Thanks mechtech. I should clarify, 'sequence harmonics', specifically positive, negative, and zero sequence harmonics are essentially, in the mechanical sense, torques. Positive sequence torques are the useful components that contribute positively to the engine's output. Zero-sequence torques represent internal losses, while negative sequence harmonics represent torque components that actually work to oppose useful work.
Essentially through the phenomena of constructive interferance, vibrations manifest themselves as combination of 3rd, 5th, and other higher-order odd-harmonic components which exist in naturally unbalanced engines.
An engine designer using an unbalanced design will attempt to incorporate mechanical filters, ie: flywheels, torsional springs, torsional dampers, etc., in order to attempt to filter out as much of the bad stuff as possible. But since engine load is dynamic, and filters cannot be re-tuned (mechanical filters are physical manifestations), the actual regions of passbands and stopbands of the harmonic filter will change as a function of overall engine load.
Where the 'science' really comes in is that through Fast Fourier Transform methods of analyzing vibration waveforms, we can actually determine the overall wear condition of individual parts and make predictions as to the conditions of individual cylinders without removing an engine from service. Online vibration analysis is a practice of maintenance engineering that is set to explode in the next few years as energy becomes signficantly more expensive and plants look for ways to improve their costs significantly. The gear and tools required have also dropped dramatically in price recently.
[ January 13, 2005, 07:03 AM: Message edited by: pitzel ]
