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Old 01-07-2015, 01:17 AM   #1 (permalink)
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Default Does My Car Stereo Amp Cause a MPG Hit?

Is there a way to quantify the effect my car stereo amp has on my MPG, if any? I have an OBDIICNC with ELD displayed, but I can't resolve the effect of the turning the amp on and off using that data.
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Old 01-07-2015, 01:26 AM   #2 (permalink)
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Where have you got the amp wired in?

Is the current it's using passing through the ELD?

Unless you are bouncing along the road due to the Bass drawing tens of amps constantly I doubt it has any significant effect.
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Old 01-07-2015, 02:06 AM   #3 (permalink)
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Originally Posted by retepsnikrep View Post
Where have you got the amp wired in?

Is the current it's using passing through the ELD?

Unless you are bouncing along the road due to the Bass drawing tens of amps constantly I doubt it has any significant effect.
I have a wire running to the + battery terminal from the amp which is under the passenger seat. My amp has a 15amp fuse so thats my upper limit. The average power will be much lower.

I just realized from reading your comment that the OBDIICNC probably has no way to register the power my amp is pulling.
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Old 01-07-2015, 03:49 AM   #4 (permalink)
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I'm pretty sure your setup would simply have the amplifier power draw show up as part of the 12V load. So, for example, if you were sitting at a stop light in autostop, pumping some serious bass, you might see Bam fluctuating a tiny bit. I guess if you had, say, a 20watt base load with 100watt peaks, it'd be something like 20w/144V to 100w/144V, or 0.14A to 0.70A Bam blips on top of the rest of the 12V load running at the time (which might be something like 0.5A, or maybe 1A, maybe up to 3.5A if you've got a bunch of stuff running)...

In general, I'm sure there's a way to quantify the 'mpg hit' based on the power the amplifier draws - convert the power to hp, do some other calculations to convert that to fuel economy. Beyond me. But it's probably very very small. I mean, take my 100w example above: 0.7A (off the IMA pack). The car regularly does 7A background charges and it's not very noticeable cruising along. Now divide that impact by 10 and you've got the impact of a 100w amplifier, I guess...
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Old 01-07-2015, 01:27 PM   #5 (permalink)
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When I have my double DIN radio running (large display lit up) I notice that the +12 volt battery drops a lot faster between charge voltage and no charge voltage, requiring the car to charge it more often, so it obviously take power, and that power ends up coming from gas.

Depending on the type of amp, it may draw a lot of power even when the volume is very low.

The best way is to put an ampmeter and see how much current it is drawing under typical operating levels.
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Old 01-07-2015, 03:38 PM   #6 (permalink)
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When I have my double DIN radio running I notice that the +12 volt battery drops a lot faster between charge voltage and no charge voltage, requiring the car to charge it more often, so it obviously takes power, and that power ends up coming from gas. Depending on the type of amp, it may draw a lot of power even when the volume is very low....
But you're not implying that that amount of power imposes a... 'substantial' hit to fuel economy, are you? I'm not claiming to be an expert here, nor do I have a great handle on power conversions between electric, gas, etc. Yet, my example of how the car does routine background charges of 7A+, i.e. about 1kW of power, with little noticeable impact on fuel economy, leads me to believe that the fuel economy impact of an audio amplifier, that probably has a lot smaller power draw, would be much less noticeable.

True, in both of these cases there has to be an impact, in an absolute sense, but it wouldn't be noticeable with the typical amplifier (100w, 250w?); it would probably be very difficult to measure even if you tried very hard... It'd actually be kind of interesting if someone could come up with some 'power conversions/equivalents and fuel economy' table: it seems like any 'load', whether electrical or mechanical or whatever - could be converted to horse power or something, and then you could use that to calculate some theoretical impact on fuel economy... Anyone know the formula, the steps to take to get there? It seems similar conceptually to a drag coefficient - so if someone knows how to calculate the loss in fuel economy due to wind resistance - a type of load or drag - then it seems like you could use a similar formula for a fixed load, like electrical power...

[edit] Here's a couple numbers that you'd probably use 'somewhere' to figure out fuel economy impact of 'X' wattage electrical load:

1kW=1.34hp
1.34hp=3.52 pound-feet of torque at 2000 RPMs...

