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Aviation History
1933
1933 - 0208.PDF
SUPPLEMENT TO FLIGHT 4$ JULY 27, 1933 THE AIRCRAFT ENGINEER THE ARMSTRONG SIDDELEY ENGINE RESEARCH UNIT : A "Tiger" cylinder on test, coupled up to a Heenan & Proude Brake. (FLIGHT Photo.) The general principles of an internal-combustion engine are well known and the science of thermo dynamics helps us to understand the behaviour of the cycle of operations and to predict the result of changes in compression ratio, mixture strengths and so forth. There are many things we cannot calculate which are of the utmost importance to efficient running. The best that we can do is to make the design which previous experience has shown to be likely to succeed, and then to determine the exact characteristics of an actual engine and finally to modify the design in view of this experience. A subject of which little is known is the behaviour of the mixture inside the cylinder, the degree of turbu lence set up and finally the rate of propagation of the flame during the firing stroke. Cylinders which are comparatively similar often vary to a marked extent in the amount of spark advance needed to obtain the best results; in fact, the accurate determination of the best ignition point is a good indication of the luck with which the designer has been rewarded in so shaping his ports and combustion space as to obtain the best degree of turbulence. It is generally considered that the less advance needed at a particular speed to obtain maximum power, the more satisfactory is the cylinder likely to be. Turbulence and volumetric efficiency are both influ enced by valve timing, and while it is generally possible to obtain a good average result by using a standard timing on many types of engines, it is nearly always possible to obtain a slight increase in output and effi ciency by finding out for each type of cylinder what is 754 the ideal valve setting. The Armstrong Siddeley research engine is unique in many respects, chiefly in that it is possible to vary the lift of the valves, their period of opening and also the timing of their opening while the engine is running, and this is done with fixed cams of standard shape. The exact method of carrying out these operations was devised by Mr. Viale, whose ingenuity in designing mechanisms is inexhaustible. The engine is illustrated by photographs showing in particular the dials on which can be read the timing of the valve gear. The unit consists of a substantial cast-iron body, to which can be fitted crankshafts of varying stroke and cylinders of different sizes to cover the range of Armstrong Siddeley aircraft engines. The engine is coupled to a Heenan & Froude water brake of standard design. It is cooled by a draught supplied by an electrically driven centrifugal fan. The compression ratio cannot be varied while the engine is running, but with the attachment used on all Armstrong Siddeley engines it is an easy matter to alter the ratio by fitting lock rings of different sizes. The valve is lifted through a series of levers se arranged that by screwing in or out a sliding fulcrum the lift of the valve is varied through wide limits. At the same time the initial setting of the valve alwaj remains the same, so that the clearance in the mechanism is constant. It is an obvious advantage that the minimum valve lift shall be used that will give the volumetric efficiency required, and this can be deter mined with great accuracy by using the variable '" mechanism. The method of obtaining variable timing is not easy
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