Could LED lights improve MPG?

[desura]

An appreciable amount?

 

[joutlaw]

Highly doubtful if this is strictly based off less power consumption and therefore less requirements of the alternator.

 

[lxskllr]

Probably about the same as taking out the floor mats to shave weight.

 

[freeskier93]

An appreciable amount? No.

 

[jdoggg12]

The best modification to engine efficiency that you can attain:

 

[JCH13]

It is measurable, at about 1.6g of CO2 per mile.

That's worth about 0.0002 gallons per mile of saved fuel consumption. Your wallet won't notice.

 

[RichUK]

Lol.

 

[Squisher]

A penny saved is a penny earned.

 

[Yuriman]

Let's assume you have 55w headlight bulbs, so 110w in total. Assume your alternator is about 60% efficient - most alternators have terrible efficiency (hybrids can generate 12v much more efficiently). Let's also assume you find LED bulbs that are 35w each, 70w in total.

1HP = ~750w

110w - 70w = 40w saved

40w/750w = ~0.05HP

0.05 / 0.6 (efficiency loss of alternator) = ~0.09HP

So, changing the bulbs may net a tenth of a horsepower less used while cruising. Considering most passenger cars take between 15 and 25HP to cruise on the highway, this is roughly a 0.5% savings. If you're getting 30mpg on the highway, you're looking at getting about a 0.15mpg improvement.

In and of itself, it amounts to almost nothing. However, I look for 1600 ways to save an ounce, rather than one way to save 100lbs, which is how I can get over 100mpg cruising in my car.

 

[RearAdmiral]

I feel like I remember on Top Gear them saying if you shave your moustache you will save $5 in fuel a year!

 

[JCH13]

At $2/gallon, driving 15,000 miles per year with the headlights on, you'll save $6/year!

Doing the power-based calculation using Audi's initial numbers (Yuriman, I don't like how you converted to fuel savings, though your math to that point was reasonable):

55W power savings
Average mixed driving speed - 43mph, from my own lifetime vehicle calculation
Average highway speed - 65mph, est. from TomTom's data: http://www.autoblog.com/2010/01/26/tomtom-data-reveals-u-s-drivers-average-speed-fastest-highway/
Alternator efficiency - 70-80% (Bosh automotive handbook)
Engine Thermal Efficiency - 0.3
Energy in a gallon of gas - 132MJ

I'm looking to compute fuel consumed per mile, so it will depend on the speed the vehicle is traveling.

55J/s/.75/.3 = 244J/s of fuel consumed

(3600s/h)/43mph = 84s/mile, OR, (3600s/h)/65mph = 55s/mile

244J/s * 84 s/mile = 20.5kJ/mile, OR, 244J/s*55s/mile = 13.4kJ/mile

Converting to gallons/mile

20.5kJ/mile / 132MJ/gallon = .00016 gallons/mile, OR, 13.4kJ/mile / 132MJ/gallon = .00010 gallons per mile

Alternatively, using the Bosh Automotive Handbook approximation:

100W of electrical load is worth .17L of fuel over 100km

55/100*.17L/100km -> .0004 gallons/mile

If you're getting 30mpg, or .0333 gallons/mile, you'll see an improvement of:

.0333 - .0001 (or .0004) = .0332 to .0329 gal/mile -> 30.12mpg to 30.4mpg

Hats off to Yuriman, by skill or luck his result looks good!

But... what does this mean to your wallet? Driving 15,000miles per year, with your headlights on:

15,000 miles*0.0001 (to .0004) gallons/mile*2.28 $/gallon = $3.42 - $13.68 per year saved

LED light conversions cost $80+ per pair, so $160 for brights and lows. Payback periods will be 10+ years best-case. Not at all worth it IMO.

 

[Yuriman]

In my opinion 80% is very generous for an alternator, and 30% thermal efficiency will only be true in a very narrow window; I don't really think it should be used as a constant.

On the other hand, added load to an engine from the alternator will probably be partially eaten up by a reduction in pumping losses, so my estimated 0.15mpg gain is probably optimistic in most cases.

