cooling system

Air cooling uses air as the cooling medium to reject heat directly or indirectly to the environment. It is often simpler than liquid cooling, but its performance is limited because air has low heat capacity and density. Air cooling is used for electronics, small engines, battery modules with low power density or auxiliary coolers. Air volume flow, pressure loss, fan power, ducting and uniform airflow are decisive.

A battery cold plate is a cold plate that removes heat from battery cells or battery modules. It must ensure a uniform temperature distribution because temperature differences influence ageing, performance and safety. The cooling channels must be designed so that all cell regions are supplied sufficiently. Besides thermal performance, sealing, crash safety, electrical insulation, weight and manufacturing are important.

A battery cooler removes heat from the battery cooling circuit. It can be designed as an air-to-coolant heat exchanger, chiller connection or another heat exchanger type. The goal is to keep the battery within a temperature window that supports power, fast charging, safety and lifetime. Not only average temperatures matter, but also temperature uniformity between cells and modules.

Battery cooling keeps battery cells within a safe and performant temperature range. It is decisive for fast charging capability, continuous power, lifetime, ageing and safety. In addition to maximum temperature, temperature uniformity between cells is especially important. Battery cooling can be implemented using cold plates, coolant circuits, chillers, air cooling or direct cooling methods.

The boiling point is the temperature at which a fluid changes from liquid to gas at a given pressure. It strongly depends on pressure and is therefore important for coolants, refrigerants and phase-change cooling. In a cooling system, the boiling point affects the risk of vapour bubbles, cavitation and local overheating. Increasing pressure or selecting a suitable fluid can extend the usable temperature range of a cooling circuit.

A bypass is a flow path that routes part of the coolant around a component. It is often used to bypass the radiator during warm-up or to ensure minimum flow rates. Bypasses improve system controllability, but if poorly designed they can create unwanted short-circuit flows. In simulation, the bypass is important because it strongly influences temperature distribution, pressure loss and flow split.

A charge air cooler reduces the temperature of compressed intake air downstream of the compressor. Colder charge air has higher density and therefore improves cylinder filling. It can also reduce knock tendency and thermal loading. However, a charge air cooler also causes pressure loss, so cooling performance and flow resistance must be optimized together.

A chiller is a refrigerant-to-coolant heat exchanger. It uses the refrigerant circuit to actively cool a coolant circuit below ambient temperature or below the level achievable with a normal radiator. In electric vehicles, a chiller is often used for battery cooling, fast charging or high continuous power. Its design must consider cooling capacity, pressure loss, control, condensation and coupling with air conditioning and heat pump operation.

A cold plate is a component that absorbs heat through a flat contact surface and transfers it to a coolant. It is often used for battery cells, power electronics, inverters or other planar heat sources. Key factors are contact thermal resistance, channel geometry, temperature uniformity, pressure loss and manufacturability. A poorly designed cold plate can cause local hot spots even if total coolant flow is sufficient.

A condenser is a heat exchanger in the refrigerant circuit where the refrigerant rejects heat and changes from gas to liquid. In vehicles, it is often placed in front of or within the cooling module. Its performance affects air conditioning, battery cooling through the chiller and overall thermal management. Because the condenser influences airflow and pressure loss in the front-end, it must be considered together with radiators, fans and ducting.

Conduction is heat transfer within a solid body or stationary medium through molecular interaction. It depends on temperature gradient, thermal conductivity, geometry and material thickness. In cold plates, housings, battery cells, engine components and power electronics, conduction is often the first step from the hot spot to the coolant. Poor conduction or unfavourable contact surfaces can cause high local temperatures.

Convection is heat transfer between a surface and a flowing fluid. It depends on flow velocity, fluid properties, temperature difference, surface area and flow regime. In radiators, cooling channels, water jackets and air ducts, convection is the central mechanism for heat removal. In CFD simulations, convection is used to evaluate local heat transfer and temperature fields.

