tailieunhanh - Internal Combustion Engines Fundamentals P2

Engine specific weight and specific volume - Correction factors for power and volumetric efficiency | 52 INTERNAL COMBUSTION ENGINE FUNDAMENTALS With units H 61 P kW or sfc g kW h sfc lbm hp h or sfc lbm hp h 2 x 103 sfc g kW h F hp Low values of sfc are obviously desirable. For SI engines typical best values of brake specific fuel consumption are about 75 gg J 270 g kW-h Ibm hp h. For CI engines best values are lower and in large engines can go below 55 tg J 200 g kW h Ibm hp h. The specific fuel consumption has units. A dimensionless parameter that relates the desired engine output work per cycle or power to the necessary input fuel flow would have more fundamental value. The ratio of the work produced per cycle to the amount of fuel energy supplied per cycle that can be released in the combustion process is commonly used for this purpose. It is a measure of the engine s efficiency. The fuel energy supplied which can be released by combustion is given by the mass of fuel supplied to the engine per cycle times the heating value of the fuel. The heating value of a fuel QHy defines its energy content. It is determined in a standardized test procedure in which a known mass of fuel is fully burned with air and the thermal energy released by the combustion process is absorbed by a calorimeter as the combustion products cool down to their original temperature. This measure of an engine s efficiency which will be called the fuel conversion efficiency rif f is given by Wc PnjffN P m nt n NV m i m QnV mfnK N QnV mfQtrV where mf is the mass of fuel inducted per cycle. Substitution for Plmf from Eq. gives f This empirically defined engine efficiency has previously been called thermal efficiency or enthalpy efficiency. The term fuel conversion efficiency is preferred because it describes this quantity more precisely and distinguishes it clearly from other definitions of engine efficiency which will be developed in Sec. . Note that there are several different definitions of heating value see Sec. . The numerical values .