Measuring engine torque

Discussion in 'Firewall Forward / Props / Fuel system' started by ArcticDave, Aug 19, 2017.

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  1. Jun 9, 2019 #41

    Armilite

    Armilite

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    If you want to know what HP your Engine is making at different Rpm, just Buy Desktop Dyno ($48 on eBay) and Input all your Engine Spec's.
    http://www.proracingsim.com/desktopdyno.htm

    If you want to know your Props Static Thrust Numbers at your Engines Max Rpm, enter your Prop Lenght & Pitch Specs, it's Max Rpm if using a Reduction Drive, Temperature, Type & Number of Blades in this Simple Calc. All you need is a Simple Tach to verify your rpm.
    http://godolloairport.hu/calc/strc_eng/index.htm

    Desktop Dyno your Engine Spec's page you fill out.
    DESKTOP DYNO INFO NEEDED.jpg
     
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  2. Jun 9, 2019 #42

    proppastie

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    be interesting to see if the real numbers are the same as the what this software says.
     
  3. Jun 14, 2019 #43

    ArcticDave

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    Well I can confidently say, this doesn't work quite as I intended. Even a 3/4 throttle run with a 55x22 prop... the needle barely budged at 4k rpm, getting nowhere near where it should have been pointing. Good thing I needed a stand anyway and it didn't cost anything extra!
     
  4. Jun 15, 2019 #44

    Vigilant1

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    Sorry it didn't work out. I scanned this thread briefly--did you consider and reject the idea of using Eiffel's technique of spinning precisely measured clubs? That will tell you what the engine can do at any RPM (so, it's better than using a prop in this respect)
     
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  5. Jun 15, 2019 #45

    ArcticDave

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    I think I have to. The engine seems to be producing a little more power than the 40 hp I was anticipating...and the prop is sized for. Before I send this propeller back, I'll try building a wood bar to run on it. Thanks for reminding me of that! I had forgotten it completely.
     
  6. Jun 15, 2019 #46

    proppastie

    proppastie

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    did you run the 40 hp on the stand before you upgraded the engine?....have you calibrated the "needle"....did you get more RPM than the 40 hp? One data point is not a good comparison.
     
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  7. Jun 16, 2019 #47

    ArcticDave

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    Indeed. I agree, one data point is not enough. I have the "wood bar dyno" spreadsheet that was posted on the Eagler's Nest forum. Looking it over, it appears a 34" bar that is 2.27" square should be the right size for testing maximum output. A few shorter bars, with smaller cross sections...would help define the torque curve.

    In answer to your questions:
    A) The 40 hp # was pure SWAG. Going off the HP ratings of Mr. Casler's fine engines...I figured 40 HP was a nice conservative estimate for a 94 x 82 stroker engine at our elevation here(2000 ft). The engine was entirely built by me. The last time it ran...it was running on all 4 cylinders(at least until the exhaust valve broke on #2 and it blew a hole in the piston.)
    B) Yes, the needle was calibrated with a scale. I based the calibration weight off the (T)orque=(F)orce x (L)ength equation.

    After some thought, I believe my basic premise of measuring torque in this fashion may be flawed. It will only measure the excess torque, produced above and beyond what is required to turn the propeller. Change the prop, and the numbers would change.

    I can't deny the temptation to just put this in a bucket and ***k it.o_O

    I know a 40hp prop will fly the little bird I'm building well enough. It would be easy to make a throttle stop to limit rpm to reasonable number. The engine would certainly run cooler not being loaded as hard, but can I walk away and leave a possible performance gain on the table? Hahaha
     
  8. Jun 16, 2019 #48

    Vigilant1

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    Some observations, this is pretty much backup to what you've probably already figured out.

    The dyno chart below was produced by Sonex for their 2180cc Aerovee. The max HP figures have been subject to a lot of discussion/dispute here on HBA, but it's not a bad place to start for your project. We don't know if your cam profile is the same, etc, but . .. .Let's just cut all the figures in half for your 2 cyl VW. [​IMG]
    The maximum available torque stays pretty flat (let's just round it to 63 lb ft (85.4Nm for the rest of the world)for your engine) from 2500 RPM to 3400 RPM.

