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Aviation History
1940
1940 - 1590.PDF
Supplement to THE AIRCRAFT ^ ENGINEER No. 172. (yol^Jne^1X) 15th YearNo. 5 May 30, 194!) THE TWO-STROKE AERO ENGINE Some Considerations of Its Advantages and Drawbacks : How Direct Petrol Injection Helps By DR. ING. P. F. MARTINUZZI PETROL injection is now a practical proposition, andthe time has come to consider the problem of thetwo-stroke aero engine. In the past, two-stroke meant compression ignition. In that field the two-stroke, engine is pre-eminent, but the future of the C.I. aero engine, particularly for war purposes, is doubtful and dis- tant. A petrol two-stroke aero engine is difficult to imagine without petrol injection; the fuel consumption would be too high, owing to loss of fresh mixture through the exhaust. Petrol injection, which allows scavenging to be effected with pure air, places the petrol engine on the same footing as the C.I. engine. The main argument in favour of the two-stroke engina is that, all things being equal, it should give twice the power of the corresponding four-stroke engine. The snag is that all things are not equal. The maximum m.e.p. and piston speed in the best possible two-stroke cylinder are certainly lower than in the best four-stroke ; the power absorbed by scavenging reduces the net available output. The two-stroke is generally supposed to be bulkier, and the petrol consumption is always higher. All the same, there is so ample a margin in favour of the two-stroke that the question is well worth examining, the more so as much development work on two-strokes is being done in many countries; the appearance of two-stroke engines in practical aviation use is perhaps nearer than most people think. Power and Size The two-stroke is particularly interesting at the present time, £3 it allows more power to be obtained from cylinders of a manageable size. The 2,000-3,000 h.p. engine is urgently needed. To obtain that power with a reasonable number of four-stroke cylinders, the bore would have to be much larger than the present standard maximum of about 6in. There is, of course, no reason why a larger cylinder, of perhaps 8in. bore, should not be successfully developed ; the limit in cylinder size is certainly not yet reached. Conditions in a large cylinder are very different, however, mainly due to the changed surface/volume ratio. The development period would be long and expensive ; adapting the 6in. bore cylinder to the two-stroke cycle appears easier. A 6in. bore, 12-cylinder two-stroke engine, about the size of the Rolls-Royce Merlin and of the Hispano, would give 1,800 h.p. at 2,500 r.p.m., with a b.m.e.p. of only about 1301b./sq. in. The question of the b.m.e.p. obtainable in a two-stroke engine is crucial and deserves closer consideration. The two-stroke cycle differs from the four-stroke only in the way the exhaust and intake are effected ; compression and expansion remain the same, giving similar conditions, and the ni.e.p. should also be unchanged. In practice, several causes concur in making the maximum m.e.p. slightly lower in a two-stroke; but there is no difficulty in making a two-stroke petrol engine give i6o/i8olb./sq. in. b.e.m.p. Unfortunately, an engine must get rid of waste heat; that is the limiting factor. Trouble would soon develop in a two-stroke engine running continuously at iSolb. b.m.e.p. To obtain a reliable engine, the waste • heat, and consequently the power and m.e.p., must be reduced. This can be done because the two-stroke engine gives the same power as a corresponding four-stroke with only half the b.m.e.p., so that there is an ample margin. The question of the maximum- b.m.e.p. that can be expected under service conditions will be ex- amined later. For the moment it must be noted that the relatively low m.e.p. of the two-stroke aero engine ia not a disadvantage ; on the contrary. A low m.e.p. implies low mechanical stresses and a lower supercharge ratio. This means that the full rated power of a two-stroke engine can be obtained with a fuel of relatively low octane num- ber. This is a most important point, particularly in war- time, considering the cost and scarcity of very high octane fuel. Besides, as the normal m.e.p. must be kept low, there is a wide margin for a temporary power increase for the take-off; wider than in a four-stroke engine. Qeneral hay out Before going into technical details, the question of the most likely general layout of the two-stroke aircraft engine must be examined. The opposite-piston type (Junkers) is not suitable for pstrol engines ; the heat stresses on the ex- haust piston are too high. Even in C.I. engines, notori- ously very cool, special precautions must be taken to make the piston last. Tests made years ago by an Italian firm on a petrol engine of this type proved conclusively that the pistons cannot stand the heat. In any case, the opposed-piston engine, though super- ficially attractive, is not really good. The heaviest part of any engine is the crankshaft, and the opposed-piston type has more crankshaft per h.p. than any other engine; the gears connecting the crankshafts are a further bad point. Kven for automotive and stationary C.I. use, where weight does not matter much and where the inherent balance of the opposed piston engine is a real advantage, this type has had no real success. It may, perhaps, sur- vive in the C.I. field because the Junkers people have: more actual experience of two-stroke aero engines than anybody else. The U-type cylinder has the same disad-, vantages as the opposed-piston type. The most likely aircraft two-stroke engine will have uni- flow scavenge, with the inlet near B.D.C. and exhaust in the head. This type can have either ordinary poppet exhaust valves or a sleeve valve. As will be seen, separate scavenge and supercharge ports with asymmetrical timing will almost certainly be used in aero engines. The single
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