[UND_Sioux]

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.

 

[piotrsko]

Somebody here was parting out hybrid E450 chassis perhaps in the classifieds.

 

[Kevin Sharpe]

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.

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

 

[brian_]

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.

 

[brian_]

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.

 

[brian_]

... 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

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.

 

[brian_]

... 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.

 

[Kevin Sharpe]

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.

The people in question regularly travel across Europe where power is widely available both at rapid chargers and at camp sites. They kept a daily log last year when they traveled from UK to Greece and back over a six week period, at no point would an RV with a 250 mile range have been a problem for them

Remember that most of mainland Europe has three phase power available to the consumer... even your 'standard' charging station at a rest stop is wired at 11kW or 22kW

 

[Kevin Sharpe]

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 agree and my colleagues have chosen 'small' Tesla drive units because the overall package is so small

 

[brian_]

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.

 

[UND_Sioux]

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

I'm still young enough to drive more than that daily.....for now.

 

[UND_Sioux]

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.

 

[UND_Sioux]

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?

 

[Kevin Sharpe]

I'm still young enough to drive more than that daily.....for now.

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

Here's the RV conversion candidate in Poland a couple of years ago

 

[brian_]

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

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".

 

[brian_]

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?

 

[brian_]

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.

 

[WolfTronix]

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.

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.

 

[UND_Sioux]

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.

 

[brian_]

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.