Application of an Improved Model for the Determination of Acoustic Resonances in Indicator Passages for Combustion Pressure Measurements

The acoustic resonances in indicator passages are often modeled using either a Helmholtz or a so called organ pipe acoustical model. However, in practice these models often indicate natural frequencies which are too high. This paper proposes the Bergh and Tijdeman model [1] which is more accurate and which was originally developed for pressure measurements in turbomachinery. This paper presents the theoretical basis for the Berg / Tijdeman model and then uses it to explore signal distortion from a variety of indicator passage geometries. In order to validate the approach, a flush-mounted water-cooled Kistler reference transducer was used to measure accurate in-cylinder combustion data in an automotive Diesel engine. An additional sensor was recess mounted with passages of different geometries. The Bergh Tijdeman model was then applied to investigate the acoustic distortion of the indicator passages. The results show excellent agreement with the experimental data, which are much closer than using the Helmholtz or the organ pipe model. Further the Bergh and Tijdeman model is applied to complex indicator passage geometries with multiple cavities. Again, for comparison, a flush-mounted Kistler reference transducer was used to measure accurate in-cylinder combustion data in a large-bore natural gas engine. Three additional sensors were mounted using different indicator passage geometries. The engine was operated under base line and knocking combustion conditions. The Bergh Tijdeman model was then applied to model the acoustic distortion of the three indicator passages and again showed good agreement with the experimental data. Finally, the paper proposes simple rules for implementing indicator passages in large gas engines.