Combustion of internal combustion engines using air or liquid to remove heat from the internal combustion engine. For small or special purpose engines, cooling using air from the atmosphere makes the system light and relatively simple. Boats can use water directly from the surrounding environment to cool their engines. For water-cooled engines on aircraft and surface vehicles, waste heat is transferred from a closed loop of water that is pumped through the engine to the surrounding atmosphere by the radiator.
Water has a higher heat capacity than air, and thus can move heat faster than the engine, but the radiator and pumping system add weight, complexity, and cost. Higher-powered engines produce more waste heat, but can move heavier, which means they are generally water-cooled. Radial engines allow air to flow around each cylinder directly, giving them the advantage of cooling air through straight engines, flat engines, and V engines. Rotary engines have similar configurations, but the cylinder also keeps spinning, creating airflow even when the vehicle is stationary.
Stronger aircraft designs favor lighter and air-cooled designs. Rotary engines were very popular on planes until the end of World War I, but had serious stability and efficiency issues. Radial machines were popular until the end of World War II, until gas turbine engines replaced them. The aircraft blades modern propellers with internal combustion engines mostly still air-cooled. Modern cars generally prefer power rather than load, and usually have a water-cooled engine. Modern motors are lighter than cars, and both cooling fluids are commonly used. Some sport bikes are cooled with air and oil (sprayed under the piston head).
Video Internal combustion engine cooling
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The hot engine generates mechanical power by extracting energy from the heat flow, just like the water wheel extracts the mechanical forces from the flow of mass that falls through the distance. The engine is inefficient, so more heat energy goes into the engine than out as an engine power; the difference is the waste heat that must be discarded. The internal combustion engine discharges waste heat through cold inlet air, heat exhaust gas, and explicit engine cooling.
Machines with higher efficiency have more energy left over as mechanical movement and less as heat waste. Some waste heat is very important: it guides heat through the engine, just like a water wheel works only if there is some exit speed (energy) in the waste water to take it away and make room for more water. Thus, all heat engines require cooling to operate.
Cooling is also necessary because high temperatures damage engine materials and lubricants and become more important in hot climates. The internal combustion engine burns the fuel more heat than the melting temperature of the engine material, and is hot enough to burn the lubricant. The engine cooling eliminates energy fast enough to keep the temperature low so that the engine can survive.
Some high efficiency engines run without explicit cooling and only incidental heat loss, a design called adiabatic. Such machines can achieve high efficiency but sacrifice power output, duty cycle, engine weight, durability, and emissions.
Maps Internal combustion engine cooling
Basic principles
Most internal combustion engines are air-cooled fluids (liquid liquids) or coolant fluids that flow through air-cooled (radiator) heat exchangers. Marine engines and some stationary engines have ready access to large volumes of water at the appropriate temperature. Water can be used directly to cool the engine, but often has sediments, which can clog cool channels, or chemicals, such as salts, which can chemically damage the engine. Thus, the cooling machine can be run through a water-cooled heat exchanger.
Most liquid-cooled engines use a mixture of water and chemicals such as antifreeze and rust inhibitors. The industrial term for an antifreeze mixture is the cooling machine . Some antifree do not use water at all, but use liquids of different properties, such as propylene glycol or a combination of propylene glycol and ethylene glycol. Most "air-cooled" engines use some liquid oil cooling, to maintain an acceptable temperature for both essential parts of the engine and the oil itself. Most of the "liquid-cooled" engines use some air conditioning, with the incoming air step cooling the combustion chamber. The exception is the Wankel engine, where some parts of the combustion chamber are never cooled by the intake, requiring extra effort for a successful operation.
There are many demands on the cooling system. One of the main requirements is to serve the whole machine as a whole, because the whole machine fails if only one part is overheated. Therefore, it is very important that the cooling system keeps all components at low temperatures. Liquid-cooled engines are able to change the size of their passage through the engine block so that the cooling stream can be adjusted to the needs of each area. Locations with high peak temperatures (narrow islands around the combustion chamber) or high heat flow (around drain holes) may require substantial cooling. This reduces the occurrence of hot spots, which are more difficult to avoid with air cooling. Air-cooled engines can also vary their cooling capacity by using cooling fins closer to the area, but this can make the manufacturing difficult and costly.
