Comprehensive study on unregulated emissions of heavy-duty SI pure methanol engine with EGR

Abstract

Methanol is a potential alternative fuel for heavy-duty engines to effectively reduce carbon emissions. On the other hand, unregulated emissions from methanol-fueled engines are harmful to the environment. The formaldehyde emission mechanism was investigated in this paper using a premixed laminar combustion experiment and a plug flow reactor (PFR) simulation with a detailed mechanism. Furthermore, the engine test was carried out on a heavy-duty spark ignition (SI) pure methanol engine with EGR to study the effects of the injection strategy, exhaust gas recirculation ratio (EGR), engine load, and speed on unregulated emissions. The high-pressure loop EGR system and port fuel injection system were adopted. Unregulated emissions were measured using a GC-2010 gas chromatography analyzer with a pulsed discharge helium ionization detector (PDHID) and Gs-OxyPLOT. Formaldehyde emission is the post-oxidation production of unburned methanol, according to the experiment and simulation, and it increases first, then decreases as the temperature rises. Unregulated emissions mainly consist of methanol and formaldehyde. The injection timing can be divided into two groups: 120°CA to 300°CA and 360°CA to 780°CA, where 0°CA represents fire TDC. The BTE at αinj of 120°CA to 300°CA is lower than that at other αinj, while methanol and formaldehyde emissions are the opposite. The temperature of unburned methanol post-oxidation is affected by engine speed, load, and EGR ratio, affecting methanol and formaldehyde emissions. The higher exhaust temperature caused by the higher engine speed, load and a lower EGR ratio can reduce methanol emissions. The nonmonotonic trends of formaldehyde emission with the EGR ratio and the different influences of EGR on the formaldehyde emission under different engine loads are the result of the nonmonotonic relationship between formaldehyde emission and temperature. The engine experiment results are consistent with the PFR simulation.