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Water Dyno question


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#1 johnnycomelately1

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Posted 13 August 2015 - 21:57

Does anyone know (not speculate) if air in the line of the hydraulic line from the master cylinder on the water brake and the gauge will give the wrong reading?



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#2 gruntguru

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Posted 13 August 2015 - 22:17

Yes. (No.)



#3 johnnycomelately1

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Posted 13 August 2015 - 22:23

Yes. (No.)

is it right that you are saying air in the line does give an incorrect reading?



#4 gruntguru

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Posted 14 August 2015 - 00:51

No it will not give a wrong reading.



#5 bigleagueslider

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Posted 14 August 2015 - 04:36

The "brake" used on "water" dynos to apply a load is a hydraulic impeller/turbine arrangement. It does not rely on fluid pressure in the circuit to determine brake torque. Instead it uses a load cell linked to a torque arm attached to the turbine to measure its torque reaction. There is no master cylinder in the hydraulic brake system, and any small amount of air entrained in the circuit would have little effect.

 

2000px-Dyno_schematic.svg.png



#6 gruntguru

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Posted 14 August 2015 - 05:21

I think he is referring to a hydrostatic circuit used to convert torque to hydraulic pressure and display on a Bourdon tube gauge. Primitiv,e but still out there on some old dynos.



#7 johnnycomelately1

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Posted 14 August 2015 - 08:07

yes, it has a master cylinder, thats why i said it.

thanks for the answers.



#8 bigleagueslider

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Posted 16 August 2015 - 05:45

If the "master cylinder" is a linear cylinder connected to a moment arm on the brake, that uses a measurement of fluid pressure in the cylinder to indicate torque reaction produced at the brake, then a small amount of air entrained in the cylinder fluid will not pose much of a problem. Pressure is pressure, regardless of whether its in a gas or liquid. The only way it would matter is if there was a large relative amount of gas in the working fluid volume and high frequency measurements of the cylinder pressure were required. In this case the compression/expansion efficiency of the gas content would have an effect. But I doubt this would be the case with the type of load measurment arrangement used on this dyno system.



#9 gruntguru

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Posted 16 August 2015 - 23:21

A lot of air might introduce a compliance issue. The accuracy would suffer if the dyno housing rotates too far under high torque and changes the geometry of the force measurement linkage.



#10 johnnycomelately1

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Posted 17 August 2015 - 13:03

my thought are that the compression of the air takes part of the energy hence not moving the hydraulic fluid



#11 Joe Bosworth

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Posted 18 August 2015 - 01:05

JCL1, until you explain what the master cylinder is doing your respondents can only guess.

 

Slide is ASSUMING that it is being used as a pressure transmitting device which makes his response 100% correct.

 

If you can't explain its usage and you don't know the difference between force and energy then there is no hope for your thread.  ;-)

 

Regards



#12 bigleagueslider

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Posted 18 August 2015 - 03:17

Thanks JB. Exactly what I meant. The entrained gas is compressible, so it will affect piston travel but not gauge pressure in the fluid that is being measured. All this assumes the cylinder piston and brake are free to travel in both directions as required.



#13 Robin Fairservice

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Posted 18 August 2015 - 04:02

If there is enough pressure, and not a lot of gas, then wouldn't the gas go into solution and then have a negligible effect?



#14 bigleagueslider

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Posted 20 August 2015 - 02:37

I don't this would be a concern with entrained air at the pressures/temperatures existing in the fluid circuit of the example described.



#15 Lee Nicolle

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Posted 28 August 2015 - 01:24

Is their any waterbrake dynos left now? My local dyno shop had one for decades but have replaced it with a 4 wheel drive Dyno Dynamics unit. Mostly because of the 4 wd capability for so many modern vehicles but it also cops more power too. And is just  more efficient to use.

The old one though always worked ok.

 

Watching US clips of dyno runs it seems that many places only use one drum, not the two like all the ones I have ever seen here in Oz. Why? it does seem quite dangerous as I know how hard it is to keep a car from climbing out of two drums, the torque recaction too tends to make them want to go sideways.

Powerfull cars are dangerous on any chassis dyno but one drum would scare me!


Edited by Lee Nicolle, 28 August 2015 - 01:28.


#16 bigleagueslider

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Posted 28 August 2015 - 03:34

There are still water brakes used for engine dynos. Water brakes are durable and cheap, and it is easy to dissipate the large amounts of heat absorbed by the brake with a simple cooling tower. For example, a basic 500hp water brake engine dyno installation might cost $75K. While a basic 500hp eddy current brake engine dyno installation might cost twice as much.



#17 gruntguru

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Posted 28 August 2015 - 04:33

Rotational speed limit is an issue with eddy current retarders (read air-cooled truck retarders) as is continuous power rating if air-cooled.



#18 johnnycomelately1

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Posted 29 August 2015 - 23:42

we have two water dynos, one engine , one wheeled.

speed is not an issue, if it is you go back a gear and recalibrate.

the engine dyno readily handles 12,000 rpm and the wheel dyno 200kph no problem.

there seems to be a lot of theory in here, from that i was simply asking for sure if anyone knew if air in the line affects the accuracy.

there seems to be a tendency when a straight answer is not known to try and baffle with science.

if the forum has a use, it would be to advance knowledge, both practical and theory.

the master cylinder can only be in one place that i know of, if someone cant work that out why query.

also the term energy is just a generic descriptor to illustrate the the question, again if that cant be interpreted why give smart alec answers.

does any one know if air in the line gives a false reading? simple.



