A four-stroke petrol engine completes one operating cycle through intake, compression, power and exhaust strokes, converting part of the fuel’s thermal energy into crankshaft work.
Learning Outcomes
The four strokes are intake, compression, power and exhaust.
During intake the piston moves down and the inlet valve admits the petrol–air mixture.
During compression the piston moves up with both valves closed and compresses the mixture approximately adiabatically.
A spark ignites the mixture near the end of compression.
Hot gases expand rapidly during the power stroke and drive the piston down.
During exhaust the piston rises and pushes residual gases through the open outlet valve.
Intermediate Level
Purpose and Efficiency
A petrol engine obtains heat from burning fuel and converts part of it into mechanical work at the crankshaft. The remaining energy is rejected to the atmosphere and cooling system. The textbook gives a typical efficiency of about 25–30%.
The Four Strokes
Stroke
Piston motion
Inlet valve
Outlet valve
Main event
Intake
down
open
closed
fresh mixture enters
Compression
up
closed
closed
mixture compressed
Power
down
closed
closed
burning gases deliver work
Exhaust
up
closed
open
residual gases expelled
Energy Conversion
Chemical energy in the fuel becomes internal energy of the hot gases. Their high pressure exerts force on the piston, and the connecting rod and crankshaft convert piston motion into rotary mechanical work.
Key equation
F=PA
Key equation
W=FΔ y=PΔ V
Why Only One Stroke Is the Power Stroke
The expanding combustion gases directly supply energy to the crankshaft only during the power stroke. A flywheel and the other cylinders help carry the engine through intake, compression and exhaust.
Identify the Stroke
Explanation: The spark occurs near the end of this stroke.
Common Mistakes
The intake valve opens during intake, while the exhaust valve opens during exhaust.
The piston moves down in intake and power, but up in compression and exhaust.
The spark begins the power-producing stage near the end of compression.
Only the power stroke directly delivers net work from combustion.
Short-Question Preparation
Q. Name the four strokes in order.
A. Intake, compression, power and exhaust.
Q. What happens during compression?
A. The piston rises with both valves closed and compresses the petrol–air mixture approximately adiabatically.
Q. Why is the third stroke called the power stroke?
A. Expanding hot gases force the piston down and transfer work to the crankshaft.
Q. What happens during exhaust?
A. The piston rises while the outlet valve opens and residual gases are expelled.
Sticky Notes
The four strokes are intake, compression, power and exhaust.
During intake the piston moves down and the inlet valve admits the petrol–air mixture.
During compression the piston moves up with both valves closed and compresses the mixture approximately adiabatically.
A spark ignites the mixture near the end of compression.
Hot gases expand rapidly during the power stroke and drive the piston down.
During exhaust the piston rises and pushes residual gases through the open outlet valve.
60-Second Revision
The sequence is intake, compression, power, exhaust. Piston motion is down, up, down, up. The mixture enters through the inlet valve, is compressed and spark-ignited, drives the piston during expansion, then leaves through the exhaust valve.
Summary
A four-stroke petrol engine completes one operating cycle through intake, compression, power and exhaust strokes, converting part of the fuel’s thermal energy into crankshaft work.
Ask-the-Tutor Modes
Explain it simply.
Trace the energy or stroke sequence.
Explain the diagram.
Solve a numerical.
Test me with an MCQ.
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.
Exam preparation
Exam Focus
Important points
The four strokes are intake, compression, power and exhaust.
During intake the piston moves down and the inlet valve admits the petrol–air mixture.
During compression the piston moves up with both valves closed and compresses the mixture approximately adiabatically.
A spark ignites the mixture near the end of compression.
Hot gases expand rapidly during the power stroke and drive the piston down.
During exhaust the piston rises and pushes residual gases through the open outlet valve.
Common mistakes
The intake valve opens during intake, while the exhaust valve opens during exhaust.
The piston moves down in intake and power, but up in compression and exhaust.
The spark begins the power-producing stage near the end of compression.
Only the power stroke directly delivers net work from combustion.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.
Four-Stroke Petrol Engine
Sticky Notes · Quick Revision
1.
Key Point 1
The four strokes are intake, compression, power and exhaust.
2.
Key Point 2
During intake the piston moves down and the inlet valve admits the petrol–air mixture.
3.
Key Point 3
During compression the piston moves up with both valves closed and compresses the mixture approximately adiabatically.
4.
Key Point 4
A spark ignites the mixture near the end of compression.
Ready to test yourself?
You've completed the learning section.
8 concept questions
Easy → Medium → Hard
Explanation after each answer
Exam Practice
Important Question
Q1. Name the four strokes in order.
Important Question
Q2. What happens during compression?
Important Question
Q3. Why is the third stroke called the power stroke?
