Troels Dyhr Pedersen.indd - Solid Mechanics

Since the total equivalence ratio did not reach the same level at 1800 RPM as the 1000 RPM case, the BMEP
did not reach the same level either. To increase the torque it would be required to increase the amount of DME
and balance the combustion timing with methanol.
THE EFFECT OF EGR ON COMBUSTION TIMING
Figures 10 and 11 shows that the combustion timing was retarded approximately 12 CAD when the EGR ratio
was increased from 0 to 65 % for both 1000 and 1800 RPM.
There was no notable effect on the heat release rate within this interval. The peak pressure was however
reduced 8 bar in the 1000 RPM case and 13 bar in the 1800 RPM case. At 1000 RPM the turbocharger is not
active, and hence the pressure reduction is due both to the increase in specific heat capacity resulting from
increasing concentrations of water and carbon dioxide, as well as the inlet throttling required to force additional
EGR back to the inlet manifold. At 1800 RPM the pressure increase from the turbocharger is reduced when
EGR is increased, so the pressure reduction is also due to a decrease in inlet manifold pressure.
BMEP is increased with increasing amounts of EGR (figure 3), despite the decrease in peak pressure. This is
due to lower heat losses and increased expansion work which are the consequences of later combustion. The
BMEP is only slightly higher at 1800 RPM, despite a notable increase in compression pressure at this speed due
to the turbocharger.
It was not possible to retard the combustion as much as desired. The optimum timing would have been
somewhere around 10 CAD after TDC, but the latest timing achieved was a few CAD before TDC.
It was noted that the combustion timing became very sensitive to changes in the EGR at the highest ratios used,
where the exhaust gas oxygen concentrations were low. Changing the EGR rate from 65 to 70 % caused a delay
of 5 CAD, the same result as changing from 0 to 60 %. It is most likely due to the fact that the oxygen
concentration is more important for the reaction rate when it is closer to the stoichiometric limit.
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300
200
100
BMEP [kPa]
1000 RPM
1800 RPM
EGR ratio
0
0.0 0.2 0.4 0.6 0.8
Figure 3: BMEP vs. EGR ratio
It is also observed that the peak pressure is greatly reduced as the amount of EGR increases. This does however
not mean that the power is reduced. Both torque and IMEP increases with increasing EGR percentage. The
reason is that the compression work is reduced at a greater rate than the expansion work. A secondary effect is
that friction work is also reduced slightly as the cylinder pressure decreases.