Comparison of Cylinder Pressure Measurements on a Heavy-Duty Diesel Engine Using a Switching Adapter

In this study, a variety of piezoelectric pressure transducer designs and mounting configurations were compared for measuring in-cylinder pressure on a heavy-duty single-cylinder diesel engine. A unique cylinder head design was used which allowed cylinder pressure to be measured simultaneously in two locations. In one location, various piezoelectric pressure transducers and mounting configurations were studied. In the other location, a Kistler water-cooled switching adapter with a piezoresistive pressure sensor was used. The switching adapter measured in-cylinder pressure during the low pressure portion of the cycle. During the high pressure portion of the cycle the sensor is protected from the high pressure and high temperature gases in the cylinder. Therefore, the piezoresistive sensor measured in-cylinder pressure highly accurately, without the impacts of short term thermal drift, otherwise known as thermal shock. Additionally, the piezoresistive sensor is an absolute pressure sensor which does not require a baseline or “pegging” on every engine cycle. With this measurement setup, the amount of thermal shock and induced measurement variability was accurately assessed for the piezoelectric sensors. Data analysis techniques for quantifying the accuracy of a piezoelectric cylinder pressure measurements are also presented and discussed. It was observed that all the piezoelectric transducers investigated yielded very similar results regarding compression pressure, start of combustion, peak cylinder pressure, and the overall heat release rate shape. Differences emerged when studying the impact of the transducer mounting (e.g., recessed vs. flush-mount). Recessed-mount transducers tended to yield a more accurate measurement of the cycle-to-cycle variability when compared to the baseline piezoresistive sensor. This is thought to be due to reduced levels of thermal shock, which can vary from cycle-to-cycle.