The combustion instability at low loads is one of the key technology risks that needs to be addressed with the development of gasoline compression ignition (GCI) engine. The misfires and partial burns due to combustion instability leads to excessive hydrocarbon (HC) and carbon monoxide (CO) emissions. This study aims to improve the combustion robustness and reduce the emissions at low loads. The GCI engine used in this study has unique hardware features of a spark plug placed adjacent to the centrally mounted gasoline direct injector and a shallow pent roof combustion chamber coupled with a bowl in piston geometry. The engine experiments were performed in a single cylinder GCI engine at 3 bar indicated mean effective pressure (IMEP) and 1500 rpm for certified gasoline with research octane number (RON) = 91. Enabling strategies such as internal exhaust gas recirculation (i-EGR), compression ratio (CR = 16 & 18) and spark assisted ignition were investigated to improve the combustion robustness. Several other control variables such as intake temperature, intake pressure, fuel injection pressure, fuel injection timing, and exhaust back pressure were used to optimize the operation under low load conditions.
The experimental study revealed that exhaust rebreathing supports the combustion stability but with the difficulty of precise back pressure control. For CR = 16 piston, exhaust re-breathing (i-EGR) combined with spark assistance enables to achieve a coefficient of variation (COV) below 2% with NOX emission of around 0.1 g/kWh. At higher CR = 18, pressure and temperature conditions are favorable to support the auto-ignition driven combustion without exhaust rebreathing. For improved combustion stability, spark assistance is needed with CR = 18 piston. Optimum spark shortened the burn rate, improved the combustion stability, and reduced the HC emissions. The effects of the flame initiated by the spark plug on the overall combustion is limited to the vicinity of spark arc. Overall, when fuel mixture is stratified under partially premixed compression ignition (PPCI) conditions, spark is needed to strengthen the ignition behavior without disturbing the major auto-ignition combustion behavior.