Heat resistant contact mic? (for engine diagnostics)

[likas]

Background:
I'm running a 111cc two stroke two cylinder engine. It can be temperamental depending on environment and need to find out whether one or both of the cylinders are skipping. I essentially need to check for pressure in one way or another.

Idea:
Contact microphone on the outside of both mufflers seems to be the most plausible option (inside the engine isn't an option and inside the muffler would be quite messy and would probably kill any sensors). However most piezo elements apparently don't stand up to heat because of the glue used for holding the two plates together.

Has anybody come across contact microphones that can handle some heat? Other thoughts?

Engine photo: http://www.dle-engines.com/dleg0111-...g0111-main.jpg
Engine video http://www.youtube.com/watch?v=A42WApgIGEg

[Ron E]

Maybe try an accelerometer fixed to the crankcase?

[likas]

Good idea. Not 100% sure it'll work though - it's a boxer and with a synchronous ignition on both cylinders, so the movements cancel each other out.

Even if it just fires on one, then the other would also move by way of being connected. Though I suppose smaller level vibrations and timings might give that away. Will perhaps try that too.

[CopperTop]

I think an engine knock sensor is what you want. These are piezo sensors designed to be bolted to the engine block:

Bosch - Knock Sensors
http://www.bosch.com.au/content/lang...sors_knock.pdf

[likas]

Engine knock is detonation ahead of ignition by too much compression. This engine is designed to operate on lower octane ratings than are available at gas stations, so it's very unlikely to detonate.

It's just occasionally unreliable and may miss a beat when atmosphere etc conditions change (it's on a RC plane).

But thanks for the suggestion - I wasn't aware of such sensors and it'll come in handy on another project.

[CopperTop]

Originally Posted by likas

Engine knock is detonation ahead of ignition by too much compression. This engine is designed to operate on lower octane ratings than are available at gas stations, so it's very unlikely to detonate.

It's just occasionally unreliable and may miss a beat when atmosphere etc conditions change (it's on a RC plane).

But thanks for the suggestion - I wasn't aware of such sensors and it'll come in handy on another project.

Yes, but these particular knock sensors are simply like microphones with a frequency response of 1-20kHz I think it says. Some knock sensors are mechanically tuned to the knock frequency, but I think these ones are broad band and it is up to the electronics to filter everything but the knock frequency when used for knock sensing.

As the Bosch blurb says:

"The main areas of application for these sensors include:

Knock control for internal combustion engines.
Machine tool protection.
Cavitation detection.
Monitoring of pivot bearings.
Anti theft systems. "

So it seems that you can use them for applications other than knock sensing, and they are designed to be robust enough to be bolted to the engine.

[Andrew Eckhardt]

You could put a condenser mic on a mechanics stethoscope.

You could mic the exhaust and get data with crank angle or ignition pulses.

[likas]

Originally Posted by CopperTop

So it seems that you can use them for applications other than knock sensing, and they are designed to be robust enough to be bolted to the engine.

I missed that part, that's useful. Many thanks for pointing it out!

[Original Burnedfingers]

I'm running a 111cc two stroke two cylinder engine. It can be temperamental depending on environment and need to find out whether one or both of the cylinders are skipping. I essentially need to check for pressure in one way or another



What? You don't check your plugs? You can certainly see if you are dropping a cylinder by looking at the tach. Pull a plug wire and watch it drop or quit. Take a compression test before starting it up to make sure the cylinders are not hurt. Things that can cause missing are mechanical damage(loss of compression, fuel(lack of), ignition Weak or loss of) ignition spart plug,wiring. If you are unwilling to check the obvious then invest in a temp gauge that will monitor the exhaust temp. An air fuel sensor and gauge will show a lean or rich condition. The sensors can be put down stream in the exhaust pipe.

[likas]

I do check plugs, carb filter on occasion etc. It's time consuming, but helps.

What it doesn't help with well is carb tuning depending on changing conditions in this climate (extra complicated because of changing altitude - it's on a plane). Two strokes are hard on air-fuel ratio meters, especially when wishing to install it permanently.

Good permanent diagnostics would help making life a lot more carefree. If a cylinder drops totally, that's easy to detect. If one intermittently skips (while on the same timing and carb with the other one), then it isn't easy to detect. Once I have diagnostics set, I'm looking to dynamically correct carb settings in-flight.

Temp gauge suggestion is good - I'll add that as well. Thanks.