1990-02-01

Cylinder-Pressure-Transducer Mounting Techniques to Maximize Data Accuracy 900171

Engine cylinder-pressure data is generally acquired using piezoelectric pressure transducers because of the many advantages they offer. Unfortunately, a serious and difficult-to-detect disadvantage is variability introduced into the raw data by thermal shock occurring at the transducer face. Transducer manufacturers are aware of this problem and have produced special designs intended to minimize thermally-induced output drift. Although improvements have resulted, the problem continues to exist. However, thermal shock can be reduced by properly mounting the transducer. This study reviews the theoretical principles dictating performance of remote-mounted transducers and examines the influence of the mounting scheme on pressure-data quality.The three feasible mounting schemes studied were flush mount, remote mount via a single passage, and remote mount via multiple slots. A fourth mounting scheme, namely remote mount via a sintered porous-metal interface, proved infeasible because of excessive pressure drop across the porous metal. The multiple-slot mounting adapter performed best. When properly designed, this adapter can maintain data accuracy while considerably reducing transducer-induced variability relative to flush mounting. For example, measured cyclic variability in indicated mean effective pressure is cut in half at a medium-load condition.LACK OF ACCURACTE CYLINDER-PRESSURE measurement can produce erroneous trends in experimental correlations and/or limit the extent to which engine performance can be examined. Quartz piezoelectric transducers are generally used to measure cylinder pressure because they have a high frequency response, small size, light weight, low power consumption, and are relatively insensitive to environmental conditions [1]* . However, they are far from perfect devices. Their two main operational disadvantages are (1) variations in output during thermal transients (thermal shock) and (2) the necessity to relate all measurements to an absolute reference pressure. This paper examines strategies to reduce the effects of disadvantage (1), whereas disadvantage (2) is addressed in a separately reported study [2]. The analysis procedure consists of carefully examining experimental pressure data to isolate specifically how, when, where, and why intracycle pressure measurement is affected. Individual-cycle heat release is used to assess the effects of raw-data variability on combustion-performance parameters.

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