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Making a school bus electric?

24K views 69 replies 18 participants last post by  Tk1968 
I assume that we're talking about a full-sized school bus, which is a medium-duty truck chassis with a 30 to 40 foot long body on it.

Fitted out as a motorhome, this is likely a ten-ton vehicle. So, with that mass and the huge frontal area, it seems reasonable to me to assume that it will take about five times as much energy to move this vehicle as it would take to move a compact car the same distance.

So... 120 mile range will require five times the battery capacity of a typical battery-electric car with that range. The Nissan Leaf has somewhat less range than 120 miles with its 30 kW-h battery... so, perhaps 180 kW-h, or about double what a Tesla Model S has. You could use six complete Leaf batteries. :eek:

A small school bus is on a commercial van chassis, and could be substantially smaller, with correspondingly lower energy needs.
 
Now what type of motor would the bus require?
Huge! :D

But seriously, accelerating all that mass at an acceptable rate requires a lot of peak power. My motorhome is on the Ford F53 chassis, the only chassis available in North America for Class A motorhomes with a gasoline engine, and comparable in size and weight to a school bus - it has (like all current and recent F53) a 365 hp V10 engine, and while it performs well, it's not quick by car standards.

Keeping that big box moving through the air at highway speed requires a lot a continuous power... in my experience, about four times what my van takes, so about six times what a typical EV car takes. A typical electric car motor won't do the job.

Climbing grades while maintaining highway speed, and perhaps accelerating to pass slower trucks, takes at least half of my engine's 365 hp, so it looks to me like a continuous rating of over 150 hp or 120 kW would be suitable.... and a higher peak power is needed.

If you want to use a single motor (to work with the stock bus axle), then a motor from the rear of a Telsa, or some industrial motor, seems likely. None of the common motors sold for DIY conversions are big enough. There are various electric trucks and buses which have been made in limited production, and the motors from them might be usable. There are some Siemens motors made for Azure Dynamics which are still around; the largest of them might be adequate if run at the motor's maximum voltage, but that's only a guess.
 
Bus selection considerations

By far the easiest way to arrive at a large EV motorhome would be to start with a motorhome, not a bus; however, a cheap motorhome may have body issues that you would rather not deal with.

One difference between typical Class A motorhomes and the classic school bus configuration (called a "conventional" or "Type C") is that in the motorhomes the driver usually sits over the front wheels (cab-over-engine, with the engine under a hump between the driver and passenger seats), while the buses usually have a conventional hood with the driver behind the front wheels. The cab-over-engine layout provides more interior space for the same overall length, while the conventional layout provides easier engine access.

For an electric conversion, "engine" access seems like a non-issue, so cab-over-engine has an advantage... although the ride for the driver might not be quite as nice. School buses are available in a "flat front" configuration (which they often call a "transit" style or "Type D"), which is either cab-over-engine or rear-engined. The buses do that so the driver can see kids crossing in front of the bus, but it might also be the better choice for a motorhome. Some flat-front school buses have a rear engine (like an intercity coach, such as those run by Greyhound), but those are likely to be more expensive. And a final note - the front passenger seat setup might not be what you want in a flat-front bus (which normally doesn't have a seat beside the driver - just a entrance door and stair).

Some school buses have storage compartments underneath, which would be useful in a motorhome and could potentially house EV components. Even if there are no compartments, they can be added - all of these things have a very high floor so there's lots of room underneath... all of which you'll probably use between batteries, RV equipment, and storage.

Also - and this is unrelated to EV conversion - check the headroom on any school bus being considered. Kids are short, so ceiling height in some of them may not be comfortable.
 
My question is: If I were to buy 2 front end collision Model X performance editions, from one of the auction sites.
My math says those rear axles will be powerful enough to replace the stock diesel.
Although I mentioned Tesla motors when this discussion was active four years ago, Tesla Model S induction motors have one major problem: they can't take sustained high power without overheating. They can produce lots of peak power, but keeping the rolling at highway speed, especially up a grade, might be a problem.

Another issue is that Tesla motors are not easily separated from the transaxle (gearbox and differential), although some aftermarket suppliers are now making housings to make that possible.

