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A big ol bus

2948 Views 14 Replies 10 Participants Last post by  piotrsko
I'm trying to size up the conversion of a bus.

I don't yet have said bus but it's likely to be a double decker or single decker if it has to be. Think Leyland Olympian (bus) or Setra s328 (coach).

Average MPG of these jobbies (diesel) is 8MPG

HP for these busses seems to range from 300-500hp and torque around 700nm - 1500nm

Let's say that when it's loaded up I would expect total mass of between 10 and 15 tonnes

Now for my use I would be looking to achieve a range of around 60 miles between charges, used on mostly flat roads but with the capability to make it's way up a mountain once in a while (who wouldn't want to take their bus skiing!)

50mph would be more than sufficient.

So, where would you begin with finding a motor and batteries to carry out such a project?
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Since lots of medium-duty trucks and similarly heavy buses have been converted or built as EVs, the obvious source is whatever components they used, perhaps salvaged from a vehicle which has been scrapped (usually because the battery is dead or the owner has given up on the idea of an EV for that application).

Typically, they use either a very large-diameter motor, or a motor with a fixed-ratio (single-speed) reduction gearbox, mounted where the transmission would normally be.

A lot of surplus electric drive hardware came from the bankruptcy of Azure Dynamics, but their only heavy enough vehicle was the Balance hybrid system for Ford chassis; it used a big induction motor and reduction gearbox, but I don't know if any of those are still available somewhere in the salvage market.

A few people have tried or at least considered projects with the components of the Smith Newton truck. It used a reduction gearbox.

TM4 (now a part of Dana) specializes in motors for large vehicles, but I doubt that it is practical (if it is even possible) to buy a single new motor from them. In their range, you would presumably want a Sumo MD. An alternative from another part of the same company's product ragne would be to replace the whole drive axle with one with an integrated TM4 motor, from Dana Electrified.

Most proposed DIY projects of this size don't get built, likely due to the high cost of the required enormous battery, but if you search in the forum for "bus", "RV", or "motorhome" you should be able to find some previous discussions.
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Why not do a tesla front and rear motor of and x or s.
Even both drive units of a Model S or X Performance Dual-Motor might not handle the sustained power demand of a bus weighing 10 to 15 tons. These motors can put out a huge amount of power for few seconds, but then overheat if you try to sustain it.

You could use whatever batteries you wanted...
You can get a wrecked chevy spark ev for under 3k and it comes with a decent 21kwh battery.
That would not be nearly enough battery to be useful. The entire pack from one of the longest-range electric cars might work... certainly at least the 60+ kWh of a Bolt or Leaf Plus or Model 3, and better the 85+ kWh of a higher-capacity Model S or X or one of the very new other SUVs (Audi e-tron, etc).

You could then just plugin at rv parks and juice up and power both propulsion and the rv off the batteries.
I mentioned RVs because previous discussions in this forum dealt with RVs of similar weight, but this proposal is for a bus... no mention of RV use.
The Azure Dynamics van is a pipsqueak 3 tons and only has a 28kWh battery -- did they make a medium duty?
The only medium-duty by AZD was the Balance hybrid powertrain in an F-450 (stripped chassis or chassis-cab). Because it was a hybrid, it still wouldn't have large battery, but the motor was inline in the propeller shaft of a loaded F-450... unfortunately with only enough power to slowly accelerate the vehicle by itself and only up to 22 mph. Good catch - it wouldn't be up to driving a ten-ton vehicle as a pure battery EV.
Gearing could be your savior. What is the transmission in this bus? Final drive? Tire diameter? If you had 3+ gears designed to max out at 10k RPM at 50mph, even a puny Nissan Leaf motor putting out 280Nm might still be sufficient to get the thing moving. I feel like the "comparison" here is to calculate torque to the axles of the stock drivetrain (including transmission and diff), and see what that looks like if it were geared very tall for a lower top speed.
I agree that gearing needs to be considered, that suitable gearing is the solution to making available motor torque work for the load to be driven, and that multiple gear ratios can keep the motor in its full power output speed range... but that's probably not enough. Even with ideal gearing, a salvaged Leaf motor (with stock controller/inverter) is good for 80 kW, and that will mean slow acceleration and very limited grade climbing. Accelerating ten tons to highway speed would take minutes.

Solutions to the lack of power include
  • using the latest Leaf motor and controller/inverter instead of the common 80 kW version
  • bypassing the 80 kW power limit with an alternate controller (and probably inverter)
  • using a motor salvaged from a different EV (most are now more powerful than the original Leaf)
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