The term Four Stroke stems from the fact that the piston must pass through four complete strokes before events within the combustion chamber are repeated. Some use the term 4 cycle. During this time the crankshaft rotates through 720 degrees or 2 revolutions. Each of the four strokes are given a name which best describes the event occurring within the combustion chamber. These are, the Inlet (Intake), the Compression, the Power and the Exhaust strokes and they occur in that order.

Our brief description starts at the beginning of the Inlet Stroke where the piston is at the tip of the cylinder, the Inlet valve is opening and the exhaust valve is closed. As the piston moves down (carried by stored energy within the flywheel) the combustion chamber volume increases producing a low pressure within. Atmospheric pressure now begins to force its way through the inlet passages into the cylinder.

As the piston passes through the bottom of its first stroke the inlet valve closes, and the Compression stroke begins. The piston rises, decreasing the volume of the cylinder and compressing the air fuel mixture. At a predetermined point when the piston is near top dead centre (the top of its stroke) ignition starts. Depending on the amount of fuel (throttle position) combustion continues as the piston passes top dead centre, generating sufficient heat to expand the gasses which in turn forces the piston down on the Power stroke.

Towards the bottom of the stroke all of the useful expansion force of the gas has been converted to mechanical energy, so the exhaust valve opens. The piston, entering the Exhaust stroke, passes bottom dead centre and moves up the cylinder driving out the exhaust gas.

The forces of Detonation, Pre-ignition and Compression Ratio apply to four stroke engines in the same way as described for two stroke engines.

The main differences between two and four stroke engines are that a two stroke engine has a Power stroke for every crankshaft revolution where as the four stroke engine has one for every two crankshaft revolutions. Therefore two stroke engines are more powerful than four strokes. However more air enters the cylinder during the four stroke Inlet stroke than is the case with the two stroke, hence more pressure is generated after combustion. More air means more oxygen and this means more fuel can be burnt. The result of this is that the piston is forced down the cylinder a greater distance than is the case with two strokes. Hence four stroke engines produce more torque than two stroke. In practise this means a four stroke would be most suited to spinning a large prop at slow speed and a two stroke a small prop at high speed.

As a rule, 4 cycle engines run hotter than 2 cycle. Although a 4 cycle engine only needs 10 to 15% oil for lubrication, it is often best to use 20 to 25% oil to aid heat diapason, especially for pattern flying when using big props and low RPM.