Headlight use effect on MPG?

They have never been proven to be a lifesaver. It just doesn't happen. There's no way to prove such a thing anyway.

Or the lack thereof.

On Wed, 18 Apr 2007 20:58:03 -0400, jcr rearranged some electrons to form:

Probability and statistics.

As I said, the data is controversial, but it comes from accident report statistics. The studies I know about were done in Denmark.

A few statistics with actual data have to hold at least the same impact as your personal opinion.

You may be right. The NHTSA has been "studying" DRL's for over a decade now and apparently have yet to find enough "proof" one way or the other to rule on the matter (one way or the other). So, we're stuck all the while with dealing with a mixed environment (some cars have them and some cars don't...which is probably even a worse situation!).

I don't DISagree with either of you- but I think DRLs are useless.

The point is that if being seen is a problem, then it is incumbent on drivers TURN ON THE STINKING LIGHTS!!

There's no need on earth to waste energy and annoy oncoming traffic on every bright clear sunny day, which is how DRL's operate 99% of the time. And the OTHER thing DRLs do is make ig'nernt drivers forget to turn the headlights (and the rest of the lights) ON as it begins to get dark. I've NEVER EVER seen so many cars driving without their lights on after dark as I've seen since DRLs started becoming common.

Rear-end collisions were one of the accident types documented several years back (by the HLDI in 1997 I believe it was) as having increased rates with DRL-equipped vehicles vs. non-DRL equipped vehicles. This is probably one reason GM now has auto light control sensors on even some of their low-end models so drivers can't drive without tail lights when it's dark...even on purpose. Although, the auto sensors don't often help in daytime fog situations still.

DRLs are also useless when sitting hours-on-end in commuting traffic...when the maximum speed might be spurts of 15-20MPH max between being dead-stopped.

If these knotheads would turn on the lights, then there wouldnt be a problem.

I have been watching them a lot in this state, and on the average only about one in three turns on the lights during heavy rain, twilight conditions, etc (which is mandated by law).

People like this perhaps shouldnt be driving. But the roads are full of them.

Some GM vehicles switch lights on with the wipers as well (but not as many as those that gave the ambient light sensor only). Of course the annoying part of that is your dash digital display devices go dim when ever you use the washer-fluid to clean the windshield. ;-)

While I cannot argue with your equations I have to ask if they take into account over-generation by the vehicles alternator? Do you know if alternators do indeed only put out the minimum required power for any given situation? I would think not as they need to over-generate to recharge the battery. How much excess power is typically generated and how much would that reduce the over-all fuel consumption due to DRL use?

NO excess power is generated. The voltage is held constant at about a volt or so above battery voltage to keep it charged, and this difference is there whether or not any lights are on. If the battery has a resting voltage of about 13 volts, the alternator will put out

Dan

"Over-generation?" That makes no sense. The alternator puts out enough current to maintain the bus voltage, no more.

Absolutely. That's the function of the voltage regulator. If the alternator just put out its max power all the time, you'd have 60 volts when driving at 70 mph with no lights or AC on, and 12 volts at idle with the high-beams, wipers, AC, blower fan on max, brake lights, and stereo on.

Nope. The alternator restores the charge of the battery after the engine is started, then throttles back so that it is providing all the power for the electrical loads of the vehicle and nothing more.

With full battery voltage to the alternator field and the engine RPM up around, say, 2000, the alternator will generate well over 100 volts. You used to be able to buy a kit to install in your vehicle that cut the regulator out of the circuit and disconnected the alternator output from the battery, and with some means of keeping the RPM fixed, you could run power tools (with series-wound motors only, since it's 110V DC), lights, and so on until the battery got weak and you had to switch back to recharge it. I had one of these things, andbefore i installed it I opened it to see why it cost so much. A DPDT toggle switch, a neon light with dropping resistor that fired at

The regulator holds the alternator's output voltage constant, and with the battery run down a bit its voltage will be lower so it'll allow more current through it. The current decreased as its voltage rises and opposes the alternator voltage. All the regulator senses (and controls via field current) is output voltage.

Dan

The "output voltage" is the same as the voltage at the positive terminal of the battery, neglecting wiring resistance. If the internal charge state of the battery is low, the set-point voltage of the regulator will result in a charging current flowing into the battery. If the battery is fully charged, the voltage on the bus doesn't change, but the charging current no longer flows into the battery so the regulator is able to throttle back the field current and therefore the output current (not voltage) of the alternator. The term "voltage regulator" seems to confuse some people. Yes, the voltage is what is being regulated- but "regulated" actually means "held constant" in this case. The current out of the alternator is what varies in order to make the voltage stay constant.