displacement. A naturally aspirated engine is simpler, often has very direct response and lower thermal complexity. A turbocharged engine enables downsizing and high power density, but requires boost pressure control, charge air cooling, turbocharger matching and thermal component protection. In spark-ignition engines, boosting often increases knock requirements. In diesel and hydrogen engines, boosting is especially important for excess air, power and emission control. The choice depends on power, drivability, cost, efficiency, packaging and development target.

NOx formation describes the formation of nitrogen oxides, mainly NO and NO2, during combustion. NOx is formed especially at high temperatures, with excess oxygen and sufficient residence time. Lean combustion can improve efficiency, but depending on temperature control it can also increase NOx. In hydrogen engines, NOx is the central exhaust emission because the fuel itself contains no carbon and therefore produces no CO2, CO, HC or soot.

Partially premixed combustion lies between purely premixed combustion and purely diffusion combustion. Part of the fuel is already mixed with air before ignition, while further mixing and reaction occur during combustion. This principle is relevant in modern diesel and low-temperature combustion concepts. The goal is often to reduce soot and NOx while maintaining efficiency and combustion stability.

Particulate emission refers to solid or liquid particles in the exhaust gas. It mainly results from soot, ash, oil components, wall films or incomplete combustion of carbon-containing fuels. In direct-injection spark-ignition engines, particulate emission is important because of local rich zones and wall wetting. In hydrogen engines, fuel-derived particles are strongly reduced, although oil or wear-related particles may still be relevant.

A passenger car engine is an internal combustion engine for passenger vehicles. It must meet a broad range of requirements including drivability, efficiency, emissions, cost, packaging, noise behaviour and everyday usability. Modern passenger car engines are often turbocharged, hybridized and strongly influenced by emission legislation. In development, transient operation, cold start, part-load efficiency, knock resistance and thermal management are especially important.

Peak cylinder pressure is the maximum pressure occurring in the cylinder during one engine cycle. It is an important quantity for component loading, sealing, bearing forces, knock limit and mechanical design. High peak cylinder pressures can increase power density, but require robust pistons, connecting rods, crankshaft, cylinder head and head gasket. In calibration, peak cylinder pressure is often monitored as a hard operating limit.

Pilot injection is a small fuel quantity injected before the main injection. It is mainly used in diesel engines to reduce ignition delay, pressure rise rate and combustion noise. Pilot injection thermally and chemically prepares the combustion chamber for the main combustion event. Correct calibration of quantity and timing is crucial because it affects emissions, efficiency and combustion stability.

The piston transfers combustion pressure to the crankshaft through the connecting rod. At the same time, it bounds the combustion chamber, carries the piston rings and influences charge motion, heat transfer and compression. The piston geometry, especially the crown and bowl, directly affects mixture formation and combustion. Pistons are highly loaded thermally and mechanically, so cooling, friction, weight and durability must be carefully designed.

The piston bowl is a shaped recess in the piston crown that strongly influences the combustion chamber and charge motion. It is especially used in diesel engines, hydrogen engines and direct injection concepts to control mixture formation and combustion. Bowl shape, depth, rim design and position influence swirl, tumble, spray-wall interaction, flame propagation and wall heat losses. An unfavourable piston bowl can significantly worsen emissions, efficiency, knock tendency or thermal loading.

A piston ring seals the gap between piston and cylinder wall. It reduces blow-by, controls the oil film and oil consumption, and supports heat transfer from the piston to the cylinder wall. Piston rings influence friction, emissions, wear, compression and engine durability. Their design is a compromise between good sealing, low friction losses and reliable oil management.

Port fuel injection means that fuel is introduced upstream of the intake valve into the intake manifold or intake port. This usually provides more time for evaporation and mixing than direct injection. The method is robust and can support homogeneous mixture formation. Disadvantages can include wall film formation, poorer cylinder to cylinder distribution or displacement of intake air when gaseous fuels are used.

Post injection occurs after the main injection and is used depending on the engine concept for emissions, exhaust temperature or aftertreatment. In diesel engines, it can support particulate filter regeneration or thermal management of exhaust aftertreatment. It also affects soot oxidation, hydrocarbon emissions and efficiency. Poorly calibrated post injection can cause oil dilution, wall wetting or increased fuel consumption.

