Thermal Stress on a Piezoelectric Pressure Transducer in the Combustion Chamber of an SI-Engine 850375
Cylinder pressure traces of internal combustion engines measured with piezoelectric pressure transducers are significantly influenced by long- and shorttime temperature changes. Transducer sensitivity changes due to temperature up to 1 %/100°C are possible. This requires calibration of pressure transducers regarding the real temperatures given by engine operating conditions.
Experience at Volkswagen Research with small transducers without separate watercooling - often the only possible alternative because of space problems in small engines - showed damage due to thermal stress when operated under knock conditions. This gives an indication of high temperatures in the front part of the transducer which are considerably higher than those in the surrounding material of the cylinder head. Regarding the design of pressure transducers this is understandable: The transducer is a vacuum tube partially filled with ceramic and quartz material which has relatively bad heat conductivity compared with the cylinder head material (aluminium for instance). Additional influence on thermal stress of pressure transducers is caused by local differences in flame propagation and heat flow.
Results of temperature measurements in the diaphragm and in the quartz material of an uncooled transducer mounted in the combustion chamber of an SI engine in two different locations will be presented. Temperatures are measured by fast thermocouples with a time response of several milliseconds, enabling measurement of the influence of gas temperature oscillations. In order to investigate the bandspread of thermal stress on the transducer, important engine parameters like load, spark advance and coolant temperature are varied. Additional measurements are carried out to show the influence of different heat flow to the walls caused by charge motion. The two combustion chamber types which were used represent extremes: The production HCS - combustion chamber (High Compression and Squish), a Heron shape, has optimized high squish flow for best efficiency. The other type, the disk shaped combustion chamber, has no squish at all.
The data collected is the basis for the design of the improved Volkswagen Research pressure step calibration unit. This unit will be able to simulate the average temperatures measured in the quartz material during engine operation. A description of the general pressure step calibration unit for all types of pressure transducers and the modified version for higher temperatures including the semiautomatic data acquisition is given.
PIEZO-ELECTRIC PRESSURE TRANSDUCERS have been utilized for more than 40 years measuring the in-cylinder pressures of combustion engines. During the last ten years decisive progress in fast, process-computer controlled high speed data acquisition systems with improved accuracy and resolution and a multitude of application-oriented evaluation programmes has been made. Yet there is no “breakthrough” for a wide use of this measurement procedure as actually required in the field of engine research and development.
Mainly two reasons are assumed for this lack of use: Firstly there is still the relatively high expense for preparation, performance and evaluation of those measurements. Secondly, there is a low confidence in the accuracy of this measurement technique.
Apart from procedure errors the pressure transducer itself is now the weakest link in the measurement chain. Errors concerning the measurement procedure and the transducer have been described in several publications (1, 2, 3).