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Alternator linked to mileage?

In today’s show (7-11-09), Tom and Ray said that disabling daytime running lights would actually improve mileage. I disagree, and here’s why. Unless alternators now have clutches, they run all the time, regardless of whether or not the car is using more electricity. So, it stands to reason that disabling the DRL’s would have no effect…the alternator is still turning. Am I wrong?

What affects the resistance to turning the alternator? How much power do you figure it takes to power the DRL’s? How much change in fuel mileage would have to be achieved to answer this question yws or no?

Are you telling me that when an alternator puts out more current, it becomes harder to turn?

You are on to something,now work on the other two questions.

If the answer is “yes”, then the other two questions are moot points. So you’re telling me that the operation of the voltage regulator actually affects resistance to turning? If that’s what you’re saying, is that your opinion or fact? Can you refer me to a source to verify that info?

This info is available in Automechanics 101. Amazon has a deal,I just got a e-mail today from them.

Listen to Oldschool, he’s right. Think of it as a ‘conservation of energy’ thing. Turn on the lights, that energy comes from the alternator, which comes from the engine. Turn off the lights, either the alternator puts out less energy (reality) or it continues to put out the same energy (fiction - where is all that energy going? Some hidden toaster under the hood?).

Yes, an alternators and generators are really easy to turn when nothing is drawing power from them and get harder and harder to turn as the assessorys draw more and more amps from them. I often demonstrate this with a hand cranked generator to people who think that generators make free electricity.
On some of the small cars I have owned, you can actually detect a small change in idle speed when you switch the headlights on or off.

I realize that the voltage regulator controls how much power goes out. I just didn’t realize that there is an increase in resistance to turning as more power is generated. I’m still skeptical…guess I’ll go do some research.

This is the answer I was looking for. Thanks!

If you want to crunch the numbers for how much gas DRLs would take, here goes:

  1. 1 horsepower = 750 watts (approx.)
  2. A fairly efficient IC engine can produce 20hp for 1hr with 1gal. gas.

So, take the current draw of DRL in W, divide by the efficiency of the alt (some # <1), divide that number by 750, then divide that number by 20 to get gal. gas required/hr.

(Apologies if they did something similar on the show; yet to hear it.)

It just goes to show than an alternator isn’t there for decoration. It’s more than just bearings and cooling fins.

If you have one of those emergency home generators, plug an electric bathroom heater into it and listen to the engine suddenly have to work really hard.

I think this was on a previous car talk links list, but here is my short calculation of the effect of DRL on mileage.

Basically, you will save more gas by slowing down than you would by turning off the DRL.

It doesn’t make any sense to me that increasing the load on the altenator would burden the engine any more.
There is no clutch, so the altenator spins at a speed corresponding to the engine. The magnetism is stable, since to produce electricty you run coiled wire though a magnetic field.
When spinning, it produces the same amount of electricty, but what isn’t used is shunted to ground and wasted (released as heat). When you turn on your lights, it taps the excess electricity produced, so less is wasted as heat, but that’s dealt with in the regulator, and doesn’t affect the altenator.

There is no “free” electricity, but there is wasted electricity, again, released as heat.
A similar thing happens with a portable radio. Your batteries won’t last longer if you listen at low volume. When you increase the volume, a potentiometer decreases resistance allowing more power through to the headphones. If you decrease it, there is more resistance, decreasing the amount of power passing through, and dissapting the rest as heat. It’s a small amount of heat, but it’s there.

Please read all of the above comments. Here’s a question for you - you say the unused electricity is ‘shunted to ground and wasted (released as heat)’. Where, exactly? Where is the large glowing lump of metal that’s accomplishing this?

And you’re wrong, listening at high volume does use more power (maybe not enough to tell without careful measurement, but it’s there). Adding resistance to a series circuit (the turned-down volume control) decreases current (Current = Volts / Ohms), decreasing power consumed (Power = volts X current).

“is that your opinion or fact?”

It’s a fact Jack. Listen to oldschool.

Please look, read and learn:

[My first reply isn’t showing up.]

If you don’t think an increase in current causes an increased load on the alternator, then stop by your local Museum of Science and get on the stationary bicycle generator that powers a 100 watt household light bulb. I remember how tired I got just trying to pedal to keep it lit at the 75 watt level.

Also, some heavy trucks have a special electrical brake sitting right next to the transmission. It’s basically a generator, driven by the transmission shaft, that sends current through resistors to ground. When activated, the load on the generator causes the truck to slow down.

All these replies are spot on. Any increase in current causes an increase in load.

This was true on my '65 (or '69?) BSA Royal Star motorcycle with a generator, not an alternator. The excess heat was given off by a zener diode mounted to a massive aluminum heat sink, up near the front fork.

But an alternator is different. The magnetism isn’t stable - it is controlled by the voltage regulator. Instead of wasting unneeded power as heat, the alternator is told to produce less power when it isn’t needed. IOW, the power produced is NOT simply dependant on RPMs.