Comparison of Excess Air (Lean) vs EGR Diluted Operation in a Pre-Chamber Air/Fuel Scavenged Dual Mode, Turbulent Jet Ignition Engine at High Dilution Rate (~40%)

Charge dilution is widely considered as one of the leading strategies to realize further improvement in thermal efficiency from current generation spark ignition engines. While dilution with excess air (lean burn operation) provides substantial thermal efficiency benefits, drastically diminished NOx conversion efficiency of the widely used three-way-catalyst (TWC) during off-stoichiometric/lean burn operation makes the lean combustion rather impractical, especially for automotive applications. A more viable alternative to lean operation is the dilution with EGR. The problem with EGR dilution has been the substantially lower dilution tolerance limit with EGR and a consequent drop in thermal efficiency compared to excess air/lean operation. This is particularly applicable to the pre-chamber jet ignition technologies with considerably higher lean burn capabilities but much lower EGR tolerance due to the presence of a high fraction of residuals inside the pre-chamber. Dual Mode, Turbulent Jet Ignition (DM-TJI) technology with its unique ability to work with high external EGR dilution (up to 40% wet/mass basis) due to its additional air delivery to the prechamber offers a viable alternative to the lean burn strategy. DM-TJI could be the technology pathway to realize high EGR diluted combustion with comparable dilution limits to those of the lean burn strategy while still enabling effective use of TWC technology. Present study compares the excess air versus EGR dilution strategy under identical level of dilution (up to 40 %) in a DM-TJI equipped single cylinder engine operating on a high (13.3: 1) compression ratio. The results show that compared to the lean burn operation, EGR dilution provides marginally lower but still comparable thermal efficiency benefits with a marked improvement in NOx reduction, especially in a high compression, knock limited situation. This study showcases that high EGR dilution rates comparable to lean burn operation can be maintained with the DM-TJI system to achieve high thermal efficiency while still operating at stoichiometric air-fuel ratio.