Experimental Investigation of a Methanol–Diesel RCCI Engine: Combustion and Emissions Characteristics under Various Methanol Substitution and EGR Rates at Different Altitudes

Given the global efforts to reduce pollutant emissions and achieve carbon neutrality, methanol–diesel dual-fuel reactivity-controlled compression ignition (RCCI) combustion has become an essential approach for the technological innovation and sustainable development of diesel engines, owing to its capabilities of reducing emissions and maintaining higher thermal efficiency. In addition, the lower atmospheric pressure at high altitudes adversely affects a diesel engine’s in-cylinder combustion process, leading to significant degradation in overall performance and increased emissions. To compensate for the limited number of studies with regard to the performances of the methanol–diesel dual-fuel engines at high altitudes as well as further extending the high-load operation range, the effects of methanol substitution rate (MSR) and exhaust gas recirculation (EGR) on a methanol–diesel RCCI engine in terms of the combustion performances and the emissions were investigated at different altitudes. According to the results, the methanol–diesel dual-fuel mode exhibits relatively higher brake thermal efficiency (BTE) and lower NOx emissions. With the MSR rising from 0% to 25% without EGR, the BTE rose by 0.9%, 1.1%, and 1.0% at 0, 1,000, and 2,000 m altitude, respectively. The increase in MSR was accompanied by higher peak cylinder pressure (PCP) and heat release rate (HRR), lower exhaust gas temperature, slowly increased CO, THC, MeOH, and HCHO emissions, and obviously decreased NOx emissions at different altitudes. With the EGR rate rising from 0% to 25% under dual-fuel mode with 20% MSR, the THC emissions decreased by 13.4%, 27.2%, and 35.7%, respectively, and the NOx emissions reduced by 74.1%, 74.7%, and 72.7%, respectively, the MeOH emissions reduced by 21.5%, 32.6%, and 38.0% respectively, the HCHO emissions dropped by 20.1%, 19.2%, and 22.4% respectively, and the BTE decreased by 3.0%, 4.3%, and 6.0% respectively at 0, 1,000, and 2,000 m altitude. Using EGR in RCCI combustion could reduce THC, NOx, unburned MeOH, and HCHO emissions, promote the exhaust gas temperature (EGT), and decrease the PCP and HRR at high loads. In the case of the MSR and EGR rates remaining the same, as the altitude level rises from 0 to 2,000 m, the PCP and HRR decrease, the exhaust temperature increases, and the BTE reduces. The CO, THC, NOx, MeOH, and HCHO emissions all present a decrease with an increase in altitude. Methanol–diesel dual-fuel RCCI engines supported by EGR or operating at high altitudes have the potential to reduce unburned MeOH and HCHO emissions owing to the delayed combustion phasing and higher exhaust temperatures.