Numerical study of formaldehyde and unburned methanol emissions of direct injection spark ignition methanol engine under cold start and steady state operating conditions

Abstract

The effects of overall equivalence ratio, injection timing, and ignition timing on formaldehyde and unburned methanol emissions, cylinder temperature histories, and formaldehyde emission histories of a spark ignition direct injection stratified charge methanol engine during cold start and steady state conditions were simulated using computational fluid dynamics coupling the methanol chemical kinetics reaction mechanism. The model results show that the overall equivalence ratio is less than 0.43, and unburned methanol significantly higher for cold start mode. For steady state mode, when the overall equivalence ratio is less than 0.4, formaldehyde and unburned methanol emissions increase rapidly. When the overall equivalence ratio is larger than 0.4, formaldehyde and unburned methanol emissions are very low. Formaldehyde and unburned methanol emissions for steady state mode are significantly lower than for cold start mode. For steady state mode at engine speed 1600rpm and brake mean effective pressure 0.67MPa formaldehyde and unburned methanol emissions are reduced 90% and 98%, respectively, compared to cold start mode at the same overall equivalence ratio (0.5). Cylinder temperature is the main factor that affects formaldehyde generation and consumption. There is a rapid decrease due to oxidation at the corresponding position of the maximum cylinder temperature, after which formaldehyde is quickly generated for any operating conditions.