Improvement of the EGR Dilution Tolerance in Gasoline Engines by the Use of a HSASI Pre-Chamber Spark Plug

Charge dilution in gasoline engines reduces NOx emissions and wall heat losses by the lower combustion temperature. Furthermore, under part load conditions de-throttling allows the reduction of pumping losses and thus higher engine efficiency. In contrast to lean burn, charge dilution by exhaust gas recirculation (EGR) under stoichiometric combustion conditions enables the use of an effective three-way catalyst. A pre-chamber spark plug with hot surface-assisted spark ignition (HSASI) was developed at the UAS Karlsruhe to overcome the drawbacks of charge dilution, especially under part load or cold start conditions, such as inhibited ignition and slow flame speed, and to even enable a further increase of the dilution rate. The influence of the HSASI pre-chamber spark plug on the heat release under EGR dilution and stoichiometric conditions was investigated on a single-cylinder gasoline engine. The performance of the HSASI spark plug was compared with a passive pre-chamber spark plug (PPCSP) and a conventional spark plug (SI) for different combustion phasings. Detailed combustion analyses were conducted and the signal from an ion current sensor located inside the pre-chamber as well as the engine out NO emissions were examined. Operation with the HSASI pre-chamber spark plug allows the ignition timing to be advanced when misfiring occurs with PPCSP and unstable engine operation prevails with SI. With the same combustion phasing, HSASI allows for retarding the ignition timing compared to PPCSP and SI by up to 28 °CA due to a faster flame development. HSASI operation shows less NO emissions than SI and similar to PPCSP for same combustion phasing. However, HSASI has higher efficiency losses due to incomplete combustion and higher wall heat losses compared to SI.