Only fixed parts of the machine, such as blocks and heads, are cooled directly by the main cooling system. Moving parts such as pistons, and to lower levels of cranks and rods, must rely on lubricating oil as a coolant, or to a very limited amount of conduction into the block and from there the main coolant. High-performance machines often have additional oil, beyond the amount required for lubrication, sprayed onto the bottom of the piston for additional cooling only. Air-cooled motorcycles often depend heavily on oil cooling in addition to cooling air from cylindrical tubes.
Liquid-cooled engines usually have a circulating pump. The first machine relies on thermo-siphon cooling only, where the heat cooler leaves the top of the engine block and is passed to the radiator, where it is cooled before returning to the bottom of the machine. Circulation is supported by convection only.
Other demands include the cost, weight, reliability, and durability of the cooling system itself.
Conductive heat transfer is proportional to the temperature difference between the materials. If the metal machine is at 250 Â ° C and air at 20 Â ° C, then there is a temperature difference of 230 Â ° C for cooling. Air-cooled engines use all these differences. In contrast, a liquid-cooled machine may discharge heat from the engine to the liquid, heating the liquid to 135 ° C (boiling point of 100 Â ° C water standard can be exceeded because of the pressurized cooling system, and using mixtures with antifreeze) which is then cooled by 20 ° C air. At each step, the liquid-cooled engine has half the temperature difference and initially seems to need twice the cooling area.
However, the cooling properties (water, oil, or air) also affect cooling. For example, comparing water and oil as a coolant, one gram of oil can absorb about 55% of heat for the same temperature rise (called specific heat capacity). The oil has about 90% water density, so the volume of oil supplied can only absorb about 50% energy from the same water volume. The thermal conductivity of water is about four times that of the oil, which can help heat transfer. The viscosity of the 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 a very low heat capacity per gram and per volume (4000) and less than a tenth of conductivity, but also a much lower viscosity (about 200 times lower: 17.4 ÃÆ'â € "10 -6 PaÃ, Â · s for air vs. 8.94 ÃÆ'â € "10 -4 PaÃ, Â · s for water). Continuing the calculation of the above two paragraphs, air cooling requires ten times the surface area, therefore the fins, and air require 2000 times the flow velocity and thus the recirculating air fan requires ten times the strength of the recirculating water pump. Moving heat from the cylinder to a large surface area for air cooling can create problems such as the difficulty of producing the required shapes for good heat transfer and the space required for free flow from large volumes of air. Boiling water at the same temperature desired for engine cooling. It has the advantage that it absorbs a lot of energy with very little temperature rise (called heat evaporation), which is good for keeping things cool, especially to pass one cooling stream to some heat object and reach a uniform temperature. Instead, passing air over several hot objects in series warms the air at each step, so the first may be too cooled and the latter less cooled. However, once the water is boiling, it is the insulator, which causes a sudden loss of cooling where the bubbles form (for more, see heat transfer). Steam can return to the water as it mixes with other coolants, so engine temperature gauges can show acceptable temperatures even when local temperatures are high enough that damage is occurring.
The machine needs different temperatures. The inbound channels include a turbo compressor and in the incoming trump and the inlet valve should be as cold as possible. The exchange of heat opposite to forced cooling air does the job. The cylinder wall should not heat the air before compression, but it also does not cool the gas at the time of combustion. The compromise is wall temperature of 90 Â ° C. The oil viscosity is optimized only for this temperature. Any cooling of the exhaust and turbine from the turbocharger reduces the amount of power available for the turbine, so the exhaust system is often isolated between the engine and the turbocharger to keep the flue gas as hot as possible.