Convective heat transfer commonly describes heat transfer between a wall and a fluid in engineering contexts. It is decisive for radiators, cooling channels, water jackets, cold plates and airflow. Heat transfer depends on flow regime, velocity, fluid properties, surface geometry and temperature difference. In simulations, it is often described using the heat transfer coefficient or locally through CFD results.

Coolant is the fluid that absorbs and transports heat in the cooling system. In vehicles, a water-glycol mixture is often used because it transfers heat well and provides freeze protection. Coolant properties influence heat transfer, pressure loss, pump power, corrosion protection and electrical safety. For batteries and power electronics, additional requirements for conductivity, ageing and material compatibility may be relevant.

A coolant circuit transports heat using a liquid coolant through pipes, components and heat exchangers. It can use water-glycol mixtures, special fluids or other suitable media. The circuit must provide sufficient volume flow without causing unnecessarily high pumping losses. Deaeration, freeze protection, corrosion protection, sealing and temperature control are important practical design topics.

Coolant mass flow rate describes the mass of coolant flowing through the cooling circuit per unit time. Together with heat capacity and temperature difference, it determines how much heat can be transported. Too little mass flow can cause local overheating, while too much mass flow increases pump power and pressure losses. For thermal management and simulation, mass flow is often more meaningful than pure volume flow because it accounts for coolant density.

A coolant pump generates the volume flow in the cooling circuit. It must provide enough flow to remove heat from critical components. At the same time, it consumes power and can promote cavitation, noise or unnecessary pressure losses if poorly designed. In modern thermal management systems, the pump map is an important element for control, efficiency and transient temperature behaviour.

A coolant valve controls, blocks or distributes coolant flow between different paths. It can be used to connect or disconnect radiators, bypasses, batteries, cabin heating, chillers or engines selectively. In modern thermal management systems, valves play a central role in temperature control and energy efficiency. A poor valve strategy can cause hot spots, long warm-up times or unnecessary pump power.

Coolant volume flow rate describes the coolant volume flowing through the cooling circuit per unit time. It is often given in litres per minute and is important for pump sizing, cooling channels and heat exchangers. Volume flow affects flow velocity, heat transfer and pressure loss. Not only total volume flow is decisive, but also its distribution across parallel paths and critical components.

Cooling describes the controlled removal of heat from components, fluids or systems. In vehicles and machines, it protects components from overheating and keeps them within a suitable temperature window. Good cooling considers not only maximum heat flow, but also temperature distribution, pressure loss, packaging, control and operating conditions. Especially in engines, batteries, power electronics and high-performance applications, cooling is a central design topic.

Cooling air ducting describes the targeted guidance of air to and through heat rejecting components. It includes ducts, seals, guide vanes, air guides, openings and separations against leakage flows. Good cooling air ducting increases the useful air mass flow through the heat exchangers and reduces unwanted bypass flow. It is especially important when packaging space is limited or several heat exchangers are stacked in one cooling package.

The cooling air inlet is the opening through which ambient air enters the cooling air path. Its size, position and shape influence cooling air mass flow, pressure level, inflow quality and aerodynamic drag. An inlet that is too small can limit cooling performance, while an oversized inlet can increase vehicle drag. The optimal design depends on cooling demand, vehicle speed, fan operation and front-end aerodynamics

Cooling air mass flow rate describes the mass of air flowing through a radiator, condenser, charge air cooler or the complete cooling package per unit time. It largely determines how much heat can be rejected to ambient. Cooling air mass flow depends on vehicle speed, fan operation, front-end geometry, ducting and cooling package pressure loss. In vehicle CFD, it is a central target quantity because it affects both cooling performance and aerodynamic drag.

The cooling air path describes the route of air from the inlet through the cooling package, fan and engine bay to the outlet. It determines how uniformly and with what pressure loss the heat exchangers are supplied with air. An unfavourable cooling air path can cause recirculation, dead zones, local undersupply or increased drag. For vehicle cooling and aerodynamics, the cooling air path is therefore an important optimization area.

