From time to time a leading technician has forecast in
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the technical press that the petrol engine will make a comeback for long-haul work. I have followed with special interest the progress that has been made in the application of turbocharging to diesel engines, with particular regard to the improved fuel consumption it provides and, in the case of a customtorque engine, the higher low-speed torque the unit develops. Would it not be possible to obtain a better fuel consumption and a higher torque from a petrol engine by turbocharging? Has turbocharging been applied to a petrol engine?
ATurbocharging has been applied to
petrol engines to obtain an increase in output, notably to cars competing in the Indianapolis race in America. The potential of turbocharging a petrol engine is less than the gain obtainable from turbocharging a diesel. While the compression ratio of a turbocharged diesel is often lower than the ratio of its naturally-aspirated counterpart, the loss of thermal efficiency that results from reducing the ratio of a diesel from, say, 17 to 1 to 15 to 1 (two ratios) is small compared with the loss from a reduction in ratio from 9 to 1 to 7 to 1 in the case of a petrol engine, which might well be necessary if a typical petrol engine were turbocharged. The thermalefficiency gain obtainable by increasing the ratio of either a petrol engine or diesel obeys the law of decreasing return. Above about 10 or 12 to 1, (depending on the size of the engine) any increase in nominal thermal efficiency may be offset by the higher friction losses involved. The ratio of the majority of diesel engines would probably be reduced if it did not create starting difficulties.
Despite these apparent limitations, it should be possible to develop a turbocharged petrol engine with fuel injection that would be more economical to run. if the vehicle were engaged on stop-start work, than a naturally-aspirated counterpart led with mixture by a carburetter. In this case, using the turbocharger to improve the output by a small amount as well as the torque output at low speeds might enable gear changing to be reduced.
It is known that experimental petrol engines have been produced that give a fuel consumption at loads in excess of 40 per cent of the peak load that is very nearly as good as the consumption of a diesel. If a vehicle equIpped with an advanced type petrol engine were exclusively employed on motorway runs at sustained high loads, a relatively low fuel consumption should be obtainable, but a fuel having a high-octane rating would be required. And this would cost considerably more than diesel fuel.