Above: 4-stroke Diesel Engine timing diagram

Stages	Piston Motion	Temp. Of Compressed Air (°C)	Pressure of Compressed Air (Bar)	Exhaust Valve	Scavenge Port
Induction	Downwards	–		Open	Closed
Compression	Upwards	550	80-90	Closed	Closed
Expansion	Downwards	1650	180	Closed	Closed
Exhaust	Upwards	–		Closed	Open

Fig: Four-stroke cycle diagram

 Induction Stroke:

·         Piston moves down with air inlet valve open and all other valves closed.

·         Fresh air is drawn into the cylinder from the atmosphere and suction valve is closed once the cylinder is filled up.

 Compression Stroke:

·         Piston moves up with all valves closed.

·         Swept volume of air is compressed to 80-90 bar with temperature raised to about 550°C.

·         High pressure fuel is then injected into cylinder at around 10-15° before TDC to give sufficient time for the fuel to mix with air for proper combustion (ignition delay).

Expansion Stroke:

·         When the piston is close to TDC, the compressed air–fuel mixture is ignited by the heat and pressure of compression.

·         The pressure from the combustion of the compressed fuel-air mixture drives the piston back towards the BDC, with all valves in the cylinder head close.

·         Gas in the cylinder reaches a high temperature of  about 1650°C.

·         Energy from the combustion process is imparted on piston, thus converting the reciprocating motion to rotary motion.

Exhaust Stroke:

·         When piston goes past TDC, burnt gases are pushed out of cylinder through opened exhaust valve.

·         Inlet valves opens towards the end of exhaust stroke to cool the combustion chamber and assist in pushing the remaining gases out of the cylinder.

·         Exhaust valves closes about 40° after TDC and new cycle begins.