Effects of compression ratio on combustion characteristics in a six-stroke spark-ignition engine using water direct injection

Abstract

Despite the rise of electrification, internal combustion engines remain essential due to technological and infrastructure limitations. Improving efficiency and reducing emissions are thus still crucial. Six-stroke engines with water injection have been proposed to enhance thermal efficiency by utilizing water's expansion effect and waste heat recovery. However, prior research often lacked rigorous thermodynamic validation and direct comparison with conventional engines for steam expansion effect. This study proposes a novel six-stroke spark-ignition gasoline engine concept that separates cooling via water injection into additional strokes to suppress knock and enable higher compression ratios, differing from previous concepts focused on steam expansion. A six-stroke single-cylinder spark-ignition engine model was developed using 1D simulation and validated using experimental data. Valve timings were optimized to minimize pumping losses, and water injection timing and mass were varied to evaluate temperature reduction, knock mitigation, and indicated mean effective pressure (IMEP) improvements, with increased compression ratio. Results showed that optimized valve timing improved gross IMEP by 1 % and reduced pumping losses by 8 %. Water injection significantly lowered in-cylinder and wall temperatures, reducing knock probability, as indicated by the normalized induction time integral, by up to 48 % compared to a four-stroke engine. Increasing water injection mass allowed raising compression ratio to 21.4, but also increased pumping losses, limiting net IMEP gains. The best performance was achieved with 10 mg water injection at a compression ratio of 18.2 and injection timing advanced to 290° after top dead center, resulting in a 6.62 % increase in net IMEP over the four-stroke engine.