Cylinder Pressure Data Quality Checks and Procedures to Maximize Data Accuracy 2006-01-1346
Cylinder pressure data is so completely integral to the combustion system development process that ensuring measurements of the highest possible accuracy is of paramount importance. Three main areas of the pressure measurement and analysis process control the accuracy of measured cylinder pressure and its derived metrics:
Association of the pressure data to the engine's crankshaft position or cylinder volume
Pegging, or referencing, the pressure sensor output to a known, absolute pressure level
The raw, relative pressure output of the piezoelectric cylinder pressure sensor
Certain cylinder pressure-based metrics, such as mean effective pressures (MEP) and heat release parameters, require knowledge of the cylinder volume associated with the sampled pressure data. Accurate determination of the cylinder volume is dependent on knowing the rotational position of the crankshaft. New, objective methodologies for use with commercially available top dead center (TDC) location probes are presented. These allow precise and robust phasing of a crankshaft position sensor (encoder) to engine crankshaft rotational position.
While cylinder pressure pegging accuracy has no effect on calculated mean effective pressures or location of peak pressure, it does influence other critical combustion metrics such as burn durations and Polytropic coefficients. The influences of pegging errors on these metrics are compared, and the sensitivities of effected metrics are presented.
Many sources of error can that affect the raw, relative pressure signal of the piezoelectric sensor. Among these are thermal shock and variations in sensitivity as a function of the mean operating temperature and pressure of the transducer.
Thermal shock refers to a short term (effects exist within a single engine cycle) shift in sensor output brought about by high rates of heat flux to the sensing element.
While modern, high-end transducers incorporate design features to minimize these effects, they can still exhibit significant errors if they are not properly mounted and the data is not collected and analyzed with the proper care and techniques. Techniques and metrics which are useful for evaluating cylinder pressure data plausibility and quality are introduced. These metrics allow quick and robust identification of the presence of thermal shock.