In Fig. 2 it is given the comparison between calculated and experimental cylinder pressure traces for 0%, 10% and 20% EGR at 1130 rpm engine speed, 100% load using an injection timing of 91 ATDC. As observed there is a good coincidence between calculated and measured values for all EGR rates examined. The simulation manages to capture the effect of EGR on the cylinder pressure trace, which results to a small reduction of cylinder pressure during compression and a more intense one during combustion and expansion. This results from the increase of charge specific heat capacity due to the presence of exhaust gas, to the reduction of O2 availability that has a negative effect on the combustion rate, and to the dissociation of CO2 and H2O. It should be mentioned that for the cases examined (full load) air-fuel ratio (AFR) values are close to their lowest limit. Thus the presence of recirculated exhaust gas in the engine intake reduces further oxygen availability, which in the present case is a dominating factor in the upcoming combustion event. Due to this, peak cylinder pressure values are reduced, as the percentage of EGR inside the engine cylinder increases. However, at part load a different behaviour can be experienced at high EGR temperatures (hot EGR) where