Experimental Study on the Combustion and Emission Characteristics of Methanol/Gasoline Fuels in Direct Injection Miller Cycle Gasoline Engines

This study explores the thermal efficiency of high compression ratio Miller cycle engines and the impact of methanol and methanol/gasoline blends on combustion and emissions. Comparative experiments were conducted to investigate the thermal efficiencies of the Miller cycle compared to the conventional Otto cycle at different compression ratios and how methanol affects combustion and emissions. The results show that under high-speed and high-load conditions, the Miller cycle offers higher thermal efficiency and better tolerance to high compression ratios than the Otto cycle. In experiments conducted at 2000 rpm and 0.66 MPa GIMEP, using the Miller cycle with compression ratios of 11.5 and 14.5 increased thermal efficiency by about 0.6 and 0.8 percentage points compared to the Otto cycle. Using methanol/gasoline blends can advance the combustion phase without changing the load, further improving the engine’s thermal efficiency. Burning pure methanol under heavy load significantly improves combustion; it increases in-cylinder pressure by about 30%, thermal efficiency by 7.2 percentage points, and NOx emissions by 80% compared to gasoline. Furthermore, using methanol fuel significantly increases nucleation mode particles and decreases accumulation mode particles, with peak values shifting to smaller diameters.