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Aviation History
1917
1917 - 0486.PDF
.MAY 17, 1917- AIRCRAFT AND MOTOR CAR ENGINE DESIGN." CONTRASTED FROM THE STANDPOINT OF A DESIGNER AND MANUFACTURER OF BOTH TYPES. LOUIS COATALEN.ByIN addressing myself to the members of the Aeronautical Society of Great. Britain, the oldest institution of its kind in the world, I have to bear in mind that a section only of that membership is composed of designers of internal combustion engines. Therefore I have striven to make my remarks this evening as intelligible as possible to those who have not had such training. At the outset, too, I take the opportunity to thank the Admiralty for giving permission for a paper to be read on so instructive a subject with a view to its being dis- cussed within the limits which it is necessary to impose on a topic of this sort in face of our being engaged in the greatest war in history. That belief which appears to obtain in some quarters to the effect that the design and production of an aircraft engine is akin to that of a motor-car one proves, on even casual investigation, to be what the old writers would have styled a vulgar error. By taking a few points which come most obviously to mind, we discover at the very outset that the problems involved by the two propositions are fundarnenfally different. Chief Characteristics of a Car Engine. Consider for a moment the chief characteristics of an internal combustion engine for motor-car service :—(1) Weight is practically no object. (2) Cost is of the utmost importance, therefore there must be the minimum of machin- ing, as instance the fact that the connecting rods of a motor- car engine are not milled, nor are the crankshafts machined all over. (3) It must be capable of production in great quantities at minimum cost ; otherwise, with the least amount of labour. (4) It must be silent to the extreme of what is practicable. (?) The maximum effort of which the engine is capable is not needed to be maintained for long at a spell. It seldom works at full power, and the brief duration of such effort explains the extraordinary reliability of even the inferior types of car engines. (6) Flexibility, giving a constant torque at a crank- shaft speed from 300 r.p.m. to 2,000 r.p.m. (7) Of course, this is a torque which corresponds to a very low mean effective pressure, namely, 80 lbs. (8) The compression is relatively low and the valve area small, the cam forms being easy and the valve springs light. (9) The system of lubrication, wherein the oil is carried in the base-chamber of the crank case, suffices. (10) The maximum horse-power required to be developed by any one engine rarely approached 100. In the vast majority of cases it does not exceed 30. And One for Aircraft Service. By contrast, the factors governing the design of an aircraft engine may be enumerated thus :—-(i) Weight is of prime importance. (2) Cost is not the deciding factor provided the necessary amount of power is obtained for the given overall dimensions of the engine, for its weight both as regards material and fuel, water and lvibricant consumption ; and that the desired degree of reliability is obtained. (3) The amount of labour necessary to produce a satisfactory aircraft engine of high output is, and will be, always many times what is necessary in the case of a car engine, and is a matter of secondary importance provided the desired results are obtained. No machining is too expensive if it saves weight. (4) Silence is relatively unimportant. (5) The aircraft engine does all its work at practically full power. (6) Flexibility, or evenness of torque, is of very secondary importance, because an aircraft engine is required to develop maximum torque at prac- tically one speed only, or, at most, at an extremely narrow range of speed. (7) But its working range of speeds is such as to call for a very high brake mean effective pressure, say 130 lbs. (8) The compression must be relatively high and the valve area large, while the valve springs must be stronger than for a motor-car, due to the cam form imposed. (9) The high mean effective pressure necessary, coupled with the fact of the engine doing nearly all its work at full power, involves a completely different point of departure in determining details of design and, notably, the exploitation of new methods of achieving lubrication. Experience has demonstrated abun- dantly that when the base-chamber is used as an oil well, as in motor-car practice, the lubricant soon becomes too hot, therefore too fluid, resulting in reduction of pressure to the main bearings ; hence the evolution of the dry sump system tor lubricating aircraft engines. Oil viscosity varies greatly with the temperature. Evolution of the Dvv Sump Lubricating System. I would mention in passing that the history of the engine dry base lubricating system is neither more nor less * A Paper read before the Aeronautical Society on May i6th, 1917. than the story of my racing experience on the Brooklands track. In the course of long runs on it years ago it was found that the oil pressure went down mote and more the longer each run was continued. Naturally we tried one brand of oil after another with the view to discovering which would retain its viscosity most effectively. Of course, castor oil gave greatly superior results to mineral oils. Even so, however. it soon became plain that the problem was one that could not be solved entirely by the "use of a vegetable oil. Indeed, results were quite unsatisfactory, notwithstanding that we greatly increased the effectiveness of the pump employed. Therefore my next step was to use the same pump to force tin1 oil out of the base-chamber through two i-in. copper pipes arranged round the car. We returned the oil direct from that process of cooling to the service of the bearings under pressure. This proved a great advance as regards main- taining pressure ; but the scheme involved all the incon- venience of a long circuit for the oil in connection with which all the cooling was achieved under pressure, because the oil passed quite round the car before being returned to the bearings. Therefore the next stage was to employ two pumps. One forced the oil out of the base-chamber through a cooler, from which it passed into a tank placed at the back of the car. In this tank the oil was not under pressure of the pump, for the tank itself was merely under atmospheric pressure. In practice it was found that this was re4Hy a notable improve- ment. Thus the bulk of the oil was kept all the time in the tank, which itself was in a draught while the car was travelling, while the base-chamber itself was kept quite empty. From the tank the oil passed to the pump, and was so forced by it into the bearings. Therefore the oil was under pressure only for a short distance, namely, from the pump to the bearings, because, as has been explained already, the tank itself was under atmospheric pressure. Only when we had arrived at this stage was it found that racing cars with engines of high output could be run for more or less indefinite periods without the temperature of the lubricating oil attaining more than 66° C, at which warmth a very good working viscosity was retained. (10) Lastly, in contrasting the standard car-engine, the racing-car engine, and the aircraft engine propositions under the headings that have been selected for the sake of illustration it is to note that the total amount of horse power required to be developed by practically all aircraft engines to-day is about 100 minimum, while the maximum totals several hundred horse-power per unit. Two Distinct Propositions. It will be seen from those ten points of contrast which, doubtless, might be increased in number, that the aircraft engine of to-day is not akin to the standard motor-car one. Admittedly, the twain are collaterals, both deriving from a common stock, the four-stroke cycle, petrol internal combus- tion engine. For the rest, the aircraft divine of to-day is. perhaps, as little like the standard motor-car one as that resembles the variety used on a commercial motor vehicle or that installed in a motor boat. In fine, it may be said that, as the stationary gas engine resembles the portable petrol variety to that meagre degree, and scarcely more, does the motor-car engine resemble the aircraft type. It cannot be proved that the aircraft engine has been developed from, the touring car variety. On the contrary, it can be demon- strated abundantly that the aircraft engine is quite a distinct branch of the development of the internal combustion engine. Hence many firms that have been strikingly successful in producing car engines for either touring or commercial use have experienced great and, in 'some cases, unsurmounted difficulty when called upon to change over to the manufac- ture of power plant for aircraft. The differences apparent in the design become even more pronounced when they art- translated into manufacturing problems in the shops. On the other hand, we may not lose sight of the likelihood that the very rapid evolution of the aircraft engine during this war, and the extraordinary manufacturing experience and developments of which that is the outcome, will at some future time exercise a more or less temporary effect on the design and manufacture of engines for car service. Be this as may be, in broad terms I am of opinion that the two schools of design, one concerned with each of these problems, will continue to advance for the most part along two distinct lines which will rather become more than less 486
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