Pre-ignition describes ignition of the mixture before the intended spark timing. It can be triggered by hot spots, glowing deposits, oil droplets, overheated components or local chemical reactions. Pre-ignition is particularly dangerous because the pressure rise can occur very early during the compression stroke. It can cause knock, super knock, high component temperatures and severe engine damage.

Premixed combustion means that fuel and air are largely mixed before combustion starts. The flame then propagates through a prepared combustible mixture. Premixed combustion is typical of many spark-ignition engines, gas engines and hydrogen concepts. It can be very fast and efficient, but is sensitive to knock, pre-ignition, backfire and local mixture variations.

Pressure rise rate describes how quickly cylinder pressure increases during combustion. High pressure rise rates can indicate fast combustion, harsh combustion, knock or unfavourable combustion phasing. They influence combustion noise, mechanical loading and durability. In engine development, pressure rise rate is limited to balance efficiency, refinement and component protection.

The pressure trace describes cylinder pressure over crank angle or time during one engine cycle. It shows compression, combustion, expansion and gas exchange portions of the engine process. Heat release rate, mean effective pressure, pressure rise rate, peak cylinder pressure and knock indicators can be derived from it. In engine development, the pressure trace is one of the most important measurements for evaluating combustion and engine performance.

The quenching distance is the smallest distance between two walls or to a wall at which a flame can still propagate. If the distance is smaller, so much heat is removed from the flame that it extinguishes. The quenching distance depends on fuel, pressure, temperature, mixture composition and flow. Hydrogen has special requirements for crevices, backfire protection and combustion chamber design because of its high reactivity and small quenching distances.

A racing engine is an engine developed for motorsport requirements. The main targets are usually maximum power, fast response, low weight, high engine speed and a usable power band. At the same time, thermal management, durability, knock limit, lubrication and regulations must be controlled. Simulations help design the combustion chamber, air path, exhaust system, boosting and cooling specifically for the intended racing application.

A range extender is an auxiliary unit that generates electrical energy in an electrified vehicle to increase driving range. It is often a small internal combustion engine driving a generator. Because the engine usually operates in a limited number of defined operating points, it can be strongly optimized for efficiency, noise, emissions and continuous operation. Range extender concepts are especially interesting when battery size, charging infrastructure or range requirements are critical.

The residual gas fraction describes the fraction of burned gas that remains in the cylinder after exhaust or is reintroduced through exhaust gas recirculation. Residual gas can reduce combustion temperature and NOx emissions, but can also impair combustion stability and ignition behaviour. It is influenced by valve timing, exhaust back pressure, scavenging, EGR and gas exchange. Its control is especially important at part load, lean operation and with alternative fuels.

Rich combustion occurs with excess fuel, meaning lambda below 1. With hydrocarbon fuels, it can be used for component cooling or power enrichment, but often increases CO, HC and soot emissions. In spark-ignition engines, rich combustion can reduce exhaust temperature, but worsens fuel consumption. In hydrogen engines, the fuel does not produce CO or soot, but local rich regions can influence combustion stability and NOx formation.

A rich mixture contains more fuel or less air compared with the stoichiometric composition. It can be used for high load, component cooling, ignition stability or specific operating strategies. With hydrocarbon fuels, rich combustion can lead to increased CO, HC and soot emissions. With hydrogen, the emission logic is different, but local rich regions can influence combustion, NOx and abnormal ignition phenomena.

Rightsizing describes the targeted design of an engine for its actual operating profile rather than simply reducing displacement. Power, torque, efficiency, emissions, cost, durability and duty cycle are evaluated together. Unlike aggressive downsizing, the engine should be efficient not only in a test cycle but also in real operation. Rightsizing is especially relevant for heavy-duty vehicles, hybrid powertrains, range extenders and alternative fuels.

The Sauter mean diameter is a characteristic droplet diameter that describes the ratio of droplet volume to droplet surface area. It is commonly used as a measure of the evaporation and reaction capability of a spray. A small Sauter mean diameter means a large surface area per fuel volume and promotes rapid evaporation. For injection, spray CFD and combustion, it is an important comparison quantity for different nozzles and operating conditions.

