Car radio sapping mileage?

Cecil's Columns/Staff Reports
1

I just wanted to say that I think there is an effect on overall mileage, however miniscule. Anyway, I took him up on his final suggestion (burn rubber while blasting some tunes). Smashing Pumpkins (in honor of that fair cit-ay, Chicago) on a 500-watt standard sound system in my black 2001 Pontiac Trans Am (top speed somewhere in the neighborhood of 160mph - untested personally, so I’ll take their word for it). Disclaimer I didn’t exceed the speed limit in any way, shape or form. That black blur you thought you saw? Maybe it was a stray piece of Mir finding its way back to terra firma?

I wonder how many dinosaurs I burned on my way into work (I am aware of Cecil’s long-ago Q&A, BTW)? And I wonder what effect the vocal ministrations of Billy Corgan ‘et al’ had upon my daily commute. My wife says his voice is very grating, so would I have gotten better mileage out of something more soothing & polished, like Chicago (the band) or Cheap Trick - assuming volume levels are the same?

2

The link to the Staff Report is: Does using your car radio reduce your gas mileage?

It’s helpful to provide the link to the Staff Report when you’re starting a topic, so that everyone who wants to respond will be on the same page (so to speak.)

[Edited by C K Dexter Haven on 03-29-2001 at 07:45 AM]

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Thank you.

4

If you paid close attention in high school physics you could figure this one out.

one horsepower is about 750 watts.
a medium sized car uses about 15 horsepower at 50mph.
so, if the radio uses 105 watts ( I think that is near full volume for a decent stereo) that is .15 horsepower, which is equal to 1% of the power consumption of the car. Of course it is unlikely that the alternator is 100% efficient. Lets say it is 50% efficient, that would be a 2% reduction in horsepower. Of course no-one actually drives 50 mph, if power consumption doubles when you drive 80mph, you are looking at a 1% decrease in mileage. If your car got 30 miles to the gallon, you’d burn an extra gallon on a cross country drive.

5

…so if you left the radio off during your cross country drive, you’d save $1.50 on gas, but you’d spend an extra $500 on munchies to keep your mind occupied while you’re driving 'cause you’re not using the radio.

6

It really depends on the radio in question. I installed a stereo in my friends Wifes little mustange years ago(about 83-84 or so), that had two fairly massive amps, and an Eq. Her milage was significantly effected by the stereo. the system also pulled more power than the alternator on that little four cylinder would put out at idle. If left idleing for two long, it would drain the battery.
Another friend of mine in my electronics classes in college was really into CB radio crap. He had some massive (and illegal) amp on the output of his CB, and on his base station at home. he used to sit in the parking lot of the school and talk to his wife about 50 miles away. When he keyed up the CB, it would kill the engine in his car if he didn’t keep his foot on the gas(just from the load on the alternator). His was a little four cylinder also, but he had a friend that bragged he could kill the V8 in his pickup in the same manner(much bigger amp).

7

Ezra, very neat, but doesn’t have the credibility or accuracy of Dogster’s formula that uses cool stuff like mu-cosine-theta. (Dang, I dunno the codes to print Greek)

8

I don’t believe using your radio affects your gas mileage. My guess is that the alternator runs at a constant speed (to save on wear and tear) and generates a constant amount of electricity. When the alternator and regulator charge your battery, extra electricity is automatically generated. This extra electricity, when not used, is turned into heat. When you turn on your radio, or any other electrical device in your car, you’re actually just using electricity that would normally be wasted if used solely to run your car. If your auxillary electrical equipment did require more power than your alternator was generating, it would end up draining your battery, which doesn’t normally seem to happen. Of couse, if you DID consume all the power normally generated by the alternator, it MIGHT compensate by generating more electricity (and hence more resistance for the engine) lowering your gas mileage. In normal circumstances, however, I don’t think it does.

9

watch it for yourself sometime. If you try to jumpstart a car with a compleatly dead battery, as soon as you connect up the cables you will hear your engine lag. Most of the older generators would have the same drag regardless of how much load was on them. Alternators, however, are turned on and off with by the regulator as needed. rather than just having fixed magnets in them, the have electro magnets. therefore being able to be turned off. When turned off, there is no load on the alternator and it freewheels. when turned on, ther is considerable drag, depending on how much of a load it is having to run. As per my previous post, I have seen this kill a car motor at idle…Think what that kind of a load would do to your milage. On a stock radio, this is no big deal. It doesnt pull much, and you probably will never notice the differance.

The alternator will normally kick as needed, so you are not useing electricity that would “normally be wasted”.

10

Without actually running the figures per Ezra, I believe my big V8 with 310 SAE hp (@ 5200rpm) could possibly get 40-50 miles per gallon if I simply didn’t turn on my stereo!!!

