Engine Dyno Practise

[topcatproduction]

Hi all, first post here and joined as I have read another good thread on water brake engine dyno's and looks like a really helpful place!

I'm looking to build a really simple dyno and had a thought which I cant find anywhere! Is there any reason why you couldn't just mount the engine on a good frame, rig up a clutch of some kind, have a set length bar (say 5') from centre of clutch plate to a point on some scales and when you hold the engine at set rpm's, push the clutch in to engage and note the highest reading the scales go to. I'm assuming as the revs dip the load will slightly reduce so you could get an accurate reading? I know this would eventually burn the clutch out but only be a couple of seconds a time...

This way all you would need to do is multiply the reading by 5 (5ft bar) and you have ftlbs at set rpms to plot onto a graph.

Simple in theory but would it work?! I attached a very basic drawing.

Also all this talk of using truck eddy brakes- how do you take any sort of reading from them? I'm getting a bit confused!

 

[Warpspeed]

All that is very true Pat. Even the illustrious James Watt was building dynos long before even the infernal combustion engine came along.

But something like a peaky high output motorcycle engine coupled to a very low inertia power absorber, can either suddenly stall dead on you, or destructively run away with remarkable ease and swiftness.

It probably would not matter, except that exploding parts can be both dangerous and expensive.

 

[patprimmer]

I presume these same engines are controllable enough via a clutch so they can be moved from a standing start without stalling or exploding. It does require a basic skill to simultaneously control power and load application in either case.

Regards
Pat
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[izzmus]

"The only thing I'm not sure about is how you would know if it was at its full potential or whether you could increase resistance a fraction more to show more torque before the revs dropped further..."

Playing with a dyno two years ago, I was able to get markedly different torque figures by pulling the engine speed down instead of letting it rise. ~50-60 ft-lb difference on a 550 ft-lb engine.

Figures don't lie, but liars can figure?

 

[topcatproduction]

Well a simple dyno as discussed should be able to give accurate WOT maximum torque readings which would be helpful, and I guess with further testing and modifications to its operation it may be the case where it becomes possible to get accurate incremental readings too. With any luck!

I could fit a simple 2:1 reduction for use with a retarder/eddy brake.

 

[patprimmer]

The only power figure you can trust is one obtained at steady state. Once you get it stable at WOT, you hold it while doing small adjustments until you get it stable at the rpm you want to measure.

To get peak power, you hold it at higher and higher rpm increments until it starts to drop off power then you keep going between the points where it increases and decreases.

Regards
Pat
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[dicer]

What you need is a transmission that uses a fluid coupling not a torque converter, hmmm just remove the stator and you have one.
Lock it in high gear so its direct at the output shaft. Then read the torque at the output shaft, lots of ways to do that. A simple bar and scale. If you have a way to control the oil into the fluid coupling then you can control the load. Its going to generate lots of heat.

 

[topcatproduction]

The downsides I can see with that is that it will not be easy adapting different engines to the auto box used, and to be safe it would have to be a reasonably sized one- which guzzle power. A good friends plymouth was rolling road'ed a couple of weeks back and his transmission ate over 80bhp...

 

[gruntguru]

Good. That is a bit less power your dyno will need to guzzle. (That's what a dyno is - a device designed to guzzle power). As long as you measure your power before the coupling, you will be OK. Note, a fluid coupling has torque output equal to torque input - its the rpm that drops to create the power loss. The fluid coupling will also eliminate the enemy of all engine dynos (particularly when testing 4 cyl 4 strokes) - torsional vibration.

 

[topcatproduction]

How can you measure power before the coupling- when the coupling is there to provide the mechanism for measuring power?

 

[Warpspeed]

Think of the power absorber as a "mystery black box" with an input shaft going into one end.
You measure the shafts torque reaction on the frame of this black box, and the rpm of the input shaft, and calculate Kw/Hp.

But when you look inside that mystery black box you discover a gear reduction and.... shock horror, the bit that actually does the retarding only turns maybe half as fast as the actual dyno input shaft.

