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Morning, forgive my ignorance but:

Does an alternator place the same load on the engine (and therefore fuel efficiency) regardless of what electrical load is required of it?

ie. Is an alternator with nothing to power requiring as much energy from the engine to rotate it as an alternator with a 60amp electrical load?

And if the answer is no, how does it work?

-Mark B)

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The power requirement varies according to load. Switching on or off power consumers like lights, rear window heater and similar can be noted on the idle speed.

How much it really matters when at speed is beyond my knowledge and experience.

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The power requirement varies according to load. Switching on or off power consumers like lights, rear window heater and similar can be noted on the idle speed.

How much it really matters when at speed is beyond my knowledge and experience.

Is that because as the electrical load increases, the magnetic field becomes stronger and therefore harder to turn?

Mark

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No, load on the engine increases as electrical load is increased, actually explaining why is slightly more difficult, involving flux, fu and general wizardry, I am sure you would find more info with a quick google, in fact, there's a short answer here: http://answers.yahoo.com/question/index?qi...07030213AABHiA6

In short you don't get something for nothing in life/physics so more power drawn = more load on engine, which is why if your alternator belt is slack and you switch on the heated rear window and the headlights the belt squeals more.

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Very roughly you could get an idea from the load e.g. a 100 amp alternator will be producing 14V x 100A = 1400W which is a little under 2hp. I doubt whether alternators are anywhere near 100% efficient so if you guess at maybe 50% of the power drawn from the engine comes out as electricity, the power draw might be double or about 4hp from the engine at full load. Anybody got a figure for alternator efficiency? :unsure:

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Hi,

This is hearsay so not that usefull.

I recall a comment in the 90s that running a Volvo with permenant lights on was costing 1/2 a mile per gallon?

Marc.

A myth. However Egyptian drivers have taken this to heart and drive generally with the lights off at night. It can make for an interesting taxi ride between towns... In practice the fuel consumption used by having lights on is negligible.

(I'd put an alternator efficiency at closer to 70%....)

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The load that dominates the alternator power consumption at higher speeds is the fan power to draw cooling air through the machine. You can actually control the mechanical load that the alternator draws by manipulating the "battery sense" voltage (since the load of the headlights etc induces a voltage drop in the battery and that's what the alternator compensates for). BMW's EfficientDynamics package has a clever computery thing between the battery and the alternator, so the alternator only 'sees' the need to charge the battery when the engine's in overrun - micro-mini hybrid!

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(I'd put an alternator efficiency at closer to 70%....)

<geek>

Bosch claim the maximum efficiency of a typical aircooled alternator is around 65%. Mean efficency in normal use is typically 55%.

For a given current load, a larger alternator (ie higher power capability) will usually be more efficient than a smaller one. It will be electrically more efficient, but that is offset a little by the increased rotational mass requiring increased energy during acceleration.

Copper losses in the field coils and diodes are constant wrt to rotational speed, but mechanical losses, stator copper losses and Iron losses are roughly proportional to rotational speed.

</geek>

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