From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The formula for calculating the power charged to a supercharger.
| Identifier | ExFiles\Box 140\1\ scan0211 | |
| Date | 28th March 1938 guessed | |
| -7- Power Charged to Supercharger. Since the amount of power required to drive a blower is a function of the design features, it becomes apparent that the results obtained by the use of a separately-driven blower can be made applicable to any super-charged unit in which the super-charger is driven by the engine, provided proper accounting is made for the net power required by the blower. If the tests data had been obtained with a unit in which the supercharger was driven by the engine, they would have been, in a sense, the results of a special rather than fundamental investigation of supercharging. In applying the test data to an installation with the supercharger driven by the engine, the power to be charged to the blower subtracted from the engine brake power was calculated from the following adiabatic air horsepower formula: (1) Horsepower = [144 x n x Q / (33000 x (n-1) x E)] * P1 * [ { (P2 / P1) } ^ ((n-1)/n) - 1 ] where: n is the adiabatic exponent of compression, 1.4 Q is the quantity of air handled by the blower in cubic feet per minute at atmospheric pressure. E is the adiabatic efficiency of the blower. (Assumed value of 50 percent which is about average for most blowers, although aircraft superchargers show as high as 70 percent adiabatic efficiency. At the lower blower speeds the efficiency is probably lower than 50 percent, while at higher speeds it may be greater than the value chosen). P1 is the supercharger inlet pressure in pounds per square inch, equal to atmospheric pressure less that pressure drop through the carburettor, assumed to be equal to atmospheric pressure for these calculations. P2 is the blower discharge pressure in pounds per square inch. For these calculations the discharge pressure was assumed to be equal to atmospheric pressure plus the total boost pressure. | ||