We were asked the following question…
I want to perform some cylinder head and inlet manifold vibration analysis, what should I do?
First we need to consider sensor selection
A 100g accelerometer is probably a bit low for a cylinder head, I have often seen values up to 400g, but it depends on the engine in question.
I’d recommend a higher acceleration range, perhaps 500g on any accelerometer selected. It is however not a simple choice. Too high in the available amplitude range and you’ll loose low amplitude resolution. So the best advice is to try some experiments to confirm the best range.
Temperature will be an issue, especially on the inlet manifold. Most accelerometers will only work up to 150°C (IEPE type). Above that and they will be damaged. Higher temperature options (charge type) are available. It depends on the requirements.
First, decide the frequency range that you want to examine. This is called the measurement bandwidth. For example, if you want to study up to 1kHz then you need 1kHz of measurement bandwidth.
The Nyquist rate is 2 times that value. I would recommend using 2.5 times the measurement bandwidth.
Using a factor of 2.5, a 2kHz sample rate will give you 800Hz frequency range.
So for a 1kHz bandwidth you’d need 2.5kHz sample rate or 2500 samples per channel per second.
In this case, at the 1kHz cut off point, the anti-alias filters will have already attenuated the signal by 3dB (that’s called the 3dB point).
So, therefore, you need to raise the sample rate a little. I’d recommend 4000 or 5000 samples per second per channel for vibration. You can always downsample later in software if you need better low frequency resolution for your frequency analysis.
Data Capture & Measurement – What can you actually get?
When working with engines you’ll need a tachometer input. This will normally be measured on the crankshaft or camshaft. A once per revolution sensor is usually sufficient. A laser sensor is the best but there are many other types available, like infra red or magnetic. You might even be able to use an electronic signal from the coil/electronics.
When you have a vibration signal and a tachometer signal it allows you to measure the vibration levels at different engine speeds. This allows you to see which rotating components are causing the vibration. For example, when the crank shaft rotates once, the cam shafts will rotate twice. Therefore, a vibration that occurs twice per crankshaft revolution is likely to be caused by something to do with cam shafts. The same can be said of various systems in the engine, like pumps or timing gears and so on.
To sum up we need to
- select an accelerometer with the correct amplitude range
- select a sensor that will work at the required temperature
- choose the frequency range we wish to analyze and thus choose a sample rate to measure at
- configure a reliable tachometer signal to measure the rotational speed of the engine
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