The operation of the Four Stroke Engine

Four-stroke engine:

1st stroke: induction stroke: while the inlet valve is open, the descending piston draws fresh petrol-and-air mixture into the cylinder;

2nd stroke: compression stroke: While the valves are closed, the rising piston compresses the mixture to a pressure of about 7-8 atm.; the mixture is then ignited by the sparking plug;

3rd stroke: power stroke: While the valves are closed, the pressure of the gases of combustion forces the piston downwards;

4th stroke: exhaust stroke: the exhaust valve is open and the rising piston dis charges the spent gases from the cylinder;

Since power is developed during one stroke only, the single-cylinder four-stroke engine has a low degree of uniformity, i.e., the rotation of the crankshaft is subject to considerable accelerations and decelerations during a cycle.

More uniform - that is to say, smoother - running is obtained with multi-cylinder engines because the ‘cranks’ of the crankshaft are staggered in relation to one another, so that the various cylinders do not develop their power strokes simultaneously, but successively (and sometimes in an overlapping sequence).

Depending on the cylinder arrangement, various types of engine are to be distinguished: in-line engine, horizontally opposed engine, vee engine, and radial engine.