Optimizing air-fuel ratio for balancing thermal efficiency and emissions in a methanol direct injection engine under diverse operating conditions

This experimental study investigates the influence of air-fuel ratio (λ) on the combustion and emissions characteristics of a methanol direct-injection engine under different load conditions. The results demonstrate that λ significantly affects multiple performance parameters. As λ increases, the cylinder pressure and heat release rate profiles transition from narrow-peak to wide-peak distributions, accompanied by an advancing combustion phasing. The peak cylinder pressure (p_max) and heat release rate (HRR_max) exhibit a biphasic response, reaching their maximum values at λ = 0.9 before decreasing with further increase in λ. The flame development period (FDP) and combustion duration (CD) show a linear increase with λ, particularly during flame kernel formation and early flame propagation stages. For example, at an IMEP of 0.70 MPa, FDP increase from 18.4 °CA to 27.6 °CA, and CD extends from 23.5 °CA to 33.4 °CA as λ rises from 0.8 to 1.4. Combustion cycle variability, quantified by the coefficient of variation of p_max and IMEP, increases with λ, with smaller loads exhibiting more pronounced sensitivity to λ changes. Pump mean effective pressure decreases linearly with increasing λ, whereas combustion efficiency and indicated thermal efficiency (ITE) initially improve and then stabilize. At IMEP = 0.70 MPa, ITE reaches 35.91 % at λ = 1.4, representing a 16.8 % increase compare to λ = 0.8. NO_x emissions show a non-monotonic trend, peaking at λ = 1.0. To mitigate NO_x emissions, rich mixtures are more suitable for high-load conditions, while lean mixtures are preferable for medium-load scenarios. HC and CO emissions decrease monotonically with increasing λ, with CO reductions exceeding 98 % across the λ range of 0.8–1.4 due to enhanced combustion completeness. These findings offer critical insights for optimizing fuel-air mixture strategies in methanol direct-injection engines, enabling a balanced approach to thermal efficiency enhancement and emissions control across diverse operating conditions.