Tinkerer's Corner: Homemade "Hybrid" Zero Turn Radius Riding Mower

[samclem]

I've been kicking this idea around for awhile and thought I'd draw some comments (and spears) from the tinkerers here.

This is a long post that describes a harebrained project that won't interest most people at all.

Overview: The project is a small ride-on lawnmower (approx 40" wide cut) that would have an onboard gasoline-powered 5500 watt generator (13 HP). This would power two drive motors, one for each rear wheel. Front wheels are free castering, steering is by differential drive of the rear wheels. The grass is cut by two or three rotating blades directly connected to AC motors (just like a plug-in corded lawnmower).

Layout: (I wish I knew Sketch-up!) Generator is at the rear of the vehicle, driver sits in front of it and low. The rear wheels are located outboard of the driver and generator, The mower deck is directly below the operator's legs. Castering wheels are at the front corners. The mower deck could be free-floating with its own adjustable wheels.

Below is a picture of the ($5000+ ) Zeon rechargeable ZTR mower. The layout I'm envisioning is similar, but with a generator back there in the bustle in place of batteries and a slightly lower seat. The "lap bars" would be replaced by a forward/stop/reverse handle on each side. And there would be a canopy and fuzzy dice. The Zeon has electric motors directly spinning the blades and two drive motors that operate through transmissions to power the rear wheels.





Power Factors: We've got 5500 watts max continuous at 120 VAC = 45 amps available. Standard corded AC lawnmowers with a 21" cut are typically rated at 13 amps max, so we'd need 26 amps total for two similar motors. That leaves 19 amps (2280 watts) for our two drive motors. That converts to a little over 3 HP total, or 1.5HP per side.

Estimated Weight: 750 lb
-- Generator: 170 lbs
-- 2 drive motors and 2 cutter motors: 60 lbs
-- Wheels and axles: 40 lbs
-- Frame, mower deck, blades, seat, wires: 300 lbs
-- Operator: 180 lbs

Cost of components: No good estimate possible. The 5500 watt generator is about $425. The mower deck could be salvaged from an old riding mower, mount the electric motors where the blade spindles used to be. The frame would need to be welded up, but without any steering components or suspension, it would not be complicated.

As a plus, during development all the sub-parts can be tested and run from the house using a (beefy) extension cord.

Why do it?
- Fun project
- Compared to a regular gasoline version this has no clutches, complicated transmission, steering mechanism, etc.
- If the generator gives me trouble I can easily remove it from the chassis and have it serviced--or replaced.
- A "real" small ZTR mower starts at about $2200
- It might actually mow the lawn faster than me and my push mower
- If we lose electricity, I've got a mower that can power my well pump or my fridge and furnace.
- I can add a small electric string trimmer to a front corner and trim the borders as I mow
- I've only got a few years left on my term life policy--this is a chance to make DW rich!
- I'm thinking there must be a way to get federal money for this. It's a hybrid! And, it could be "zero emissions" (Add the optional ballast weight in place of the generator, order the optional 2 AWG extension cord that conveniently plugs in where your clothes dryer gets power). Virtually silent mowing! Get Green bragging rights, a tax credit, and probably some sort of carbon offset payments.

Unknowns:
1) Are 1.5 HP drive motors (one per side) enough? I'm not building a racing machine, but if I can't go up a moderate grade or accelerate to 3-4 MPH (about 5 FPS), then this might not work. I've heard a lot about the fantastic low-end torque of electric motors, but is that only high $$ DC permanent magnet types? 3 HP doesn't sound like much. But, here's a 3/4 HP (4.5 A at 110VAC) electric winch that lifts 220 lbs (straight up) at 33 FPM. At our approx 750 lb all-up weight, a 1:5 slope, and 3 HP available, that would give a vertical lift rate of 5.6 FPM (which is faster than the vehicle's target speed on level ground). Obviously, that's just a back of the envelope approach, (I didn't do the MGS math) and there will be lots of other losses, but maybe these motors will be enough.

