I rarely agree with Bob, but many seem to be getting in the weeds, and mostly out of fear because they are exaggerating towing conditions. Some comments make it seem like a motorhome towing an electric car is like a tractor pull.
Let’s look at worse case based on realistic assumptions and facts, then we can exaggerate from there. To make it worst case, let’s assume all energy is supplied by motorhome engine directly.
First let’s use electric RWD Mustang as a toad because there’s lots of data on it. Based on its EPA rating which has been independently confirmed, it uses 15 kWh of battery capacity to travel 50 miles.
I’m going to stick to 50 miles per day for a toad to run to grocery store, go out to dinner, maybe a show, laundry mat, etc. If you feel you drive 100 miles, simply double the estimate.
The RWD electric Mustang will therefore use approximately 15 kWh of battery energy. It’s not that much. An AWD Mustang will use a little more, but not worth worrying about in this context.
Modern vehicle powertrains are typically at least 70 percent efficient in delivering engine power to the ground. That’s to say that when a motorhome engine makes 100 HP, at least 70 reach the ground through the driven wheels.
Likewise, during regenerative charging, we can roughly estimate that at least 70 percent of power at wheels reaches battery as extra charge.
Within “reason”, we should expect that at least 50 percent of motorhome incremental engine power due to charging load ends up in toad’s battery. For those that didn’t follow, 70% X 70% is 49%, or rounded to 50% to keep numbers simpler. So if motorhome works 10 HP harder to charge, about 5 HP should reach toad’s battery. The rest is wasted.
Getting back to a toad the size of an electric Mustang, if it consumes 15 kWh of battery energy to travel 50 miles the previous evening, the motorhome’s engine will need to produce twice that, or 30 kWh of work energy.
For us Americans, 30 kWh converts to 40 HP-hour at the engine.
A modern diesel should make 20 HP for one hour per gallon of diesel fuel (based on BSFC), which means it would burn an extra 2 gallons of diesel fuel to charge the Mustang’s battery back up.
A modern gasoline engine should make 15 HP for one hour per gallon of fuel, so it would burn about 2.7 gallons of gas.
If an electric Mustang drove 50 miles and then it took 2.7 gallons of extra fuel the next day to charge it back, the equivalent MPG isn’t that bad for something done occasionally.
Most important, if the incremental required 40 HP-hour from motorhome engine is spread over a 5 hour drive, it works out to just an extra 8 HP on engine. When a Ford gasoline V8 is rated up to 350 HP, adding an extra 8 HP to charge toad’s battery doesn’t seem that big a problem.
This is reasonable worst case because all incremental toad energy comes from motorhome’s engine, and even that is pretty good. Better yet, if motorhome tows electric Mustang to top of Rockies like any other car, and only then during descend is the Mustang’s battery charged, not only is energy from Mustang but also from motorhome going downhill is recoverable to charge the Mustang’s battery. That’s free energy which would have been wasted otherwise.
This is NOT meant to be exact, just to “roughly” quantify power needs so we don’t end up in the weeds thinking it will end up using way too much fuel, or so much power that motorhome will overheat. Nothing I see suggests it is that big a problem.
The concept is so simple and makes so much sense that it’s hard for me to imagine it is patentable.
) need things to fill their day so they tend to wade in the weeds. Us old down South hicks like to call it "making a mountain out of a mole hill".
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