Turn the radio off, you may be wasting my gas!
February 7, 2008 12:59 PM Subscribe
Do electrical devices running on your car's electrical system affect gas mileage?
Do you use more gas, all else being equal, with the lights, radio, plug-in accessories running than without?
Do you use more gas, all else being equal, with the lights, radio, plug-in accessories running than without?
The question is whether this is a noticeable drain. Your car is producing electricity whether you're using it or not. Thats what the alternator does. How much stuff would you have to run to see a difference sounds like a question for someone with the time and equipment to run some decent experiments.
posted by pupdog at 1:07 PM on February 7, 2008
posted by pupdog at 1:07 PM on February 7, 2008
Best answer: The Straight Dope: Does using your car radio reduce your gas mileage?
posted by iconomy at 1:08 PM on February 7, 2008
posted by iconomy at 1:08 PM on February 7, 2008
Some of the energy in the gasoline burned by your engine turns an alternator which, in turn, generates the electricity used by lights and gadgets. Alternator efficiency is only 50-65%, so the amount of gas used by lots of devices may be non-trivial.
posted by sindark at 1:09 PM on February 7, 2008
posted by sindark at 1:09 PM on February 7, 2008
Do you? Yes. Is it measureable? No. Even the strongest alternator, at say 135 amps, is drawing 135*12.6/745=2.3 hp, when it's balls-out, which is pretty much never. You could gain that back by putting two more PSI in your tires. It's miniscule.
posted by notsnot at 1:10 PM on February 7, 2008
posted by notsnot at 1:10 PM on February 7, 2008
Best answer: My theory: it would technically put more resistance on the alternator, which is run by a pulley on the engine's crankshaft, and through which it would send some feedback to the engine. In theory, just like the a/c compressor. But unlike turning on the a/c, I think the effect of plugging in a laptop would be negligible, and hard to measure, in gas used or in noticeable engine strain.
posted by M.C. Lo-Carb! at 1:11 PM on February 7, 2008
posted by M.C. Lo-Carb! at 1:11 PM on February 7, 2008
yeah, so in other words, what iconomy said.
posted by M.C. Lo-Carb! at 1:15 PM on February 7, 2008
posted by M.C. Lo-Carb! at 1:15 PM on February 7, 2008
What about A\C? I often here people saying it reduces fuel economy. Is that because it does something more than pull electricity?
posted by jeffamaphone at 1:28 PM on February 7, 2008
posted by jeffamaphone at 1:28 PM on February 7, 2008
jeffamaphone: The air conditioner is belt driven, not electric, in 99% of cars. (Certain hybrids and electric cars have electric air conditioning.) Turning on the air conditioner in a normal car activates the compressor (engaged via a clutch) and places additional load on the engine. So yes, it does reduce your available engine power and your fuel economy.
posted by knave at 1:36 PM on February 7, 2008
posted by knave at 1:36 PM on February 7, 2008
As someone who drives stick it's very easy to tell that the A/C puts additional load on the engine; it's much easier to stall it when the AC is on.
posted by aubilenon at 1:45 PM on February 7, 2008
posted by aubilenon at 1:45 PM on February 7, 2008
Here's a little example: say your stereo runs at 40W continuous (a new Honda Civic stereo runs 160W max) and your alternator is 55% efficient (Wikipedia says 50-62%). You need to 40W/0.55 = 73W of power delivered to the alternator. Your engine is about 20% efficient (Wikipedia again) and a litre of gasoline has 34.8MJ of energy. 73W = 73 J/s ~ 260000 J/h, so you need (260000J/h)/((34.8MJ/l)*0.2) = 0.038 litres of gasoline per hour to power your stereo. So if you are cruising down the highway at 100km/h in your Civic (which burns about 7l/100km on the highway), your stereo will take about 0.038l/7l = 0.5% of that.
