Author Topic: Dynamometer and teststation  (Read 1069 times)

Offline lohring

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Re: Dynamometer and teststation
« Reply #15 on: November 06, 2019, 03:05:05 PM »
Jim Allen has built both a water and fan brake for model engine dynos.  See the bottom of https://www.intlwaters.com/media/users/jim-allen.2129/albums?page=3 for the index to the pictures.

Lohring Miller

Offline AlexS

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Re: Dynamometer and teststation
« Reply #16 on: November 06, 2019, 08:01:59 PM »
Thanks guys, this is stuff to think about. I agree that it is difficult to estimate what the output of the pump would be. Also At some point you should just build it and make adjustments on the way. But I need to dig in deeper to estimate absorbed power with this setup.
« Last Edit: November 06, 2019, 08:19:41 PM by AlexS »

Offline lohring

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Re: Dynamometer and teststation
« Reply #17 on: November 07, 2019, 03:21:51 PM »
Another quick way to estimate power is with calibrated propellers.  I can't post the .pdf file.  See below for some excerpts.  Send me your email for the complete paper.

Lohring Miller


Offline Allen Smithee

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Re: Dynamometer and teststation
« Reply #18 on: November 07, 2019, 06:02:39 PM »
Many years ago I used to have a set of Kavan calibrated props, but I sold them because I didn't use them enough. These were (IIRC) machined and ground steel props which each had a calibration table of RPM vs Torque.

It's not that hard to make your own using ordinary commercial propellers. The calibration table can be established using an electric motor because it is relatively easy to derive the power absorbed by the load of a BLDC motor using just a voltmeter and a clamp-meter. For wooden, composit or plastic propellers it's necessary to measure over a wide range of RPM because they flex, whereas the Kavan metal ones can be assumed to be rigid over their intended speed range, but that just means plotting a table or graph rather than using the single P number as above.

To measure the power absorbsion just set up the prop on a known electric motor. You need to know (or measure) the following:

Motor Constant Kv (in rpm/v)
No Load Current I0 (in Amps)
Motor winding resistance Rm

You then fit the prop and measure the RPM (s), current (i) (with the clamp-meter) and applied voltage (v) as you raise the voltage over the desired range. Output power can then be calculated from simple equations:

P(out) = P(in) - Losses

 = (v x i) - Losses

 = (v - voltage losses) x (i - current losses)

= (v - (s/Kv)) x (i - i0)

Or if you want the answer in BHP rather than watts it becomes:

= ((v - (s/Kv)) x (i - i0)) / 745.7


So you can take some measurements on a set of props and record the data in a table or graph. Not that this tells you the power DIRECTLY. If an engine turns that prop at the RPM recorded in your table/graph then that is the power it is developing.

If you use it in this way - to provide a set of calibrated props - you will need to apply factors to correct for aur density and viscosity (there are standard correwction formulae for these), but the beauty of this approach is that you can use it as a direct test reference. Put a prop on your engine, measure the RPM. Then take the prop off the engine and immediately fit it to the electric test motor. Increase the voltage until the RPM is the same as for your engine and then the calculated power will apply to both - no corrections of any kind are needed. It also means you don't need to spend time testing/calibrating and checking a whole family of props in case you need to use them!

AS
Quidquid latine dictum sit altum sonatur

Offline dieselpilot

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Re: Dynamometer and teststation
« Reply #19 on: November 07, 2019, 10:12:01 PM »
Allen have you done this in practice? How was accuracy? The problem with this approach is calculated vs real world efficiency (or motor constants).

Offline Allen Smithee

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Re: Dynamometer and teststation
« Reply #20 on: November 07, 2019, 11:06:57 PM »
I've done it, and I got a number that was in the expected range - I have nothing to calibrate it against so I can't make any claims on accuracy. But I would take issue with comments about "calculated" efficiency - with electric motors all these things come down to measurable fundementals: voltage, current, resistance and rpm. Of these the only one that is "tricky" is the winding resistance because it's a very low value (a few tens on milliohms) and as such should really be measured using a four-wire or bridge method, and for absolute accuracy the resulting number should be factored by the thermal resistivity coefficient for the (measured) winding temperature under load. But the differences are tiny.
You mention motor constant - this is very easy to measure, so I don't see the problem.

The beauty of this approach is that all those normally troublesome-to-measure losses (bearing friction, magnetic losses, copper losses, air drag etc) are directly accrued into the measureable parameters. The no-load current represents the sum of all the power lost to friction and magnetic effects. In this way you can directly calculate both input power and output power.

AS
Quidquid latine dictum sit altum sonatur


Offline lohring

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Re: Dynamometer and teststation
« Reply #22 on: November 10, 2019, 03:15:44 PM »
All these "propeller" brake methods are approximate, but have been used for a very long time.  Nearly all nitro engine development was done by measuring rpm with a select group of propellers.  The big advantage of inertial dynos is that in the time it takes to make one run on a brake dyno, you can get a full power curve.  You can test dozens of things in an afternoon.

The main problem is people that don't understand physics don't understand how they work and why they accurately measure power.  In the days where engines ran at the same rpm with a fixed load, brake dynos simulated the real world.  Where engines are constantly accelerating and decelerating as in racing, an inertial dyno better simulates the situation.

Lohring Miller

Offline Allen Smithee

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Re: Dynamometer and teststation
« Reply #23 on: November 10, 2019, 09:07:32 PM »
I agree with you - the inertial dyno is beautifully simple and fundamentally (intrinsically) accurate. I didn't use them on FAI-F3d engines for two reasons:

1. I was concerned that I would not be able to stop the engine running past the upper operating point (call it 30,000rpm) quickly enough to prevent it over-revving an un-necessarily grenading engines.

2. With the very high Q of this kind of engine/pipe combo I was never convinced that the mixture (and thus ignition timing) would "settle" to a representative condition on the way through, so the curve as measured may not have been representative if the in-race condition

3. For FAI-F3d engines we very much DID want a single, constant power setting - the engines weren't equipped with throttles and wouldn't have responded to them in any meaningful way even if they were.

OK, so that's three reasons - sticking within a bidget was never one of my strengths...

AS
Quidquid latine dictum sit altum sonatur