Headlights on means less mpg?

SydneyRoverP6B

SydneyRoverP6B

Well-Known Member
Staff member
You never get something for nothing as they always say, so I was thinking about headlights recently and whether or not using them would have an effect on fuel consumption. I was thinking along the lines that it may well do, afterall the alternator is working to return the current being used to the battery, hence an additional load is being placed upon it, and as such this will follow through to the engine having to cover the additional load and so on....

Did a search and found this article. See what you think.

http://mb-soft.com/public/headlite.html

Ron.
 
been looking at instaling LED light into mine for brake indicator park and taillights (plus reversing) however havent found a suitable H4 main/dip replacement LED yet.

Graeme
 
Makes sense. When I used to sprint and hillclimb an Escort the other guys used to unplug their alternator for the run and put it back in at the finish trying to eek out the last ounce of puff :)
 
I can never understand drivers in saloon car racing turning on their lights when chasing for the lead. As said earlier you never get something for nothing, and any possible distraction to the leading driver, (unlikely with any driver worth his salt) would be far outweighed by the loss of performance. A good way to prove the point is to turn on all the ancillaries on an idling engine and watch the tickover fall. The same thing applies, albeit to a lesser extent when you switch on the lights, with both performance and MPG.
 
The government we suffer in Spain will put in force the DTRL for the year 2012, if I remember well.

I understand that the Day Time Running Light could be fine for North Europe countries, where the day is very short, but not here. 20 Dec sunset is around 17:30 h.

You cannot get something for nothing. Of course the decrease in mileage when you use the DTRL is just some yards per gallon, but...

If you multiply these additional drops of fuel by the 25.000 Km annual average and by the millions of cars on the road...


The money the goverment can take because of fuel taxes is HUGE!!!

So, with the excuse of safety increasing, eventually the DTRL is just another way of taking the money out of our pockets.
 
Probably running aircon on a modern car uses more fuel than the lights ,but I'd rather pay the difference than sweat
 
Hi
I'm missing something here , please help me .
The alternator is a lump of rotating metal . Surely it will generate power whenever it rotates .
The mechanical power used to drive the alternator is a function of engine speed ?
The electrical power generated is a function of engine speed?
The power used by accesories is provided by the battery and is not routed directly to the user from the alternator
Power is either fed to the battery or dissipated to the atmosphere ???
There is no direct correlation between electric power generated and electric power used to run the electrical system ??
The mechanical power absorbed to drive the alternator is a function of the alternator's speed of rotation and not electrical power used ?
So it running with headlights on has no effect on fuel consumption ????

Air conditioning is a different case you either drive a stonking great compressor or let it idle on the compressor drive clutch

RVW
 
How come when you put more load on the battery the alternator ups its charge rate, as seen on the ammeter, and as it does so it takes more effort to drive it, so the engine uses more fuel. If this isn't the case, and you're right, congratulations you've just worked out how to get something for nothing. I'll have half the royalties please! We may be talking small amounts but nevertheless it goes back to not being able to get something for nothing.
 
Absolutely right of course...but I'd sooner use an eggcup more fuel if it helps another driver (or some half-asleep twonk with still-iced/misted windscreen) see me coming towards him in the semi-twilight or semi-dawn.

I still find it mind-numbingly stunning that some people drive around thinking they only need their lights on if they need them to see the road, rather than to be seen on the road. Amazing...and alarming.
 
Hi
I dont claim that " you get something for nothing "! I'm saying that the power used to drive the alternator has nothing to do with the electrical load on the battery .So fuel consumption is not affected by using the headlights
My theory is the alternator absorbs power from the engine as a function of it's speed of rotation ie engine revs . The power absorbed from the engine is not dependent on the electrical load on the alternator, but on the rotational speed the alternator ?
I also think the alternator's total electrical power production depends on the alternator's speed , and not on the power consumed by the car's electrical systems ?
The alternator feeds the battry in an attempt to balance the condition of the battery and the power drain from the battery ?This may result in excess power being provided by the alternator ??
If the above is correct any excess power generated must be dissipated some where??> Where does it go ?
If I were to race my car and wanted to deliver maximum power to the wheels the logical thing would be to fit a fully charged stonking great battery and remove the alternator drive all together, or even the alternator , to save weight ??
RVW
 
ripvanwinkle said:
Hi
I dont claim that " you get something for nothing "! I'm saying that the power used to drive the alternator has nothing to do with the electrical load on the battery .So fuel consumption is not affected by using the headlights
My theory is the alternator absorbs power from the engine as a function of it's speed of rotation ie engine revs . The power absorbed from the engine is not dependent on the electrical load on the alternator, but on the rotational speed the alternator ?
I also think the alternator's total electrical power production depends on the alternator's speed , and not on the power consumed by the car's electrical systems ?
The alternator feeds the battry in an attempt to balance the condition of the battery and the power drain from the battery ?This may result in excess power being provided by the alternator ??
If the above is correct any excess power generated must be dissipated some where??> Where does it go ?
If I were to race my car and wanted to deliver maximum power to the wheels the logical thing would be to fit a fully charged stonking great battery and remove the alternator drive all together, or even the alternator , to save weight ??
RVW
Click to expand...

