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).
Otto cycle efficiency, specific fuel consumption, and the effects of supercharging on engine performance.
| Identifier | ExFiles\Box 140\1\ scan0213 | |
| Date | 28th March 1938 guessed | |
| -9- The efficiency of a theoretical Otto air cycle is independent of charge density and of the amount of heat supplied, and is determined by the expression: Efficiency = 1 - (1/r)^(n-1) where r equals the compression and expansion ratio of the engine and n equals the compression and expansion exponent. Such a cycle is represented by area 2-3-4-5-2 of Figures 25A and 25B. For an actual engine supplied with air from a separately-driven blower, the work corresponding to this area, which is the work derived from energy released by combustion of the charge, is equal to the sum of the measured brake output work of the engine and the measured work necessary to motor the engine at the speed and intake manifold pressure considered, as will be shown later. It would therefore be expected that the specific fuel consumption of such an actual engine based on the sum of these two measured work quantities would be substantially constant at various intake manifold pressures and any given speed. Figure 17 shows curves of speed versus such horsepower sums for the engine unsupercharged and with an intake manifold pressure of ten inches of mercury above atmospheric. Also shown on Figure 17 are curves of engine speed versus specific fuel consumption based on these horsepower sums, and it is interesting to note that there apparently is no significant difference between the specific fuel consumption figures for the two manifold pressures. However, although the specific fuel consumption figures calculated on this basis are substantially independent of boost, as theoretical considerations predict, the fact remains that the data show lower net brake specific fuel consumption for the supercharged engine at the higher speeds, which is of primary importance from the practical standpoint. Table II shows the distribution of energy in the engine during operation at various speeds with the 5.55 to 1 compression ratios unsupercharged and with the 5.55 and 4.25 to 1 compression ratios supercharged to an intake manifold pressure of ten inches of mercury above atmospheric. It will be noted that the percent of the total energy in the fuel which was dissipated as heat to the cooling water in general was less for the engine when it was supercharged at either compression ratio than when it was unsupercharged at the 5.55 to 1 compression ratio. | ||