Internal combustion engine cooling refers to the cooling of an
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
, typically using either air or a liquid.
History
The change of air cooling to liquid cooling occurred at the start of World War II when the US military needed reliable vehicles. The subject of boiling engines was addressed, researched, and a solution found. Previous
radiatorRadiators are used for cooling internal combustion engines, chiefly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of such an engine....
s and
engine blocksThe cylinder block or engine block is a machined casting containing cylindrically bored holes for the pistons of a multi-cylinder reciprocating internal combustion engine, or for a similarly constructed device such as a pump...
were properly designed and survived durability tests, but used water pumps with a leaky
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
-lubricated "rope" seal (
glandA gland is a general type of stuffing box, used to seal a rotating or reciprocating shaft against a fluid. The most common example is in the head of a tap where the gland is usually packed with string which has been soaked in tallow or similar grease...
) on the pump shaft. The seal was inherited from steam engines, where water loss is accepted, since steam engines already expend large volumes of water. Because the pump seal leaked mainly when the pump was running and the engine was hot, the water loss evaporated inconspicuously, leaving at best a small rusty trace when the engine stopped and cooled, thereby not revealing significant water loss. Automobile
radiatorRadiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...
s (or
heat exchangerA heat exchanger is a device built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants,...
s) have an outlet that feeds cooled water to the engine and the engine has an outlet that feeds heated water to the top of the radiator. Water circulation is aided by a rotary pump that has only a slight effect, having to work over such a wide range of speeds that its impeller has only a minimal effect as a pump. While running, the leaking pump seal drained cooling water to a level where the pump could no longer return water to the top of the radiator, so water circulation ceased and water in the engine boiled. However, since water loss lead to overheat and further water loss from boil-over, the original water loss was hidden.
After isolating the pump problem, cars and trucks built for the war effort (no civilian cars were built during that time) were equipped with carbon-seal water pumps that did not leak and caused no more geysers. Meanwhile, air cooling advanced in memory of boiling engines... even though boil-over was no longer a common problem. Air-cooled engines became popular throughout Europe. After the war, Volkswagen advertised in the USA as not boiling over, even though new water-cooled cars no longer boiled over, but these cars sold well, and without question. But as air quality awareness rose in the 1960s, and laws governing exhaust emissions were passed, unleaded gas replaced leaded gas and leaner fuel mixtures became the norm. These reductions in the cooling effects of both the lead and the formerly rich fuel mixture, led to overheating in the air-cooled engines. Valve failures and other engine damage was the result. Volkswagen responded by abandoning their (flat) horizontally opposed air-cooled engines, while Subaru took a different course and chose liquid-cooling for their (flat) engines.
Today practically no air-cooled automotive engines are built, air cooling being fraught with manufacturing expense and maintenance problems. Motorcycles had an additional problem in that a water leak presented a greater threat to reliability, their engines having small cooling water volume, so they were loath to change; today most larger motorcycles are water cooled with many relying on
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
circulation with no pump.
Overview
Heat engineA heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle...
s generate mechanical power by extracting energy from heat flows, much as a
water wheelA water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, the development of hydropower. In the Middle Ages, waterwheels were used as tools to power factories throughout different counties. The alternatives were the windmill and human and animal...
extracts mechanical power from a flow of mass falling through a distance. Engines are not perfectly efficient, so more heat energy enters the engine than comes out as mechanical power; the difference is waste heat and must be removed. Internal combustion engines remove waste heat through cool intake air, hot exhaust gases, and explicit engine cooling.
Engines with higher efficiency have more energy leave as mechanical motion and less as waste heat. Some waste heat is essential: it guides heat through the engine, much as a water wheel works only if there is some exit velocity (energy) in the waste water to carry it away and make room for more water. Thus, all heat engines need cooling to operate.
Cooling is also needed because high temperatures damage engine materials and lubricants. Internal-combustion engines burn fuel hotter than the melting temperature of engine materials, and hot enough to set fire to lubricants. Engine cooling removes energy fast enough to keep temperatures low so the engine can survive.
Some high-efficiency engines run without explicit cooling and with only accidental heat loss, a design called
adiabaticIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid. The term "adiabatic" literally means impassable, coming from the Greek roots ἀ- , διὰ- , and βαῖνειν ; this etymology corresponds here to an...
