Four Stroke Engine Working Principle

A four-stroke engine also known as four-cycle is an internal combustion engine in which the piston completes four separate strokes which constitute a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction. In four stroke cycle engines the four events namely suction, compression, power and exhaust take place inside the engine cylinder. The four events are completed in four
strokes of the piston two revolutions of the crank shaft. This engine has valves for controlling the inlet of charge and outlet of exhaust gases. The opening and closing of the valve is controlled by cams, fitted on camshaft. The camshaft is driven by crankshaft with the help of suitable gears or chains. The camshaft runs at half the speed of the crankshaft. The events taking place in I.C. engine are as follows:
1. Suction stroke 2. Compression stroke
3. Power stroke 4. Exhaust stroke

Suction stroke
During suction stroke inlet valve opens and the piston moves downward. Only air
or a mixture of air and fuel are drawn inside the cylinder. The exhaust valve remains in
closed position during this stroke. The pressure in the engine cylinder is less than
atmospheric pressure during this stroke.

Compression stroke
During this stroke the piston moves upward. Both valves are in closed position.
The charge taken in the cylinder is compressed by the upward movement of piston. If
only air is compressed, as in case of diesel engine, diesel is injected at the end of the
compression stroke and ignition of fuel takes place due to high pressure and
temperature of the compressed air. If a mixture of air and fuel is compressed in the
cylinder, as in case of petrol engine, the mixture is ignited by a spark plug.

Power stroke
After ignition of fuel, tremendous amount of heat is generated, causing very high
pressure in the cylinder which pushes the piston downward. The downward
movement of the piston at this instant is called power stroke. The connecting rod
transmits the power from piston to the crank shaft and crank shaft rotates. Mechanical
work can be taped at the rotating crank shaft. Both valves remain closed during power
stroke.

Exhaust stroke
During this stroke piston moves upward. Exhaust valve opens and exhaust gases
go out through exhaust valves opening. All the burnt gases go out of the engine and the
cylinder becomes ready to receive the fresh charge. During this stroke inlet valve
remains closed.
Thus it is found that out of four strokes, there is only one power stroke and three
idle strokes in four stroke cycle engine. The power stroke supplies necessary
momentum for useful work.