Disconnecting alternator from car....

I would suspect something on the order of 1/100 gallon per mile increase. In other words about 0.025%

while you are at it disconnect the A/C belt too!

BTW the battery wouldn't last very long.

while you are at it disconnect the A/C belt too! =======================================

Theoretically, a good idea. Why not drive the A/C off a sep. battery/charging system? No belts req'd.

BTW the battery wouldn't last very long. =========================================== Good point. Same problem with plug-ins, eh??

Which all depends on how much gas you actually save --which might make all this moot, depending on what is the hp drain of an alternator actually is.... No load? "Full load" -- headlights, defrosters, etc....

I would estimate the loss just due to friction, belts, etc is about 1/2 hp. Then, each 10 amps of draw is about 1/6 hp -- don't know how much juice a car really pulls. But let's round off, and say 1 hp, or double it, for 2 hp.

In a 150 hp car, that's like 1.3%, so presumably mpg's would increase by

1.3%. Paul said "0.025%", but I think he meant 0.025 or 2.5%, which is actually more generous than my above rough calc.

So what I thought I observed, a significant increase in mpg's, was proly blip of some sort, or just confabulation.

But..... but...... WAIT......

Cars are rarely driven at their maximum hp!!!! The old VWs cruised around with a max hp of 42, so you figger highway hp for a small car is about 20-25 hp, less around town.... Depending on the CdA, you actually might need only 5 or so hp to maintain 60 mph....

So, 2 hp out of 20 is a full 10%..... Now THAT is getting significant....

So I proly was NOT confabulating.... mebbe the idea is worthwhile. Mebbe sep. battery packs, perhaps a lithium ion for running the car, and for everyday total discharges, with the lead acid for starting, would be a worthwhile/economical mod....

So you think the AC would require less electrical energy than it requires mechanical energy? I dont think so.

You'd eliminate frictional losses, belts, bearing wear, etc. And, if the batteries driving the A/C were separate, so they were not charged by gas power but hydroelectricity, you wouldn't have thermodynamic inefficiencies -- a *substantial* savings.

In a sense, you are hybridizing the car "from the other end" -- the non-powertrain end.

Don't forget the efficiency of the power grid & battery charger, plus batteries themselves are not 100% efficient, nor are electric motors. Electric motors have bearings too.

The bottom line is it's a lot of hassle for a nearly unmeasurable gain, if any, so why bother? Oh, and don't forget the extra weight and expense of the MUCH larger battery you'd need, plus the weight of the electric motor to drive the A/C compressor.

Stupid, just like most incomplete analyses of transportation system efficiencies that don't look at the complete system end-to-end, which is invariably because of either ignorance, or a political or economic axe to grind.

"Existential Angst" wrote in news:4d6e34af$0$12977$ snipped-for-privacy@cv.net:

you are way over estmating the elect draw hp loss on the mpg of the car. any un hooking of the alt will result in very short time on the batt unless you turn off everything but the eng. and any night use will be impractical at best. and forget AC use. KB

I doubt very seriously that it would be substantial. Alternators are about 75-80 percent efficient. Home battery chargers much less. I encourage you to try it, but would bet a weeks pay that you would gain .1 mpg improvement or less. HTH, Ben

What you gain one place, you probably lose at another. This is "too much sugar for a dime".

I agree with the other posters that, whatever you do, you may only realize a very very small improvement in economy, if any.

Yeah, you brought up some minorly valid points, but I don't see a whole spreadsheet full of numbers supporting YOUR assertions.... or did I miss an attachment or something?? So because you are a clairvoyant genius, you KNOW there wouldn't be any payoff, right?

Now, as to the other much more civilized doubters, yeah, I wouldn't bet the farm on anything. But, ito of efficiencies, I'm not talking about alternator efficiency or efficiency of any components per se, but rather the efficiency of the *gas being combusted* to drive the alternator. A gas engine is, bottom line, about 30% efficient, and possibly much less.

So getting *anything* off the "gas grid" is a 70+% savings, right off the bat. How much the inefficiencies of alternative methods might eat into that, such as the charging efficiency, battery weight, etc remains to be evaluated, but I can't see that stuff eating THAT much into that immediate 70% savings from the thermodynamic losses.

Again, I don't know exactly what the frictional losses are in these belt drive systems, but I do know those belts are effing TIGHT, and a 1 hp loss doesn't seem that unreasonable. And 1 hp is a *subtantial* fraction of what it takes to keep a small car going at 60 mph -- not far at all from that

15-20% hike in my mpgs that I thought I observed years ago. And, THAT was just with a blown alternator, not with a removed belt!!

