Numerical investigation of hydrogen-enriched spark ignition methanol engine

Abstract

The majority of transportation and power generation depends on internal combustion engines, which primarily operate on fossil fuels and significantly contribute to greenhouse gas emissions.These engines release harmful pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC) through their exhaust,posing serious health and environmental risks.To mitigate these effects,the exploration of clean fuels for internal combustion engines is essential.Methanol,with its high knock resistance and latent heat of vaporization and hydrogen,with its high flame speed and superior energy content,present promising alternatives that can enhance efficiency and reduce environmental impact.This study explores detailed investigations using various strategies to reduce exhaust emissions and enhance engine performance by utilizing methanol in a spark-ignition engine. A detailed 1-dimensional and 3-dimensional computational fluid dynamics (CFD) model of a methanol spark-ignition engine was developed, incorporating varying proportions of hydrogen addition.Moreover, the combined effects of CO and hydrogen addition on the performance and emissions of the methanol spark-ignition engine were analyzed. The first part of the study was conducted using a 1-dimensional single-cylinder two-zone model to analyze methanol with varying proportions of hydrogen addition, employing both single and multi-Wiebe functions, validated against experimentally published data. The laminar flame correlation for methanol, developed by Xiaolong Liu et al, was incorporated into the single and multi-Wiebe models to simulate a 50-liter genset engine from the Ricardo WAVE engine database. The effects of boosting under low and high load conditions on a methanol spark-ignition engine with hydrogen enrichment were investigated. The results indicated that higher power operation with hydrogen addition reduced CO emissions and increased NOx emissions, along with higher in-cylinder pressure, heat release rate, and improved indicated thermal efficiency. Furthermore, increased boosting of methanol with hydrogen addition reduced both CO and NOx emissions, while also increasing the indicated thermal efficiency, maximum in-cylinder pressure, and heat release rate. In the second part of the study, a 3-dimensional computational fluid dynamics (CFD) model of a single-cylinder methanol direct injection engine with hydrogen addition was developed using the Converge CFD solver. The SAGE solver, Reynolds-Averaged Navier Stokes (RANS) model with k- turbulence, and the O′Rourke and Amsden heat transfer sub-model were utilized to analyze in-cylinder behavior. The Extended Zeldovich mechanism and Hiroyasu-NSC model were applied to evaluate NOx and soot emissions, respectively. The effect of injection timing during the compression stroke was examined, revealing that retarded injection reduced indicated specific CO and soot emissions but increased NOx emissions. Hydrogen enrichment further enhanced hydroxyl radical concentration, shortened combustion duration, and reduced CO and soot emissions, while increasing NOx emissions. Additionally, advancing methanol injection during the intake stroke with hydrogen addition caused an earlier rise in in-cylinder pressure, improved fuel-air mixing, and enhanced flame propagation, reducing combustion duration and lowering CO and soot emissions, though NOx emissions increased. Hydrogen enrichment also extended the lean burn limit, improved IMEP, and decreased specific CO, soot, and HC emissions compared to neat methanol, with shorter combustion duration and better fuel-air mixing near the spark plug. However, increasing hydrogen beyond 3% had little impact on combustion efficiency, though lean burn operation led to increased CO, HC, and soot emissions with reduced NOx emissions. Lastly, the combined effect of a small proportion of CO and hydrogen addition under fixed load conditions was investigated. Results showed reductions in CO and NOx emissions and improved thermal efficiency compared to neat methanol. While hydrogen addition alone resulted in a greater reduction in CO emissions and higher thermal efficiency, it also led to increased NOx emissions compared to the combined CO and hydrogen addition.