Not much, but once you add all the electrical devices together, it probably adds up to a few percent.
posted by ssg at 1:54 PM on February 7, 2008 [1 favorite]
Not much, but once you add all the electrical devices together, it probably adds up to a few percent.
posted by ssg at 1:54 PM on February 7, 2008 [1 favorite]
Related: daytime running lights burn a significant amount of gasoline.
posted by Wet Spot at 2:08 PM on February 7, 2008
posted by Wet Spot at 2:08 PM on February 7, 2008
For some cars the increased load on the alternator is noticeable. On my old Honda, for example, when you turned on the headlights the idle speed dropped a bit.
posted by Killick at 2:16 PM on February 7, 2008
posted by Killick at 2:16 PM on February 7, 2008
As someone who drives stick it's very easy to tell that the A/C puts additional load on the engine; it's much easier to stall it when the AC is on.
This definitely depends on the car. I usually drive manual cars, and with my current 220HP V6, I couldn't tell you if the AC was running or not, just based on the driveability. When I drove a 92HP Civic, I could definitely tell the AC was dragging the engine.
posted by knave at 3:00 PM on February 7, 2008
This definitely depends on the car. I usually drive manual cars, and with my current 220HP V6, I couldn't tell you if the AC was running or not, just based on the driveability. When I drove a 92HP Civic, I could definitely tell the AC was dragging the engine.
posted by knave at 3:00 PM on February 7, 2008
First law of thermodynamics... energy is not free.
If you use it, it has to come from somewhere. The somewhere in a car is gasoline (or equivalent.)
As others have said, it's not a major draw, but it is non-zero, and that is the proper answer to your question, as phrased.
posted by FauxScot at 6:26 PM on February 7, 2008
If you use it, it has to come from somewhere. The somewhere in a car is gasoline (or equivalent.)
As others have said, it's not a major draw, but it is non-zero, and that is the proper answer to your question, as phrased.
posted by FauxScot at 6:26 PM on February 7, 2008
On some newer cars the generator charge rate is cut back if the battery doesn't need to be charged to save fuel economy. It takes some getting used to when you're driving along at highway speeds with the voltage guage reading 12 volts instead of the usual 14.
posted by rfs at 8:41 PM on February 7, 2008
posted by rfs at 8:41 PM on February 7, 2008
Okay, so that Straight Dope article has me wondering what the actual numbers would be. The rear defroster is going to put significant load on the electrical system (and hence the alternator will resist a bit more in order to meet that load). They didn't directly answer what the effect on mileage would be, though, just some estimates.
If you could find out how much current is required to run every electrical device, you could then get the efficiency ratings of the alternator and hence how much torque (or revs) is required to generate 1 amp. Using that, you'd know roughly the ratio of alternator load to engine torque output.
Now where the hell would you get all of these figures?
posted by spiderskull at 10:51 PM on February 7, 2008
If you could find out how much current is required to run every electrical device, you could then get the efficiency ratings of the alternator and hence how much torque (or revs) is required to generate 1 amp. Using that, you'd know roughly the ratio of alternator load to engine torque output.
Now where the hell would you get all of these figures?
posted by spiderskull at 10:51 PM on February 7, 2008
Response by poster: Sorry I couldn't hang out for this one, had some work to do. Interesting discussion.
I understand that energy isn't free, but my question centers around whether the alternator has to work harder when there are more things on, or if it just kind of maintains a steady state, delivering a certain amount of current to the battery whenever the motor is running at a certain speed, regardless of whether electrical devices are running. Turns out that the resistance increases when you turn devices on, so theoretically there is increased use of fuel.
Also looks like no one can agree on whether it is trivial or not; I'll bet that can vary dependdeing on the car you're driving.
Thanks!
posted by Mister_A at 9:36 AM on February 8, 2008
I understand that energy isn't free, but my question centers around whether the alternator has to work harder when there are more things on, or if it just kind of maintains a steady state, delivering a certain amount of current to the battery whenever the motor is running at a certain speed, regardless of whether electrical devices are running. Turns out that the resistance increases when you turn devices on, so theoretically there is increased use of fuel.
Also looks like no one can agree on whether it is trivial or not; I'll bet that can vary dependdeing on the car you're driving.
Thanks!
posted by Mister_A at 9:36 AM on February 8, 2008
This thread is closed to new comments.
posted by These Premises Are Alarmed at 1:01 PM on February 7, 2008