Chainsaw Dyno bring saws to there knees. Build from start to end with video

[drkptt]

One thing I haven't noticed mentioned that would account for the inaccuracy of the reading, friction and inertia aside, is the increase in temperature of the hydraulic fluid. I've built a few small engine dyno in the past and the simplest is driving a positive displacement pump. Hydraulic pumps are simple and cheap to come by and can stop a very large load. I used have a 400 HP rig that used a 50 HP hydraulic pump out of an old combine. There was only one source of measurement in power and that was a pressure/temp gauge. As the pressure went up rolling resistance was placed on the engine and by calculating the temperature increase of the fluid at a given rpm one could determine work done and torque applied. I cannot for the life of me find the old files I had with the formulas in a spread sheet but know I found them online very easily.

So if you take the fluid temp increase, along with the friction of the chain, slippage in the clutch, resistance of a viscous fluid through the pump, and bearing friction all into account. You should have a very accurate setup. As someone who has spent the last ten years developing import race engines and countless hours of dyno work I must say nice work!

You must have missed the torque arm and scale on this dyno. Change in fluid temperature won't affect the accuracy of the measurement. It will cause difficulty in load control because the viscosity will change and the pressure drop across the regulating valve will drop as the oil heats. This will require adjustment of the valve to maintain constant speed.

I can't imagine using temperature change to calculate dyno load. Pressure and flowrate would do it more accurately.

 

[peacmar]

You must have missed the torque arm and scale on this dyno. Change in fluid temperature won't affect the accuracy of the measurement. It will cause difficulty in load control because the viscosity will change and the pressure drop across the regulating valve will drop as the oil heats. This will require adjustment of the valve to maintain constant speed.

I can't imagine using temperature change to calculate dyno load. Pressure and flowrate would do it more accurately.

The temperature rise can be calculated as

dt = Ps (1 - μ) / cp q ρ (1)

where

dt = temperature rise in the pump (oC)

q = volume flow through the pump (m3/s)

Ps = brake power (kW)

cp = specific heat capacity of the fluid (kJ/kgoC)

μ = pump efficiency

ρ = fluid density (kg/m3)

 

[Hddnis]

The temperature rise can be calculated as

dt = Ps (1 - μ) / cp q ρ (1)

where

dt = temperature rise in the pump (oC)

q = volume flow through the pump (m3/s)

Ps = brake power (kW)

cp = specific heat capacity of the fluid (kJ/kgoC)

μ = pump efficiency

ρ = fluid density (kg/m3)

That is not how this dyno is designed to work and such a calculation is less than useless for this setup.



Mr. HE

 

[drkptt]

That would be a silly way to calculate power for any dynamometer. Note the (1 - μ) term. Your dt is only due to pump inefficiency, not the amount of work put into the system from the engine. The higher the pump efficiency the smaller the temperature rise would be--if the pump was 100% efficient the dt would be zero. How would you go about determining the pump efficiency to use this method?

You do know the big temperature rise occurs across the valve when the pressure drops back to atmospheric, right?

The temperature rise can be calculated as

dt = Ps (1 - μ) / cp q ρ (1)

where

dt = temperature rise in the pump (oC)

q = volume flow through the pump (m3/s)

Ps = brake power (kW)

cp = specific heat capacity of the fluid (kJ/kgoC)

μ = pump efficiency

ρ = fluid density (kg/m3)

 

[Hddnis]

...You do know the big temperature rise occurs across the valve when the pressure drops back to atmospheric, right?

The temp rise would only be from friction and not from the pressure drop.




Mr. HE

 

[Naked Arborist]

Hat's off to you chadihman...
Big rep bomb sent your way...
Now...
Less egg head talk, and more vids...

Oh, and air filter off = more horsepower... Not necessarily from being too lean, just from easier flow. A two stroke motor is just a pump... Take the screen of and let it suck!!!

On to the 461!!!!!!

I thought we'd all be egg heads after reading all this fine technical information. On with the testing please.

 

[peacmar]

OK before this gets out of hand I only posted the formula for information sake and I understand that this is a steady state resistance load type of dyno. The formula I posted is the basis of some of the oldest small engine dyno there have been. Basically a purposely inefficient hydraulic system that measures power by calculating the inefficiency. Direct drive, minimal loss, most accurate from an empirical stand point. Simply a matter of conversation and things for the OP to ponder in his creation as it grows.


Having worked with hydraulics I do still see one flaw that could hinder performance on large saws of high output. If the fluid heats quickly enough or one would want to do some heavy duty testing of a large displacement saw it could become hard to control such said saw once the fluid does heat up. As the fluid heats and viscosity decreases, so does the torque transmittance to the load cell through the torque arm. The pump may not be able to handle enough pressure to create enough resistance to take a reading.

