Experimental study of the polytropic index of the compression stroke for a direct injection hydrogen engine

Hydrogen is regarded as a promising alternative fuel in the future. The direct-injection (DI) hydrogen engine has been demonstrated to offer large power without the risk of abnormal combustion. For the engine test of combustion analysis, the piezoelectric transducer measures the dynamic cylinder pressure data rather than the absolute one. The traditional method of absolute cylinder pressure correction is normally based on the polytropic index. This paper investigated the polytropic index effects by injection duration, start timing of injection and speed based on a 2.0 L direct-injection hydrogen engine. The experiments found that, since the injection of large volume hydrogen leads to approximately 0.8 bar increase of cylinder pressure in the compression stroke, the traditional correct method is not suitable for the DI hydrogen engine. What's more, the polytropic index drops from 2.2 to 1.22 with the changes of the crank angle and is sensitive to the concentration of the hydrogen. The study of the instantaneous polytropic index can not only be used to calculate the accurate heat release rate but also guide the research of the heat transfer of the DI hydrogen engine. The average polytropic index of 1.32 provides a new reference for the absolute cylinder pressure correction of the DI hydrogen engine.