Investigations on the Transient Wall Heat Transfer at Start-Up for SI Engines with Gasoline Direct Injection

The introduction of CO₂-reduction technologies like Start-Stop or the Hybrid-Powertrain and the future emissions regulations require a detailed optimization of the engine start-up. The combustion concept development as well as the calibration of the ECU makes it necessary to carry out an explicit thermodynamic analysis of the combustion process during the start-up. As of today, the well-known thermodynamic analysis using in-cylinder pressure traces at stationary condition is transmitted to the highly dynamic engine start-up. Due to this approximation the current models for calculation of the transient wall heat fluxes by Woschni, Hohenberg and Bargende do not lead to desired results. But with a fraction of approximately 40 % of the burnt fuel energy, the wall heat is very important for the calculation of energy balance and for the combustion process analysis during start-up. The paper shows how the transient heat fluxes to the combustion chamber wall can be measured during the engine start-up. Needing an offset correction which can not be carried out properly at engine start-up conditions, the indirect heat flux measuring method can not be used. Due to the high engine speed gradients, a novel heat flux sensor is used for the time-resolved heat flux density measurements. The working principle of this Atomic-Layer-Thermopile sensor is based on a thermoelectric field created by the transverse Seebeck effect. Because of the sensors high frequency response, highly time-resolved local heat flux density measurements up to 1 MHz range can be realized. Results of investigations at homogenous and stratified engine start-up in a DISI engine are presented and the influence of the measured wall heat flux on the cylinder pressure analysis is explained. It is obvious that only a novel heat flux model for the engine start-up process makes the cyclic resolved thermodynamic analysis good enough to allow an efficient optimization of the engine start-up. The shown experiments help understand the physical effects and set an extensive data base for modeling the wall heat transfer during the engine start-up.