Troels Dyhr Pedersen.indd - Solid Mechanics

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16.7 kW @ 3000 rpm
17.9 kW @ 3600 rpm
Maximum torque 55 Nm @ 2000 rpm
Table 2: Engine data and performance
An eddy-current dynamometer was used as engine load. Since the dynamometer did not have motoring
capability, the cylinder on the engine not used in the test was run in normal diesel operating mode to keep the
engine at operating temperature and at a constant speed.
CYLINDER PRESSURE MEASUREMENTS
An optical pickup was used to measure the cylinder pressure. The sensor measures the diaphragm deflection
with light and outputs the reading in a voltage between 0.5 and 5 volts.
The pickup is designed for peak pressures of 350 Bar, which is necessary for long life in knocking conditions.
Sensitivity is however adjusted so that the highest output corresponds to 70 Bar, to obtain the best accuracy.
The stiffness of the diaphragm provides a high resonance frequency of 120 kHz. The transducer is designed to
detect frequencies up to 25 kHz.
It is critical that the resonance frequency is not exited even under strong knocking conditions, as the sampling
frequency is insufficient to avoid aliasing at frequencies above 36 kHz. It was verified that the sensor resonance
frequency is not present, by sampling the sensor output at 400 kHz at higher amplitudes of knock than used in
the tests.
It was chosen to treat the cylinder pressure oscillations as sound pressure. This simple approach means that the
sound pressure in the cylinder can be compared directly to the sound pressure of the radiated noise. The
difference may then be expressed as attenuation.
VIBRATION MEASUREMENTS
The engine surface acceleration is measured by a ceramic axial shear accelerometer mounted in the top corner
of the engine block, on the HCCI cylinder side. This position is favorable for measuring the frequencies
transmitted from the chamber, but less favorable for measuring the engines lower resonant frequencies which
are strongest in the middle of the large unsupported engine surfaces. The position was chosen since the direct
transmission of sound from the cylinder was considered most relevant.
SOUND MEASUREMENTS
The instantaneous sound pressure was recorded with a sound level meter. The microphone was placed 1 meter
away from the engine, perpendicular to the crank shaft and at the same elevation as the top of the cylinders.
The walls and the floor in the test cell were partly covered with 50 mm insulation materiel to reduce the sound
reverberation. With this material thickness and some distance to the wall it is possible to eliminate reflection of
frequencies higher than approximately 1 kHz. Lower frequency noise could not be reduced much under the
given circumstances. The noise of interest was however that which was generated by engine vibration, which