. For example, 10,000 mile-per-gallon "cars" for the Shell economy challenge are insulated, both to transfer as much energy as possible from hot gases to mechanical motion, and to reduce reheat losses when restarting. Such engines can achieve high efficiency but compromise power output, duty cycle, engine weight, durability, and emissions.
Basic principles
Most internal combustion engines are
fluidA fluid is a substance that continually deforms under an applied shear stress. All gases are fluids, but not all liquids are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
cooled using either air (a gaseous fluid) or a liquid coolant run through a heat exchanger (
radiatorRadiators are used for cooling internal combustion engines, chiefly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of such an engine....
) cooled by air. Marine engines and some stationary engines have ready access to a large volume of water at a suitable temperature. The water may be used directly to cool the engine, but often has sediment, which can clog coolant passages, or chemicals, such as salt, that can chemically damage the engine. Thus, engine coolant may be run through a heat exchanger that is cooled by the body of water.
Most liquid-cooled engines use a mixture of water and chemicals such as antifreeze and rust inhibitors. Some use no water at all, instead using a liquid with different properties, such as
propylene glycolPropylene glycol, known also by the systematic name propane-1,2-diol, is an organic compound , with a faintly sweet taste, and is a colorless, nearly odorless, clear, viscous liquid that is hygroscopic and miscible with water, acetone, and chloroform...
or a combination of propylene glycol and
ethylene glycolEthylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet tasting liquid....
. Most "air-cooled" engines use some liquid oil cooling, to maintain acceptable temperatures for both critical engine parts and the oil itself, Most "liquid-cooled" engines use some air cooling, with the intake stroke of air cooling the combustion chamber. An exception is
Wankel engineThe Wankel engine is a type of internal combustion engine which uses a rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that is...
s, where some parts of the combustion chamber are never cooled by intake, requiring extra effort for successful operation.
There are many demands on a cooling system. One key requirement is that an engine fails if just one part overheats. Therefore, it is vital that the cooling system keep
all parts at suitably low temperatures. Liquid-cooled engines are able to vary the size of their passageways through the engine block so that coolant flow may be tailored to the needs of each area. Locations with either high peak temperatures (narrow islands around the combustion chamber) or high heat flow (around exhaust ports) may require generous cooling. This reduces the occurrence of hot spots, which are more difficult to avoid with air cooling. Air cooled engines may also vary their cooling capacity by using more closely-spaced cooling fins in that area, but this can make their manufacture difficult and expensive.
Only the fixed parts of the engine, such as the block and head, are cooled directly by the main coolant system. Moving parts such as the pistons, and to a lesser extent the crank and rods, must rely on the lubrication oil as a coolant, or to a very limited amount of conduction into the block and thence the main coolant. High performance engines frequently have additional oil, beyond the amount needed for lubrication, sprayed upwards onto the bottom of the piston just for extra cooling. Air-cooled motorcycles often rely heavily on oil-cooling in addition to air-cooling of the cylinder barrels.
Liquid-cooled engines usually have a circulation pump. The first engines relied on thermo-syphon cooling alone, where hot coolant left the top of the engine block and passed to the radiator, where it was cooled before returning to the bottom of the engine. Circulation was powered by convection alone.
Other demands include cost, weight, reliability, and durability of the cooling system itself.
Conductive heat transfer is proportional to the temperature difference between materials. If engine metal is at 250 °
CCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
and the air is at 20°C, then there is a 230°C temperature difference for cooling. An air-cooled engine uses all of this difference. In contrast, a liquid-cooled engine might dump heat from the engine to a liquid, heating the liquid to 135°C (Water's standard boiling point of 100°C can be exceeded as the cooling system is both pressurised, and uses a mixture with antifreeze) which is then cooled with 20°C air. In each step, the liquid-cooled engine has half the temperature difference and so at first appears to need twice the cooling area.
However, properties of the coolant (water, oil, or air) also affect cooling. As example, comparing water and oil as coolants, one gram of oil can absorb about 55% of the heat for the same rise in temperature (called the
specific heat capacitySpecific heat capacity, often shortened to specific heat, is the measure of the heat energy required to increase the temperature of a of a substance by unit degree. The term originated primarily through the work of 18th-century physicist Joseph Black who conducted various heat measurements and...
). Oil has about 90% the density of water, so a given volume of oil can absorb only about 50% of the energy of the same volume of water. The thermal conductivity of water is about 4 times that of oil, which can aid heat transfer. The viscosity of oil can be ten times greater than water, increasing the energy required to pump oil for cooling, and reducing the net power output of the engine.
