Piezoelectric pressure transducers produce a charge relative to change in pressure
levels, therefore a method is required to peg recorded pressure data to absolute
levels. Randolf^{2} describes a number of methods for pegging absolute cylinder pressures.
The two main types of pegging described are setting a point within the engine cycle to a
known or estimated pressure and fitting the compression to a polytropic process.

Randolf^{4} notes that determination of IMEP, variability in IMEP, PMEP, maximum rate of
pressure rise and location of peak pressure do not require pegged data. It is only absolute
metrics such as peak pressure that require pegging.

Randolf^{2} notes that short-term pegging (i.e. every cycle) removes the problems of longterm drift
inherent in piezoelectric devices and those techniques that use mechanical
switching as an indicator for absolute pressures are too slow for cycle-resolved use.

Brunt^{1} notes that higher absolute accuracy is generally needed for combustion analysis at
low load and under slow burn conditions and that high accuracy is only achievable with the
absence of all other sources of error throughout the whole cycle. Thermal shock, long term
drift and sensitivity errors mean accurate pressure referencing will occur over a limited
portion of the engine cycle.

Randolf^{2} notes that intracycle drift, the drift that occurs between the beginning and end of
a single cycle, is of greater importance than long-term drift. He identifies that intracycle drift
can be measured by the difference in transducer output at IBDC (before pegging) for any
two consecutive cycles.

Brunt^{1} therefore concludes that it is necessary to decide which part of the cycle needs
accurate referencing. Accurate referencing of induction and exhaust pressures for example
will be of critical importance for breathing and friction studies but will be of much less
importance for combustion analysis.

Randolf^{2} found that for the engine used in his study referencing the transducer output at
inlet bottom dead centre (IBDC) to intake manifold pressure (MAP) performed best.
However, this is only true for engines with untuned intake systems or at very low speeds in
tuned systems. He notes that any type of runner will generate tuning effects, thereby
limiting this method to low engine speeds. To reduce the effects of noise the inlet manifold
pressure was the average of the transducer output at one degree before IBDC, at IBDC
and one degree after IBDC.

Lancaster et al^{3} says: Cylinder pressure data is pegged by assuming the pressure at BDC
after the intake stroke is equal to the mean intake manifold pressure.

Through experience Hayes et al^{5} set the cylinder pressure 40 degrees before IBDC equal
to the manifold pressure.

Pressure correction is applied to each point of cycle data by:

`p_"actual"=p_"measured"+p_"correction"` (Equation 1)

where

`p_"correction"=(p_2-p_1)/((V_1/V_2)^n-1)-p_1` (Equation 2)

where P_{n} and V_{n} are the pressures and volumes respectively of data in the compression
region.

Randolf^{2} used a constant polytropic coefficient, n, of 1.32, and quotes Hohenberg and
Killmann as using 1.32 for homogeneous-charge engines and 1.27 for Diesel engines).

Randolf^{2} suggests that to minimize variability from slope computation (of polytropic
coefficient) it is advisable to maximise the number of measurements and the crank angle
spread of those measurements when calculating the polytropic compression coefficient.
However, it must remain between intake-valve closure and ignition.

Brunt et al^{1} quotes AVL as suggesting a polytropic index of 1.32 between 100 and 65
degrees BTDC, whilst Kistler's 5219A signal conditioner uses 1.35 between 120 and 70
degrees BTDC. Brunt et al^{1} also notes that the polytropic method is more sensitive to
noise spikes than pressure referencing, but this can be reduced by increasing the upper
crank angle. Their conclusions are that polytropic indexing is the best method for pegging,
however it is unsuitable for situations where a polytropic index is unknown, such as weak
mixtures.

catoolRT Offset Correction.pdf

The following calculation are made available in catool:

- POFF - Pressure offset

1. Brunt, M. F. J., Pond, C. R., "Evaluation of Techniques for Absolute Cylinder Pressure Correction," SAE Paper 970036, 1997.

2. Randolph, A. L., "Methods of Processing Cylinder-Pressure Transducer Signals to Maximize Data Accuracy," SAE Paper 900170, 1990.

3. Lancaster, D. R., Krieger, R. B., Lienesch, J. H., "Measurement and Analysis of Engine Pressure Data," SAE Paper 750026, 1975.

4. Randolph, A. L., "Cylinder-Pressure-Based Combustion Analysis in Race Engines," SAE Paper 942487, 1994.

5. Hayes, T. K., Savage, L.D., "Cylinder Pressure Data Acquisition and Heat Release Analysis on a Personal Computer," SAE Paper 860029, 1986.