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Old 12-20-2013, 10:08 AM   #1 (permalink)
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Generator trailer?

A few years back, I was discussing electric cars with a friend, and came up with an idea; what if you had an electric vehicle with a generator that could be used to keep the batteries full, so you could drive long distances without stopping, while being able to run short hop entirely off house-based recharges.

That part has since been realized with the Volt, albiet well out of my price range, but I can hope that someday someone will come up with a sub 20K version of that plan.

But another part of my dream scheme was that the generator would be modular; unplug it from the car, slide it out, store it in the garage, when you know you won't be needing it. This way you don't have to haul around all that extra weight, so your battery charge lasts longer and you have more cargo room.

The Volt doesn't do that; you carry the generation system with you everywhere. You can also run off the ICE itself, which wasn't part of my scheme, but it makes it easy to see why you wouldn't be able to easily disconnect.

OK enough backstory.

So reading around on the forum today, I ran across the Pusher trailer idea. Basically, build a separate self contained motorized trailer using the "other" system, whether it be an electric trailer designed to allow short range cruising with your ICE car's engine off, or whether an ICE trailer designed to allow an all-electric to drive in the "infinite range" of an ICE without needing recharge, just refueling.

I'm sure it's been thought of before, but the idea was new to me... what if you put a generator in the trailer instead of a pusher? The same idea as the ICE pusher, giving "infinite" range to a straight electric. And similar to the ICE pusher, and my "modular" idea, it could be left at home for normal use, added to the vehicle only when the extended range is needed. But unlike my "modular" idea, it would be simple to physically remove; all you have to be able to do is dismount a trailer from the hitch, no heavy lifting. And easy to hook up, again, just mount a trailer on the hitch, instead of heavy lifting and struggling to get fitted into it's slot in the vehicle.

Why would this be better than the pusher ICE? Well without actually doing the math, thought experiment says there's a whole lot of weight and engineering involved in getting the power to the ground, and that cars aren't really designed to be pushed around. I know people have done this, but it still sounds, frankly, terrifying. But more to the point, I would think ICE plus generator would weigh less than ICE plus everything it takes to get power to the ground. I could easily be wrong there.

Anyway, obviously running physical power up from a trailer to the wheels of the car itself would be even more difficult... but running electric power would require nothing more than a cable and the plug that the vehicle already has.

So, I'm going to look into this idea some more, but I thought I'd get it out there, see what people think... and of course have folks point me to the 20+ threads where other people are already doing this.

Questions:
1) What's the idea car to do this with? (probably unanswerable... but maybe some are ruled out as clearly being unable to lug a trailer around)

1a) in order to answer 2) we need to pick some actual car and work backwards. Answers for 2) will be different for other cars, but should at least be in the range of "possible" or "impossible"

2) How big would the trailer have to be?

2a) How big would the ICE, generator, and fuel tank have to be

2a1) How much power would the generator have to supply?

2b) What sort of trailer would work? Ideally we're talking a streamlined motorcycle one wheel, lightweight, low cD, easy to maneuver. Worst case we're talking double tandem tractor-trailer rig, which would make the endeavor pointless for anything short of an electric Semi.

3) would an alternative to the trailer be better

3a) back to my "slide in modular plan". Benefit, would not alter the drag of the vehicle, would not add the difficulty of driving that any trailer does, and would presumably be lighter than any trailer. Detractions, harder to mount/dismount, takes up internal room, and what to do about the exhaust of the ICE? Oh, noise inside the cabin, too.

3b) rooftop mount version of the modular plan. Assuming this is light enough, it could be used on any electric capable of supporting a roof rack

3c) trailer hitch rack. Again, if light enough, would work on any car.

3d) bed mount, in pickup-style electrics. Maybe even do conversion on regular electric cars to turn them into pickups to facilitate this.

3bc&dI) since these aren't inside the vehicle, perhaps a simple overhead lift system could be used to mount and dismount, more easily than the "slide in"

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Old 12-20-2013, 12:46 PM   #2 (permalink)
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The main issue that I see is that to push a family car down the road requires around 13kw delivered to the wheels. That's without a trailer, measured in mechanical power. Towing a trailer and allowing for conversion inefficiencies and you are probably talking 18kw. There's plenty of lightweight engines that can make that with decent efficiency, but where do you get a generator head that can take that kind of load? I mean assuming your battery is 200v, that's a cool 900 amps...
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Old 12-20-2013, 01:19 PM   #3 (permalink)
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How is the Volt doing it? Or is my understanding of how it works all wrong, and the Volt isn't generating electric power from its engine, but rather putting power from the ICE strictly to the wheels, in sort of an either-or-both configuration?

Without doing a whole lot of detailed work, just looking at the first item on a google search for 18000 watt generator, I found the Winco "Big Dog" 18000 watt portable generator, selling for $4350 and with a shipping weight of 500 lbs. Just guessing, can probably get a trailer for it cheaply enough to put the finished price of the build at $6000. As an added benefit, you've got a generator set to run your house in event of grid outages, or ready to loan out to red cross emergencies.

Sadly, 500 lbs probably puts it outside of my top of roof alternative... but it does seem like maybe motorcycle hauler hitch mount territory, I see some rated for that. And the motorcycle pulled trailer style? Certainly seems doable.

Looking around motorcycle pulled trailer topics on the web, I see folks are building them for under $500, combining a harbor frieght flatbed trailer and a cartop cargo carrier.

Where are you getting that 18000 watt figure, by the way? Is that peak load, or average? Because peak wouldn't be necessary, the key would be to generate more than the average requirement, so you'd be recharging the battery while operating, but the battery would be supplying the power needed when you're drawing more than the generator makes.

