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Homemade dynomometer.

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Arcstarter

Mon Nov 01 2010, 11:16PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

For most of my life i have been into mechanics, as well as electronics. In recent years i have been tuning engines up for more output power, while still using stock parts.

While i can tell that the modifications do help, because of throttle response, and other signs, i do not know what kind of power gain i am actually getting. Nor can i tell what modifications help with, be it high end or low end power, RPM, or what the power band looks like.

So, i was wondering about the feasibility of making a simple dynomometer. I was thinking something along the lines of an electromagnetic clutch like that used for the radiator fans in automobiles, something to read the RPM, and something to read the torque, such as a foot long piece of metal welded to the clutch for foot/pounds. The clutch's voltage could be changed for different loads. After the testing, calculating the numbers would be easy, just T*S/5252 where T = torque in foot pounds and S = speed in RPM.

First off, how big would an electromagnetic clutch that could handle something like 10-20hp at 6.5krpm (where i limit most of my engines)? Also, what kind of price would i be looking at? I found some clutches on eBay, such as

, but i have no idea how to figure out what kind of rotational force/speed they can safely handle.

Would i be better off with some other way to vary load? I could short an alternators output windings and vary the field voltage for variable load, but i don't know how much an alternator would like 7450 watts of power... Something like an eddy current brake would also work, but i think that would be too hard to make, and i really don't have any money at all

I could use a centrifugal force clutch, but since the load is not variable, i could only check the HP for whatever RPM the clutch engages at.

Thanks,
Matthew.

Renesis

Tue Nov 02 2010, 01:22AM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Google "Prony Brake", that's basically what you're trying to build.

I have been playing with the idea of constructing a dynamometer myself, but instead of a clutch or brake i'd go with an electric generator with a variable external load. The benefit of this approach is that i dont need any fancy watercooling, and there's no wearing of parts.

Instead of the weights used in the prony brake i'd use a hydraulic cylinder, and by measuring the pressure inside the cylinder i can calculate the torque.

Would'nt it be awesome if you replaced the engine mounts with load cells, and attached an accurate tachometer to the drive shaft? Then you could measure power output while driving, and display it on a screen inside the car.

Hey, check this out... VROOOOOUUMM

Arcstarter

Tue Nov 02 2010, 02:48AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Renesis wrote ...

Google "Prony Brake", that's basically what you're trying to build.

I have been playing with the idea of constructing a dynamometer myself, but instead of a clutch or brake i'd go with an electric generator with a variable external load. The benefit of this approach is that i dont need any fancy watercooling, and there's no wearing of parts.

Instead of the weights used in the prony brake i'd use a hydraulic cylinder, and by measuring the pressure inside the cylinder i can calculate the torque.

Would'nt it be awesome if you replaced the engine mounts with load cells, and attached an accurate tachometer to the drive shaft? Then you could measure power output while driving, and display it on a screen inside the car.

Hey, check this out... VROOOOOUUMM

Well, i thought about variable load too, but that takes higher power parts... Something like an electric clutch would require like 12 watts at full load

. To control that, i could use a simple variable linear voltage regulator. And then, an electric clutch would be relatively small.

Price is the biggest problem... That clutch i linked is affordable, but idk what kind of power it could withstand, so i need to do some research on them. So far i have not seen too much about them.

Renesis

Tue Nov 02 2010, 03:32PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Price is the biggest problem... That clutch i linked is affordable, but idk what kind of power it could withstand

How about an old drum/disk brake from a scrapped car? That should be affordable, and should be able to handle quite some power for short intervals.

radiotech

Wed Nov 03 2010, 09:09AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Parts list: Spring balance 0-2 kG
Stick 1 M long with cM/mM (a tape measure stuck on)
An Arbor to fit your motor shaft capable of
clamping cloth buffing wheel.
Two pieces on plywood big enough to
just cover the buffing wheel.
Two bolts with wingnuts.
Optical tachometer

Drill a hole through both pieces of plywood
so you can clear the shaft and flange nuts of the arbor

Assemble so buffing wheel is between the plywood
with two pieces loosely held in place by the bolts
in opposite corners. Now fasten the arm with tape
measure attached. Attach arbor to motor shaft so
buffing wheel spins between plywood and friction
pushes arm down on the spring scale, Use a small
triangular block to point to arm length.

