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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.
 

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A 100kW generator would be huge and incredibly inefficient...
While far short of the efficiency of a mechanical transmission, I don't know if "incredibly inefficient" is an appropriate term.

How about putting some numbers on this? The generator would presumably be sized and geared to be running in its "sweet spot" for efficiency at the planned continuous engine power output, so combined controller and generator efficiency will be better than 90%. The power goes to an inverter and motor which are typically further off the sweet spot, as with any EV, so wildly guess 85%. The combination of the two is 77% or better. That's terrible compared to about 95% for a mechanical transmission, and the loss is very significant to the overall efficiency of the system; whether that should be called is "incredibly inefficient" or not matters less than the actual numbers.

The engine would run somewhat more efficiently than it would with a mechanical transmission, because it would be at a more optimal speed (although that's not a big difference when considering modern transmissions with eight or more ratios), and because it would run only when required at near optimal load (although that sort of load-smoothing implies generation, storage, and motor losses).

Although a series system is attractive for packaging, it would be worth considering a parallel hybrid instead for cruising efficiency; the engine could still run when desired in neutral as a stationary generator.
 

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... 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 think that either they will "camp" at serviced (with power) sites, or they will run the generator, because even a 100 kWh battery is unlikely to be sufficient for a return trip (not just one way) plus power for duration of the stay.

I participate in a forum for travel trailer owners, in which someone proposed using a Tesla Model X to travel with a trailer. The outcome of the discussion was that it would be viable only if staying at sites with 50-amp 240 V power (which is a common offering here, but only in a minority of sites within each campground, and only in some campgrounds). The need for these sites makes this mode of RV travel far too constrictive for most people, but it would suit some.
 

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... 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.
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.
 

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The people in question regularly travel across Europe where power is widely available both at rapid chargers and at camp sites.
So it's the first of my two options. Glad that works for them. :)

This is a good reminder of why geographic context matters - the charging situation in North Dakota (as well as typical driving distances) will likely be quite different from Europe.
 

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They can 'easily' drive 500 miles in a day with a single charging stop (for lunch?)... they could use the ~20kW REx for charging but in reality I think they'll just use the public charging infrastructure because it's faster and a lot cheaper than petrol/gas :cool:
Again, good for them. :) If UND_Sioux finds that there are public charging stations (which will fit the RV) at typical mid-day stop locations, and high-capacity electrical service at campsites, then that could work. My guess is "no".
 

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This particular RV is not completely like a barn door, but it is heavy.
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?
 

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Maybe two smaller motors would be better. Can modern controllers sync two motors accurately?
Yes, EV manufacturers have no difficulty coordinating two motors on separate axles; they even run two motors on the same axle (one for each wheel) in some hybrids and some limited production EVs. It's not difficult for the technology, but I don't think that off-the-shelf controllers for DIY builders typically have control logic to handle this... although I may be mistaken. Not many DIY builders use more than one motor.

An interesting exception in DIY controllers would be Curtis Instruments, which designed its controllers for industrial vehicles (such as forklift trucks), and built in logic to handle two motors on one drive axle; that requires two controllers (one dedicated to each motor), communicating with each other, with one acting as the master (that's what you connect your accelerator pedal to) and the other as a slave. Curtis calls this "Dual-Drive".

You could connect two motors together, driving the same shaft. The HPEVS "AC" series includes dual AC-34 and dual AC-35 motors, which are two motors built on the same shaft and in the same housing, but the same thing has been done with two separate motors mechanically connected together. Coordination of the two is easy, because they will always run at the same speed.

If I were going to go the expense and complication of having two motors and two controllers, I would want to get 4WD or at least independently driven wheels on one axle for my trouble.
 

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It does not matter, since you command torque, not speed.

Each motor will output the desired torque, independent of each other.

My truck has two motors and two controllers, and they don't talk to each other at all.

Each one takes in a throttle signal from a dual gang pot.
Assuming that this is your Solectria E-10 pickup, this is the easiest case of the motors driving the same shaft (to the rear axle in this case):
You could connect two motors together, driving the same shaft.
...
Coordination of the two is easy, because they will always run at the same speed.

If you have low enough expectations of the control system, it does not need to be sophisticated even if the motors are not mechanically linked. If one motor spins due to loss of traction, you can just let it go. If one controller has a problem, you can just let the other one drive... hopefully you're not using them to drive individual wheels.
 

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I was trying to keep the Make and Model under the radar, but it will be in a classic mid-70's GMC Motorhome.
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.
 

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I was hoping to keep the motors down low either driving the differential or ideally direct drive with the range extender mounted above.
I'm not sure what you mean by "direct drive", but you certainly need a reduction drive (gearbox) between the motor and the axle shaft, so that you can use a motor of a reasonable size. A motor which turns barely over 700 rpm at 60 mph (the rotational speed of the stock size of tire) would have to be massive - you want the motor turning about ten times that fast.

(The GMC Motorhome came with 8.75R16.5 tires, which are the same height as a more modern 225/75R16.)
 

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... I can wait until technology improves or at least gets less expensive ...
Maybe a Tesla Semi will be crashed in a junkyard by then :)
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.
 

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Well, I could possibly use two motors end to end and adapt to the original "final drive" differential.
End-to-end is very long, even for the relatively wide motorhome. In the case of Tesla drive units, it would help to remove the inverter from the gear case (and move it, using extended cables). The Leaf, in contrast, has the inverter on top of the motor, so it is taller but not so wide.

