A single-cylinder four-stroke piston engine spends three- quarters of its running time exhausting burned gas, draw- ing in fresh mixture and com- pressing it.

On only one of the four strokes—the power stroke—is any energy produced and this makes the output of a single- cylinder four-stroke engine very uneven.

This can be smoothed out if more cylinders, with their pis- tons driving a common crank- shaft, are used. A twin-cylinder four-stroke, for instance, will produce one power stroke for each revolution of the crank- shaft, instead of every other revolution as on a single-cylin- der engine.

If the engine has four cylin- ders it produces one power stroke for each half-turn of the crankshaft and at no time is the crankshaft free-wheeling’ on one of the three passive strokes.

Even better results can be obtained using six cylinders, as the power strokes can be made to overlap, so that the crankshaft receives a fresh impulse before the previous power stroke has died away—on an in-line six-cylin- der engine the crankshaft receives three power impulses each revolution.

In theory, the more cylinders you can use to drive the crank- shaft, the smoother the power output, and 8-and 12-cylinder engines are used on some of the more expensive cars.

A large number of cylinders can pose practical problems. An engine with eight cylinders in a straight line for instance would have a very long crank- shaft which would tend to twist and be more likely to break at higher engine speeds. The car would also need a long bonnet to enclose the engine.

So in the interests of crank- shaft rigidity and compact- ness, 8-and 12-cylinder engines have their cylinders arranged in aV, with two cylin- der heads and a common crankshaft.

There are also V-6 and V-4 cylinder engines.

The other layout in popular use is where the cylinders are horizontally opposed in two flat banks, with the crankshaft be- tween them. Its low build makes the flat’ engine particu- larly suitable for rear installa- tion. In 4-or 6-cylinder form, the flat engine has excellent mechanical balance as move- ment of a piston assembly in one direction is perfectly balanced by movement of simi- lar components operating in the opposite direction.