Here’s where I sorta figure this. My car (Pontiac T/A - V8 standard) in 6th gear at 60mph runs at 1800rpm. My average speed this morning turned out to be 58.8mph (distance / time). I can’t pull up the website this morning (Damn slow servers) with the actual dyno figures to give an exact number, but an identical T/A at 1800rpm puts out about 220-230 SAE rwhp (rear-wheel horsepower). My sound system is rated at 500w maximum. I don’t know much about sound systems, except that 500w is REALLY LOUD. Assuming I had SP blaring at 2/3 of total volume this morning (still quite LOUD) I arrive at 333w (again I plead ignorance if there is some kind of non-linear correspondence between volume level & wattage - please enlighten me if there is).

Soooo, assuming (lots of assumptions here)

333w = .444 hp (from Ezra’s 1 hp=750w)

Assuming no further drive-train losses and .70 alternator efficiency (70% is a good rough number for electrical loss over short distances), we get:

.444hp/225hp = 1.97% x 1/.70 = 2.82% reduction of horsepower

225hp x 2.82% = 63.45hp lost simply because I had my radio playing.

Methinks something is wrong in these calculations. My head hurts too, now.

11

No way you were blowing 333w continuous. If you were, you’d be deaf.

The ratio between volume level and power is as such: a 3dB change in volume requires double the amount of power. 3dB is considered “noticeable”. To double the amount of volume you need a 10dB change. This requires 10 times as much power.

There are many variables (such as the car’s complex acoustics and the speakers’ efficiency) that make it very difficult (if not impossible) to calculate power output based on perceived volume. If you had connected a watt meter to your amp’s outputs, you’d probably find that you were running a continuous output of maybe 1-5 watts, with peaks at 10 times that.

However, keep in mind that amplifiers are not 100% efficient (it varies greatly depending on the type of amp, but there will at least some percent of power lost)

12

Yep a number of things are wrong with your calculations.

Your T/A isn’t using all 225 HP. Just because it can put out 225HP at 1800 RPM doesn’t mean it is. It means at 1800 RPM it is capable of producing 225HP. Example going down hill, the RPM is still 1800, put the power output is quite low.

There is a very non-linear connection between the position of the volume level and wattage output. Volume controls are normally audio taper variable resistors, which means turning it 60% doesn’t give 60% output. Kinda like your gas pedal. If your car can go 125MPH, and you want to go 62 MPH, you don’t press and hold the gas pedal half way down.

.44 can’t be 1.97% of 225, (since it’s less than one it must be less than 1% of a number greater than 100) it’s more like .247% (note: 1.97% of 225 is ~4.44)

Just because many a car radio is sold under interesting specs, If your car amp doesn’t have at least a 50 Amp fuse on the 12V input line, you’ll never get 500 Watts output. Assuming 80% power transfer which isn’t likely.

As I’m on a roll, Generators don’t have the same load regardless of power output, they get harder to turn as they generate more flow. Alternator’s don’t run at a constant speed, as thier hooked up to the crank. Nor do they generate a constant amount of electrical flow.

My thoughts on the topic are, The car radio does affect gas milage, not much, but it does. It’s additional power out of the system, gotta come from somewhere. Ahh I’ll stop there. Feel free to set me straight. I’m no engineer, just an old tech.

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Thanks frogstein & danvanf. I was oversimplifying.

Also, welcome danvanf! You’ll like it here.

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You guys have conveniently forgotten that huge energy sink we call a battery.

Some things that may be important:

An alternator creates AC, not DC.

An alternator produces uniform power once it gets above a threshold RPM, at which point RPM is no longer relevant.

Assuming the alternator is capable of producing more than enough power to charge the battery at idle (most modern cars), and the drain on the battery is less than the power required to charge it, the alternator is not having to produce extra power. Assuming some stop and go driving, use and non-use at different timews of accessories, there will be times when the charge depletion on the battery is greater than the charge addition from the alternator. A good example is an older car with a weak alternator and an old battery. Turn on the lights and the defroster and the engine bogs, the lights dim, etc.

But just a radio on a modern car(whoami has more than that, methinks) you shouldn’t be drawing more juice than the alternator provides to charge the battery. You could in fact be using so little that the times when you turn it off you allow a complete recharge, even at idle.

This also has a little to do with the way the circuit is designed, though. I wouldn’t put it past the European carmakers to mess this up completely :smiley:

15

Speaking from experience, my '84 Mustang had an Amp meter on the dash that would go left or right with the changing load and I did notice that certain accessories affected it. Radio: almost nothing. Headlights: big jump. Heater fan: big jump. Rear defroster: big jump. Once my alternator went out and I really had to watch that guage. I did notice that even then, running the radio didn’t matter much.

Sure the heat is practically free, but you still gotta get it into your car. That fan does take some juice.

If anyone else’s car has a ampmeter, maybe they could check it out and confirm this.

