Originally Posted by Ralf S.
Do BMWs have weak alternators then.. you often see them on dark mornings, driving around with just the sidelights on...?
However, I'm not convinced that using the electrics would give you less mpg. The load on the engine comes from driving the alternator and the alternator is turned regardless of whether all the lights are on or not.
It's true that the "energy" being converted from petrol to leccy has to go somewhere (not being able to destroy it) but it goes into the battery. When the battery is fully charged by the process then the electrolyte increases in resistance.
The regulator does something at this stage (probably converting the excess current into heat, judging by the huge fins they normally have on them) but I don't see how the alternator becomes any easier for the engine to turn as a result.
It doesn't have any back-channel process to monitor what's happening at the battery and even if it did, how would it be easier to turn? It would need some kind of viscous coupling, clutch or differential in it to become easier to turn. I can't see it myself.
Air-con works via a valve that is closed and causes the compressor to start compressing when you switch it on. That acts as a "clutch" of sorts, so explains why A/C does use more gas.
But alternators... ? It's "free" electricity as far as I can tell.
Not so sure Ralf.
What about when you've just started a car up on a cold dark morning, and you switch the main beam on? You definitely see and hear the revs drop for a moment, then recover. That suggests there's definitely an extra load on the engine.