2) Source of drive motors/associated controllers: I'm guessing the optimum motors won't be the cheap motors. There are some (big) plug-in hand tools that could be pressed into service in the hillbilly approach: electric impact wrenches, etc. These come with their own gearing and controllers, just take the power from that 1/2" square drive and rig up a mechanical linkage to the trigger, plug 'em right in to the generator. Be sure to buy the extended warranty.

3) Brakes: I'll probably need some. Maybe a set from a small motorcycle would do okay.

"What do you do all day?" indeed.

Okay--ready for the darts!

[Walt34]

Well, like my Dad used to say "damnifiknow" but is sounds like a good idea.

[ERD50]

Darn it! I just bought a new riding mower this spring. This idea is so much cooler than my my Craftsman T2600 (46", 19HP)!

Hmmmm....

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Why do it? - Fun project - Compared to a regular gasoline version this has no clutches, complicated transmission, steering mechanism, etc.

Yes, the direct drive motors do offer advantages. But you need ~ 2HP per blade - that's a big motor isn't it? I have not looked at what the 21" corded use (OK first one I found at amazon is $210 for the whole mower, and 53# total, so maybe not too crazy?).

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- If the generator gives me trouble I can easily remove it from the chassis and have it serviced--or replaced.

Yes, it really bugs me that I have this 19HP engine that is single purpose.

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- A "real" small ZTR mower starts at about $2200

I researched ZTR mowers before I bought this craftsman. I was steered away from them (pun intended). They said they were actually rather difficult to get accustomed to, and the homeowner models just were not that great.

But being electric drive might be better?

BTW, my Craftsman is a 'Tight Turn' model - 6" radius, so I figured that is a good enough, and it is very nice.

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- If we lose electricity, I've got a mower that can power my well pump or my fridge and furnace.

A plus, for sure. And unlike a stand-lone generator, this will get a regular workout, so you will probably have more confidence that it will start when you need it.

I've got a power inverter (two now, see below), but that won't handle my well pump.

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- I can add a small electric string trimmer to a front corner and trim the borders as I mow

This was a recent DIY project, far down the scale from your attempt. I bought a second power inverter, mounted it to aboard to make it a bit more convenient, and I hook it to my tractor and use the electric, corded string trimmer out at the corners of the yard. Nicer than dragging all that cord out there, and it might come in handy as a second unit if we have an extended power outage.

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Virtually silent mowing!

In a recent review of a battery powered riding mower, they were surprised how noisy it was - those spinning blades are a pretty big component of the overall noise.

Wait a minute- a transmission for each side, how will you control the turns? Won't you need finer control? My Craftsman has the variable-pulley-size CVT, is that what you are planning on? The CVT is nice in many ways, but I'm not totally thrilled with it - it's a little jerky and slips in/out of moving and not moving when you in the lowest speeds and are close to neutral. That could be a real problem trying to make turns?

For low-end torque, yes, I think it depends on the type of motor. I'm not really up on motor types, I always need to google when I need details. But a power tool like a drill certainly has low-end torque ('universal' motors with brushes & commutator?). But a brushless induction motor (like in a furnace blower, or desktop fan), has low starting torque.

What colors will it come in? When is the IPO?

-ERD50

[M Paquette]

Oh yeah! More power! Giant whirling razor sharp blades! Huge batteries! And it's a vehicle!

What could possibly go wrong?

[Telly]

Oh boy! I'm gonna feel like a killjoy

Relating the hoist to the mower, did you mix up FPM and FPS somewhere there?

Over half of the available power is going for cutting, not vehicle drive... and it's ~750 lbs., and up a slope. Seems woefully inadequate on wheel drive HP. And will need to have serious gear reduction between wheel motors and drive axles. You can probably come up with a ball-park axle torque requirement via wheel radius, mass, acceleration, reserve factor, etc. etc.

Remember that the drive system needs to be continuous duty rated, able to get rid of heat, and operating in a dusty/fibrous environment.

