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Aviation History
1940
1940 - 1594.PDF
.5OPPLEMENT TO FLIGHT 499c MAY 30, 1940 THE AIRCRAFT ENGINEER the same in a two-stroke as in a four-stroke; but the two-stroke has twice the number of valves, and they remain open only half the time, thus absorbing only half the heat. Besides, though the exhaust gas is slightly hotter, the large volume of excess salvage air escaping through the exhaust lowers the average temperature con- siderably ; even at equal m.e.p. and piston speed the two- stroke valves would be cooler. On the other hand, the mechanical stresses, especially on springs and valve gear, are very high, owing to the limited time available for opening, and the consequent brutal accelerations. Fortunately, racing car practice gives abundant data on really fast valve operation; as silence is not required, there will be no real difficulty. Masking the valves can be useful to increase the real open- ing time. Anyhow, the two-stroke aero engine will prob- ably run slower than the four-stroke; 14 m/s (2,750 feet/min.) would be quite satisfactory. In a two-stroke engine, the piston has two distinct func- tions : the crown and rings withstand the working pres- sure and get rid of excess heat; the skirt acts as an inlet distributor. The piston is undoubtedly the weakest point of the two-stroke petrol engine, the success of which de- pends on a satisfactory solution of the piston problem. Considering again the two similar cylinders, the amount of heat communicated to the piston crown during expan- sion is the same for both two and four-stroke type. Most of the heat leaves the piston via the top rings and lands; as in the two-stroke cycle the two idle strokes fall out, and the time available to cool the piston is halved. On the other hand, the four-stroke piston receives more heat during the exhaust stroke, while in a two-stroke with uniflow scavenge the incoming fresh air isolates the piston from the hot exhaust gas, even before the end of the expansion stroke, and helps actively to cool the piston. With petrol injection, the latent heat of the fuel can also be used to cool the piston. Even so, the piston tempera- ture is bound to be higher in a two-stroke, and everything must be done to reduce it; m.e.p. and piston speed must be decreased, and a high expansion ratio must be used, i.e. a high compression ratio and a low degree of super- charge, thus also improving fuel consumption. Good design can also help. The piston top must be quite flat, to present the minimum surface to the flame. Luckily in a two-stroke the valves open while the piston is near bottom stroke. In a flat four-valve head, such as is generally used in liquid-cooled engines, the red-hot valve heads face directly the piston crown and irradiate on it a considerable amount of heat; a steep pent-roof head would probably be better. Finally, the piston can be cooled by spraying the ribbed inner surface with oil; this method, used in the General Motors' two-stroke C.I. engine, subtracts heat directly from the hottest point, the centre of the crown, and so considerably relieves the top rings and lands. Piston Temperature The piston temperature question is serious, but must not be exaggerated. Experiments by Baker have proved that the maximum temperature of a four-stroke piston increased from 220 deg. to 262 deg. C. when the power output was doubled by raising the r.p.m. from 2,000 to 4,000. If the doubled output had been obtained at 2,000 r.p.m. by going over to the two-stroke cycle, the piston temperature increase would certainly have been smaller, both for the reasons given above and because doubling the piston speed causes a considerable increase in piston fric- tion and heat. Piston length and connecting-rod length are vital factors affecting two-stroke distribution. The connecting-rod length, or rather its ratio to the crank radius, governs the law of motion and therefore the time available for scaveng- ing. Fig. 1 shows piston displacement in relation to crank angle for an infinite rod (R = 00 ) and for a very short rod (R = 2, too short in practice). The infinite rod gives pure sine law piston motion, the top and bottom halves of the stroke being symmetrical. The short rod causes the Influence of piston and connecting-rod lengths on engine size. The normal two-stroke piston and rod are shown in B. piston to move faster during the top half of the stroke and slower during the bottom half, during which scaveng- ing is effected ; the time available is therefore increased. Ordinary two-stroke engines, having very long pistons, must also have long connecting rods, usually R = 4.5/5. With short pistons R = 3 can be used without excessive side thrust. Fig. 2 shows the time integrals, that is, the effective port areas, for a 170 mm. stroke piston with R = 3 and R = 4.5 respectively. It will be seen that the short rod gives an average 10 per cent, increase in port area, and consequently in piston speed and power, within the usual scavenging angles. The exhaust is also similarly helped. In ordinary two-stroke engines the piston skirt is longer than the stroke, as the scavenge ports must be closed even with the piston right up to avoid loss of air. Actu- ally, a second scraper ring is generally placed below the ports to prevent too much oil reaching them and being blown into the cylinder during scavenge. Piston, connecting rod and cylinder liner must therefore be very long, and the overall dimensions of a two-stroke engine far exceed those of a four-stroke of the same bore and stroke. The consequent increase in weight and cross- section make this type of two-stroke engine practically useless for aircraft. The two-stroke aero engine must have a short piston and must not be any larger. A short piston can be used in a two-stroke, provided the scavenge air is- controlled by a distributor to prevent air blowing into the crankcase while the piston is near T.D.C. Unfortunately, the usual type of distributor cannot prevent the uncovered inlet ports being filled with lubricating oil. The problem is not easy but it can certainly be solved ; the ingenuity of designers will undoubtedly find several ways of over- coming this difficulty. Fig. 3 shows the influence of piston and rod length on. engine size. Of the two sketches, which are to scale, A shows a 150 x 170 cylinder with short piston and rod (R = 3). B shows one with normal two-stroke piston and rod (R = 4.5), the design being in each case the most com- pact possible. It can be seen that lengthening the piston starts a vicious circle, the increased dimensions of one part causing an increase in another. Thus, although the cylin- der liner in A reaches much lower than in B, the connecting
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