A Temperature-dependent Model of Ratio of Specific Heats Applying in Diesel Engine

Abstract

Rate of heat release is a standard tool when engineers tune and develop new engines. The ratio of specific heats $\gamma$ is considered an essential parameter for achieving accurate rate of heat release calculation as it couples the engine system energy and other thermodynamic properties.The $\gamma$ model is a function of various factors, such as temperature, air-fuel-ratio, pressure, etc. To improve the accuracy of ROHR calculation in the Scania diesel engine in NTNU machinery laboratory since constant-value $\gamma$ model is applied there, three existing temperature-dependent $\gamma$ models are investigated in this work. Some other more complex models considering other factors are not discussed here as real-time computation is required. An engine experiment comprising of constant speed and propeller curve test cycle is carried out for supplying the raw data.\\

A reference model based on the theory of Zacharias's formulas is established to evaluate the accuracy of three candidate models. The models are evaluated in both $\gamma$ domain and ROHR domain. Three evaluating criteria including MRE, RMSE and NRMSE are carried out to evaluate the error of models. A retrofit $\gamma$ model is proposed by applying $\gamma$ offset value to further improve accuracy in the end.