Troels Dyhr Pedersen.indd - Solid Mechanics

METHANOL TEST PROCEDURE
In the first part of the experiment the amount of DME injected was kept constant. Methanol was then added to
the intake manifold in increasing amounts. The injector opening time was increased in steps of 1 millisecond
and the equivalence ratio of methanol to air calculated subsequently. The increased amount of methanol
retarded the combustion as expected. Ultimately partial combustion and misfire was observed. This was
indicated by increasing levels of CO and THC in the exhaust, as well as unsteady operation and decreasing
torque. This meant that the optimum amount of methanol had been passed, which concluded the test.
The equivalence ratio of DME was kept at approximately 0.25. This equivalence ratio results in moderate
engine knock in the given engine, particularly when combustion timing is advanced as was the case before
methanol or EGR was applied. By adding a sufficient amount of methanol the experience was however that
combustion noise was reduced, despite the increase in total fuel amount.
A major problem with port fuel injection of methanol is that it is difficult to obtain a good vaporization into the
air stream. A part of the methanol will enter the engine as droplets rather than as a gas. These droplets are likely
to dissolve into the lubricating oil when they come into contact with the cylinder wall.
During the experiment it was observed that methanol was evaporating from the crankcase ventilation, which
had been disconnected from the inlet manifold. This meant that the amount taking part in the reaction could
only be calculated by an exhaust carbon balance. The equivalence ratio of methanol in the combustion was
therefore calculated from the carbon balance, with the equivalence ratio of DME being determined without
injection of methanol. The method given by Heywood [15] for determining equivalence ratio from exhaust gas
composition of fuels containing oxygen was used. The carbon and hydrogen numbers (m, n and o) in the
calculation were adjusted from the mole fraction determined. The mole fractions of the calculated species
differed from the measured species by less than 2 %.
EGR TEST PROCEDURE
The test with EGR was also conducted with an equivalence ratio of approximately 0.25 for DME. The
equivalence ratio was measured without EGR. The amount of DME injected was then kept constant by
maintaining the injection duration.
EGR was gradually increased in steps of 10 percent. The exhaust back pressure sufficed to ensure that the EGR
was guided back to the inlet manifold up to about 60 % EGR. Hereafter the inlet air was throttled slightly to
force additional EGR uptake. The amount of EGR was calculated from accurate inlet air flow measurements
while assuming a constant volumetric efficiency. This was justified by the inlet temperature being constant due
to the efficient water cooled EGR cooler.
At very low concentrations of remaining oxygen in the exhaust gas the combustion efficiency started to drop
with increasing emissions of CO and HC, at which point the test was ended. The effect of increasing the EGR is
observed from figures 5-8.
EMISSIONS MEASUREMENT
All emissions were measured with a Horiba MEXA 7500 DEGR. This analyzer has two direct lines. The gas
was collected before and after the DOC in the exhaust pipe. The measured gases, detection technology and
range are found in table 2.
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