Research on the various lean burn strategy for high CR methanol engines and multi-objective optimization control based on RSM

Methanol is a promising alternative fuel for new energy vehicles, but the problem of coordinated control of emissions and economy urgently needs to be solved. This research investigated the influence of various lean burn strategies on performance of a high compression ration spark ignition engine fueled with methanol, and applied response surface methodology to conduct multi-objective optimization of key parameters. Five lean burn strategies were designed and experimentally compared, and the coupling lean burn were further studied by simulation. The findings reveal that internal exhaust gas recirculation(EGR) via early exhaust valve closing provides the most significant enhancement in IMEP and fuel economy due to reduced pumping losses and retained high-temperature active species. In contrast, cooled external EGR demonstrates superior NOx suppression owing to its strong thermal dilution effect. Furthermore, by coupling lean burn by exhaust gas and air, IMEP can be improved by about 10% compared to the initial point with excess air ratio in 1.2–1.4. Using a cubic polynomial response surface model under custom experimental design, multi-objective optimization was performed, yielding tailored combinations of ignition timing, λ, exhaust valve timing, and cooled EGR rate. The optimized strategies achieved remarkable simultaneous improvements for brake specific fuel consumption was reduced by up to 24.82%, while NOx, THC, and CO emissions were decreased by approximately 90%.