Numerical study of HCCI combustion in diesel engines using reduced chemical kinetics of n-heptane with multidimensional CFD code

摘要

The homogeneous charge compression ignition (HCCI) is one of the alternatives to reduce significantly engine emissions for the future regulations. The combustion process in HCCI engines does not involve flame propagation or flame diffusion as in conventional internal combustion engines. Many studies have confirmed that during this mode the combustion process is mainly controlled by chemical kinetics. However, a coupled CFD and detailed chemistry simulation requires substantial memory and CPU time which may be very difficult with current computer capabilities. Thus a reduced mechanism is required to simulate the engine cycle during this operating mode to achieve more accurate analysis. In this study reduced chemistry was used with an engine CFD code combustion (Star-CD/Kinetics) to study combustion process in homogeneous charge compression ignition (HCCI) engines. The Chemkin code was used at first to develop a reduced autoignition model for n-heptane using a systematic sensitivity analysis approach. The reduced mechanism with 37 species and 61 reactions reproduces the ignition delay times of the mixture over a wide range of engine operating conditions (inlet pressure and temperature, air-fuel ratio, engine speed, compression ratio and EGR rate). The results obtained have been validated by comparison with detailed mechanisms developed by Lawrence Livermore National Laboratory, Chalmers University and the shock tube experiments of Ciezki and Adomeit. The comparison shows that the reduced mechanism behaves in the same way as these detailed models for the description of the two stages phenomenon (cool flame zone followed by main ignition), but also for the temperature and pressure inside the cylinder. The reduced reaction scheme has been implemented in Star-CD/Kinetics CFD code. As the computational time for the original mesh (3D - 38000 cells) was too high on a work station, we have used a simplified mesh (2D - 700 cells) with only 14 hours computational time which reproduced similar results. With the simplified mesh and the reduced reaction scheme, the numerical simulations in homogeneous mode show that heat release is very steep whatever the operating conditions (equivalence ratio, EGR rate, inlet pressure) which is in agreement with the massive self-ignition of the homogeneous charge. The simulations with different EGR rates confirm that the dilution by EGR delays the ignition timing when the gas temperature is not changed.