Aircraft piston engine

Aviation piston engines rely on the reciprocating motion of the piston in the cylinder to complete the thermodynamic cycle of the gaseous working medium, converting the chemical energy of the fuel into mechanical energy. They are basically the same as the piston engines used in general automobiles in structure and working principle. According to the stroke, they can be divided into four-stroke and two-stroke; according to the cooling method of the cylinder head, they can be divided into liquid (water) cooling and gas (air) cooling; according to the different arrangements of the cylinders, they can be divided into in-line, parallel, V-type, X-type and star-type. Usually, V-type and in-line types are mostly liquid-cooled, and star-type types are all air-cooled.

The war factor stimulated the rapid development of aviation engines, and ushered in the "golden age" of piston engine development before and after World War II. In order to increase the speed of the aircraft, it is necessary to first increase the output power of the engine. The simplest and most direct way to increase the engine power is to increase the number of cylinders, thereby increasing the engine displacement. The engine of the "Flyer 1" had only 4 cylinders, and later it developed more, up to 28 cylinders (the US "Hornet" liquid-cooled piston engine), equipped with the US B-29 "Super Fortress" long-range bomber. In addition to increasing the number of engine cylinders, the following measures can also be taken to increase the power of the engine:
First, we start with fuel to improve the anti-knock performance (i.e. octane number) of gasoline during combustion, thereby increasing the compression ratio of the cylinder oil-gas mixture before combustion, and improving thermal efficiency and output power. This method is still widely used in automotive piston engines. Secondly, we use afterburner technology to spray a mixture of water and methanol into the cylinder to greatly increase the engine power in a short period of time. This technology was widely used in fighter engines at that time. Another important measure is to use a turbocharger to introduce the high-temperature and high-pressure exhaust gas of the piston engine with a temperature of up to 600-700℃ into a turbine, and the turbine drives the compressor to supercharge the inlet air flow, and then introduce the supercharged air into the cylinder for combustion. In World War II, on the famous Hump Route jointly opened by China and the United States, it was with the help of the engine's turbocharger that the aircraft was able to fly over the Himalayas, known as the roof of the world. Turbocharger technology is still widely used in diesel engines used in large trucks today.

However, as the flight speed increases further, the engine power increases further, and the weight of the piston engine also increases rapidly, which can no longer meet the requirements of high-speed flight; on the other hand, the efficiency of the propeller drops sharply after the flight speed exceeds 700 kilometers per hour. Both of these aspects limit the increase in flight speed. Therefore, the flight speed of an aircraft using a piston aircraft engine-propeller combination cannot reach or exceed the speed of sound. In order to increase the flight speed, it is necessary to develop a new type of aircraft engine with greater power and lighter weight - jet engines and gas turbine engines.