This is due to the combination of the different influences
imposed on engine operation by EGR. In the present full
load operating case, the effect of thermal throttling (that is
reduced amount of air and oxygen during the inlet stroke)
is important and increases as EGR temperature is
increased to higher values. This obviously has a negative
effect on combustion. At the same time, the average
temperature level of the cylinder contents increases due to
the high EGR temperature resulting to an increase of heat
losses. Finally, due to the presence of EGR, oxygen
concentration of the intake air is reduced having a
significant negative effect on combustion. The above
observations explain adequately the reduction of brake
thermal efficiency observed in Fig. 7.