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 (aluminum 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.