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[Sarcassidy]

Hi Everyone - I compete at the top level in Stihl Timbersports/Lumberjack Sports competitions. One of our events is the hot saw where we are using Rotax or Honda CR 250-350 cc motors for our power source. Here is a video https://youtu.be/G9DiMPGsH9U?t=12037. We are typically racing through three cuts of white pine or other soft woods 16-20" in diameter. Top times are 5-7 seconds.

For a while now I've thought that going electric could be a game changer for a few reasons. There would be no warm-up or start issues, there should be less vibration making for smoother operation, and the motor would reach top speed faster. Maintenance and reliability should be easier/better as well. Stihl recently gave me their top of the line lithium ion saw, the MSA-220C, to test and it checks all the boxes above compared to a gas saw of the same size, albeit a tuned up gas aw of the same size would be still be faster.

The two potential cons I see could would be that the electric motor could have so much torque that it throws the chain when I hit the throttle, and the potential weight of the battery back. The weight issue could be mitigated by either a small battery pack (I'd be running saw for 7-12 seconds max per run) or a battery pack that stays stationary on the ground and connects to the saw via cables, thus the battery pack does not need to be lifted. I"ve heard that supercapacitors might be better than batteries as they can dump a lot of power quickly.

I'd like to know what types of electric motors and power packs I should be looking into for the build. Our saws are topping out at 55-60 hp at 9000-10000 rpm. I am unsure of the torque. Chain speed maxes out around 200mph or 290 feet per second. The typical saw weighs between 50 and 60 lbs. Anything larger can slow down the operators ability to move fast through the cut transitions.

The normal heat time for our races is 6-9 seconds depending on size and hardness of the wood. The max range for a very large and hard piece of wood would be 12-14 seconds.

I'd like to know what the electric motor experts think about the potential for an electric racing saw build. Will I be able to match or exceed max power and chain speed of our two stroke motors? If so, what motors should I look into? Would the increased torque be an issue for throwing or breaking the chain? How about power packs? What types of batteries or supercapacitors should I look into? Finally, what would be the weight and cost of the recommend motor and batter components be?

Thanks!

[Doug S]

What a fascinating application! This is great sort of stuff for poking out the corners of what a technology is capable of. I'm going to be thinking about that for days, I feel sure. Here are a few thoughts off the top of my head.

First, don't get married to Li-ion batteries. I'd very seriously look at NiMH batteries, which can charge and discharge extremely fast. Your application isn't going to need much capacity -- it's over in seconds, after all -- but you'll need a very high discharge rate. Yes, supercaps can do that...but they're pretty expensive, and their voltage sags as you discharge them. I don't think you want your saw getting weaker during the run. NiMH batteries can give you very high discharge rate even with a fairly small battery pack. For instance, my RAV4 hybrid uses NiMH batteries and is spec'ed to have a ONE KILOMETER electric-only range, so you can see the battery is very small (they don't give the actual capacity). But the electric portion of the drivetrain is rated at well over 100 horsepower because of how fast the battery pack can discharge, given its small size. It also charges very fast when the ICE is running.

If you do want to go with lithium, look at LiFePO (Lithium Iron Phosphate) batteries. They don't have as much energy density as some of the other Li-ion formulations, but again, you don't need much capacity. LiFePO is capable of far higher discharge rates than most other Li-ion formulations, as well, which is what you really want.

As far as the actual capacity you'd need, for reference, if you wanted to put out 100 hp, that's about 75 kilowatts. If you need to do it for 20 seconds, say (just to be sure), that's only 0.416 kW-hr. The energy density of LiFePO is about 130 Wh/kg, so your LiFePO battery pack could weigh as little as 3.2 kG, or seven pounds. NiMH stands about half that, so you might have a 14-pound battery. Just a week ago, a team of researchers announced they'd achieved a record energy density for supercaps, at 88 wH/liter. I don't know how heavy a liter of supercap is, but it sounds like it's at best competitive with the LiFePO -- and it's not available OTC yet, it's just in the laboratory. I'm sure it's REALLY expensive, too.

As far as motors go, you're probably in the right place. Some of these electric motorcycle motors are very much in the ballpark of where you want to be in terms of power, and they're not all that heavy. I'm guessing you could find something in the 35-40 pound range with somewhere in the 100 horsepower output, lighter than your Rotax with more power and torque.

And I'm thinking you're right, all that torque might be a bit of a beast to handle, and hard on stuff too. Fortunately, however, everything's controlled electronically, so it seems like you'd be able to keep rpm from ramping up too fast, or exceeding a safe maximum, which would solve your startup and runaway problems (which you might experience if a chain breaks).

It sounds like a lot of fun, and honestly, a good application to take advantage of the very high output per pound of a good electric motor. I hope you'll keep in touch with us and let us know how it goes.

[Fran K]

You would need a new class hot battery saw.

Those hot saws do not run on gasoline.  What mix do you run in yours?

Dream on if you think that top of the line  Stihl 36 volt battery saw runs with the 35cc 201 either top or rear handle Stihl with the same bar and chain. I take it your idea of tuned up is different than stock.  The Husky t536lixp gets a time of 1 compared to the 338xpt of 0.6.  Basically the same physical size.  My testing showed that the skinny chain my battery saws come with isn't the fastest in bar close to burried compared to 63ps3 or 13rm.  Never experimented with that baby 1/4 inch pitch Stihl chain.

I doubt the chain questions are going to get answered here, obviously folks make v8 engine hold with two people creations.

I will add pertaining to the fear of run away.  These kind of electric motors and controlers make the maximum torque pretty much from real slow until max hp is reached and then there is bunch more rpm but hp stays the same so torque drops off.

