# Offset Correction

#### Background

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. Randolf2 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.

Randolf4 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.

Randolf2 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.

Brunt1 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.

Randolf2 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.

Brunt1 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.

#### Reference Pressure

Randolf2 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 al3 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 al5 set the cylinder pressure 40 degrees before IBDC equal to the manifold pressure.

#### Polytropic

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 Pn and Vn are the pressures and volumes respectively of data in the compression region.

Randolf2 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).

Randolf2 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 al1 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 al1 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.

#### catool

The following calculation are made available in catool:

• POFF - Pressure offset
Implementation: See offset_correction_channel() in cat.c

#### References

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.