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Headlights affecting mpg- how does it work?
2

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?

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

RE: Headlights affecting mpg- how does it work?

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

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

RE: Headlights affecting mpg- how does it work?

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

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RE: Headlights affecting mpg- how does it work?

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

RE: Headlights affecting mpg- how does it work?

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...

RE: Headlights affecting mpg- how does it work?

Are the headlights smaller and therefore less drag??

RE: Headlights affecting mpg- how does it work?

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

- Steve

RE: Headlights affecting mpg- how does it work?

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

Come one, Kenat.  Give us something here!

RE: Headlights affecting mpg- how does it work?

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

RE: Headlights affecting mpg- how does it work?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Headlights affecting mpg- how does it work?

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...

RE: Headlights affecting mpg- how does it work?

2
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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Headlights affecting mpg- how does it work?

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.

RE: Headlights affecting mpg- how does it work?

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

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RE: Headlights affecting mpg- how does it work?

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
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RE: Headlights affecting mpg- how does it work?

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

RE: Headlights affecting mpg- how does it work?

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).

RE: Headlights affecting mpg- how does it work?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Headlights affecting mpg- how does it work?

Ooops. Ok obviously that´s the case.

RE: Headlights affecting mpg- how does it work?

I'll echo Pat's sentiments about their value being longevity.  I am shocked every day how many people drive around with only one of their 3 or 4 brake lights functioning.  With less and less routine maintenance and far fewer trips to the shop for maintenance, modern drivers don't ever seem to check their lights.  All brake lights as a minimum should be mandated to be LED.  

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?

Thanks Pat, that was the case, and it was simply a gross swag to get some numbers into the discussion.

TTFN

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RE: Headlights affecting mpg- how does it work?

So... the original claim seems to be in the right ballpark?

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Headlights affecting mpg- how does it work?

I will take a swag at about 1 MPG, presuming all lights wipers, demister etc are on, but not recharging starter draw etc.

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  

Regards

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

RE: Headlights affecting mpg- how does it work?

(OP)
OK I was hiding behind Valeo in my original question to mask my ignorance of automotive electrical systems.  So I gather that alternators _do_ increase their mechanical requirements in proportion to load demand- just like A/C comressors.  I once laughed at someone who said she turned off the radio when she was afraid of running out of gas, but I guess she was on to something . . .

 

RE: Headlights affecting mpg- how does it work?

drwbb, simplistically, for an IC (including regular hybrids)powered car, all of the energy I believe comes from the gasoline/petrol/diesel/LPG (barring when you install a new battery or go down hill with the wind behind yousmile).

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?

if that weren't the case, alternators would be mighty hot most of the time (much hotter than the underhood air), since they'd have a mechanical power input equal to (100/40)x peak demand, so maybe 3.7kW, and an electrical output much lower than that.  

 

RE: Headlights affecting mpg- how does it work?

From Wikipedia:    "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."

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?

Found the map, it is in the Bosch book.

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Headlights affecting mpg- how does it work?

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.

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?

'cause it was suggested that the mechanical input doesn't vary, regardless of e load (a few posts up)
 

RE: Headlights affecting mpg- how does it work?

I think that there's some misunderstanding.  

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?

IRstuff

I think Ivymike and you are both saying the same thing, you are just coming at it from opposite directions.

Regards

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

RE: Headlights affecting mpg- how does it work?

wow, should I dump my 1000 watt stereo?  NAW, I love it.

RE: Headlights affecting mpg- how does it work?

So I guess excessive use of the electrically heated seats in our new Miata is causing the low gas mileage and not our exhilarating driving style.

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