Investigation on cold start issues of methanol engines and its improvement from the perspective of droplet evaporation

[Display omitted] •Methanol droplets could not completely evaporate at temperatures below 263 K due to its hygroscopicity.•Raising ambient temperature from 243-283 K to 293 K was optimal for promoting evaporation.•The evaporation of binary mixtures could be regarded as the evaporation of a pseudo single component.•The water absorption of methanol droplets was inhibited by reducing droplet diameter.•Increasing fuel temperature could not promote droplet evaporation. The cold start issue of methanol engines limits their wide application in cold seasons and regions. To explore the underlying mechanisms and propose effective improvement measures, the evaporation characteristics of methanol at 243–303 K were first investigated by the single droplet method in this study. The effects of ambient temperature, initial diameter, fuel temperature and intake air flow velocity on evaporation of methanol droplets were quantitatively analyzed and their guidance for improving the cold start performance of methanol engines were discussed. The results revealed that the evaporation process of methanol droplets in low temperature and humid environment showed two-stage feature due to its hygroscopicity, including pure methanol evaporation and water-dominated evaporation of methanol–water mixture, but it could be regarded as the evaporation of a pseudo single component and the same was true for other binary mixtures. The existence of water in methanol droplets not only led to their incomplete evaporation at temperatures below 263 K, but also caused the linear change of their evaporation rates with temperature, which was different from the exponential change of pure methanol evaporation rate with temperature. Increasing the ambient temperature from 243-283 K to 293 K was optimal for promoting methanol evaporation. Reducing droplet diameter inhibited the water absorption of methanol droplets, thus enhancing their evaporation rates. Increasing fuel temperature could not promote droplet evaporation, but mainly influenced water absorption of methanol droplets. The promotion effect of air flow on methanol evaporation became weaker with the increase of intake air flow velocity, especially when it exceeded 3 m/s. The findings of this study suggest that multiple methods should be combined to improve the cold start performance of methanol engines.