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Aviation History
1920
1920 - 0085.PDF
JANUARY J5, 1920 •••<*. ••*!"*•' THE VALUE OF SUPERCHARGING THE following report is based upon a test made in the Altitude Laboratory of the Bureau of Standards to determine the effect produced upon the horse-power output of an aeroplane engine by the introduction of air to the carburettor at a higher pressure than the exhaust or back pressure. Such a condition is easily produced in'this laboratory, as the engine under test is enclosed in a chamber, the air pressure in which may be controlled independently of that on the carburettor inlet, by means of suitable pipes and valves leading to the atmosphere and to a suction blower. A stock 150 h.p. Hispano-Suiza engine, built by the Wright-Martin Aircraft Corporation, New Bruns- wick, N.J., having a compression ratio of 5-3 to 1 was used in making the tests. The Claudel carburettor', with which the engine was equipped, was adjusted in each case to give maximum power. i 1 Two runs were made with approximately a constant pressure on the carburettor, and with varying exhaust back pressures in each case. The first run was made with the valve on the intake wide open, so that the highest pressure could be obtained on the carburettor. The pressure within the chamber, which is the same as the exhaust back pressure, was adjusted, and readings taken at values of approximately 62, 50, 38 and 34 cm. Hg. The data obtained are listed in Table I, and the results are plotted as Curve " A " in Fig. 1. In the second run, the valve on the intake was partially closed, so that the pressure on the carburettor was the equivalent of 20 cm. Hg. below the prevail- ing atmospheric pressure, and readings taken at approximately the same points as before.. The results of this run are given in the second half of Table I, and are plotted as Curve " B " in Fig. 1. In the first run the pressure of the intake air was not constant throughout, varying from 72-67 to 70-22 cm. Hg., so that it was necessary to correct the results to some constant pressure. The correction was made to a pressure of 70 cm. Hg. by interpolation, which is based on the assumption that the increment of horse-power developed at a given back pressure is proportional to the increment of pressure of the carburettor air. This correction gave Curve " C " in Fig. 1. All horse-powers were corrected to 1500 r.p.m. and 0° C. This temperature correction was made in accordance with the results of a series of tests performed in this laboratory to determine the variation of horse-power with temperature. In order to determine the engine performance under different conditions of carburettor pressure and exhaust back pressure, a family of curves at carbu- rettor pressures of 76, 70, 65, 60 and 55 cm. Hg. were plotted against the exhaust back pressures. These were derived from the two experimental curves by interpolation, based on the assumption, as mentioned above. As it is desirable for purposes of design to know the engine output under different conditions as a function of the maximum output on the ground, the ratio of horse-power taken from the above-mentioned curves to the horse-power at a carburettor and ex- haust pressure-of 76 cm. Hg., taken from Curve " D," is computed and plotted in Fig. 2. These curves give the engine performance under the different conditions on the basis of a constant temperature of air at the carburettor. -4-Lj- To compute the horse-power that would be de- veloped by an engine equipped with a supercharging device, under a given set of conditions of altitude,
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