Experimental Study of the Polytropic Coefficient for an Air-Cooled, High-Compression-Ratio, Spark-Ignition Engine Fueled with Natural Gas, Biogas, and a Propane–Syngas Blend

The polytropic coefficient is an important variable for determining errors in pressure and volume measurements and for apparent heat release calculation in engine combustion analysis. For commercial gasoline-fueled spark-ignition engines and diesel-fueled compression-ignition engines, a wide understanding about the thermodynamic models and values of the polytropic coefficient exists; however, in other technologies, in which gaseous fuels are used, the pressure treatment strategies and heat transfer models should be adjusted to allow for a better calculation of the polytropic coefficient. This paper presents research on the effects of fuel composition, spark timing, and engine load on the polytropic coefficient in an air-cooled, spark-ignition engine with a high compression ratio (15.5:1). The fuels tested were natural gas, biogas, and a propane–syngas blend. The experimental results suggest that, during compression, the appropriate crank angle interval for polytropic coefficient estimation is between 50 and 30 crank angle degree (CAD) before top dead center (BTDC), and during expansion, the appropriate crank angle interval is between 40 and 60 CAD after top dead center (ATDC). It was found that the polytropic coefficient is lowered during compression and increased during expansion with advanced spark timings. Cycle–cycle variations tend to increase the polytropic coefficient during compression and reduce it during expansion.