Using Oxygen Sensors
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Using Oxygen Sensors
STANDARDIZED COMPARISONS -
One of the difficulties when comparing the running of two different engines [or the same engine built by two different builders in two different places] is knowing whether they are really running the same. I have proposed before that we set some standard for running so they can be compared, but the trouble is accurately estimating identical gas condition in the running engine. If we could accurately measure this, though, we could eliminate the guesswork associated with comparing 'maximum thrust' or what constitutes 'lean running'.
It turns out there are some places on the 'net that sell automotive oxygen sensors for less than $40 US. These are normally mounted in the car's exhaust manifold, so they're made to take the heat and gas mixtures associated with combustion. In one of our engines, you'd have to put them in a location where you could assume practically complete combustion, but not so far aft that they would be influenced by the air pulled back into the tailpipe, so in most engines the only logical place would be in the tailpipe just aft of the exhaust nozzle - not a very good place for keeping the flow undisturbed, but what else can we do? A threaded fitting would need to be provided, but that's no more complicated than a spark plug mount.
Using the device is simplicity itself: The device creates a voltage depending on the amount of oxygen freely available. This can be measured by a sensitive voltmeter. An extremely lean mixture provides a voltage of about 0.2 volt, and extremely rich mixture about 0.9 volt. 0.5 volts indicates a supposedly optimal mixture [for a piston engine, that is].
Our engines will normally be run on the lean side, except for a situation where we're really pushing for absolute maximum thrust [gas hog mode]. We would simply find a voltage reading that corresponds to what we consider good lean running of some commonly built engine type. This would become our Gold Standard for how a pulsejet is run for comparison purposes, and fuel input would be tuned to hit that voltage before thrust and thrust specific fuel consumption are measured. Since they're a little expensive, one could be used for several engines, if you're only testing one engine at a time. Presumably, a short metric bolt of appropriate size could be found to use as a plug when the sensor isn't wanted.
There are some negatives to the use of these devices:
- The device is rather heavy, big and clunky, for small engines.
- The part that's in the exhaust flow stream is fairly streamlined, but probably a lot to put in the way of the flow on a small engine.
- A good voltmeter that can read accurately in the 0-1 volt range is needed.
- Connection leads to the device will need to take exposure to high temperature radiant heat, or be well protected.
ANOTHER USE: FLAMEOUT PREVENTION -
An engine actually used for propulsion could benefit from this device, as follows. A sensor in good condition is supposed to react very quickly to changes in oxygen level. The voltage shift could be used to control a fast-acting servo that would provide 'air throttling' to prevent flameout of an engine that is suddenly throttled back in flight [i.e. the fuel input is suddenly reduced under high air input conditions, creating a super-lean condition]. The servo could cause a choking or shunting action to radically reduce excess air as soon as the change is detected.
COMMENTS? Criticisms? Suggestions?
L Cottrill
One of the difficulties when comparing the running of two different engines [or the same engine built by two different builders in two different places] is knowing whether they are really running the same. I have proposed before that we set some standard for running so they can be compared, but the trouble is accurately estimating identical gas condition in the running engine. If we could accurately measure this, though, we could eliminate the guesswork associated with comparing 'maximum thrust' or what constitutes 'lean running'.
It turns out there are some places on the 'net that sell automotive oxygen sensors for less than $40 US. These are normally mounted in the car's exhaust manifold, so they're made to take the heat and gas mixtures associated with combustion. In one of our engines, you'd have to put them in a location where you could assume practically complete combustion, but not so far aft that they would be influenced by the air pulled back into the tailpipe, so in most engines the only logical place would be in the tailpipe just aft of the exhaust nozzle - not a very good place for keeping the flow undisturbed, but what else can we do? A threaded fitting would need to be provided, but that's no more complicated than a spark plug mount.
Using the device is simplicity itself: The device creates a voltage depending on the amount of oxygen freely available. This can be measured by a sensitive voltmeter. An extremely lean mixture provides a voltage of about 0.2 volt, and extremely rich mixture about 0.9 volt. 0.5 volts indicates a supposedly optimal mixture [for a piston engine, that is].
