The analysis of dynamic engine vibration and the accurate measurement of angular vibration is a non-trivial task, as a more in-depth analysis of boundary conditions reveals.
Tools for engine vibration analysis
A number of different solutions are available on the market
systems based on continuity in time, like laser vibrometers
systems based on event detection, like the passing of teeth of a toothed structure
others based on an (inefficient) continuous high-speed time-based acquisition
or (more efficient) high time resolution counter/timer based data acquisition
to mention the most popular ones.
However, there is no ‘magic bullet’ to hand, and no one solution performs best under all conditions, no matter what any ‘solutions provider’ pretends.
A practical example of engine vibration analysis
This author has gained many years of experience with many sensor concepts, data acquisition systems and data analysis software solutions and writes from that experience. The findings were often unsatisfactory, and not many packages actually provide true fidelity (i.e. accurate and accountable results in angular vibration measurement and analysis).
This application note covers an aspect of rotational analysis where high fidelity, accuracy and credibility are paramount. That is high-performance engine analysis. We will show how to conduct joint data analysis from various types of inputs.
First, the determination of angular vibration from toothed structures, e.g. gear wheels, utilizing VRS (variable reluctance sensors) coupled to tacho signal channels on a Prosig data acquisition system.
Second, input from several analogue signals sampled concurrently, sensing data, like cylinder pressure information with the 24-bit resolution ADC channels.
Third, imported data from the vehicle’s CAN bus merged into the acquired dataset.
For many years and with the advent of extensively programmable motor management electronic control units (ECU), it became “en vogue” to modulate the engine cycle parameters for several purposes.
From the acoustical point, designated ‘soundscapes’ addressing the desires of sporty motor enthusiasts were created featuring a racy, fierce growl and all sort of designed engine exhaust sounds.
For vehicle handling and traction control, vibrations can be induced, giving the driver a better “feeling” for controlling the vehicle with abundant power and limited tire grip.
But especially with today’s high-performance engines, for their high dynamics and low mass and inertia designs, these programming attempts can have severe side effects, mostly unimagined, unintended, unnoticed, and underestimated.
The following data analysis shows that the engine cylinder combustion cycle’s individual programming can, and usually will, lead to angular vibration in the powertrain.
Oscillation of gear flanks can lead to abnormal wear, surface deterioration, material defects, unexpected engine vibration and, for severe and long-lasting cases, the breakage of bearings and shafts.
In Figure 1, the detailed speed signal from two rotating shafts inside an engine is shown, and the cylinder pressure signals.
Detailed speed is a little more accurate than the average speed, and frequencies are already discernible as nonuniformities over time.
Here, speed signals calculation from pulses detected from tooth passing of rotating gear wheels (ppr: pulses per revolution) is performed and shown as a resampled time signal combined with pressure signals, which were initially time sampled signals.
Figure 1 observes the short intervals of speed ramping (upper view) versus controlled cut-off for one cylinder (lower view – four-cylinder pressures), thus creating a distinct vibration pattern, detectable as speed variation.
Figure 2 shows an engine-run with phases of different throttle positions and modulation of engine cycle parameters.
Note the somewhat unusual representation of data in the “rotational angle” domain, instead of velocity, as it shows more intuitively the oscillatory behaviour (“angle retarding or advancing”) superimposed on the “steady” rotation of the rotating wheels, which can give you the picture of a “grinding process” on tooth flanks of the gear wheels and stresses on shafts.
In Figure 2, the upper view shows a colour-coded spectrogram, showing cylinder pressure vs engine cycle (720° crank angle) vs evolved engine cycles (800 x 720° deg crank angle): designated identification of distinct, cut-off cylinders. The lower view shows the angular vibration of the crankshaft.
The modulation of the engine cycle by cutting-off cylinders creates varying low-frequency crankshaft rotational vibration. The rotational vibration propagates through the power train (Figure 3).
Using Prosig DATS systems
As we have discussed, dynamic engine vibration analysis is not simple and requires a pragmatic and nuanced solution. The flexibility and depth of analysis tools in DATS make it an ideal tool for this task.
Prosig data acquisition systems are modular, offering possibilities for multi-channel signal acquisition sampled in the time domain and utilizing counter/timer based signal acquisition, e.g. detecting the passage of gear wheels with toothed structures or similar operating in the rotational or angular domain.
Engineers and technicians working in the field of rotational vibration will be able to judge the fidelity of DATS software, providing rotational vibration analysis from these brief visualizations.
The DATS Analysis Rotating Machinery Suite comprises several so-called “modules”, basically data analysis algorithms employed to perform dedicated angular vibration analysis tasks.
Prosig are experts in the measurement and monitoring of noise and vibration. They provide data capture and analysis systems for a wide range of applications with particular focus on noise & vibration, NVH and acoustics for the automotive, aerospace and power generation Industries. The company designs and develops its own products and its engineers have decades of experience in solving real-world noise and vibration problems for major organizations throughout the world.