To release the chemical energy in diesel effectively, an atomized form of the fuel is made to contact with high temperature and high pressure air. A diesel engine usually operates by repeating a cycle of four-stage or strokes combustion principle, also known as the otto cycle. The chemical energy release (combustion) is effectively transferred as mechanical rotational energy. The operation of a diesel engine is all about producing high temperature and high pressure air continuously.
Piston, connecting rod, crank and cylinder form a mechanism called slider-crank mechanism. The linear motion of the piston is transformed to a rotary motion at the crank. During the motion of the piston, the top most point it can reach is called Top dead centre (TDC) and the bottom most position the piston can reach is called as Bottom dead centre (BDC). This mechanism is properly supported in an engine block. Cylinder head, valves and fuel injector are fitted above the engine block.
When the piston moves downwards, inlet valves open and fresh air from outside is sucked in (suction stroke). During the return stroke, inlet and exhaust valves are closed and the air inside the cylinder get compressed. During the compression stroke, the piston does work on the air. So the temperature and pressure of the air will rise to a level which his higher tha
t the self-ignition of the diesel. An atomized form of diesel is injected into this compressed air. The fuel gets evaporated and undergoes an uncontrolled spontaneous explosion, as a result the pressure and temperature rise to high level values.
The high energy fluid pushes the piston downwards, the hot air does work on the piston and energy in the fluid is converted to the mechanical energy of the piston. This is the only stroke where the piston absorbs power from the fluid. Due to inertia of the system, the piston moves upwards again, this time the exhaust valves open and the exhaust is rejected, with the suction stroke taking place
again. This cycle which has a total 4 strokes, is repeated over and over for continuous power production.
You may have noticed that a bowl is provided on top of the diesel engine piston. During the compression stroke this bowl helps produce air that is rapidly swirling. Thus the injected fuel gets mixed with the air effectively.