Speaking of drive axles, lawn tractors have a transaxle or transaxle/hydro unit that besides providing reduction gearing, also provides the axle support and wheel bearing function, a semi-floating axle. With a per-wheel motor and gear reduction unit, you will have to create the wheel-axle support function, and it will have to be very sturdy in all directions, all the while keeping side-loads off of drive components not meant to take it. Not a simple task!

Weight include gas tank and many gallons of gas, and battery to start generator?

A lot of energy conversions here, are conversion inefficiencies taken into account at the top-level power budget?

Series hybrids are excellent for locomotives - weight (within reason) is not a negative, as it aids traction; No ratio-varying transmission is needed (the #1 reason for the diesel-electric design); Plenty of room for forced-air cooling, etc.
Diesel-electric hybrids have moved on to mining loaders and trucks for many of the same benefits as locomotives when getting really big. I doubt if it scales down nicely...

[FIGuy]

On the "are the 1.5hp motors enough" question:

1hp = 550 ft-lbs / s. This means you can push 550lbs 1 ft in a second or 1lb 550 ft in a second. So, given 2x1.5 hp you have 3*550 =1650 ft-lb / s. 3 mph is about 5 ft / s, so you've got a thrust of 1650 / 5 = 330 lbs. Now, since you'd have to find a extremely high slope to come anywhere near that, I highly doubt you'll see a problem. Electric motors are rated "continuous" as well so you've got a burst of 2-3 x that available for short periods. You could probably move a F250 around at 5mph on 3hp electric motors (geared to that speed).

Now, converting gas to electric to motion is too complex for this task IMO, but it sounds like you just want a fun hobby here. In this case, I'd find a gas one with a dead engine and just convert that to electric. I've been thinking a small tractor converted to electric and carrying a PV array overhead for power and shade would be a fun project myself. Now, if I only had a yard...

[Ronstar]

Seems doable. Weight is in line with gas powered riding mowers. I say give it a try

[REWahoo]

Sam, if you do go forward with this project you'll have to provide ongoing updates with photos. I'm not skilled enough to attempt something like this but enjoy admiring the efforts of those who are - from a safe distance.

[samclem]

Thanks for all the feedback so far. Keep it coming!

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Originally Posted by Ronstar Seems doable. Weight is in line with gas powered riding mowers. I say give it a try

Yes, but it doesn't sound right that all this stuff would weigh about the same as a simple single-engined riding mower. Maybe my estimates are off.

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Originally Posted by FIGuy You could probably move a F250 around at 5mph on 3hp electric motors (geared to that speed).

Interestingly, there are small handheld "tugs" powered by regular cordless drills. These are used to move 3000+ lb airplanes around on airport ramps. But, the situation is optimum: billiard-table flat, high-pressure tires, and max speed required = "slow walk".

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Originally Posted by FIGuy Now, converting gas to electric to motion is too complex for this task IMO, but it sounds like you just want a fun hobby here. In this case, I'd find a gas one with a dead engine and just convert that to electric.

Oh, there wouldn't be anything efficient about this, all the conversions (chemical to kinetic to electricity to kinetic) will assure that. The compensating advantages I hope for would be 1) elimination of mechanical complexity (steering rack and differential/transmission) 2) the low-speed torque of electric motors is an advantage in this application.

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Originally Posted by Telly Oh boy! I'm gonna feel like a killjoy

Nope, I welcome the spears! Most of my "bright ideas" have a flaw, and when I identify it I always learn something.

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Originally Posted by Telly Relating the hoist to the mower, did you mix up FPM and FPS somewhere there?

I think you are right. So, how's this:
1) 3/4 HP hoist will raise 220 lbs at 33 FPM, so with proper gearing would raise our 750 lb mower at 9.6 FPM
2) If a 3/4 HP motor will do 9.6 FPM. our available 3 HP will do 38.4 FPM
3) That's 38.4 FPM straight up. On a 1:5 hill, that works out to an "up the incline" over-the-grass speed of 192 FPM = 3.2 FPS Hmmm. That's too slow. But, if I gear it so that it can climb a 1:7 slope it could still go 4.5 FPS, which I think is good enough (over 3 MPH). I guess steeper hills could get mowed on a diagonal.

