Topic 8.5.18 min read
Four-Stroke Petrol Engine
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.
Choose Your Level
Beginner Level
A petrol engine repeatedly draws in fuel and air, compresses the mixture, ignites it to push the piston, and removes the burnt gases. These four piston movements make one cycle.
Recommended for Class 11
Four-Stroke Petrol Engine
Overview
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.
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.
Exam Focus
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.
Important formulas
- F = PA
- W = PΔV
- typical petrol-engine efficiency ≈ 25–30%
Common exam traps
- 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.
Long question structure
Definition → labeled parts or stages → operation → equation → example.
Common Mistakes
Common Mistakes
- Mistake
The intake valve opens during intake, while the exhaust valve opens during exhaust.
- Mistake
The piston moves down in intake and power, but up in compression and exhaust.
- Mistake
The spark begins the power-producing stage near the end of compression.
- Mistake
Only the power stroke directly delivers net work from combustion.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
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.
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.
- Definition
- 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.
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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?
Important Question
Q4. What happens during exhaust?