96, dispenses with the carburetor and sparking plugs of the petrol engine. The gas inside the cylinder on the compression stroke is pure air, which is compressed to 1 : 14 to 1 : 20 of its initial volume - a much higher compression ratio than is used in petrol engines. At the top of the compression stroke a fine spray of oil fuel is injected into the cylinder. As gas is compressed its temperature increases, so that the oil spray meets the air charge at a temperature sufficiently high to ignite it spontaneously.Because of its high compression ratio the compression-ignition or diesel engine is more efficient than a petrol engine. But for the same reason it must be more heavily built, thus offsetting the advantage somewhat. Diesel engines offer economies in fuel consumption at the expense of a loss in performance; they are particularly suited to frequent stop and start duties, and as a result are widely used in taxis, buses and lorries.The gas turbine, a completely different kind of engine, was first devised at the beginning of the twentieth century (1900s) and perfected in the 1930s. It usually has a single shaft carrying a series of propeller-like fans divided into two groups, the compressor and the turbine.In an operating gas turbine air is drawn in the compressor fans and its pressure increased. The compressed air is mixed with fuel and ignition takes place, further increasing temperatures and pressures. The burned mixture leaves the engine through the turbine, driving the blades round. The compressor, which is often driven directly by the turbine, takes up much of the power produced, but enough is left to make the gas turbine exceedingly powerful form of engine. Efficiencies are not high, but the good power-to-weight ratio of a gas turbine makes it suitable for aircraft propulsion. A gas turbine is about three times as powerful as a piston engine of the same weight....
