Methods of Processing Cylinder-Pressure Transducer Signals to Maximize Data Accuracy

Two disadvantages of using piezoelectic pressure transducers to measure cylinder pressure are the sensitivity of the transducer to temperature and the necessity to reference the output to absolute pressure (pegging). Transducer drift driven by the combustion event, often referred to as thermal shock, enhances measured cyclic variability by exaggerating the effects of actual cyclic variations in combustion temperature. Any artificial variability that persists until the portion of the cycle used for pegging offsets all of the referenced measurements of that cycle by the magnitude of the variability. This study reviews several methods of pegging, and examines the extent to which the four most viable methods propagate thermally-induced intracycle and intercycle measurement variability.

It is preferable to peg when artificial variability is minimized, which occurs when the transducer output is least affected by the thermally transient nature of the engine cycle. Unfortunately, the majority of thermally-induced measurement variability persists throughout the engine cycle and is thus inherent in all pegging procedures. However, this study shows that the pegging procedure selected does have a secondary influence. For the engine used in this study, referencing the transducer output at inlet bottom dead center (IBDC) to intake manifold pressure (MAP) performed best. Equating the average measured pressure during the exhaust stroke to the exhaust backpressure or forcing a fixed polytropic compression coefficient were slightly worse, while forcing a variable polytropic compression coefficient was considerably worse.