How does a four-stroke internal combustion engine work?

The principle of operation of the internal combustion engine is based on the effect of thermal expansion of gases that occurs during the combustion of the fuel-air mixture and ensures the movement of the piston in the cylinder. The piston internal combustion engine operates cyclically. Each working cycle takes two crankshaft revolutions and includes four strokes (four-stroke engine): intake, compression, power stroke and exhaust. During the inlet and power strokes, the piston moves downward, and the compression and exhaust strokes move upward. The working cycles in each of the engine cylinders are out of phase, which ensures the uniformity of the ICE operation. In some designs of internal combustion engines, the working cycle is realized in two strokes – compression and working stroke (two-stroke engine).
On the intake stroke, the intake and fuel systems provide an air-fuel mixture. Depending on the design, the mixture is formed in the intake manifold (central and distributed injection of petrol engines) or directly in the combustion chamber (direct injection of petrol engines, injection of diesel engines). When the intake valves of the gas distribution mechanism are opened, air or the fuel-air mixture is supplied to the combustion chamber due to the vacuum generated by the downward movement of the piston.
During the compression stroke, the intake valves close and the air / fuel mixture is compressed in the engine cylinders.
The working stroke is accompanied by the ignition of the fuel-air mixture (forced or self-ignition). As a result of the ignition, a large amount of gases are formed, which press on the piston and make it move downward. The movement of the piston through the crank mechanism is converted into rotational movement of the crankshaft, which is then used to drive the vehicle.
During the exhaust stroke, the exhaust valves of the gas distribution mechanism are opened, and the exhaust gases are removed from the cylinders to the exhaust system, where they are cleaned, cooled and noise reduced. Then gases enter the atmosphere.
The considered principle of operation of an internal combustion engine makes it possible to understand why the internal combustion engine has a low efficiency – about 40%. At a given moment in time, as a rule, useful work is performed only in one cylinder, in the rest – providing strokes: intake, compression, exhaust.