Fuel economy and headlights

Any electrical load has to be provided by burning fuel, but the extra amount for running your lights is trivial, probably so little as to be not easy to accurately measure, you would get as much variation as you do day to day from atmospheric pressure variations, and less than getting stuck in a traffic jam for a little while or having a soft tyre.

Thus spake Angus Manwaring (angus@angusm_antispem_.demon.co.uk) unto the assembled multitudes:

Hmmm, that may explain the number of idiots who drive in twilight with no lights on at all, or in rain/fog with just their sidelights on...

It doesn't however explain the number of idiots who drive with their fecking rear fog lights on when it's *not fecking foggy*. Grrr!

Headlights usually take 110 watts which is approx 0.15HP. Alternators are pretty efficient - say about 90% - so lets be generous and say 0.2 HP.

Not sure how many HP the average car uses when cruising at 70 mph - say 50?

You are forgetting that when the headlights are on, so are umpteen other lamps of 5 watts each or so. You would need to add at least another 50 watts for those.

Still absolutely trivial in the greater scheme of things of course.

Chris

Ah, probably true in general terms but automotive ie car (and truck) style alternators are _very_ inefficient.

Julian.

Although most of the losses are the fan, so additional loading from the headlamps is going to be closer to 90% effecient.

Straying completely off topic - but related.

It is well known that fuel economy at slower speeds (eg 70 vs 80) improves due mostly to wind resistance.

But, if running the A/C, then it consumes an energy of a CONSTANT power x time, ie proportional to time used. Using the A/C degrades fuel economy. This implies "get there quicker / drive faster" to save fuel.

Any thoughts on quantitising the two opposing effects ???

Jim

The amount that a/c uses is trivial compared to moving at a significantly higher speed.

Doesn't sound right to me, The first result from Google:

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges; at part load, efficiency is between 50-62% depending on the size of alternator, and varies with alternator speed.

Julian.

OK so double the power used and add a bit, that takes it to half a horsepower, still not as significant as cheapo tyres, a roof rack, a window open or any of the other things that reduce fuel efficiency. I would hazard a guess that a car at about 60 is using about 30 horsepower, so an extra half a horse is a little over 1 percent additional fuel. the difference between 50 mpg and 50.5 .

Mrcheerful

I'd say the main inefficiency is in the drive - and you can't do much about that.

On 06-Feb-09 09:20:13, A.Clews said

Yeah, I really hate that - you're in a line of traffic, moving along at less than 30 mph, able to see at least 3 cars ahead, and the guy just in front of you has his fog lights on. Grrr indeed.

All the best, Angus Manwaring. (for e-mail remove ANTISPEM)

I need your memories for the Amiga Games Database: A collection of Amiga Game reviews by Amiga players

On 06-Feb-09 09:58:48, News said

So broad brush, with lights on in average use, are we saying you would use less than 1% more fuel?

All the best, Angus Manwaring. (for e-mail remove ANTISPEM)

I need your memories for the Amiga Games Database: A collection of Amiga Game reviews by Amiga players

On 06-Feb-09 13:33:05, Mrcheerful said

Okay.... but as I understand it, the amount of extra fuel consumed by having the a/c on is not trivial = is that right? Rough Percentage? :)

All the best, Angus Manwaring. (for e-mail remove ANTISPEM)

I need your memories for the Amiga Games Database: A collection of Amiga Game reviews by Amiga players

A double wrong there :-)

Manufacturers have changed from V to poly V belt which permits cooler running and higher drive efficiency, IIRC the poly V offers high drive transmission efficiency. See my other post for the reasons for poor efficiency from the automotive alternator.

Julian.

OK, 1/2 a horsepower it is, that's about 400 watts, next to bsugger all.

But would you leave electrical loading on around your house to the tune of

Julian.

it seems to make the most difference on small engine petrol cars, I have seen 10 per cent quoted, certainly the compressor coming in on an escort

from personal experience it makes no noticeable change to the mpg on a diesel passat, lexus gs300 and ls400.

the average car journey does not last long compared with household lighting duration.

Tyre quality and maintenance, coupled with good driving habits can give a massive improvement in MPG, leaving the lights on will make scarcely any difference.

Which is independent of load

ditto unless they're knackered

of which the diodes are probably the biggest loss at _1.5v/15V so about 10%