How a Four Stroke Engine Works Page 3
The power cycle is the only point at which the engine creates energy. Now
that the gas is compressed and heated at the top of the cylinder the spark plug emits a spark. This has been timed perfectly through the mechanisms of the engine to occur just after the piston has reached the highest point in its motion and is about to begin going down in the cylinder again, this point is called TDC (top dead centre).
It is a similar idea as pushing a child on a swing – you don’t push them as the are coming towards you, you wait until they are just about to begin travelling away from you again, this way you can transmit a lot of energy to the child on the swing for the least amount of work.
As you can see from the image below, the engine has forced the piston
down inside its cylinder and has expended all of the energy from the rapidly expanding gases.