Cooling air temperatures can range from well below freezing to 50 ° C. Furthermore, while machines on long-haul ships or rail services can operate on a stable load, road vehicles often see loads that vary widely and quickly. Thus, the cooling system is designed to vary the cooling so that the engine is neither too hot nor too cold. Cooling system settings include adjustable baffles in airflow (sometimes called 'shutters' and generally run by 'pneumatic shutterstat'); a fan operating either independently of the machine, such as an electric fan, or having an adjustable clutch; thermostatic valve or simply 'thermostat' which can block the flow of coolant when it is too cold. In addition, motors, coolers, and heat exchangers have some heat capacity that smooths the temperature increase in short sprints. Some engine controls turn off the engine or limit it to half throttle if it gets too hot. Modern electronic engine controls adjust the throttle-based cooling to anticipate the temperature rise, and limit the engine power output to compensate for limited cooling.
Finally, other concerns may dominate the design of the cooling system. For example, air is a relatively poor coolant, but the air conditioning system is simple, and failure rates typically increase as the square of the number of points of failure. Also, the cooling capacity is reduced only slightly by small air-cooled leaks. Where reliability is of the utmost importance, as in airplanes, it may be a good trade-off to provide efficiency, longevity (interval between engine reconditioning), and calmness to achieve a slightly higher reliability; the consequences of a damaged aircraft engine is very severe, even a slight increase in reliability is worth handing over other good properties to achieve it.
Air-cooled and liquid-cooled engines are both commonly used. Each principle has advantages and disadvantages, and certain applications can support one from the other. For example, most cars and trucks use liquid-cooled engines, while many small aircraft and low-cost air-cooled engines.
Generalization difficulties
It is difficult to make generalizations about air-cooled and liquid-cooled engines. Air-cooled diesel engines are chosen for reliability even in extreme heat, because air cooling will be simpler and more effective in the face of extreme temperatures during winter and summer highs, rather than water cooling systems, and is often used. in a situation where the machine runs unattended for months at a time.
Similarly, it is usually desirable to minimize the number of heat transfer stages to maximize the temperature difference at each stage. However, Detroit Diesel's two-cycle cycle engines generally use water-cooled oils, with air-cooled water.
Coolants used in many liquid-cooled machines must be periodically updated, and can freeze at regular temperatures causing permanent engine damage when expanding. Air-cooled engines do not require cooling service, and do not suffer freezing damage, two commonly cited benefits for air-cooled engines. However, refrigerants based on propylene glycol are liquids up to -55 ° C, cooler than those found by many machines; shrinks slightly when crystallized, thus avoiding damage; and has a service life of over 10,000 hours, basically a lifetime of many machines.
It is usually more difficult to achieve low emissions or low noise from air-cooled engines, two more reasons most road vehicles use liquid-cooled engines. It is also often difficult to build large air-cooled engines, so almost all air-cooled engines are below 500 kW (670 hp), while large liquid-cooled engines exceed 80 MW (107000 hp) (WÃÆ'¤rtsilÃÆ'¤-Sulzer RTA96 -C 14-cylinder diesel).
Air conditioning
Cars and trucks using direct air cooling (no intermediate fluid) were built for a long time from the beginning and ended with minor technical changes and are generally unknown. Before World War II, water-cooled cars and trucks routinely overheated when climbing mountain roads, creating boiling water geysers. This was considered normal, and by that time, most of the recorded mountain roads had car repair shops to serve overheated machines.
ACS (Auto Club Suisse) retains historical monuments to that era at Pass Susten where two radiator refilling stations remain. It has instructions on cast metal plaque and watering the ball bottom can hang next to the water tap. The bottom of the ball is meant to prevent it from being lowered and, therefore, becomes useless around the house, regardless of where it was stolen, as the picture shows.
During that period, European companies such as Magirus-Deutz built air-cooled diesel trucks, Porsche built an air-cooled agricultural tractor, and Volkswagen became famous for air-cooled passenger cars. In the United States, Franklin built air-cooled engines.
Over the years, air cooling is preferred for military applications because liquid cooling systems are more susceptible to damage by shrapnel.
