Hydrogen helps promote combustion, its ignited jets can hence serve as pilot flames, to enhance spark ignition and combustion characteristics of low-grade gases in engines. The time difference between hydrogen injection and engine spark ignition can alter the shape of the hydrogen jet flame, thereby influencing its spark ignition efficiency. This study uses numerical simulation to investigate the effect of variations in the hydrogen injection time on engine combustion parameters and flame shape at a fixed engine spark ignition time. The results indicate that with hydrogen injection and ignition intervals is less than approximately −2.5 crank angle degrees (°CA), the ignition kernel first ignites the coalbed methane and hydrogen mixture, which then forms a quasi-spherical flame distribution. As the interval extends beyond −2.5° CA and approaches 5.5°CA, the ignition flame exhibits a distinct jet source flame. Both spark ignition flames can significantly enhance the engine combustion characteristics. However, the quasi-spherical jet flame is less effective at igniting coalbed methane compared with the jet source flame. Reasonable control of the hydrogen injection-ignition timing can increase the indicated thermal efficiency of the engine by about 4% compared with the original engine, and shorten the ignition delay period and combustion duration by 63% and 66%, respectively. This study provides valuable guidance for using hydrogen jet ignition in large-bore coalbed methane engines.
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