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Engine Dyno Practise

Engine Dyno Practise

Engine Dyno Practise

(OP)
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!

RE: Engine Dyno Practise

That's pretty much how all dynamometers work.
The trick is that you have to get your data before the clutch smokes.
The problem is that in order to get repeatable, accurate numbers, you have to wait for the oil and coolant temperatures to stabilize at the applied load level.  No clutch will last that long.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Dyno Practise

(OP)
So would it work using a torque converter type pump, or water or oil pump constantly slipping thus pushing on the scales or is that exactly how they work anyway?!

RE: Engine Dyno Practise

That's exactly how a classical dynamometer works:
Some kind of mechanism with a controllable slip torque (the classical mechanism is called a Prony brake), coupled to a lever arm that bears on a scale.  The brake is free to rotate, restrained only by the lever arm.
Reaction torque is computed from the scale reading and the lever arm length.
RPM is measured separately.
HP is computed from RPM and torque.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Dyno Practise

Topcat - I have tried making a simple dyno similar to that you described - your idea should work quite well.
 Rather than a clutch I used a front wheel drive (Morris 1100) drive shaft, suspension upright and disc brake. I think a disc brake is better able to take the punishment and is also easier to load more progressively than a clutch.
 One problem I didn't forsee (and you will have to allow for this) is the enormous amount of smoke, sparks, dust etc. that come off the disc brake when it was working hard under load.
 You will also probably need two operators to juggle the throttle, brake load and read off the RPM and the weight scale.

RE: Engine Dyno Practise

(OP)
Cool thanks for the replies.  If I used some kind of water brake or eddy brake which was constantly dragging, then should I be able to just use a hand throttle (non-sprung) and increase the revs in increments, taking readings at each, or would I also need to constantly increase or decrease the drag induced by the brake?

If it was dragging a fair amount then would the engine ever show it's full potential or would it not have enough chance to get there?

Basically I'd want to put Rover V8's on, some land rover diesels and maybe even little A series engines, so I'd also need an easy way of adapting to different flywheels.  The A-series might not be possible actually as the gearbox has to be attached as they share the same oil (but ignore that as it will go way off topic!!)

RE: Engine Dyno Practise

(OP)
Ah- so run at full throttle, quickly bring in the brake to hold at required revs at WOT, then could use the same brake resistance for all the testing through the scale?  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...

I can see it being a balancing act where you could run at WOT but having so much resistance it only runs at half speed but shows plenty or torque, where you could also run at the same speed with half throttle and a less resistive braking effect whilst showing much less torque.

Would the best way to test it to take to required revs, then dump the power onto the scales taking the highest reading, release, rev to next increment, repeat process etc?  Not how the big expensive dyno's do it but hey, were not talking thousands of £'s investment here...

RE: Engine Dyno Practise

NO

you test at full throttle. You adjudt the load to control the speed. You change the load to allow the speed to change for the the different rpm increments, but still at full throttle.  

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: Engine Dyno Practise

(OP)
Really? Didn't know that.  So for example an engine will scream to say 7k but peak power will be much less, say 6k, do you push to WOT (7k) and then increase the load until it shows the highest reading (around the 6k mark), then keep increasing the load will show less and less torque as the revs cant keep up?  And continue right down the rev range or is it only good for measuring peak?

RE: Engine Dyno Practise

You will never be able to get a steady enough reading on your scales to be useful or readable. You will be juggling that throttle and brake lever, and the rpm and torque will both be wobbling up, down and sideways, all over the place.

And as another poster has said, the noise, smoke, sparks, and smells will be really exciting. It may even fly apart and hurt you. But as a serious repeatable measurement tool for any useful engine development testing, it will be all but useless.

Get yourself either a water brake, or an eddy current retarder, mount it all in a very sturdy frame, and save yourself a lot of grief.
Then build a closed loop speed control system for it.

You set the target speed to maybe (?) 4,800 rpm.
Below that rpm, the dyno offers zero resistance, and you can start and warm up the engine under no load.

When you open the throttle, the rpm quickly rises to 4,800 rpm, and as it tries to rev any higher, the dyno load comes in harder and harder, so it holds that rpm rock steady at any throttle opening above idle.

