Thermodynamic and Chemical Effects of EGR and Its Constituents on HCCI Autoignition 2007-01-0207

EGR can be used beneficially to control combustion phasing in HCCI engines. To better understand the function of EGR, this study experimentally investigates the thermodynamic and chemical effects of real EGR, simulated EGR, dry EGR, and individual EGR constituents (N₂, CO₂, and H₂O) on the autoignition processes. This was done for gasoline and various PRF blends. The data show that addition of real EGR retards the autoignition timing for all fuels. However, the amount of retard is dependent on the specific fuel type. This can be explained by identifying and quantifying the various underlying mechanisms, which are: 1) Thermodynamic cooling effect due to increased specific-heat capacity, 2) [O₂] reduction effect, 3) Enhancement of autoignition due to the presence of H₂O, 4) Enhancement or suppression of autoignition due to the presence of trace species such as unburned or partially-oxidized hydrocarbons.

The results show that the single-stage ignition fuels iso-octane and gasoline are more sensitive to the cooling effect of EGR, compared to the two-stage ignition fuels PRF80 and PRF60. On the other hand, the two-stage ignition fuels have much higher sensitivity to the reduction of O₂ concentration associated with the addition of EGR. Furthermore, H₂O has a pronounced ignition-enhancing effect for the two-stage fuels that tends to counteract the strong cooling effect of H₂O. Finally, for the single-stage ignition fuels, gasoline and iso-octane, partially-oxidized fuel tends to enhance the ignition, thus counteracting the retarding effect of EGR. On the other hand, the trace species present for operation with the two-stage ignition fuel PRF80 increased the retarding effect of EGR.

In addition to the experiment, corresponding chemical-kinetics modeling was performed to evaluate the predictive capabilities of detailed iso-octane and PRF chemical-kinetics mechanisms from Lawrence Livermore National Laboratory (LLNL).