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As engines generate mechanical Power they also generate waste heat energy because they are not perfectly efficient. The engine must therefore be cooled to prevent it from cooking in its own heat. Although some waste heat goes out with exhaust gases in most conventional Internal Combustion Engine s, further cooling is needed otherwise some components will get so hot that materials or lubricants will fail. BASIC PRINCIPLES Most internal combustion engines are " Air-cooled " or " Liquid-cooled ". Each principle has advantages and disadvantages, and particular applications may favor one over the other. For example, most Cars and trucks use liquid-cooled engines, while most small Airplane engines are air-cooled. Most liquid-cooled engines use a mixture of water and other chemicals such as antifreeze and rust inhibitors. Some use no water at all, instead using a liquid with different properties, such as Propylene Glycol or the combination of Propylene Glycol and Ethylene Glycol . Although the term "liquid-cooled" is used here, most air-cooled engines also use some liquid oil cooling, and most liquid-cooled engines subsequently cool the hot liquid with air. Conductive heat transfer is proportional to the temperature difference between materials. If an engine metal is at 300° C and the air is at 0°C, then there is a 300°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 150°C which is then cooled with 0°C air. Thus, in each step, the liquid-cooled engine has half the temperature difference and so may need as much as twice the cooling area. 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 Exchange 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. Air cooling is reduced by removing the fins. Cooling-water is then admitted to regulate the temperature. Any cooling of the exhaust and the turbine of the turbo reduces the amount of power available to the turbine. The motor block and the material of the heat exchanger have some heat capacity which smooth out temperature increase in short sprints. Modern electronic regulates the water valve also based on throttle to anticipate a temperature rise and compensate for the finite Thermal Conductance . GENERALIZATION DIFFICULTIES For all these reasons, it is difficult to make generalizations about air-cooled or liquid-cooled engines. Air-cooled Volkswagen Kombis are known for sometimes "eating engines", with both rapid wear in normal use and sometimes sudden failure when driven in hot weather. Alternately, air-cooled Deutz Diesel Engine 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. It is usually more difficult to achieve either low emissions or low noise from an air-cooled engine, two reasons why 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-C 14-cylinder diesel). A reliable generalization is that for a given power level it is more difficult to build a light liquid-cooled engine. Weight is one reason air-cooling is common in aircraft engines, though reliability through simplicity and ready access to cool air are two other reasons. WHY AUTOMOBILE ENGINES WERE AIR-COOLED Passenger and industrial vehicles using direct air cooling, without an intermediate liquid, were built over a long period beginning with the advent of the mass produced passenger car and ending with a small and generally unrecognized technical change. That water cooled cars and trucks routinely created geysers while climbing mountain roads before World War II was taken for granted. At the time, most noted summits had auto repair shops to minister to overheating engines. ACS (Auto Club Suisse) maintains historical monuments to that era on the Susten Pass where two radiator refill stations remain. {Link without Title} 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 Magirus-Deutz {Link without Title} built air-cooled diesel trucks, Porsche {Link without Title} built air-cooled farm tractors and Volkswagen became famous with air-cooled passenger cars, not to say there weren't others in the field, but these were the most visible in Europe while in the USA, Franklin built air-cooled engines. What changed and when? The change 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. Although previous Radiator s and Engine Blocks were properly designed and survived durability tests, they used water pumps with a leaky Graphite -lubricated "rope" seal ( Gland ) on the pump shaft. This seal was inherited from steam engines, where water loss is accepted, the process being open flow, expending large volumes of water. Because the seal leaked mainly when the pump was running and the engine was hot, it evaporated its losses inconspicuously, leaving at best a small rusty trace when the engine stopped and cooled, thereby not revealing significant water loss. Automobile Radiator s (or Heat Exchanger 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 is inefficient because it must work over a wide range of engine speeds. 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. Water circulation ceased and water in the engine boiled. 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... that were no longer built. Once begun, air-cooled engines became popular throughout Europe. After the war, Volkswagen advertised in the USA as not boiling, in spite of new water-cooled cars that no longer did so, but it sold well, and without question. 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 although today nearly all are water cooled. However, many motorcycles rely on Convection circulation without a pump. SEE ALSO |
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