A cooling channel is a flow passage through which coolant is guided to absorb heat. It can be integrated into engine components, cold plates, power electronics, housings or additively manufactured parts. Geometry, cross-section, bends and roughness influence heat transfer and pressure loss. In CFD design, cooling channels are often checked for hot spot avoidance and uniform flow distribution.

A cooling circuit is a closed or partly closed fluid loop that transports heat from a source to a sink. The coolant absorbs heat at components and rejects it through radiators or heat exchangers. Important quantities include volume flow rate, pressure loss, temperature difference, pump power and heat transfer. Vehicles often have several coupled cooling circuits with different temperature levels.

Cooling system pressure loss describes the pressure drop caused by pipes, hoses, radiators, valves, bends, manifolds and cooling channels. It determines the pump head and electrical power that the coolant pump must provide. Excessive pressure loss can reduce volume flow and cause hot spots or non-uniform temperatures. In design, pressure loss must always be evaluated together with cooling capacity, temperature uniformity and pump efficiency.

Cylinder head cooling removes heat from one of the most thermally loaded engine components. Critical regions include exhaust valves, valve seats, spark plug, injector, combustion chamber roof and exhaust ports. Insufficient cooling can promote knock, pre-ignition, material fatigue or local hot spots. Especially in high power density engines, hydrogen engines and motorsport applications, cylinder head cooling is a central development area.

Direct cooling means that the coolant absorbs heat directly at or very close to the heat source. This can reduce thermal resistance and improve control of high heat fluxes. Examples include direct oil cooling of electric motor windings, direct battery cooling or cooling channels close to power semiconductors. The challenges are sealing, electrical safety, fluid compatibility, contamination and manufacturable integration.

An electric coolant pump is driven electrically and independently of engine speed. This allows coolant flow to be controlled according to demand, even when the combustion engine is stopped or during electric operation. It is especially important for hybrid vehicles, electric vehicles, after-run cooling and precise thermal management. Its design must consider head, volume flow, efficiency, lifetime, noise and controllability.

Electric motor cooling removes heat from stator, windings, rotor, magnets, bearings and housing. Winding temperatures and magnet temperatures are especially critical because they influence power, efficiency and lifetime. Depending on power density, jacket cooling, oil cooling, direct cooling or housing cooling can be used. Good electric motor cooling enables high continuous power and prevents thermal derating.

Engine cooling keeps the combustion engine, cylinder head, engine block, oil and adjacent components within the allowed temperature range. It must remove high heat flows from the combustion chamber, exhaust region, friction and oil circuit. At the same time, it should not keep the engine unnecessarily cold because that can worsen efficiency, friction and emissions. Modern engine cooling is therefore a controlled thermal management problem, not only a maximum cooling capacity question.

An evaporator is a heat exchanger in the refrigerant circuit where the refrigerant absorbs heat and evaporates. In vehicles, it is classically used for cabin cooling. It can also be indirectly connected to other thermal functions through refrigerant circuit strategies. Its performance depends on airflow, refrigerant state, humidity, temperature difference and control.

Forced convection occurs when a fluid is moved by a pump, fan, vehicle speed or another external driving force. It enables significantly higher heat transfer than natural convection. Typical examples include coolant flow in cooling channels, airflow through radiators or fan operation at standstill. The design must consider heat transfer and pressure loss together because higher flow velocity usually requires more pump or fan power.

Fuel cell cooling keeps the fuel cell stack within a narrow temperature window. Excessive temperatures can damage the membrane, catalyst and lifetime, while temperatures that are too low can reduce efficiency and disturb water management. Uniform stack temperature is especially important because local hot spots can overload individual cells. Cooling is closely linked to air supply, water management, cold start, load changes and system efficiency.

A heat exchanger transfers heat from one medium to another without necessarily mixing the media. Typical combinations are coolant-to-air, oil-to-coolant, refrigerant-to-coolant or air-to-air. Its design depends on heat flow, temperature difference, surface area, material, flow arrangement and pressure loss. In thermal management systems, heat exchangers are central coupling elements between different circuits.

