Spark image

Internal combustion engine

1. Induction stroke

The piston moves downwards carried by the inertia of the flywheel, and the petrol/air mixture is sucked into the cylinder through the open inlet valve. The outlet valve is closed.
The petrol air mixture is quite cold at this point, its temperature depends on the air temperature outside the engine.
Notice the piston rings, these help to seal the cylinder.

2. Compression stroke

The flywheel moves round, pushing the piston up and so compressing the mixture in the cylinder. As it does so the mixture will warm up a little depending on how much it is compressed. Both the valves are closed during this stroke.
The ratio of the volume of the cylinder when the piston is fully down to that when it is fully up is called the compression ratio, usually about eight.

3. Power stroke

When the piston reaches its highest position the spark plug fires. At this point the mixture has its smallest volume and when it is ignited by the spark it expands and this forces the piston rapidly down the cylinder.
Both valves are closed during this stroke.

4. Exhaust stroke

As the piston moves up again the burnt gases are forced out of the open exhaust valve. The inlet valve stays shut.
Because the petrol/air mixture is burnt in the enclosed cylinder with a limited amount of air the burning is not very efficient.


The diesel engine

In the diesel engine there are no sparking plugs. The compression ratio is very large, up to 15:1, and the fuel is pumped into the cylinder as a fine spray.

On the compression stroke the fuel/air mixture is compressed so much that it heats up a great deal. It is so hot that it catches fire on its own without the need for a spark.

Diesel engines have a heater to heat them up on starting and they are also rather heavy because the cylinders have to withstand high pressures. However, they are very reliable and have a higher efficiency than other petrol driven internal combustion engines.


schoolphysics reciprocating engine animation

To see an animation of a reciprocating engine click on the animation link.

© Keith Gibbs 2020