    Using Eiffel's equations, if your engine performs like 1/2 of the Aerovee and you use the 34"L x 2.27" square club prop, the engine will be able to spin that club to about 2960 RPM. At that point, the engine will achieve that 63 lb ft of torque level and it won't turn any faster, sp you won't know how much power your engine can make. As you can see on the graph above, at 2950 RPM the Aerovee was about 10 HP short of its max HP (reached at about 3400 RPM). So, in your case that is about 5 HP equivalent. >IF< your engine does maintain the ability to produce 63 lb ft or torque through 3400 RPM (as 1/2 of the Aerovee numbers), then you'd need a club that was 32.17" long x 2.15" square (817mm long and 54.5mm square) for the engine to reach that RPM (at which point it would be producing 41.25 HP). Whatever the math shows, you'll want to keep cutting down the club (maintaining the same 15:1 ratio between club length and square side dimension) until the HP stops going up or you're not interested in turning it any faster. I know with 4 cyl VWs, few people routinely run them faster than 3600 RPM (though the racers routinely do, and the max RPM for the Aerovee is listed as 4000 RPM).

    I think it's fair to say that many people believe the Aerovee dyno chart may be optimistic, and if they are right, then you'll need to cut your test club down even more to reach 3400 RPM.

    If you'll be making changes to the engine in the future (trying new carbs, new induction runners, various ignition advance settings, etc), consider making multiple clubs rather than cutting one down. A full set could be used sometime in the future for comparison, etc. A very nice thing about the test clubs is that they can be run with the engine in the plane, so building a new dyno chart after making some changes is not too hard--if you've got a set of various size clubs handy.

    Another comment: I wouldn't junk your test stand and homebrew torquemeter yet. Maybe modify it with a longer arm and weaker springs/counterwights, etc so that the needle does deflect appreciably with the force you expect this engine to make. As long as the needle deflections are consistent (i.e. there's no random error like a bearing that sticks sometimes and not others), you can record the needle deflections as you run the test clubs on the engine--do each club several times to see if the needle deflects the same amount each time. At the end of the testing with the clubs, the RPMs they produced with each club will tell you precisely the torque it was making at that time. If you know your arbitrary needle reading, if there's noting fundamentally wron with your rig you may be able to associate it with a torque. Build a chart and you might have something useful in the future: After you re-mount a real propeller, the needle deflection will tell you the torque is being produced at various RPMs and you'll know how much HP you are making (at least static HP).

    Oh, I'm not sure how altitude compensation works with these Eiffel clubs. Obviously, the air is thinner at your 2000'MSL than at SL, and it takes less torque to spin a particular club in thinner air. Likewise with temp variation from standard. OTOH, you engine's power would be greater at lower altitude. I don't know if the two curves (engine power vs DA and torque-required-to-spin-the-club vs DA) are the same--probably not. I'm sure somebody knows.

    Good luck! Get those sticks well balanced before you start spinning them at hundreds of RPM.
     
    Last edited: Jun 16, 2019
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  9. Jun 16, 2019 #49

    poormansairforce

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    The prop absorbs the torque so there is no excess. You'll have to work around the pulses as mentioned i.e. read between the lines so to speak.
     
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  10. Jun 16, 2019 #50

    akwrencher

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    This is a good video of a homemade dyno. Perhaps you have already checked this stuff out though. I'm still trying to wrap my head around wether or not this same thing should work with a prop or not. Not sure why but my brain is confused.

     
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  11. Jun 16, 2019 #51

    GeeZee

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    William Wynne has had a dyno as part of his engine test stand for years. He claims it works well and is accurate. There may be more detailed info in his archives but this link shows some photos and he talked about comparing an O-200 to a Corvair. Turns out O-200s are really only about 81 Hp....
    https://flycorvair.net/?s=Dynamometer+
     
    Last edited: Jun 17, 2019
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  12. Jun 17, 2019 #52

    proppastie

    proppastie

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    You are doing this because it is fun......Solving the problems is the real fun.....anyone can buy a kit or built-up engine from a dealer.
     
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  13. Jun 17, 2019 #53

    ArcticDave

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    Mr. Wynne's dyno appears to a similar concept! His is closer to the Whatley stand than mine, but measuring the same thing.

    Vigilant1, that is a really good idea, referencing the indicator needle to test clubs! I had thought about lighter springs to give the needle more range in order to observe changes in output, but I couldn't think of a way to tie needle position to a known value.

    I will build the 34" test club next and see what the maximum torque is and mark it on the dial. I could then use the needle to spot changes in torque output as changes are made.

    Here is the chart section for the 34" test club I obtained from the Eagler's Nest.

    RPM HP
    2000 11.08
    2250 15.77
    2500 21.64
    2750 28.80
    3000 37.39
    3100 41.26
    3200 45.38
    3300 49.77
    3400 54.43
    3500 59.38
    3600 64.61

    I will post up my numbers when I can get it done, If you still don't mind running them through your calculator. It may be more accurate than the above chart.
    I also have doubts on the accuracy of the tach, and I'm fighting sealing the valve covers without resorting to a sealant. They drip and that of course gets blown all over EVERYTHING! I'm going to order a TinyTach for it, as both of the ones I have here give entirely different readings and I'm unsure which is right.

