Evaluating the influence of the compression ratio on the combustion process using methanol, gasoline, and methanol-gasoline blends

Abstract

In response to global energy and environmental challenges, this study systematically evaluates performance of methanol-gasoline blends (M100, M85) in comparison to pure gasoline in a spark-ignition (SI) engine. Through experimental data and numerical simulations, the combustion characteristics, energy flow distribution, and thermomechanical conversion processes of the three fuels were analyzed across compression ratios (CRs) ranging from 11 to 15. Under full-load conditions at 3500 r/min, gasoline showed 7.58 % and 8.69 % higher power outputs than M100 and M85, respectively. Additionally, when operating at the same speed under high loads (exceeding 125 Nm), methanol demonstrated better fuel economy than gasoline. This improvement stems from increased high-pressure cycle work, reduced heat transfer, and incomplete combustion losses. Both the effective expansion ratio (EER) and energy economy efficiency (EEE) peaked at a CR of approximately 13, beyond which efficiency gains plateaued while knock propensity intensified. Between 2500 and 3500 r/min with CR exceeding 13, methanol blends exhibited 18–23 % higher efficiency than gasoline, owing to their superior knock resistance resulting from greater octane sensitivity. The combustion characteristics of M85 fell between those of gasoline and M100, highlighting its potential as promising alternative fuel. These findings provide critical insights into enhancing methanol-gasoline engines through CR optimization, combustion phasing (CA50) calibration, and implementation of an effective thermal conversion process. The study systematically analyzes the combustion characteristics and energy flow synergy of high-proportion methanol-gasoline blends under high CR, along with the interactive effects of CR, load, and combustion timing.