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Thread: EGT/CHT vs A/F sensor for Engine Leaning

  1. #1

    EGT/CHT vs A/F sensor for Engine Leaning

    There are now available air/fuel mixture ratio sensors available to aid engine tuning in automotive applications. Aircraft still seem to rely on EGT peaking to get the same result. Since the A/F ratio sensors directly detect the lean/rich condition, isn't this a more accurate approach to engine leaning for peak performance and maximum engine life?

  2. #2

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    For better or worse, in airplanes, the pilot takes the place of the engine computer that cars today have. In order to make use of the technology that you are talking about, you will have to have an engine control computer FAA certified. There are a very few single control engine management systems certified at this point, but given the poor state of the aviation industry and the low sales volume, don't expect to see lots of airplanes flying with them.

    The FAA certification of electronic engine controls for the existing certified aircraft fleet is way too large a task for any of the manufacturers. In the homebuilt world you do see folks working on electronic ignition systems and between that and the electronic engine monitoring systems out there, I would not be surprised to see someone try to combine all that into a system that makes use of the parts that you mention.

    Gluing new technology into airplanes is not for the faint of heart.

    Best,

    Wes
    N78PS

  3. #3
    Wow, had not even thought about airfuel ratio and here I am in the middle of trying to buy a engine monitoring system for my airplane that primarily keeps tabs on the
    EGT/CHT, OP OT etc. I actually installed a AF meter in my sports car about 3 years ago and it shows a digital readout and also has a circular arc of l.e.ds with red yellow
    and green indicators. I will now surely put one of these in my aircraft now, probably not right away but in the near future for sure. First I have to do base line testing
    then add a PowerFlow muffler and retest. Once that is done and all is stable I will install a AF meter . A good one will instantaneously give the readings as I recall and that
    is a narrow or wide band device. Only problem may be the lead in AvGas, I'll check that out first. Anyway, here is an article that I just copied.....worth a look .


    Theoretically, the ideal stoichiometric A/F mixture (the chemically ideal mixture of air and fuel that is required to provide a complete burn) for a properly tuned engine running on pure gasoline is 14.7:1; that is, 14.7 lbs. of air to 1 lb. of fuel. However, because of operating losses in the induction system due to intake runner and cylinder wall wetting, plus the fact that fuel may not fully vaporize in the combustion chamber, a 14.7:1 A/F mixture is often too lean for actual operating needs. A more realistic light-load, cruise A/F mixture for a stock carbureted engine running on reformulated unleaded gasoline is in the 14.1:1 range.
    The A/F mixture always varies from cylinder to cylinder, therefore we tend to tune the average A/F mixture slightly on the rich side to avoid engine misfire in the leanest cylinder. It is possible to target an A/F mixture leaner than 14.7:1 for maximum fuel economy but this can lead to driveabilty problems if any one cylinder is leaner than the others. The power mixture we target for maximum horsepower is in the 12.2:1 - 13.5:1 A/F range, depending on the engine package and its combustion chamber design.

  4. #4

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    Ok... But a car is water cooled.
    Isn't the reason you use EGT / CHT is to also monitor the temp to ensure you don't burn anything up... Right?

    So, you are working to get to the best lean without causing engine damage.

    If you did it purely by air / fuel mixture, wouldn't there be a potential of excess heat ?

  5. #5
    steveinindy's Avatar
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    If you did it purely by air / fuel mixture, wouldn't there be a potential of excess heat ?
    Isn't the reason you use EGT / CHT is to also monitor the temp to ensure you don't burn anything up... Right?
    I'm not an "engine guy" but that's the way it has always been explained to me. Personally, there's a reason why I like FADEC systems so damn much. To me at least, the cost is justified by the reduction in workload in the cockpit and the headache in the build phase (as compared to trying to put a non-standard electronic system on an engine).
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  6. #6
    I have to go along with roxiedog13 - these in cockpit AF meters should help monitor one of the critical engine parameters. I don't think they will replace any of the others (EGT, CHT, etc.) but I have to believe they provide a more accurate and instantaneous view of the A/F ratio. The pilot is still responsible for setting mixture correctly (unless you want to add FADEC), but it just seems more accurate than the ole ' lean of peak/rich of peak' guessing game.

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    The discussion above is based on the incorrect assumption that you always want to run the engine at the "ideal" fuel air ratio. For an air cooled aircraft engine, that assumption is not correct.

    Aircraft engines work harder than auto engines and as alluded to above, they have different issues. Water cooled engines are heavier, but cool the cylinder heads more efficiently. For air cooled engines, heat removal is a problem. So we run aircraft engines watching temps. If you have a turbo for instance, too high and exhaust gas temp will create the need for an expensive replacement.

