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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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 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.

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 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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 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.

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.