The question is it possible to mount those motors onto the bus? What garage would have that capacity?
How would you want salvaged EV motors (such as from Tesla... but any of them) to connect to the rear axle?

Tesla built custom axle housings and gearboxes to mount Model 3 motors onto a heavy truck axle for the Semi prototypes (who knows what they'll use if they ever actually put it in production), which is a somewhat common approach for heavy commercial vehicle propulsion, e.g.:
"e-Axle" products at Dana Electrified

Some of these (including the Tesla Semi) use one motor per wheel, mounted either out at the ends of the axle (as in buses) or near the middle of the axle (to fit with a typical truck - or school bus - frame and suspension).

The alternative is to mount a motor to the frame, and use a normal drive shaft. There's no need for the motor to sit where the engine did - it could be as close to the axle as the closest hanger for a multi-part shaft.
 
I thought the Model X performance addition had the most powerful motor they produce so far, with its towing capacity.
Of the original series of Tesla induction motors (which are being discontinued), yes... but the difference is largely the inverter and its programming (not the motor itself), and peak power is irrelevant anyway. It is what the motor can handle for an extended period which matters with this heavy vehicle.

Does the Model 3 have that same rear axle power for towing?
The Model 3 motors (and the motors now in other models, derived from this one) are entirely new designs. The rated peak power of any single Model 3 motor is much lower than the old large Model S/X rear induction motor, but the continuous power is probably comparable. The Model 3 rear motor is a permanent magnet design, which doesn't have the rotor cooling problems of an induction motor.

The school bus, outfitted as an RV, will be much heavier than a Model X with the maximum allowed trailer.

My original thought was to buy at least 2 salvaged vehicles and place the motor a motor gearbox system directly in line with the tire.
...
Are there any shops I could speak to, that use the axle to tire method?
That is the approach taken by some of the commercially available "e-axles" that I link to; did you look at those? No one routinely does this as a custom service.

That leaves 150k for the EV conversion. The same price as the 300mile Tesla Semi is supposed to be.
The price of any unreleased Tesla model is entirely fiction, and the promised price of one that should have been built two years ago but is still nowhere in sight is particularly suspect. Real electric heavy duty trucks are much more expensive than that.
 
I've seen a couple electric motors pushing 100Hp so I'm not sure that power is the issue here, but i could be doing the math wrong in my head, i feel like it'd be more the amount of wattage you'd need to carry (obviously could be wrong)
Power is certainly an issue, and 100 horsepower isn't much for a bus. Fortunately, there are much more powerful motors than that - essentially every production EV has at least 100 HP (75 kW) of motor output. It's not technically difficult to install a powerful enough motor, but the builder does need to choose enough power, and it is expensive.

What do you mean by "the amount of wattage you'd need to carry"? A watt is a unit of power measurement, but if you mean energy in a battery (which would be watt-hours, or kilowatt-hours), then that (battery size and weight) is a concern, as we have discussed.
 
My last one was a client who wanted to convert a bus to electric and I told him it can be done but outside of the bus manufacturers we were in unkempt territory. I talked him into going with a bluebird ordered straight from the factory with no interior what so ever and highway gearing. We effectively doubled the battery and it works great but only about 170 miles per charge because the manufacturer designs and builds the bus for low speed stop and go, not long range...

Doubling a 155 kWh battery (to 310 kWh) for 170 mile range implies 1.8 kWh/mile or 1.1 kWh/km... ouch! It's not surprising: that's four times the consumption a large car, but my Class A motorhome (similar to the bus but a bit wider and substantially lighter) uses about four times as much fuel as our minivan, and that seems reasonable.

The brochure lists a TM4® SUMO™ motor as part of the Cummins PowerDrive 7000 "propulsion system". The TM4 motors are now part of the Dana Electrified product line, and come in Light Duty, Medium Duty, and Heavy Duty versions; the bus is a Class 7 vehicle so it should have an HD (their interesting 9-phase design).... that's a 340 kg motor. Perhaps they skimped and went MD... only 6 phases and 180 kg or 225 kg. From the specs in the ElectricDreamBus site linked to the Cummins website, it looks like they're using the SUMO MD HV3000-6P, which is the larger "L2" variant of the MD.