You can then read off the exact steady torque on your scales at 4,800 rpm, and that is it......

Rinse and repeat at a different rpm setting.

 

RE: Engine Dyno Practise

(OP)
Thats exactly what I was thinking- big strong frame with scales mounted well and eddy brake, but I may not have actually stated that since my original post...

Still not sure how you would know how much load to put on the brake (eddy/water etc) as it must be adjustable, and too much wouldn't let the engine rev enough and not enough it will just rev past it without showing much load on the scales, or do you just juggle it with WOT so it revs at a certain speed but always WOT, then you always have 2 accurate reference points- 0 and WOT.

RE: Engine Dyno Practise

That is exactly why you need a closed loop speed control system.

It will automatically adjust the dyno load resistance to whatever speed you wish to test your engine at.
All commercial dynos do it that way.

RE: Engine Dyno Practise

Simple dynos (e.g. a manually adjusted water brake) will provide torque as an increasing function of speed.  There will be a speed at which the brake torque matches the engine torque.

As you open or close the dyno, its torque vs speed relationship will change - the curve will steepen or shallow.  This will move the equilibrium point and the engine will naturally speed up or slow down.

In practice it's quite easy to run a power curve by gently winding a dyno in or out to control the engine speed.  Of course if you want a part-load point, that's more complicated and where closed loop control comes in.

- Steve

RE: Engine Dyno Practise

Topcat - It is interesting that you mention A-Series engines. This is what I was testing. I used a Mini subframe with Morris 1100 drive shafts and disc brakes. The subframe pivoted on pieces of steel pipe. I used bathroom scales under the gearlever extension to measure the load.
Oddly enough it is not too difficult to balance the throttle and brake load.
 My main problem was deciding just how long the lever arm was, where to measure it from etc. With a Mini engine/gearbox you also have to factor in the diff ratio and gear ratio (if not in top gear). Mini engine/gearboxes are (in)famous for losing almost half the engine power on the way to the wheels - so I was never sure what I was measuring.  
 Another problem was to decide whether the torque/power readings I got were the equivalent of power at the flywheel, at the wheels or whatever. But as a comparison with a standard test engine the method worked very reproduceably and consistently.     

RE: Engine Dyno Practise

(OP)
Well even if I can only get a WOT reading at say 1000rpm increments then it will be good enough to be handy, so I will start hunting for an eddy brake!  I'm a bit put off by water brakes as I can't see cavitation not causing problems.

RE: Engine Dyno Practise

An eddy current retarder will be far simpler and much easier to control, because it can be controlled electrically.
The only down side to using one as an engine dyno, is the rpm limit on most is typically around 4,000 rpm.

But you can always fit the gearbox to your engine and run it in a suitable gear.

While a water brake will certainly increase it's resistance with rpm, if the torque also rises steeply with rpm, accurately controlling the speed can still be tricky. It works far better with a flat or a falling torque curve.

RE: Engine Dyno Practise

For many years before electronic controllers where even thought of, let alone invented people ran water brake dynos with a hand held string to pull on the throttle and a simple water flow valve to control the load.

You pull the string and turn up the load. If the revs are dying you back off the load before you stall it. If the rpm is increasing to fast you ease off the throttle to give you time to wind on more load then apply more throttle.

Once you have a steady rpm at full load, you slowly change the load and read the rpm. You can do this at however many rpm increments you like, but keep in mind the resolution of your instruments so you can keep it realistic.

I really can't simplify it any further.

Fancy controllers are nice, but dynos existed and operated long before these controllers existed.

I can remember dynos where you slid a weight out along the arm attached to the brake so as to measure the reaction. Load cells either electronic or hydraulic are a lot easier.

The device does need to be durable.

The load does need to be able to be maintained in a stable manner. A water brake or eddy field or even an adjustable electric generator set with somewhere to dissipate the charge can all do that.

Monitoring of temperatures and humidity is important to get accurate reproducible results.

Regards
Pat
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RE: Engine Dyno Practise

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.