16

I saw a nifty demonstration with a hand-cranked generator about a year ago. Generator, switch, and light bulb, to be exact. With the switch in the ‘off’ position, the handle rotated freely. When ‘on’, however, it took a fairly decent effort to light the bulb.

Moving magnetic fields create electron motion, which is what makes generators and alternators function. But moving electrons create their own magnetic fields. In a generator, these fields repel, thus creating the mechanical resistance.

But guess what? Electrons don’t move through a radio which is turned off, so there’d be less resistance, and thus less work on the engine. As has been mentioned already, most car radios don’t need a whole lot of power to work, so the overall effect on gas milage is very tiny, but it does have an effect.

As to regulators, they convert AC to DC and regulate the voltage coming from the alternator, which can vary wildly.
The overall power from the alternator will depend on the RPM, to a great extent, but the regulator circuitry attempts to feed a little more than 15 volts through, yes, a so-called “12-volt” system. It’s the amperage, on the other side of the regulator, which changes. When you try to draw too many amps - more current than the alternator and regualtor can supply (from turning on the defroster, the blower, and the high-beams, while trying to light a cigarette) - then the voltage drops and everything dims out dramatically.

Even then, there will only be a noticable difference in engine performance if your battery (a good source for extra power when you need it) is low on charge.

I was in a car with a dead alternator once, at night. I suddenly realized the headlights appeared to be dim and yellow. I aksed the driver to turn on the high-beams, and there was absolutely no difference in the amount of light spread out of the road in front of us. I said “uh-oh.” Sure enough, about five mintues later the battery decided it couldn’t even keep the spark-plugs firing (at least not with the headlights on), and the car died. Turning the key was a joke (didn’t even get a click out of the solenoid), but the guy driving didn’t fully comprehend what had happened to his car. The most difficult part was explaining to him why the radio still worked when the lights were turned off (and the radio didn’t last very much longer).

Now that I think a little now, I saw an even better demonstration 16 years ago, at work. A double-E coworker had wired up a massive capacitor (with ceramic-insulted terminals, no less) with a buttload of diodes straight into a 110-volt outlet, and let it charge for a while. He’d made a big O of 6-guage copper wire (seven or eight loops), and connected one end to one of the terminals, the other bare end dangling about an inch from the other terminal. In the middle of the loop, he placed an empty Coke can.

He unplugged this contraption, put the shaft of a screwdriver on the second terminal, and slowly made contact with the bare end of wire from the coil. POW! Coke can hits the ceiling, then falls to the floor. Where it was closest to the wire, it was crushed to about a half-inch in diameter - wasp-waisted, if you will.

The massive magnetic field from the discharge of the capacitor set up an electric current in the can, which produced its own, similar magnetic field in the aluminum. The two fields repelled each other, and the thin aluminum was no match for the massive copper wire.

Anyway, there exist folks with car stereos capable of so much power they actually install a second alternator and regulator just to power the sound system (stock alternators tend to be capable of about 1,000 watts, total, I think).

Years ago, there was a radio contest in my area where the grand prize was a pickup truck with a 1,700-watt stereo system. Now, riot police sometimes find that 2,000 watts of sound blasted at a crowd in open air will drive everyone away because of the pain. I can’t imagine what 1,700 watts in a few-dozen cubic feet of enclosed cabspace would do…

But the bottom line really is, when there’s no work to do, electrically, then power supplies such as alternators and generators simply don’t do the work that they are capable of. No “power supply” does. A human pulling a rope can do so for many miles without breaking a sweat. Put just a 25-pound weight on the other end of that rope, though, and you’ll see quite a difference.

In other words, just because an alternator is capable of putting out 1,000 watts doesn’t mean it always does.

“My dream is of a day where every SDMB poster will have a quote of mine in their sig.” - Arnold Winkelried

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No, unless your definition of “wildly” is measured in millivolts per volt.

No, it depends on the load. Been through this at length in analysis of underdrive pullies for decreased power loss.

Yes, but now you are contradicting your previous statement about voltage varying wildly. An alternator puts out roughly 115 volts AC (domestic ones at least), at a current determined by the load. The “voltage regulator” that most people refer to on a car is actually a rectifier (usually a bridge type) and a regulator.

When you draw more power than the battery can supply, the load on the alternator increases because it has to make up for that excess drain in addition to charging the battery.

Absolutely. And I don’t have a problem with most of your post, although some of it got a little off-path (I am NOT the person who should point fingers on this, hahaha!).

