DAYTIME RUNNING LIGHTS

October 29, 2003

Comments

Slate’s Explainer takes on a recent survey published by GM showing that daytime running lights helped avoid 37,000 accidents since 1995. GM’s methodology sounds reasonably sound.

What’s interesting to me is this tidbit:

[Opponents claim] that GM is merely trying to make a buck by making lights burn out faster and upping fuel consumption (since lights do consume an iota of gas). The carmaker has countered that low-intensity DRLs do not significantly decrease bulb life and that the fuel-consumption hit will amount to less than $10 per year, per vehicle. Multiply that by 200 million vehicles in the United States, though, and it amounts to a healthy chunk of change.

DRLs use gasoline?! I always presumed they ran strictly off the battery which, as I understand it, is kept charged by the alternator. So, does running DRLs make the alternator work harder, or what?

31 responses to “DAYTIME RUNNING LIGHTS”

Teri

October 29, 2003 at 3:54 pm

How would GM benefit by increasing fuel consumption? Is there some conspiracy whereby all GM executives or shareholders are also oil investors?

I would think that it would be to GM’s benefit to REDUCE gas consumption, because even the tiniest nth of a teaspoon would adversely affect their CAFE averages. So, in other words, it seems silly to claim that GM is deliberately increasing fuel consumption.
James Joyner

October 29, 2003 at 3:57 pm

Yeah, that puzzles me, too. I don’t know if GM also owns fuel interests.
Fred Boness

October 29, 2003 at 3:58 pm

Any load on the alternator is a load on the engine driving the alternator. So, yes, any extra electrical load burns more gas.

My own opinion is that when a few cars have running lights they will stand out but, when all cars have these lights it is just more noise in the driving environment.
joy

October 29, 2003 at 4:07 pm

I actually burn my lights on my non-GM jeep during the day even though I have to pull the control myself.

I think that cars with lights are easier to see.
Kate

October 29, 2003 at 4:15 pm

We’ve had a “lights on for life” campaign here for years, and now new vehicles are mandated to have running lights.

Having everyone put them on does not create more “noise”, but it does put those who do not have them on at a distinct visual disdvantage. When you notice how much, it tends to make you reach down and put your own lights on.

Kate
James Joyner

October 29, 2003 at 4:21 pm

I tend to run my lights, including fog lights, most of the time as well. I figure it can’t hurt.
jen

October 29, 2003 at 4:31 pm

My Toyota has DRLs. I love the feature – mostly because the bonus is that when it’s dark the regular lights automatically trigger on. I never have to touch that lever unless it’s for the turn signal.
Fred Boness

October 29, 2003 at 4:36 pm

During WWII there was a camoflage scheme called Yehudi. (For you younguns, Yehudi is the little guy who turns on the light when you open the refrigerator door.)

Patrol bombers hunting subs in the North Atlantic could be seen a long way off. Yehudi hid the bombers in the background sky light.

It works like this: there was a row of lights along the leading edge of the wings and around the nose of the plane. The brightness of the lights was controlled by a rheostat to match the brightness of the sky. The bomber would blend into the background and could get a lot closer before being spotted.

So, you drive with your lights on all the time? Does that include when you are driving out of the sunset? A motorcycle can disappear completely under those circumstances.
Sam

October 29, 2003 at 5:09 pm

I’d guess that running lights all the time may increase fuel consumption a bit, but it may be down in the noise. The alternator is basically always charging the battery, when the battery is at or near full capacity, almost all of the charging power gets turned into heat. With DRL, some of it gets diverted to powering the lights, but probably almost all of it would have been turned into heat anyway.
I’d believe that DRL would reduce the lifetime of the lights.
Bill

October 29, 2003 at 5:29 pm

The batteries only use is to start the engine. After the engine is started, the alternator supplies the electrical power, e.g, keeping the battery charged and running all the other electrical doodads. The more doodads in use requires the alternator to work harder,which in turn puts a heavier load on the engine. Therefore gas consumption increases.
My car has had two headlight bulbs burnout in 80,000 miles(oddly enought it always the right side bulb.) To me,always on headlights are a waste of watts. This “feature” was useful when they first appeared, but now (from experience)they are mainly ignored by other drivers.
Director Mitch

October 29, 2003 at 5:40 pm

Keep in mind that power consumption from lights will be reduced to less than 10% of what they are today with the introduction of Light Emitting Diode (LED) headlights.