The Czech-based company, Tatra, is known for its large-capacity air-cooled V8 car engine; Tatra engineer Julius Mackerle published a book about it. Air-cooled engines are better adapted to very cold and hot environmental weather conditions: You can see air-cooled engines starting and running in freezing conditions that confiscate water-cooled engines and continue working when water-cooled starts producing steam jets. Air-cooled engines may be an advantage from thermodynamic point of view due to higher operating temperatures. The worst problem encountered in air-cooled aircraft engines is the so-called "Shock cooling", when the plane enters the dive after rising or flying level with the throttle open, with the engine under no load while the dive plane produces less heat, and airflow which cools the engine up, a terrible engine failure can occur because different engine parts have different temperatures, and thus different heat expansion. Under such conditions, the machine can seize, and sudden changes or imbalances in the relationship between heat generated by the engine and heat dissipated by cooling can lead to increased engine wear, as a consequence of the difference in thermal expansion between machine parts. , the liquid-cooled engine has a more stable and uniform working temperature.
Liquid cooling
Currently, most of the automotive and larger IC machines are liquid-cooled.
Liquid cooling is also used in maritime vehicles (ships,...). For ships, sea water itself is mostly used for cooling. In some cases, chemical coolant is also used (in a closed system) or mixed with seawater cooling.
Transition from air conditioning
Changes in air cooling to liquid cooling occurred at the beginning of World War II when the US military needed a reliable vehicle. The subject of the boiling engine is discussed, researched, and solutions found. Radiators and engine blocks were previously designed properly and resistant to endurance tests, but use water pumps with sealed "graphite" sealed seals inside the pump shaft. The seal was inherited from the steam engine, where water loss was received, because the steam engine had already consumed large volumes of water. Because the pump seals leak especially when the pump is running and the engine is hot, the water loss evaporates without attracting attention, leaving a small trail of rusting when the engine stops and cools, thus not revealing significant water loss. Car radiators (or heat exchangers) have an outlet that supplies cooled water to the engine and the engine has an outlet that supplies hot water to the top of the radiator. Water circulation aided by rotary pumps which have only a small effect, should work on a wide range of speeds that impellers have only minimal effects as pumps. While running, the leaking pump seal dries the cooling water to a level where the pump can no longer return water to the top of the radiator, so the water circulation stops and the water in the engine boils. However, due to water loss causing overheat and further water loss from boiling, the hidden original water loss.
After isolating the problem of pumps, cars and trucks built for war effort (no civilian cars built during that time) equipped with carbon water seal pumps that did not leak and did not cause more geysers. Meanwhile, advanced air cooling in the boiling engine memory... although boiling is no longer a common problem. Air-cooled engines are becoming popular throughout Europe. After the war, Volkswagen is advertised in the United States as not boiling, though new water-cooled cars are no longer boiled, but these cars sell well. But when air quality awareness improved in the 1960s, and laws governing exhaust emissions were passed, unleaded gas replaced leaded gas and a slimmer fuel mix became the norm. Subaru chose liquid coolers for their EA (flat) series machine when it was introduced in 1966.
Low heat resistance machine
Special classes of experimental prototype internal combustion piston engines have been developed for decades with the aim of improving efficiency by reducing heat loss. This machine is widely termed adiabatic machine, because of better approach of adiabatic expansion, low heat rejection machine, or high temperature machine. They are generally a diesel engine with a combustion chamber coated with a ceramic thermal barrier layer. Some use titanium pistons and other titanium parts due to their low thermal conductivity and mass. Some designs can eliminate the use of cooling systems and associated parasitic losses altogether. Developing a lubricant that is able to withstand the higher temperatures involved has become a major barrier to commercialization.
See also
- Radiator
- Heating core
References
Source
- Biermann, Arnold E.; Ellerbrock, Herman H., Jr (1939). Fin design for air-cooled cylinder (pdf) . NACA. Report Nº. 726.
- P V Lamarque: "Cooler Fin Design for Motor-Cycle Machine". Report of the Automobile Research Committee, Institution of Automobile Engineers Magazine , March 1943 edition, and also in "The Institution of Automobile Engineers Proceedings, XXXVII, Sessions 1942-43, pp. 99-134 and 309-312./li>
- "Air-cooled Cooling Machine", Julius Mackerle, M. E.; Charles Griffin & amp; Company Ltd., London, 1972.
- engineeringtoolbox.com for physical properties of air, oil, and water
External links
Source of the article : Wikipedia
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