Page:EB1911 - Volume 20.djvu/60

 Watson, which clearly prove that in some cases as much as 30% of the whole heat of the petrol is lost in the exhaust gases by imperfect combustion. This opens a wide field for improvement, and makes it probable that with better carburettors motor cars would

not only discharge purer exhaust gases but would work on very much less petrol than they do at present.

. 4.

. 5.

Practically all modern petrol engines are controlled by throttling the whole charge. In the earlier days several methods of control were attempted: (1) missing impulses as in fig. 1 of the Daimler engines; (2) altering the timing of spark; (3) throttling petrol supply, and (4) throttling the mixture of petrol and air. The last method has proved to be the best. By maintaining the proportion of explosive mixture, but diminishing the total volume admitted to the cylinder per stroke, graduated impulses are obtained without any, or but few, missed ignitions. The effect of the throttling is to reduce compression by diminishing total charge weight. To a certain extent the proportion of petrol to total charge also varies, because the residual exhaust gases remain constant through a wide range. The thermal efficiency diminishes as the throttling increases; but, down to a third of the brake power, the diminution is not great, because although compression is reduced the expansion remains the same. At low compressions, however, the engine works practically as a non-compression engine, and the point of maximum pressure becomes greatly delayed. The efficiency, therefore, falls markedly, but this is not of much importance at light loads. Experiments by Callendar, Hopkinson, Watson and others have proved that the thermal efficiency obtained from these small engines with the throttle full open is very high indeed; 28% of the whole heat in the petrol is often given as indicated work when the carburettor is properly adjusted. As a large gas engine for the same compression cannot do better than 35%, it appears that the loss of heat due to small dimensions is compensated by the small time of exposure of the gases of explosion due to the high speed of rotation. Throttle control is very effective, and it has the great advantage of diminishing maximum