Bottom line, when driving, you place different loads on the electrical system just as you do the fuel system. Over time a radio could but won’t necessarily affect milage, because it doesn’t necessarily consume power that would not have been produced.
(post edited to fix formatting of quotes)
[Edited by Arnold Winkelried on 03-30-2001 at 04:55 PM]

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As an electrical engineer, I thought I might throw a few comments in here:

To scott: A few things about alternators:
First, the output of the alternator does vary according to the RPM of the engine. This can be easily seen if you turn on the headlights and rev up the engine. The lights get brighter as the engine revolves faster. If you measure the electrical system with a volt meter, expect the DC voltage to jump by a volt or two as the engine RPMs increase. This increase in voltage does cause unregulated devices (like headlights) to draw more power. Car radios with simple linear power supplies will also draw more power as the engine revs faster, but higher power radios with switching regulators (needed for higher efficiency) will tend to draw the same amount of power no matter what the RPM of the engine, because they regulate themselves to a constant voltage internally.

By the way, this is also how to check to see if your alternator is busted. Put a meter on your battery and rev up the engine. If the voltage doesn’t change, something is broke. All too often, car mechanics will put the meter on and say “well, it’s above 13 volts, must be charging” not realizing that the battery voltage for a typical car battery is up around 13.6 volts. That 12 volt number assumes 1.5 volts per cell, which is not quite accurate. But, I digress.

Also, the frequency output of an automobile alternator varies along with the RPM of the engine. It is not a constant value, like you expect with home AC.

As long as the radio is on, it’s going to be drawing power. In fact, even when it is off, it will still draw power due to the clock (not true for old fashioned analog radios of course). I tend to believe the less than 1 percent calculations as a rough ballpark, and it’s going to be almost immeasurably low if all that is running is the digital clock. The 750W = 1hp conversion should make this fairly obvious, when comparing this to the typical hp output of an engine. A little 25 watt radio played at a low volume will draw significantly less power from the engine than a 500 watt radio blasting at full volume, so it should also be obvious that the effect on gas mileage varies depending on the type of radio also.

No matter how much or how little power the radio is drawing, though, it is always drawing power. There is no “free” engery in the car’s electrical system, even when it is sitting at idle. The power generated by the alternator depends on the voltage of the alternator (which varries with its rotation speed), and the current drawn by everything in the system (battery, radio, the little computer that controls the engine, lights, fuel pump, etc).

By the way, to DaveW, a 100 watt guitar amplifier with the speaker placed against a garage window can blow the window out (as proven by the guy I sold my guitar amp to about 10 years ago). That should give you an idea of what 1700 watts inside of an automobile cabin can do.

To zzupdown, the alternator is connected to a belt which connects to the main shaft of the engine, and thus it spins at a speed that is proportional to the overall speed of the engine. It is not constant. The amount of power it generates is determined by the voltage it generates (which varies a bit according to the speed of the engine) and the current draw, which is the sum of the currents drawn by every device in the system, as I mentioned above. For simple DC stuff, power is voltage multiplied by current.

If it wasn’t late on a Friday night, I’d inject a lot of cosine theta stuff, but I’m tired. Sorry Dex, no fancy equations from me this time around. Long equations with fancy greek letters are very important to engineers. Hence the old saying, if you can’t dazzle them with brilliance, baffle them with… well… never mind.

19

Hmmm, Scott…

My mistake, I should have said that the power available from the alternator varies with RPM. If the mechanical work isn’t going into it, the electrical power won’t be available coming out.

I don’t see how. I said that the power coming out of the alternator varies, and that the amperage coming out of the regulator varies. Two different, non-contradictory, things.

Now the first part I have a hard time believing, since I’ve never seen a transformer in an automotive voltage regulator - they’ve been using transistor regulators for quite some time. And in a transistor regulator, the voltage difference tends to be dispersed as heat, since the regulator represents a resistive load on the source (the alternator). Dropping from 115 volts to 15 is very extreme, and I’m surprised that regulators aren’t absolutely plastered in heat sinks. Especially since we’re talking about a lot of watts of power being delivered to the rest of the car, potentially (haha!).

IF, however, that 115 is a typo, and you meant 15, then there’d be no need for a regulator if the variance was on the order of millivolts. Just a rectifier would do, along with a capacitor to smooth out the ‘bumps’. If the power source is always at or near the proper voltage, a regulator is a waste.

Hopefully, I’ll have a few minutes to spare during daylight hours this weekend to take a multimeter out to the '98 Saturn’s eletrical system.

“My dream is of a day where every SDMB poster will have a quote of mine in their sig.” - Arnold Winkelried

20

Automobile electrical systems vary quite a bit in their design, but quite often the system relies on the battery for a great deal of the voltage regulation and filtering. Hence there is no capacitor in the circuit, and quite often the “regulator” is little more than just a diode rectifier.

115 VAC at 60 Hz is home AC, not automobile AC. An automobile alternator is not going to put out that high of a voltage, and as previously stated, the frequency is variable according to the rpm of the engine, not fixed.

Regulation is necessary since the output of the alternator varies according to the speed of the engine. There is more than a few millivolts of difference involved.