Our engines will normally be run on the lean side, except for a situation where we're really pushing for absolute maximum thrust [gas hog mode]. We would simply find a voltage reading that corresponds to what we consider good lean running of some commonly built engine type. This would become our Gold Standard for how a pulsejet is run for comparison purposes, and fuel input would be tuned to hit that voltage before thrust and thrust specific fuel consumption are measured. Since they're a little expensive, one could be used for several engines, if you're only testing one engine at a time. Presumably, a short metric bolt of appropriate size could be found to use as a plug when the sensor isn't wanted.
There are some negatives to the use of these devices:
- The device is rather heavy, big and clunky, for small engines.
- The part that's in the exhaust flow stream is fairly streamlined, but probably a lot to put in the way of the flow on a small engine.
- A good voltmeter that can read accurately in the 0-1 volt range is needed.
- Connection leads to the device will need to take exposure to high temperature radiant heat, or be well protected.
ANOTHER USE: FLAMEOUT PREVENTION -
An engine actually used for propulsion could benefit from this device, as follows. A sensor in good condition is supposed to react very quickly to changes in oxygen level. The voltage shift could be used to control a fast-acting servo that would provide 'air throttling' to prevent flameout of an engine that is suddenly throttled back in flight [i.e. the fuel input is suddenly reduced under high air input conditions, creating a super-lean condition]. The servo could cause a choking or shunting action to radically reduce excess air as soon as the change is detected.
COMMENTS? Criticisms? Suggestions?
L Cottrill
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Re: Using Oxygen Sensors
Hi,
Oxygen sensors vary in output voltage with both temperature and air/fuel ratio. Calibration at a pulsejet-specific temperature would be necessary for accurate results. A temperature sensor ( thermocouple ) should be placed very close to the oxygen sensor, to provide the necessary data for calibration.
Al Belli
Oxygen sensors vary in output voltage with both temperature and air/fuel ratio. Calibration at a pulsejet-specific temperature would be necessary for accurate results. A temperature sensor ( thermocouple ) should be placed very close to the oxygen sensor, to provide the necessary data for calibration.
Al Belli
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Re: Using Oxygen Sensors
Thanks, I was wondering about some of the details of these sensors. I think it's an essential sensor for any serious test stand. I didn't know it was so easy to get data from. Kind of amazing, really.
Mike Often wrong, never unsure.
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Re: Using Oxygen Sensors
It's also interesting to see that the device is only linear over a certain range! Note, however, that in that linear range, it's remarkably close to the same line for all temps! It's only out at the ends that the temp really messes you up. But, it could be that's where you'd have to operate in a lean-running pulsejet. Some experimentation required.
Thanks, Al!
Man - how/where do you come up with this stuff?
L Cottrill
Thanks, Al!
Man - how/where do you come up with this stuff?
L Cottrill
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Re: Using Oxygen Sensors
Ooh ... another thing I just noticed:
The thing is almost a snap-action switch, in terms of air/fuel ratio. That is, there is only a range from 14.5 to 15.0 that lies between the knee of the curve and the foot! That's not much useful range - 14.5 to 15 is pretty rich for pulsejet combustion, though probably not bad enough to disqualify it as a standard. It's probably a mixture that can be run on just about any fuel.
Note that at about 0.25 volts, the curves all come together, i.e. the temperature differences iron out and that setting would give you an accurate setting for easy comparison. As long as that isn't ridiculously rich, there's no reason that couldn't be our Gold Standard value, meaning just a hair below 15:1 ratio for hydrocarbon fuels.
Note also that the steepness of the curve in the linear range means that it may be pretty doggone hard to achieve and maintain precisely that setting - the slightest tweaking or variation in fuel flow will yield a huge percentage of change in the voltage. That's good for accuracy - but bad for manageability. This is going to be especially true for small engines, where settings are pretty touchy as it is. A decent needle valve and good pressure regulation should get it, though, in most cases.
