Troels Dyhr Pedersen.indd - Solid Mechanics

from the engine vibration measurements was significantly above 1 kHz. Therefore the low frequency noise was
removed from the measurement with a digital high pass filter in the subsequent analysis.
The noise from the diesel combustion event 360 CAD before the HCCI combustion did not influence the
acoustic SPL measured outside the engine since all high frequency noise was efficiently attenuated in the test
cell. The acoustic SPL for one complete cycle is found in figure 23, from which it can be seen that the acoustic
SPL has decreased to around 70 dB before the HCCI cylinder combustion event. Since the acoustic SPL from
HCCI combustion increases the
DATA SAMPLING
The amplified signal from the cylinder pressure transducer was measured simultaneously with the engine
vibration and sound pressure. The sampling clock was supplied by the engines crank angle encoder which
supplies 10 pulses per CAD. At 1200 rpm this results in a sample frequency of 72 kHz which is sufficient to
detect frequencies up to 36 kHz. This range fully satisfies the need, since frequencies above 15-20 kHz only
appear at very weak amplitudes.
TEST PROCEDURE
The engine was preheated while running on the diesel cylinder alone. HCCI combustion was initiated on the
other cylinder with a fixed quantity of DME per cycle, which was maintained during the test.
The quantity of DME was controlled by having a fixed injector opening time of 4.5 ms per cycle, which results
in 20.4 mg of DME per injection. The resulting equivalence ratio was calculated to be 0.35, corresponding to an
excess air ratio of 2.85. The engine can tolerate higher fuelling rates and hence higher pressures than this. The
service life of the pressure transducer is however severely shortened when very strong knock occurs, which is
the case at higher fuelling rates. With an injection time of 5.5 ms per cycle the amount of fuel per cycle
increases to 24.0 mg and the equivalence ratio becomes 0.41. At this point, pressure pulsations with peak
amplitudes of more than 10 bars are observed. At higher fuelling rates there are indications of developing
detonations, such as extremely rapid pressure rise rates which can only be associated with shock waves. These
can rapidly destroy the pressure transducer and are therefore avoided.
After HCCI combustion was established, the inlet air manifold was coupled to an air cooler which supplied air
at approx. -13 °C. The inlet air temperature was measured at the inlet manifold, before the injector position. The
engine was then allowed five minutes to adjust to the new inlet condition. When the HCCI combustion was
stabilized in its retarded position, the cooling unit compressor was switched off to allow a slow increase in inlet
air temperature. Due to the large thermal mass of cooling liquid and heat exchanger, the temperature increment
was in the order of 1 °C per two minutes. This allowed ample time for the engine to adjust thermally, as well as
measurements to be made at near steady conditions.
The first measurement was made at -10°C and the subsequent in 5°C intervals. The last measurement was made
at +15°C. The cylinder pressure sensor, accelerometer and microphone output were sampled simultaneously in
0.1 CAD intervals for 20 consecutive cycles in each recording.
The equivalence ratio is slightly affected by the reduction in inlet temperature. Table 3 shows the calculated
equivalence ratio as function of inlet temperature. It has been assumed that the volumetric efficiency relates to
inlet temperature as:
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