Fig. 15 shows the variation of smoke emissions with various
first injection pulse durations and last injection pulse timings.
We see that generally smoke emission levels are low, at all the
operating conditions of the engine. However, the change in smoke
emission levels is marginal with changes in the engine operating
conditions. It is known that Smoke emissions are negligible in
the HCCI combustion mode [29]. From Fig. 15, we see that, the
smoke levels are lowest, when the last injection pulse is most
retarded i.e., at TDC. At this condition, a major portion of the fuel
is already burnt as observed by the HRR curves.