Start your motor with wattmeter attached. Now with
the two wingnuts tighten evenly to give you the desired torque. At the wattage you decide, use a tachometer to get speed. 3 students did this. l

This Prony Brake will be a dynamometer for up
to 1/2 Horsepower at 1800 RPM.

The cloth will give you about 20 seconds, then
let it cool down.

This is my dynamometer that I used for 15
years in my electric motor course and it works.
It was used to test efficiency of motors and
costs about $10. (tach extra)

Arcstarter

Wed Nov 03 2010, 07:12PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

radiotech wrote ...

Parts list: Spring balance 0-2 kG
Stick 1 M long with cM/mM (a tape measure stuck on)
An Arbor to fit your motor shaft capable of
clamping cloth buffing wheel.
Two pieces on plywood big enough to
just cover the buffing wheel.
Two bolts with wingnuts.
Optical tachometer

Drill a hole through both pieces of plywood
so you can clear the shaft and flange nuts of the arbor

Assemble so buffing wheel is between the plywood
with two pieces loosely held in place by the bolts
in opposite corners. Now fasten the arm with tape
measure attached. Attach arbor to motor shaft so
buffing wheel spins between plywood and friction
pushes arm down on the spring scale, Use a small
triangular block to point to arm length.

Start your motor with wattmeter attached. Now with
the two wingnuts tighten evenly to give you the desired torque. At the wattage you decide, use a tachometer to get speed. 3 students did this. l

This Prony Brake will be a dynamometer for up
to 1/2 Horsepower at 1800 RPM.

The cloth will give you about 20 seconds, then
let it cool down.

This is my dynamometer that I used for 15
years in my electric motor course and it works.
It was used to test efficiency of motors and
costs about $10. (tach extra)

That would be easy to make and cheap, but the problem is that i could be testing up to 20hp...

A disk or drum brake would probably be the easiest thing to use... I could just test the torque from the caliper. But, the biggest problem there is finding a way to fix the rotor to the engine's shaft... It would be best if i used a couple bearings so that the rotor would be fixed without the need for being fixed onto an engine, that way i would not have to make a whole new setup for testing different engines. But again, i would need to have different couplings for each different engine.

I also do not know anywhere around here i could get a caliper and rotor and a master cylinder for load adjustment. As for power, i am sure that even an old worn out disk brake would easily test higher than 20hp, as long as it is not allowed to get too hot.

If i had a big induction motor or something along those lines, i would simply make a generator type dyno. The highest HP motor i have is 2hp, and it is a universal motor. I could easily vary load with one, using the rotor as a field winding such as in a car, but the power it could handle would be limited to around 1kw.

Arcstarter

Wed Nov 10 2010, 12:48AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, now i have 2 rotors from a small car, with a radius of about 6 inches. Now i just need a caliper and master cylinder or some other way of compressing the hydraulic fluid, and i can make it. A 6 inch steel bar would be connected to the caliper with some sort of scale, so that there will be no calculating required.

I guess i will use something like one of those fishing scales, or some other scale with a simple spring... Load cells are total overkill. A simple linear spring scale is fool proof.

As for the tachometer, i'm not sure. I could use a pickup coil around the spark plug wire, but i am unsure how to get an accurate reading from that. I guess i would just use my DMM and multiply Hz by 60, should be around 1% accurate. That would also be fool proof.

The main thing that i need to figure out is a way to hook up the caliper to that it can move radially, but not axially. If i had steel and a good welder, that'd be no problem, but i basically have nuts, bolts, nails, and wood.

I have yet to decide upon how to make the rest of it, anyway. I would like to make the dyno separate from the engine, so that i could measure different engines. But that would still be problematic, because i would need a way to couple different engine shaft sizes to the dyno.

So simple in theory, but without raw materials, not so simple in practice!

radiotech

Wed Nov 10 2010, 03:05AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Stare at these for inspiration. Myself I would have use a twoshoe drum brake needing only 1 cylinder and I would have got
the whole thing off a junkyard car, pedal,master cylinder , wheel .cylinder. If its old enough you can fix all cylinders easily.

As for the tach , sparkplug pickups are messy and work best with
engine repair analysers. Use a magnet glued to the shart, a pickup coil and F meter in DVM, but you need four decimal places, so
proper frequency counter, set to period would be best.

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