During the last few years the use of the Diesel engine for aircraft has become important; it has advantages and some disadvantages when compared with the petrol engine, and what we are directly concerned with is overcoming, or at least reducing, the disadvantages. The most obvious of these is the relatively high weight-power ratio of the engine, but it is to be hoped that this will be overcome as rapidly as it was in the development of the petrol engine. In any case, the unfavourable weight-power ratio of the compression-ignition engine is, for long flights, offset by the weight of fuel required to be carried.