Exhaust fumes, any useful information to be had with light?

[Dyslexicbloke]

OK this is probably a little off the wall but ….

I just went to start my genny, its dark so I had a small torch with me and happened to notice that the beam was visible in the exhaust.

Now in the day or under defuse light the exhaust looks completely clean IE no visible smoke but in the concentrated torch light (LED) you can clearly see puffs of scattered light as if you were shining a torch through mist.

The effect is only slight but I find my self wondering if any useful information could be extracted by looking at the absorption or reflection of light through an exhaust stream.

I guess it would be like a pulse-ox meter for an engine ….

I am sure there would be sufficient signal to detect RPM even with simple cheap components and I cant help wondering if measuring the density of exhaust gas particulates might be useful for something else.

Any thoughts? Is it an effect worth further investigation?

Al

[esquilax]

I am sure there would be sufficient signal to detect RPM even with simple cheap components and I cant help wondering if measuring the density of exhaust gas particulates might be useful for something else.

Any thoughts? Is it an effect worth further investigation?

Al

I suspect that anything you could measure optically this way (so not counting exhaust oxygen and useful stuff like that ), you could measure even easier some other way.

If I wanted to measure RPM optically, I'd put a bit of shiny tape on the crankshaft and point my laser/LED and photodiode at that. You would end up with far better SNR than you'd get from the exhaust gas particles.

I'm sure you're just asking hypothetically anyway...

[Ivan]

Yes, I'm sure you could get the rpm from the exhaust pulses. I remember as a child watching cars on frosty mornings as they set off, to see the point where the exhaust coming out of the exhaust pipe was going backwards at exactly the same speed as the car was moving forwards, and so left several individual pulses of steam/exhaust in the cold air.

MOTs test the particulate content of diesel exhausts, and I'm pretty sure it's simply by shining a light through the gas and measuring absorbance. I think the main problem is that the particulates coat the surface of the light, requiring cleaning and/or regular calibration.

You might be able to identify concentrations of some gases by measuring degree of rotation of polarisation if you used a polarised light source.

[Dyslexicbloke]

Yep just musing ..... saw the effect in the torch light and simply had to shine it closer just to see what it looked like ....
I'm sad like that.

I'l be using a reflective shaft endoder for RPM

Didnt kbow about polerised light based sensors though ... I can feel a google session coming on just for fun.

Al

[Ivan]

http://www.springerlink.com/content/326x2453x20l41r1/

This kind of thing. I don't pretend to understand it, though.

[KLD]

Ivan,

Do you have access to the full text article? Somethings a bit odd with what they claim in the abstract. Under total internal reflection (TIR) , both polarisation directions are reflected to 100% according to the Fresnel Equations. There is a way of influencing this, for instance by Surface Plasmon Resonance (SPR), as used in commercial instruments by Biacore and others. In the abstract and first page they only talk about TIR, without any metallic coating?

Klaus

[Ivan]

Sorry Klaus, I don't have access to the article. I just remembered that polarisation is something that can be altered when light travels through certain things. Beyond that, I have no knowledge or understanding.