See my above post agreeing with you 8) and we used to use solar panel (no electric supply) battery chargers inbetween runs to top up the battery :)

Incidentally, when you put your lights, fan, heated rear window on etc you can hear the engine note change with the increased load from the alternator. Is this not the regulator cutting the alternator in as required? :?
Somemodern cars have an auto idle adjust to compensate for this change in speed at tickover.
 
Physics doesn't lie: Energy cannot be created.

Therefore if you use headlights, air conditioning, radio or anything else you need energy. The more devices you use the more energy you need.

And in a car there is only one source of energy: the fuel.


Regs
 
There is very little resistance on an alternator which is not being used to charge the battery.

It is only when charge is required that the alternator's resistance is increased and therefore uses power from the engine and uses fuel.

Richard
 
I was told years ago that when you set idle speed on a motor, you should do so with the cars headlights on highbeam, as the alternator will load down the motor with the extra current load from the headlights.

I tested this out on a early 80's Ford 4 cylinder car in Australia, and there was a noticable RPM drop going from off, to low beam to high beam. So I would expect that since the alternator loads down the motor, with high beam, then it would result in a decrease in Mile per gallon.

James.
 
ripvanwinkle wrote,..
Hi
I dont claim that " you get something for nothing "! I'm saying that the power used to drive the alternator has nothing to do with the electrical load on the battery .So fuel consumption is not affected by using the headlights
My theory is the alternator absorbs power from the engine as a function of it's speed of rotation ie engine revs . The power absorbed from the engine is not dependent on the electrical load on the alternator, but on the rotational speed the alternator ?
I also think the alternator's total electrical power production depends on the alternator's speed , and not on the power consumed by the car's electrical systems ?
The alternator feeds the battry in an attempt to balance the condition of the battery and the power drain from the battery ?This may result in excess power being provided by the alternator ??
If the above is correct any excess power generated must be dissipated some where??> Where does it go ?
If I were to race my car and wanted to deliver maximum power to the wheels the logical thing would be to fit a fully charged stonking great battery and remove the alternator drive all together, or even the alternator , to save weight ??
RVW
Click to expand...

Hello RVW,

I provided a paragraph from one of my Physics books...."Physics for Scientists and Engineers" by D C. Giancoli. This is in particular reference to an AC generator which is essentially what an alternator is complete with voltage regulated output. "In a generator, the mechanical turning of the armature (in an alternator, the armature is the stator which is fixed, and the magnetic field which is induced rotates within the stationary armature) induces an emf in the loops which is the output. If the generator (alternator) is not connected to an external circuit, the emf exists at the terminals but no current flows. In the case it takes little effort to turn the armature (alternator rotor). But if the generator is connected to a device which draws current, then a current flows in the coils of the armature. Because the current carrying coil is in a magnetic field, there will be a torque exerted on it, and this torque opposes the motion. This is called a counter torque. The greater the load, ie the more current that is drawn, the greater the magnitude of the counter torque. Hence the external applied torque (ie the engine turning the alternator) will have to be greater to keep the generator (alternator) turning. This of course makes sense from the conservation of energy principle. More mechanical-energy input is needed to produce more electrical-energy output."

Ron.
 
Thanks
Now I understand . I was wrong , the alternator 's power production IS a function of thje electrical power load . The alternator power demand from the engine is therefore determined by the electrical load .
Cheers
RVW
 
It's a hard one to get your head around, but sydneyrover has explained very well with aid of Giancoli's book. If you look at wiring diagrams for alternators and see how the power arrives at the alternator and leaves and bear in mind the above post from sydneyrover all should hopefully become clear.
 
Gents

I rediscovered this thread whilst searching for something else to do with the current thread on batteries elsewhere on the forum. I thought it might be interesting to resurrect it, particularly as we move into the dark/cold/wet autumn/winter days, to see if people were experiencing this effect of increased mpg with increased usage of wipers/demisters/lights etc.

It might be a bit early to tell but for those of us driving our cars through the winter (or is it just me who's mad :twisted: ) it would be interesting to see physical evidence of this. I keep a log of all my fuel and mileage for business use so I'll have a good picture of my own fuel consumption.

......and before people start mentioning the other variables that can affect fuel consumption.....the proof of the pudding will be if our fuel consumption improves again as we move into spring/summer next year.

Dave
 
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