Alternator

[h5djr]

I wonder if any of you knowledgeable chaps can answer a query in connection with the alternator.

The alternator is driven via a belt by the engine. Does an alternator take more engine power, and thus more fuel, to drive it when it's actually charging the battery as opposed to when the battery is already fully charged.

In a similar way the air-con compressor is driven, via a belt, by the engine but it has a electrically controlled clutch which only engages when output is required from the compressor. Obviously the compressor uses a little more engine power when the clutch is engaged than when it's not, but does the same apply to the clutch-less alternator.

My reason for asking is that the extra fuel used is often given as a reason for not introducing Daytime Running Lights but can an extra 42w make that much difference.

 

[Twizzler]

The power required to drive the alternator increases in proportion to the load. Unfortunately we don't get energy for free. The electricity, ie. energy, generated by the alternator is produce by converting it from the fuel.

 

[consilio]

Not sure thats quite right if the vehicle is in motion. The speed that the alternator will turn at when being driven will generate more then enough power for all equipment. I dont think that turning on more stuff increases the friction on the alternator either

I think the only time you will consume more fuel is when the engine is at idle speeds

 

[steve184]

I think it does make a difference, BMW have introduced technologies into their cars now that disengages the alternator when the battery is fully charged, however can't see this being very often as i doubt nowadays with the amount of electricals in modern cars, pumps, fans, lights, stereos, heated seats, computers, etc etc that a car could be driven that long without the battery constantly being topped up. Its probably one of those things that makes a tiny difference, but, as they say lots of tiny differences add up to bigger differences in terms of improving fuel economy.

 

[Dandle]

Any time the alternator is producing electricity it is using power from the engine. The more power required the more load is put on the engine, meaning more fuel is being used to produce that power(although 42w isnt going to be anything you will notice).

BMWs new LCI cars (LifeCycle Impulse) use many methods to increase effiency one of them is brake regeneration where the alternator will only charge the battery when the car is decreasing speed and braking, this in effect is free energy as the engine is coasting and still turning the alternator through the engine revs being powered pretty much by the inertia of the car moving along the road. Essentials that need to be powered like the lights and heating etc still take their power from the alternator all the time. Its this technology thats enabled BMW to bring their 3.0l(280bhp) petrol engines CO2 levels down to that of alot of smaller engined deisel cars(not that anyone here is interested really).