Effects of EGR and its constituents on HCCI autoignition of ethanol

The thermodynamic and chemical effects of real EGR, simulated EGR, and individual EGR constituents (N-2, CO2, and H2O) on the HCCI autoignition processes of ethanol have been investigated experimentally and computationally. The results for ethanol were compared in detail with existing data for gasoline, iso-octane, PRF80, and PRF60. The data show that addition of EGR retards the autoignition timing for all five fuels when the intake temperature is maintained constant. However, the amount of retard is dependent on the specific fuel type, with ethanol showing the lowest sensitivity to the addition of clean simulated EGR gases. The response to EGR can be explained by quantifying the various underlying mechanisms. The results show that the single-stage ignition fuel ethanol is quite sensitive to the reduction of compression heating that occurs with EGR due to the higher heat capacity of the EGR gases compared to air. This high sensitivity to the cooling effect of EGR is similar to that of gasoline and iso-octane, which also are single-stage ignition fuels under these conditions. On the other hand, ethanol is very insensitive to the reduction of O-2 concentration associated with the addition of EGR. Both of these characteristics relate to ethanol's molecular stability - it does not react much until just before the hot-ignition point is reached. Consequently, ethanol has a low intermediate-temperature heat-release rate, which leads to a low temperature-rise rate prior to hot ignition, and therefore a high sensitivity to the cooling effect of EGR. Also, the relative lack of intermediate-temperature heat release prevents [O-2] from having much influence on the temperature rise prior to hot ignition, leading to a low sensitivity of the autoignition timing to changes of [O-2]. Finally, both H2O and trace species have significant ignition-enhancing effects for ethanol that to some degree counteract the retarding effect of EGR. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.