The internal combustion engine in most cars works on the four-stroke principle. This means that to produce one pulse of power the piston must travel up and down the cylinder four times.

Each stroke of the piston per- forms a separate function in the cycle as follows:

Induction stroke Begins the process. The inlet valve is open and rotation of the crank- shaft is moving the piston down the cylinder, sucking in a mix- ture of fuel and air which travels from the carburettor, along the inlet manifold and past the open valve.

Compression stroke Both val- ves are shut and the rotating crankshaft now raises the pis- ton, compressing the mixture above it into the combustion area.

Power stroke Both valves remain shut, and a spark jump- ing across the electrodes of the spark plug has set the mixture alight. It burns rapidly, expand- ing very quickly just as the piston begins its downward movement. The energy released rams the piston to the bottom of the cylinder, driving round the crankshaft half a turn.

Exhaust stroke Spent gases from the power stroke leave the combustion chamber through the open exhaust valve, helped out by the pressure created by the rising piston.

When the piston reaches the top of the cylinder atthe end of this stroke, the exhaust valve will close, the inletvalve will open and the cycle begin again with another induction stroke.

During the four strokes, the crankshaft rotates twice, but since the valves only need to operate once during each cycle, the camshaft that opens them is driven at half crank- shaft speed and rotates only once every four strokes.

Compression ratio

The power that an internal com- bustion engine develops depends on how much energy can be released above the pis- ton at each power stroke. This in turn depends on the quantity of fuel/air mixture in the cylin- der and the efficiency with which it is compressed.

The amount that the mixture is squeezed up is referred to as the compression ratio. This is the difference between the vol- ume of the mixture in the cylin- derwhen the piston is at the bottom of its stroke, and the volume when the piston is at its highest position. If the upward movement of the piston reduces the mixture to one-eighth its original volume, the compression ratio is 8:1.

In theory the more the mix- ture is compressed, the more energy it releases when it burns. In practice, however, very high compression ratios result in knocking or pinking in which some of the mixture furthest away from the spark plug explodes or detonates causing uneven burning, over- heating and loss of power. For maximum efficiency, burning of the mixture should occur rapidly but smoothly.

Valve overlap

So far we have assumed that the incoming mixture rushes past the inlet valve as soon as it opens. In practice, the mix- ture is slow to accelerate, and in order to fill the cylinder as completely as possible, the inlet valve is opened a little early, when the piston is near the end of the exhaust stroke, and while the exhaust valve is still open. This is called valve overlap.

It might seem that opening the inlet valve early would offer an alternative exit for the exhaust gas, but provided the amount of overlap is carefully chosen, the opposite happens and the last wisps of exhaust leaving the cylinder help drag the fresh mixture in past the inlet valve.

Once it is moving, the inlet mixture does not stop automati- cally when the piston reaches the bottom of the cylinder, and if the closing of the inlet valve is delayed, the cylinder fills more completely, even though by now the piston has started to rise on the compression stroke.

In practice, in order to make the most of the momentum of fresh mixture and exhaust gas flowing in and out of the cylin- der, the exhaust valve opens before the piston reaches the bottom of the cylinder and closes after it has reached the top. Similarly, the inlet va~ve opens before the piston reaches the top of the cylinder and closes after the piston reaches the bottom.