Modeling and Experimental Validation of a Pure Methanol-Fueled Small SI Engine: Impact of the Laminar Flame Speed Correlation

In a context of growing concern for vehicle-related CO₂ and pollutant emissions, non-conventional fuels like methanol (CH₃OH) represent a valid alternative to fossil fuels to decarbonize the transport sector in a reasonable time. This is mainly due to its lower carbon content than conventional gasoline and diesel. Moreover, methanol can be obtained either from biomass or CO₂ capture from the atmosphere, which makes the latter a renewable fuel. Given the possibility of being stored in liquid phase at standard temperature and pressure (STP), methanol is very suitable for Light Duty Vehicles (LDVs), in which the need to contain fuel tank dimensions is relevant. Regarding the deployment of methanol as a fuel, it is not very challenging, as it can be adopted in current production Internal Combustion Engines (ICEs) either in pure form or in blend with other fuels without any significant modifications.

Within this context, the present work aims to assess, in both experimental and simulation environments, the benefits of pure methanol fueling of Spark Ignition (SI) engines. Particularly, a small single-cylinder production SI engine has been tested on the engine test bench at different speeds and air-fuel ratios, at unthrottled conditions. Besides the test campaign, a 1-D model of the engine under study has been developed, with a particular focus on the simulation of methanol combustion. Particularly, different correlations for the laminar flame speed, implemented in the combustion model, have been investigated to evaluate their effect on the prediction of performance, efficiency, and pollutant emissions. Combustion, heat transfer, and pollutant emissions sub-models have been tuned and validated based on the experimental data. Both experimental measurements and simulation results evidence that methanol fueling allows for improving engine efficiency, while reducing regulated pollutant emissions from a current production SI engine, if compared to conventional gasoline, especially under lean-burn operating conditions. This work is useful for future activities, which will concern the optimization of the Ignition Timing (IT), as well as the investigation of the knock tendency of methanol-fueled SI engines.