HCCI/SCCI Load Limits and Stoichiometric Operation in a Multicylinder Naturally Aspirated Spark Ignition Engine Operated on Gasoline and E85

Abstract

To meet demands for improvements in the CO2 emissions and fuel economy of gasoline passenger car engines advanced combustion strategies, to replace (or combine with) conventional spark ignition, must be developed and implemented. One possible strategy is homogeneous charge compression ignition (HCCI) achieved using negative valve overlap (NVO). However, several issues need to be addressed before this combustion strategy can be fully implemented in a production vehicle, one being to increase the upper load limit. One constraint at high loads is that the combustion becomes too rapid, leading to excessive pressure-rise rates and large pressure fluctuations (ringing), causing noise. A potential solution to this is to use charge stratification, but charge stratification normally gives rise to increased NO x emissions. Tests with a multicylinder engine reported here confirmed that there is significant potential to increase the upper load limit using charge stratification. In addition, the possibility of operating the engine in stoichiometric conditions, using a combination of NVO and external exhaust gas recirculation (EGR) (thus allowing the increased NO x emissions to be countered using a three-way catalyst) was investigated. Stoichiometric operation was found to be possible for both homogeneous and stratified modes, across a wide operating range, with small compromises in maximum load and fuel consumption. Nevertheless, delaying the need for a mode shift, and operating in stoichiometric conditions when entering a mode shift, should be beneficial in a drive cycle.