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.

中文翻译：

---

天然气，沼气和丙烷-合成气混合燃料的空冷，高压缩比，火花点火式发动机的多方系数实验研究

多变量系数是一个重要变量，用于确定压力和体积测量中的误差以及发动机燃烧分析中的表观放热计算。对于商用汽油燃料火花点火发动机和柴油燃料压缩点火发动机，人们对热力学模型和多变系数值有广泛的了解。但是，在使用气体燃料的其他技术中，应调整压力处理策略和传热模型，以便更好地计算多方系数。本文介绍了在高压缩比（15.5：1）的空冷式火花点火发动机中，燃料成分，火花正时和发动机负载对多方系数的影响的研究。测试的燃料是天然气，沼气，以及丙烷与合成气的混合物。实验结果表明，在压缩过程中，用于多方系数估计的适当曲柄角间隔在上死点（BTDC）之前为50至30之间的曲柄角度（CAD），而在膨胀过程中，适当的曲柄角间隔在40至50之间的曲柄角度之间。上止点（ATDC）之后60 CAD。已经发现，随着先进的火花正时，多变系数在压缩期间降低，而在膨胀期间增加。循环间的变化往往会在压缩过程中增加多变系数，而在膨胀过程中会降低多变系数。上止点（ATDC）之后，适当的曲柄角间隔应在40到60 CAD之间。已经发现，随着先进的火花正时，多变系数在压缩期间降低，而在膨胀期间增加。循环间的变化往往会在压缩过程中增加多变系数，而在膨胀过程中会降低多变系数。上止点（ATDC）之后，适当的曲柄角间隔应在40到60 CAD之间。已经发现，随着先进的火花正时，多变系数在压缩期间降低，而在膨胀期间增加。循环间的变化往往会在压缩过程中增加多变系数，而在膨胀过程中会降低多变系数。