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Discussion Starter · #1 · (Edited)
I apologize if this post gets long winded. I'm looking at building a powertrain setup for a mid-size motor home. I'm not doing it do be super green, or save money (because I know that won't happen); I'm doing it because I think it may be the best layout for an updated RV. Motorhomes already have stuff that makes them conducive to a range extended electric powertrain. They already have the need for large batteries, and they already have a generator.

Here is the basic idea. It will be a series hybrid type. Small V6 gas or diesel engine (leaning toward gas for convenience) mated to a generator (alternator) right at the flywheel. Then a large electric motor to power the drive wheels. The generator would have to be large enough to charge the batteries and power the vehicle; so my rough math says around 75-100 kw minimum. The battery bank would only need to be big enough to run overnight the motorhome roof AC units (about 10A each), low voltage lighting, small on demand water heater for a short time, maybe a LCD TV. If the battery were to get low, the engine driven generator would start, and recharge while sleeping off grid.

Most small modern V6 or powerful 4 cylinder engines are actually quieter than the Onan type RV generators. I'd ultimately like enough battery capacity to run 10-20 miles on electric only and would definitely like to run the "house" overnight without running the engine/generator.

Finding batteries from a Volt/Kia Soul/Tesla isn't that hard anymore. The inverter to run 110v appliance off of high voltage is easy to find too because of solar installations. The thing I am having a hard time finding is a genset for the engine that doesn't weigh a ton. TM4 makes a bunch of different units that are made for busses or small commercial vehicles, but they don't sell to the general public.

Anybody know where I can find something like that? I'm considering the Model S drive unit because it has been hacked already, and the power/torque is appropriate for my size vehicle.

Thanks in advance for the input.
 

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Discussion Starter · #11 ·
A 100kW generator would be huge and incredibly inefficient.... you'd get much better performance using the gas/diesel to drive the wheels directly.

That said, I have a colleague who is working on an electric RV. They are using two 'small' Tesla motors (FWD) and a complete Tesla battery pack. They are also using the BMW i3 REx (~20kW) for emergency range extension. They estimate electric only range at more than 250 miles using the Tesla 100kWh battery and said they will rarely drive beyond the battery range in a single day... it will be interesting to see if the REx is ever installed :rolleyes:
I'm still young enough to drive more than that daily.....for now.
 

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Discussion Starter · #12 ·
The good news:
  • The idea can certainly work - there are many series hybrid buses on the road to prove that this powertrain configuration can work for this size of vehicle.
  • Having a large battery pack and a good (quiet, efficient) generator in an RV is obviously a good thing.
  • If you only need 100 kW, it doesn't even need to be a V6 engine.

Now the issues:
  1. Hybrids get their efficiency advantage in stop-and-go urban traffic, but RVs spend most of their driving time cruising highways; the inefficiency of turning mechanical energy into electricity then back into mechanical energy is a problem.
  2. The biggest performance challenge for an RV is typically climbing mountain highway grades at high speed; even people who live in flat areas tend to go to mountainous areas for recreation. That means that the vehicle must sustain high power for a long climb, which is exactly the wrong situation for a hybrid - high electric motor power and large battery capacity are required.

Of course nothing is ever that simple. The RV won't see urban use, but there would be some (small) advantage of the hybrid system during regenerative braking on mountain descents. Climbing grades might be viable; for instance, a 24 kWh (Leaf or whatever) battery adding 80 kW (Leaf continuous power rating) to 100 kW of generator output might be enough, and could be sustained for 10 minutes.
I'll give up a little efficiency for better packaging. If my RV was electric with a range extender, I could ditch the onboard small gas generator for 110v, which is one more thing to maintain, and it would open up some storage space.
 

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Discussion Starter · #13 ·
For the generator, the largest EV motors (even the new-for-2018 Leaf) should be suitable for continuous output at the desired level (up to 100 kW).

For the drive motor, I think the solution is multiple motors, at least one per drive axle, to avoid the need for a single high-power motor. That could be an entire Tesla or Leaf drive unit at each axle.

I doubt that any of the Tesla Model S/X motors are actually suitable as a single motor for the motorhome, because the heavy vehicle with barn-door aerodynamics will require substantially higher continuous power than the less massive and much sleeker Tesla car. Under continuous heavy load, the motor is likely to overheat... and the Tesla-powered Cobra race car people have discovered.
This particular RV is not completely like a barn door, but it is heavy. Maybe two smaller motors would be better. Can modern controllers sync two motors accurately?
 

