PETROL ENGINE: 4 STROKE INTERNAL COMBUSTION ENGINE

Highly simplified 4-stroke petrol engine cycling through the intake, compression, power and exhaust strokes. These engines use controlled explosions (combustions) of petrol (gasoline) and air mixtures to rotate wheels.

1. INTAKE STROKE: piston moves down and the petrol air (gasoline-air) mixture enters the cylinder (shown as a rotating spiral entering the engine through the purple inlet).

2. COMPRESSION STROKE: piston moves up and compresses the air-fuel mixture. The sparking plug (spark plug) then fires (an electrical discharge) and the compressed mixture explodes (by contrast, diesel engines use the heat of compression to ignite the mixture).

3. POWER STROKE: The explosion drives the piston downwards in what is called the power stroke.

4. EXHAUST STROKE: The piston then returns to the top of the cylinder in the exhaust stroke and drives the exhaust gases out of the engine. The exhaust gases can be seen leaving the (orange-yellow) exhaust tube as a cone of pale orange.

The flow of gases is controlled by valves (not shown) whose movements are timed to allow the inflow of fuel and air and the outflow of spent gases. The intake valve (which would be at the point the purple fuel line enters the top of the cylinder) opens during the first (intake) stroke and allows the fuel-air mix to enter the cylinder. It then closes to seal the cylinder during the remaining part of the cycle. The exhaust valve opens during the final (fourth) stroke to allow the exhaust gases to leave.