THE DIESEL ENGINE

Evening Star, Issue 25620, 22 October 1945, Page 7

THE DIESEL ENGINE

HOW IT WORKS

The average motorist has little or no knowledge of the operation oil’ the diesel type of engine. Its four-stroke cycle follows closely the four-stroke petrol engine, except that on the induction stroke, it draws in only air without restriction by the throttle butterfly, the air intake being' consistent at all speeds. After admission into the cylinder the air is compressed into one-fourteenth or one-sixteenth of its area—in other words, the compression ratio is 14 or 16 to 1. The pressure in the combustion chamber is then in the neighbourhood of 4501 b to 5001 b per square inch, and the temperature of the air so compressed is about lOOdeg Fabr. Shortly before the top of the compression stroke is’ reached, the 'fuel oil is injected directly into this compressed air. the heat of which causes it to ignite. The fuel injection pressure must be much more than the 5001 b per square inch reached by compression, and is generally in the region of 2,2001 b to 2,5001 b per square inch. The timing of the injection calls for much greater accuracy than the ignition timing on a petrol engine, for the fuel must be injected at the exact moment when the temperature of the compressed air is at its maximum.

When the piston reaches its maximum velocity, about halfway up its stroke, the heat of compression is increasing, but as the piston slows down oil approaching the top of its stroke, the air begins to cool off. Therefore, ilf injection of fuel is delayed until the top of stroke is reached the compressed air is too cool to give rapid ignition. Diesel engine “ knock,” which most motorists have no doubt noticed, says the ‘ Dunlop Bulletin.’ is usually caused by ill-timing of the fuel injection, but also through incorrect injection pressure or use of an unsuitable type of injector nozzle which sprays the fuel oil to the wrong place in the combustion head.

Illustrative of the fine degree to which the amount of fuel injected must be metered according to the power required, is the ifact that an average diesel engine in a road vehicle turns over at idling speed on about one pint of fuel an hour. An idling speed is about 400 revolutions a minute—equivalent to approximately 200 fuel pump r.p.in. The pump makes some 12,000 revs, in an hour. Thus an aggregate of six injections into the cylinders per pump revolution means 72,000 injections from one pint of fuel at idling speed per hour. The fuel-injection equipment has to measure out and deliver, in correctly pulverised form, the required quantity of fuel to the air contained in the combustion head. Delivery of fuel in the •case of a multi-cylinder engine bv the injectors'should be metered within - per cent, of the average, and for accurate timing it must start within one degree of the moan. . So finely fitted is the plunger which forces the fuel through the injector nozzle that no packing is possible the plunger and barrel being hand-lapped together within a'limit of tolerance in the region of one-tenth of a thousandth part of an inch—a dimension which may be visualised as one oiio-hundred_ and-twentieth part of the thickness ot a human hair.