Author Topic: Dynos  (Read 3921 times)

Offline lohring

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Dynos
« on: July 22, 2014, 02:03:14 PM »
I've spent a lot of time with a partner, Mike Bontoft, testing and improving model boat engines.  In the beginning we decided that a dynamometer was an essential tool.  I picked the inertial style dyno and have been very happy with its ease of use.  We were developing engines to set straight line speed records.  The characteristics of the dyno were designed to mimic the acceleration that's typical of those runs.  The huge advantages of an inertial dyno are ease of construction and complete power data from a less than 10 second full throttle run.  This avoids wear and tear on engines as well as allowing a lot of testing in a short time.  We often logged 40 runs in an afternoon.  Of course you can't do endurance or other tests that a brake dyno is good for.  See http://www.dtec.net.au/Inertia%20Dyno%20Design%20Guide.htm for an explanation of how inertial dynos work.  It's critical to check the flywheel design for adequate strength and balance.  We threw a magnet through a wall.  I can't imagine the damage a burst flywheel would cause.

Below are pictures of our dyno.  We used Performance Trends data loggers (http://performancetrends.com/dtm-dyno.htm) for data gathering, but it's possible to build a data gathering system from the physics of the flywheel.  The first picture shows an engine mounted for testing. 

The second picture shows the drive line.  We used a square key between two Aeromarine square drive adapters for the engine connection.  This turns out to be a great fuse.  We seized an engine at over 15,000 rpm and the key twisted 180 degrees before the clutch released.  A Lovejoy coupling was then used to dampen vibrations.  It may not really be needed.  Finally, an RC car clutch was used to allow the engine to idle and prevent the flywheel from driving the engine.  The third picture shows the engine mount detail for a Fugi engine.  The mounts could be changed for different engines, but most engines used a common front mounting pattern.  Finally, the last picture shows a somewhat simpler version built by Dave Marles.

A complete description can be found in issue 15 of Model Engine Builder magazine.  I would be glad to answer any questions and provide more information.

Lohring Miller

Offline rockknocker

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Re: Dynos
« Reply #1 on: July 22, 2014, 03:27:10 PM »
Thanks for sharing! I worked with small engine dynos in the 5-ish HP range and remember having a lot of trouble with love joy couplers at high speeds, they would start throwing chunks of rubber at about 13,000 rpm. We were using the highest quality love joy spiders available, but the rubber in yours looks a bit different. Have you had a similar experience? We ended up designing a custom coupler.

Its hard to get much simpler than an inertial dyno, there are no water tanks or resistor banks to cool and no worry about leaks, shorts, control loops, and the like. Your build looks great, very sturdy and easy to set up!

How long does it take for the engine and flywheel to "motor down" after a run?

Offline lohring

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Re: Dynos
« Reply #2 on: July 23, 2014, 02:07:50 PM »
We ended up using the hardest available material in the Lovejoy coupling.  I believe it was urethane with a Shore hardness of 98.  We've run the coupling to 3 times it's rated rpm with no problems.  We haven't needed to replace it since 2004.  Again, I'm not sure the coupling is really needed.  If you look, there's a plate that pivots beside the flywheel.  It brakes the wheel after a run.  We also use it as a light load to warm up the engine.

Lohring Miller
« Last Edit: July 23, 2014, 02:14:48 PM by lohring »

Offline RcTintin

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Re: Dynos
« Reply #3 on: December 22, 2017, 05:23:31 PM »
Hi Lohring,

I'm building a new dyno and have a question for you, or any one else who has built one, re the clutch you have used.

I was going to use the stock Zenoah pair of clutch shoes from a RC Car clutch, but I see theses nicely machined boat clutches for sale, and would fit my build perfectly. My concern is that the shoes appear to be steel to steel bell! Thats what Im not so happy about. Do any of you guys have any experiance or issues with steel/steel clutch?

http://www.arrowshark.com/ClutchV3-Reverse.php

Offline lohring

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Re: Dynos
« Reply #4 on: December 23, 2017, 01:46:10 PM »
I bet the clutch shoes are a sintered material, not steel.  That whole assembly looks ideal for testing Zenoah style engines.  I would drive the flywheel with a square key out of a square collet.  Another thought would be to drive a one way clutch connected to the flywheel.  My experience has been that the centrifugal clutch won't release fast enough if the engine seizes.  Something like this should work:
https://www.ebay.com/itm/NEW-TRACTOR-OVER-RUNNING-COUPLER-PTO-OVER-RUN-CLUTCH-1-3-8-FEMALE-MALE-NEW-/380663450927

Lohring Miller

Offline RcTintin

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Re: Dynos
« Reply #5 on: December 24, 2017, 03:32:44 PM »
Thank you for your reply sir

I had thought of using a oneway, but a friend of mine who is a little further ahead than me on his dyno had issues with centrifugal force releasing the oneway at around 11-12K rpm. We need upto 20K rpm.

Looking at various manufacturers spec sheets, there’s nothing that looks like it can take anything like those speeds, they are more like 7-8K range. Do you know of what’s inside the one you linked above? I was looking at the Sprag style, because at least they are not going up a ramp, but more of a rocking motion that locks the clutch. Do you think this will unlock with centrafugal force?

Thanks, Rob.

Offline lohring

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Re: Dynos
« Reply #6 on: December 25, 2017, 03:17:02 PM »
I believe the one pictured has the construction shown below.  It would lock up solidly with centrifugal force.

Lohring Miller



Offline RcTintin

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Re: Dynos
« Reply #7 on: December 28, 2017, 01:23:06 PM »
That looks good, thank you for the link.

 

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