[gregj]

A big capacitor in line or parallel could be use to tame the initial startup accelleration. Could be worth experimenting.
 Of course it may not be a problem. You don't need max revs ( for the torque) to cut and could have a more sensitive accelleration device ( spring loaded wide angle lever?). You could rev up to speed as you approach the initial cut ( to get the angular momentum into the chain to help you get a smooth cut)?

[Sarcassidy]

I dug pretty deep into the specs of various e-motors and battery configurations the past 24 hours. Max rpm seems to be an issue. I would need a motor that weighs 30 lbs or less that can crank out 8000+ rpm. E-motors like the ERMAX 208 look awesome as far as raw power and size go, but a 6000 max rpm means I would have to use a monster sprocket to get the required chain speed. A giant sprocket means a giant bar which disrupts the weight and balance factors.

Are there high speed e-motors out there? Could one be custom built? If so, how much?

[TheRan]

Couldn't you just use a multi stage gearbox if it's all going to be custom made?

[NEW2elec]

A more aggressive tooth chain to take advantage of the torque?  The bigger chunks wouldn't bog down the motor.  Chew more wood with each tooth so RPMs are less important.
 

[togo]

Smoothness of start and stop with a well-tuned electric motor might give you better chain handling, perhaps you could run a faster cut rate with good electronic controls, but this is an inherently dangerous activity, be very very careful.  A 2013+ Zero motor with a Sevcon/Zero controller powering a chainsaw would be amazing, but heavy.

[Doug S]

You don't need a gearbox, just a single gear -- two sprockets and a chain. And I don't see why they'd need to be giant sprockets. IIRC, the motor in my 2014 Zero SR maxes out around 5000 rpm, so if you want 10,000 rpm, you just need a 2:1 sprocket and chain set. I wouldn't think it would have to be that big, either. True you've got a lot of torque and horsepower to handle, but only for extremely short bursts.

Do the Rotax/Honda engines use direct drive? Certainly makes things easier but you don't have to do it that way. Also, same question NEW2elec has -- can the cutting chain be sharpened with a more aggressive profile, so it cuts as fast at a lower speed?

@togo: Thanks, mom, but I suspect he knows it's a dangerous hobby/profession.    But seriously, why do you think a first-gen Zmax and Sevcon controller would be heavy? IIRC that motor's around 25 pounds, and I can't imagine the controller is any more than 5 pounds. With a 15-pound battery and a sprocket/chain set, I think he's in the ballpark of the Rotax or Honda solutions.

[Biff]

The motor that Alta used seems like it might be a good candidate, as it spins fast like two stroke engines, and is very small and light..  If one (which can probably do 50hp for 12 seconds or so) isn't enough, you could use two.  I think they are less than 15 lbs each. I suspect you will be able to do more with less electric horsepower than with ICE horse power, as I doubt you are actually using the peak power of the ICE engine.

Another option would be to use the largest RC airplane motors as they are very light and powerful for short periods of time.

The  Emrax motor mentioned earlier would be a good option.

As far as chain breakage goes, I wouldn't be worried about that, electronic speed controllers can control the acceleration of the motors very accurately, you could dial it in to ensure you don't rip the chain apart if just going wide open as fast as possible ends up breaking chains.

-ryan

[togo]

> ...I wouldn't think it would have to be that big, either. True you've got a lot of torque and horsepower to handle, but only for extremely short bursts. ...

Indeed, a bunch of momentum in moving metal.

> Do the Rotax/Honda engines use direct drive? Certainly makes things easier but you don't have to do it that way. Also, same question NEW2elec has -- can the cutting chain be sharpened with a more aggressive profile, so it cuts as fast at a lower speed?

Electric typically will run at a higher speed than internal combustion

> @togo: Thanks, mom, but I suspect he knows it's a dangerous hobby/profession.    But seriously, why do you think a first-gen Zmax and Sevcon controller would be heavy? IIRC that motor's around 25 pounds, and I can't imagine the controller is any more than 5 pounds. With a 15-pound battery and a sprocket/chain set, I think he's in the ballpark of the Rotax or Honda solutions.

Haha you funny, if you use any of of my ideas you better bet there'll be a disclaimer there, I don't want to be sued, and I'd feel real bad if somebody got hurt because I was too free with advise.

[Fran K]

If you watch enough of the video it becomes apparent that only the top 6 of all entered in the event get to do hot saw.  You better have your chopping and manual saw technique down.  I have only seen those things at state or county fair type demonstrations.  They do have that Timbersports competition at the Paul Bunyan show I have seen some of the events Jack and Jill two person team manual saw, spring board three times and chop off the top etc, missed the hot saw.

It does not appear that those things even have the centrifugal clutch normal chainsaws have.  One does tune the depth gauges which controls how thick of a slice each tooth takes.

The Alta motor is 15,000 max if I recall so it is likely around max hp in the range those things run at.  There is an outfit that makes sprockets of pins or teeth of any number one might want.

[Apriliarsv]

Sarcassidy
Our saws are topping out at 55-60 hp at 9000-10000 rpm. I am unsure of the torque.

57.5 hp at 9,500 rpm means Tq is right around 31.8 lbft

[gregj]

One way of helping to tame the torque reaction and help the down cut ( although it would hinder up cuts) would be to increase the weight at the end of the bar. That would help move the centre of gravity towards the end and increase pendulum mass/distance relative to the centre of the rotation.
 Perhaps a more bulbous end on the bar with a greater end radius? think more like a circular saw. Not sure how the rules would handle that. Would help with smoothing twist twist as well (I think).
I know how savage torque from electric hand held motors can get after being given an undamped 1.2kw router. The guy that gave it to me wasn't strong enough to hold it steady when it started and had to go and buy a soft start one.
Suspending the motor on high hysteresis mounts my help as well ( butyl rubber or polyureathane os the right type).