Comparative study on EGR and lean burn strategies employed in an SI engine fueled by low calorific gas

• EGR has a narrower window of dilution than lean burn in low-Btu gas engines.
• Lean burn shows more intensive heat release and higher peak than EGR.
• EGR affects the O2 fraction rather than specific heat in a low-Btu gas engine.
• EGR is effective in NOx reduction, but it also increases THC faster.
• Lean burn shows higher efficiency than EGR while meeting the legal standard.

In this study, a naturally aspirated spark ignition engine fueled by low calorific gas (LCG) was tested in both exhaust gas recirculation (EGR) and lean burn modes, and the effects of these modes on engine performance and combustion and emission characteristics were evaluated and compared. The LCG, composed of 40% natural gas (NG) and 60% nitrogen (N2), was used as the main fuel, and a dilution rate was employed to carry out a comparison between the two modes under identical levels of dilution. The engine test results demonstrate that the dilution range was narrower when running with EGR at stoichiometry than when running with lean burn, while more intensive heat release and higher peaks were obtained in lean burn than in EGR under similar operating conditions. Analysis of the in-cylinder mixture condition shows that introducing EGR to an LCG engine mainly affected the O2 fraction rather than the specific heat, owing to the presence of a large amount of inert gas. This is one of the major reasons for the difference in combustion characteristics between EGR and excess air operations. The engine test results also indicate that an improvement in thermal efficiency was possible in the lean burn mode when using LCG fuel, whereas the use of EGR barely improved inferior fuel economy. EGR was more effective in reducing NOx emissions, but it also increased total hydrocarbon emissions faster. Lean burn as well as EGR successfully satisfied the legal emission regulation when the level of dilution was increased to the dilution limit, although there was a slight reduction in efficiency.