An example with completely made up and meaningless numbers and values:

An imaginary saw can generate 30 lb/ft of torque at maximum power

A pump will transmit rotational torque to the body that is equal to the force applied on the shaft

Said pump requires 30 lb/ft torque to generate 2000 psi pressure if the fluid is at 80 degrees, pump will absorb power of the saw.

Fluid is now at 150 degrees, pump is able to generate 2000 psi with only 7 lb/ft torque. Doubling the pressure to 4000 psi will only require 20 lb/ft of torque. Pump maxed out and cannot hold back saw until fluid has cooled again.


Again these numbers are made up but the real life specifics are available from the pump manufactures and are something to consider for anybody planning to build one.

 

[ozflea]

Keep going fellas this puppy will be worth a mint ............. be satisified .............. its not the space shuttle you know

 

[Mastermind]

Not sure if I can ask this but here goes. Would anybody be interested in a dyno if the cost was $1500. Maybe I could make them cheaper on the next build. I'm just trying to figure out if this would be worth manufacturing.

I'm interested.......work out the bugs and it's a deal. :msp_thumbsup:

 

[chadihman]

I'm interested.......work out the bugs and it's a deal. :msp_thumbsup:

Holy crap I thought you were never leaving that boat. Welcome back! The bugs are getting worked out. My first problem was the drive pins in my 24 tooth sprocket were sliding out. I fixed that with a spot weld on every pin. Second problem was my shaft rpm sensor was not working. Got that working now. Now I'm wating on some vibration dampening pads for the scales to sit on. The vibration made the scales jump a bit. The pile of rags on the video helped a lot. I think I'm going to put a oil cooler on it because the oil got to max temp after I broke in a 460 for one tank of fuel. Wow the 460 bb drinks fuel when its under constant load. This thing sure is sweet for breaking in a saw.

 

[tony56]

Got the dyno done for the most part. I have lots of pictures of the build and a video of a MS 460 begging for mercy.

 

[tony56]

Would sure like to know where you got the driven sprocket... Is it 3/8", or .325 ?

 

[Paragon Builder]

Would sure like to know where you got the driven sprocket... Is it 3/8", or .325 ?

I don't think chad has been around for quite some time. http://www.danzcoinc.com/html/chain_saw_sprockets.html
Has some, or can make them. I'm sure there are others...


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[Firemoore98]

Buying cool is one thing, making something cool is what impresses me the most. Solid work!!! Thanks for sharing

Jason


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[rd35]

Wow, I just found this thread. Been working up a design for a dyno (in my head mostly) and the OP dyno is EXACTLY what I was planning to build in terms of the concept. Hydraulic pump to absorb the power. Shaft sensor tach and a scale to read the floating pump torsion created against a lever arm pushing on a digital scale. This is, in my opinion the most simple, yet fully functional and accurate method available to determine power from a chainsaw. Only error is the loss in the pillowblock bearings. I was hoping to be able to measure torsion on the entire assembly to get a more accurate torque reading. HOWEVER, I have not lifted a finger yet. I have been all talk-no go! Awesome job Chad! You actually did it! That thing is just too cool!!!

 

[sweepleader]

VERY NICE, excellent! Maybe in future models you might want to mount the pump sprocket outboard so it could be serviced easily and without messing with the pump shaft or bearings. It would be overhung but with the loads involved that would not seem to be a problem. The bearings themselves could be a source of friction losses, oil lubed bearings would have less drag than greased bearings. Shielded bearings have reduced drag compared to sealed bearings too. You might also want to reduce the chain contact with the bar to a minimum needed to guide the chain between the sprockets.

Maybe you have thought of these things already, in that case, never mind.

Love it, thanks for building it and sharing with all of us. Beautiful.

 

[tomdcoker]

A hydraulic motor is very inefficient and thatis basically what you have. I learned this while running a 5 HP 220 3 phase motor to pull a hydraulic pump on a surface grinder with a phase converter. After 30 minutes the motor was smoking, but a 50 an 20 microfarad running capacitor connected in parallel stopped that. Tom

 

[67L36Driver]

The pump don't have to be efficient. Just provides the variable resistance so you can measure torque and speed.

 

[BrettS]

Chainsaw Dyno, bring saws to THEIR knees.

 

[sweepleader]

Just noticed the plate on the scale should have a pivot joint, to prevent binding when the arm from the hydraulic motor rotates. It will not be much but it could throw off the readings quite a bit. The arm will tend to tip the plate with a solid joint causing uneven loading.

This thing is GREAT! Have you run it?