For real working-process simulations it is essential to know the caloric properties of the working fluid, such as the specific enthalpy and the real gas constant. When using standard-fuels there are established models which describe the caloric variables as functions of temperature, air/fuel-ratio and pressure. In each case, these models were developed for a certain fuel composition and their application to alternative fuels is limited or not valid at all. Thus, an approach is discussed, which is valid for any user-defined fuel.
This work presents the formulations for the caloric modelling of burnt gas and fuel vapor, respectively. An algorithm is introduced that provides a very fast calculation of the chemical equilibrium condition of burnt gas. The influence of the equilibrium coefficients and the fuel composition on the chemical equilibrium composition is discussed. This is the basis for a component-based method to calculate both specific enthalpy and individual real gas constant of burnt gases. Comparing the results with established approaches, there is an exact match for standard fuels. The sensitivity of this caloric approach on the chemical equilibrium coefficients, the fuel composition and the “freeze-temperature” is investigated.
Also a new detailed component approach for the internal energy of fuel vapour is given and discussed. In many cases there are no detailed informations about the composition of fuel vapour available. Therefore another approach is discuessed which requires less inputs about fuel composition.
Last but not least an equation for the conversion efficiency of the combustion (fuel heat release efficiency) is proposed as a function of temperature, pressure and air/fuel ratio and is compared with the well-known approximation by Vogt. An improved approximation equation is given, which is meant to be used below 1.800 K for any user-defined fuel.