Impact of Spark Ignition Duration, Energy and Phasing on Combustion and Performance in a Gasoline Turbocharged Direct Injection Engine near the Dilute Limit

High dilution combustion is a promising technology to continue improving gasoline spark-ignition (SI) engine fuel conversion efficiency and for the reduction of nitrogen oxide emissions. Dilution is principally limited in SI engines by ignition, flame kernel development, successful transition to the turbulent propagating flame during the bulk burn portion of combustion process and the resulting combustion stability. However, the SI gasoline turbocharged directed injection (GTDI) engine ignition requirements, especially at the combustion stability limit, are not well understood and must be better characterized to enable improved designs. A multi-discharge, electronically control, inductive ignition system was employed on a multi-cylinder GTDI engine to control and quantify the ignition and combustion processes under high dilution operation. The ignition system, developed by Ford Motor Company and integrated on the engine, is a Variable Output Ignition System (VOIS) enabling flexible spark discharge patterns with control of ignition energy, duration, and phasing. In this study two coil discharges were examined under both continuous and discontinuous settings by delaying the second discharge with respect to the first. Ignition secondary voltage and current were measured with a high speed analog to digital recorder to measure and quantify the breakdown voltages, ignition duration, ignition energy, and other ignition metrics over 300 cycles of continuous engine operation. Simultaneously 300 cycles of crank-angle resolved cylinder pressure and other data was recorded to analyze cycle based work and combustion rates and to correlate ignition with combustion. With respect to ignition analysis and correlations, the results show that the breakdown voltage of the second discharge was correlated to the second coil discharge timing. The second breakdown voltage increased when the second coil discharge timing was retarded. It was also found that the glow energy was highly correlated to the ignition duration on a cycle-by-cycle basis. The ignition duration decreases with higher ignition voltages during glow discharge giving higher total glow energy as a result of hypothesized arc stretching. With respect to ignition and combustion correlations, the flame kernel development period was found to be weakly correlated to both the ignition duration and energy. The breakdown and arc energy of the second discharge had higher correlation to the combustion phasing than the glow energy of the second discharge. With respect to engine performance in a dual-coil multi-discharge ignition, an optimal ignition energy phase delay time exists with a fixed total ignition energy. A delayed second coil discharge at the dilution limit can convert abnormal combustion cycles to normal burn cycles. The combustion phasing of these converted cycles is dependent on the phasing of the second discharge. The results also showed that combustion phasing was strongly correlated to the flame kernel development period. The gross indicated mean effective pressure (IMEP) was negatively correlated to combustion duration. Longer flame kernel development period and bulk burn duration resulted in delayed combustion and reduced IMEP