Power (accessory type) consumption and FE

[SVOboy]

I know there's a link, and I think the link is pretty obvious, but can someone explain it in a more technical and direct fashion? Like, what kind of power draw makes for what kinda of fuel consumption difference? And how such things like removing and alternator or a/c belt would help? I'm kinda out in the woods here, sorry,

Thanks though!

[JanGeo]

Basic Electric Power 101

Alternator generates 3 phase electric from stator windings that feed through a set of 6 diodes to may DC out to the car's 12 volt battery and electrical system. There are voltage drops across the diodes thus energy loss so lest work with some more accurate numbers than the 13.9 volts that usually is being generated at the battery terminals. In reality add about 1.6 volts to the 13.9 = 15.5 volts, next the amps that you draw times this voltage makes watts and 746 watts is 1HP but the alternator is not that efficient - maybe 60-80% at the extreems (low and high). Using 60% efficiency then 1HP into the alternator gets you 746*.60/15.5 = 28.877 amps out. There is about 9KWH of energy in 1 liter of Gasoline so with a 25% efficiency engine figure about 1 gallon for 9KWH of usable output energy - Electric here in Newport is about $0.24 per KWH or about $2.16 of energy equivalent to a gallon of Gasoline. Suddenly the electric car is cost effective since they usually go about 5 miles per KWH of energy or about 45 miles for $2.16 of electric and these electric rates are the highest in the country. Back to the gasoline 9KWH/746 watts yields 12 hours of output at 28.877 amps on a gallon of gasoline. If you car has a steady electric load like the fuel pump and computer and ignition then driving really slow burns the gas to make the electric power - going faster and burning more fuel to move the vehicle vs less making power for the electric results in higher efficiency to a point until the air drag increases lowering the over all efficiency again.

[Silveredwings]

All good info JanGeo, but man that's expensive electricity.

Some of the inefficiency of the alternator comes from the fact that you have to use electricity to make electricity. In other words, the voltage regulator sends 12V DC current from the battery into the field rotor coil to create the necessary electromagnetic fields. When the alternator spins it induces current into the stator coils which create the 3-phase output. By selectively energizint the field, the alternator can be periodically turned on or off depending on the battery's state of charge. When the alt. is off, it draws no electric current and neglible drag to the engine (except for resistance from bearings, brushes and the belt itself).

I recently saw a product that shuts off the alternator field when the engine is at high power. It does this by detecting low vacuum in the intake manifold that occurs when you accelerate or climb a hill. This leaves more power where you need it: for locomotion. As you lift the gas pedal, the vacuum rises and re-enables the alternator to make use of surplus hp from the engine. I believe this could help get higher FE.

Has anyone tried this? I'd like to if I can find a vacuum sensor or switch.

[Sludgy]

There are lots of vacuum switches out there, but few are rated for high amps. I saw one that was rated at 3 amps DC, but you'd need one rated for about 100 amps for most alternators.

Probaly need a capacitor too keep the contacts alive for laon as well.

[Matt Timion]

A few insight drivers have done this with the air conditioning. Instead of constantly turning the AC on and off depending on driving style (hills vs. flat land, etc.) one of them hooked up a vacuum switch to the AC switch.

Under certain loads the AC turns off and won't turn back on until the load is gone.

[MetroMPG]

Originally Posted by Sludgy

There are lots of vacuum switches out there, but few are rated for high amps. I saw one that was rated at 3 amps DC, but you'd need one rated for about 100 amps for most alternators.

you're thinking alternator output (and 100 amps is a heavy duty alternator). you just need to switch the field wire. i can't tell you how many amps it is, but it's much much less than the max amp rating of the alternator.

[Sludgy]

Aren't most new alternators "one wire" with the voltage regulator inside the alternator casing? This would make cutting the field windings difficult.

[MetroMPG]

hmm, mine's got 2 external, but they arrive in the same cable casing.

[Silveredwings]

Originally Posted by Sludgy

Aren't most new alternators "one wire" with the voltage regulator inside the alternator casing? This would make cutting the field windings difficult.

Doh, that would make it more difficult, wouldn't it.

Back to the drawing board.

Originally Posted by MetroMPG

hmm, mine's got 2 external, but they arrive in the same cable casing.

If it's got only 2 wires (a big one and a small one), then the small one is probably for the idiot light.

[SVOboy]

So then, I must ask, what is the difference between not using the a/c and not using it with the belt removed?