Do not understand how Alternator low voltage load increase MPG.

kozaz · 11-14-2007, 05:18 PM

I understand if you disconnect the belt from the alternator you will get better mpg.

What I do not understand, is how does regulating the voltage required to keep your car running increase mpg?

I also can see replacing batteries would be a cost increase.

I've read about switching to LEDS and some newer cars that lower the amount of voltage drawn from the alternator if the car is not requiring the full amount of voltage.

I need schooling!

An after thought. What about running the alternator off one of the drive axels? It would not turn when at a stop, (battery support only) then when you drive the axles would turn the alternator.

A pulley could be machined, from the inside out, to the outer diameter of the drive axle and welded. You could hang the alternator off the body and you would not have to extend the wiring that far. Only issue is, you would have to detach your drive axle from the output flange on your transmission to change a belt.

Regards,

Ryan

ajohnmeyer · 11-14-2007, 05:37 PM

I'll give this a shot, but anyone feel free to jump in and correct me if I'm wrong.
well, it's not really voltage that you're trying to decrease, the alternators output voltage should remain relatively constant, it's the amperage that you're trying to keep down. The way I understand it is that the more amperage that you are demanding from the alternator (or any generator for that matter) the more physical resistance that the alternator is presenting to the engine. So, the more amps you draw, the more HP required to turn the alternator. So the overall HP load of the alternator should (relatively) match the amperage load of your electrical system.

It's not quite like the other main parasitic accessory the a/c compressor. With a compressor it's either on or it's off. When it's off there's only a slight load due to the increase in turning the extra belt and clutch wheel. When it's on there's a lot of resistance because you're having to compress all that freon, which takes a lot of HP to do.

csrmel · 11-14-2007, 05:38 PM

voltage isnt the measurment of power. wattage is. when you lower the running voltage you reduce the wattage load from most electrical devices.

runing an alternator off an axle wouldnt work because axles don't spin quick enough at slow speeds or even highway speeds to make an alternator charge. they have a certain minimum rpm as which they will charge. besides that, axles move up and down with suspension. there would be no way to keep a pulley installed on an axle inline with the alternator installed on the car.

running led's doesn't lower the ammount of voltage drawn from the electrical system, it lowers the ammount of amperes drawn. less amps couples with the same voltage means less wattage. again, wattage is how we determine load.

with that said, i think the gains from running at a reduced voltage is low. personally i don't like dim headlights and slow turning fans. i keep my system running at 14.5v.

Snax · 11-14-2007, 05:51 PM

You need to think outside the box a bit csrmel. I've seen a RWD driveshaft driven alternator work just fine. In this case, it was with an independent rear suspension, so the driveshaft did not move vertically. Likewise however, any transaxle that employs a carrier bearing and stub axle setup could easily do the same thing. In the case of the MR2, it would have to be driven off of the transaxle side of an inboard CV joint, but nonetheless, possible.

Alternator output must also be regulated to avoid overpowering whatever loads are connected - like a fully charged battery for instance.

8307c4 · 11-14-2007, 06:29 PM

Now I can't remember if this would be practical for anything but racing.
But you might check into some aluminum / high performance pulleys...?
They're not cheap thou, and reading up on it is fairly required.

As a general rule they weigh less, also in some cases they're a different diameter, the idea being that it reduces the accessory's rpm some.

DracoFelis · 11-14-2007, 06:32 PM

As other have said, you want to lower the WATTS (power used) in your car, because virtually every electrical watt you draw is at the expense of putting a little more drag on the engine (and therefore a little more fuel used) to generate that extra (electrical) power. The thing you have to remember is alternators aren't simple on/off devices, they (like virtually all mechanical to electrical generators) are proportional devices (i.e. the more electrical power you draw, the more mechanical load you have on your mechanical source, in this case the gas engine). So the goal is to lower watts used.

Now: Watts (power) = amps (current) X volts (voltage)

Which means that there are two ways to lower total watts used. You can either lower the amps, or you can lower the voltage (or both). Most of us take the lowering of the amps approach. For example, using LEDs (which are more energy efficient), instead of normal car incandescent bulbs. Also turning off fans/etc when they aren't needed.

In theory you could also lower the voltage, but there are problems there. Remember, most of the car's electrical system is DESIGNED to run at 11.5 to 14.5 volts. So running at a lower voltage risks lights being too dim, problems with ignition, problems with charging the battery, etc. Lowering voltage could (in theory) save some power (watts), but in general it's likely to be more of a PITA than it's worth. And that's even more true, when you consider the power losses (inefficiency) that happens when you convert from one voltage to another using a simple circuit. i.e. if you lower the voltage in a car's electrical system without using a very fancy/decent circuit, you may actually lose more power (from the voltage lowering circuit) than you would gain by running on a lower voltage! So all things considered, I think playing with the voltage (in a car) is usually more trouble then it's worth.

However, lowering the current (by lowering the total watts of all electrical consumers in a car) can be worthwhile. And there are many practical ways to help with lowering total watts used. For example, I saved a lot of power by converting my normal car bulbs (not the headlights themselves, but most other car lights) to LED technology (which last for a very long time, and use a fraction as much electricity as the bulbs they replaced). And I've also got an adjustable radiator fan sensor, that is set to get noticeably hotter than stock (but still cool enough I don't think I'm significantly overheating my engine) before the radiator fan kicks on (thereby often saving the huge power of the radiator fans). And I generally avoid using the interior vent/heater fan, unless I really want the air blow (i.e. I don't leave it on all the time, like many drivers do), etc. And I'll only use my main headlights after dark when they are needed for seeing (and even then I avoid the extra power hungry "high beams", except when I actually need that extra light), and instead only use my (energy efficient LED) secondary lights if/when I just want other cars to be able to see me!

The point is, such electricity saving tricks can make a noticeable (if not huge) difference in FE, because using more electricity in a car really does translate into slightly more fuel used as well (as the electricity has to come from somewhere, and that "somewhere" is extra mechanical drag on the gas engine).

csrmel · 11-14-2007, 06:58 PM

Quote:

Originally Posted by Snax

You need to think outside the box a bit csrmel. I've seen a RWD driveshaft driven alternator work just fine. In this case, it was with an independent rear suspension, so the driveshaft did not move vertically. Likewise however, any transaxle that employs a carrier bearing and stub axle setup could easily do the same thing. In the case of the MR2, it would have to be driven off of the transaxle side of an inboard CV joint, but nonetheless, possible.

Alternator output must also be regulated to avoid overpowering whatever loads are connected - like a fully charged battery for instance.

show me a drive shaft driven car alternator that works just fine.

DarbyWalters · 11-14-2007, 07:15 PM

Quote:

Originally Posted by 8307c4

Now I can't remember if this would be practical for anything but racing.
But you might check into some aluminum / high performance pulleys...?
They're not cheap thou, and reading up on it is fairly required.

As a general rule they weigh less, also in some cases they're a different diameter, the idea being that it reduces the accessory's rpm some.

In racing you change pulley sizes on the crank, waterpump. alternator to slow down the water pump to prevent cavitation at high rpms and lower the alternator draw at high rpms. Increases horsepower...usually run no fans except in 12 and 24 hour enduros for the same reason.