Experimental Studies Aimed at Lowering the Electrical Energy Requirements of a Plasma Jet Ignition System for M100 Fuelled Engines

This paper describes experimental studies carried out as part of a program to develop a neat methanol (M100) version of a GM 4-cylinder light truck engine. The engine was originally intended for variable fuel applications with fuels containing up to 80% ethanol. To permit M100 operation, a variable energy ignition circuit and special recessed surface gap ignitors have replaced the standard ignition components. This is referred to as a “plasma jet” ignition system, and is employed both to overcome the cold starting difficulties inherent with neat alcohol fuel and to permit less enrichment to be used during start-up in the interest of reduced hydrocarbon emissions.

The plasma jet ignition systems used in previous related studies suffered from excessively high ignition energy requirements which would be detrimental to ignitor durability. Thus efforts were carried out to optimize the circuit and ignitors with the aim of limiting ignition energy to conventional levels during warm running, while permitting intermittent duty operation during cold starting at approximately ten times the warm running energy level. These studies included both bench tests to determine electrical energy transfer to the spark gap through high speed voltage and current measurements, and engine tests examining cyclic variation and heat release characteristics derived from cylinder pressure measurements. As a result of this work, it was shown that improvements in warm idle combustion characteristics could be achieved with the complete plasma jet system at energy levels below that of the standard ignition system. Use of the plasma jet ignition circuit in combination with conventional spark plugs did not result in similar improvements.