The Four-Stroke Cycle

Intake, compression, power and exhaust — the beating heart of the engine.

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What you will learn

Describe each of the four strokes
Explain the role of the piston, rod and crankshaft
Relate ignition or injection timing to the power stroke

Introduction

Almost every car engine you will diagnose runs a four-stroke cycle: intake, compression, power and exhaust. The piston moves twice up and twice down for each combustion, and the crankshaft makes two full turns. The camshaft, driven by a belt or chain at half crankshaft speed, times the valves.

Stroke	Piston	Valves	What happens
Intake	Down	Inlet open	Air (and fuel) fills the cylinder
Compression	Up	Both closed	The charge is squeezed and heated
Power	Down	Both closed	Combustion pushes the piston
Exhaust	Up	Exhaust open	Burnt gases are expelled

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A skipped timing belt or chain on an interference engine can bend valves in seconds. Never turn a suspect engine by hand without confirming timing.

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Key points

One complete cycle = two crankshaft revolutions
Compression raises temperature before ignition
The spark or the injection event starts combustion

All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.