As known, the use of EGR is an efficient technology for
the reduction of NOx emissions. Towards this direction,
Fig. 8 presents the variation of NO as function of EGR
temperature for various EGR rates. As revealed, NO
emissions at full load remain almost constant when altering
EGR temperature (in the range examined). A small
increase is observed only at high EGR rates [34].
Considering that the formation of nitrogen oxide is
temperature and O2 sensitive, it is concluded that the
temperature increase inside the combustion chamber, as
shown later on in Fig. 10, due to the increase of EGR
temperature is compensated by the reduction of AFR ratio.
Observing the results of NO variation with EGR rate for
the upper and lower EGR temperature values examined it
is verified that EGR temperature, in the range examined,
has no significant effect on NO for all engine speeds leading
to a small increase only at high EGR rates. On the other
hand the effect of EGR rate is slightly higher at low engine
speed. This, results from the lower AFR observed at low
engine speed, as already mentioned. In the case of hot EGR
(no cooling) the increase of charge temperature would be
significant and is expected to lead to an increase of NO
compared to the cooled EGR case examined here [5,6,27].