Novel Transient Wall Heat Transfer Approach for the Start-up of SI Engines with Gasoline Direct Injection

The introduction of CO₂-reduction technologies like Start-Stop or the Hybrid-Powertrain and the future emissions limits require a detailed optimization of the engine start-up. The combustion concept development as well as the calibration of the ECU makes an explicit thermodynamic analysis of the combustion process during the start-up necessary. Initially, the well-known thermodynamic analysis of in-cylinder pressure at stationary condition was transmitted to the highly non-stationary engine start-up. There, the current models for calculation of the transient wall heat fluxes were found to be misleading. But with a fraction of nearly 45% of the burned fuel energy, the wall heat is very important for the calculation of energy balance and for the combustion process analysis. Based on the measurements of transient wall heat transfer densities during the start-up presented in a former work, the paper describes the development of adaptations to the known correlations by Woschni, Hohenberg and Bargende for the application during engine start-up. To demonstrate the high accuracy of the introduced equations, the results of the cyclic-resolved thermodynamic analysis using the presented novel approaches are compared with the results of the measurements. It is shown, that the usage of the novel heat flux models for the engine start-up process make the cyclic-resolved thermodynamic analysis good enough to allow an efficient optimization of the engine start-up.