Combustion phasing of residual-effected homogeneous charge compression ignition

Abstract

Residual-effected homogeneous charge compression ignition (HCCI) is defined as an approach which relies on the reintroduction of combustion products to achieve both the required dilution and sensible energy rise to enable compression ignition. While convenient, this strategy introduces a coupling between the initial charge temperature and composition which is unique to this approach. Increasing the residual fraction tends to increase the initial charge temperature while simultaneously lowering reactant (fuel-air) concentrations. While intuition into chemical kinetics suggests that the initial thermal state should have the most significant impact on combustion phasing, experimental data for residual-effected HCCI show that increasing residual fraction delays phasing, despite higher apparent initial charge temperature. A kinetics-based model of residual-effected HCCI shows that two regimes of operation exist: A thermally-limited regime, in which phasing is most sensitive to the initial charge temperature, and a concentration-limited regime, in which phasing is most sensitive to the residual fraction. Because HCCI operation is practically limited by the rate of pressure rise as dilution decreases, the model suggests that residual-effected HCCI can occur solely in the concentration-limited regime. Therefore, control strategies for residual-effected HCCI should consider the dilution level as an input of primary importance.