Headlights affecting mpg- how does it work?
Headlights affecting mpg- how does it work?
(OP)
The April Automotive Engineering Intl. has an article on new LED headlights that cites "Valeo estimates that replacing halogen lights with LEDs is worth about 0.25 mpg, and replacing daytime running lights as well boosts the savings to 0.50 mpg," and seems to imply this is a direct result of LEDs higher energy efficiency (lm/W). Does this assume a smaller, lighter alternator with lower mechanical power requirements would also be installed, or can a mpg credit be realized from a lower drain on a comparable electrical system requiring lower mechanical input from the engine?





RE: Headlights affecting mpg- how does it work?
- Steve
RE: Headlights affecting mpg- how does it work?
KENAT, probably the least qualified checker you'll ever meet...
RE: Headlights affecting mpg- how does it work?
That's less than 0.1 mpg difference, not 0.25 mpg, and that's just turning them off altogether.
TTFN
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RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
It's definitely true, you should press your local congressman/senator/MP... to put forward legislation to mandate their use...
KENAT, probably the least qualified checker you'll ever meet...
RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
- Steve
RE: Headlights affecting mpg- how does it work?
Come one, Kenat. Give us something here!
RE: Headlights affecting mpg- how does it work?
And that's just the brakes. If you were to replace all four corner sidemarkers, etc., at 5W each stock, that's another 15-20W of savings. Now consider an electrical system that requires 50W+ less... a smaller alternator is an easy next step.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Headlights affecting mpg- how does it work?
Total Std lighting is about 250 to 350 watts. Say that is cut to 100 to 150 watts, saving say 175 watts average if the lights are on high beam 100% of the time.
You might reduce an alternator from 80 to 65 watts.
You might save a little in battery size and gauge of wire to the lights.
We measure motor output at kilowatts, even at cruise throttle settings.
The possible reduction in electrical systems will be small.
Modern halogen headlights are quite aerodynamic in shape and conform to existing body lines.
I find it highly unlikely that the temperature of the headlight surface has a measurable effect on aero.
Without doing the sums, I see a measurable but not significant saving, so tests might support the claim under extreme conditions, like low speed top gear cruise with all lights, but nothing else on.
Regards
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RE: Headlights affecting mpg- how does it work?
I suppose semi-trucks here in the US have a lot more lights but I still have trouble believing they add up to that much.
KENAT, probably the least qualified checker you'll ever meet...
RE: Headlights affecting mpg- how does it work?
Hmm. That's too neat.
Car batteries /are/ designed specifically for each model, or more accurately the OEM battery is tweaked. They are primarily sized for cold weather starting I believe (I could find out but then I'd have to shoot you).
Alternator size seems to be driven entirely by worst case load at idle, which would include the lights I guess.
Cheers
Greg Locock
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RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
The listed alternators run in the neighborhood of 90A capacity. That means that reducing the headlight load is irrelevant to the sizing of the alternator, since it's sized for something else.
TTFN
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RE: Headlights affecting mpg- how does it work?
I expect the fuel savings from downsizing alternator and/or battery and wire gauge would be insignificant so it is a mute point in this arguement.
However, electrical draw on the alternator will be effected by the difference in the light efficiency and will give a small improvement only when running with lights on.
Of course durability of the LEDs is another factor and seems a very good reason to use them. I am somewhat biased in this view at the moment having just had to remove quite a few under bonnet components and losing some skin and blood while replacing a parking light bulb on my 1989 model Honda Integra. It had just failed a roadworthy certificate test because of the blown bulb.
Regards
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RE: Headlights affecting mpg- how does it work?
Two 55 W headlights for 60 minutes = 0.396 MJ/hr electrical energy
0.396 MJ/hr electrical energy assuming 70% alternator eff = 0.57 MJ/hr (45-85% typical)
0.57 MJ/hr alternator input assuming 97% drive eff = 0.58 MJ/hr @ crank pulley
0.58 MJ/hr @ crank drive pulley assuming 35% engine eff = 1.67 MJ/hr of fuel
1.67 MJ/hr of fuel @ 145 MJ/gal = 0.011 gal/hr
30mpg becomes 30miles/0.989 gallons = 30.33mpg
20mpg becomes 20.22mpg
15mpg becomes 15.17mpg
Best case: 45 wall low beam = 0.98 MJ/hr (used 85% eff alternator, 40% eff engine)
0.98 MJ/hr of fuel @ 145 MJ/gal = 0.0068 gal/hr
40mpg becomes 40.27mpg (Prius)
30mpg becomes 30.20mpg
20mpg becomes 20.14mpg
Worst case: 135 watt high beam = 7.41 MJ/hr (used 45% eff alternator, 30% eff engine)
7.41 MJ/hr of fuel @ 145 MJ/gal = 0.051 gal/hr
30mpg becomes 31.61mpg
20mpg becomes 21.07mpg
15mpg becomes 15.81mpg
ISZ
RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
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RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
As I say this, I shudder to think of what a replacement brake light assembly will cost if they do mandate them....
RE: Headlights affecting mpg- how does it work?
TTFN
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RE: Headlights affecting mpg- how does it work?
I think even that surprises most of us.
So, here's a freeby. Typical automotive alternators are only 40% efficient, at typical operating conditions. There is no real reason why that couldn't be >90%, if you want to pay for it. What's that worth in mpg?
Cheers
Greg Locock
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RE: Headlights affecting mpg- how does it work?
Of course it will be more if the battery is depleted for any reason and will be a lot less on a warm dry sunny day with no lights, wipers, demisters etc running.
Typical average I would guess is about 0.3 mpg
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RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
Hence the electrical energy comes from that source too, via the engine/alternator/battery.
So reducing vehicle electrical consumption should reduce fuel consumption.
All those of you smarter than I, is that not pretty much correct?
KENAT, probably the least qualified checker you'll ever meet...
RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
A fully loaded 100 amp alternator puts around 1360W (assuming 13.6V), so at worst case 50% efficiency it consumes 2720W of mechanical power.
The 50% efficiency implies that the alternator must get hot (hotter than the under hood temp) so it can shed the 1360W loss. That is why they have a fan built into the sheave.
I cannot vouch for the accuracy of Wikipedia, but their numbers sound reasonable.
Timelord
RE: Headlights affecting mpg- how does it work?
Typically a 90A alternator is most efficient at half its rated current output, maxes out at ~60% at 4000 alternator rpm, droppping to 42% at 15000 rpm, and then at max output its efficiency drops to about 37%.
So I misremebered it, 60% is a better approximation than 40.
Cheers
Greg Locock
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RE: Headlights affecting mpg- how does it work?
yeah, now imagine if the amount of heat it shed just went up and up as load went down (and the mechanical input stayed constant, so at zero draw it still required 2720W input but had 0 electrical output)...
RE: Headlights affecting mpg- how does it work?
TTFN
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RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?
An alternator that's not supplying any power presents minimal load to the engine and dissipates minimal heat. Only when there's and electrical load will it load down the engine and dissipate heat. The minimal load at zero output power is the bearing friction and wind drag of the attached fan, if any.
TTFN
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RE: Headlights affecting mpg- how does it work?
I think Ivymike and you are both saying the same thing, you are just coming at it from opposite directions.
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RE: Headlights affecting mpg- how does it work?
RE: Headlights affecting mpg- how does it work?