Heh, it's an easy enough experiment to try, right? Just keep a spare battery in the trunk.... :) Altho, I was not able to find out how to "unplug" the alternator on my '04 Nissan Frontier pickup, so I have not yet been able to re-do the experiment.

I used to remove the A/C clutch electrical plug every fall on my cars, and tape it out of the way. GM vehicles. Didn't want the compressor running in defrost mode in the winter, because I didn't need it to clear the windows. I've heard since that it's good to run the compressor in the winter, but pulling the plug never caused me an difficulty I know of. Probably saved some gas doing that. Don't fool around with your alt. That's a dead end.

--Vic

Let's jsut assume you have a 100 amp alternator. So at 13.5 volts, you could produce as much as 1350 watts. This is roughly equivalent to two horsepower or 85 BTUs per minute. Say your engine plus alternator drive is 10% efficient, so you need 850 BTU per minute of energy from gasoline to generate this power. A gallon of gasoline has roughly 115,400 BTUs per gallon. So to run your alternator, you need something on the order of 0.007 gallons of gas per minute or approximately 0.4 gallons per hour. If your average speed is 30 mph, then the alternator might affect your gas mileage to the tune of 0.02 mpg......a lot less than 15-20%.

You need to go back and check your mileage.....

Ed

Don't forget to check the tire pressure.Be sure the brakes aren't dragging, too much.

The only times I ever mess with the alternator is when/if it quits charging and I take it to a parts store and trade it in for a rebuilt alternator. cuhulin

Well, in a subsequent post, I pointed out that a car's mpg's arise from from maybe 15-25 *regular* hp... mebbe even less. So a 2 hp draw is a substantial fraction of this, quite in the ballpark of my vague 15-20% recollection.

And your last set of numbers (which I haven't checked) would seem to bear this out: Suppose you drove for an hour at 30 mph, and suppose your mpg's were 30 mpg. That would be 1 gal of gas per hour, right? Your .4 gal per hour is a whopping 40% of that 1 gal!!!

Ergo my point: The alternator, from a number of calculational perspectives, seems to gobble up quite a bit of fuel, %-age-wise. Mebbe not

40%, but I'll bet somewhere between 10 and 20%.

In message , Existential Angst writes

In an old car with a Vee belt the belt itself would account for 7% of the load it is driving, further, if you car is that old it's also likely that the alternator has a fan which is also labour intensive. Modern alternators are different, they are driven by Poly-vee belts and don't have cooling fans owning to better quality of insulation at a higher temperature. Now this is an interesting point because current in any wire causes heat and there are many yards of thinly coated wire all adding heat to the internals without cooling. The only answer can be that whilst the alternator runs at a sizeable load to put back what the starter has taken out of the battery, once this is done, (1 to 2 mins) it only trickle charges the battery, maybe 2 to 3 amps. The only other load on the alternator is the "Hotel load" which is the items using electricity, lights, fan etc. The heating is done by hot water from the engine and cooling is by the compressor driven by the engine. The total normal load on an alternator under normal conditions wouldn't exceed 10 amps or 120 watts. The load of an alternator is caused by the electrical load on itself, very little, I don't think you'd notice the difference.

Fantasy figures. You're just guessing. Why dont you take your alternator and head to Mexico and carefully monitor your fuel economy. Dont call us...Call AAA

As are you, if you can't refute specific calculations/estimations.

In fact, CE White's li'l calculation more than agrees with my li'l theory here, but you haven't said much about that...

Why would I need to call AAA??

You are certain of your assertions, but you cant figure out how to disable your alternator?? Get off the computer...try your theory out for a few tankfuls and let us know how it goes. Otherwise you are just playing pocket pool... Ben

I don't recall expressing certainty. But no one is throwing out numbers, either, and the one who did wound up supporting my thesis.

but you cant figure out how to

I could if I could goddamm see it/the wiring. No clear fuse for it either, tried a few things. That was going to be my next Q. This is not the 1960s or 1970s, where shit is obvious, or even meant to be fixed. You do know that many parts are now RFID'd, so you cain't even change many of your own pats, without the dealer.

Actually, if I could disconnect the alternator, I could tell you almost immediately, because I have a ScanGauge that will read mpg's realtime.

You seem to miss the point of (attempted) intelligent discussion.