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?

 

[RossABQ]

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....

 

[IRstuff]

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

TTFN

FAQ731-376

 

[GregLocock]

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

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

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[drwebb]

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 . . .

 

[KENAT]

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 you).

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...

 

[ivymike]

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.

 

[Timelord]

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

 

[GregLocock]

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

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

 

[ivymike]

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)...

 

[IRstuff]

huh? Why?

TTFN

FAQ731-376

 

[ivymike]

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

 

[IRstuff]

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

FAQ731-376

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[dcasto]

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

 

[TheBlacksmith]

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.