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Aviation History
1938
1938 - 0521.PDF
FEBRUARY 24, 1938 13 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT e TWO-STROKE AERO ENGINES A* Summary oj Their Advantages and Disadvantages, and Some Speculations on Future Developments By F. UMPLEBY THE two-stroke cycle engine is passing through, orjust emerging from, a period of transition in itsdevelopment. In this new departure resonatingoscillations are created in the exhaust pipe, by the first blast of exhaust gas, and the oscillations are utilised to scavenge and charge the cylinder. This may lead to the production of a simple, light, powerful and reliable engine which can be manufactured at a most economical figure, since, except for the crankshaft, the only two essen- tial moving parts required, per cylinder, are the piston and connecting-rod. All the usual mechanically driven scavenge pumps or blowers are eliminated, except, perhaps, a small inertia blower to be used when starting up the engine. Engines operating on this system are being marketed by Petters, Ltd., and their possibilities are being investi- gated by Armstrong Securities, Ltd., in England, and by Kadenacy in France, Westinghouse in U.S.A. and Wilin in Poland. Unsupercharged, these engines would be suit- able for moderate altitudes. Larger engines for higher altitudes would, of course, be supercharged by means of an exhaust-gas turbo-supercharger designed to utilise the first blast of exhaust gas energy without destroying the depression created behind the blast. , •-•;"•'... Comparisons When four-stroke and two-stroke cycle engines oi the same bore and stroke, running at the same speed, are com- pared, the flow of heat energy to the two-stroke, if the same amount of fuel and air are being completely con- sumed per cycle, will be double that of the four-stroke. The flow of heat going to waste will also be double that of the four-stroke, and it is the waste heat that makes trouble, which manifests itself in various ways. Tn engines provided with overhead poppet exhaust valves the valve heads burn out and the cylinder heads crack between the valve seats. In addition, it is almost impossible to operate the valves efficiently at high speeds, as their period of opening is only about 120 deg. of crankshaft travel against 240 deg. in the four-stroke, and they must operate at double speed if the engines are to be run at the same speed. The flow of exhaust gas being double, steps must be taken to cool the barrel of the cylinder and the exhaust port belt. This is done on the Junkers engine by locally increasing the velocity of the cooling water. Fortunately, it is the intensity of the heat and not the quantity which gives most trouble. Where the piston is used to control the exhaust ports it has to contend with its ordinary double share of waste heat as a piston and act as an exhaust valve in addition. Ihus, here again, special precautions must be taken to facilitate the flow of heat from the piston crown to the cooler walls of the cylinder. Where a ring of piston-con- trolled exhaust ports is used, the length of the piston skirt must be increased to prevent the ports being uncovered at the top of the stroke. This also involves an increase in the length ot the cylinder barrel and connecting-rod, accompanied by a corresponding increase in engine weight. From an aero-engine point of view, this increase in weight is not at all welcome, but it is not altogether a disadvantage from an endurance point of view, because the thrust per square inch of piston in contact with the cylinder wall is reduced, and the area of contact for heat flow is increased The piston must also carry an effective oil scraper ring at the bottom of the skirt to prevent loss of lubricating oil through the ports which are overrun by the gas seal nngs each stroke. The gas forces in a two-stroke engine predominate over the inertia forces and may give lubrica tion troubles at the gudgeon pin The constancy of pressure in one direction also increases the severity of the conditions imposed upon the top piston ring, as the ring is held constantly, by gas pressure and inertia, against the lower edge of its groove. The conse- quence is that the stagnant oil behind the ring may be baked into a carbonaceous cement which finally sticks the ring in its groove. The gudgeon-pin lubrication trouble may be overcome by using a knuckle joint instead of the usual type of gudgeon pin, and Junkers use a solid L-shaped top piston ring, something like a Gnome obtura- tor ring, neither of which rings is required in a soundly designed cylinder barrel. The inertia forces on the crankpin of a four-stroke radial engine are a limiting factor against high speed, but there is little doubt that there should be a substantial advantage here in favour of a two-stroke radial engine. Hitherto one of the outstanding difficulties in connection with a two-stroke radial engine has been the difficulty of scavenging and charging the engine, since crankcase com- pression was impossible; but exhaust gas velocity and turbo-scavenging have changed the whole outlook. Recent developments, now to be described, have solved this problem by even more simple and efficient means than the scavenging stroke of the four-stroke cycle engine. These developments in connection with two-stroke cycle engines are described more fully in articles by S. J. Da vies, D.Sc, M.I.Mech.E., in Engineering, June 18th and 25th, 1937. The research was carried out on engines de- signed by M. Michel Kadenacy. During the tests it was found that immediately the exhaust port opened the ex- haust gases left the cylinder as a compact mass with a ballistic velocity exceeding that of sound in high-tempera- ture gases. This rapid evacuation of more than 50 per cent, of the cylinder contents leaves behind it a dapression in the cylinder exceeding half an atmosphere, with the exhaust gas residue moving rapidly towards the exhaust ports. The evacuation was completed in a definite period of time, about 0.003 sec., and did not depend upon the size or length of the exhaust pipe, provided steps were taken to prevent return surges of the air or gas in the pipe re-enter- ing the cylinder. This point is quite neglected in the ordinary four-stroke engine. Confirmation These preliminary conclusions were confirmed on a commercial type of Junkers engine modified in accordance with the " Kadenacy " patents, one of which modifications was to remove the Junkers scavenge air pump. The ex- haust pipe was fitted with a pitot tube so arranged that the gas pressure variations could be observed and measured stroboscopically in relation to the speed of the crankshaft in degrees Two hypothetical gas pressure curves, A and B, Fig. 1. are given. The early parts of the curves are based on two of Dr. Davies' curves, and the latter parts are slightly smoothed out and modified to suit the port action of the common combustion chamber engine pistons. Dr. Davies found that after a short delay, following the opening of the exhaust port, the pressure in the exhaust pipe in- creased rapidly to 33^in. of Hg. and then decreased with equal rapidity (see curve A, Fig. 1), until at 400 it reached atmospheric pressure. From this point the curve is based upon the common combustion chamber engine. The pressure continues to fall, and there is first a slight depression, and between 850 and 950 a depression of 5|in. of Hg. Aftei 950 the depression decreases until at 1150 it
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