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.
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.
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.
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.
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.