That's all I got. The pound-feet of torque is probably what gets us closer to the actual calculation, as scanning over a couple things, it looks like 'drag' and such are expressed in lbs. Something on this page linked below might help; it has some formulas, explains things, calculating power required for top speed, incorporating air resistance and rolling resistance, and also mentions calculations for fuel consumption. It's not as transparent as it could be, though:
http://www.eng-tips.com/viewthread.cfm?qid=354919

Seems like there might be a really easy, straight-forward calculation, if we started with some known values for the Insight. Like, for example, 1kW=1.34hp, and 1.34hp is X percent of the total power being used; so if you impose a 1kW load (or 100w, or 250w or whatever), it simply represents an X percentage increase in gas power needed to overcome that load - and maybe it's the same percentage increase in fuel consumption, all else being equal?? You'd probably have to choose some conditions to do the calculation, for the impact at say Y speed and Z engine speed, to come up with only one value for those specific conditions. And then you'd need to do a bunch of calculations over varying conditions to get a real estimate... Something like that...
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Old 01-07-2015, 05:14 PM   #7 (permalink)
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I have a gen 2 and love loud music. If I listen to it in traffic and I will see a force recharge with the ima battery empty and ac wont work til it recharges.

When on the highway no sooner I turn the radio on I see the soc immediate go from 60% to 90%. I cant measure it, but it does seem to have an effect.



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Old 01-07-2015, 05:42 PM   #8 (permalink)
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I did this pretty lazily, but I like calcs like this so here it goes. Figure the Insight converts gas to electricity roughly as efficiently as a portable generator. The random one I picked had these stats: 14.4h run time to produce 2500W out of 8 gallons of gas. So 2500W * 14.4h / 8 gallons = 4500 Wh/gallon. The Insight may have a substantially different level of efficiency here, but let's figure this is +/- 30% (maybe??) which is probably plenty for our purposes.

Let's figure that you have an accessory that draws 100W average per hour. So it consumes 100 W / 4500 Wh/gal = 0.0222 gal/h. You're averaging 45 mph and 60 mpg in your Insight. 45 mph / 60 mpg = 0.75 gal/h for the car. 0.75 gal/h + 0.0222 gal/h = 0.7722 gal/h for car + accessory. 45 mph / 0.7722 gal/h = 58.27 mpg. You lost 1.73 mpg in this scenario. The slower your average speed or the higher your mpg the bigger the hit.
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Old 01-07-2015, 06:16 PM   #9 (permalink)
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Nice work. I found this little blurb on wikipedia - which includes a figure similar to yours, -1.7mpg for a 200 watt load:

"Overall, the sources of energy loss in moving a vehicle may be summarized as follows:.... Electrical systems. Headlights, battery charging....[etc.] can significantly increase fuel consumption, as the energy to power these devices causes increased load on the alternator. Since alternators are commonly only 4060% efficient, the added load from electronics on the engine can be as high as 3 horsepower (2.2 kW) at any speed including idle. In the FTP 75 cycle test, a 200 watt load on the alternator reduces fuel efficiency by 1.7 MPG.(41) Headlights, for example, consume 110 watts on low and up to 240 watts on high. These electrical loads can cause much of the discrepancy between real world and EPA tests, which only include the electrical loads required to run the engine and basic climate control....

"Fuel-efficiency decreases from electrical loads are most pronounced at lower speeds because most electrical loads are constant while engine load increases with speed. So at a lower speed a higher proportion of engine horsepower is used by electrical loads. Hybrid cars see the greatest effect on fuel-efficiency from electrical loads because of this proportional effect...."
Fuel economy in automobiles - Wikipedia, the free encyclopedia
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Old 01-07-2015, 08:14 PM   #10 (permalink)
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I know that turning the hvac fan on or off affects how often the 12 v charges, and headlights on/off are really noticeable. I assume that my radio is probably drawing 5 amps or so (just a guess, but the display is fairly bright, gps tracking location, etc). Getting 75-80 mpg in the summer there is a noticeable affect driving before dark or after dark. During the day I can very easily keep the ima topped off so it shuts off regen, at night I have to work to keep it topped off. That's without hvac, radio, etc running.
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