Car Parts and Accessories

Fuel burning

Heat-Energy produced by the combustion of petrol and air is converted into mechanical power by the pistons, connecting rods and crankshaft of an engine. The engine’s efficiency depends on how much of this energy becomes useful power.

The more petrol / air mixture that can be drawn into a cylinder, and the more it can be compressed, the higher will be the specific output of the engine.

A comparison of the volumes of gas in the cylinder before and after compression takes place is expressed as the compression ratio. The average family car has a compression ratio of about 9: 1, which means gas in the cylinder is compressed to one-ninth of its original volume.

The compressed mixture, when ignited by a spark, should burn rapidly but progressively and smoothly across the top of the piston. It should not explode.

If the compression ratio is too high for the grade of petrol being used, the burning will not be progressive. Some of the mixture furthest away from the spark- plug will explode violently. This detonation results in a ‘ping’ from the engine.

Apart from the loss of power, this detonation can cause overheating and if it continues will damage the engine.

Loss of efficiency and overheating can also occur in an engine from pre-ignition —ignition before the timed sparking occurs at the spark-plug. This can arise from defective or incorrect spark-plugs, or it can be caused by deposits in the combustion chamber which have grown hot and glow continuously. Pre-ignition, like detonation, can cause extensive damage, as well as reduce engine power.

The action cycle in most car engines is the 4-stroke or Otto cycle, with the power stroke of the piston occurring once every four strokes. During one revolution of the crankshaft, the piston descends on its induction stroke and rises on its compression stroke. During the next revolution of the shaft, the piston is forced down on its power stroke, then rises on its exhaust stroke to expel burnt gases.

Since the inlet and exhaust valves must open only once in each cycle, the camshaft operating them is driven at half the speed of the crankshaft, which makes two revolutions during the 4-stroke cycle.

A few makes of cars are fitted with 2- stroke engines, in which combustion takes place on each downward stroke of the piston, or once in each revolution of the crankshaft. This cycle is simpler, but less efficient, than the 4-stroke cycle.

Valve overlap speeds the flow of gases

It might be expected that valves would open and close when the piston is at the top or bottom of its stroke. But in practice there is a slight overlap in the timing of the valves.

The exhaust valve opens just before the piston reaches the bottom of its stroke and closes just after it reaches the top; the inlet valve opens just before the top and closes shortly after it reaches the bottom.

During the overlap period, both valves are open together, and the momentum of gases flowing in and out of the cylinder helps to fill the cylinder with petrol/air mixture and complete the release of exhaust gases.

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