Does anybody have an opinion as to whether this is worth doing? Here's our chance, guys, to set our own international standard for an issue that's pretty important to almost everybody that's really serious about jets ...
L Cottrill
The thing is almost a snap-action switch, in terms of air/fuel ratio. That is, there is only a range from 14.5 to 15.0 that lies between the knee of the curve and the foot! That's not much useful range - 14.5 to 15 is pretty rich for pulsejet combustion, though probably not bad enough to disqualify it as a standard. It's probably a mixture that can be run on just about any fuel.
Note that at about 0.25 volts, the curves all come together, i.e. the temperature differences iron out and that setting would give you an accurate setting for easy comparison. As long as that isn't ridiculously rich, there's no reason that couldn't be our Gold Standard value, meaning just a hair below 15:1 ratio for hydrocarbon fuels.
Note also that the steepness of the curve in the linear range means that it may be pretty doggone hard to achieve and maintain precisely that setting - the slightest tweaking or variation in fuel flow will yield a huge percentage of change in the voltage. That's good for accuracy - but bad for manageability. This is going to be especially true for small engines, where settings are pretty touchy as it is. A decent needle valve and good pressure regulation should get it, though, in most cases.
Does anybody have an opinion as to whether this is worth doing? Here's our chance, guys, to set our own international standard for an issue that's pretty important to almost everybody that's really serious about jets ...
L Cottrill
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Re: Using Oxygen Sensors
Just one more little thing:
The location I've described above to mount this device is where the experiments Ben and I did showed heavy oxidation and erosion of materials in the Dynajet !!!
As I always say, some experimentation required ...
L Cottrill
The location I've described above to mount this device is where the experiments Ben and I did showed heavy oxidation and erosion of materials in the Dynajet !!!
As I always say, some experimentation required ...
L Cottrill
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Re: Using Oxygen Sensors
Hi Larry,
I keep any interesting information, that I run across, that I feel I might be able to use in My "tinkering". I also have been an avid reader of technical literature and have an extensive collection of technical books. My experience in product and process design has exposed Me to many varying disciplines. I am always glad to help anyone with any information that I might have.
Al Belli
I keep any interesting information, that I run across, that I feel I might be able to use in My "tinkering". I also have been an avid reader of technical literature and have an extensive collection of technical books. My experience in product and process design has exposed Me to many varying disciplines. I am always glad to help anyone with any information that I might have.
Al Belli
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Re: Using Oxygen Sensors
I'm not sure I agree about the need for air/fuel ratio measurements.Larry Cottrill wrote:STANDARDIZED COMPARISONS -
One of the difficulties when comparing the running of two different engines [or the same engine built by two different builders in two different places] is knowing whether they are really running the same. I have proposed before that we set some standard for running so they can be compared, but the trouble is accurately estimating identical gas condition in the running engine. If we could accurately measure this, though, we could eliminate the guesswork associated with comparing 'maximum thrust' or what constitutes 'lean running'.
The pulsejet automatically maintains its own A/F ratio, except at/near rich extinction. This condition would be sensed by SFC curve deflections during that portion of testing.
After you know what the A/F ratio is, there's not a darned thing you can do about it in a pulsejet!
Bill H.
Acoustic Propulsion Concepts
".......some day soon we'll be flying airplanes powered by pulsejets."
Re: Using Oxygen Sensors
About 15 years ago, I lived in Las Vegas and my car failed the emissions test. I put some methanol additive in my gas tank and the same guy said this time my car was running as clean as a brand new engine. I'm a criminal.
Mark
Mark
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Re: Using Oxygen Sensors
Hmmm ... is this always true, regardless of geometry, etc.? It is always true, within high and low limits of air flow?hinote wrote:The pulsejet automatically maintains its own A/F ratio, except at/near rich extinction. This condition would be sensed by SFC curve deflections during that portion of testing.