LEDs are already used on all new traffic lights in the U.S, saving replacement and electricity costs (look at the next stop-light you are at and see that it is made of a bunch of smaller lights – these are the LEDs). In cars, LED headlights can already be found in the very high end. In five years or so, LED headlamps will become standard features, meaning you will probably not need to replace them during the normal life of your car and electrical consumption for this component will go down.

Ofcourse, total electronic use inside cars is dramatically increasing – GPS, airbags, ABS, etc. – making headlight power decreases sort of meaningless. Essentially the car is turning from a hydrolic-mechanical device into an electrical device, with “concept cars” being all electrical systems (so called “drive by wire”).

Eh, I guess I should do a posting on this – I wrote a 50 page paper on this six months ago…
Anonymous

October 29, 2003 at 6:26 pm

> My Toyota has DRLs. I love the feature – mostly because the bonus is that when it’s dark the regular lights automatically trigger on. I never have to touch that lever unless it’s for the turn signal.

Jen, better take a second look. While Toyota’s may be different – most cars with DRL are headlights only – meaning, tail lights only come on with the manual switch.

> It works like this: there was a row of lights along the leading edge of the wings and around the nose of the plane. The brightness of the lights was controlled by a rheostat to match the brightness of the sky. The bomber would blend into the background and could get a lot closer before being spotted.

> So, you drive with your lights on all the time? Does that include when you are driving out of the sunset? A motorcycle can disappear completely under those circumstances.

I own a motorcycle. They all have running lights, and believe me, if your bike has left the ground in the manner you describe, the last thing you have to worry about is being seen by opposing traffic.

Kate.
Fred Boness

October 29, 2003 at 6:44 pm

Uh, that’s wheels on the road driving out of the sunset. I owned motorcycles for years and discovered the disappear in the sunset effect more than thirty years ago when Wisconsin adopted a headlights on law for motorcycles (“safety” you know). People would turn in front of me like they didn’t see me and well, they didn’t.
Paul

October 29, 2003 at 9:20 pm

The batteries only use is to start the engine. After the engine is started, the alternator supplies the electrical power, e.g, keeping the battery charged and running all the other electrical doodads. The more doodads in use requires the alternator to work harder,which in turn puts a heavier load on the engine. Therefore gas consumption increases.

WRONG WRONG WRONG WRONG

How can an alternator work harder? The alternator spins at the same rate off the engine no matter the load. There is no clutch or variable gear ratio, it is a fixed drive. The load on the engine will only change as a function of RPM.

I have no idea who at GM would claim it burns 10 bucks a fuel a year. Not an engineer for sure.

As another commenter said, it would just become heat.

The only way you could POSSIBLY make the case is if GM had to install a bigger alternator to handle the extra load. That is patently silly. At best they they use 20 watts of power. A typical alternator is making 720 watts of power at 35mph.

If you actually believe it burns gas then wouldn’t your stereo burn more gas considering in a modern car they are about 40 watts.

Have ya ever heard someone say to turn down the radio to get better milage?

geeze.

P
Bill

October 29, 2003 at 11:44 pm

Of course the alternator runs at some ratio of drive pulley sizes

Manufactors have installed higher capacity alternators to handle increased loads of computers, high energy ignition systems, more powerfull radios,etc. Typical alternators can generate over 900 watts in current cars.

A stereo can put out a claimed 40 watts, but it may take 100 watts to generate that 40 watts.
The missing 60 watts is heat-ever notice the large heatsinks on the high power radios and computer CPUs. I would not be surprised if the various computers did not use 200-300 watts. The ignition might use another 100-200 watts. the headlights and running lights another 150+ watts.

When all this stuff is running the alternator has to work harder to satisty the load.What makes it
work harder is back EMF (electomagnetic force)–there ain’t no free lunch. The more demand on a alternator (regardless of speed) the higher the back EMF, which means more work to maintain the load, which makes the engine work harder.

Yes, you do use more gas playing the radio, the amount would be miniscule.

When I was in the service, we had a field radio powered by a hand cranked generator. When the radio was in receive mode-it has very easy to turn the crank. But when it was transmitting it was a bear to turn due to higher power demands and increased back EMF at the same cranking speed as when receiving.
melvin toast

October 29, 2003 at 11:48 pm

WRONG WRONG WRONG paul.

A car going 40 miles an hour up a hill is working harder than a car going 40 miles an hour on a flat. Work is distance times force, or in this case torque. You’re leaving
out torque.