Scavenging describes the displacement of residual gas from the cylinder by fresh charge. It is especially relevant in two-stroke engines, turbocharged engines and operating points with valve overlap. Good scavenging can reduce residual gas and improve cylinder filling. Excessive or poorly controlled scavenging can cause fresh charge losses, higher emissions or reduced efficiency.

An SCR catalyst reduces nitrogen oxides in the exhaust through selective catalytic reduction. Ammonia is usually supplied from a urea-water solution. SCR systems are especially important for diesel engines and lean-burn engines because excess oxygen makes conventional three-way NOx reduction difficult. Effectiveness depends on temperature window, dosing, mixing, engine-out NOx and avoidance of deposits.

Sequential turbocharging is a form of multi-stage charging in which turbochargers are activated or bypassed depending on the operating point. A small turbocharger can provide fast response at low engine speed, while a larger turbocharger supplies sufficient air mass at high load. This enables a broad usable engine speed range. The challenge is clean switching without torque gaps, pressure surges or thermal overload.

A single-turbo system uses one turbocharger for the engine. A twin-turbo system uses two turbochargers arranged in parallel, sequentially or by cylinder bank. The single turbo is usually simpler, less expensive and easier to integrate. However, it must cover the entire operating range alone, which can create trade-offs between response and maximum power. A twin-turbo system can combine small and large turbochargers or use one turbocharger per cylinder bank. This can improve response, air mass flow and packaging depending on engine layout. Twin-turbo systems are more complex in terms of control, exhaust routing, boost control and cost. The choice depends on engine size, cylinder count, power target, packaging and desired drivability.

Soot formation occurs during incomplete combustion of carbon-containing fuels in locally rich regions. It is especially relevant in diesel combustion, direct injection, poor mixture formation and wall wetting. Soot formation strongly depends on injection strategy, oxygen availability, temperature, pressure, turbulence and evaporation behaviour. Hydrogen does not produce fuel-derived soot because it contains no carbon.

Spark ignition means that combustion is started by an external ignition source, usually a spark plug. It is typical of spark-ignition engines, gas engines and many hydrogen engines. Spark timing can be actively controlled and influences pressure trace, efficiency, knock and exhaust temperature. Spark ignition must match local mixture composition, turbulence and in-cylinder flow so that a stable flame kernel forms.

A spark-ignition engine is an internal combustion engine in which the air-fuel mixture is usually ignited by a spark plug. It typically operates with gasoline, natural gas, hydrogen or other ignitable fuels. Important development parameters include spark timing, compression ratio, mixture formation, knock limit, charge motion and exhaust emissions. Compared with a diesel engine, its combustion is more strongly characterized by spark ignition, flame propagation and knock tendency.

A spark-ignition engine usually operates with ignition by a spark plug. A diesel engine operates with compression ignition because fuel is injected into highly compressed hot air. A spark-ignition engine is usually controlled through air flow, throttling, boost pressure and spark timing. A diesel engine is mainly controlled through injected fuel quantity and injection strategy. In spark-ignition engines, knock, pre-ignition, spark timing and mixture formation are key development topics. In diesel engines, ignition delay, injection pressure, soot, NOx and exhaust aftertreatment are more central. Spark-ignition engines are often smooth and high-revving, while diesel engines often provide high efficiency and high torque. The technical difference is therefore not only the fuel, but mainly the combustion process.

The spark plug generates the spark that initiates the flame kernel in spark-ignition engines. Its position, electrode geometry, heat range and ignition energy influence early flame development. An unfavourable spark plug location can promote slow combustion, knock, misfire or high cycle-to-cycle variation. In hydrogen engines, thermal management and avoidance of hot spots are especially important because ignition tendency is high.

Spark timing describes when the spark is triggered relative to crankshaft position. It is usually specified in crank angle degrees before or after top dead centre. Advanced spark timing can increase efficiency, but also increases knock tendency and pressure rise. Retarded spark timing reduces knock risk and pressure peaks, but can reduce efficiency and increase exhaust temperatures.

