Bridging the Gap between HCCI and SI: Spark-Assisted Compression Ignition

Homogeneous charge compression ignition (HCCI) has received much attention in recent years due to its ability to reduce both fuel consumption and NO

emissions compared to normal spark-ignited (SI) combustion. However, due to the limited operating range of HCCI, production feasible engines will need to employ a combination of combustion strategies, such as stoichiometric SI combustion at high loads and leaner burn spark-assisted compression ignition (SACI) and HCCI at intermediate and low loads. The goal of this study was to extend the high load limit of HCCI into the SACI region while maintaining a stoichiometric equivalence ratio. Experiments were conducted on a single-cylinder research engine with fully flexible valve actuation. In-cylinder pressure rise rates and combustion stability were controlled using cooled external EGR, spark assist, and negative valve overlap. Several engine loads within the SACI regime were investigated. Heat release analysis of these points showed a distinct region of flame propagation followed by rapid auto-ignition of the remaining charge. With proper control of the combustion variables, this region of flame propagation became larger as load was increased, eventually resembling conventional spark-ignited combustion. Using this strategy, a maximum engine load of ~7.5 bar NMEP was achieved while maintaining good efficiency and complying with emissions regulations.