When more electricity is being draw from a generator,
it becomes more difficult to turn it. If you ever used
a gas powered generator and plugged in a high current
peripheral, you’ll notice the engine bearing down to work
harder.

Alternators also have voltage regulators. When the
battery is full they’ll cease to draw current and basically
allow the alternator to spin freely. Actually they can’t because there’s some wind resistence and bearing friction
but you get the idea.
Paul

October 30, 2003 at 6:56 am

Apparently neither of you guys get it.

I started to make a longer post explaining it in greater length but I didn’t.

In typing this reply, I put down a whole bunch of words to explain it. Then I decided to do it right. I deleted all the words and left the numbers.

–

I tell ya what.. I went to engineering school for a reason, let’s get out the calculator.

[First, pure theory] A gallon of gasoline has a fixed amount of energy. That is about 60kWh per gallon. By the time you convert it from chemical energy to mechanical energy to electrical energy yields about 20kWh of energy. Let’s say a car is used 500 hours year and the lights draw 20 watts. (10kWh)

EVEN IF you have no loss for heat, no excess power being dumped, even if you account for every millijoule you are talking half a gallon of gas per year. [in pure theory]

In reality, it does not work that way, we dump megawatts of power into heat on the alternator because we generate more than we use.

IF IF IF you wanted to put a true number on it and if you wanted to make the case that sometimes it will increase the load, when would that be?

The only time that would happen is when the alternator was in a state where energy demand exceeded the base capacity. The only time that would be true is at startup when we charge the battery. (After that we dump energy by the pound.) If you say the average person starts their car 4 times per day. Let’s do the math.

It takes about 5 minutes for the alternator to recharged the battery. So that is 20 min a day * 365 = 122 hours per year the system is in a regressive state. * 20 watts that is 2440 wh per year.

Since you get about 20,000 Wh from a gallon of gas EVEN IF I give you every benefit of the doubt that is 1/10 of a gallon of gas per year. You spill 40 times that when you fill up.

Even in pure theory it is only 1/2 gallon of fuel per year. In reality it would be less than 1/10th of a gallon, EVEN giving you the argument that it does not come from excess generation. (which it does)

The calculator don’t lie. You can not make the case that DRL burn an accurately measurable amount of fuel per year.

Paul
Charlie on the PA Turnpike

October 30, 2003 at 7:29 am

Personally, while I’ve long been an ‘automatically put on my headlights day or night’ kind of person, I find the idea of mandating DRL through law a waste of legislature time and resources.
jen

October 30, 2003 at 12:28 pm

Jen, better take a second look. While Toyota’s may be different – most cars with DRL are headlights only – meaning, tail lights only come on with the manual switch.

october, I did check that the full lights came on without my having to flip the switch. They do.
Gary Renaud

October 30, 2003 at 12:43 pm

Whilst I agree that the DRLs do not burn a MEASUREABLE amount of fuel, they most assuredly burn a small increase.

The alternator does NOT generate (OK, convert) a constant amount of power, with the excess dumped as heat. There is a regulator which adjusts the field current so there is a constant voltage at the sense point. Each load added would drop the voltage slightly if there were no regulation, so the alternator DOES slightly increase its current output and thus increase the shaft load. It’s NOTHING compared to what the A/C pulls down.

Normally, I start work around 0600, so part of the year I drive at dawn. I have never seen a case in which a lit vehicle would merge with the background lighting to be less visible than a dark one, though I’m not going to deny that it might happen.

I HAVE had innumerable cases in which I see the road clear, start a pass, then suddenly see lights flash on a grey or blue car in the opposing traffic lane.
Paul

October 30, 2003 at 1:43 pm

Well, you inadvertently gave me ammo Gary. I showed, thru sound methodology, that it was MAYBE 1/10th of a gallon per year. (even giving you guys a faulty argument)

I failed to consider much of the driving is done at night. I assumed all daylight driving.

If you consider that (wild guess) 30% of the driving we do is at night, now you have about 0.07 gallons per year that is about 1/10,000th gallons per day. You lose more than that to vapor when you open the gas cap.

Now consider this…. If just one person per year sees the DRL and does not pull in front of you making you hit the brakes and waste energy, the DRLs will pay for themselves.

When you go to engineering school they make you think of these things.
melvin toast

October 30, 2003 at 6:33 pm

I don’t know what engineering school you went to paul but you don’t seem to know how cars are engineered.

