Vibrations Monitoring as a Predictive Maintenance Tool for Reciprocating Engines
The vibrations signature of a four-stroke four-cylinders inline
engine was measured. An acceleration transducer was mounted on the engine
block and while the engine was running the output signal was amplified
and filtered with an anti-aliasing filter. The measurements were stored
on the hard disk of a host computer after being converted by an analog-to-digital
board. To perform an accurate FFT analysis, the sampled data had to be
synchronized with the position of the crankshaft. This was achieved without
complicated auxiliary equipment by synchronizing the data with samples
that were acquired simultaneously from the primary coil line.
Parameters such as consistency of measurements, location and direction
of vibrations transducers, and the required frequency band were examined
during the experiments.
During normal operating condition the primary harmonic component
appears at twice the revolution frequency which is, essentially, the combustion
frequency of the engine.
When one of the spark-plugs is disconnected so that the cylinder doesn’t
undergo combustion, the primary harmonic components appear at one half
and at three halves the revolution frequency.
Owing to the ability of the engine’s block to oscillate in response
to the crankshaft motion when one of the engine’s supports is released,
a severe harmonic component at the revolution frequency appears.
Appreciable harmonic components can be distinguished at one half, three
halves and five halves the fundamental frequency when a small amount of
ambient air leaks into the inlet manifold. There is also a severe dispersion
of energy in the sub-harmonic range of frequencies and in the high frequency
range of eight to ten times the revolution frequency.
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