All About Engines

208 All About Engines of a fine spray, and the first main difference be* tween a petrol engine and a gas engine is the neces- sity, in the former case, of what is called a carburettor. A typical one is shown diagrammatically in Fig. 117. It consists of a small vessel into which petrol flows by gravity from a reservoir at a higher level. Inside is a float, attached to levers and a needle valve in such a way that when the float rises beyond a cer- tain point the valve is closed. By this device the Fig. 117.—Diagrammatic section of a carburettor petrol in this chamber is always at the same level, and the pressure under which it flows along the passage to the nozzle is always the same. The nozzle is in a space to which air has free access, and which is warmed—at any rate, when the engine is running —by a surrounding jacket of hot water from the cylinder jackets or hot gases from the exhaust. In the cold, when starting, the petrol leaves the nozzle in the form of spray, but when running it is rapidly vaporised and mixed with the air also drawn in by the suction of the engine. Ordinarily, in the type of carburettor shown, the air needed for combus- tion enters the orifice at the bottom, but when run- ning at high speed additional air is obtained through the small valve on the right.