Troels Dyhr Pedersen.indd - Solid Mechanics

At lambda 4, IMEP peaks at a compression ratio of
10.2 for 1000 RPM, while at 2000 and 3000 RPM a
much higher compression ratio of 11.4 must be
used. At this point IMEP at 1000 RPM is still close
to the peak value, which makes 11.4 the best
solution for all speeds.
For all equivalence ratios and compression ratios it
is found that increasing engine speed from 1000
RPM to 2000 RPM significantly increases IMEP
The benefit of further increasing to 3000 RPM is
less, probably because engine knock is higher at
this speed.
Whereas the optimal compression ratio should
ideally be that which results in a combustion
phasing slightly after TDC, it is found that at over
advanced combustion phasing does not have a
very large effect on the IMEP. It may be explained
by better combustion efficiency when higher
compression ratios are applied.
IMEP [Bar]
6
5
4
3
2
1
0
IMEP for lambda 2.5
8.8 9.0 9.2 9.4 9.6 9.8 10.0 10.2
Figure 28: IMEP for lambda 2.5
IMEP [Bar]
6
5
4
3
2
1
0
IMEP for lambda 3
8.8 9.0 9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0 11.2 CR
Figure 29: IMEP for lambda 3
IMEP [Bar]
6
5
4
3
2
1
0
IMEP for lambda 4
Figure 30: IMEP for lambda 4
9.8
10.0
10.2
10.4
10.6
10.8
11.0
11.2
11.4
11.6
11.8
12.0
12.2
12.4
12.6
12.8
CR
CR
1000
2000
3000
1000
2000
3000
1000
2000
3000
Indicated efficiencies
Figures 31-33 contain the calculated values of
indicated efficiencies. As the amount of fuel per
cycle was kept constant for each lambda, the
graphical representation is also the same as that
for IMEP. The effects of engine speed and
compression ratio on the indicated efficiencies are
of course the same as are observed for IMEP.
High indicated efficiencies of 40-45 percent are
observed at lambda 3 and 4 at engine speeds of
2000 and 3000 RPM. This is believed to be a result
of both a higher compression ratio and less heat
loss due to engine speed.
The indicated efficiency at lambda 2.5 is more
modest, around 30-35 percent. This is most likely
due to the lower compression ratio and higher heat
losses due to higher temperatures.
IMEP [Bar]
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Indicated efficiency for lambda 2.5
8.8 9.0 9.2 9.4 9.6 9.8 10.0 10.2 CR
Figure 31: Indicated efficiency for lambda 2.5
IMEP [Bar]
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Indicated efficiency for lambda 3
8.8
9.0
9.2
9.4
9.6
9.8
10.0
10.2
10.4
10.6
10.8
11.0
11.2
11.4
Figure 32: Indicated efficiency for lambda 3
IMEP [Bar]
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
9.8
10.0
10.2
10.4
10.6
Indicated efficiency for lambda 4
10.8
11.0
11.2
11.4
11.6
11.8
12.0
12.2
12.4
12.6
Figure 33: Indicated efficiency for lambda 4
12.8
CR
CR
1000
2000
3000
1000
2000
3000
1000
2000
3000