Power machinery can be divided into three categories: wind machinery, hydraulic machinery and heat engine according to the way that different energy in nature is converted into mechanical energy.
There are sails, windmills (wind turbines), wind mills, etc. In the twentieth century, power generation devices with direct application of wind power appeared, but they were limited by the distribution of natural wind regions, because it is generally considered that the wind speed is greater than 4 m/s. It is estimated that the wind energy on the earth is about 10 GW, which has been used less than one percent. Therefore, wind energy has great prospects for development.
There are waterwheels, water mills, turbines, etc. Since the 20th century, hydropower stations using hydro turbines have been increasing, because hydropower stations have the advantages of low operating costs, no pollution, and inexhaustible use. However, the construction of reservoirs and dams has a large initial investment and a long construction period, and it also has an impact on ecological balance and geological force balance. China's hydropower reserves are about 680 megawatts, ranking first in the world, and there is room for development and utilization.
Including steam engines, steam turbines, internal combustion engines (gasoline engines, diesel engines, gas engines, etc.), hot air machines, gas turbines, jet engines, etc. Internal combustion engines are the most widely used in industries such as industry, agriculture, transportation, mining, and military. Ships, locomotives, automobiles, material handling machinery, earthmoving machinery, tanks, irrigation and drainage machinery, small power generation equipment, etc. are all powered by internal combustion engines.
The gasoline engine is powered by gasoline and is electrically ignited. The speed is generally between 3,000 and 6,000 rpm, and even as high as 10,000 rpm. Power ranges from a few hundred watts to hundreds of kilowatts. It is widely used as a power for tea picking machines, lawn mowers, machine guns, sprayers, brush cutters, machine saws, etc. in agriculture and forestry; it is used as a power source for motorcycles, cars and boats in transportation.
In addition, small generator sets for communication and film projectors, mining rock drills, construction smashers, etc. are all powered by small gasoline engines. The early aircraft were powered by large gasoline engines and were largely replaced by turbines, especially jet engines. Gasoline engine emissions are very toxic to the human environment.
The diesel engine is fueled by diesel fuel and self-ignited by compression heat. The rotation speed is generally from 100 to 5,000 rpm, and the power is from several kilowatts to tens of thousands of kilowatts. Widely used as power for automobiles, tractors, tanks, ships, warships, locomotives, generator sets, material handling machinery, earthmoving machinery, etc.
Since the 1960s, due to the worldwide oil crisis and the high thermal efficiency of diesel engines, the scope of application has also expanded. Some areas that used gasoline engines in the past, such as cars and light trucks, are increasingly using diesel engines.
The gas engine is powered by gas, natural gas and other combustible gases, and is electrically ignited. It is also fired by a small amount of diesel fuel. Due to the limitation of the source of gaseous fuel, coupled with the large size and difficulty of carrying the gas engine itself, its application is far less than that of gasoline engines and diesel engines. Gas engines are mostly used in stationary power units, but gas fuels are also used for liquefaction or liquefaction bottling for transportation vehicles, but they are not promoted due to inconvenient use.
A steam engine is a thermal device that converts thermal energy in steam into mechanical energy. Due to the low efficiency of work, it has been basically eliminated except for a few countries still used for locomotives. Steam turbines are power units that are widely used in large generator sets and large ships.
The hot air machine is also called the Stirling engine. It is a heat engine that uses air, hydrogen and helium as working fluids, and returns to the heat-closed thermal cycle to perform periodic compression and expansion work. The hot air machine is an external combustion engine, which can adopt various fuels, and has the advantages of low noise, low vibration and less sewage discharge. The main disadvantages are large radiator, difficult sealing and high cost.
A gas turbine is a device that directly drives a turbine to work as a gas produced by combustion of a fuel. The speed can be as high as tens of thousands of revolutions per minute and the efficiency is also high. Gas turbines are divided into open cycle and closed cycle, which are used as power for generators, locomotives and aircraft.
The jet engine is a heat engine that uses the reaction force generated by the fuel combustion gas discharge process, and is mainly used in aviation and aerospace. Jet engines can be divided into two categories, air jet engines and rocket jet engines.
A jet engine that takes in air from the outside as a working fluid and uses oxygen contained in the air as an oxidant is called an air jet engine. It can be divided into two types: airless air jet engines and air compressors with compressors. The most widely used gas turbine jet engine in modern aviation belongs to the latter.
Both the fuel and the oxidant are jet engines carried by the engine or the aircraft itself, called rocket jet engines, or rocket engines for short. According to the fuel used, it is divided into solid fuel rocket engine and liquid fuel rocket engine, which are mainly used as the power of weapons and space shuttles.