Investigating the relationship between heating value and combustion/emissions and residual gas in a small spark-ignition engine powered by pure propane

The influence of the heating value (HV) on combustion characteristics in an experimental engine setup was examined in this study. A comprehensive analysis of engine performance parameters, including residual gas concentrations, effective release energy, brake mean effective pressure (BMEP), brake-specific fuel consumption (BSFC), engine brake torque, and engine power, was conducted under varying HVs. An experimental system was installed with a dynamo testing system, and a simulation model was established using AVL Boost software. In this research, the ignition timing was increased from a 10-degree to 40-degree crank angle before top dead center. The engine testing speed band is from 3000–10000 rpm. The findings revealed an interplay between HV and combustion completeness, with a critical threshold at an ignition timing of 35 degrees. Notably, the highest residual gas concentrations were consistently observed at an ignition timing of 35 degrees, indicating incomplete combustion. Combustion completeness improved at 20 degrees, accentuated by the pivotal role of the HV. The effective release energy exhibited a consistent increase up to an ignition timing of 30 degrees, underscoring the intricate relationship between ignition timing and energy release. The BMEP peaked at 30 degrees across all HVs, with HV significantly increasing the combustion efficiency. Conversely, the BMEP reached its nadir at 35 degrees. The BSFC displayed optimal combustion efficiency at 25 degrees. The engine demonstrated maximum brake torque and power at specific HVs and ignition timings. A consistent decrease in NOx emissions with advancing ignition timing, reaching a minimum at 40 degrees, was particularly pronounced with a HV of 11200 kcal/kg. Simultaneously, CO emissions exhibit a fluctuating pattern, with the highest peak at 35 degrees, emphasizing the influence of both ignition timing and HV. In the case of HC emissions, an initial rise was observed from 10 to 20 degrees, followed by a plateau or dip at higher timings, with the peak consistently at 40 degrees, notably accentuated by the 10800 kcal/kg HV.