The trick to using a gearbox in your dyno, is you solidly mount the gearbox casing so it becomes a physical part of the power absorber. You then measure the torque going into the gearbox input shaft.

Any gearbox losses just add to the power absorbing function without adding any error, because you are measuring the torque reaction of the gearbox + retarder, measured at the input shaft.

 

[topcatproduction]

Yep sorry got you! So really it is ideal- apart from having to mount a torque converter on every engine I test...

I was wondering if I could use a big generator of some kind and ditch the power somewhere would that work the same- just measure the reaction of the genny? Maybe ditch it into a testla coil- that would be a crazy looking dyno!

 

[Bribyk]

You can use anything that will absorb rotational energy: a friction brake, air brake (fan), water brake, eddy current, alternator/generator, etc... or a combination of these. There's at least 8 common types of dynamometers but 4 of them are most common. They all have inherent advantages and disadvantages.

A water brake is probably the cheapest and is reliable - a friction brake is usually tough to control and wears rapidly (trust me, I tried to break in a 70cc quad engine using the rear brake disc - it got smokin' hot fast). They need rpm to make torque so they don't work as well at low speeds as an alternator/generator (eddy current is in between). They are readily available from a number of manufacturers as well.

You need to ask yourself, "do I want to test/develop engines or dynamometers?"

Brian Bobyk - Hoerbiger Canada

 

[dicer]

You would not want to use a torque converter, as it multiplys the torque, and also would create more of a loss. Hmmm how many dynos take that loss into consideration? Since a dyno is generating heat that is a loss.
As far as reading the power, any kind of scale that reads high enough and an arm attached to the output or on the swiveling load absorber will do it. Then its a simple math deal.

 

[topcatproduction]

Having spoken to a lorry mechanic he says he can't remember the last time he saw an eddy brake on a truck- they use other types built into the engine now for endurance braking. So I may look for a water brake- but I have no experience of them- how do you control the load and is cavitation ever a problem?

 

[MikeHalloran]

You control the load by controlling the amount of water that's in the brake at any given time. I think they supply a specialized valve to take care of that.

Given that a water brake is a pump with a lot of air in the impeller, I'm sure they cavitate. So what? All it will do is eventually destroy the impeller, which puts a finite limit on its lifetime. ... but that doesn't affect its performance until the impeller is completely gone. In the meantime, you really don't care what's happening inside.

The water brake is connected to a sump and a big radiator; you just make sure the sump has water in it. Or use a pond or other _really_ big sump.





Mike Halloran
Pembroke Pines, FL, USA

 

[evelrod]

Agreed, Mike, that cav can destroy an impeller...

However, the gentleman that has done my dyno work since the early 1980's uses an old Clayton water brake. He does several hundred cars each year and the thing seems to still work well enough. Worrying about impeller wear would seem a bit of "too much about too little" and the least of the possible wear points.

To the OP...In the late 60's I built a small engine dyno (well, helped build it) to test Go Kart engines. Used a spring scale and a centrifugal clutch. Worked fine for a very short period...it ate clutches. Looked pretty much like your sketch. Later the unit was converted to a simple chain driven water pump/ball valve/garden hose---not sure if it worked better, I was not there to see it function!

Rod

 

[patprimmer]

If the water brake is designed to run in cavitation, it can be quite durable. The impeller does not need to be light weight or have high efficiency.

Regards
Pat
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[gruntguru]

Here ya go.

http://www.youtube.com/watch?v=oQSYwIzy-AM

 

[evelrod]

Interesting setup...somewhat more interesting than the 'thing' he seems to wish the engine to be in...

One thing...From a personal experience...While working on my son's old 60 something Dodge 318 V8 engined pickup...leaning over the fender and adjusting the carb...had the privilege of seeing a connecting rod poke out the side of the block just below my face...REALLLY gets your attention!

Rod

 

[patprimmer]

Rod

I had that thought as he casually walked around with uncovered shafts and engines at high rpm full load and no shrapnel protection.

His HO Holden 1 tonner while not at all interesting is normally useful for utilitarian use and durability. They have an inexplicable cult following here.

Regards
Pat
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