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Old 28-12-04
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Thumbs Up Electricity consumes petrol?

Is it or is it not true, that electrical devices in a car (when on) make the car consume more petrol?

my dad and my girlfriends dad say they do... but I'm having hard time figure out why would the car consume more petrol per 100km when the mainbeams (for example) are on!?!

Please PLEASE enlighten me
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Old 28-12-04
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Re: Electricity consumes petrol?

Because something has to make the electricity to run those items.

The Battery is only a chemical storage device for starting and spreading the load .The rest of the needed electricity is made using an Alternator and you get nothing for nothing. ie an altenator requires energy to be expended in turning it over in order for it to make electricity
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Old 28-12-04
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Re: Electricity consumes petrol?

So if I but the mainbeams on the alternator becomes more harder to turn for the engine so it consumes more petrol?
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Old 28-12-04
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Re: Electricity consumes petrol?

Doesn't the alternator spin the same ammount regardles of how much electricity the cars devices consume?
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Old 28-12-04
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Re: Electricity consumes petrol?

But the back EMF produced when Electricity is actually being produced causes the altenator to be harder to turn over ..or something like that
Bugger but it is 30 years since I looked at the theory of Alternators

Last edited by Stori; 28-12-04 at 09:53.
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Old 28-12-04
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Re: Electricity consumes petrol?

Easiest way to think about it (for me anyway!) is the extra load placed on the engine by aircon. Switch it on and the engine will rumble and possibly increase revs...to compensate for the extra load being placed on the system.

You will also apparantly see a 2-3mpg loss using the aircon for this reason.

The answer is yes, electricity uses more fuel....

wrinx
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Old 28-12-04
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Re: Electricity consumes petrol?

But someone needs to explain me why and how!

how does the alternator work?
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Old 28-12-04
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Re: Electricity consumes petrol?

http://www.vtr.org/maintain/alternator-overview.html
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Old 28-12-04
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Re: Electricity consumes petrol?

http://www.straightdope.com/mailbag/mradiomileage.html
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Old 28-12-04
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Re: Electricity consumes petrol?

....or forgeting how an alternator works and looking at the car as a whole... the law of conservation of energy...

"Energy can be converted from one form to another, but it cannot be created or destroyed."

So the energy to run all electrical devices in a car has to come from somewhere and that source of energy is petrol (gasoline). which in turn has to generate electricity using the engine then the alternator.
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Old 28-12-04
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Re: Electricity consumes petrol?

Thank you stori... that answered my question
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Old 28-12-04
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Re: Electricity consumes petrol?

Wow, I didn't know that...
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Old 28-12-04
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Re: Electricity consumes petrol?

Neither did I... Live and learn
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Old 28-12-04
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Re: Electricity consumes petrol?

I wonder how much money the headlights cost when turned on...
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Old 28-12-04
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Re: Electricity consumes petrol?

I already knew this ... but it's always interesting to get more detail
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Old 28-12-04
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Re: Electricity consumes petrol?

Of course you should know, with that eye of yours.
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Old 28-12-04
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Re: Electricity consumes petrol?

Originally Posted by Antonio Chico
I wonder how much money the headlights cost when turned on...
6% more, according to this site http://mb-soft.com/public/headlite.html
There is even a formula to calculate it, if you want.
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Old 28-12-04
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Re: Electricity consumes petrol?

Wow, that's a lot after a while...
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Old 28-12-04
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Re: Electricity consumes petrol?

Originally Posted by Antonio Chico
Wow, that's a lot after a while...
Even more with "Climate control" running........but I know what I won't be turning off on a 40 C day

Still the 2.0 TS 147 gives 8.8 km/liter(32 mpg) with A/C on and my SEAT ibiza GTi 2.0 gave 7.4 km/liter (38 mpg) with the A/C on. (Same Road)
So the extra 20% weight of the 147 over the SEAT is an even bigger factor than the Lights.
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Old 29-12-04
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Re: Electricity consumes petrol?

New question: If you drive a bike with a dinamo on... how many donuts extra you burn in every mile?
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Old 29-12-04
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Re: Electricity consumes petrol?

Originally Posted by Enrique Lacoss
New question: If you drive a bike with a dinamo on... how many donuts extra you burn in every mile?
about 8%
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Old 29-12-04
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Re: Electricity consumes petrol?

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.

Ralf S.


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Old 29-12-04
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Re: Electricity consumes petrol?

also, don't forget that in all the processes involved in generating the electricity, there will be some energy loss due to electrical resistance, heat transfer, belt and bearing friction etc
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Old 29-12-04
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Re: Electricity consumes petrol?

"Energy can be converted from one form to another, but it cannot be created or destroyed."
Pasi,

In my case it's the energy of flipping my headlight switch with my finger that is converted to electricity for the headlights to work

Mind you, it's 12 volt

By the way, I'm glad someone was keen enough to invent that actuator thingy
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Old 29-12-04
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Re: Electricity consumes petrol?

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.

Ralf S.
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.
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