This study investigated how the amount of dilution applied can be extended while maintaining normal engine operation in a GDI engine. Adding exhaust gases or air to a stoichiometric air/fuel mixture yields several advantages regarding fuel consumption and engine out emissions. The aim of this paper is to reduce fuel consumption by means of diluted combustion, an advanced ignition system and adjusted valve timing.
Tests were performed on a Volvo four-cylinder engine equipped with a dual coil ignition system. This system made it possible to extend the ignition duration and current. Furthermore, a sweep was performed in valve timing and type of dilution, i.e., air or exhaust gases.
While maintaining a CoV in IMEP < 5%, the DCI system was able to extend the maximum lambda value by 0.1 - 0.15. Minimizing valve overlap increased lambda by an additional 0.1.
For dilution by exhaust gases, an increase of 9% was noted for the ignition system and a further increase of 9% was obtained by minimizing the valve overlap at 1500 rpm/5.00 bar BMEP. This exchange of internal combustion residuals for external dilution resulted in a further decrease of fuel consumption but increased engine out NOx.
Using air as a diluter resulted in a 6% fuel consumption benefit over exhaust gas dilution, mainly due to the enhanced combustion efficiency arising from higher oxygen concentrations. However, the lower oxygen concentration when using exhaust gases as a diluter led to 50% lower engine out NOx levels at the dilution limit.