The introduction of CO₂-reduction technologies like Start-Stop or the Hybrid-Powertrain and the future emission legislation 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. Therefore, adaptations to the start-up conditions of the known models by Woschni, Hohenberg and Bargende were introduced for calculation of the wall heat transfer coefficient in SI engines with gasoline direct injection.This paper shows how the indicated values can be measured during the engine start-up. Furthermore, the methods of deriving the piston positions and the engine speed from the time-based data are described. With these, the transient burn rate analysis is performed using the introduced approaches for the wall heat transfer calculation.For the validation of the thermodynamic analysis start-up behavior of 3-, 4-, and 6-cylinder SI engines is investigated and the results are discussed. In addition to this, a 3-cylinder CI engine start-up is analyzed to verify the novel wall heat transfer approach for diesel engines.Optimization of a 6-cylinder engine start-up is shown using the transient burn rate analysis results which demonstrate the efficiency potential for the application process.