Cepstral Analysis for the Recovery of Apex Seal Interaction Forces in Rotary Engines

A new method has been developed to recover apex seal interaction forces from housing acceleration measurements in rotary engines. The development was motivated by a need to rapidly compare and evaluate seal design concepts during rotary engine development when engine builds and test conditions change frequently. The new method is based on cepstral analysis techniques and offers several advantages compared to conventional signal processing techniques. By transforming information from the time domain to the cepstral domain, computations required to recover interaction forces are simplified. Furthermore, windowing in the cepstral domain can be used to minimize the effects of the vibration transfer path making this procedure relatively insensitive to changes in engine structure and operating conditions. The method was tested and verified using a beam and a rotor housing before being applied to an operating rotary engine. The transfer path of an assembled single rotor engine was measured and used to recover the apex seal interaction forces under motoring and firing conditions. The results are encouraging, suggesting that this method might have broader applications to other engine monitoring and diagnostic situations, but additional development work is needed.