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I have a Rotax 914, a type of aircraft engine which is basically a 1980s turbocharged carbureted 4-cylinder boxer. There is no knock sensor, but considering the absolute devastation that detonation can cause it seems a knock sensor would be useful. Even though I couldn't close the loop by adjusting spark plug timing, at the very least it could warn of impeding doom by alerting pilots to a knock situation.

However, while there are many human-in-the-loop solutions, I cannot find any good information on permanent DIY knock sensors. There are chips to help, e.g. https://www.ti.com/lit/an/slyt580/slyt580.pdf, but I haven't found an A-to-Z guide. What would the process be to add a knock sensor?

Is it possible to do this without a specific hardmount, perhaps with a hose clamp or appropriate adhesive?

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Some of the knock sensors are designed to fit into a bolt hole.

Some are designed as bolt through ie a "ring" for the sensor where the bolt goes through.

Either way, the issue will be having something to read the sensor and then display the result in a meaningful fashion.

I had friends design (student engineers find lots of ways to apply their studies :) :) ) a simple circuit once to use a Hall effect sensor to drive the rev counter that was designed for a coil input.

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A knock sensor is an accelerometer, with a restricted bandwidth. The location and the bandwidth and the sensitivity/threshold are important. For a production engine we run it at light knock (detected with a sophisticated apparatus consisting of a piece of brake pipe jammed into a plastic coffee cup at each end) and do a survey of the practical locations on the cylinder head, at various speeds. Here's the instructions for a DIY sensor https://www.spoolstreet.com/threads/independent-knock-feedback-knock-ears.6504/ There were many hits for DIY knock sensor, that was the first.

You can use dental cement https://www.hbm.com/de/2961/dehnungsmessstreifen-klebstoffe-x60/ to stick the knock sensor to the head. Ordinary glues are not stiff enough typically, although if you've got a nice flat surface epoxy will probably do.

Sorry I can't directly help with designing the readout box, it's basically going to be a tunable filter and threshold, and some sort of warning light.

The difficulty with tuning the system is that you are detecting the high frequency resonances of the head and block, and anything impacting the block will set those off, not just pre-ignition. So a rattly old engine will set the sensor off.

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  • $\begingroup$ Thanks for the answer, I am specifically looking for permanent DIY solutions, and have updated my question to make that clear. Very neat to know that this can be done so easily with a coffee cup and a human ear. $\endgroup$ Commented Nov 23, 2023 at 0:05
  • $\begingroup$ Dental cement is permanent! $\endgroup$ Commented Nov 23, 2023 at 2:21

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