As we've been saying... big motor, although these Sumo's are bigger than a motor using an additional reduction gearbox would be.


A note on the ElectricDreamBus site information: the FAQ includes an item which says
Both our Type C and Type D buses use 155 kWh Li-ION NMC/G cell batteries (lithium, nickel, manganese, cobalt, and gel).
This is likely untrue, as lithium-ion batteries don't use gelled electrolyte, and the term NMC/G normally means nickel-manganese-cobalt cathode and graphic anode. "NMC/G" is normally just called NMC since lithium-ion cells normally use graphite anodes, but XALT seems to like the "/G" terminology.
 
Honestly I really don't know. The runs were short (like maybe 3 or 4 miles at most as I remember) but very steep, narrow, and twisty. I was seated up front. The driver was pretty proud of his bus.
Gets me wondering if they still use the same buses.
Not likely - most battery-electric commercial vehicles (which are mostly trucks) seem to be scrapped before their diesel-engine equivalents would be, as batteries and electronics become obsolete and unrepairable. It would be remarkable if anyone had anything from four decades ago still in service.
 
The motor aft of the rear axle is an interesting setup.
I hadn't even noticed that, in this photo.
Engineering Machine Auto part Composite material Metal

I assume that they did it to free up more space for the battery pack.

It says batteries are between the frame rails. That must mean the "frunk" has nothing in it, so maybe a chance to double the range by stuffing it with bags of money (batteries)? Or an ICE range extender to make it into a HEV (engine/generator combo)
Yes, you can see the bottom of the battery pack - it's the largest thing visible between the axles in that photo. The space under the front hood in EV-converted commercial vehicles is usually surprisingly full of the various supporting systems (motor controller, charger, DC-to-DC, etc). In this case, with the rear motor, the inverter will in the rear, not the front.
 
Looks to me like they're using Borg Warner HVH cartridges (250 and 410) and the N*3 phases are independent drives for each of n cartridges/resolvers. Rotating the cartridges would reduce cogging torque.

The length can be explained by its planetary gearset and likely a Dexron/(water)Glycol heat exchanger and Dexron pump in that case.

It's a nice solution for hooking straight to an existing driveshaft and only providing DC power and (water)glycol-only cooling system.

The price list would be interesting. I'm going to guess 50-60k for a "6-phase" setup. The two HVH250 cartridges are about $9k. The 410's would push the solution north of $100k, since they're over $20k, iirc.
No, TM4 was an entirely independent company (from Dana or BorgWarner or anyone else other than their owner, Hydro-Québec). Their high-torque Sumo motors are outer-rotor radial flux single-core designs. Dana TM4 is a joint venture - the motors are still the original TM4 design. HVH motors and cores are from BorgWarner, a competitor to Dana. There is no planetary gearset in the Sumo motors, which are high-torque low-speed designs. The six or nine phases are all separately driven from one inverter unit, matched to the motor.

The dimensions of the Sumo motors are given in the web pages that I linked, or the spec sheets linked to them. They're much larger in diameter than the HVH250 motors, shorter than a dual-core HVH motor, and even heavier than a dual-core HVH250.

I have no idea what any of the TM4 motors cost, but the Sumo units must be expensive. Almost nothing could be as expensive as a dual-core HVH motor with Cascadia inverters.
 
Would you trust any of these motors with your vehicle?
The Wuling units listed generally look very light-duty - small pickup truck size or smaller, perhaps, and typically very low power - but real information is lacking. If they were sized appropriately they would probably be fine, but I wouldn't trust the source.

This is from one of the listings ("Wuling Motors LCVA4O40 electric rear axle "), and is obviously a heavy truck axle (even heavier than you would want)... but it's also just a computer rendering.
 
Contact Wrightspeed (and report back here) and see if they'll sell just their motorized axle...

My contact there left the company or I would do it.
I didn't realize that Wrightspeed was still going.

