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The engine

The object of the petrol engine is to develop a maximum of power in a small space with a minimum of waste. It uses petrol which evaporates quickly, and which, when mixed with about 15 times its own volume of air, forms an explosive gas, which is exploded inside a cylinder by means of an electric spark genereted by a high-tension coil. The essentlal parts of an internal combustion engine:

Cylinderss which are usually cast in a single unit with the greater part of their crancase. The cylinder head is made a separate| casting and secured to the cylinder block with studs and nuts.

Pistons, which slide up and down inside the cylinders. The piston forms the movable wall of the combustion chamber, allowing that chamber to increase and decrease in volume.

A crankcase, which serves as the main structural part which| supports the cylinders and the crankshaft and is in turn supported upon the vehicle frame.

A crankshaft, which is mounted on bearings in the crankcase and is free to rotate.

A connecting rod, which is a connecting link between the crankshaft and the piston. It serves to transform the reciprocating motion of the piston into rotary motion of the crankshaft.

Inlet valves, through which the gas enters the cylinders from the carburettor.

Exhaust valves, through which the exploded gas escapes from the cylinder.

A camshaft, which opens and closes the valves as required.

The complete cycle of operation in an ordinary four-stroke cycle petrol engine occupies two revolutions of the crankshaft, which means that between explosions (in any one cylinder of the engine) the piston goes up and down twice each way, or four times altogether. These four strokes are:

1. Downward — suction stroke

2, Upward — compression stroke

3. Downward — explosion (power) strike

4. Upward — exhaust stroke

After each cycle of operation the cylinder must be refilled with gas from the carburettor, which is done on the downward suction stroke of the piston. Almost immediately after the piston begins to descend, the inlet valve is opened by means of a cam and a charge of gas is drawn in from the carburettor. Shortly after the piston reaches the bottom pf the suction stroke, the inlet valve closes.

The piston now rises in the cylinder, compressing the gas into a small space termed the combustion chamber and whilst on this stroke both inlet and exhaust valves remain closed. When the piston has reached the top of its stroke, and the gas is fully compressed the latter is ignited by an electric spark across the points of the sparking plug.

The effect of the explosion is to cause a great increase in the volume of the gas and the additional pressure in the small space acts on the top of the piston, driving it down the cylinder, thereby rotating the crankshaft.