 

[JCH13]

70-80% is from the Bosch Automotive Handbook, 8th ed. My 7th ed says 60-70%. *shrug*

Thermal efficiencies are stated anywhere between 21% and 38% depending on the maker/source/type of rating. I picked something on the higher-end of average because the alternator doesn't see drivetrain losses, and some sources were calculating a tank-to-road sort of efficiency, which isn't appropriate for this calculation.

 

[agent00f]

Let's assume you have 55w headlight bulbs, so 110w in total. Assume your alternator is about 60% efficient - most alternators have terrible efficiency (hybrids can generate 12v much more efficiently). Let's also assume you find LED bulbs that are 35w each, 70w in total.

1HP = ~750w

110w - 70w = 40w saved

40w/750w = ~0.05HP

0.05 / 0.6 (efficiency loss of alternator) = ~0.09HP

So, changing the bulbs may net a tenth of a horsepower less used while cruising. Considering most passenger cars take between 15 and 25HP to cruise on the highway, this is roughly a 0.5% savings. If you're getting 30mpg on the highway, you're looking at getting about a 0.15mpg improvement.

In and of itself, it amounts to almost nothing. However, I look for 1600 ways to save an ounce, rather than one way to save 100lbs, which is how I can get over 100mpg cruising in my car.

At $2/gallon, driving 15,000 miles per year with the headlights on, you'll save $6/year!

Doing the power-based calculation using Audi's initial numbers (Yuriman, I don't like how you converted to fuel savings, though your math to that point was reasonable):

55W power savings
Average mixed driving speed - 43mph, from my own lifetime vehicle calculation
Average highway speed - 65mph, est. from TomTom's data: http://www.autoblog.com/2010/01/26/tomtom-data-reveals-u-s-drivers-average-speed-fastest-highway/
Alternator efficiency - 70-80% (Bosh automotive handbook)
Engine Thermal Efficiency - 0.3
Energy in a gallon of gas - 132MJ

I'm looking to compute fuel consumed per mile, so it will depend on the speed the vehicle is traveling.

55J/s/.75/.3 = 244J/s of fuel consumed

(3600s/h)/43mph = 84s/mile, OR, (3600s/h)/65mph = 55s/mile

244J/s * 84 s/mile = 20.5kJ/mile, OR, 244J/s*55s/mile = 13.4kJ/mile

Converting to gallons/mile

20.5kJ/mile / 132MJ/gallon = .00016 gallons/mile, OR, 13.4kJ/mile / 132MJ/gallon = .00010 gallons per mile

Alternatively, using the Bosh Automotive Handbook approximation:

100W of electrical load is worth .17L of fuel over 100km

55/100*.17L/100km -> .0004 gallons/mile

If you're getting 30mpg, or .0333 gallons/mile, you'll see an improvement of:

.0333 - .0001 (or .0004) = .0332 to .0329 gal/mile -> 30.12mpg to 30.4mpg

Hats off to Yuriman, by skill or luck his result looks good!

But... what does this mean to your wallet? Driving 15,000miles per year, with your headlights on:

15,000 miles*0.0001 (to .0004) gallons/mile*2.28 $/gallon = $3.42 - $13.68 per year saved

LED light conversions cost $80+ per pair, so $160 for brights and lows. Payback periods will be 10+ years best-case. Not at all worth it IMO.

55w saving is likely in comparison to lower efficiency incan/halogen, not similar hid/arc lighting.

 

[razel]

The best modification to engine efficiency that you can attain:

LED bulbs > DEI > Super Charger > Turbo

 

[JCH13]

Using these numbers the Audi power consumption correlates to...

LED
80W*50Lumen/Watt = 4,000 lumens

HID
135W*30Lumen/Watt = 4,050 lumens

The stated power consumption from Audi correlates to LED and HID systems of equivalent light output.

agent00f, care to rebut? Or just extract your foot from your mouth?

 

[agent00f]

Using these numbers the Audi power consumption correlates to...

LED
80W*50Lumen/Watt = 4,000 lumens

HID
135W*30Lumen/Watt = 4,050 lumens

The stated power consumption from Audi correlates to LED and HID systems of equivalent light output.

agent00f, care to rebut? Or just extract your foot from your mouth?