It appears that Wrightspeed has changed their drive hardware, but it's hard to tell because in the past they didn't show it clearly. The Geared Traction Drive that they have shown for years is a complex four-speed drive unit, and the provided images don't even really show whether it is mounted on a beam axle to the frame. The Route vehicle configurations now show a two-motor drive axle, likely the same as shown in more detail in a photo on their home page:

While the Route page calls this unit the Geared Traction Drive, that name seems to be used for two different generations of hardware. Here's the original GTD, from a 2014 article:


Of course a small vehicle manufacturer wouldn't likely build this sort of component when it is commercially available. In this case, in 2017 Wrightspeed made a deal in which "Wrightspeed’s GTD (Geared Traction Drive) pairs with a custom axle from AxleTech International" (although the motor and gearbox unit looks nothing like the original GTD); this is the current unit.

The net result is that this unit would probably need to come from Wrightspeed rather than AxleTech (especially since AxleTech is now part of Meritor), and while it couldn't hurt to ask the likelihood of being able get one seems small to me. It would be well-suited to a large school bus or Class A motorhome, except that it is set up for air brakes.

AxleTech's axle product line includes two electric axles, neither of which is the Wrightspeed unit, so I doubt it is available to anyone other than through Wrightspeed. Of the AxleTech products, one (EPS785) is far too heavy for a bus, and the other (5000 Series EISAS™) is an independent suspension steer axle. They also have (in the Electric Vehicle Systems product category) a motor and gear unit (AFE series) which installs into their regular axle housings, but again only axles which are too heavy for a school bus. AxleTech doesn't sell directly to individual customers, and I doubt any of this could be found from their aftermarket parts source even if it were suitable.
 
Not sure what Ian Wright (founder of Tesla, Wrightspeed, and creator of the Ariel Atom, and its X1 EV variant) is up to these days.
Wright's company built the X1, but he is not the creator of the Ariel Atom. From Wikipedia:
The Atom began as a student project by Coventry University transport design student Niki Smart. Known then as the LSC (Lightweight Sports Car), it was developed at the university in 1996 with input and funding from various automotive industry members, including British Steel and TWR. Ariel Motor Company boss Simon Saunders was a senior lecturer whose responsibility for the project was primarily as financial manager and design critic for Smart, whom he described as "The best all-round design student I've ever seen." The car was first shown publicly at the British International Motor Show at the NEC in Birmingham in October 1996.[4]
https://en.wikipedia.org/wiki/Ariel_Atom#cite_note-4
 
I understand there are Electric BlueBirds out there, they are designed and noted for school systems with a lot of stop-and-go driving.
There are battery-electric school buses from many manufacturers now, including both the traditional companies and new companies specializing in EVs. Few private individuals could afford one (they're usually subsidized when a school operator buys them), but they provide design examples.
 
My questions are what happens to the battery pack requirements when doubling the motors? Does the Voltage in the equations double?
No, unless you connect the motors in series with other. If doubling motors doubles your peak power consumption, your battery needs twice as much power capability (so twice as much current if the same voltage).

When using two motors I'm guessing it would need an adaptor of some kind before attaching the transmission?
Connecting any motor to a transmission means the need for an adapter to connect the housings (cases) and a coupler to connect the shafts. Connecting two motors to a transmission obviously means more of the same. The most common way to do this is to use one motor which has a shaft that sticks out at both ends (what NetGain calls a "double ended shaft"), couple it to the transmission, and couple the second motor to the extra shaft end of the first one. Then of course you need an adapter to connect the case of the first motor to the transmission, and another adapter to connect the motor cases to each other.

You might be able to find this whole setup in a museum of early EV drag racing conversions somewhere. ;)

This next part is a joke. Why hasn't anyone patented a plug and play axle system, where you add the motors you're using to a modular transfer case gearbox mechanism to straight bars that go into the wheels, have multiple sizes for different size vehicle types...
You're joking, but that's essentially what AxleTech is doing with their AFE Series, which I mentioned earlier. The motor and gearbox unit is the eCarrier, and it goes in one of their axle assemblies instead of a conventional pinion carrier for a propeller shaft from a transmission.
 
One may be able to make the bus 4 wheel drive... I.E. two drive motors.
That would require a complete front axle replacement... but of course that can be done (and has been done many times, just not for EV conversions).

As far as I know, all commercially produced battery-electric buses of school bus size and style are RWD only... they just use an appropriate motor.
 
