All About Engines

232 All About Engines used in lamps to produce light. This oil is heavier than petrol, and requires a higher temperature to convert it into vapour. When once it is converted into vapour and the vapour is mixed with air, the mixture is highly explosive, and just as useful as the lighter and more volatile petrol for producing mechanical power. Moreover, and this is all-impor- tant, it is cheaper. So the oil engine is similar to the petrol engine, but works on a cheaper fuel. It is, therefore, more suitable for larger powers, and in construction it resembles a gas engine rather than the dainty little motor which is used in the car, the boat, and the aeroplane. In the Priestman engine the oil was vaporised in a chamber heated by a lamp before it entered the cylinder, and the explosion was caused by an electric spark. Before it had been in use more than three or four years a new form arose. The Hornsby- Ackroyd engine embodied a totally new principle, which can be most easily explained by reference to Fig. 131 on Plate 23. At the back of the cylinder is a space called a combustion chamber which, before the engine is to be started, is heated by a blow lamp, and into which the oil intended for combustion is sprayed, together with the requisite quantity of air. This takes place on the outward stroke of the piston. The oil is immediately con- verted into vapour, and the explosive mixture of this vapour with air, which has expanded into the cylinder, is compressed in the combustion chamber by the returning piston. Now, it will be remembered that