Insights into the potential of EGR for reducing unregulated emissions and enhancing the performance of methanol-diesel RCCI engines at different altitudes

The unburned methanol and formaldehyde emission levels at low loads partly restrain methanol fuel application in diesel engines. Therefore, the potential of exhaust gas recirculation (EGR) for reducing unregulated emissions and enhancing the plateau adaptability of a methanol-diesel reactivity controlled compression ignition (RCCI) engine was investigated by changing the EGR ratio and altitude at different engine speed and load conditions. The results revealed that the altitude environment and EGR ratio exert notable impacts on the combustion behaviors and unregulated emissions of methanol-diesel RCCI engines. At A25 condition (25% engine load of 1400 rpm), the increase in EGR ratio and altitude reduces the excess air coefficient, improving the overly lean mixture quality in the cylinder and enhances combustion efficiency. At altitudes of 1000 and 2000 m, a concentrated heat release phenomenon is observed, with the peak heat release rate (HRR) increases as the EGR ratio increases, leading to improved thermal efficiency and fuel economy. Additionally, the combustion phase delay effect caused by changes in specific heat capacity due to increased EGR ratio is partially compensated by the combustion-promoting effect of the diesel-methanol mixture, resulting in little changes in the combustion phase with increasing EGR ratio under A25 condition. At B50 condition (50% engine load of 1800 rpm), the increase in EGR ratio continues to reduce the excess air coefficient. The cooling effect and increased specific heat capacity due to higher EGR ratio dominate, slowing down the combustion rate. A more noticeable combustion phase lag is observed under B50 condition. Regardless of the operating conditions, increases in EGR ratio and altitude effectively reduce unburned methanol emissions. Formaldehyde emissions are primarily related to exhaust temperature. At A25 condition, the lower exhaust temperature leads to a slight increase in formaldehyde emissions with increasing EGR ratio, while at B50 condition, formaldehyde emissions decrease with increasing EGR ratio. The rise in exhaust temperature due to increased altitude enhances the oxidation reaction of formaldehyde in the exhaust pipe, reducing formaldehyde emissions in both A25 and B50 conditions. Under A25 condition, at 1000 m with 40% EGR ratio, the brake thermal efficiency (BTE) is the highest, and fuel economy is optimal, the BTE is 35.95%, the equivalent specific fuel consumption (ESFC) is 235.65 g/(kW·h), the methanol emissions are 640.0 ppm, and formaldehyde emissions are 100.0 ppm. Under B50 condition, at 2000 m with 20% EGR ratio, the thermal efficiency is the highest, and fuel economy is optimal, the BTE is 38.09%, the ESFC is 222.40 g/(kW·h), the methanol emissions are 654.7 ppm, and formaldehyde emissions are 100.7 ppm.