I also see a problem in that, with my theory, I'd still end up running out of power if climbing up some serious mountain ranges. But I guess that's not really a crippling issue, just pull over and wait till the battery is fully charged again, and then start over. Stored battery power plus continuously generating enough for average running should mean a fairly good range, and when I run out, I still don't have to find somewhere to plug in, at least until the gas runs out (and that's no different from running dry in an ICE only configuration, right?)
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Old 12-20-2013, 03:20 PM   #4 (permalink)
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Quote:
Originally Posted by Swordsmith View Post
How is the Volt doing it? Or is my understanding of how it works all wrong, and the Volt isn't generating electric power from its engine, but rather putting power from the ICE strictly to the wheels, in sort of an either-or-both configuration?
The Volt is using planetary gears to split the mechanical energy so the gas engine or the electric motor can drive it.

Quote:
Where are you getting that 18000 watt figure, by the way? Is that peak load, or average? Because peak wouldn't be necessary, the key would be to generate more than the average requirement, so you'd be recharging the battery while operating, but the battery would be supplying the power needed when you're drawing more than the generator makes.
My 1,400 pound electric car draws 6,000 watts while going 35mph on a flat straight road, 18,000 seems high as that is 24hp and I thought that the average car uses 15-20hp at highway speeds.

This same topic just came up on our electric auto club email list, turns out the Nissan Leaf does not like towing trailers, so while a pusher trailer would work fine a generator or battery trailer would not work well.

There is a wide agreement that pusher trailers will also use less fuel then a generator trailer and weigh less, most common that I've seen is the diesel VW front end with an automatic transmission, a regular gear box left in 5th gear should also work if you have control over the clutch, the transmission should even weigh less then a generator with fewer losses then size and tune it to run most efficiently at cruising speed.

The only issue with a pusher trailer is backing up, but if you use the front end of a small car then you have steering and people have made simple steering set ups for backing up trailers that I hear work really well.

Battery trailers are the other option, but the one example I read about took a vehicle with 100 mile range and put the same battery pack in a trailer to "double" it's range and they got a 180 mile range instead of 200.

I like the battery idea because the rest of the time it can be used to power other stuff or as a back up to power your house, all without burning extra fuel.

I of course want to build a battery powered pusher trailer that could be used for cargo as well behind any vehicle down to a bicycle.
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Old 12-20-2013, 03:25 PM   #5 (permalink)
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Quote:
Originally Posted by Ryland View Post
The Volt is using planetary gears to split the mechanical energy so the gas engine or the electric motor can drive it.
The Volt uses a generator connected to the ICE, the ICE isn't mechanically connected to the wheels. I think you're thinking of the way the Prius and Ford's hybrids work (possibly others).

Edit: Nevermind, Wikipedia mislead me until I read more.
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Old 12-20-2013, 05:53 PM   #6 (permalink)
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I pulled the 13kw figure from an actual dyno reading under the same speed/load conditions that one large family car used on the highway at 100km/h. I then added to that based on towing, and again based on conversions not being 100% efficient. It also doesn't hurt to have a small buffer. The car that the measurements were made on was a 1996 Ford Falcon (1550kg four door, 4L Six cylinder, 0.32 drag co-efficient, 7.5l/100km highway standard).

As for how does the volt to it? I'm assuming that it uses higher than 200v for the traction battery (I have no idea really) and it would have a separate AC speed controller for the starter/generator head.

I suppose if you had a convenient battery voltage you could just attach a bridge rectifier to a gen-set for your DC output though...
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Old 12-20-2013, 07:10 PM   #7 (permalink)
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Quote:
Originally Posted by vskid3 View Post
The Volt uses a generator connected to the ICE, the ICE isn't mechanically connected to the wheels. I think you're thinking of the way the Prius and Ford's hybrids work (possibly others).

Edit: Nevermind, Wikipedia mislead me until I read more.
Right, that was the Volt design of 5+ years ago, if you draw the Prius drive train in diagram form and the Volt drive train they look nearly identical, they are just different enough that they can each have their own patents.

I believe that most electric cars on the road have a battery pack voltage of around 300v.

Running a generator you are going to have losses, I believe around 30% engine to wheels, where if you have a mechanical connection it's less then 10% engine to wheels.
Where the mechanical connection has draw backs is when you try to vary the vehicle speed a great deal and thus vary the engine speed and load a great deal, but if you turn on your long range trailer at 65mph and have it tuned for that speed then you are set!
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Old 12-20-2013, 09:04 PM   #8 (permalink)
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This topic has been hotly debated in several threads.

As already pointed out, a generator is not as efficient as a pusher due the mechanical -> electrical -> mechanical conversion. A pusher generates mechanical energy and applies it directly to propelling the vehicle.

The advantages I see of using a generator:

1. Backup home power in the event of outages
2. Generator does not push the vehicle and introduce dangerous driving dynamics
3. Generator can be sized for the average load and run at a constant speed, utilizing battery power for acceleration and hills

Either way, a generator or pusher is impractical since most families have multiple vehicles, one of which could contain an ICE. Furthermore, the cost of a pusher or generator exceeds the cost of a normal used car. There just isn't a point to either a generator or pusher, since better alternatives for distance travel already exist.
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Old 12-20-2013, 09:08 PM   #9 (permalink)
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Cool

Make your pusher trailer out of a gen 2 prius, problem solved.
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Old 12-20-2013, 11:59 PM   #10 (permalink)
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Just make the range extending trailer out of an electric golf cart with larger wheels for higher speeds so it can be a self propelled utility trailer or a home back up power supply, then it can replace a pickup truck and take you long distances.

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