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Originally Posted by Telly With a per-wheel motor and gear reduction unit, you will have to create the wheel-axle support function, and it will have to be very sturdy in all directions, all the while keeping side-loads off of drive components not meant to take it. Not a simple task!

I'm probably overlooking something, but I'm envisioning each wheel on a short shaft with a bearing near the wheel, then the driven sprocket, then an inboard bearing. A 16" (dia) tire would turn 65 RPM to give us our desired 4.5 FPS speed. Single-phase AC induction motors turn at about 1700 RPM, so we need about 1:26 reduction. Gears would be nice, but two chains with a jackshaft (1:5 driving a 1:5) would be simpler and easier to modify as needed. If we used a universal motor at high speed, then gears would probably be required.

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Originally Posted by Telly Weight include gas tank and many gallons of gas, and battery to start generator?

Recoil start, baby!

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Originally Posted by Telly A lot of energy conversions here, are conversion inefficiencies taken into account at the top-level power budget?

Yes, I think, because of the caveman "analysis" I've used. The genset specs include losses, and similarly since I've used real equipment for examples I've just used their stated input wattage and the work they do (the mower heads cut 21" of grass, the hoist does X amount of work, etc).

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Originally Posted by Telly Series hybrids . . . Diesel-electric hybrids . . . I doubt if it scales down nicely...

Yep, it's definitely not efficient or elegant.

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Originally Posted by ERD50 A plus, for sure. And unlike a stand-lone generator, this will get a regular workout, so you will probably have more confidence that it will start when you need it.

Right. Part of the appeal of this project is that I already own a generator, and I'm not good about firing it up and properly shutting it down. I'm thinking it's more likely the beast will start when I need it if it's in regular use.

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Originally Posted by ERD50 I bought a second power inverter, mounted it to aboard to make it a bit more convenient, and I hook it to my tractor and use the electric, corded string trimmer out at the corners of the yard. Nicer than dragging all that cord out there, and it might come in handy as a second unit if we have an extended power outage.

Cool. For this project I'd probably hard-mount the string trimmer at a corner of the mower deck and use an on/off trigger switch to trim that last pesky 2" up against fences, borders, rocks, etc that is unmowable due to the blade guard.

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Originally Posted by ERD50 Yes, it really bugs me that I have this 19HP engine that is single purpose.

I've stewed on that, too. There should be some way to put a single engine to work for mowing, blowing snow, making AC power, augering postholes, chipping branches, splitting logs, pumping water, etc. Too bad the small lawn tractors don't have a PTO.

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Originally Posted by ERD50 Wait a minute- a transmission for each side, how will you control the turns? Won't you need finer control?

I'm not sure if my idea will work. I'm planning to steer by varying the RPM (incl reverse) of the rear wheel motors as needed, the motors are hard-connected to the wheels/tires (via gears/chains/toothed belts). No slip, no differential, no variable ratios, etc.

I'm gonna have to study the motor choices some more. Easy RPM control, light weight, small size, and high starting torque (so we can eliminate a clutch) all point to use of a universal motor to drive the wheels. But, they are noisy (e.g. circular saws/drills) and their high speeds might be a problem. Induction motors would be a heavier and have lower starting torque (maybe a deal-killer for use as the drive motors), but apparently modern circuits allow their RPMs to be controlled without much trouble. They would be quieter and more efficient.

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Originally Posted by REWahoo Sam, if you do go forward with this project you'll have to provide ongoing updates with photos. I'm not skilled enough to attempt something like this but enjoy admiring the efforts of those who are - from a safe distance.

I'm not skilled enough to attempt it either--or smart enough to steer clear of it. Anyway, it's staying on my list of "fun projects to do someday", meanwhile I'll continue to push my little 21" mower.