Improved Passage Design for a Spark Plug Mounted Pressure Transducer

Combustion chamber pressure measurement in engines via a passage is an old technique that is still widely used in engine research. This paper presents improved passage designs for an off-set electrode spark plug designed to accept a pressure transducer. The spark plug studied was the Champion model 304-063A. Two acoustic models were developed to compute the resonance characteristics. The new designs have a resonance frequency in a range higher than the fundamental frequency expected from knock so that the signal can be lowpass filtered to remove the resonance and not interfere with pressure signal components associated with combustion phenomena.

Engine experiments verified the spark plug resonance behavior. For the baseline engine operating condition approximately 50 of 100 cycles had visible passage resonance in the measured pressure traces, at an average frequency of 8.03 kHz. The resonance frequency was found to be highly dependent on the air/fuel ratio due to the temperature dependence of the sound speed. Variation of other engine operating parameters resulted in smaller changes to the resonance frequency.

New passage designs were created using the eigenfrequency analysis mode of the commercial code COMSOL Multiphysics. 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 designs increased the 1^st-mode eigenfrequency by about 60%, according to the model. A linear acoustic analysis conducted with the time-harmonic application mode in COMSOL was used to model the gain and phase delay due to the passage and to quantify the effects of fluid viscosity.