    The paragraphs above are preparation for explaining that fuel is used to cool the engine. On takeoff, we run excess fuel for cooling. Even in cruise, you may have seen Mr Busch write huge amounts of material about running leap of peak EGT. That is efficient and it creates less heat. Running rich of peak is less efficient but also keeps CHT's OK. You will find no one advocating running at peak EGT, which is a product of the "best" fuel air ratio.

    The problem that engine controller designers have is trying to read the mind of the operator. The engine operator who stays in the traffic pattern doing touch and goes is a different operating profile than the guy to takes off, cruises 3 hours as 12,000' and then lands, which is different than the guy who takes off, spends 40 mins at high power pulling positive and negative G's. I would like to be a fly on the wall of the guy who takes an engine controller to the FAA that includes artificial intelligence.

    So the problem has complexities in multiple dimensions. A mixture meter is not a one size fits all solution and it is redundant to the gauges that we already have. If it floats your boat then put one on your panel, but if you fly a lot, and you do not simply fly the same hour 1000 times, you will likely find its utility limited.

    But building a smart FADEC is a challenge worth attacking. Which one of you has the time and $$ to glue together a EI or JPI unit to something like an Airflow Performance fuel control, an E-Mag, and a computer to make that real for experimental aircraft?

    Best of luck,

    Wes
    N78PS

  8. #8
    steveinindy's Avatar
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    But building a smart FADEC is a challenge worth attacking. Which one of you has the time and $$ to glue together a EI or JPI unit to something like an Airflow Performance fuel control, an E-Mag, and a computer to make that real for experimental aircraft?
    I hate to put this argument out there but why would you (unless you're just trying to save money) want to "reinvent the wheel" when systems already exist for many certified engines? Maybe it's just my unwillingness to screw with the firewall forward aspects anymore than "Fuel line A to connection B...." but it just seems to be kind of more of a headache and potential risk than the benefit most of us would get out of it. The breadth of engines that people use also makes it difficult to extrapolate from one to another so at best you're looking at something that might be useful to a small slice of the experimental fleet and at worst you're looking at something that can wreak havoc on the engine of your own aircraft if you aren't very careful.

    The simplest choice is if you want the top of the line so far as fuel mixture et cetera goes, you pay for it and get yourself a certified engine with a proven FADEC or something along those lines.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



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    I'll bite. Have I been asleep? Have I missed a FADEC system for a Continental O-200? Who makes it? Continental does not.

    Thanks,

    Wes
    N78PS

  10. #10
    steveinindy's Avatar
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    I'll bite. Have I been asleep? Have I missed a FADEC system for a Continental O-200? Who makes it? Continental does not.
    Did I specify that one? I simply said that they are out there for many of the engines we use.

    Honestly, if you're only running a hundred horsepower, why would you need anything but the standard ignition and fuel control system? Adding complexity to one of the most basic pieces of aircraft propulsion gear you'll come across just seems like asking for more trouble than it's worth. I mean, that's a lot of work to replace the carburetor with an injection system and figure out all the other stuff that goes along with the exchange from a traditional fuel metering system. It's worked well (outside of carb icing issues and people not maintaining them properly) for sixty-five years and honestly there aren't many airplanes running the O-200 series that exactly have a high workload given how slowly they fly (the Cassutts notwithstanding but they are more of an anomaly than anything else). If something works well, why screw with it when the alternative is not likely to produce a vast improvement in reliability, safety or efficiency? So far as I am concerned, unless you're doing something beyond doing pattern work or doing $100 hamburger runs there exists no real need for a FADEC or anything more complicated than the standard "throttle/mixture/pitch" controls. Some may want it and think it is worth the effort but we need to be careful in chasing such desires that we do not inadvertently cause more problems than we solve.

    Remember Wes, not all of us are as skilled with engines or the underlying science as you appear to be. In fact, most of us probably have little business doing extreme modifications because of a lack of experience with anything beyond the most basic of maintenance. That coupled with a poor assessment of one's abilities- something many of us are guilty of when it comes to actually turning an idea into a reality- is one of the biggest problems that homebuilding faces.

    We don't want to unnecessarily quash constructive experimentation but at the same time we have a duty to our fellow pilots and homebuilders to keep them from doing anything too outlandish or risky. It is a very blurry line sometimes, especially when you get into the more long-held dreams of homebuilding. The best example of this is the repeated discussion and attempts to produce an economically viable "flying car". Given that we're a group prone towards looking at the "let's make it fly!" side of the issue, we don't like to be reminded that the biggest hurdles are not the technical ones of making something that you can drive down the road and fly (hell, Taylor proved this was possible decades ago). The thing that is going to kill any flying car project is the need to comply not only with the FAA certification standards but with the federal and international regulations on car safety. Couple that with the cost of a "flying car", the widespread fear of flying, the NIMBY a**holes and the fact that not everyone can be trained to be a pilot in a safe and efficacious manner....there goes that dream. But yet, we have the dreamers who keep on...economic viability isn't everything when you've got a mission in mind sometimes.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



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