    On the good side of things, the engine normally starts on the first flip and has 25 psi hot oil pressure at idle and a steady 45 psi with throttle. At this rate I'll be flying this bird in a couple more years. :cool:

    Thanks for the input guys! I appreciate it.
     
  14. Jun 17, 2019 #54

    akwrencher

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    20 bucks for an optical tach on Amazon. Or a contact one isn't much more. Nice to have to verify things.
     
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  15. Jun 17, 2019 #55

    Vigilant1

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    Yes, the calculator gives the same HP numbers as your chart (within .1 HP, probably a difference in rounding).
    Two big points:
    -- These charts/numbers are for sea level, standard day conditions. Your location at 2000' MSL and any temps different than standard (which is 51.9 F degrees at your elevation) will obviously make the air density different and these charts will need correction for that. I'd guess the values will be quite sensitive to air density. You should see if there's a correction for density altitude available (most likely you'll find a conversion chart or correction equation for various values of air density, and you'll need to convert again for density altitude). Your engine will also make less power in thinner air, but I don't know that the curves will be the same and cancel out.
    -- Again, each test club will just give you one point (a single RPM and a single torque value, which gets converted to HP.) Torque, not HP, is what will limit each test run. With a 34" club, at sea level standard day, at 2960 RPM an engine needs 63 lb ft of torque to turn that club--and this is likely the limit for your 1/2 VW (based on the Aerovee numbers previously posted). That will be 35.8 HP, and your engine can probably make more power than that (at higher RPMs). If you want to know the engine's true limit, you'll need a smaller club as well, to allow it to turn faster (make more power) with that same 63 lb ft limit. A 32.17" long x 2.15" square club, at SL std day, would take 63 lb ft of torque to turn at 3400 RPM (= 41.25 HP). And even smaller club might be needed to allow your engine to reach 3600 RPM, if you are interested in turning it that fast in use.
    Maybe the torque needed to turn a 34" club at 3400 RPM >at your density altitude< will calculate out to be just right, but at sea level std day it looks like your engine won't be able to turn it faster than about 2960 RPM.
     
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  16. Jun 18, 2019 #56

    ArcticDave

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    I was taught to estimate a 3-4% loss in power for every 1000 feet above MSL. You touched on something important in density altitude. We live within 40 miles of downtown Phoenix AZ, so elevated summer temps are the norm. According to the cheat sheet I have for density altitude, today was a cool 98 degrees and DA this afternoon was over 4500 feet. That will make a significant impact on produced torque.

    akwrencher....That is an outstanding idea. I might pick one of those up soon. I have this old "Tach Chek" thingamabob that I was going to try and figure out. A friend gave it to me in a couple of large boxes of AN hardware he donated to the cause. Internet search didn't come up with much. It does have directions on the back, so it shouldn't too difficult to figure out. 20190617_181527[1].jpg
     
  17. Jun 18, 2019 #57

    BBerson

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    That Tach Chek isn't much use. You need another tach to get within 100 rpm to start with.
    It isn't a tach for use alone. Just for checking an airplane tach and not much good for that either.
     
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  18. Jun 18, 2019 #58

    Vigilant1

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    Right, but also the bar will produce lower drag in thinner air at the same RPM. If we assume (I don't know) that the test club resistance can be expressed by the formula for drag D = Cd * A * .5 * r * V^2, the "r" "rho" or air density.
    At sea level, standard day, rho is .002378 slugs/cu ft. If your density altitude during your test is 4500' as it was today, then rho = .002080. The bar will take 13% less torque to swing at any given RPM. That's a pretty close fit for your estimated 3-4% loss in engine power per thousand feet, so maybe using the bar is self calibrating to SL--the torque and HP you'll read off the chart is >not< what your engine is actually producing during the test, but what it would produce at sea level std day.
     
  19. Jun 18, 2019 #59

    ArcticDave

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    I was hoping somebody was familiar with it. I won't waste much time with it then. I ordered a TinyTach yesterday and I will probably get a non contact model to verify everything.

    That was something I hadn't even considered!
    I pulled some wood out of the barn yesterday for test clubs, but it is likely to take a couple weeks to get it done. I'll post up what I find out though. I expect to find my tach is wrong and I'm not turning the rpm's I thought I was. I should have the new tach by the time the test clubs are done.
     
  20. Jun 18, 2019 #60

    rv6ejguy

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    Ideally you want to correct for humidity and barometric pressure as well and be aware that output is also affected by reduced exhaust back pressure as altitude increases. This offsets the density loss somewhat.
     

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