Heat flow describes the amount of heat transferred per unit time. It is usually given in watts and is a central quantity for radiator design, battery cooling, engine cooling and power electronics. High heat flow does not automatically mean high temperature if cooling surface and heat transfer are sufficient. For design, heat flow must always be considered together with temperature limits, pressure loss and duty cycle.

Heat flux describes heat flow per area. It is especially important when high heat output occurs on a small surface, for example at power semiconductors, battery cell contacts, combustion chamber walls or piston crowns. High heat flux can quickly create local hot spots if conduction and convection are insufficient. In CFD and thermal simulation, heat flux is an important boundary condition and result quantity.

A heat sink is a region, component or medium that absorbs or removes heat from a system. Examples include ambient air, coolant, radiators, chillers, heat exchangers or massive components with thermal storage capacity. The capability of a heat sink depends on temperature, surface area, heat transfer and heat capacity. In system design, the heat sink must match the heat source and the duty cycle.

A heat source is a region or component where heat is generated or introduced into a system. Examples include combustion, electrical losses, friction, chemical reactions, battery internal resistance or power semiconductors. For thermal management, it is decisive how much heat is generated, where it is generated and when it occurs. A wrong assumption about the heat source often leads to incorrect temperature fields and unsuitable cooling design.

Heat transfer describes the transport of thermal energy due to temperature differences. The main mechanisms are conduction, convection and thermal radiation. In technical cooling systems, these mechanisms usually occur together, for example conduction through a component and convection to coolant or air. Heat flow, temperature difference, surface area, material and flow condition are decisive for design.

The heat transfer coefficient describes how intensively heat is transferred between a surface and a fluid. It links heat flux to the temperature difference between wall and fluid. High values mean strong heat transfer, but often result from higher flow velocities and therefore higher pressure losses. In cooling CFD and 1D thermal management models, the heat transfer coefficient is a central evaluation quantity.

A high-temperature circuit operates at a higher temperature level and is often used for combustion engines, cylinder heads, engine blocks or certain power components. It can reject heat efficiently to ambient because the temperature difference to the environment is relatively large. At the same time, materials, seals, coolant and control must be designed for high temperatures. In hybrid or complex vehicles, the high-temperature circuit is often coupled with low-temperature circuits and cabin heating.

A high-temperature radiator rejects heat from a high-temperature circuit to ambient air. It is typically used for combustion engines, engine blocks, cylinder heads or other components operating at higher temperature levels. Because coolant temperature is high, the temperature difference to ambient is larger, which supports heat rejection. Nevertheless, packaging, airflow management, fan operation and pressure loss must be designed carefully.

A hot spot is a local region with significantly increased temperature. It often results from a high local heat source, poor flow, low heat transfer, poor contact or unfavourable material layout. Hot spots can damage components, accelerate ageing, limit performance or create safety risks. In cooling design, finding and avoiding hot spots is often more important than considering average temperatures alone.

Immersion cooling means that components are fully or partially immersed in a dielectric cooling fluid. This allows heat to be absorbed directly from many surfaces, enabling high power density and good temperature uniformity. The method is especially interesting for batteries, power electronics and data centres. Critical topics include fluid cost, material compatibility, sealing, serviceability, electrical safety and long-term fluid stability.

Indirect cooling means that the heat source is not directly touched by the coolant. Heat is first transferred through material layers, contact surfaces or thermal interface materials and then removed by the coolant. This principle is robust and controllable, but introduces additional thermal resistance. Indirect cooling is often used in battery cold plates, power electronics housings, cooling jackets and conventional heat exchangers.

Inverter cooling removes heat from power semiconductors, circuit boards, DC link and housing. The inverter converts electrical energy between battery and electric motor and generates heat losses in the process. Excessive temperatures reduce efficiency, lifetime and allowable continuous power. Cooling must handle high local heat fluxes while ensuring electrical insulation, sealing, packaging and low pressure loss.