The pickups might be a little lightweight for the bus project, but they are supposed to be able to tow a fair amount which may be similar to moving a heavy bus around.
Yes, it is similar. The weight of the empty vehicle (curb weight) and the Gross Vehicle Weight Rating (GVWR) - which is what the vehicle can carry on its own axles - are irrelevant to the powertrain.

The number relevant to suitable capacity in this case is Gross Combination Weight Rating (GCWR), which is how much the vehicle's powertrain can haul. In a car, that's typically not much more than the car's Gross Vehicle Weight Rating (GVWR) but a truck is intended to haul trailer weight as well. The engine (or motor) and transmission don't know or care whether the mass they're accelerating is carried on the same axles as the powertrain, or even on the same chassis of two linked vehicles (truck and trailer).

Unfortunately, even though an F-150 Lightning can tow (a suspiciously round number of) 10,000 pounds of trailer, so the powertrain can handle at least 16,000 pounds (given a 6,000 lb base curb weight), that's still too low for a full-size school bus... and that's using both front and rear drive units. The Rivian R1T and Silverado will be similar; the Hummer EV is heavier but might not have any higher GCWR.

Some of the motors from these pickups might be suitable for higher load if equipped with lower (more reduction) gearing and adequate cooling, but that's getting very far from just transplanting salvaged drive units into another vehicle.
 
I was wondering about gear reduction.

There is 4.5 to 1 reduction gear set for the Tesla Large Drive Unit, designed to be coupled to a differential. So coupling that to a 4.10 differential, that gives one about 18.45:1, or about double the stock Tesla.

It would help a lot with the low end torque.

But, overall, the gearing may not make a big difference if it takes 200 HP to make a vehicle move at speed as the kW load may be the same. However, it could help to split it to a front and rear drive unit using differentials donated from large 4x4's & 6x6's.
Splitting the drive to two axles is good for traction, if you need that for this bus, but it doesn't help with the ability of the motor to handle the load, and it adds drag.
 
Just went and looked up the cummins motor for my old schoolbus: 1990 bluebird 50 passenger. 250 hp 525 lbft torque and that bus was a slug crawl going up hills even empty. Otoh, it made those hp and torque specs for the 1/2 hour it takes to get from Reno to Truckee although not at 65 mph. The equivelant electric motor is going to need 186,500 watts and dissipate a high amount of heat. So maybe 800 volt battery pack and you require 233 amps minimum for that half hour trip, 466 @ 400v. Your battery has to be at least 100 kwh useable or a bunch of tesla packs stacked to get to Truckee, and it's going to be a while recharging even at a supercharger.

This comment was spitballing numbers and you can pick it apart all you want, but as F Zappa said:"Reality is a mother*"

The city has electric busses that in theory can go to Truckee, but they only go empty for an occasional demo and I do not know about additional support equipment like gensets.
Excellent reality check. (y)

A note on motor or engine performance specs: torque doesn't matter by itself, only in combination with speed. 525 lb-ft at 2,500 RPM or 175 lb-ft at 7,500 RPM is the same ability to do work (250 HP). Yes, that's 186 kW.

By the way, that spec combination is strange: peak power for an engine is never at the same speed as peak torque; 525 lb-ft peak torque is plausible for a Cummins B5.9 (likely at 1600 RPM) but it won't maintain that torque to 2500 and so won't produce 250 HP, a 250 HP B5.9 (likely at 2500 RPM) or B8.3 (at 2400 RPM) will produce much more than 525 lb-ft of peak torque (over 600 lb-ft, at 1300 to 1600 RPM).​

Reno to Truckee is only 32 miles one way. In Edmonton we have battery-electric city buses here that could do that round-trip five times... but that have 675 kWh batteries and the cost about one million dollars more than the diesel equivalent. You can have the range you want, but there's a price...

RTC Washoe appears to have two models of battery-electric buses:
  • Proterra EcoRide BE35 - this is presumably the short-range model mentioned, with a UQM PowerPhase 150 permanent magnet motor (that is occasionally available used) and a 368 volt battery with capacity of 54 to 72 kWh weighing 1650 to 2200 lb (750 to 1000 kg)
  • Proterra Catalyst BE40 - later, bigger, longer-range model
Edmonton uses a newer version of the Proterra Catalyst.
 
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