Modern farm tractors and automobiles are powered by piston-type internal combustion engines. An internal combustion engine is one in which the thermal energy supplied by a burning fuel is directly transformed into mechanical energy by the controlled combustion of the fuel in an enclosed cylinder behind a piston. Such engines are classified according to the following basic features:
(a) by the method of mixing fuel and air: engines with external mixing - carburettor engines and gas engines; engines with internal mixing-diesel engines;
(b) by the method of ignition of air-fuel mixture: spark ignition engines - carburettor engines and gas engines; compression ignition (self-ignition) engines-diesel engines;
(c) by the number of piston strokes in one complete cycle of operations: four-stroke engines and two-stroke engines;
(d) by the kind of fuel used: gasoline (petrol) engines, gas engines, and diesel engines;
(e) by the cooling method: liquid-cooled engines and air-cooled engines;
(f) by the number of cylinders: single cylinder engines and multicylinder engines (double-cylinder, four-cylinder, six-cylinder, eight-cylinder, and twelve-cylinder engines);
(g) by the cylinder arrangement: single-row engines and double-row engines including V-type ones in which the two rows of cylinders are arranged at an angle to each other.
It takes oxygen to burn the fuel, therefore the latter is mixed with air. A mixture of atomized fuel and air in certain proportions by weight is called air-fuel mixture. The air-fuel mixture entering a cylinder of a running engine intermingles with unscavenged exhaust gases and thus forms what is known as the combustible charge.
Engines using a special device- carburettor- to mix fuel with air are referred to as carburettor engines. In such engines, the air-fuel mixture is ignited by a spark. In diesel engines, the air-fuel mixture forms inside the cylinder and is self-ignited by the heat of compressed air. Carburettor engines are used mainly on light and medium trucks and cars and also on tractors where they serve to start the main engines. The main engines powering modern tractors and heavy trucks are of the diesel type.