# RV conversion



## madderscience (Jun 28, 2008)

how about towing an electric "dinghy" behind the RV? put a couple KW of solar on top of the RV to charge it to avoid running generators. Should be plenty of space.

You would have to spend a lot of money to build a diesel electric hybrid setup for an RV that would provide any real benefit. Best bet might be to find a diesel electric bus drivetrain and figure out how to retrofit it.

Good luck.


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## StayPuff (May 30, 2008)

Good Idea, thanks.
I was looking at using the on-board generator as the power plant to drive the electric motor that would turn the wheels. Maybe converiting it (the generator) to run on propane... The thing I am chewing on is what size of motor would I need to push this beast down the road at 60 mph, and how much power the genny would have to put out.


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## madderscience (Jun 28, 2008)

not knowing how big the RV is, I'm just guessing here but typically when somebody is looking at converting a large vehicle or truck, we recommend either a warp 11" or warp 13", coupled with a zilla or soliton controller if you are looking at DC motors. An aforementioned trolley bus or hybrid bus drivetrain would be the only place you are going to find an AC traction motor big enough to propel a large motorhome at freeway speeds.

If you wanted to run this with a generator (not recommended for primary propulsion, its going to be less efficient probably than a standard drivetrain) the generator is going to have to be big enough (accounting for all losses) to cover the average energy usage of the motor plus the recharging rate of your battery bank. So you are probably looking at 50-100KW. Generators that big weigh around 1000lbs and come mounted on trailers. Battery bank if lithium at least 500lbs, if lead at least 1500lbs to do anything significant. warp 13" motor weighs around 300lbs I believe. Figure another 500lbs for everything else. Considerable engineering will be required to make everything play well together. Not impossible, but its not going to be plug and play either.

Good luck.


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## ewdysar (Jun 15, 2010)

Stay Puff,

One way you can guesstimate the power requirements is to convert your gas mileage at 60 mph. Take your mileage and convert that number to gallons per hour, i.e. 10mpg is 6gph, 15mpg is 4gph, etc. 

There are some basic fuel consumption to hp calculations based on thermal efficiency, assuming a 25% thermal efficiency (maybe optimistic) 6gph is about 66hp. Since these calculations are linear, 1gph is about 11hp. Therefore, 30mpg @ 60mph works out to about 22hp. 22hp is about 16.5kW or an average of about 135A @ 120V and that sounds about right from existing conversions in normal driving conditions. 

Back to your motor home, 10mpg (66hp) is about 50kW or 415A @ 120V! Even 15mpg (44hp) is 33kW or 275A @120V which is pretty severe continuous duty for most of the drive systems discussed here. In my opinion, you're better served by sticking with a conventional ICE drive in a motor home. The suggestion of getting a small electric runabout instead for local driving makes a lot of sense to me.

Electric drives can be a good answer to the right question, but they are not a universal replacement for ICE propulsion. 

Eric


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## ewdysar (Jun 15, 2010)

ewdysar said:


> Stay Puff,
> 
> One way you can guesstimate the power requirements is to convert your gas mileage at 60 mph. Take your mileage and convert that number to gallons per hour, i.e. 10mpg is 6gph, 15mpg is 4gph, etc.
> 
> ...


Looking at my numbers, I realize that 30% thermal efficiency (TE) is rarely achieved apart from aircraft and racing engines operating at peak efficiency. So using 25% TE (probably still too high) the numbers come down a bit. Here's the same paragraph using the new figure.

There are some basic fuel consumption to hp calculations based on thermal efficiency, assuming a 25% thermal efficiency (maybe optimistic) 6gph is about 66hp. Since these calculations are linear, 1gph is about 11hp. Therefore, 30mpg @ 60mph works out to about 22hp. 22hp is about 16.5kW or an average of about 135A @ 120V and that sounds about right from existing conversions in normal driving conditions. 

Back to your motor home, 10mpg (66hp) is about 50kW or about 415A @ 120V! Even 15mpg (44hp) is 33kW or 275A @120V which is pretty severe continuous duty for most of the drive systems discussed here.


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## Tronthor (Aug 2, 2010)

Since we are on the subject, and since I am considering the same idea, I'd love to continue the discussion.