A spray is the spatial distribution of an injected liquid fuel into many droplets. It is influenced by injection pressure, nozzle design, fuel properties, ambient pressure, temperature and in-cylinder flow. Spray behaviour determines penetration depth, atomization, evaporation, wall wetting and mixing. In engine development, sprays are often analysed experimentally and using CFD because they strongly shape subsequent combustion.

The spray pattern describes the shape, direction and spatial distribution of a fuel jet or spray. It shows where the fuel travels and how it spreads in the combustion chamber or intake port. A good spray pattern avoids undesired wall contact and supports fast, uniform mixture formation. For multi-hole nozzles, direct injection and hydrogen injection, the spray pattern is a central design parameter.

Squish flow occurs when gas is forced from narrow regions between piston and cylinder head into the combustion chamber near the end of compression. It can generate turbulence and influence mixing and flame propagation. Squish areas are deliberately used to support fast and stable combustion. At the same time, excessively narrow gaps can increase wall heat losses, knock tendency or mechanical risks.

A stationary engine is an internal combustion engine used for fixed applications rather than primarily for vehicle propulsion. Typical applications include generators, combined heat and power units, pump drives, industrial plants or emergency power systems. Stationary engines often operate for long periods at defined load points, making efficiency, durability, emissions and maintenance effort especially important. Gas engines and hydrogen engines are particularly relevant concepts in this area.

Stoichiometric combustion occurs with exactly the amount of air theoretically required for complete chemical conversion of the fuel. In spark-ignition engines, stoichiometric operation is especially important for the function of a three-way catalyst. It enables simultaneous reduction of CO, HC and NOx in the exhaust gas. For lean gas engines, diesel concepts or hydrogen engines, operation is often deliberately outside stoichiometry.

A stoichiometric mixture contains exactly the amount of air theoretically required for complete chemical combustion of the fuel. For gasoline, the air-fuel ratio is approximately 14.7 to 1 by mass, while other fuels have different values. Stoichiometric operation is especially important for spark-ignition engines with a three-way catalyst. For hydrogen, natural gas or lean-burn concepts, operation is often deliberately outside stoichiometry.

A stratified mixture deliberately has different fuel-air ratios within the combustion chamber. Often, the region around the spark plug is ignitable or richer, while other regions are leaner. This concept can provide fuel consumption benefits at part load, but requires precise injection, charge motion and ignition. Risks include increased particulate formation, local over-rich zones, unstable combustion or higher NOx emissions.

A supercharger mechanically compresses the intake air, usually driven directly by the engine or by an electric motor. In the classical engine context, supercharger usually means a mechanical charger, not the compressor stage of a turbocharger. Its advantage is very direct response because it does not need to wait for exhaust gas energy to build up. The disadvantage is the power required for driving it, which can reduce efficiency.

Super knock is a particularly severe and potentially damaging form of abnormal combustion. It is characterized by extremely high pressure rise rates and high peak cylinder pressures. Super knock is often linked to pre-ignition or local ignition sources and cannot be controlled solely by spark timing like normal knock. It is a critical development risk especially in highly turbocharged spark-ignition engines and hydrogen engines.

Super knock describes an extremely severe combustion event with very high pressure rise rates. Pre-ignition describes the premature start of combustion before the intended spark timing. The two phenomena are often related, but they are not identical. Pre-ignition can be a trigger that subsequently leads to super knock. Super knock then primarily describes the consequence in the pressure trace and the mechanical loading. Pre-ignition more strongly describes the timing and cause of the unintended ignition. Early pre-ignition can be especially critical because combustion works against the still upward-moving piston. For engine development, it is important to avoid triggers such as hot surfaces, oil droplets, deposits, hydrogen pockets or unfavourable ignition sources.

The surge line describes the region in a compressor map where the compressor becomes unstable and mass flow can periodically collapse or even reverse. Surge occurs when pressure ratio and mass flow no longer match in a stable way. In engine operation, this can cause noise, pressure oscillations, poor drivability and high mechanical loading of the turbocharger. Boost pressure control and turbocharger sizing must therefore maintain sufficient margin to the surge line.

