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1. What is a Four-Stroke Engine?

A four-stroke engine is an internal combustion engine where the piston completes four separate strokes (movements up and down) to complete one power cycle. The four strokes are:

1. Intake
2. Compression
3. Power (Combustion)
4. Exhaust

It is widely used in cars, motorcycles, and generators because it’s efficient and durable.

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2. Main Parts of a Four-Stroke Engine

1. Cylinder – A chamber where fuel combustion occurs.
2. Piston – Moves up and down inside the cylinder, transferring energy to the crankshaft.
3. Connecting Rod – Connects the piston to the crankshaft and converts reciprocating motion into rotational motion.
4. Crankshaft – Rotates and delivers power to the vehicle’s drivetrain.
5. Cylinder Head – Covers the cylinder; contains intake and exhaust valves.
6. Valves
   • Intake valve – Allows air-fuel mixture to enter.
   • Exhaust valve – Lets burnt gases exit.
7. Spark Plug (in petrol engines) – Ignites the fuel-air mixture.
8. Camshaft – Operates the valves in synchronization with the piston movement.
9. Flywheel – Maintains engine momentum and smooth operation.

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3. Working of a Four-Stroke Engine

Stroke 1: Intake Stroke

• Piston moves downward.
• Intake valve opens, allowing air-fuel mixture (in petrol engine) or air only (in diesel engine) into the cylinder.
• Exhaust valve remains closed.

Stroke 2: Compression Stroke

• Piston moves upward.
• Both valves are closed.
• Air-fuel mixture gets compressed, increasing pressure and temperature.

Stroke 3: Power Stroke

• Spark plug ignites the compressed mixture (petrol engine).
• Rapid combustion forces the piston downward, producing mechanical power.
• This is the stroke that actually drives the vehicle.

Stroke 4: Exhaust Stroke

• Piston moves upward again.
• Exhaust valve opens, pushing burnt gases out of the cylinder.
• Intake valve remains closed.

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Cycle Repeats

• After the exhaust stroke, the cycle starts again with a fresh intake stroke.
• Two revolutions of the crankshaft are required to complete one power cycle in a four-stroke engine.

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✅ Key Points:

• Produces power every two revolutions of the crankshaft.
• More fuel-efficient and smoother than a two-stroke engine.
• Common in cars, motorcycles, and light trucks.

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