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  #1  
Old 06-07-2013, 10:23 PM
vwvwvw vwvwvw is offline
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Analysis- Fuel consumption at idle

Hey I did some experiments to measure how much fuel my 335i uses at idle. The reason is that I was curious how much fuel the auto start/stop routine actually saves over time. I thought the community might be interested in the results.

I measured fuel usage at idle by coasting at constant speed in neutral with my foot off the accelerator. I recorded fuel economy using the instantaneous MPG reading on the dash. The engine had been run for about 20 minutes and was at normal temperature. The air temperature was 68 degrees. The car is a 2013 335i 6MT with about 500 miles on it.

For the experiments I picked a road with a slight downhill so the car would be at roughly constant velocity. Then I waited for the MPG reading to stabilize and recorded its value. To find fuel consumption rate, an example calculation is:
10miles/hour / 37miles/gallon = 0.270 gallon/hour.

Here are the results:
AC off:
10mph / 37.0mpg = 0.270 gallon/hour
20mph / 74.1mpg = 0.270 gallon/hour
25mph / 88.5mpg = 0.271 gallon/hour

MAX AC on:
23mph / 48.5mpg = 0.474 gallon/hour

So at $4/gallon, for the AC off idling costs $1.08/hour. If you assumed 5 minutes of idling daily, the monthly cost of idling would be $2.70/month. For the AC on it's $4.74/month.

The data here is a bit rough, but it should give you an idea of idle fuel consumption and whether the auto start/stop feature is worthwhile to you.
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  #2  
Old 06-08-2013, 07:29 AM
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rdkind62 rdkind62 is offline
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I don't have this option on my current car but when I test drove one a couple of weeks ago I was pretty annoyed by the ASS. When I get my 435i I plan on just turning it off and never using it, and your data (if correct, which I have no idea, but I believe you) just gives me a better reason for not using it.
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Old 06-08-2013, 08:09 AM
Bratters123 Bratters123 is offline
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Analysis- Fuel consumption at idle

Great work with this test. A pity BMW don't have a gallons per hour readout on the OB computer like some other cars have. This F30 Forum is obsessed with ASS and EPS. I've driven BMW's in the UK with ASS for the last 6 years, both manual and auto. Your the driver and you have the choice at every single stop to allow it to engage or not. So why re-code?


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Old 06-08-2013, 08:24 AM
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Dave 20T Dave 20T is offline
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2-4 years ago, I saw a Ford Trucks website that gave a figure. I think it was something like 1 gallon per hour. At the time, it made sense to me as it was like a 20 mpg pickup truck traveling at 20 mph using as much gas as just idling.
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Old 06-08-2013, 08:25 AM
vwvwvw vwvwvw is offline
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I actually like the idea of ASS, especially with a manual. But it would be better if it defaulted off and the driver gave a signal to shut the engine off so it's not turning off at every light. For long lights it's good though.

But the fuel numbers above show she's not blowing through much fuel idling. It's just cool to sit with be engine off.
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Old 06-08-2013, 08:37 AM
Bratters123 Bratters123 is offline
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Analysis- Fuel consumption at idle

BMW quotes that the engine needs to be off for at least 4 seconds otherwise no fuel is saved.

It's not about the the volume of fuel you save it's the ***163;'s. Here in the UK fuel is far far far more expensive than the US.

I'd like to see a survey of the % of drivers who code to turn off in both countries. I bet it's considerably lees here in the UK.


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Old 06-08-2013, 08:39 AM
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Analysis- Fuel consumption at idle

Thanks for the Detailed MPG calculation!!!


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Old 06-08-2013, 08:51 AM
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Quote:
Originally Posted by vwvwvw View Post
I actually like the idea of ASS, especially with a manual. But it would be better if it defaulted off and the driver gave a signal to shut the engine off so it's not turning off at every light. For long lights it's good though.
I don't mind ASS, but I have 6MT. I just leave the clutch in if I think the light will change soon or when creeping up for right turns. I may have a different opinion if I had 8AT.
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Old 06-08-2013, 09:27 AM
Bratters123 Bratters123 is offline
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Analysis- Fuel consumption at idle

8AT is just as easy as 6MT. When the car comes to a halt put extra force on the brake pedal release slightly and reapply to prevent the car creeping forward.


