Lean Air-Fuel Ignition System Comparison in a Fast-Burn Engine 850076
Ignition experiments were performed with a fast burn, single cylinder engine with the objective of comparing lean air-fuel ignitability characteristics. Ignition system variables investigated were: plug type, ignition energy, ignition power input, firing duration, and a plasma jet ignitor.
Lean air-fuel Ignition limits were determined for four different ignition system combinations. The ignition limit at a given A/F condition was determined by first setting the engine at its subjective MBT condition at a constant speed, load, and A/F ratio and then advancing the spark angle until misfire occurred. The misfire limit was analyzed via the location of the angle of peak cylinder pressure over 1800 engine cycles. Misfires were assumed to occur for all peak cylinder pressure angles less than or equal to top dead center of compression.
The spark angle sensitivities of the various ignition systems were further compared at a constant A/F ratio of 24:1 at constant flow rates for MBT operation at 1500 RPM, 58 pai IMEP. Variables investigated were specific fuel consumption and emissions, 0-10 % and 10-90% burn times, and coefficient of variation of the peak cylinder pressure.
The ignition limit of the long duration ignition system (40-60 crankangle degrees) used with a surface-air gap plug was found to have the greatest spark angle tolerance. The plasma jet ignition system had the leanest projected lean misfire limit. Adding capacitance to a conventional plug to enhance the initial discharge was found to increase the initial rate of burning, but had little effect on the ignition limit.
The rapid energy discharge system had the fastest 0-10 % burn time with the most retarded MBT+1 % ISFC spark advance angle. It also had the best combustion stability and lowest ISNO at its MBT+1 % condition. All the high energy ignition systems had comparable ISFC values. The air-fuel ratio at which the minimum IFSC occurred, however, was observed to shift to a leaner operating condition as the burn rate of the mixture was enhanced by the ignition system.