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Discussion Starter · #19 ·
Do you mind sharing the make and model, or at least the size and type (length and Class A, B, or C) of the RV? It would help provide context.

Also, I've been guessing that "UND_Sioux" refers to the University of North Dakota, and that school's traditional name for its sports teams. That would place the likely location for the use of this RV in North Dakota. Is that correct?
I was trying to keep the Make and Model under the radar, but it will be in a classic mid-70's GMC Motorhome. They are front wheel drive based on the Oldsmobile Toranado package. The transmission basically wraps around the left side of the engine and then it is directly mated to a differential (final drive). That is one of the reasons for looking at electric motors. Nobody makes a transmission quite like the original TH425 that can handle the weight of a 10,000 lb motorhome. Unfortunately, without something really new and unique, they are stuck with the big block Oldsmobile 455 with a 3 speed and no overdrive.

I went to UND, but I live in Texas.
 

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Discussion Starter · #23 ·
Nice! :) For those who are not familiar with this classic, just do a web search for "GMC Motorhome".

That's a substantial vehicle. Packaging is a challenge, because it has a low floor which is great for getting in and out, and for overall height, but doesn't leave much space under the floor. There is the big engine space in front, of course, and with front wheel drive it does make sense to pile the weight up there.

Driving more than the front wheels would be problematic, because
  • the rear suspension and hubs are not designed to be driven,
  • the rear floor is (deliberately) low so it would probably be impractical to mount drive motors inboard,
  • there are tandem rear axles (that's a lot of motors to drive all of the wheels), and
  • the space between the frame rails at the leading rear axle is filled with a tank.
I suppose that if you're okay with radial mechanical changes, you could set up inverted portal drive axles at the rear.

Aerodynamically it is better than a typical class C, and roughly comparable to a Class B or other Class A's. Of course it isn't as good as a modern car, and has over double the frontal area of even a large car, so it's going to use perhaps two to three times the power (for a given highway speed) as large electric car.

It certainly is hard to find an alternative transmission for this unusual configuration, or at least one which is suitable for the 12,000 pound GVWR. That alone may justify, in a way, a series hybrid configuration, but on the other hand finding a suitable electric motor and transaxle or pair of motors with gearboxes (for separate left and right wheel drives) is a challenge, too. If you can find the right electric drive, and squeeze the battery in somewhere further back, there should be space for the engine+generator set ahead of the front axle.
There is a little room under the floor between the frame rails for some of the battery. The rear of the coach under the bed area typically has the onboard generator and the propane tank, both of which I would toss. That leaves more room for more battery. I was hoping to keep the motors down low either driving the differential or ideally direct drive with the range extender mounted above. Theoretically, the range extender could be relocated to the back.
 

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Discussion Starter · #24 ·
Also, I should note that I am planning to do a pretty hefty restoration, so if it is not feasible right away, I can wait until technology improves or at least gets less expensive and I 'll just enjoy it with the ICE in the mean time. Maybe a Tesla Semi will be crashed in a junkyard by then :)
 

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Discussion Starter · #27 ·
You're just kidding, but the Tesla Semi is planned to use two Tesla Model 3 motors per axle. You can similarly use two Tesla or other EV motors for the front axle, if you can sort out a pair of gearboxes for them.
Well, I could possibly use two motors end to end and adapt to the original "final drive" differential.
 

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Discussion Starter · #33 ·
Well I officially own the GMC so that is the ride if I do this. It weighed in at 10,500 with mostly full tanks.

The drag coefficient of the GMC is around .31 without the AC units I believe.
 

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Discussion Starter · #36 · (Edited)
Interesting testing here. I'm starting to think that a Model X motor could pull my GMC around just fine. It can pull it's own 5000 lbs plus a 5000 lb trailer. My GMC weighs in at 10,500, and when I'm done with it, it will weight considerably less. The amount of steel frame couches, and heavy wood cabinets plus the marine plywood floor that will be upgraded to much lighter materials will take out some weight. Removing the generator will take out some weight. I'm guessing I could get it down under 9000 lbs. Without the engine and transmission, it would be closer to 8000 lbs....maybe less. The shell itself is pretty light. It is all the stuff in it that weights it down.

edit: Forgot that the Model X is AWD. Definitely need two motors.

https://insideevs.com/tesla-model-x-energy-consumption-towing-various-trailers-video/
 
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