  

RE: Engine Dyno Practise

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: Engine Dyno Practise

"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?

RE: Engine Dyno Practise

(OP)
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.

RE: Engine Dyno Practise

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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RE: Engine Dyno Practise

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.  

RE: Engine Dyno Practise

(OP)
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...

RE: Engine Dyno Practise

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.

RE: Engine Dyno Practise

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

RE: Engine Dyno Practise

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.



 

RE: Engine Dyno Practise

(OP)
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!

RE: Engine Dyno Practise

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

RE: Engine Dyno Practise

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.  

RE: Engine Dyno Practise

(OP)
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?

RE: Engine Dyno Practise

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

RE: Engine Dyno Practise

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

RE: Engine Dyno Practise

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

RE: Engine Dyno Practise

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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RE: Engine Dyno Practise

Depending on what you want the dyno to do, you could just use a big flywheel and measure the acceleration rate of said flywheel. As Pat already mentioned, there is always some error if you do not measure at steady state but if you are doing this for tuning purposes then it would suffice. You could use a proximity sensor and a cheap PLC with a high speed counter input to get accurate data.

RE: Engine Dyno Practise

(OP)
I would like the flywheel idea- but testing engines up to about 400bhp it would need to be so big it would be dangerous!  The Rover V8 I last built and is in my current car is 370bhp and I want more...

Will look for water brakes

RE: Engine Dyno Practise

You could use the Earth as a flywheel and the car tyres to turn it. Use the crank angle sensor or ignition pulses instead of a prox ensor.

RE: Engine Dyno Practise

There's an iPhone app for that.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Dyno Practise

Yeah, I have an app that attaches to the front seat cover that seems to work reasonably well...and...it's free!

Rod ;o)

RE: Engine Dyno Practise

Oh, good lord yes.......

-----When I retired in 98 I was 185lbs.  I zoomed up to 235 in two years and have now "slimmed" to a svelte 210 !!!


Rod

RE: Engine Dyno Practise

(OP)
Hey all, I have found a choice of 2 telma/eddy brakes, one is about 11" diameter, 12v and from a 7.5t lorry, the other is bigger at about 15" diameter, 24v and from an 18t fire engine, I cant find ANY information from the serial numbers etc, but could anyone hazard a guess as to what sort of KW each would be rated to?  I'm thinking the bigger one should be ok- but not sure yet!  

If I put a gear reducer infront then it will help absorb some power which is good- but dont want it to be too big a set up.

RE: Engine Dyno Practise

Information on these Telma retarders is hard to come by, but this is what I have scrounged so far :

CC50, 368 ft/lb, limiting rpm 4500.
CC80  589ft/lb, limiting rpm 4500, rotor diameter 340mm
CC100, 737ft/lb, limiting rpm 4500
CC125, 921ft/lb, limiting rpm 4000
CC160, 1179ft/lb, limiting rpm 4000, rotor diameter 570mm
CC200, 1474ft/lb, limiting rpm 4000
CC220, 1842ft/lb, limiting rpm 3500
CC270, 1990ft/lb, limiting rpm 3500
CC300, 2233ft/lb, limiting rpm 3500

The only reason I happen to know the rotor diameters for the CC80 and CC160 is that I have a CC80 in my own dyno, and a friend has a CC160 in his.
 

RE: Engine Dyno Practise

(OP)
Thanks, unfortunately the bigger one is not a Telma, the only lettering appears to be IC or KC, but not seen it myself yet.

The smaller one is a Telma, CE30 12v which would appear to be much smaller, so you are actually running one in an engine dyno?

RE: Engine Dyno Practise

My CC80 is direct coupled to some rollers and used as a chassis dyno.

RE: Engine Dyno Practise

(OP)
Does it work well with smaller and bigger engines?  I wasn't sure how well a little engine would work on the same dyno as a big V8!

RE: Engine Dyno Practise

This is a home dyno, used only for personal engine tuning use, on my own four cylinder everyday turbo car.  
A dyno cannot really be too big, because you can turn down the load resistance right down to zero.

RE: Engine Dyno Practise

(OP)
Thanks.

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