Dyno Design

[1337engie]

Hello,

I'm building a small dyno as a home project for low powered scooters and bikes.

I thinking of coupling the drum with a motor and simply reading the current drawn than using P=IR to find the power.

Or using a car brake with the housing attached to a lever arm and load cell.

Will this work? What size roller will i need? Any comments welcome.

Tim

 

[GregLocock]

The motor idea is better. You'll need a heat sink of some form, in the past I've seen electric water heater elements or just resistance wire. You may want to use a force balance even with the motor as you don't know how efficient it is.

The car brake will be a pain, its resistance will change as it heats up.



Cheers

Greg Locock

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[tbuelna]

1337engie,

You don't want a motor for your dyno brake (absorber), you want a generator. The dyno needs to put a load on your test article, not drive it.

Whatever type of brake device (electric, friction, hydraulic, etc.) you employ to load your engine, you will need to dissipate the energy absorbed somehow. Getting rid of excess electrical energy (in your garage) would seem to be more expensive and problematic than pumping a supply of water past a restricting valve and measuring the torque reaction about the pump axis. A simple hydraulic brake would also be more tolerant of driveline dynamics than an electric device. These torsional modes always seem to cause problems with IC engines driving dynos. And the first indication of a problem is usually broken dyno parts.

Regardless, it sounds like fun. Just watch your budget. The cost of these "hobbies" can get out of hand quickly!

Good luck.
Terry

 

[patprimmer]

A DC motor is a generator if driven.

You then need to drive something with the charge produced. A very large array of heating elements could suffice.

Obtaining a correct size DC motor could be a problem.

Regards
Pat
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[1337engie]

Wow, thanks for the tips guys. My 1st post on this forum, many helpful replies.

With the electric generator, could I simply just run it to a resistor bank (like a regenerative braking system) and measure the current and voltage across that using a multimeter?


Or in terms of the car brake option, to get a reading from the load cell, is all i need to purchase a digital display that the cell can plug into?What would be the best option to measure RPM of the driving shaft, I had a handheld tachometer in mind.

Cheers,
Andy

 

[SomptingGuy]

The car brake isn't really a good option unless you like fires. It really has to be water or electric and there must be a way of dissipating the energy you remove from the engine.

- Steve

 

[patprimmer]

Measure the torque reaction of the generator or the motor with a load cell just as you would with a brake.

This will only work for a very small motor unless you can get a very powerful motor and a big electrical load to absorb the energy.

Regards
Pat
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[evelrod]

I think I posted some reference to a small engine dyno a few years back. It was built by a friend who was building go kart engines, late 60's. His shop was in the Long Beach/Signal Hill area. Anyway, his was a very simple "automatic clutch", an oil filled device used on the karts, and a lever arm attached to a spring scale. It was quite primitive, but it seemed to do the job for the small hp engines. I don't recall any heat problems but, looking back, I cannot see how that would not have been a major issue.

Rod

 

[GregLocock]

Yup using a torque converter as an oil filled water brake has some definite advantages.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Greg

I read Rod's statement as a multi plate wet clutch with centrifugal action.

I guess a lot of oil and a big cooler and short pulls and long recovery times.

Either way, controlling the applied load will be an issue. A multi plate wet disk with a control lever rather than fully centrifugal or a control valve to adjust oil level in the torque converter maybe.

Regards
Pat
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[EdDanzer]

The RPM you want to test at, the accuracy and the duration of the tests will impact the methods you choose and the cost to build and operate.
If you plan on operating over 3400 rpm the generator or pump may have to be belt driven or a very expensive generator acquired. The best accuracy is to measure the shaft torque, next is the reaction torque, last, is the output of the pump or generator. The advantages of a friction brake is low cost, the difficulty is cooling and consistent loading. As a brake warms up the torque will change. With electric or hydraulic the longer you run or the more accurate results, the more important cooling will be.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[dicer]

Where to get some detailed pictures of a water brake?
They must be very similar to a fluid coupling. That is like a torque converter without the stator.

 

[Bribyk]

Try the patent or industrial design databases, perhaps. I doubt too many manufacturer's have there designs available for viewing. Pretty much like a torque convertor though, rotor with fins pointing one direction (i.e. CW) , stator with fins in the other direction (i.e. CCW).

 

[btrueblood]

Sorta like a centrifugal pump too...hey, there's a thought.

 

[bluebeast]

How about one of those bicycle trainers, They have a display that shows you how many watts you are putting out.
They are available in magnetic resistance, viscous and in air resistance. It would probably be easier to adapt one of those to this project. They start at about $100.00 and go up from there, but you don't need the one that connects to your tv and virtually race the tour de franks!!

 

[dgallup]

All the hot rod chassis dynos are simple inertial drums. You just calculate the power produced from the rate of acceleration of the inertial mass. No power to dissipate. Just make sure the thing doesn't burst from overspeed.

 

[ScottyUK]

A standard 3[Φ] induction motor with a regenerative drive feeding power back into the mains, something like an ABB ACS800 or similar? Would be useful if you have more load elsewhere than you have regeneration so you don't get into a power exporting condition at the utility metering point.


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If we learn from our mistakes I'm getting a great education!

 

[PedroCG]

if you want to measure the output of a generator you need to know the efficiency of the generator, plus even for small power generators can get expensive, if you use inertia dynos you are not taking in acount the inertia of the engine and transmition and you need to map rpm vs time accurately witch can be expensive, if you use the brake-lever-spring you dont know how much of the power is being wasted in heat form, using some type of brake (electric (eddie current brake), water or other fluid) and then measuring the rotation between two sections of a shaft is what i'd do, i would measure rotation and rpm at same time using data aquisition software like HPVEE or a simple osciloscope (rotation would be calculated using the phase angle between signals and rpm trougth frequency.

 

[evelrod]

Let us get back to what the OP was asking---a "SIMPLE" dyno...

All the above reasons are why I use "dyno" figures for comparison only. A small chassis dyno "rolling road" is, for me, an acceptable tool for tuning and/or comparison...not much else. With an assortment of added tools, a dyno run can shortcut the setup of a new race car, thereby saving me mucho bucks. I just do not live and die by the numbers I get.

Rod

 

[ScottyUK]

Rod -

1 motor, 1 cable to the drive from the wall, 1 cable from drive to motor, 1 drive. Simple!


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If we learn from our mistakes I'm getting a great education!