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Aviation History
1933
1933 - 0209.PDF
49 T,;V 27, 1933 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT THE VALVE PHASE INDICATOR : The exact timing is read off on this. (FLIGHT Photo.) to follow by diagrams, but the general principles can be understood easily enough. The cam is driven through an epicyclic gear in such a way that the angular posi tion of the cam relative to the crankshaft can be advanced or retarded. This is effected by turning one of the three hand wheels that can be seen in the photo graph. The alteration to the period of opening, which is the most original feature of the engine, is effected by introducing into the drive of the cam a mechanism which can impart to the cam an auxiliary motion which is not uniform. When the motion of the cam is speeded up while it is operating the valve, then the period of valve opening is reduced. If it is retarded, then the period of opening is increased. This result is achieved by imparting to the cam a harmonic angular motion in addition to its uniform angular motion, and this is effected by turning one of the other hand wheels at the side of the engine. Both of these operations can be carried out while the engine is running, and the valve timing can be read off in degrees on the dial. There is an exactly similar gear for each valve, so that the timing of the exhaust and inlet valves can be carried out separately. In actual practice the engine runs very well indeed, and the variable mechanism can be used for long periods without losing its calibration. I he method of carrying out a research test is, first of 0 run the engine for a considerable time until the temperature of the engine and of the oil has reached a predetermined figure. The engine is then run for a tlme on full load until the reading on the brake is utely steady. Variations in valve timing are then iiled out, keeping the engine at a constant speed, ^nf' hese are combined with different ignition settings, the - 1 ease < is the are ti settin settin e number of combinations is, of course, large, but the eau be carried out fairly quickly owing to the 1 making the various changes. The same process 'epeated through a range of speeds. The results U'U analysed to find out roughly what is the best and the tests are repeated round about this m order to obtain a check and to get still a d'ff * accuracy- The whole series is then repeated at aiiteient compression ratio. sj_1 " a number of other tests are carried out on has to't er Units- Tlle best clearance for tUP P^ton hiffh ° e determ'ned, which is done by running at the tion* temperature considered safe. The oil consump- dition ssary *° keep the piston and rings in good con- slppli dT ^6 determined and on the test unit the oil ' to the piston can be varied and the consumption measured accurately. The behaviour of the valve spring? can be studied. It is found by practice that the best form of spring can be determined only by experiment, and not directly by calculation. The chief difficulty with single-cylinder testing is that it is nearly impossible to obtain accurate petrol con sumption figures. There is always a certain amount of blow-back to the carburettor, resulting in some petrol being wasted. Various schemes have been used to prevent this, but without success. The usual practice is to determine the petrol consumption for maximum power under each set of conditions by altering the mixture with an adjustable jet. The petrol consumption is determined by a flowmeter, and this is treated only as a comparative figure which will not be directly applied to the complete engine. It may well be asked to what extent are single-cylinder tests applicable to multi-cylinder engines. Experience over a great number of years has shown that, compara tively at least, the results are always of value. That is to say, the best valve timing for a single cy'inder is likely to give the best results on a multi-cylinder engine, and the cylinder which gives the most satisfactory results as a single cylinder will also give the best results on the complete engine so long as the proper attention is paid to distribution and the cooling. The mechanical efficiency of the complete engine is always higher than that of the single cylinder, consequently the brake mean effective pressure for the same induction pressure should be a little higher on the complete engine; and in fact this almost always happens. The chief thing that cannot be determined on the single-cylinder unit, and which is always an anxiei v to designers, is the distribu tion of the mixture on the complete engine. Even if this is not good, it is generally possible to obtain the power that may be expected, but the petrol consumption will be higher because the mixture has to be strong enough for the weakest cylinder and is consequently over rich for other cylinders. There is also a slight THE THREE CONTROLS : The wheel on the left alters the valve lift, that in the centre the duration, and that on the right the timing. (FLIGHT Photo.) 754 i
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