Potential of Thermal Stratification and Combustion Retard for Reducing Pressure-Rise Rates in HCCI Engines, Based on Multi-Zone Modeling and Experiments

This work investigates the potential of in-cylinder thermal stratification for reducing the pressure-rise rate in HCCI engines, and the coupling between thermal stratification and combustion-phasing retard. A combination of computational and experimental results is employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKIN III kinetics-rate code, and kinetic mechanisms for iso-octane. This study shows that the potential for extending the high-load operating limit by adjusting the thermal stratification is very large. With appropriate stratification, even a stoichiometric charge can be combusted with low pressure-rise rates, giving an output of 16 bar IMEPg for naturally aspirated operation. For more typical HCCI fueling rates (Φ = 0.38 - 0.45), the optimal charge-temperature distribution is found to depend on both the amount of fuel and the combustion phasing. For combustion phasing in the range of 7 - 10°CA after TDC, a linear thermal distribution is optimal since it produces a near-linear pressure rise. For other combustion phasings, non-linear distributions are required to achieve a linear pressure rise. Also, the total thermal width must be greater at higher fueling rates to avoid excessive pressure-rise rates. The study also shows that increasing the natural thermal width of the charge by 50% would allow the equivalence ratio to be increased from 0.44 to 0.60, with an associated increase of the IMEPg from 524 to 695 kPa for naturally aspirated operation. It was also found that the naturally occurring thermal stratification plays a major role in producing the experimentally observed benefit of combustion-timing retard for slowing the combustion rate. Reduced chemical-kinetic rates with combustion retard are found to play a lesser role.