Which stroke is the power stroke in a four-stroke engine?

Which stroke is the power stroke in a four-stroke engine?

Option:

a. Intake stroke
b. Compression stroke
c. Expansion stroke
d. Exhaust stroke

Correct Answer : c. Expansion stroke

Explanation:

In four-stroke engine, the piston completes four separate strokes which include a single thermodynamic cycle. One stroke is defined as a single travel of the piston in cylinder in either direction.



The four strokes are given below.

(1) Intake: In this stroke the piston moves down from top dead centre of the cylinder by allowing the air-fuel mixture inside the cylinder through inlet port. The exhaust port is closed in this stroke.

(2) Compression: In this stroke both the inlet and exhaust port are closed and the piston moves from bottom dead centre to top dead centre of the cylinder with compressing the air-fuel mixture in the cylinder head.

(3) Expansion: In this stroke the compressed air-fuel mixture in spark ignition engine is ignited by providing spark through spark plug. Due to instant ignition of the compressed fuel high pressure is produces and this pressure forces the piston to downside at high thrust. Therefore this is a power stroke.

(4) Exhaust: In the exhaust stroke the piston again returns to the top dead centre while exhaust valve is open and this stroke allows the spent fuel-air mixture through the exhaust port.