After you know what the A/F ratio is, there's not a darned thing you can do about it in a pulsejet!
It seems to me if the engine was this good at self-adjustment, all our engines [that run] would be widely throttleable, wouldn't they? I'm thinking they would all be able to set up optimal explosions for any fuel volume delivered, over a wide range. But doesn't that turn out to not be true, especially for changes in air intake geometry and fuel delivery setup?
And suppose it is true as you say, "except at/near rich extinction" - doesn't that put us in the place where everyone's trying to get "maximum thrust", somehow, without regard to the actual operating condition? What then SHOULD we do to compare how different builds or different designs are operating?
L Cottrill
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Re: Using Oxygen Sensors
Makes me think we need to be able to adjust air flow as well as fuel. Like making a chinese with two intakes, that will run on one, and put a butterfly valve in the other. Mmmm, that would be cool.
Mike Often wrong, never unsure.
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Re: Using Oxygen Sensors
Unless misunderstood, I have it on good authority.Larry Cottrill wrote: Hmmm ... is this always true, regardless of geometry, etc.? It is always true, within high and low limits of air flow?
That would be true, if it were the only variable that affected throttleability.Larry Cottrill wrote: It seems to me if the engine was this good at self-adjustment, all our engines [that run] would be widely throttleable, wouldn't they?
The old ogre of fuel delivery is responsible for a lot of problems, as well as designed-in inflexibilities.
I have been fortunate to see turndown ratios of at least 8:1 in each of my 3 successful valveless pulsejets--I believe this should be possible in any good design. Larry, even your FWE is displaying good ratios, isn't it?
That's why I harp so much on measurements and testing; there's no use just burning as much fuel as possible--the engine is well past its max level of efficient power, and will only put out slightly more thrust than the max thrust with stoichiometric F/A ratio.Larry Cottrill wrote: And suppose it is true as you say, "except at/near rich extinction" - doesn't that put us in the place where everyone's trying to get "maximum thrust", somehow, without regard to the actual operating condition? What then SHOULD we do to compare how different builds or different designs are operating?
If each designer/builder were able to present a TSFC curve for his engine as a result of real-life tests, then we would have something to discuss.
Then he could say, "I can produce this amount of max thrust at this fuel consumption", and, "I can produce this amount of thrust at this most efficient point on the TSFC curve".
Even the small engines (such as for model airplanes) need to have such information. The flier is going to want to know how much thrust his engine can produce (so he can design his model around that power) and what the fueling rate is (so he can build-in the appropriate amount of fuel capacity).
My current project (the mini-Escopeta) will soon be fully documented in this regard, AFTER which I will begin to apply modifications and/or "improvements", and record how they affect thrust and fuel flow.
This is the only real way to educate ourselves about how these things respond.
Bill H.
Acoustic Propulsion Concepts
".......some day soon we'll be flying airplanes powered by pulsejets."
Re: Using Oxygen Sensors
Here's a spark plug that might work to see how hot a pulsejet is running in an exact spot or region.
http://cgi.aol.ebay.com/ebaymotors/ws/e ... otohosting
Mark
http://cgi.aol.ebay.com/ebaymotors/ws/e ... otohosting
Mark
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Re: Using Oxygen Sensors
I think you'd burn it up. IC engines are a much more benign environment.
Mike Often wrong, never unsure.
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Re: Using Oxygen Sensors
The spark plug unit would probably be OK if it is quartz in the plug, the plug would melt before the quartz, however the optic cable attachment, if that's how it works, might overheat quickly. Perhaps an anti-fouling adapter on the plug to stand it out and away from the pulsejet more would work. It could act as an extention.
I've seen some high temperature viewing glass fittings on eBay, maybe you could use part of the above device with a simple viewing "monocle" screwed into the side of the pulsejet?
Mark
I've seen some high temperature viewing glass fittings on eBay, maybe you could use part of the above device with a simple viewing "monocle" screwed into the side of the pulsejet?
Mark
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