Numerical investigation of intake oxygen enrichment effects on radicals, combustion and unregulated emissions during cold start in a DISI methanol engine

Oxygen enrichment combustion is one of the attractive methods to improve combustion and reduce emissions in spark ignition engines. The effects of intake oxygen enrichment on intermediate radicals, combustion, and unregulated emissions of formaldehyde and unburned methanol during cold start were numerically calculated by using CFD simulation coupled with a detailed methanol chemical kinetic model for a DISI methanol engine. Results show that the concentration of OH, O, and H radicals during cold start is obviously increased and CA50 is slightly advanced with increase of intake oxygen concentration (C-O2), resulting in accelerated chain reaction in methanol combustion, and further mitigating unburned methanol emissions. When Coe varies from 21% to 36%, cylinder temperature increases due to the post-combustion. Cylinder temperature reaches the maximum value at 36% C-O2. After further increasing C-O2 to 41%, the in-cylinder temperature start to decrease as a result of the decreased OH concentration and inhibited post-combustion. In general, C-O2 is the accelerant in formaldehyde formation at initial stage and in formaldehyde oxidization during post-combustion. By increasing C-O2, formaldehyde emission rises and unburned methanol emission diminishes during cold start.