[remy_martian]

The key to maximizing an RV's gas mileage?

Park it

10HP motors on a 10,000lb land yacht is a fart in the wind.

 

[piotrsko]

Going to need commercial bus class stuff to push that down the road, aero efficiency is your other enemy.

Loose the AC & stuff on the roof, round leading corners, or just accept 8mpg because the efficiency mods should double the original purchase price.

 

[Electric Land Cruiser]

How much gas can you buy with the same money spent on 30 200Ah lithium batteries? 8 MPG is actually pretty good IMO. I say convert it to E85 and drive it.

 

[Jordan325ic]

The 'easiest' way to accomplish this would be a BAS hybrid system (aka "mild hybrid") which attaches an electric motor to the crankshaft via an accessory belt, just like the AC or power steering, etc... It would switch to 'generator mode' when braking to recharge the batteries and would apply power when accelerating at low speeds. I know there were several companies developing these sort of systems to be added on to large trucks for fleet usage, but I'm not sure where that stands today. I suspect that it could be done by a very capable DIYer (getting the system to work well with the ICE system would be very challenging, clever programming likely required) or perhaps installed for a few ten thousand dollars by a company which specializes in this sort of thing.

Likely it would only net you a few MPG, best case scenario, and be quite expensive. If this conversion is for purely financial reasons, I wouldn't bother. There are easier ways to improve the MPG of a combustion engine. Is this a diesel? Do a veggie oil conversion. Learn how to hypermile. Buy a "Vehicle limited to 50mph" sticker and stick to the right lane...

Another thing to consider is that most hybrid systems make the most difference at low speeds, when accelerating and decelerating often, and when idling (or not idling more accuartely). The typical RV spends most its time on the highway, not much can be done at those speeds.

 

[remy_martian]

The typical RV spends most of the time parked - they put on around 4,000 miles a year if you look at the odometer of used ones.

That's 500 gallons of fuel a year, $2,000. A small motor assist, as proposed, will likely yield a quarter mpg, optimistically say a half, in a heavy vehicle like that. That saves 30 gallons of fuel a year or $120.

If it was worth it, it would be in the RV chassis already as a competitive advantage.

What I would do, and I'm crazy that way, is get a Nissan Leaf and rig it as a pusher on a towbar...

 

[DMPstar]

...and run a belt to an alternator off one of the Leaf's tires. Alternator then powers the fridge thereby reducing the load on the RV engine. Better MPG

 

[remy_martian]

Canadian engineering.

Luckily, you all live in igloos and the fridge is a hole in the snow outside the front door. Energy saving chilling the Labatt's, eh...

😂

 

[DMPstar]

Labatts is less common on this side of the bay, but yes beer in the snowbank or near the drafty door seam if it's -30 or less

 

[brian_]

The 'easiest' way to accomplish this would be a BAS hybrid system (aka "mild hybrid") which attaches an electric motor to the crankshaft via an accessory belt, just like the AC or power steering, etc... It would switch to 'generator mode' when braking to recharge the batteries and would apply power when accelerating at low speeds.
...
Another thing to consider is that most hybrid systems make the most difference at low speeds, when accelerating and decelerating often, and when idling (or not idling more accuartely). The typical RV spends most its time on the highway, not much can be done at those speeds.

I agree, with both the most practical approach and the most effective use of hybrids, but there is one additional use case to consider: regenerative braking on grade descents. While most highway driving is relatively flat, RVs tend to be used for trips into mountain areas, and braking for descending long grades is common. I would like to turn that energy into battery charge, whether to use later to reduce fuel use or to use while camped.

A mild hybrid system can be attached to the transmission output, but putting it on the engine allows it to take advantage of transmission gearing. I have considered these systems for my motorhome, although without any serious intention to build one, and simply mounting anything to engine is problematic enough to consider something inline with the propeller shaft (mounted at the steady bearing between shaft sections), but producing enough torque at propeller shaft speed while accommodating maximum highway speed is a challenge.

 

[remy_martian]

Most motorhomes dinghy-tow a car behind them. There's your brake/charger. And pusher...

 

[MattsAwesomeStuff]

It might be possible to improve a little bit of fuel economy by removing the engine and installing a smaller, undersized, more efficient engine, along with the EV stuff.

Basically a giant Prius.

You don't need most of the horsepower for general travelling, but for respectable acceleration and some hill climbing. So size a motor that can kind of keep you up at highway speeds, and then throw in the EV with some minimal amount of battery, enough to help accelerate and maintain some speed while climbing shorter-ish hills.

I don't know how big of an efficiency difference there would be between your stock engine and whatever the minimally-sized engine could be, if both demanded the same power when travelling flat at highway speeds. But, some.

Enough to be worth it? Absolutely not unless you're city commuting in the thing.

Headed through the foothills? It'll be okay. Headed through long uphills of mountain passes? You'll be puttering along.

 

[brian_]

I don't know how big of an efficiency difference there would be between your stock engine and whatever the minimally-sized engine could be, if both demanded the same power when travelling flat at highway speeds. But, some.

Likely some, but maybe not. If the original vehicle has the engine running a high load and low speed, replacing it with an engine that has to run faster to produce the same power might make efficiency worse. RVs are typically not like passengers cars and passenger cars with big open trunks (called "pickup trucks") which have vastly more powerful engines than they need.

My ten-ton motorhome has a 6.8 L V10 engine, with about the same rated power as a typical light-duty ("half-ton" full-size) pickup truck. To maintain 110 km/h on the highway, it only has to turn around 2,000 RPM, and is probably running near its peak efficiency; it might even be better with a higher top gear than the cheapest-thing-we-make-that-won't-grenade 4-speed transmission provides. Of course it's still a gas hog, because it's a house being pushed down the road. The current version of the same chassis has a 7.3 L V and slightly less antiquated 6-speed. I'm not convinced that a smaller engine would help, but I would be interested if anyone has objective data.

 

[Electric Land Cruiser]

It might be possible to improve a little bit of fuel economy by removing the engine and installing a smaller, undersized, more efficient engine, along with the EV stuff.

Basically a giant Prius.

You don't need most of the horsepower for general travelling, but for respectable acceleration and some hill climbing. So size a motor that can kind of keep you up at highway speeds, and then throw in the EV with some minimal amount of battery, enough to help accelerate and maintain some speed while climbing shorter-ish hills.

I don't know how big of an efficiency difference there would be between your stock engine and whatever the minimally-sized engine could be, if both demanded the same power when travelling flat at highway speeds. But, some.

Enough to be worth it? Absolutely not unless you're city commuting in the thing.

Headed through the foothills? It'll be okay. Headed through long uphills of mountain passes? You'll be puttering along.

Unfortunately not. When you need power, you need power. The RV the OP is talking about has the new Ford 7.3L gas V8 which is designed to be fuel efficient in a broad powerband. No way you are going to beat that with anything homebuilt. In fact probably you would be worse off.

Here is a good visualization; BMW M3 V8 vs. Toyota Prius. Prius running as fast as possible and BMW just has to keep up. Which do you think uses more fuel?

 

[remy_martian]

My brother's Sprinter is pretty good on fuel, but it's smaller frontal area, lower weight, and uses a turbo four banger diesel. It also breaks down a lot with all that highly stressed fine German engineering that Mercedes put together. That's another thing that kills mileage -- reliability.