Offset Active Prechamber (OAP): A Strategy to Enable Low Load GCI Operation

High fuel stratification gasoline compression ignition (HFS-GCI) strategies allow for the use of ignition control methods similar to those used by diesel-fueled compression ignition (CI) engines while offering the emissions benefits of gasoline-like fuels. Despite this benefit, low load GCI operation requires ignition assistance viz. intake boosting, intake heating, cylinder deactivation, etc. for consistent autoignition. A novel ignition assistance methodology using an offset active prechamber (OAP) is proposed in this work to enable low load GCI operation. A 1.5cc OAP with a pressure-sensing spark plug and gaseous fuel injection system is designed and mounted in a medium-duty single-cylinder test engine based on the Cummins ISB engine. The prechamber is provided with two holes designed to ignite the fuel spray from the centrally mounted direct injection (DI) fuel injector. Gasoline was used as the main chamber fuel and methane was used as the prechamber fuel. A detailed parametric optimization for both prechamber and main chamber parameters are performed and presented. Results show that the OAP improves the combustion stability at low load GCI operation with no thermal efficiency penalty.