#19 Greg Locock

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Posted 29 August 2015 - 23:49

I dunno, most of your questions are so badly worded and hypothetical it is hard to tell what sort of answer you are looking for.

 

So, handbiter, air in a hydraulic pressure line will not affect the steady state pressure in the line, in theory, and probably in practice.

 

And if you are so bloody clever why not do the experiment and find out? You do /calibrate/ your dynos don't you?



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#20 bigleagueslider

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Posted 30 August 2015 - 01:32

There are very low-cost, simple water brake dyno systems and expensive, sophisticated water brake dyno systems. But they all now employ some form of electronic load cell to measure brake torque, rather than the hydraulic cylinder and mechanical pressure gauge arrangement described in the OP. Electronic load cells (piezoelectric, strain gauge, etc.) and digital data acquisition systems are now very capable and easily within the budget of anyone performing dyno testing. It would probably only cost a couple hundred bucks to retrofit the dyno with a strain gauge load cell and a PC-based data acquisition system.



#21 gruntguru

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Posted 31 August 2015 - 02:22

 

the engine dyno readily handles 12,000 rpm

 

Which is why water brakes are more common for engine dynos. An eddy current retarder capable of 12,000 rpm is very expensive. Similarly, the power absorption of a water brake is continuous, for an air-cooled eddy current retarder, the continuous rating is typically <25% of the maximum rating.



#22 bigleagueslider

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Posted 01 September 2015 - 05:39

I believe DC dynos are preferred for applications like mapping/calibrating of high rpm race engines. DC dynos can be used to both motor and brake the engine. The problem with water brakes is they can only absorb in one direction of rotation. Eddy current brakes use water cooling to reject the absorbed power. And while eddy current brakes don't have the speed capability of water brakes, eddy current brakes provide much more precise control at low speed/high load for applications like performance mapping of diesel engines.



#23 Canuck

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Posted 01 September 2015 - 15:36

2 vs 1 roller. Assuming you're talking rollers per axle as opposed to rollers per vehicle, I've used both. The dual-roller put much more heat into the tire than did the single, as the tire rides on top, rather than sort of pinched between. Any coming off the roller(s) is a result of poor application of tie-downs or other vehicle restraints.

#24 gruntguru

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Posted 01 September 2015 - 22:18

The extra heat is more likely due to smaller diameter on the two roller dyno. For high power work the car needs to pull forward onto the front (drive) roller anyway, making the two roller into a single roller dyno.

 

Two roller dynos are much more convenient for the many times an average workshop wants to drive on, do some low power (or zero power) checks and drive off.



#25 Lee Nicolle

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Posted 04 September 2015 - 08:14

2 vs 1 roller. Assuming you're talking rollers per axle as opposed to rollers per vehicle, I've used both. The dual-roller put much more heat into the tire than did the single, as the tire rides on top, rather than sort of pinched between. Any coming off the roller(s) is a result of poor application of tie-downs or other vehicle restraints.

Having had 500 RWHP cars on modern dynos they are trying to climb up and out of the rollers. That is why those single roller scare me.

As for tyre problems?  IF you are using very soft dirt track tyres it can be a problem but I have never had problems with road race tyres ever. Unless you are doing 10min power runs, not the 30-60 sec that is usual.

Look at some of the clips on You Tube, lots of scarey accidents as cars get off the rollers. 

Tie downs are webbing straps and do stretch.  I have heard of one breaking too. They really are not rated to drive against, just tie down goods. And sometimes the lack of tie down points can be a real issue too.



#26 Greg Locock

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Posted 04 September 2015 - 12:11

We always use slave tires if the cars are going to be on twin rollers for long. The reverse curvature at the contact patches is reputedly very bad for the tire structure. I don't know what incident caused that rule to be introduced but it was pretty strictly enforced.

 

Restraining cars on 2m single rollers is quite an art - full throttle runs on late 80s FWD turbos were always exciting. Obviously at FoA we were very used to RWD, just clamp the front wheels down and use a crossed pair of long tie downs at the rear. Never had an incident, but then we'd all been doing it for years. Now they've got some fancy 4wd rig, never played with it.



#27 Lee Nicolle

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Posted 04 September 2015 - 23:17

We always use slave tires if the cars are going to be on twin rollers for long. The reverse curvature at the contact patches is reputedly very bad for the tire structure. I don't know what incident caused that rule to be introduced but it was pretty strictly enforced.

 

Restraining cars on 2m single rollers is quite an art - full throttle runs on late 80s FWD turbos were always exciting. Obviously at FoA we were very used to RWD, just clamp the front wheels down and use a crossed pair of long tie downs at the rear. Never had an incident, but then we'd all been doing it for years. Now they've got some fancy 4wd rig, never played with it.

There is a You Tube clip of a serious Front Drive [are they really serious!] Drag car. High growth tyres restrained down by the straps and the tyre expodes. You can see it going to happen.

That actually is another drama with using at least race tyres on a chassis dyno. They all grow considerably. Even  some road race tyres do to some degree though I doubt they are really a problem.

Dirt tyres, Drag tyres, some off road tyres [especially the wide tyre skinny rim trend] do grow alot.

The dyno challenge at car shows etc too are sometimes dangerous as the rollers are not well restrained [or unrestrained totally] and it seem often go walkabout!