For starters, putting the hybrid option to play has dual benefit in this configuration - you get to toss out the genny that you are usually just dragging around.

I have a 1984 class A motorhome as my potential donor. The ICE is a Chevy big block 454: 240hp, 355 ft-lbs of torque in the standard config as far as I have been able to figure out.

Based on some numbers that I have run, I believe that I'm using closer to 100 hp to cruise at 65 mph (equivalent flat plate whetted area is about 72 ft^2).

I have also been looking into the Sterling Engine. Not as a prime mover, but as a means of using all the heat energy that is being lost (now - big time, but with the power, either hybrid or total EV, being used there will be heat.) But as a means of recovering wasted energy.

Also looking at the idea of water-ammonia absorption chiller for A/C, again, putting waste heat to use (and improving overall efficiency of most parts of the game)

Again based on the numbers I've played with, I think that a consistent 25 or so % of the cruising power can be gotten by hooking a Sterling Engine to a gen and running it just off the heat collected on the roof - on a sunny day of course.

Anyway, the hybrid idea works here I think. The duty cycle that we are talking about is 8-10 hours of driving in a day. By taking the ICE away from the primary traction drive, you allow it to run at a steady state, and you remove the need to worry about not having the right gears - I'm stuck with the 1984 3 speed automatic that Detroit thought was good enough for anyone. Bullet proof yes, but efficient? Not. Most of what I'm doing is just spinning the engine fast enough to make the tires turn.

I really don't think that I need 7.4L of ICE displacement for most of my driving - though I have not been into the mountains. However, there are times when you would need that sort of power. The GWV is 15,000lbs. Based on the very diligent work done by the builders, you are very much at 80+% of this 100% of the time.

Clearly some streamlining of the basic box would be in order as well - to reduce that cruising power requirement - but I think that a hybrid configuration with the option to run straight EV - in and out of the campground as mentioned for example, especially if you arrive late in the evening, or pull out early.

Regardless, I'd love to see the discussion continue. There is another thread where the discussion is about converting a long haul rig (80,000 lbs) to full EV - there is a place for an ICE-EV hybrid option if ever there was one.

As much as I would love to be full EV, there is no real way to do so - especially since you can't fully recharge overnight - at best you get a 50A/220V plug in when you get to a campground, and if camp management glommed onto what you were up to, they would double or triple your nightly rate due to the current draw you would have to recharge your pack for a full day's drive.

Again, there are benefits - especially if you add in something like a Sterling Engine powered by the sun and waste heat (same engine) also hooked to a genn - granted you are looking at 2 gens here I think...but maybe not, I don't know yet.

Food for lots of thought I hope.


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## DavidDymaxion (Dec 1, 2008)

Here are a couple of parallel ideas:


Tow an electric car, but use regen on the car to help slow the RV every time you hit the brakes. After a day of driving, the little EV will probably be fully charged.
Get rid of the gas stove, gas heater, and propane system, and replace it with electric. You could have an electric stove (or just use a microwave) and electric heat. With solar panels and a windmill and enough batteries you could live off the grid, possibly never needing a gas generator.
Use a Chevy Hybrid truck to tow a lightweight trailer (might not do better than a diesel, though). I actually considered using a Chevy hybrid truck to tow my electric car to events, but the tow rating was too low.
Build a super aerodynamic motorhome, or find the most aerodynamic one you can and improve it as you can.
Remember, hybrids get better mileage in the city than the highway, but the highway mileage is about the same as a similar economy car. Motorhomes tend to not be city vehicles. The extra weight of an electric drivetrain could actually use more gasoline!


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## Tronthor (Aug 2, 2010)

The city/highway issue is big obviously, and you are correct.

The RV is a start it up, go all day long, shut it down and sit for a while (days possibly), then reverse and when you get it home, park it for a long time.

I had also planned to use whatever passive generation potential it ended up with to offset grid usage while at home.

The streamlining is big. Just adding gears to the ICE drive train to get the ICE back into it's very comfortable range as well.

Lots of options. I'd just love to clean up the emissions and avoid the big hit to the vacation budget when I have to feed the ICE - as well as improve off grid (ergo "dry") camping potentials, so solar and or small wind (stowed while driving obviously) would be part of it.

Thanks for the input. Keep it coming.


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