As far as soot emissions are concerned an increase of
their value is observed when increasing EGR temperature,
as demonstrated in Fig. 9. This results mainly from the
reduction of AFR. Oxygen concentration is reduced
affecting both soot formation and oxidation. The effect
of EGR temperature is almost linear and more pronounced
at high EGR rates and low engine speeds. In the same
figure is given the corresponding relative variation of soot
emissions with EGR rate compared to the zero EGR case,
for the three engine speeds and the upper and lower limits
of the EGR gas temperatures examined. The effect of EGR
rate on soot is stronger at high EGR temperatures.
Furthermore, as shown, the effect of EGR temperature is
higher at low speed where the maximum values of soot
variation are observed [5,35]. In the case of soot emissions
it could be expected that using a higher EGR temperature
would enhance soot oxidation leading to a reduction of
emitted soot. This may be the case for a heavy duty diesel
engine operating at low load since in this case oxygen
availability even when using EGR is high due to the AFR
values used. However, for full load operation this is not
the case. As EGR temperature increases it has a negative
effect on AFR and thus there is not enough O2 to oxidize
soot.