30 lumen/watt from a 120 source is absolutely worse garbage scenario that's maybe indicative of shit aftermarket systems or your arguments. As another example of the latter, typical max HID bulbs are ~35w nominal.

 

[JCH13]

What's a '120 source' in this context? Do you have any evidence or a source to show that 30L/watt is a bad efficiency?

At ~35W per bulb, two bulbs for lows, tow bulbs for brights... ~35w*4 ≈ 140w. As far as I know 140w ≈ 135w, further supporting the notion that Audi compared HID to LED.

 

[rstrohkirch]

Considering the source, I expect the article to be skewed a bit. However, they've listed efficiency losses on the LED rather accurately. From my experience in DIY LED head lamp and bar lamps, you're looking at 8-10% loss for protective cover, 10-15% loss on your optic/reflector and 10-15% loss with your driver. If you design your heat sink well or use active cooling you can keep losses to heat low unless you're just pouring the current to them. Stated efficiency of 100 lm/w is probably pretty accurate or just a tiny bit low when using max rated output.

However, I know nothing about HID or care to. I also don't know what kind of LED or configurations are being used in vehicle lights. It does seem common practice though to have active cooling on LED head lights which also run after the vehicle is off. The fans on my vehicle run for over a minute. But again, don't really care enough to see what kind of fans are being used to calculate current consumption.

 

[JCH13]

I looked up a few more sources that compare HID and LED.

http://www.ansorg.com/en/expertise/...ciency/comparison-hid-and-led-technology.html States 60L/W for HID and 94L/W for LED. About the same ratio as my original source, but perhaps more efficient overall because it's indoor lighting.

http://www.stouchlighting.com/blog/lighting-comparison-led-versus-hid notes that HIDs have a great principal efficiency, over 100L/W, but due to system inefficiencies the effective output is <30L/W, the number I used. It also notes LEDs range from 37 to 120L/W, and I used 50, definitely on the lower end of things.

https://www.energy.gov/eere/ssl/led-basics shows up to 104 L/W for HID, but this likely doesn't account for application losses. Some LEDs are lower, some are higher, but they generally agree with the other sources I found.

LED as a technology is only a little more efficient at making light than HID, but LED wins out in car-like applications because their light is naturally directed, whereas HID lights lose a significant portion of their output to all of the reflecting they need.

 

[agent00f]

What's a '120 source' in this context? Do you have any evidence or a source to show that 30L/watt is a bad efficiency?

At ~35W per bulb, two bulbs for lows, tow bulbs for brights... ~35w*4 ≈ 140w. As far as I know 140w ≈ 135w, further supporting the notion that Audi compared HID to LED.

The efficacy of the light source, ie arc. Sure there are loses, just like for LED. Like for optics, discharge arcs are nearly ideal point sources for collimation, in contrast to the relative lower surface brightness of led's, which limits range for given optical surface per https://en.wikipedia.org/wiki/Etendue#Maximum_concentration. Mirror reflectors or lenses for the former are unquestionably more efficient than TIR optics or such used for the latter, and any argument for LED would need to center around beam control and not total output. You literally have ~zero knowledge of these systems yet continue to bluster, such as the brights on cars are typically implemented with halogens because ballasts are not fit for frequent switching.

 

[JulesMaximus]

No

 

[JCH13]

So, no sources to suggest that 30L/W is unreasonable.

It is true that HID isn't the best for brights. Maybe Audi compared 70W of HID lows and 70W of Halogen brights. *shrug*

Not sure why you went off on a tangent with reflectors...

 

[rstrohkirch]

I believe you guys are a bit off topic. It doesn't matter what the actual efficiency of the system is in total lumens. The topic is about reducing wattage draw from the headlights to increase MPG. The information that people should be trying to obtain is the total system draw from each type of headlight. It really doesn't matter if one type gives you more output or not.

 

[JCH13]

It absolutely matters how much light a system outputs. In order to draw a holistic conclusion one must make a comparison between two systems of equal output/performance. Comparing a 10,000 lumen LED system to 1,000 lumen HID system would be silly.