Fig. 1.—Diagram to Explain the
Meaning of “Suction.”
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.
Fig. 2.—Diagram to Explain the
Meaning of “Compression.”
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 The fact that the piston is being withdrawn and no more air or gas admitted above it to fill up the volume it has displaced on its descent causes a partial vacuum in the cylinder.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.
Fig. 3.—Otto Cycle.
The Suction Stroke.