Recent research has shown that increased levels of EGR in spark ignition engines can enable operation at higher compression ratios and loads than were previously possible, due primarily to a reduction in knock tendency. Increasing the amount of dilution by increasing the air/fuel ratio has also been shown to have similar effects.
Implementation of these features gives rise to the problem of ignition and flame propagation at these increased dilution levels. Several companies now sell igniters designed to improve ignitability and promote faster burn rates in the engine. One specific example of these igniters is the torch-style plug (also referred to as a “pre-chambered” plug), sold by several manufacturers. This plug works by having a pre-chamber in the plug where the mixture is ignited. The burning gasses expand, pushing first cold and then hot burned gases from the pre-chamber into the cylinder. The jets that issue from the pre-chamber mix with the un-burned gases and promote more rapid burn rates than stock igniters.
Today's torch-style plugs have two drawbacks that prevent them from being useful in high EGR applications. The first is that the expanding gasses in the chamber push cold, unburnt gases out first, resulting in an ignition delay that must be factored into the engine calibration. The second is that they do not work at very high dilution rates, probably due to heat losses.