Troels Dyhr Pedersen.indd - Solid Mechanics

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0
-5
-10
-15
-20
-25
Equivalence ratio
ENGINE KNOCK REDUCTION WITH METHANOL
0 0.03 0.06 0.09 0.12
Figure 1: Cooling effect of methanol on inlet air
HCCI combustion of DME creates a very hard sound, though it is not harmful to the engine in the same way as
SI engine knock. The temperature and pressure is not high enough to damage the engine at lean conditions. The
sound is however undesired and sometimes unacceptably high. Reduction of the knock is therefore a high
priority.
The rate of reaction for the high temperature reaction increased to more than twice the initial value as the
equivalence ratio of methanol was increased to 0.12 in the 1000 RPM test. The pressure rise rate increased up to
about 10 Bar/CAD with the highest amount of methanol. The sharp knocking sound observed with pure DME
combustion was however reduced with increasing amounts of methanol. There was no equipment available to
verify this tendency, so no data can be presented. A possible explanation is however that methanol not only
increases ignition delay, but also the octane rating of the fuel [16, 17]. As the octane rating is increased, so is
the fuels ability to resist detonation which is the primary cause of knock. The increased pressure rise rate is thus
not a problem, since it is more uniform and hence does not cause any transmission of noise through the engine.
BMEP
The increase in BMEP (figure 2) is caused both by the increase in total fuel amount and the retarded
combustion. The increases are very linear in both cases and follow the same line.
500
400
300
200
100
BMEP [kPa]
1000 RPM
1800 RPM
Equivalence ratio of MEOH
0
0.00 0.03 0.06 0.09 0.12
Figure 2: BMEP vs. MEOH equivalence ratio