Homemade Chainsaw Dynamometer

[timberwolf]

Took me a while, but built one. (right thread this time)

Still working out some improvments, but so far so good, smaller 50cc saw is enough to pull it up to speed in 20-30 seconds and holding up to 100 plus cc saws.

Testing an 026 in pictures and a ported 066BB in video. Ran it up to 11,500 in the video and 12,000 on a later run.

http://www.youtube.com/watch?v=fatrSgdW6hs

 

[woodgrenade]

Wow

That looks awesome. What kind of software are you using? How are you measuring output, with some kind of photogate?

 

[timberwolf]

Yes, using optical sensor to track RPM and then calculating torque and HP from established formulas.

Still a work in progress, able to track other temperatures and pressures as well but have just played with that part a bit so far.

 

[Plan-b]

Nice work Timberwolf!

 

[woodgrenade]

Cool, very very cool.

 

[Haywire Haywood]

Does it work well for artificially tuning "in the cut".

Ian

 

[timberwolf]

Not so much, a break or pump type dyno would be better for steady state tuning. But it does look promising for tuning to get maximum HP, then it's the operators job to put it in the powerband and the in the cut speed will come on its own.

 

[blsnelling]

That's way cool Brian!

 

[brncreeper]

That's definitely an inspiration to the do-it-yourselfer, great job Brian!

How did you do the software and (sensor to computer) interface?

 

[Ol' Brian]

I think that a big burly, robust truck alternator with something like 200 amps output could be used to make a saw dyno...

Control the field to vary the output, and just use a resistive load, like a heating element in a water tank, to absorb the power. Then it would be extremely easy to measure the volts and amps, and calculate the horsepower produced. And, with a big heat sink like a water tank, you could hold the saw at constant output for extended periods to get the carb tuned just right.

Works in my head anyway!!!

 

[barneyrb]

You have a known weight/inertia and you are calculating tq by the time required to achieve a set rpm???? Have I got this right? BTW, really great job.

 

[timberwolf]

"You have a known weight/inertia and you are calculating tq by the time required to achieve a set rpm????"

Sort of, I have a known mass with known moment of inertia but calculate torque through rate of change in RPM rather than time to achieve a set RPM.

Problem with an alternator is the efficiency will change as frequency changes... bunch of reluctance and capacitance phase shift junk going on between windings and the iron core. Also a saw putting out 12 hp is cranking out close to 10,000 watts. Thats going to take a pretty big generator/alternator which is going to have significant windage and mechanical losses that will be hard to acount for.

With inertia dyno the spin down rate after a run can be tracked to generate a windage and bearing loss curve which can be used to correct the spin up data.

 

[barneyrb]

"You have a known weight/inertia and you are calculating tq by the time required to achieve a set rpm????"

Sort of, I have a known mass with known moment of inertia but calculate torque through rate of change in RPM rather than time to achieve a set RPM.

Problem with an alternator is the efficiency will change as frequency changes... bunch of reluctance and capacitance phase shift junk going on between windings and the iron core. Also a saw putting out 12 hp is cranking out close to 10,000 watts. Thats going to take a pretty big generator/alternator which is going to have significant windage and mechanical losses that will be hard to acount for.

With inertia dyno the spin down rate after a run can be tracked to generate a windage and bearing loss curve which can be used to correct the spin up data.

Are you taking into consideration the clutch bearing also? I know it's not much but is it worth figuring this into the equation. Also, would it wreck a chainbrake if you accidentily engaged it on spindown?

I think this would be a better alternative to the alternator set-up.

http://www.magpowr.com/magpowr_products_magpowr_bseries.asp

 

[timberwolf]

I am not taking the clutch bearing into the calculations, it only turns when the clutch is not locked to the drum. Anyway it saw clutch, bearings, bar and chain are all there on a working saw so having them conected in the dyno setup is like measuring rear wheel HP vs engine HP.

There is a built in fail safe in the driveline to let the flywheel overrun the engine.

They do use this magnetic partical principal in some types of dynamometers, but take a look at the specs of the ones you linked, they come up far short of what is needed for testing a saw both in RPM and total energy. I'm thinking $$$ are going to beome a factor to get something big enough in that sort of equipment.

 

[artie__bc]

If you're using a generator or alternator, an easy way to create load is to connect it to a set of electrodes and lower them into a vat of brine. The deeper you immerse the electrodes, the greater the load. This is how we tested the generators we repaired at tech school.

 

[DroppedP51]

water pump and vary the ouput nozzle size to control pressure.

measure pressure, volume, and calculate horsepower.

Some tractor pto dynos are done similar with a tap to shut the flow back with a pressure guage on them

 

[Nethercutt]

"... Also a saw putting out 12 hp is cranking out close to 10,000 watts. Thats going to take a pretty big generator/alternator which is going to have significant windage and mechanical losses that will be hard to acount for.
...QUOTE]

A large pickup-truck alternator can easily draw 15 hp.
We should be able to spin the saw up to max RPM, and increase the duty cycle on the alternator until the saw is no longer able to keep up speed, and begins to bog down.
With a specification for the alternator, it should be pretty easy to look up the loading torque for that RPM at that duty cycle, and determine the HP.

As far as coupling goes, one would want to couple directly to the crankshaft if possible, and not after the saw's clutch. A stihl 056 has a very nice 3/4" hex protrusion that fits nicely into a socket. A Stihl 031 has the same 3/4" protrusion, but it is much too shallow to couple nicely with a socket. Maybe a three-pronged adapter that would mate with the three clutch shoes may work well.

 

[FATGUY]

awesome, rep sent

 

[drkptt]

With a specification for the alternator, it should be pretty easy to look up the loading torque for that RPM at that duty cycle, and determine the HP.

As far as coupling goes, one would want to couple directly to the crankshaft if possible, and not after the saw's clutch...

If you go to the trouble of setting up an alternator brake for steady-state testing it would not be much harder to mount the alternator housing in a trunnion bearing so it freely rotates about its shaft axis. Attach a torque arm to the housing to keep it from rotating and measure the force at the end of the torque arm with a load cell (or even a postal scale). Now you're measuring torque directly; no worries about errors from measuring electric power or accurately knowing the efficiency of the alternator.

As far as the clutch it's been my experience that it's absolutely necessary to have one to disengage the dyno drive at idle. The hammering of an idling 2-stroke twists more shafts than WOT runs.

 

[Nethercutt]

If you go to the trouble of setting up an alternator brake for steady-state testing it would not be much harder to mount the alternator housing in a trunnion bearing so it freely rotates about its shaft axis. Attach a torque arm to the housing to keep it from rotating and measure the force at the end of the torque arm with a load cell (or even a postal scale). Now you're measuring torque directly; no worries about errors from measuring electric power or accurately knowing the efficiency of the alternator.
[\QUOTE]

You're right.
Crap, now I have to build one.
Just what I needed... another project.

Although I disagree with you on how to couple it to an engine. The nice part about the alternator setup is that you can just set the duty-cycle back to zero, and effectively decouple it from the engine. The inertia of the rotating assembly should be negligible. (I hope I don't use that last sentence as the caption for a picture of a bent crankshaft some day...).