Troels Dyhr Pedersen.indd - Solid Mechanics

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8.2 Cylinder pressure transducers
The first transducers used were Kistler 6052 CU piezo electric transducers. This
transducer is not water cooled, but still maintains a high precision at elevated
temperatures. It is not susceptible to thermal shock due to a low diaphragm area and front
sealing, which results in a low heat transfer through the pickup. Since it is connected to
the combustion chamber through a small diameter bore, combustion is quenched before
reaching the transducer.
The piezo electric transducer responds to pressure changes, not absolute pressure. A
change in pressure on the diaphragm results in a small charge of electrons. As insulation
resistance is never perfect, some of the charge is lost before it reaches the charge
amplifier. This causes the amplifier signal to drift, especially when changing the engine
load. To fix the signal, a constant leakage current through the charge amplifier is
established by coupling a large bleeder resistance to the transducer signal. The discharge
current stabilizes the signal at an RMS value around zero, without affecting the
magnitude of the signal.
Kistler claims that the transducer is resilient to knocking combustion due to a reinforced
diaphragm, but this was clearly not adequate for the kind of knocking experienced in this
project. Two of these pickups were used to investigate the acoustic behavior of various
combustion chamber geometries. The continued exposure to knocking combustion
resulted in one of them to fail completely as the diaphragm collapsed, while the other was
partly damaged which resulted in reduced sensitivity.
To replace the broken transducer with a more reasonably priced option, an optical
transducer from the US based company Optrand was selected for trial. This type of
pickup measures absolute pressure by measuring the deflection of the diaphragm with
light. The sensor is connected to an amplifier through a fiber optical cable. The amplifier
outputs a voltage from 0.5 to 5 volts, with the lower voltage corresponding to 1
atmosphere. Since drift does not occur with this transducer it is not necessary to calibrate
the sensor or correct the signal for drift; it may be used directly after scaling. It was
however noted that the signal from the amplifier includes broadband noise, which is
likely caused by the amplification stage of the transducer. This means that low pass
filtering had to be applied for data that were to be used for the heat release analysis.
When the sampling frequency is high the relative error on the pressure change between
samples becomes very significant, which necessitates filtering before heat release
calculations. For frequency analysis the signal-to-noise ratio is however quite low and
filtering is not required.
8.3 Data acquisition
The signals are acquired by a National Instruments USB-6212 data acquisition unit. It has
a maximum sample rate of 400 kS/s which is shared between the active channels.
The sample clock for the DAQ is supplied externally by a ScanCon encoder connected to
the engine shaft. It delivers 3600 pulses per revolution and a separate pulse to indicate
one full revolution. This resolution, 10 samples per CAD, is necessary since the