New Apparatus Will Aid Control
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of "Diesel Knock"
TEST apparatus which, it is hoped, I will aid the quest for quietness in oil engines, was described by Mr. C. L. Edwards, North of England manager of C.A.V., Ltd., in his inaugural address as chairman of the Northeastern Centre, Institution of Mechanical Engineers (Automobile Division), at the opening of the centre's new session, at Leeds. University.
Mr Edwards said it had been established that "Diesel knock" could be eliminated: with little effect on efficiency, by obtaining control over the injection rate. That rate was primarily decided by cam profile and pump-plunger diameter, but, because of the complicated effects of spring-loaded nozzles, and resilience of the pipe-line and fuel, the rate of injection from the nozzle could differ materially from the rate of delivery at the pump.
This was readily proved by the fact that the injection period at the nozzle usually exceeded the period necessary for the completion of the effective pumping stroke, particularly at high speeds of operation.
To establish a relationship between the pump and nozzle injection rate, and to provide data to obtain ideal nozzle injection rates, an apparatus had been developed which enabled the actual rate of injection at the nozzle to be plotted in terms of fuel discharged against pump camshaft angle.
This apparatus consisted, basically, of two rotating discs driven at a rotational speed equal to that of the -injection pump. Each disc was slotted near its periphery, and the width of the clear aperture through the two slots could be varied. The nozzle was mounted adjacent to the upper disc, and a measuring glass was placed at the under side of the lower disc.
The start of injection was determined by a stroboscopic light source, and the leading edge of the lower-disc slot was adjusted to coincide with the nozzle spray at the beginning of injection. The upper disc was then adjusted so that the trailing edge of its slot was 2 degrees retarded from the leading edge of the slot in the lower disc. This gave an effective slot width of 2 degrees.
With the apparatus running, the measuring glass was put into action for a measured period of time (or number of revolutions), and from the amount of oil collected the quantity injected in the first 2 degrees was calculated.
With the lower disc left as before, the upper disc was then successively adjusted so as to increase the effective slot width by 1 degree at a time, until the whole injection period had been covered. At each adjustment, measuring-glass readings were taken as before. Results were then plotted and compared with the theoretical delivery rate at the pump. A typical graph showed considerable divergence between the theoretical delivery rate at the pump and the actual injection rate from the nozzle, and the difference between the theoretical and actual injection periods.