The surge line and choke line are operating limits in the compressor map. The surge line occurs at too low mass flow and too high pressure ratio. In this region, compressor flow can become unstable, periodically break down or even reverse. The choke line occurs at very high mass flow, where flow capacity barely increases despite additional pressure difference. Surge is mainly an unsteady stability problem with pressure oscillations, noise and mechanical loading. Choke is more of a flow capacity limit caused by high local flow velocities. For turbocharger sizing, the operating range must lie safely between both limits. Boost control must maintain sufficient margin to the surge line, especially during transient load changes.

Swirl is a rotational charge motion around the cylinder axis. It is mainly generated by the intake port, valve lift, valve timing and combustion chamber geometry. Swirl supports mixture formation, fuel distribution and combustion, especially in diesel engines, gas engines and some hydrogen concepts. Too much swirl can increase flow losses or weaken other desired charge motion patterns.

Swirl and tumble are both forms of charge motion inside the cylinder. Swirl is rotation around the cylinder axis. Tumble is a rolling motion perpendicular to the cylinder axis. Swirl is often used in diesel engines, gas engines and certain hydrogen concepts to support mixture formation and combustion. Tumble is especially important in spark-ignition engines because it can break down into turbulence near the end of compression and accelerate flame propagation. Both motion patterns are influenced by intake port, valve lift, valve timing, piston shape and combustion chamber geometry. Too much swirl or tumble can increase pressure losses and reduce cylinder filling. In development, 3D CFD is therefore used to assess which charge motion is optimal for mixture formation, burn duration, efficiency and emissions.

Thermal efficiency describes what fraction of the supplied chemical energy is converted into useful mechanical work. It is influenced by compression ratio, combustion phasing, wall heat losses, exhaust losses, gas exchange, friction and operating strategy. High thermal efficiency means that less fuel is required for the same useful work. In modern engines, it is improved through lean combustion, boosting, optimized combustion phasing, reduced heat losses and suitable cycle concepts.

A three-way catalyst reduces carbon monoxide, hydrocarbons and nitrogen oxides simultaneously under stoichiometric operation. It is typical for spark-ignition engines with lambda 1 control. To work efficiently, the air-fuel ratio must be controlled very precisely around lambda 1. Under continuous lean operation, a three-way catalyst alone is not sufficient for NOx reduction.

The throttle valve controls the air mass flow in the intake system by changing the flow area. In conventional spark-ignition engines, it is a central component for load control. A partially closed throttle valve causes pressure loss and therefore pumping losses, especially at part load. Modern concepts partly reduce these losses through variable valve timing, hybridization or alternative load control.

Transient response describes how quickly and stably an engine reacts to changes in load, engine speed or driver demand. In turbocharged engines, it is strongly influenced by turbocharger inertia, boost control, air path dynamics, injection, ignition, exhaust enthalpy and engine control. Good transient response means rapid torque build-up without overshoot, turbo lag, knock or exhaust temperature problems. In development, it is often evaluated using 1D engine simulation, test bench measurements and transient CFD for critical components.

Tumble is a rolling charge motion perpendicular to the cylinder axis. It is often used in spark-ignition engines to generate high turbulence in the combustion chamber near the end of compression. This turbulence can accelerate flame propagation and shorten combustion duration. Tumble is influenced by intake ports, valve lift, piston shape and combustion chamber geometry.

Turbine efficiency describes how efficiently the turbine converts exhaust enthalpy into mechanical shaft power. High turbine efficiency improves compressor drive and can reduce the required exhaust back pressure. This benefits boost build-up, fuel consumption, exhaust gas temperature and gas exchange. Turbine efficiency strongly depends on mass flow, pressure ratio, pulsation, turbine geometry and operating point.

The turbine map describes the operating behaviour of a turbocharger turbine in terms of mass flow, pressure ratio, speed and efficiency. It shows how much exhaust gas energy the turbine can convert into shaft power for the compressor at different operating points. In engine development, the turbine map is important for boost build-up, exhaust back pressure, transient response and exhaust gas temperature. A well-selected turbine map is essential for good turbo matching and stable engine performance across the full load range.

