Headlights

[guesswest]

I'm not very science-minded at all and I know there's a lot of people on here who are - can someone help me resolve a discussion I was having with some friends today?

I mentioned that I'd read somewhere about an alarming practice in Taiwan of driving around at night without headlights on because of a widespread belief that it uses up more gas. The article I'd read also described this belief as 'erroneous'.

The people I mentioned this to immediately objected, saying that such a belief is correct, that using headlights does consume more fuel, albeit not much. I was arguing that, although obviously there's a finite amount of energy being produced, that since the battery is being sent charge by the engine anyway it's essentially 'free' energy because the battery would otherwise be incapable of receiving charge and that energy would be wasted (I assume it'd be lost as heat). Apologies if my understanding here is completely retarded, I hang out here for the philosophy and know nothing about this stuff, so I await correction.

So can someone clear this up for me? Assuming a modern car with a properly functioning electrical system / good battery etc, do you use more fuel by having your headlights turned on while driving? Obviously I know you can with a [censored] electrical system/bad battery etc (remember my 67 bug used to stall when I turned on the headlights!). Thanks in advance.

[Matt R.]

Without thinking too much about the actual mechanism -- I don't know how a car's electrical system is actually set up -- I think that using any accessories (A/C, stereo, headlights) on a car consumes some amount of fuel over the basal rate. Someone who knows more about car engineering will have to elaborate further. When you use the car's battery to operate things like the headlights, you must use up some amount of fuel to recharge it. Maybe this extra fuel used to recharge the battery would be lost as heat anyway, but I don't think this is so. I'd need a bit more knowledge of the electrical system to say for certain though.

[soko]

Headlights = Energy consumption
Battery = Energy source for lighting headlights
Alternator = Energy source for charging battery
Motor = Energy source for alternator
Gasoline = Energy source for motor

How come your battery dies when you leave the lights on overnight but you can leave the headlights on with the motor running for 10 years and the battery never dies? The energy has to come from somewhere.

[soko]

To correct your logic here:

[ QUOTE ]
that since the battery is being sent charge by the engine anyway it's essentially 'free' energy because the battery would otherwise be incapable of receiving charge and that energy would be wasted (I assume it'd be lost as heat)

[/ QUOTE ]

The alternator absorbs energy from the motor and turns it in to battery charge. Without the alternator the motor would be able to work less hard (consume less gas) to produce the same level of forward motion, saving gasoline.

[Brainwalter]

[ QUOTE ]
To correct your logic here:

[ QUOTE ]
that since the battery is being sent charge by the engine anyway it's essentially 'free' energy because the battery would otherwise be incapable of receiving charge and that energy would be wasted (I assume it'd be lost as heat)

[/ QUOTE ]

The alternator absorbs energy from the motor and turns it in to battery charge. Without the alternator the motor would be able to work less hard (consume less gas) to produce the same level of forward motion, saving gasoline.

[/ QUOTE ]

Your logic is wrong too. We're talking about turning the headlights off, which does not result in the alternator absorbing any less engine power, and thus does not save gas.

[soko]

[ QUOTE ]
[ QUOTE ]
To correct your logic here:

[ QUOTE ]
that since the battery is being sent charge by the engine anyway it's essentially 'free' energy because the battery would otherwise be incapable of receiving charge and that energy would be wasted (I assume it'd be lost as heat)

[/ QUOTE ]

The alternator absorbs energy from the motor and turns it in to battery charge. Without the alternator the motor would be able to work less hard (consume less gas) to produce the same level of forward motion, saving gasoline.

[/ QUOTE ]

Your logic is wrong too. We're talking about turning the headlights off, which does not result in the alternator absorbing any less engine power, and thus does not save gas.

[/ QUOTE ]

Two things,

My logic is correct where you quoted me because I never mentioned headlights.

Second, the alternator does work harder when the lights are turned on and puts a larger load on the engine, consuming more gasoline. If the alternator didn't know how hard it had to work it would overcharge the battery and it (the battery) would explode.

[guesswest]

Thanks for the replies guys. Again, is quite likely I'm wrong but just to clarify the impression I had initially when this came up:

The alternator is (I think) spinning whenever the engine is running. It has a regulator so it can spin faster when it has a particularly high draw on it, like when you turn on the AC, and obviously that kind of appliance would eat into fuel because the alt would have to speed up, burning fuel. But I thought even spinning at it's base rate the alt would generate sufficient power to run the lights. Like I say, that may well be wrong, just mentioning because I don't think I explained myself well initially.

Can you overcharge the battery? And if you can is stopping the alt the only way of avoiding overcharge? I was always under the impression that alts spun whenever the engine is running, at some low/base speed, but maybe that's not so? Do alts stop completely?

I understand of course that fuel is required to generate the electricity, it was just a question of whether that energy was already being generated by the base movement of the alt, and consequently whether an additional fuel draw was required to run the headlights.

[soko]

Well by working harder I mean it has to work longer to get the battery up to it's standard voltage. The alternator switches on and off by powering the electromagnet on or off when the voltage regulator determines battery needs more charge. When the electromagnet is off the engine still rotates the internal parts but there are no resisting forces besides friction, when the electromagnet is on, the engine must overcome the resisting magnetic focres as well to create electricity, and this is where the motor must work harder and mroe gasoline is burned.

A simple rule is, the more electricity you are using in the car, the longer the alternator will be in the "on" phase and the more gas you will wind up burning

(also, the AC compressor is attached to the motor belt and is a seperate unit altogether, although the fans may still be run by the battery)

[Hiding]

If you can find a toy/ loose motor spin it by hand then short the power wires (just unconnected wires) you will be shocked how much force it takes to overcome the magentic field. At that point you have loaded the motor to its maximum, any motor is a genrator same pricniple.

I've been drinking. someone explain that better

[Hiding]

746 watts is one horsepower. One 100watt light bulb is ~1/8HP at100% efficiency. A REALLY efficient motor/generator is in the 75% range. So find the average watts of a car headlight (google) and alternator efficiancy, then the HP/gallon of gas in an avergae car and you have the actual savings