Your quote: How can an alternator work harder?

That’s what the stick we’re beating you with. As far as how
much more energy your talking about, howstuffworks.com claims about 406 millions gallons of gas each year. Admittedly not much since that’s only a couple gallons per
vehicle or so. But that has nothing to do with the above quote.
Paul

October 30, 2003 at 10:47 pm

Melvin, can you work a calculator?

I found the page you were talking about and he used a methodology pretty close to my own for much of it but he makes serval mistakes.

He said headlights burn 100 watts but DRLs do not run at full power as he says, they run at a much reduced power. Also he uses old numbers for engine and alternator efficiency. Today we get about 20kWh of energy back out that gallon of fuel. He also makes the mistake I did of not counting night time hours.

AND he does not even mention that ANY power would be used that would go off into heat. Certainly those things alone would make his number off by a factor of 20 or 30. (at very minimum)

But to answer the question, “How can an alternator work harder?” YES under large loads the EM filed will increase.

20 watts would not be measurable.

I tell ya what.. Since I apparently wasted all my time in that silly engineering school, it is your turn. Since you know so much more about engineering automobiles and magnetic fields that me, here is your assignment:

Get your calculator and figure out the increase of the electromagnetic field in an alternator to produce 20 watts of power at oh say 2000 rpm.

If you can come close to doing the math I might come close to respecting your opinion.

Engineering is applied math. The rest is dumb shit.

Paul

And as an added bonus I’ve give you another way to skin the cat. Any engineer worth his salt could actually figure out the force required without a calculator. It is a “cheating” method but it would work. If you can figure that out, I’ll still give it to you.

Of course figuring that out would prove me right so you might want to stick to the calculator.

Feel free to give me some engineering lessons.
melvin toast

October 31, 2003 at 1:05 am

Paul,

Here are your own idiotic quotes which shows that you were clearly wrong:

“How can an alternator work harder? The alternator spins at the same rate off the engine no matter the load… The load on the engine will only change as a function of RPM.

I have no idea who at GM would claim it burns 10 bucks a fuel a year. Not an engineer for sure. ”

The load on the engine doesn’t change as a function of RPM? What about torque?

As for the theoretical change in EMF due to the added load, you just take 50 watts and multiply by about 3/4 and you get 37 ft lbs divided by 2000 which is 0.0185 ft-lbs of torque. If you want to use your 20 watt number you can call it 15 ft lbs or 0.0075 ft-lbs of torque. I don’t think two 10 watt bulbs would be bright enough for DRL

Stick that in your pipe CALCULATOR MONKEY BOY!
melvin toast

October 31, 2003 at 1:10 am

Hey Paul,

Actually that’s wrong because the 2000 rpm is in minutes and my ft-lbs is in seconds so it’s really 2000/60 which
is about 33 revs per second. So for 50 watts which is
37 fl lbs, you’ve got a little over 1 ft-lb of torque. For your
twin 10 watt bulbs it would be about 1/2 a ft-lb of torque.

I’ll bet you didn’t even catch my mistake Calculator Monkey Boy!
Paul

October 31, 2003 at 1:33 am

In my best Regis Philban voice…

“Is that your final answer?”

It ain’t too late to phone a friend.
Paul

October 31, 2003 at 1:38 am

And as for your harping on that quote I have explained what I was saying 3 times. If you don’t get it, I can’t help you. I was trying to be brief. (had I only know what the future held 😉

I expand what I meant and explained it. 20 watts even 50 watts.. hell probably not even 100 watts would not even be a measurable amount of force.

If I was unclear sorry/too bad.

P
Paul

October 31, 2003 at 1:47 am

ooops Mevin- I reread what I wrote- I can see I worded that very, very poorly. Ya know how when you make a typo you read it with what you MEANT to say. I had two thoughts in my head and they collided on the way out.

I can now see why you were going nutz over it and I was ignoring you. After all you were reading what I wrote and I was reading what I THOUGHT I wrote.

That said, my math is right.

Is yours?
melvin toast

October 31, 2003 at 11:07 am

I love you man. (in a platonic way)
JCR

December 26, 2003 at 10:55 pm

Uhmmm…DRLs must consume a fair amount of fuel…why else would GM petition the DOT to test their vehicles for CAFE standards with the DRLs off? They subsiquently got approval to do just that. So, remember, when you see mileage ratings for GM vehicles they are without the DRLs being lit.