Electrical system generators.

Discussion in 'Firewall Forward / Props / Fuel system' started by pictsidhe, Jun 17, 2019.

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  1. Jun 17, 2019 #1

    pictsidhe

    pictsidhe

    pictsidhe

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    I've been pondering various UL engine possibilities and one thing keeps jumping out. The generators in current use are awful heavy. A Briggs single cylinder 5A stator I have is 11Oz, the entire flywheel is a foot breaker, but likely has several ounces of magnets. anyone wanting to go direct drive and ditch the flywheel is going to be wondering how to replace the generator.

    While there is no shortage of mechanical skill here, the electrical and electronic side is not so well represented. So, I think I can do better, a lot better. Marketable better.

    I am thinking of a small, light belt driven generator, looking suspiciously like a brushless outrunner RC motor with a ribbed outrunner-pulley. It would have a separate electronics pack to regulate the output to either lead or lithium friendly voltages. Belt driven means the actual generator can be spun fast which makes it lighter. To simplify fitting to engines, the drive pulley will be a simple flat pulley. A cylinder. The generator will need to be attached to the engine by some sort of bracket that can be adjusted for tension. It would prefer to be in cool-ish air. the hotter it gets, the less current it can generate. I am thinking of building in a temp sensor to keep it alive. Some heat soak when shut down will be OK, though below 100C/212F

    What I particularly want to know from you lot is what kind of sizes you'd be interested in and what batteries you'd want to run. A 12V 5A generator should be << 1/4lb and << $100 and do minimal charging and ignition for bare bones aircraft. Obviously, that won't run an XP1000000 full glass panel with 360 degree synthetic vision, so bigger ones too. Likely 10A, 20A, 30A maybe 40A, though that may be into motorcycle alternator territory where I may not be able to come up with something much better.

    Persoanlly, I favour using lithium tool batteries. it will save me having to have a dedicated and pricey battery for my plane. But does mean that I'll have an 18V electrical system. Many devices are 12-24V and are fine with that, others can run from step down adaptors. But the separate electronics pack for the generator means that voltages and battery types can be changed fairly painlessly, possibly by setting some DIP switches.

    Any other comments and suggestions will be entertained.
     
  2. Jun 17, 2019 #2

    BBerson

    BBerson

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    Two amps is enough. Or whatever it takes to run a stock VW distributor ignition and one GPS.
     
  3. Jun 17, 2019 #3

    Hot Wings

    Hot Wings

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    To be marketable it needs to output be 12.8V. I'd personally be happy with 5 amps for a minimalist plane that uses tablets/phones with their built in batteries for auxiliary data.

    While the permanent magnet brushless looks promising I keep coming back to a good old fashioned alternator with a regulator controlled field.
    Rather than a belt and pulley how about the way the old bicycle generators worked? Just rub the thing against the tire, or in this case a simple disk attached to the crank and the rubber wheel on the alternator.

    Another wild idea:
    Use exhaust heat?
    20hp output at 50% efficiency = 15Kw of energy going out the pipe. Tap 10% of that at 4% efficiency for the conversion = .06Kw = 60 watts. @12.8V = 4.7 amps.
    I have no idea how much this might weigh......
     
  4. Jun 17, 2019 #4

    poormansairforce

    poormansairforce

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    Are those bearings designed to run for hours as well as take side loads?
     
  5. Jun 17, 2019 #5

    Dana

    Dana

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    Don't the Briggs engines also use the flywheel magnets as part of the magneto? Or is it electronic ignition powered by the lighting coil (generator)?

    The nice thing about the flywheel generator is it's stupid reliable. Add a belt drive generator and you have more moving parts to maintain or fail.
     
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  6. Jun 18, 2019 #6

    proppastie

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    by the time one designs the brackets for how many different engines.....there might be 5 or so guys that would want to try and adapt your new generator if there were no brackets. Motorcycles had I believe an alternator on one end of the crank to charge the battery. Maybe something similar between the prop flange and engine.
     
  7. Jun 18, 2019 #7

    BBerson

    BBerson

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    Nope. The ignition magnet is on the outside of the flywheel. The generator magnets are on the inside of the flywheel.
     
  8. Jun 18, 2019 #8

    pictsidhe

    pictsidhe

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    Yep, the standard Briggs generator is rock solid reliable, but it's nudging 20lb with the flywheel. Or you can lose 10lb with a $300 billet flywheel, which may or may not have generator magnets. the Briggs ignition is from a seperate magnet on the outside of the flywheel. To use standard Briggs coils, you just need that one magnet.
    Of course, someone could always mount Briggs magnets in a much lighter flywheel and have something between 1 and 2lb. But if you're going to make parts, why not go light? Something like a half VW is will be easier to hang a belt generator on.

    Having some brackets for the common engines would be good. But a custom bracket to mount a belt driven generator should be within the abilities of most homebuilders. The cylindrical pulley probably needs a lathe, but could be improvised without one from a closed end cylinder.

    While most small bikes have a crank mounted permanent magnet generator, many of the higher performance big bikes have a gear driven alternator much like a mini car alternator. They would likely be hard to significantly beat in terms of weight. Though it's been a few decades.

    Currently, I can find 5lb 35A alternators that look suspiciously like belt driven versions of the motorcycle ones I was thinking of. That's still a chunk of weight for a 103.

    I've run some numbers on the bearings, they will outlast the engine if belt tension is set correctly and they are good quality.

    A bicycle generator might be enough for a few watts. I forget the rating. Unfortunately, they are hard to find now. I have a vague memory of plain bearings.

    A wheel drive would work.

    A thermoelectric generator is an interesting idea. They are horribly inefficient, but that isn't a problem with a ton of exhaust heat. The weight may be unpalatable, though. A quick google turned up a 300W 57kg unit. That includes 7.8kg of plutonium. Availability may be limited...

    Another option for power misers would be solar cells. If there is light where you park your plane, a small one is handy to keep a lead-acid topped off.
     
  9. Jun 18, 2019 #9

    proppastie

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    quick disconnect and easy access ......take it home and re-charge it. If you take a trip extra batteries, enough for a day, and a wall wart, to charge in the motel overnight. Probably want Lipro batteries, as spilling acid is not good.
     
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  10. Jun 18, 2019 #10

    Vigilant1

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    For somebody up against a make-or-break mandated weight and a plan to run the engine a few hours max per day, this could make a lot of sense.
     
  11. Jun 18, 2019 #11

    Derswede

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    And you probably would set off multiple NEST detectors as well. Unless you can get a flux capacitor, then the 7.8 lbs of Plutonium would be better served as you could use an airplane as opposed to a Delorean. Most ultralights can hit 88 mph....
     
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  12. Jun 19, 2019 #12

    Daleandee

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    I use a 4lbs (with the regulator), 20 amp Dynamo PM alternator that comes from a John Deere Lawn tractor. Awesomely rugged. On newer Corvair conversions they are mounted on the rear and driven direct from the crank. Mine is front mounted and belt driven. Here's some info about them from Mark Langford's site:

    http://www.n56ml.com/corvair/dynamo.html

    I'm certain this is old news to most builders here ...

    Dale
    N319WF
     
  13. Jun 19, 2019 #13

    12notes

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