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Aviation History
1944
1944 - 1438.PDF
4» /\?y> / FLIGHT Zo^rOv^^^^^, The ©pposed^^fragilie Limitations WhiclyJttclMdiritsTtslse in Aircraft By R. H. P. N<XTT •< WITH reference to the correspondence which has been appearing recently concerning t\e possibilities of opposed-piston two-stroke engine^ifor aircraft, the following observations may be of interest. All opposed-piston engines suffer from the major disad vantage of great length in one plane, which necessitates the engine being buried horizontally in the wing if excessive form drag is to be avoided in the case of engines of high power output. This would be particularly true of the exhaust-piston type of engine, advocated by some corre spondents, which is, I believe-, manufactured by Harland and Wolff for snip propulsion. In tms instance, as each cylinder is required to accommodate three pistons of approximately equal length, and the engine is double-acting with the centre piston rod passing through the lower exhaust piston, the overall length would be hopelessly excessive. The two-crankshaft engine, exemplified by the Junkers design, is satisfactory for crankshaft speeds up to at least 3,000 r.p.m., but in view of the mechanical complication of the various forms of single-crankshaft opposed-piston engine, such as the many bearing surfaces and pin joints, it would appear highly improbable that reliable running cjaruld be achieved at speeds much above 1,500 r.p.m. An in crease of crankshaft speed, which can be obtained in opposed-piston engines by a reduction of the stroke /bore ratio, was shown in the case of the Junkers engine to influ ence adversely the scavenging ability of the engine, and its correction constituted a serious problem in scavenge port layout. The Junkers engine needs a train of gears, each of which must be able to transmit most of the power output of its bank of cylinders and must be supported by bearings of proportionately adequate dimensions. For this reason, and for the reason of increased piston friction in the case of the exhaust-piston engine with one crankshaft, one would expect the<inechanical efficiency of all types of opposed- piston engine to be lower than that of orthodox designs embodying only "one piston per cylinder. Another disadvantage of the opposed-piston engine is the uneven distribution of heat and stresses throughout the length of the cylinder liner. Scavenging Efficiency - Turning now to the characteristics of the two-stroke engine per se, probably the most important design require ment is high " scavenging efficiency," defined as the weight of fresh charge in the cylinder at the time the inlet valve closes, divided by the product of total cylinder volume and atmospheric density. This is analogous to volumetric effi ciency in the four-stroke engine and its importance is obvious, considering that the maximum I.M.E.P., which determines the power output ofJJie* engine, is roughly pro portional to the product of scavenging efficiency and inlet charge density. To attain a good scavenging efficiency a relatively large quantity of air must be supplied by the scavenge blower (60 per cent, excess air by volume in the case of the Junkers Jumo 205) and this entails a high power input to the blower, which must be debited to fuel con sumption. While pressure waves in the inlet and exhaust systems of four-stroke engines have a noticeable effect on volu metric efficiency, the effect *is much greater with the two- stroke, because during the' scavenging period inlet and exhaust ports are open, together and pressure waves in the exhaust system exeptfa marked effect on the charging pro cess. Moreover, tne two stroke engine, since it scavenges in consequence of a pressure drop between inlet and exhaust, is much more sensitive to changes in exhaust back assure thap is the four-stroke engine. Hence severe fimitatiojjannay be imposed on exhaust system design. rcharging of the two-stroke engine is difficult by reason of the short period of crank angle available for supercharging after the scavenging process is completed, and, additionally, by the progressive reduction of inlet port area which occurs during the supercharging period "and causes high flow loss through the ports. Further, a high scavenge pressure is to be deprecated and in consequence the optimum arrangement can only be achieved by the use of two pressures, the lower for scaveng ing, the higher for supercharging. This results in added complication but is, nevertheless, a feature of certain indus trial and marine two-stroke diesel engines. For economical supercharging and scavenging an exhaust turbine driven blower has much to recommend it, especially as the exhaust back pressure then increases in step with increases of inlet pressure and an approximately constant"*) pressure drop is maintained across the cylinder. But here ' the interconnection of all the exhaust ports may cause pressure waves in the exhaust pipe of one cylinder to affect adversely the charging of another cylinder; hence the development of the Buchi turbo-charging system for diesel engines with the exhaust pipes coupled in groups to separate turbine nozzles. Direct Fuel Injections For a two-stroke spark-ignition petrol aircraft engine direct fuel injection into the cylinder is essential in order to avoid fuel loss during the scavenging period. This might permit the use of '' safety '' fuels, at any rate for civil air craft, but the use of abnormally heavy fuels in two-stroke compression-ignition engines, as sometimes occurs in marine practice, could not be tolerated for aircraft work. Mr The problem of cooling the two-stroke engine is also formidable, although it is generally accepted that on a basis of equal power output the two-stroke rejects less heat to the coolant than does the four-stroke, because a certain amount of cooling is done by the scavenging air which passes out through the exhaust ports. This characteristic does not eliminate the need for dissipating a far greater quantity of heat from the cylinder than in the four-stroke engine, by virtue of the higher power output per unit- cylinder capacity. Therefore, in view of the limitations on output already experienced with present-day four-stroke aircraft engines due to difficulties of cooling the cylinder assembly, it will be appreciated that this problem consti tutes one of the most serious in the development of the two-stroke engine, particularly the air-cooled type. In the light of all these limitations it is surely not to be wondered at that the two-stroke engine has made little progress so far in the aeronautical field. But the success of the mono-sleeve valve for four-stroke engines may open up renewed possibilities in the two-stroke sphere by virtue of its adaptability to a variety of port arrangements and its simplicity of operation. UNITED NATIONS PETROL PROGRAMME MR. RALPH K. DA VIES, U.S. Deputy Petroleum Adminis trator, announced in Washington recently that 80 per cent, of the authorised projects in the roo-octane aviation petrol programme of the United Nations has been completed. One hundred and eighty-nine separate projects were involved in the programme, of which 150 have now been completed. Seven of these were built in 1942, 71 in rg43, and 72 have been finished so far this year. Of the remaining units, 25 are expected to be ready this summer and the balance during the r autumn. Approximately 450 refineries and natural petrol plants are now engaged in making 100-octane or its components.
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