The unaided starting behaviour of seven diesel engines of different sizes and designs has been studied in some detail. It has been established that the cranking time required to achieve an ignition (the first fire) depends on (a) the compression temperature available for a given ambient temperature and cranking speed, (b) the increase in compression temperature per cycle when the engine is cranked and fuelled and (c) the temperature required for ignition which can be obtained from ignition delay data.
The time taken from first fire to starter-off depends on a number of factors, and these are discussed and the differences in this respect between the direct injection (D.I.) and the indirect injection (I.D.I.) engines pointed out.
Based on the above facts, the starting behaviours of the seven engines are shown to be predictable from the respective three sets of data described above and obtained experimentally.
In order to permit prediction at the engine design stage, the effects of engine geometry on heat and mass loss and hence on the compression temperature are examined in detail. The generalized data on heat and mass loss, the increase in compression temperature per cycle and the ignition delay are then presented, from which the unaided starting behaviour of any engine can be predicted.
While the accuracy obtainable is only within 5 degC in ambient temperature, such a prediction is considered to be useful when it is realized that the range of unaided starting temperature of practical engines is as much as 50 degC.
Finally, the relationships between engine size, startability and compression ratio are discussed, based on the theory developed.