A turbocharged engine uses an exhaust gas turbocharger to compress the intake air and increase cylinder filling. This enables higher power density, engine downsizing and, in some cases, better efficiency. Developing a turbocharged engine requires careful matching of turbocharger, boost control, charge air cooling, knock limit, exhaust temperature and transient response. The interaction between 1D engine simulation, test bench work and 3D CFD in the intake and exhaust path is especially important.

A turbocharger uses exhaust gas energy to drive a compressor through a turbine. The compressor increases intake air pressure and therefore the possible cylinder charge. This increases power density, torque and downsizing potential. Turbocharger design must consider compressor map, turbine map, boost pressure, exhaust temperature, response, surge limit and efficiency together.

A turbocharger uses exhaust gas energy to drive a compressor. A supercharger is mechanically or electrically driven and does not directly depend on exhaust gas energy. The turbocharger can use energy that would otherwise partly leave with the exhaust and therefore improves power density. Its disadvantage is delayed response because sufficient exhaust energy must first build up. A mechanical supercharger responds very directly, but requires drive power from the engine. An electric compressor can operate independently of exhaust gas flow, but requires electrical power and a suitable vehicle electrical architecture. For high efficiency and high power, the turbocharger is often very attractive. For immediate response, low engine speed or support of complex boosting systems, a supercharger or electric compressor can offer advantages.

Turbo lag describes the delay between a load request from the driver or test bench and the actual build-up of boost pressure and torque. It occurs because the turbine, compressor and shaft first have to accelerate, requiring sufficient exhaust gas energy. Turbo lag can be particularly noticeable at low engine speed and with large turbochargers. Measures against turbo lag include smaller turbochargers, twin-scroll systems, VGT, electric assistance, e-turbochargers, electric compressors and optimized boost control.

Turbulent flame speed describes how fast a flame propagates through a turbulent mixture. It is usually much higher than laminar flame speed because turbulence wrinkles the flame front and increases the reaction surface. In engines, it depends on in-cylinder flow, swirl, tumble, turbulence level, pressure, temperature and mixture composition. It is an important quantity for burn duration, efficiency, knock tendency and predictive combustion models.

A twin-scroll turbocharger has a turbine housing with separated exhaust gas passages. This allows exhaust pulses from different cylinders to be separated and directed more effectively onto the turbine. It improves response, turbine efficiency and gas exchange, especially in engines with suitable firing intervals. The design requires appropriate cylinder grouping, manifold geometry and valve timing.

Twin turbo refers to a boosting system with two turbochargers. The turbochargers can be arranged in parallel, sequentially or assigned to different cylinder banks. Depending on the concept, the goal is better response, higher air mass, more compact turbo sizing or improved pulse utilization. In V-engines, one turbocharger is often used per cylinder bank, while sequential twin-turbo systems can combine different turbocharger sizes.

A two-stroke engine completes intake, compression, power and exhaust within one crankshaft revolution. This allows more power strokes at the same engine speed than a four-stroke engine. The main challenges are scavenging, fresh charge losses, emissions, lubrication and thermal loading. Two-stroke concepts are used in small engines, certain high-performance applications and large marine diesel engines.

A two-stroke engine completes one working cycle within one crankshaft revolution. A four-stroke engine requires two crankshaft revolutions with intake, compression, power and exhaust strokes. The two-stroke engine can theoretically achieve high power density because power strokes occur more frequently. However, scavenging, fresh charge losses, oil management, emissions and thermal loading are challenging. The four-stroke engine separates the strokes more clearly and therefore allows better control of gas exchange, combustion and emissions. This is why it is widely used in passenger cars, commercial vehicles and industrial engines. Two-stroke concepts remain relevant in small engines, certain high-performance applications and large marine engines. The technical comparison depends strongly on whether power density, emissions, durability or simplicity is prioritized.

A valve controls gas exchange in an internal combustion engine. Intake valves open the path for fresh charge or air into the cylinder, while exhaust valves release exhaust gas. Valves must withstand high temperatures, pressure forces, accelerations and cyclic loads. Their geometry, timing and lift curve influence cylinder filling, charge motion, residual gas fraction and efficiency.