Liquid cooling uses a liquid coolant, usually water-glycol or a special fluid, to absorb and transport heat. It achieves higher heat transfer and better temperature control than pure air cooling. Typical applications include combustion engines, batteries, inverters, electric motors and fuel cells. The design must consider volume flow, pressure loss, pump, sealing, corrosion protection, deaeration and material compatibility.

A low-temperature circuit operates at a lower temperature level than a conventional engine cooling circuit. It is often used for charge air cooling, battery, inverter, electric motor or sensitive electronics. Because the temperature difference to ambient is smaller, low-temperature circuits often require larger cooling surfaces, good airflow management or active support from the refrigerant circuit and chiller. Its design is especially important for electrical power, battery lifetime and reproducible performance.

A low-temperature radiator rejects heat from a low-temperature circuit to ambient air. It is often used for charge air cooling, battery, inverter or electric motor cooling. Because the temperature level is lower, the driving temperature difference to ambient is smaller. Therefore, good airflow management, sufficient cooling surface and low air-side and coolant-side pressure loss are especially important.

Natural convection is caused by density differences in the fluid resulting from temperature differences. Warm regions rise, cooler regions sink and create a natural flow. Heat transfer is usually much lower than with forced convection. Natural convection is relevant for parked vehicles, heat soak, electronics housings, tanks and passively cooled components.

An oil cooler removes heat from engine oil, transmission oil or hydraulic oil. Oil provides not only lubrication, but also cooling for bearings, pistons, gears and friction contacts. Excessive oil temperature worsens viscosity, lubrication capability and lifetime. An oil cooler must therefore reject enough heat without causing excessive pressure loss or overly long warm-up times.

The overall heat transfer coefficient describes total heat transfer through several resistances, for example from one fluid through a wall into a second fluid. It includes heat transfer on both sides, conduction through the wall and possible contact or fouling resistances. This quantity is often used for heat exchangers, radiators, condensers and evaporators. It is important for estimating real component performance at system level.

Overheating means that a component, fluid or system exceeds an allowable temperature limit. Consequences can include power loss, material damage, ageing, seal failure, safety shutdown or thermal runaway. Causes include excessive heat generation, insufficient volume flow, air pockets, fouling, undersized cooling area or poor control. In simulation, overheating is evaluated through maximum temperatures, hot spots and transient duty cycles.

Phase-change cooling uses the high latent heat involved in evaporation, condensation, melting or solidification of a medium. This allows large amounts of heat to be transferred at nearly constant temperature. Examples include refrigerant circuits, heat pipes, boiling cooling and phase-change materials. The design is demanding because pressure level, boiling point, flow regime, dry-out, condensation and control must be safely managed.

Piston cooling reduces piston temperature, especially at the piston crown and piston ring area. Oil jets are often used to cool the piston from below. Good piston cooling protects against overheating, knock risk, oil coking, ring land damage and material fatigue. At the same time, it influences oil temperature, friction, pumping effort and the engine’s overall heat balance.

Power electronics cooling removes heat from components such as inverters, DC-DC converters, power semiconductors, circuit boards and DC links. High local heat fluxes at IGBTs, MOSFETs or SiC semiconductors are especially critical. Cooling affects efficiency, continuous power capability, lifetime and thermal derating. In addition to cooling performance, electrical insulation, sealing, pressure loss, packaging and thermal cycling resistance are important design criteria.

A radiator is a heat exchanger that rejects heat from the coolant to ambient air. It typically consists of tubes, fins, tanks and connections. Its performance depends on air volume flow, coolant volume flow, temperature difference, fin geometry and pressure loss. In a vehicle, the radiator is closely linked to the fan, front-end, air ducting and aerodynamics.

Radiator is the standard English term for a coolant-to-air heat exchanger. It usually refers to a component that rejects coolant heat to ambient air. The term is widely used in international projects, motorsport, CAE and vehicle development. In German technical glossaries, it should be explained as a synonym for Kühler to avoid translation ambiguity.

