Pressure Sensors
Load change drift bar/s
The load change drift is a slow drift
of the pressure signal after a load
change which is caused by a change
of temperature level and heat flux.
The characteristic value for the load
change drift is determined in real
engine operation, by first running the
engine at a specific load point and
then changing to motored mode by
shutting off the fuel supply thus producing
a quick change in the heating
effect on the pressure sensor (by a
sudden load change). The drift itself
is defined by the maximum change
of the pressure level per unit of time
and is called maximum zero-line
gradient dp/dt.
The resulting permanent zero-line
deviation has no relevance due to
drift compensating modes of modern
charge-amplifiers and is only mentioned
for the sake of completeness.
M
Max. temperature of plug seat °C
This temperature defines the maximum
allowed temperature of the
plug seat of a spark-plug adaptor.
Measuring range bar
This is the pressure range in which
the sensor meets the specifications.
For analysis of the cylinder pressure
this range should be at least
0…200 bar.
Under severe conditions like supercharged
engines and knocking the
maximum pressure range becomes
an issue. High pressure peaks can
fatigue the membrane and make the
sensor fail. The maximum allowed
pressure is mainly defined by the
design and the material of the sensor
membrane. The trade-off to improved
maximum pressure is in most cases
the decrease in resolution and
sensitivity which is on the other hand
required for thermodynamic analysis.
Mounting bore mm
Diameter of the indicating bore for
plug- and probe-types. The diameter
of the mounting thread is only listed
for the thread-types (see “thread
diameter”).
Mounting torque Nm
Each sensor, adaptor and threaded
connector needs to be mounted
with a specific torque. This ensures
safe operation and best performance
of all components. To apply
the right torque, tools like calibrated
torque wrenches should be used.
N
Natural frequency Hz
The natural frequency is the lowest
possible frequency of free (nonforced)
oscillations in the measuring
element of a fully assembled pressure
sensor.
This value should be at low engine
speeds at least 50 kHz.
At high engine speeds the moving
actions of the valves generate mainly
high frequency noise. This noise can
become visible as an artefact in the
measurement signal if the natural
frequency is in the frequency range
of this noise. Therefore for testing
with high engine speeds the natural
frequency of the sensor should be at
least above 100 kHz.
In contrast, with the term natural
frequency, the basic resonance
frequency defines the frequency of
the measurement quantity at which
the pressure sensor gives the output
signal with the highest amplitude.
Where there is little attenuation, as it
is generally the case for piezoelectric
pressure sensors, the basic resonance
frequency is the same as the
natural frequency 1st order.
O
Operating temperature range °C
Temperature range in which the
pressure sensor meets the specifications
of the data sheet. For typical
combustion analysis this range
should be at least 0…400 °C. The
temperature which is meant here
is the maximum temperature at the
mounting position.
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