Improved passage design for a spark plug mounted pressure transducer
Combustion chamber pressure measurement in engines via a passage is an old technique gaining new acceptance from the modern scientific community. While it was once used to facilitate pressure measurements from indicators that were too large to mount flush in the combustion chamber, it is now used to alleviate the problems associated with thermal shock and to allow measurements in combustion chambers where access is limited. However, the technique is not without its drawbacks. Combustion gases can resonate inside the passage and the signal can become distorted due to friction within the passage. This thesis presents improved passage designs for a spark plug mounted pressure transducer. These new designs have a resonance frequency in a range higher than that expected from knock so that the signal can be low pass filtered to remove the resonance and not interfere with pressure signal components associated with combustion phenomena. This thesis also attempts to quantify the effects of fluid friction within a passage using a linear acoustic analysis. The modeling performed for the thesis was verified with experimental data. For the baseline engine operating condition used for this project, approximately 50 of 100 cycles had visible resonance at an average frequency of 8.03 kHz. Of the engine operating parameters, the engine speed and ignition timing showed a significant effect on the percentage of cycles with resonance. The variation of the air/fuel ratio showed the greatest effect. The actual resonance frequency was also found to be highly dependent on the air/fuel ratio. This is due to the effect of the air/fuel ratio on the average sound speed in the combustion chamber and the directly proportional relationship between the speed of sound and predicted eigenfrequencies. The variation of the other engine operating parameters resulted in minimal changes in the resonance frequency. The most promising new passage design is a concept where the main chamber is divided into three sections: the original kidney shape and two cylinders. The best design of the concept increased the first mode eigenfrequency by 59.5%. The new designs were created using the eigenfrequency analysis mode of COMSOL Multiphysics. The linear acoustic analysis, used to quantify the effects of friction, was also performed in COMSOL using the time-harmonic application mode.