Calculation of power

[84-1074663779]

These things are actually a complete eddy current dyno power absorbing head, and are fitted into the tail-shaft of some large trucks to replace the normal centre tail-shaft bearing.

The idea is that heavily laden trucks can descend long steep hills without burning up or wearing out the normal friction brakes.

Check out google "Telma Retarder" for more information and some pictures. There are possibly a lot of these sitting out in the truck wrecking yards all over the world. They are commonly fitted to some Scania trucks and coaches.

I think you might need to be pretty lucky (like Bruce) to find a complete dyno head in a machinery junkyard, but these Telma units are really exactly the same thing and could make the basis of a very high power home engine, or chassis dyno.

[98-1074649673]

The one I got is from a company in Spain or Italy called KLAM. It looks just like a Telma. I had to make a homemade power supply for mine using a variac to control the voltage, on your adivce Tony I'm still searching for a transformer to use with the variac.

My only concern with my unit is its size for my engines. I'm afraid of the inertia from the rotating assembly. It will handle 8000rpms for a short time and 280 lb ft of torque. Way more than I need for my small engines. I'm itching to get it finished, I'm stalled on the console for the instrumentation and power supply, I'm hoping to run into a transformer before I build it.

[84-1074663779]

I believe those retarders on large trucks are specified to sink 1,000 Ft/Lbs, and some up to 4,000 Ft/Lbs continuously. Those rotors on your dyno sure look pretty big !

Are you sure your dyno is only rated to 280 Ft/Lb ? I have seen chassis dynos smaller than yours, and they are going to see a lot of torque because of rear end diff ratio in the car, and large diameter driven rollers.

How about some sort of speed reduction, as inertia is speed squared, slowing it down is going to make a very big difference to the inertia.

[98-1074649673]

The manufacturer told me the ratings on the unit I have. The rotors are around 12" if I recall (without running dowstairs to check). U can get them bigger I'm sure of it. For my application 280 is plenty big! Single cylinder motors here so I won't be close to 100 lb ft. The inertia is not a big problem once its running, if I test an engine it will always be tested on my dyno so the repeatability is what matters to me. Actual torque is not really a big thing. I'm not going to compare them to anyone elses. My only concern was when starting the engine up that the starter would spin it fast enough. We run a set timing and it has to be advanced enough to make rpm's so slow starting can be a problem. Those are bugs I'll work out as time goes by.

Yes I might have to get into a reduction of some sorts, this being more due to the rpm and not inertia though. for right now I just want to get this thing running its been way to long a project! That happens with limited cash outlay. It might have taken sometime but this dyno project is well under $1000 right now thank you eBay . . .

I've also collected over the past few years almost all the parts needed to make a hydraulic dyno setup, so if the eddy current unit doesn't work I have a backup plan.

[jsmith]

Not much interest to a man of my limited knowledge but it might be of use to you guys:

[Greg]

This thread has given me some ideas about a chassis dyno, If you set up a set of rollers with the front one connected to the retarder and the rear one just freewheeling, say a 3" shaft with 8" rollers on it and a big bearing on each side of each roller and on the end of the shaft.
How do you measure the torque/power?
Does the retarder pivot on the shaft and load up some type of scales/strain gauge/hydraulic cylinder?
I've seen the Dyno Dynamics chassis dynos which are an eddy current dyno and take up around 1500hp or more at the rollers and are a small unit, I suppose this is the type of retarder they use?
This could be another project to clog up the workshop!
Software is another problem I suppose but that must be able to be worked out..

[84-1074663779]

Yes, you are right, have a look at Bruce's engine dyno.

The body of the retarder is dragged around the shaft axis, and usually on a chassis dyno there is an arm that either rests on a weighing scale, or on a big coil spring, or on an electronic load cell.

You calibrate it by just applying weights to the end of the arm. Accurate weights are easy to come by, they are used to calibrate scales. Even a set of barbell weights from e-bay would probably be accurate enough.

You then need to know the speed of your rollers. An optical or magnetic pickup would be pretty easy to arrange and a digital tacho. To verify the speed accuracy, some painted white lines will strobe with a fluorescent light at the 60Hz (50Hz) mains frequency at any suitable calibration speed.

Power calculation can be as simple or as sophisticated as you want. A pencil and paper, or a laptop with simple spreadsheet program.

You will also need an adjustable dc power supply to set the dyno resistance, but that is pretty easy as well.

The rollers might need to be larger than eight inches if traction is not going to be a problem. The chassis dynos I have seen use rollers 18" to 24" diameter, and are usually coated with a non slip surface of some sort.

[willeng]

Bruce:

You may want to check the rpm limit of your retarder, just to make sure.

I am not sure how you are going to set it up, but you should use some sort of clutch setup this will help.
Also the clutch will help to stop crankshaft breakages due to starting misfires even though you said it is only a small retarder, the inertia of the rotors can & will damage small engines. ie if the engine fires and then for some reason cuts or backfires suddenly the inertia can break things fairly easily.
With a clutch you can warm the engine & then engage the retarder with no problems.

I built a retarder myself & controlled it with a variac, i didn't know it was a variac until Tony told me it was though, i hooked this up to a AC single phase welder & then into a AC/DC converter used for tig welding.
The second hand welder & the converter i bought for under $100-00 & it worked fine.
Not to long ago i bought a Telma retarder & then was fortunate enough to find Tony who has gone out of his way to help me understand more about the power control & what was really required.
As it turns out the Variac, welder & the AC/ DC converter supplies the correct amount of current for the retarder i have & it would i'm sure suit your application, The welder is adjustable so it should work.
To make sure i'd personally ask Tony, he's the man when it comes down to this sort of thing--believe me.
I would like to take this opportunity to thank Tony for his advice & patience with this matter.
I knew it worked but didn't know all the facts that Tony has worked out for me.

[Sandra]

I have a rolling road but I am not sure how I should calculate.

This is how I have calculated: (torque*RPM)/5252= Bhp.
My problem is that I am not sure how to calculate the torque. I have a loadcell that is assembled 30 cm from the cnter of the brake. Do I have to consider the diameter on the rolls for the calculation? The diameter is 22,5 cm.

Can I do it like this: kg(on the cell)*(0,3+11,25)*9,8=Nm?
9,8 is the number to get it from kg to Nm.

And finelly should I use the RPM on the rolls or on the wheel?