From the previous results it is evident that EGR gas
temperature increase affects the combustion mechanism
resulting to a reduction of peak combustion pressure. For
this reason it is examined in Fig. 7 its effect on brake
thermal efficiency for various EGR rates and engine speeds
at full load. As shown, brake thermal efficiency is reduced
linearly with increasing EGR temperature. The decrease of
brake thermal efficiency is due mainly to the reduction of
AFR ratio, which affects the combustion rate of fuel and to
the increase of in-cylinder mean gas temperature that
affects in a negative manner heat losses. This is clearly
revealed from the relevant curve providing the variation
(reduction) of engine brake efficiency with EGR rate
for the upper and lower EGR gas temperatures examined.