The Meaning of Compression.—Close the tap or valve and push the piston up again sharply to its original position of Fig. [1]. You will now encounter considerable resistance and experience a force pushing down against you because you are reducing the volume of the gas and thereby increasing its pressure; that is to say, you are compressing the gas, because you are now making an amount of gas that recently occupied the whole cylinder fit itself into the small space between the top of the cylinder and the crown of the piston. In technical language you would say, “the piston has now compressed the charge” of gas within the cylinder.
Fig. 4.—Otto Cycle.
The Compression Stroke.
The Meaning, of a Stroke.—In an engine such as is shown diagrammatically in Figs. [3] and [4], when the piston P moves from its topmost position in the cylinder down to its very lowest position we say it has completed a downstroke, and when it moves upwards from its lowest to its highest position we say the piston has completed an upstroke. The length of the piston’s stroke is equal to twice the length of the crank radius R, and is measured by observing the distance moved by the piston in travelling from its highest position in the cylinder to its lowest or vice versa. The space existing above the piston between it and the cylinder head when the piston has reached its highest position in the cylinder is called the clearance space. It is also referred to as the combustion chamber, or chamber in which the petrol gas is exploded. When the piston is either at the top or bottom of its stroke the crank radius R and connecting rod T are in one and the same straight line; under these conditions we say the crank is on its inner or outer dead-centre.
The Otto Cycle.—Most petrol engines operate on what is known as the “Otto” cycle, in which the cycle of events is completed once in every four strokes (or two revolutions) made by the engine. The “Otto” cycle is therefore usually referred to as the four-stroke cycle. In the accompanying diagrams (Figs. [3], [4], [5], and [6]) we show in diagrammatic form the interior of a petrol engine cylinder fitted with mushroom type valves.
Fig. 5.—Otto Cycle.
The Power Stroke.
In studying the figures we assume the engine is being cranked round by hand in the direction of the arrow while we view it from the “flywheel” end (i.e. the end adjacent to the driver’s seat), then A is the pipe which leads the mixture of air and petrol vapour from the carburettor to the cylinder and is called the induction pipe. C is the cylinder, P the piston, I the inlet valve, E the exhaust valve, T the connecting rod, R the crank, and S the sparking plug. The pipe B which leads the burnt gases from the exhaust valve to the silencer is called the exhaust pipe. The cycle of operations is as follows:—