Valve lift describes how far a valve is opened, either at maximum or at a specific point in time. Higher valve lift increases the effective flow area and can improve cylinder filling. At the same time, it affects flow velocity, swirl, tumble, valvetrain losses and mechanical loads. With variable valve lift, the valve lift can be adapted specifically to load and engine speed.

Valve overlap is the crank angle range during which intake and exhaust valves are open at the same time. It can be used to influence scavenging, residual gas fraction and turbocharger excitation. Too much valve overlap can cause fresh charge losses, unstable combustion or unfavourable emissions. In turbocharged engines and gas engines, valve overlap must be carefully matched with boost pressure, exhaust back pressure and operating strategy.

Valve timing describes the timing of intake and exhaust valve opening and closing. It is usually specified in crank angle degrees and is decisive for gas exchange. Valve timing affects cylinder filling, residual gas fraction, scavenging, pumping losses, torque curve and emissions. Depending on the operating strategy, early or late opening and closing can be used deliberately, for example for Miller or Atkinson cycles.

The valvetrain includes all components and mechanisms that control valve opening and closing. Depending on the design, this includes camshaft, valves, springs, finger followers, tappets, rocker arms and variable actuation systems. The valvetrain affects gas exchange, volumetric efficiency, residual gas fraction, emissions and friction losses. Modern variable valvetrains allow flexible adaptation to engine speed, load and operating strategy.

A variable geometry turbocharger has adjustable guide vanes or variable turbine geometry. This allows the turbine to adapt to exhaust mass flow and pressure ratio over a wider operating range. At low engine speed, VGT improves response, while at high load it can limit exhaust back pressure. The technology is especially common in diesel engines, but places high demands on temperature resistance, control and mechanics.

Variable valve lift changes valve lift depending on the operating condition. It can influence the effective flow area, charge motion and, in some cases, load control. In spark-ignition engines, it can reduce throttling losses and improve part-load efficiency. Technically, it is more complex than simple cam phasing, but provides additional degrees of freedom for gas exchange, fuel consumption and emissions.

Variable valve timing allows valve timing to be adjusted during engine operation. Intake or exhaust timing can be adapted to engine speed, load, temperature and operating strategy. The goal is a better balance of torque, efficiency, emissions, scavenging and drivability. Especially in turbocharged engines, hybrid powertrains and modern gas engines, variable valve timing is an important degree of freedom.

Wall heat loss is the portion of combustion heat transferred through combustion chamber walls, piston, cylinder head and cylinder liner to the cooling system. High wall heat losses reduce efficiency, but may be necessary for component protection and thermal control. They depend on combustion chamber geometry, surfaces, pressure, temperature, turbulence, flame position and cooling concept. In engine simulation and 3D CFD, wall heat losses are an important calibration and design parameter.

Wall quenching is a specific form of flame quenching at cold or heat-absorbing walls. When the flame front approaches a wall too closely, heat is removed from it and the chemical reaction can stop locally. This can leave unburned hydrocarbons or residual mixture in near-wall regions. Wall quenching affects emissions, efficiency, wall heat losses and the design of the combustion chamber, crevices and spark plug location.

A wastegate is a bypass valve that routes part of the exhaust gas around the turbine. This limits turbine power and controls boost pressure. Wastegates are commonly used in turbocharged spark-ignition engines. Their design influences boost stability, exhaust back pressure, turbocharger speed, component temperature and transient operation.

A wastegate is a bypass valve that routes exhaust gas around the turbine. Variable turbine geometry uses adjustable vanes or a variable turbine geometry to adapt the turbine flow area. The wastegate mainly limits turbine power and thereby controls boost pressure. VGT actively changes turbine behaviour over a wider operating range. At low engine speed, a closed VGT can improve response, while at high load it can limit exhaust back pressure. A wastegate is simpler, less expensive and often more thermally robust. VGT offers more control authority, but is more demanding mechanically, thermally and in terms of control. VGT is very common in diesel engines, while higher exhaust temperatures make its use more challenging in gasoline engines.