Quantification of Thermal Shock in a Piezoelectric Pressure Transducer

One of the major problems limiting the accuracy of piezoelectric transducers for cylinder pressure measurements in an internal-combustion (IC) engine is the thermal shock. Thermal shock is generated from the temperature variation during the cycle. This temperature variation results in contraction and expansion of the diaphragm and consequently changes the force acting on the quartz in the pressure transducer.

An empirical equation for compensation of the thermal shock error was derived from consideration of the diaphragm thermal deformation and actual pressure data. The deformation and the resulting pressure difference due to thermal shock are mainly a function of the change in surface temperature and the equation includes two model constants. In order to calibrate these two constants, the pressure inside the cylinder of a diesel engine was measured simultaneously using two types of pressure transducers, in addition to instantaneous wall temperature measurement. The thermal shock of the first transducer was assessed through comparison with the signal from the reference water-cooled transducer. Subsequently, thermal shock was evaluated in a gasoline engine using the developed equation for a range of conditions. The results indicate that the thermal shock equation provides reliable correction based on known surface temperature swing. Analysis of a large set of results enabled development of another correlation for the correction of calculated IMEP values directly from speed and pressure measurement in an SI engine.