Combustion Limits and Efficiency in a Homogeneous Charge Compression Ignition Engine

Abstract

Combustion phasing is characterized by CA50, the crank angle position where 50 per cent of the total heat release occurs. CA50 = ( a + b) [OI - OI0] where OI is the octane index defining the fuel auto-ignition quality given by OI = (1 - K) RON + K MON = RON - KS and S is the sensitivity of the fuel, (RON - MON). K, ( a + b), and OI0, which is the requirement of the engine, can be estimated from in-cylinder thermodynamic parameters and engine operating conditions. If OI is much smaller or greater than OI0, combustion will reach, respectively, the ‘knock limit’ with unacceptably high rate pressure rise or the ‘instability limit’ with unacceptably high cyclic variation. Nineteen different gasoline-like fuels with RON > 60 have been tested at many different operating conditions in a single-cylinder homogeneous charge compression ignition (HCCI) engine. This generates a data set across which the correlations between CA50 and other operating parameters such as the equivalence ratio and the inlet pressure Pin are broken. Regression analysis shows that the maximum pressure rise rate (MPRR) increases with increasing φ and Pin and decreases with increasing CA50 and S if other parameters are kept constant. X = 1.55CA50 - 52 can be used to discriminate between conditions of high and low cyclic variation. For X < 0, the coefficient of variation (COV) of indicated mean effective pressure (i.m.e.p.) is less than 10 per cent in about 90 per cent of the cases, but if X>0, COV of i.m.e.p. is less than 10 per cent in only about 12 per cent of the cases. The i.m.e.p. increases with fuel energy content per cycle. The indicated fuel conversion efficiency ηi increases with increase in i.m.e.p. With other parameters kept constant, ηi decreases with increasing φ, intake air density, and intake temperature and increases with increasing CA50 and maximum pressure. An HCCI engine can be operated with an acceptable noise level and cyclic variation only within a narrow range of CA50 between about –5 and +10° from top dead centre.