Ignition Systems for Highly Diluted Mixtures in SI-Engines

Various advanced ignition systems have been investigated in order to evaluate their efficiency to initiate combustion of highly diluted mixtures in SI-Engines (lean burn and EGR concepts). Experiments have been performed on a single-cylinder engine on basis of a modern 4 valve passenger-car engine. Several levels of tumble flow were provided by means of different intake port configurations. The flame initiation mechanisms of the ignition systems were analyzed with cylinder pressure indication, mass fraction burned calculation and optical investigation of the flow field near the spark plug and the flame kernel.

The study shows that transistorized coil ignition systems lead to better flame initiation of lean mixtures than capacitive-discharge ignition systems. Among a variety of standard spark plugs only a plug with thin electrodes and extended gap improves lean operation in comparison to the production J-plug. Surface-gap spark plugs lead to a reduced lean limit. The efficiency of a low energy plasma-jet ignition system depends on the level of charge motion. At low engine speed and load the lean limit is enhanced considerably. Swirl chamber spark plugs lead to fast energy conversion but do not improve flame initiation of diluted mixtures at part load. A swirl chamber spark plug that is scavenged with methane during charge exchange shows excellent lean operation qualities. At 2000 rpm and 280 kPa IMEP the lean limit is extended up to a relative air/fuel ratio of λ ≈ 1,96.