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Aviation History
1938
1938 - 0525.PDF
FEBRUARY 24. i<)38 15 SUPPLEMENT TOFLIGHT THE AIRCRAFT ENGINEER exhaust ports and the resultant loss of expansion ratio. Where, however, the uniflow scavenge system is used, not only is better scavenging obtained, but the inlet and ex- haust ports are not situated at the same level; therefore, the ports can be reduced in height, a later opening be obtained, the expansion ratio increased, and still have sufficient port area for high speeds. Furthermore, during the period when the ports are full open the piston stops and reverses its stroke, and it is moving with its minimum velocity for about 30 crankshaft degrees on each side of the dead centre. This movement may be prolonged on the closing side by offsetting the centres of the cylinder and crankshaft. The light thrown on the actual exhaust and inlet gas velocities and depres- sions by Prof. Davies indicates that any difficulty here can be eliminated. Port Timing On Fig. 1 is shown, approximately, the port opening path, port timing, area, and the periods during which the exhaust, scavenge, charge and supercharge may be said to occur for the three engines illustrated on Fig. 3. In the common combustion-chamber engine the exhaust port opens 72 crankshaft degrees before bottom dead centre of the exhaust piston and is almost full open when the inlet port is first opened by the inlet piston 33 degrees later. Due, however, to the differential stroke action of the pistons, the inlet piston does not reach bottom dead centre until the crankshaft has travelled 80 degrees, and as a result of the same action the exhaust port closes 30 degrees before the inlet closes. This 30 degrees is available for supercharging because, since the exhaust ports are closed, gas losses through the exhaust ports cannot occur, and it is during this 30 degrees that the depression on the inlet side is converted to a tairly high positive pressure, as is shown by curve B. On the opposed-piston type of engine the port opening and closing periods are varied and controlled by variation in the height of the ports and by setting the crankshaft connected to the inlet position so that it reaches bottom dead centre later than the exhaust piston. This is shown on Fig. 1, where the inlet piston is set 20 degrees behind the exhaust, at 60-80 degrees respectively. Both ports open at the same points as those on the common com- bustion-chamber engine, but even so the inlet port is only open for 8 degrees after the exhaust port has closed, and very little time is available for supercharging with the mlet open and the exhaust closed. 17 16 15 14 • l$ \i S'i »r JUNKERS COMMERCIAL. TWO-STROK.E ENGINE. 6S7M BORE X 210*1*1 TOTAL STROKE. 700^ 5VOL. —AT X KAO FIG.E / ENACY / FNC / f • 5INE ^—•* / / RESEd / / RCH ENG / NEERING / 8-6-: / 110 ,100 90 80 600 800 1000 1200 MOO ENGINE RPM 1600 1800 The inlet ports in the sleeved-piston type of engine, with ports cut in the sleeve, are controlled by the inwardly pro- jecting cylinder head. The position of the inlet and ex- haust ports in this engine can, of course, be reversed. They are the most shallow of the ports illustrated, but even so they open within 20 degrees of the exhaust port opening point, at which point the maximum pressure due to the first blast of exhaust gas (see curve A) has just been passed ; the inlet would be subject to heavy blow-back, since the pressure in the cylinder is still high, unless super- charged at higher pressure. The inlet also closes 10 degrees before the exhaust port, at no degrees and 120 degrees respectively. Thus, the most favourable engines, from a port operation and gas flow control point of view, are the common combustion and the opposed piston types, but they are mechanically the most awkward. The port area curves show the area of the respective inlet and exhaust port openings at any instant, read vertically from the zero line. On the right, below these, are shown the height that the respective ports are open at any instant, with the opening points below and the closing points along the zero line ; each square represents a quarter-inch of opening or closing. It will be noted from these curves that the rate at which the ports open or close decreases as the piston approaches the full open position, and that the curves cross the zero line at an acute angle. This is due to the piston travelling at its minimum velocity at this point. The same feature is manifest on the port area curves ; indeed, to some suitable scale and engine speed these curves would form a piston velocity curve. Informative Curves The whole of the curves on Fig. 1, set out as they are on a crankshaft degree travel base, are most informative and form an interesting study of the gas pressures and velocities in relation to the port sizes and the opening and closing points and their periods. There is considerable prejudice against the two-cycle engine, but, as we have already seen, modern instruments have enabled us to gain a much wider and a more correct knowledge of what really happens in the gas-flow system of the engine. The materials used in the construction of the modern four-stroke cycle engine now make it possible to build two-stroke engines able to stand up to the extra heat flow and stresses involved. One of the greatest of the obstacles to two-stroke cycle engine development has been its excessive fuel consump- tion, but a few two-stroke engines now have a fuel con- sumption equally as low as that of their corresponding four-stroke cycle prototype. One solution of the fuel-con- sumption problem lies in the use of fuel injection and compression ignition ; here the fuel is injected into the combustion chamber after all the ports are closed, and there can be no loss of unburnt fuel through the exhaust ports. It may be that in the common combustion-chamber engine operating on the compression-ignition system we have the aero engine of the future, combining, as it does, favourable port control without valves, minimum weight possibilities, and the ability to run at high speeds. ^ - - : Bi-fuel Systems Another method of reducing fuel consumption is to inject high-octane fuel into the cylinder early in the compression stroke, but after all the ports are closed, and ignite the mixture electrically. High B.M.E.P.'s and high speeds have been attained by this method. Unfortunately it adds to the complexity of the engine and involves the weight of two sets of fuel injection-ignition systems, but the extra weight is more than regained by the extra power developed, and a low ignition point fuel can be used. To obtain maximum power from an engine of a limited capacity it must be run at high speed, and a lower cruising speed must also be provided for. Both speeds must be economical fuel consumption speeds, and it is quite feasible to design the port sizes and opening periods in such a way that there is no loss of unburnt fuel through the exhaust ports. This can be done by using a sandwich system, where first pure air is admitted for scavenging, and then a
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