How much gas does the alternator take to run?

Some more things to consider and WAG about.
I'm guessing the average load on the alternator is way more than 8 amps (100W/13v). Large 4 stroke automotive type outboard engines can consume up to 30A just for the operation of the engine (seems high but that's their claim), that's almost 400W. I'm guessing car engines aren't too different. So that would be .024 gallons.

The gas to power conversion of a otto cycle engine is in the 15-20% range. Atkinson cycle is about 15% more efficient, (average 17.5% * 1.15) let's say the 2GR is about 20% efficient then... it's .12 gallons which is about a pint of gas.

If you're averaging 60mph for 2 hours at 20mpgs/, you've burned 6 gallons. If you didn't have your alternator, then you only burned 5.88 gallons, that would be 20.4mpg

.4mpg = something a car maker will definitely spend tens of millions of dollars trying to find.

 

100% agreed. I thought about digging into efficiencies, but was more than I really felt like typing

 

The mechanical load of the alternator absolutely changes with the amperage it supplies.

 

Only if the iffling pin is engaged.

 

Voltage doesn't equal power. You should educate yourself on how work/power and generators works before you dig yourself a bigger hole.

QUOTE="Sharpish, post: 25714414, member: 173981"]The voltage regulator limits the alternator output to 14.5v as that is the effective voltage to charge the battery, it doesn’t dynamically vary based on load or need.

Go start your truck with a multimeter on the battery posts, it will read 14.5ish at idle and 14.5v at 5000 RPM. It will be 14.5 if the battery is brand new or if the battery is shot and you just jumped it. There’s no intelligence to it.[/QUOTE]

Watts = HP. Volts don't. You can theoretically make 10000v with no power loss.

 

I would think the mechanical drag of the alternator spinning is a constant regardless of the output. I suppose if the truck is energizing an electromagnet inside to different levels the physical resistance to spinning would change.

You learn something new every day. I’ve had an Ultragauge in my truck for 2 years and system voltage is always 13.85 or so regardless of accessories turned on, highway use, slow crawling up hill in 4LO, towing, or sitting in traffic. It would be interesting to see, like you suggest, that alternator output Amperage is changing dynamically on the fly.

 

Think of it this way - an alternator is an induction motor working backwards, more or less. Load a motor more mechanically, it takes more electrical power. Same principle with alternators, the heavier you load them electrically, the more power they take to turn mechanically.

 

That makes sense. Thanks for the clarification.

 

All of this thinking makes my hat rack hurt. So... how does an alternator work again??

 

All electronics work on smoke... An alternator is no different.

smoke_theory (rochester.edu)

 

Since nobody in this lovely discussion has said whether or not they have any real world experience with a solar panel setup installed on their truck, I'll chime in.

I have a 160W flexible panel permanently installed on the canopy of my truck. It's wired through a charge controller to my battery. There is no change in fuel economy with it installed. Depending on what you need to do to install the panel, it can actually hurt your fuel economy based off of how much it weighs and how it's mounted. For example, if you need to install a roof rack you're going to take a hit in mileage that will be bigger by far than any advantage gained by the panel. Roof racks, and putting things on them, are one of the worst things you can do if you're trying to get better mileage. I take a bigger MPG hit when I put my canoe on the roof of my truck at freeway speeds than I do when I tow my 2000 lb teardrop trailer behind me.

The reason why the solar panel doesn't increase the mileage is simple. When the alternator is on and charging the battery, the battery voltage is high enough that the solar charge controller sees it as a full battery and doesn't send any juice that way. When you stop, the battery voltage drops down and the solar charge controller will begin sending juice to it to charge it until it reaches its target voltage.

The other issue that you'll see is that the amperage from a panel really isn't that much. My 160w will put out about 7 amps in really optimal conditions. I could have gone with a more efficient panel that might have gotten me closer to 10 amps, but to get more than that I'd be looking at covering my entire roof and probably my hood in solar panels. I'm not interested in doing that, and the 160W is almost exactly 2/3 of the size of my canopy roof. I threw my amp clamp on my truck while it was sitting idling. For giggles, I put on both seat heaters, my headlights, driving lights and fog lights, fan on high and stereo on. I was pulling 36 amps at idle, based off of adding up all of the loads that I could measure coming off of the battery positive terminal. So, based off of my panel, I'd need at least five of them (and optimal sunlight) to meet the alternator output of my truck at the time.

I'd be curious to see his tests, but I wouldn't be surprised if his methods were full of holes. The first one I can see is that he's doing his testing "around town". Around town I can get my mileage to fluctuate that much just based off of how hard I come off of a stop and how much I romp with changes in speed limit.

 

Was OP taking the drag effect of the solar panels into consideration?

 

Toyota hybrids get excellent gas mileage because they don't have alternators. No other reason!

 

That is flippin’ hilarious. I’ve heard the expression, “let the smoke out” before (thanks, AvE), but had never seen that paper. Dying

 

While out camping running a fridge, lights etc.....What’s cheaper in the long run..... buying $100’s of solar panels, extra battery, charge controller, to provide power/recharge or..... simply idling the truck for a while to recharge the battery. Carry an extra can of gas rather than a bunch of panels?
Someone savvy enough could likely make an auto start system to start truck when battery voltage went to a certain voltage to recharge. Will the solar ever pay for itself in the costs fuel idling a while?

 

Typically the alternator is producing less power at idle, and you're having to run an entire engine to turn it. Its horribly inefficient.

Besides, you can get a 100W solar panel with a built in controller for $120.

At that price point, I don't really care if its actually cheaper - I don't want to be babysitting or listening to an idling truck while I'm camping.

 

Anyone familiar with british cars is already living this joke (the electronics are soo shitty):

 

Just start the engine and pour gas in until its full.

 

No need for an extra battery. Plus solar works any time the truck is off and you don't have to be around to babysit the truck. As mentioned above, at idle the alternator is not very efficient. You'll see high-idle options on some emergency vehicles and commercial vehicles. It's usually a switch that increases the idle speed for the sole purpose of increasing the idle output of the alternator. This is also an issue with alternators that have been rewound for higher output.

For my uses, the solar panel also helps keep the battery charged when I'm at work. My truck sits an average of six months a year while I'm at sea. I'm on vacation the remainder of the year and the panel is used to keep my fridge running.