It is evident that the increase of EGR percentage at
constant boost pressure results to a decrease of the amount
of fresh air inducted per cycle. Consequently since the
amount of fuel injected per cycle remains practically
constant, AFR should decrease. A similar effect is expected
when increasing EGR gas temperature at a given EGR rate.
This is verified observing Fig. 5 providing the variation of
AFR with EGR gas temperature at various EGR rates for
all cases examined. The variation of AFR with EGR gas
temperature is almost linear. In the same figure is presented
the relative variation of AFR with EGR percentage for the
upper and lower EGR gas temperatures examined, using as
reference the value of AFR for engine operation without
EGR. As observed, the negative effect of EGR on AFR
increases with the increase of EGR temperature. For all
cases examined the effect of EGR rate and temperature is
more pronounced at low engine speed. Thus at full load the
effect of thermal throttling (reduced amount of charge to
the cylinder) is significant and increases as EGR temperature
is increased to higher values.