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A History of Aeronautics by Marsh and Vivian

The scavenging of the power cylinder


theoretically, is obtained the maximum of energy with the minimum of expenditure; in practice, however, the scavenging of the power cylinder, a matter of great importance in all internal combustion engines, is often imperfect, owing to the opening of the exhaust ports being of relatively short duration; clearing the exhaust gases out of the cylinder is not fully accomplished, and these gases mix with the fresh charge and detract from its efficiency. Similarly, owing to the shorter space of time allowed, the charging of the cylinder with the fresh mixture is not so efficient as in the four-stroke cycle type; the fresh charge is usually compressed slightly in a separate chamber--crank case, independent cylinder, or charging pump, and is delivered to the working cylinder during the beginning of the return stroke of the piston, while in engines working on the four-stroke cycle principle a complete stroke is devoted to the expulsion of the waste gases of the exhaust, and another full stroke to recharging the cylinder with fresh explosive mixture.

Theoretically the two-stroke and the four-stroke cycle engines possess exactly the same thermal efficiency, but actually this is modified by a series of practical conditions which to some extent tend to neutralise the very strong case in favour of the two-stroke cycle engine. The specific capacity of the engine operating on the two-stroke principle is theoretically twice that of one operating on the four-stroke

cycle, and consequently, for equal power, the former should require only about half the cylinder volume of the latter; and, owing to the greater superficial area of the smaller cylinder, relatively, the latter should be far more easily cooled than the larger four-stroke cycle cylinder; thus it should be possible to get higher compression pressures, which in turn should result in great economy of working. Also the obtaining of a working impulse in the cylinder for each revolution of the crankshaft should give a great advantage in regularity of rotation--which it undoubtedly does--and the elimination of the operating gear for the valves, inlet and exhaust, should give greater simplicity of design.

In spite of all these theoretical--and some practical--advantages the four-stroke cycle engine was universally adopted for aircraft work; owing to the practical equality of the two principles of operation, so far as thermal efficiency and friction losses are concerned, there is no doubt that the simplicity of design (in theory) and high power output to weight ratio (also in theory) ought to have given the 'two-stroke' a place on the aeroplane. But this engine has to be developed so as to overcome its inherent drawbacks; better scavenging methods have yet to be devised--for this is the principal drawback--before the two-stroke can come to its own as a prime mover for aircraft.

Mr Dugald Clerk's original two-stroke cycle engine is indicated roughly, as regards principle, by the accompanying diagram, from which it will be seen that the elimination of the ordinary inlet and exhaust valves of the four-stroke type is more than compensated by a separate cylinder which, having a piston worked from the connecting-rod of the power cylinder, was used to charging, drawing the mixture from the carburettor past the valve in the top of the charging cylinder, and then forcing it through the connecting pipe into the power cylinder. The inlet valves both on the charging and the power cylinders are automatic; when the power piston is near the bottom of its stroke the piston in the charging cylinder is compressing the carburetted air, so that as soon as the pressure within the power cylinder is relieved by the exit of the burnt gases through the exhaust ports the pressure in the charging cylinder causes the valve in the head of the power cylinder to open, and fresh mixture flows into the cylinder, replacing the exhaust gases. After the piston has again covered the exhaust ports the mixture begins to be compressed, thus automatically closing the inlet valve. Ignition occurs near the end of the compression stroke, and the working stroke immediately follows, thus giving an impulse to the crankshaft on every down stroke of the piston. If the scavenging of the cylinder were complete, and the cylinder were to receive a full charge of fresh mixture for every stroke, the same mean effective pressure as is obtained with four-stroke cycle engines ought to be realised, and at an equal speed of rotation this engine should give twice the power obtainable from a four-stroke cycle engine of equal dimensions. This result was not achieved, and, with the improvements in construction brought about by experiment up to 1912, the output was found to be only about fifty per cent more than that of a four-stroke cycle engine of the same size, so that, when the charging cylinder is included, this engine has a greater weight per horse-power, while the lowest rate of fuel consumption recorded was 0.68 lb. per horse-power per hour.

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