Radiator pressure drop describes the pressure loss experienced by coolant or cooling air as it flows through the radiator. It is caused by tubes, fins, flow turns, header regions and narrow flow passages. Low pressure drop reduces pump or fan power, but must not come at the expense of heat transfer. Good radiator design is therefore a compromise between thermal performance, pressure drop, packaging and weight.

Subcooling describes the state in a refrigerant circuit where a liquid is cooled below its saturation temperature at a given pressure. This ensures that liquid refrigerant is available upstream of the expansion valve and that vapour bubbles are avoided. Subcooling improves stability and performance of the refrigerant circuit, but if poorly controlled it can also indicate unfavourable control or excessive cooling. The term should not be confused with a battery or engine being too cold.

Temperature distribution describes how temperatures are distributed across a component, cooling circuit or system. A uniform temperature distribution is often important for lifetime, performance, efficiency and material stresses. Non-uniform distributions can cause hot spots, thermal gradients, ageing or distortion. Especially in batteries, fuel cells, cylinder heads and power electronics, temperature distribution is often more important than average temperature.

A temperature field describes the spatial distribution of temperature in a component, fluid or complete system. It shows where warm and cold regions are located and how strong local differences are. In CFD, CHT and thermal FEA, the temperature field is a central result quantity. It is important in development because average temperatures can appear uncritical while local hot spots already exceed component limits.

A temperature gradient describes how strongly temperature changes over a distance. Large temperature gradients can cause high heat flows, thermal stresses and material loading. In engines, batteries, power electronics and fuel cells, gradients are often critical for durability and ageing. Cooling must therefore not only reduce maximum temperatures, but also control local temperature differences.

Temperature uniformity describes how evenly temperature is distributed within a component or system. High temperature uniformity means small temperature differences and often lower thermal stresses and more uniform ageing. It is especially important for battery cells, fuel cell stacks, power electronics and cooled engine components. Good cooling must therefore not only remove enough heat, but also distribute coolant flow effectively.

Thermal management describes the overall control and distribution of heat in a technical system. It includes heat sources, heat sinks, coolant circuits, heat exchangers, pumps, valves, sensors and control strategies. The goal is not only cooling, but maintaining optimal temperatures for efficiency, lifetime, safety and performance. In modern vehicles, thermal management is especially complex because battery, electric motor, inverter, cabin, engine and aftertreatment have different temperature requirements.

Thermal radiation is heat transfer by electromagnetic radiation and does not require a fluid as transport medium. It becomes much more important as temperature increases and depends on surface, emissivity, view factor and temperature difference. In vehicles, thermal radiation is relevant around exhaust systems, turbochargers, battery shielding or engine encapsulation. At high temperatures, radiation can account for a significant share of heat transfer.

Thermal resistance describes how strongly a component or heat path impedes heat flow. It links temperature difference and heat flow and is often used to evaluate heat conduction paths. Low thermal resistance enables good heat removal, while high resistance creates large temperature differences. In batteries, power electronics and cold plates, contact resistance, material thickness and thermal conductivity are often decisive.

A thermostat controls coolant flow depending on temperature. In a conventional setup, it opens the path to the radiator only after a defined coolant temperature is reached. This accelerates warm-up and brings the engine or system into the desired temperature range more quickly. Modern thermostats can be mechanical, electrically heated or fully electronically controlled.

Vehicle cooling includes all systems that reject heat from powertrain, battery, power electronics, cabin and auxiliary components to the environment. This includes radiators, fans, airflow management, coolant circuits, heat exchangers and control valves. Vehicle cooling is closely linked to aerodynamics, packaging, energy consumption, noise and driving profile. The key is that cooling must work not only at one operating point, but also in traffic, hill climbs, towing, track use or fast charging.

The water jacket is the cooling channel system in the engine block or cylinder head. It guides coolant around thermally loaded regions such as combustion chamber, valve seats, exhaust ports and cylinder liners. Its design influences component temperatures, knock tendency, distortion, durability and heat losses. Good water jacket flow avoids dead-water regions, local overheating and non-uniform temperature fields.