Explosive Mixtures.—If a small quantity of liquid petrol or benzol be placed in an open vessel and exposed to a current of air it will quickly disappear or evaporate. We say that the liquid petrol has been vaporized or turned into petrol vapour. A mixture of air and petrol vapour can be ignited and burnt, the rate of burning being affected by the strength of the mixture. The strength of the mixture is determined by measuring the respective volumes of air and petrol vapour present in a known volume of the mixture. It is possible to form a mixture of air and petrol vapour in such proportions that when ignited by an electric spark it will be completely burnt at such a rate that the combustion is almost instantaneous, i.e., it will explode. This mixture of air and petrol vapour would then be referred to as an explosive mixture and would be suitable for supplying to the cylinder of a petrol engine.

The Meaning of Suction.—Imagine an iron cylinder A (Fig. 1) held down on a rigid base C and fitted with a gas-tight piston B. If we pull the piston down sharply to the position shown in Fig. 2 we will realize that there is apparently some force inside the cylinder which is trying to suck the piston up again. The fact that the piston is being withdrawn and no more air or gas admitted above it to fill up the volume it has displaced on its descent causes a partial vacuum in the cylinder. Now if by means of a tap or valve of some kind we could put the cylinder in communication with the atmosphere, air would rush in and fill up the cylinder until the pressure of the gases in it became equal to atmospheric pressure, when no more air could enter, because there would be no excess of pressure to force it in. In technical language we would say, “the piston has sucked in a charge of air” through the tap or valve.

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.

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.

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:—

(1) On the first downstroke made by the piston a suction effect or partial vacuum is produced in the cylinder; the air and petrol vapour in the induction pipe being at atmospheric pressure, which is in excess of that now existing in the cylinder, flow into the cylinder as soon as the inlet valve I is opened by the engine mechanism. At the end of this, the suction stroke, the inlet valve closes and traps the charge of explosive mixture in the engine cylinder. This is shown in Fig. 3.

(2) On the first upstroke made by the piston the charge of explosive mixture is compressed ready for firing. Both valves are shut. This is shown in Fig. 4.

(3) On the second downstroke made by the piston the sparking plug S passes a spark which explodes the charge at the very commencement of the downward movement of the piston. The force of the explosion drives the piston downwards, doing useful work. Both valves are shut. This is the power stroke, and sufficient power must be developed on this stroke not only to do the work required from the engine but also to tide it over the other three idle strokes. On this stroke the piston drives the crank by means of the connecting rod, but on the other three strokes of the cycle the crank has to drive the piston by means of the power or energy stored in the engine flywheel on the power stroke. Towards the end of the power stroke (or explosion stroke) the engine mechanism opens the exhaust valve E and allows part of the burnt gases to escape to the silencer along the exhaust pipe. This is shown in Fig. 5.

(4) On the second upstroke of the cycle the piston pushes the remaining burnt gases out of the cylinder through the exhaust valve. When the piston reaches the top of its stroke the exhaust valve closes. This is shown in Fig. 6. The cycle of operations then begins again, giving one power stroke and three idle strokes each time as already described.