Comparing air and water, air has vastly lower heat capacity per gram and per volume (4000) and less than a tenth the conductivity, but also much lower
viscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick," having a higher viscosity...
(about 200 times lower: 17.4 × 10
−6 Pa·s for air vs 8.94 × 10
−4 Pa·s for water).
Continuing the calculation from two paragraphs above, air cooling needs ten times of the surface area, therefore the fins, and air needs 2000 times the flow velocity and thus a recirculating air fan needs ten times the power of a recirculating water pump.
Moving heat from the cylinder to a large surface area for air cooling can present problems such as difficulties manufacturing the shapes needed for good heat transfer and the space needed for free flow of a large volume of air. Water boils at about the same temperature desired for engine cooling. This has the advantage that it absorbs a great deal of energy with very little rise in temperature (called heat of vaporization), which is good for keeping things cool, especially for passing one stream of coolant over several hot objects and achieving uniform temperature. In contrast, passing air over several hot objects in series warms the air at each step, so the first may be over-cooled and the last under-cooled. However, once water boils, it is an insulator, leading to a sudden loss of cooling where steam bubbles form (for more, see
heat transferHeat transfer is the transition of thermal energy from a hotter object to a cooler object...
). Unfortunately, steam may return to water as it mixes with other coolant, so an engine temperature gauge can indicate an acceptable temperature even though local temperatures are high enough that damage is being done.
An engine needs different temperatures. The inlet including the compressor of a turbo and in the inlet trumpets and the inlet valves need to be as cold as possible. A
countercurrent heat exchangeA heat exchanger is a device built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants,...
with forced cooling air does the job. The cylinder-walls should not heat up the air before compression, but also not cool down the gas at the combustion. A compromise is a wall temperature of 90°C. The viscosity of the oil is optimized for just this temperature. Any cooling of the exhaust and the turbine of the turbocharger reduces the amount of power available to the turbine, so the exhaust system is often insulated between engine and turbocharger to keep the exhaust gases as hot as possible.
The temperature of the cooling air may range from well below freezing to 50°C. Further, while engines in long-haul boat or rail service may operate at a steady load, road vehicles often see widely-varying and quickly-varying load. Thus, the cooling system is designed to vary cooling so the engine is neither too hot nor too cold. Cooling system regulation includes adjustable baffles in the air flow (sometimes called 'shutters' and commonly run by a pneumatic 'shutterstat); a fan which operates either independently of the engine, such as an electric fan, or which has an adjustable clutch; a thermostatic valve or just 'thermostat' that can block the coolant flow when too cool. In addition, the motor, coolant, and heat exchanger have some heat capacity which smooths out temperature increase in short sprints. Some engine controls shut down an engine or limit it to half throttle if it overheats. Modern electronic engine controls adjust cooling based on throttle to anticipate a temperature rise, and limit engine power output to compensate for finite cooling.
Finally, other concerns may dominate cooling system design. As example, air is a relatively poor coolant, but air cooling systems are simple, and failure rates typically rise as the square of the number of failure points. Also, cooling capacity is reduced only slightly by small air coolant leaks. Where reliability is of utmost importance, as in aircraft, it may be a good trade-off to give up efficiency, durability (interval between engine rebuilds), and quietness in order to achieve slightly higher reliability — the consequences of a broken airplane engine are so severe, even a slight increase in reliability is worth giving up other good properties to achieve it.
Air cooledAir-cooled engines rely on the circulation of air directly over hot parts of the engine to cool them.-Introduction:Most modern internal combustion engines are cooled by a closed circuit carrying liquid coolant through channels in the engine block, where the coolant absorbs heat, to a heat exchanger...
and liquid-cooled engines are both used commonly. Each principle has advantages and disadvantages, and particular applications may favor one over the other. For example, most
carsAn automobile, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
and trucks use liquid-cooled engines, while many small
airplaneA fixed-wing aircraft, usually called an airplane, aeroplane or plane, is an aircraft capable of flight using forward motion that causes air to pass over its wings to generate lift. Planes include jet engine and propeller driven vehicles propelled forward by thrust, as well as unpowered aircraft...
and low-cost engines are air-cooled.
