How much gas does the alternator take to run?

Clearly, not getting it.

 

So to get a real answer to this, you'd have to log the power output from the alternator over time and see what the demands on the electrical system are. I don't think the ECU does this by default.

But we can play with some numbers and get a WAG.

1 watt = 1 joule per second

1 gallon of gas = 120,000,000 joules

So if your electrical system pulled 100 watts continuously, and you replaced that with 100 watts of solar, and you drove for 2 hours per day, that would be:

2 hours * 60 minutes/hour* 60 seconds/minute* 100 watts = 720,000 joules for the electrical system.

720,000 joules / 120,000,000 joules per gallon = .006 gallons of gas saved on a 2 hour trip.


Disclaimer: I've only had one cup of coffee.

 

Just remove the alternator altogether, run off the battery as a total loss system. You won't make it too far, but mpgs should be .001% better

 

When the alternator went kaput on our old suburban we ran it for 3 and a half hours on the battery - AC off, lights off.

We had to stop when the bearings in the alternator went too, causing the rotor to go sideways, the fan to dig into the front of the housing, and the whole thing to catch fire

 

Power Nation did a youtube video (which I cannot find at the moment) with dyno tests a stroked 351 (409 CI) with and without an alternator. They found that even with a dead battery and alternator charging at 14.6V, the HP loss was tiny. Almost not measurable. They found the most HP by removing the mechanical water pump and putting on an electric one. I think it was +20HP there. Interesting video.

 

I spent many years doing trackdays on motorcycles, lots of dudes seeking every last ounce of weight saving just ran off a lightweight lithium battery. Hopefully your truck survived?

 

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.

 

It's funny to here people that obviously have NO idea how a charging system works state "facts". Do you think the alternator demand is the same when you have the blower motor running, the headlights on, and the stereo on, as when all that is turned off? If as soon as a alternator was spinning it was putting out voltage then your battery would over charge, and boil over all the time. Try taking a alternator and spinning it with a small pony motor, you won't get it to produce any voltage until you "excite" it, which is a signal that comes from the regulator or ECM. All vehicals have a Voltage regulator, older vehicals had external ones, and then they came up with the "one wire" alternator which was internally regulated. MOST new vehicals are now regulated by the ECM to maximize fuel efficiency and extend the life of the alternator. And yes, some newer vehicals also have clutches to try to maximize efficiency.

Now to the question at hand, I don't belive for one second that removing the belt form the alternator would increase fuel economy but 10-15%. Maybe 1-2% one the best of days, but even that would never be noticeable because there's way to many other factors that could change your milage more then that. Tire pressure, and having a head/tail wind could have more affect then that.

 

Reminds me of the weekend warrior cyclists who spend thousands on the lightest bike and gear under the guise of cutting weight when they could drop 10lbs of fat for free.

 

Oh yeah, no problem. Had a friend tow us the rest of the way.

Didn't get the core charge on the alternator back though

 

So you're agreeing with me without knowing it.

In order to keep it at 14.5 volts, the alternator adjusts its current output. The easiest way to do this is turn the field coil on and off based on the voltage sensed in the electrical system.

 

To explain, if you have a spinning magnet near a coil you get a voltage proportional to the rpm and current draw that's whatever the loads draws at that voltage. That obviously won't work for an engine which varies in rpm constantly.

So instead of using a permanent magnet, they spin an electromagnet. A regulator adjusts the current to the magnet, and therefore the strength of the magnet (The field winding) which maintains the voltage that way.

 

Way better mpg's with the alternator off. I've heard eventually you won't even be burning any fuel at all!. I think ima try this mod this weekend

 

Engine Masters did a similar one. I don't recall how much an alternator took but it wasn't much. They found 20+ though by removing the mechanical fan.

Man, you could easily google this to see that you're wrong. Or maybe it's just a Friday thread... with thought I'll try one last time.

https://www.carparts.com/blog/a-short-course-on-charging-systems/#:~:text=A VOLTAGE REGULATOR regulates the,right with the charging system.

"The rotor contains the powerful magnet that passes close to the many wire loops that make up the stator. The magnets in the rotor are actually electro magnets, not a permanent magnets. This is done so that we can control how much voltage the alternator produces by regulating the amount of current that creates the magnetic field in the rotor. In this way, we can control the output of the alternator to suit our needs, and protect the circuits in the automobile from excessive voltage."

"The voltage regulator controls the field current applied to the spinning rotor inside the alternator. When there is no current applied to the field, there is no voltage produced from the alternator. When voltage drops below 13.5 volts, the regulator will apply current to the field and the alternator will start charging. When the voltage exceeds 14.5 volts, the regulator will stop supplying voltage to the field and the alternator will stop charging. This is how voltage output from the alternator is regulated."

 

Shit posting is legal on Fridays.

 

You're welcome.

My point......the horse has been beaten and shot.....its dead.

 

It's called an overrunning alternator pulley. The clutch in newer alternators is there to protect the alternator from engine speed changes like deceleration or changing gears, not to bypass the alternator to be more fuel efficient.

For the purpose of this discussion, An alternator has the same mechanical load whether it is making power or not. The ECU and voltage regulator decide whether or not to supply a small amount of power to the alternator to return more power to the battery. An alternator also has the smallest pulley in an engine, meaning it is spinning 2-3 times faster than RPM of the engine.

I agree that the 10-15% is way out of line mostly because those numbers come from hyper-milers driving a Geo Metro that has 50-70hp max.

 

3.14 gallons per hour is A LOT. That's roughly what it takes to cruise on flat ground at 75 mph in top gear. An alternator should take no more than 0.1-0.15 gallons per hour even while delivering full amperage.