Headlights affecting mpg- how does it work? 2

[drwebb]

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?

 

[SomptingGuy]

It must be the latter. Unless batteries become tailored to specific vehicles (how complex & expensive would that get?). In my youth it was common practice for the 50cc FS1E boys to turn off their lights at night to go just a bit faster.

- Steve

 

[KENAT]

I'd assume latter too, just plain and simple less drain on the electricals.

KENAT, probably the least qualified checker you'll ever meet...

 

[IRstuff]

Hmm, can't get the numbers to work. Two 50 W headlights for 25 minutes = .15MJ and assuming 30% efficiency gets it up to 0.5MJ, but a gallon of gas contains about 145 MJ, and assuming 25 mpg, that gives 5.8 MJ/mi.

That's less than 0.1 mpg difference, not 0.25 mpg, and that's just turning them off altogether.

TTFN

FAQ731-376

 

[MechanicalChef]

"Valeo estimates" That's where I stopped taking the excerpt seriously.

 

[KENAT]

Given that my company in indirectly involved in production of such LEDs...

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...

 

[MiketheEngineer]

Are the headlights smaller and therefore less drag??

 

[SomptingGuy]

Forget estimates (verb or noun), impartial test results are required.

- Steve

 

[Bribyk]

Hot air off the lens covers affecting the boundary layer downstream over the rest of the car?

Come one, Kenat. Give us something here!

 

[MacGyverS2000]

Lower electrical requirements are a surprising savings for power. For example, one of my LED-based products (Animated Brakelights) uses less power when fully powered/lit than the stock incandescents do when just the running lights are on (<5W each). When the brake pedal isn't pressed, they only pull about 2W (compared to 10W for stock)... when the pedal is pressed, they pull less than 10W (compared to 50W+ stock).

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

 

[patprimmer]

The alternator still has to replace draw from cabin fans, engine cooling fans, A/C clutch, wipers, starter motor drain, ignition and fuel systems, electric seats, windows, central locking, sound system and various computers. Maybe 300 to 600 watts.

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[KENAT]

What type of vehicles are we talking about?

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...

 

[GregLocock]

Roughly speaking the effective average speed of a car is 42 mph, and needs 8 kW tractive power. So a reduction of 100W would be ~1%, so at 25 mpg that is, surprisingly, 0.25 mpg

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cibachrome]

There's no credit for reduced electrical lighting efficiency. The Federal Emissions test on which MPG is based allows lights to be off during testing. Since lights are not an issue during coast down tests, (the engine load is not a factor), and the coast down sets the load for the driving cycle, there's nothing to be gained. The lights are off. Hybrids present a new challenge. Hybrid-II may reveal some new constraints.

 

[IRstuff]

The alternator is dictated by the charging rate required to ensure that the battery can start the car after a heavy start with a minimum of charging time:

http://www.carpartswholesale.com/cpw/dodge~alternator.html

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

FAQ731-376

 

[patprimmer]

I would expect the alternator to recharge power drawn by starter in a quick time when running with everything on. That requires an excess capacity over full running load. The lights effect full running load, so a decreased output alternator could be used with more efficient lighting.

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[IceStationZebra]

Running with IRStuff's line of thought I came up with 0.14-1.6 mpg improvement with lights off vs. lights on, depending on efficiencies and original mpg assumed. The "average" scenarios where 0.17-0.33mpg. Of course you need to take some of this back to run the LED's.

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

 

[vims]

Personally I have a feeling 30% efficiency for a halogen bulbis to high. Wikipedia states 10-30%, but perhaps Irstuff actually has some knowledege in the field (compared to my skill att googling).

 

[patprimmer]

I think he is presuming 30% efficiency in converting the fuel for the engine into electrical energy for the lights.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[vims]

Ooops. Ok obviously that´s the case.