The most compact arrangement of two typical drive units would be to rotate one 180 degrees and shift them sideways, so that the axle lines (through the differentials) are in line with each other, but one motor behind the axle (the stock Tesla position) and one motor ahead of the axle (the stock Leaf, etc position). The likely problem is that the rotated unit would need to drive in reverse, which is no problem for the motor and no problem for the electronics, but the gears probably wouldn't lubricate properly.

In any use of drive units with a differential (used to drive both wheels of one axle with one motor) to drive just one wheel, the internal gears of the differential would need to be locked or replaced with a solid part, which is called a spool. A full spool replaces the ring gear carrier and all internal differential parts; a mini-spool just replaces the internal gears. Then the output facing the wheel gets a normal axle shaft (of whatever length is needed to make it work) and the other output is just capped off.

I'll attach an image of two Tesla large (rear) drive units (taken from the thread in this forum Tesla Large Drive Unit Dimensions). For scale, see that thread for dimensions (the overall width of one unit is 33.53 inches). Since this combination is almost certainly too wide, I'll note that the outer parts are the inverters (which look almost like the motors). In this quickly pasted image, you can see small dark stubs where one axle shaft would connect to each (locked) differential.
 

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At that point, I was thinking non-Tesla motors end to end with a driveshaft coupled to the RV's original differential. The reduction is only 3.07:1
This scheme is often called "direct drive", but it is really using only the final drive unit for reduction. With such a tall (little reduction) final drive ratio the motor would still be turning only 2200 rpm. That speed is great for an engine (and with newer transmissions even lower engine speeds are used), but it would be using only a fraction of the speed range of a typical motor... although a lot better than 700 rpm!

For comparison, while this final drive has a 3.07:1 ratio (and similar units range from just under 3:1 to over 5:1), the overall reduction ratio of a Nissan Leaf (which has smaller tires and so doesn't need as much reduction) is 7.94:1, and a typical Tesla has a 9.73:1 ratio. Overall ratios mentioned for the proposed Tesla Semi (Class 8 truck) are 15:1 and 23:1 (although with tires about one-third taller than the tires that a pickup would use).

Although the final drive of this motorhome's THM425 transaxle can be removed, I don't think it's necessary to stick with this unit if not using the rest of transaxle. Much higher ratios are available in other final drives, so if taking this general approach it would make sense to look at other units used in independent suspensions. A challenge is to find something strong enough to haul six tons. For instance, whatever is under the front of a Chev/GMC 3500 4X4 pickup is a candidate (because they're all independent), and the current standard ratio with the gas engine is 4.10:1... but you really want more reduction than that. It might be reasonable to use a GMC unit, with an aftermarket high-ratio ring and pinion gear set.

It is also possible to connect two motors side-by-side to one shaft (a final drive input), but that generally means gears (which are usually not practical for DIY) or chains or belts (which are good to avoid). For an example, the old Solectria E-10 (a commercially produced conversion of the Chevrolet S-10) had two AC motors side-by-side, driving a common shaft with a toothed belt each. That extra step of gears or chains or belts is an opportunity to get the desired overall ratio.
 

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The best RV for conversion is the Vixen which came with a naturally aspirated 6 cylinder Mercedes diesel and 30mpg stock

CD of .3
The diesel Vixens had a BMW M21 engine (not Mercedes). While this engine was available naturally aspirated - unusual for modern diesels - I think the Vixen used the turbo... the model name is even Vixen TD. ;) The turbo was needed for adequate performance - the 63 kW (84 hp) of the other version wouldn't have been enough.

Of course the original engine doesn't matter. :) The Vixen is a low-profile design (needing a pop-up top or extension to allow standing up), and of moderate size (reportedly only 5,100 pounds curb weight). It should be great for aerodynamic efficiency, although it isn't very big.

The rear engine design of the Vixen might also be easier to work with for conversion than the GMC Motorhome. The manual-transmission version used a Renault 5-speed, in a longitudinal rear-engine configuration (like an air-cooled VW).
 

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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.
Somebody here was parting out hybrid E450 chassis perhaps in the classifieds.
At this time I would suggest electric city transit bus salvage yards.
A small transit bus or anything on an E450 chassis would be roughly appropriately sized.

I think it would be worth looking at the buses in a salvage yard, if only to see what they use for a transmission and final drive. They may be on the hybrid E450 chassis.

I assume that the hybrid E-450 is the Azure Dynamics adaptation of Ford's E-450 stripped commercial chassis to become a parallel hybrid, which they branded as "Balance Hybrid Electric". Apparently AZD mounted a 280 volt 130 hp AC (induction) motor in parallel with the stock transmission's output. From the diagram and some descriptions, it that is was a motor with a double-ended shaft, forming part of the propeller shaft (driveshaft) and so running at transmission output / final drive input speed. The final drive ratio (in at least some of them) was 4.56:1.

This illustration was taken from the manual (linked below), which also provides system weights and dimensioned drawings of the component layout, as well as many photographs of the system installed on the E-450 chassis.

In the Balance E-450, the electric motor only handled propulsion by itself at low speeds; above a set speed (20 to 35 mph depending on report), or when the accelerator is pressed enough, the engine ran as well. It's unlikely that this motor could handle a six-ton motorhome by itself. About a thousand of these were built, with over half being Purolator delivery vans.

References:
 

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Are Model X motors any different from Model S motors? I assume that the S and X are just the sedan and tall wagon (or "SUV") body variants of the same car, with the same drive units and batteries. My guess is that towing is discussed more for the Model X simply because that's an expected capability of an "SUV".
 
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