engine dyno braking with DC generator 1

[sacem1]

I am planning on building an engine dynamometer so that power changes due the modifications done to my aircooled VW engine used in my self built tubular dune buggy can be determined.

There is no dyno service around here and the equipment is going to be used only on my car and several friends who have similar buggys.

We need to be able to buid it at almost no cost so I'm planning on using several pieces of equipment I have around my shop.

I have a big 100 Kw DC generator which I would like to use as a brake for the engines, use an old VW transaxle as a support for the engine with the spyder gears in the differential blocked so I can go to 5,000+ rpm's and make the generator spin at 3.88 times less using the gearcase and transaxle reduction.

I'm planning on mounting the DC generator on a support that allows it to tilt because of the torque transmited by the engine, read the weight that is being done on a side support with a known lever and read from there the torque that the motor is transmitting. Am I right?

If that is correct then the rest would only mean that I need to do some math and develop the power at a given RPM, however for it to work I need to increase the resistant load on the generator at each rpm setting until the engine starts to bog down and thats the max power at the particular speed, again Am I right?

If so can any one help me in suggesting a cheap way to vary the load at the generator so that I can make this work.

The alternative is going the inertia wheel type dyno but for the power transmitted (up to 120+ HP on 2,170 cc double carb engines) the inertia wheel requiered is big (around 48" diameter and 4" thick for a 10-15 second aceleration) and I like more the brake type dyno readings.

If this does not work can any one give an idea of how to build a waterbrake to replace the DC generator.

Thanks for the help you can give me.

Regards

SACEM1

 

[NickE]

Well the generator (at 100Kw) should brake up to ~130Hp. I dont know how you are going to dissipate that kind of electrical power. Maybe try this:

http://srforums.prosoundweb.com/index.php/mv/msg/5094/0/32/0

Those guys know how to sink a large amount of power! (still though only about half what you are going to have to get rid of)

You'll have to figure out how to measure the output of the generator. I think you could devise a proer voltage measuring device and then use resistanc eof the dummy load.

Here is an interesting story about why gear ratios are important when dyno'ing motors:

http://www.niquette.com/books/sophmag/bourke.htm

Since the generator is DC you should be able to measure it. Since you are measauring power the important equations are:

V=IR (of course)
P=VI
P=I^2R
(Theres proabably a few I'm forgetting)

And 1Hp is roughly 750watts

(Someone out there will be able to tell you How to vary the load on the generator to give you what you need, Im not sure stepping up the resistance will help you any. Youre still left with ~120 Horses of Power to dump somewhere. If you simply step the resistance up you may find that the current might drop off properly, but the voltage should climb. Again I'm just a metallurgist who is into cars, there is definately engineering data on the generator you will need.)

 

[sacem1]

Thanks NickE:

That site on the Bourke engine was awesome, and you are right the speed must be taken at the brake device whatever it is.

I do not intend to measure the electrical output because many variables come into the equation and precisely can cause a big mistake in total output readings.

However what I really want is to make relative readings, that is with one carb I get one reading and with two carbs I get another reading and what is the relative effect 10% better or even 15% worse performance as I am really triying to tune the engine for torque maximixation because we use them for hill climbing and the problem is not the power you start with at the begining of the climb (maximum speed as possible) but when you are nearing the summit the engine is on first gear, full throttle and it is still going in revs down to a stalled engine and you are 15 ft from the top, thats where I need the power, and thats why I want to be able to test at full throttle at different motor speeds and graph the best (for us) power curve.

I planned on connecting also a 100 Kw DC motor which I also have but if the motor is not doing work it will not apply a real heavy load, those generator testing devices would probably be useful but I don't think they are going to be cheap.

Thanks for your answer it gave me somethings to think about and that site was excellent.

Regards

SACEM1

 

[zekeman]

If you are testing for a limited time, then you might consider removing the heat generated using immersion resisters in an open tank of water which absorbs
about 1100 BTU/lb in heating from room temperature to boiling. Since
3413BTU=1KW hr you could boil
3413BTU/1100= 3.1 lb/KHhr= 3.1 lb/hr per KW
For 100 KW, you would need 310lb/hr or abou 45gal/hr.
If you take readings over say 20 minutes then you would need a water tank of 15 gallons or thereabouts

 

[sacem1]

Thaks for the answer I had received several advices for the same solution and I am planning on using it but had not done the tank size calcs yet, or even thought about how to size the tank, good advice.

Thanks

SACEM1