To provide a better explanation for the previous
observations concerning the effect of EGR temperature
on NO and soot, it is given in Fig. 10 the variation of
cylinder mean gas temperature, local zone temperature
(zone 1,1,1-1st injected periphery zone), NO and soot
formation histories as function of EGR temperature for
100% load. As observed the increase of EGR temperature
from 90 to 240 1C results to an increase of the mean gas
temperature and the individual zone temperature during
the main combustion period. The last is due to the fact that
the increase of EGR temperature results to a decrease of
the AFR inside the zone and increase of entrained air
temperature compensating the negative effect of CO2 and
H2O dissociation. The increase of gas temperature cannot
compensate for the lack of O2 availability through soot
oxidation resulting from thermal throttling leading to an
increase of emitted soot. It is evident that when using high
EGR gas temperature soot oxidation ceases earlier during
the expansion stroke due to lack of O2. This provides an
explanation for the negative effect of EGR gas temperature
increase on soot emissions. The small variation of NO with
EGR temperature is explained observing the variation of
local zone gas temperature and considering Fig. 5
providing the effect of EGR temperature on the overall
AFR. The small increase of local gas temperature with
increasing EGR temperature has a small positive impact on
NO formation, which is obviously overwhelmed by the
decrease of O2 percentage. The last two mechanisms result
to a very small increase of NO within the EGR temperature
levels examined, which is more pronounced at high EGR
rates.