100% of the energy used in your car comes directly from the gas. Headlights use energy. Vrey little energy, however. I’m guessing about a cup of gas every 20,000 miles.
Let’s assume that the headlights on is 100 watts of extra power, headlights plus taillights etc. Assuming that the average speed during 20,000 miles of driving is 40 mph, that’s 500 hours of headlight use or 50,000 watt-hours of energy. 50,000 watt-hours equals 67 horsepower hours. At max efficiency, most gasoline engines need about .4~.45 pounds of gas per horsepower hour. That would mean about 67 X .4 or 26.8 pounds of extra gas burned. This assumes an alternator that’s 100% efficient.
If you think that a 40 mph average is too slow, time your next commute to work.
Maybe I’m a lone ranger here, but I have to chime in in support of handlebarjay.
The real question is about the baseline current produced by the alternator. At idle speeds the alt much produce enough current to provide spark, run the computer, and maintain the battery charge - it probably produces a little more. At higher rpm’s it produces much more current and while the engine consumes a little more there is still an excess. It is possible that a finely tuned engine and a closely matched alternator may cause the computer to increase engine rpm at idle when the lights are turned on, but in most cases the lights and other accessories are using the excess current produced by the alternator anyway.
Rix06trix:
I think you need to study up on your physics.
The more current an alternator produces, the more difficult it is to rotate. B.L.E. had a very good analysis on this. If you don’t believe me, us or your science books, just stop by your local Museum of Science and get on one of those bicycle-powered generators - and see how much more leg power it takes to make the light bulb shine brighter.
There is no such thing as “excess current” from the alternator.
Multi-wrongs. The extra load induces additional current flow in the alternator, which interacts with the magnetic field to produce a back force–additional work the engine must do, which in turn must burn more gas to overcome. Another way to say it is that the extra energy must come from somewhere. That said, I expect that this is quite small. Well, you can estimate it. Let’s say the headlights are about 200 watts each (I have no idea if this is right). That’s about 1/2 horsepower. So maybe it could add 1%??? Then you have to add the ultimate cost of bulb replacement.
That being said, I believe this is a small price to pay for the additional safety, which has been demonstrated in research studies.
Okay, let’s take your example. I spin the bike to power the light bulb. That is a closed system where most of the power generated is used by the bulb. So I keep spinning and someone unscrews the bulb. I haven’t suddenly stopped producing energy just because there is no bulb to use it. This is excess current that is lost mostly as heat.
My point is that a car’s alternator is not so perfectly tuned that it only produces exactly the energy needed by the engine and accessories at any given moment. There is more current available that is usually lost as heat. This otherwise lost energy is sufficient to power the lights and most other accessories without consuming more gas.
The moment you unscrew the light bulb, the resistance required to pedal drops. The impact on your body is you’re now expending less calories. The impact on your car (when you shut off the lights) is that your engine now has to work less hard and you save gas. The saving is small, but it’s there.
Any current coming from the alternator to power your lights or accessories can only happen if the engine expends horsepower to drive the alternator.
What is this magic about the alternator producing “extra current” or that “most of the power generated is used by the bulb”? Exactly the amount of current used by the load is exactly the amount of current produced by the alternator at any given time! It is true that some of the power produced is wasted as heat inside the alternator, but increasing the load just increases the waste, it doesn’t shift it around. It’s real simple. Electrical power out = mechanical power in divided by efficiency. (Efficiency accounts for the heat wasted in the alternator, among other things.) Mechanical power is RPM times torque. If the RPM is fixed, then if you draw more power (current) from the alternator, then the engine has to supply more torque to it.
Look, I get the theory. You can’t use more energy than you produce. But that’s looking at the wrong side of the equation. You can produce more energy than you need. What you don’t need gets lost in other forms. The alternator is going to produce a little more energy than the car needs at any given time. What the car doesn’t need is lost. The minimal need of the headlights consumes a tiny bit of that energy that is otherwise ordinarily lost.
No. No. No. The alternate produces exactly as much electrical power as is used by the load at any given time. There is no extra that is magically laying around.
Right re: laws of physics and all. I thought the belt and alternator placed the engine under the same continuous mechanical load regardless of lights on or off. Got it now. Thanks to all. Share the pie!
I think you are right. The lights draw power from the battery. The alternator charges the battery. I suspect that there is some additional load but none more than radio or windshield wipers. Even the AC today is so efficient as not to require much additional fuel to run it. MOO. My opinion only.
Actually, you friend is correct. It does take some energy (fuel) to run them, and of course it does reduce their useful (night/rain) life. Assuming they are incandescent lights (not HIDs) - if he has DRL’s, they probably only burn around 30 watts or so, but it is something. If he just runs normal low beams, it’s more like 100 watts! And no, the alternator is not just pumping out power all the time, it adjusts to the load. Yes, there is mechanical load all the time, but powering the lights causes a electromagnetic load, slowing down the engine. More load = more fuel used.
And if you don’t think it is significant, go to a science museum some time. They usually have a display with a light bulb and a hand crank. For the power to light the bulbs bright, you could easily push your car on a flat road.
One of my cars would squeal its alternator belt in the morning as I left for work. I quickly learned that I could stop the squeal by switching off the headlights for a second.
Why didn’t I just tighten the belt? Lazyness mostly and access to the alternator was a real PITA in that car. Yes, I did eventually tackle the job and tightened that belt.
Yes it uses more gas, there is no such thing as free energy. The headlights put a draw on your battery therefore requiring the alternator to produce more power to charge the battery. The larger the draw on the alternator the more work is required to spin it. The comparison between lights on or off when it comes to gas use is like removing a drop of water from a pool.
I think the confusion comes perhaps because cars made long ago behaved as you believe. If your car had a generator and a three speed transmission, at highway speeds the engine RPMs would be high enough that the car’s momentum and the engine’s momentum would be enough to power the headlights. Automotive generators didn’t produce as much power and were not nearly as sophisticated as today’s automotive alternators. If you and your friend are betting on a 1956 Chevy, you might be right. If the car in question was made after 1990 though, I am sure you loose.
I never realized a question of this sort could generate so much postulation.
Does your friend turn off the A/C and drive with windows open? Do open windows actually create more drag and use more gas than A/C, or does this depend on ambient temperature? Time to win a pie back?
Hey! What about solar powered day-time running lights? Put a little panel up there, run a couple of wires…
Can you explain this? It doesn’t make sense. Whether you have a generator or alternator, it still takes engine power to turn it to produce electricity. The more current the generator or alternator produces, the more horsepower required from the engine to turn it.
BTW, the generator seemed to disappear from US autos in the early to mid 60s.