rv6ejguy said:
I believe many of the early widebands used the free air method to calibrate however the later sensors use a precision resistor located in each sensor connector which the box can talk to so there is no more "group" calibration. A magazine test a couple years ago found a whole point variation in AFR between free air calibrated wideband units. I couldn't find any reliable information discussing variations with altitude in any Bosch literature I have.
Here is the basic info supplied from Bosch:
http://www.wmsracing.com/o2/tech.htm
No, the resistor has always been in the Bosch wiring harnesses. Remember, the sensors were built for a mass production application. The trim resistor essentially places stoich at the proper point. The assumption is then made that the sensor can generally be fit to a calibration curve that was established empirically with a sampling of like sensors.
The Bosch sensor with the Bosch driver does what Bosch says, but it is not a high speed, precision wideband controller. Again, I'd send you to the Megasquirt site. Bowling and Grippo started working on a precision wideband controller some years ago. They lay out pretty clearly why the stock Bosch solution was not sufficient for their needs and put together some very nice documentation on the sensor technology itself. As I've said, I have some questions about the practicality of their measurement principle. If you look at their calibration page, you will see what I mean. Each sensor must be calibrated with precision gases and they have a Windows application to help you factor in atmospheric factors in calibrating the lean side with free air.
I am not endorsing the Megasquirt do-it-yourself EFI! But not only has their 'open' work on precision wideband measurement been interesting, they have made practical demonstrations of the limitations stock implementations like the Bosch one you referenced with their AFR auto-tuning features. Users can successfully close the loop with a precision wideband controller like ours, but not with the stock Bosch driver. Understand, regardless of how fast someone samples the outputs of the Bosch chip, measurements themselves are limited to about one every 100 to 200 mS because of the measurement principle used. And, the sensors virtually never fit the fixed calibration curve exactly, so measurements always become less accurate the farther you go from stoich.
I'd be glad to go into why all Bosch sensor applications care about changes in partial pressure, but I'm already writing a book here! In brief, it is a consequence of the way Lambda is actually measured. Like sensor to sensor variation, the impact on accuracy becomes greater as you move from stoich. You don't free air calibrate because you 'have to', for example, we could easily use a constant like Bosch. You free air calibrate when you are looking for maximum precision from a given sensor under specific atmopheric conditions.
I would be interested in any magazine article you still have. Again, I'm falling into ad mode, but we believe strongly in our patented measurement principle and have kicked serious tush in every speed/precision shootout that I am aware of over the last few years (including some big dollar lab stuff).
Shifting gears...
*I am very sorry to have seemingly annoyed so many people in such a short time!!!*
I'm extremely interested in your work with experimental aircraft EFI. I'd love to see some of your logs. Since you have a bung installed, etc. I'd also be happy to arrange for a loan of a wideband controller and sensor for you to play with. Just email me with the lambda or AFR range you are using (tell me what stoichiometric multiplier you are using for 100LL if AFR) for your analog input and I'll pre-configure it for you.
*Regarding leaning (again)!!!*
I'm not encouraging anyone to Best Power or Stoich at max power. I started with the context of racing, and edging towards best possible power.
In the auto racing world, this has been done for years with EGT and stories like Mr. Atkinson's are common. I'm NOT saying that this is what happened to Mr. Atkinson in Reno, but the typical scenario goes something like follows:
* The tuner tries to get as close to Max Power as possible without the engine self destructing
* One cyl (or more) is slightly leaner, hence closer to peak pressures and temperatures
* That cyl starts to detonate
* EGT goes down slightly
* The tuner interprets this as indicating a richer mixture than desired
* The tuner leans a little more, taking things closer to peak pressures and temperatures...
Right or wrong, I translated this scenario to my own GA experiences. When the mixture knob is all they way in, the engine is almost always running needlessly rich. This is not nec. a bad thing, better too rich than too lean, but in a high performance application you would want to bring this back to the optimum level that is sustainable by the engine.
We do this in, say, a new 206. We go to a fixed fuel flow for takeoff in a new T206, and use the placarded fuel flows for altitudes on a non turbo 206 for "Max Performance Takeoffs".
But this is fairly imprecise and, again, typically errors on the side of rich. My only point, really, was that I'd want an O2 sensor to tweak to the optimum point for a max performance application. I would not trust absolute EGT in this case, because there is not a really good way to derive it, and I wouldn't trust fuel flow alone. I'd actually want to combine individual cyl lambda with individual CHT and EGT, since it would give me more clues about preignition and detonation, but that is almost another discussion.
My preference for true lambda measurement probably comes from my work. If you have a big hammer, the world seems full of nails... But I did try to point out the limitations for aviation applications!
BOBM: Over 5000', a normally aspirated engine can't generate enough power to easily do harm with the red knob. That is why POHs have long been OK with it. Leaning for cruise at any altitude is a (relatively) new addition. Engine makers presumably bowed to pressure from plane makers - who care about speed and range. If you head to Mammoth Lakes, Tahoe, Flagstaff, etc. in the summer - check density altitude and performance and definately lean for takeoff.
gmcjetpilot: I hope some of the top of this covers your questions about O2 sensing. We can discuss pros and cons, but I'm trying to not hopelessly hijack the thread *and* tick everyone off! My comments about tolerances relates to manufacturing quality. I had a horrific experience with a factory reman'ed engine. Lycoming did replace it, since there were some obvious manufacturing flaws. But I still think a lot of engines fail prematurely because of things not to being manufactured to proper tolerances.
Mr. Atkinson: Again, I'm sorry to have irritated you.
In fact, I'll extend that to everyone. I am interested in the use of O2 sensing in GA, especially with 100LL at $4.50 at my home airport and the whole future of 100LL up in the air. But I'm not even completely convinced that it is either widely practical or beneficial.
My main interest is, as a long time GA pilot, if I can realistically build an RV-8 and if 180-200 HP is enough. I recently transitioned back to conventional gear after years of tricycle, but my midlife crisis is still not fully appeased. An RV-8 sounds like a possible ticket and aircraft construction and snap rolls are both a lot safer (at least in my house) than a girl friend half my age!
-jjf