This paper focuses on the combustion development portion of the Advanced Combustion Controls Enabling Systems and Solutions (ACCESS) project, a joint research project partially funded by a Department of Energy grant. The main goal of the project is to improve fuel economy in a gasoline fueled light-duty vehicle by 30% while maintaining similar performance and meeting SULEV emission standards for the Federal Test Procedure (FTP) cycle.
In this study, several combustion modes Spark Ignited (SI), Homogeneous Charge Compression Ignition (HCCI), Spark- Assisted Compression Ignition (SACI)) were compared under various conditions (naturally aspirated, boosted, lean, and stoichiometric) to compare the methods of controlled auto-ignition on a downsized, boosted multi-cylinder engine with an advanced valvetrain system capable of operating under wide negative valve overlap (NVO) conditions. The engine was operated under steady state conditions at a constant engine speed of 1500rpm and various loads (2.0 - 6.0 bar BMEP) to investigate the impact of extending the HCCI/SACI operating range and compare the results to traditional SI combustion modes. The HCCI operating mode was tested under lean conditions both naturally aspirated and with mechanically supercharged operation to increase the load range. SACI combustion was examined at both lean and stoichiometric operating conditions, with the use of external cooled Exhaust Gas Recirculation (EGR) for mitigation of combustion noise at high loads.
It was found that the effective load range of naturally aspirated lean HCCI is limited but provides good combustion stability with significant improvements in BSFC compared to the optimized SI combustion mode. Extending the lean HCCI range via forced induction improves the lean capability at higher loads but exacerbates loss mechanisms that reduce overall brake thermal efficiency. Extending the operating range through stoichiometric SACI combustion modes resulted in BSFC improvements at the mid and high load points tested.
