Yan, B., Yao, M., Mao, B., Li, Y. et al., "A Comparative Study on the Fuel Economy Improvement of a Natural Gas SI Engine at the Lean Burn and the Stoichiometric Operation both with EGR under the Premise of Meeting EU6 Emission Legislation," SAE Technical Paper 2015-01-1958, 2015, doi:10.4271/2015-01-1958.
In order to further study the effects of air and EGR dilution on the fuel economy improvement of natural gas engines under the premise of meeting EU6 legislation, a comparison between stoichiometric operation with EGR and lean burn operation with and without EGR has been conducted at 1600rpm 50% and 75% load. The conversion efficiencies of the catalysts for both NOx and CH4 emissions are assumed at 90% for lean burn operation. Experiment results indicate that under the condition of meeting both NOx and CH4 predetermined engine-out emissions limits for EU6 legislation, lean operation with a small fraction of EGR dilution enables more advanced combustion phasing compared to pure lean operation, which results in much better fuel economy, thus further improvement compared to stoichiometric operation is achieved. Additionally, this advantage is enhanced with increased load, as the higher combustion and exhaust temperatures lead to lower CH4 emissions, therefore sufficient EGR dilution is enabled for further reducing NOx emissions, which results in more favorable combustion phasing.