Generalization difficulties
It is difficult to make generalizations about air-cooled and liquid-cooled engines. Air-cooled
VolkswagenThe Volkswagen Aktiengesellschaft , also known as Volkswagen Group or as VW, is an automobile manufacturer and mobility organisation based in Wolfsburg, Lower Saxony, Germany and is the pioneer brand within the Volkswagen Group, which contains the car brands Audi AG, Bentley Motors Ltd.,...
kombis are known for rapid wear in normal use and sometimes sudden failure when driven in hot weather. Alternately, air-cooled
DeutzDeutz AG is a German engine manufacturer.-History:The company was founded by Nikolaus Otto, inventor of the four-stroke internal combustion engine, in 1848 as N. A...
diesel engineA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression...
s are known for reliability even in extreme heat, and are often used in situations where the engine runs unattended for months at a time.
Similarly, it is usually desirable to minimize the number of heat transfer stages in order to maximize the temperature difference at each stage. However,
Detroit DieselDetroit Diesel Corporation , is a diesel engine producer headquartered in Detroit, Michigan, USA. There are today two individual divisions that share this name: the off-highway division which is owned by Tognum, which EQT IV formed along with MTU Friedrichshafen, and the on-highway division which...
2-stroke cycle engines commonly use oil cooled by water, with the water in turn cooled by air.
The
coolantA coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
used in many liquid-cooled engines must be renewed periodically, and can freeze at ordinary temperatures thus causing permanent engine damage. Air-cooled engines do not require coolant service, and do not suffer engine damage from freezing, two commonly-cited advantages for air-cooled engines. However, coolant based on
propylene glycolPropylene glycol, known also by the systematic name propane-1,2-diol, is an organic compound , with a faintly sweet taste, and is a colorless, nearly odorless, clear, viscous liquid that is hygroscopic and miscible with water, acetone, and chloroform...
is liquid to -55 °
CCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
, colder than is encountered by many engines; shrinks slightly when it crystallizes, thus avoiding engine damage; and has a service life over 10,000 hours, essentially the lifetime of many engines.
It is usually more difficult to achieve either low emissions or low noise from an air-cooled engine, two more reasons most road vehicles use liquid-cooled engines. It is also often difficult to build large air-cooled engines, so nearly all air-cooled engines are under 500 kW, whereas large liquid-cooled engines exceed 80 MW (
Wärtsilä-Sulzer RTA96-CThe Wärtsilä RT-flex96C is a two-stroke turbocharged low-speed diesel engine manufactured by the Finnish manufacturer Wärtsilä. It is currently considered the largest reciprocating engine in the world, designed for large container ships, running on heavy fuel oil...
14-cylinder diesel).
Air-cooling
Cars and trucks using direct air cooling (without an intermediate liquid) were built over a long period beginning with the advent of mass produced passenger cars and ending with a small and generally unrecognized technical change. Before
World War IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
, water cooled cars and trucks routinely overheated while climbing mountain roads, creating geysers of boiling cooling water. This was considered normal, and at the time, most noted mountain roads had auto repair shops to minister to overheating engines.
ACS (Auto Club Suisse) maintains historical monuments to that era on the
Susten PassSusten Pass is a mountain pass in the Swiss Alps. The pass road, built from 1938-1945, connects Innertkirchen in the canton of Bern with Wassen in the canton of Uri.- External links :**...
where two radiator refill stations remain (See a picture
here). These have instructions on a cast metal plaque and a spherical bottom watering can hanging next to a water spigot. The spherical bottom was intended to keep it from being set down and, therefore, be useless around the house, in spite of which it was stolen, as the picture shows.
During that period, European firms such as Magirus-Deutz built air-cooled diesel trucks, Porsche built air-cooled farm tractors, and
VolkswagenThe Volkswagen Type 1 is an economy car produced by the German auto maker Volkswagen from 1938 until 2003. It used an air cooled rear engined rear wheel drive ....
became famous with air-cooled passenger cars. In the USA,
FranklinThe H. H. Franklin Manufacturing Company was a maker of automobiles in the United States between 1902 and 1934 in Syracuse, New York. Herbert H...
built air-cooled engines.
Liquid cooling
Today, most engines are liquid-cooled.
Liquid cooling is also employed in maritime vehicles (vessels, ...). For vessels, the seawater itself is mostly used for cooling. In some cases, chemical coolants are also employed (in closed systems) or they are mixed with seawater cooling.
External links