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Old 06-08-2013, 09:54 AM
vwvwvw vwvwvw is offline
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Quote:
Originally Posted by Dave 20T View Post
2-4 years ago, I saw a Ford Trucks website that gave a figure. I think it was something like 1 gallon per hour. At the time, it made sense to me as it was like a 20 mpg pickup truck traveling at 20 mph using as much gas as just idling.
I got about a quarter gallon per hour (AC off). This was inline with general estimates for cars. Trucks with big displacement engines are probably more. 4x seems high though.
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Old 06-08-2013, 10:22 AM
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I drove the car in Germany after European delivery. During that time and during break-in, I plan to shut off ASS by pushing the button above the start button. In my current car, I turn it off at stoplights about 3-4 times a week, usually at a long stop light that lasts about a minute. BMW might say 4 seconds is the breakeven point, but I'd rather use gas than have additional wear so my breakeven point is about a minute for regular cars and 30 seconds for cars with an autostop/start feature, which presumably have some beefed up components. I am pondering about having it recoded.
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Old 06-08-2013, 11:08 AM
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The idea of ASS is for global savings of gas and environment. So for the individual - eh- but if everyone has it then that is huge savings. The problem with ASS is it's technology that works great with hybrids, but not so much with motor only engines (due to the negative impacts on the engine starter), plus the delay in start (which will only get worse over time).
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Old 06-08-2013, 12:22 PM
HugH HugH is offline
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Quote:
Originally Posted by Bratters123 View Post
Great work with this test. A pity BMW don't have a gallons per hour readout on the OB computer like some other cars have. This F30 Forum is obsessed with ASS and EPS. I've driven BMW's in the UK with ASS for the last 6 years, both manual and auto. Your the driver and you have the choice at every single stop to allow it to engage or not. So why re-code?


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Cause I kept forgetting to disable the darn thing as soon as I got it started and again and when re-starting. My wife was afraid she would get rear-ended in lights that just turned
green as she just came to a stop. She was also worried about getting across a wide avenue from a stop due to the delay of the re-start!

She stopped driving the car until we had it re-coded. I can't remember anything I have ever found in a car as annoying as the ASS implementation in the 328i. To me it's inexcusable for a 40 thousand plus car to shudder the way this car does at every re-start!

Last edited by HugH; 06-08-2013 at 12:26 PM.
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Old 06-08-2013, 09:12 PM
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For me, it's a non-issue. I turn off ASS when I start my car, along with setting up sport mode and turning off DSC, turning on the lights so I can better see the dash, locking the doors, and dealing with the annoying legal disclaimer. I'm driving a BMW and I'm ready to go and have fun.
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Old 06-09-2013, 08:13 AM
jwalz1 jwalz1 is offline
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I don't have my car yet, but my order is in. Not sure if they have refined the system at all, but the literature I read when the 2012's were released to the press was horrible. I drove a 2013 and while it was weird when the engine went off the first few times, I did not find it to be that big of a deal. Maybe the early models were worse? It started smoothly, but I pull away from a light normally and am not gunning it or anything. I removed my foot from the brake and the engine came on with just a hint of a shudder and not that big of a deal. This was in a 335i. My daily commute is generally all highway, but I sit at two long lights everyday so I will probably use it. One is probably two minutes every day. Not concerned about wear on the starter, I will move on to a new car long before that becomes and issue, although I will have the dealer reprogram the car to default to last setting so if I have turned it off and set to sport mode, it stays until I change it back.

I am not disputing your calculation but here is another way I would do it based on BMW claims, and yes this is based on a "claim" and not a measurement but I am not sure your method is 100% either because the dash reading is not perfect.

BMW claims the feature saves 1-2 miles per gallon, I will use 1.25 miles per gallon to be on the low side of the middle, and I ordered a 328i. Using the Car and Driver and Motor Trend long term test average mileage of ~ 27.5 miles to the gallon adding 1.25 to that yields a savings of about 4.5%. If the average driver does about 12,000 miles per year, 1000 per month divided by 27.5 miles per gallon equals 36.36 gallons per month. OR with the ASS on it would be 1000 divided by 28.75 or 34.78 gallons per month. At the same $4 per gallon the savings would be $6.32 per month or about $76 a year.

If you average less than 27.5 mpg your savings are greater, if the price of gas is higher your savings are greater, and if you drive more than 12,000 miles per year your savings would be greater as well.

Now saving $76 a year might mean dick to most BMW owners I still plan to use the feature. I would probably turn it off in stop and go traffic, but for long lights it does not bother me at all, like that it saves gas just for the sake of burning it with no forward motion does nothing for me. I think people like to nitpick it and find reasons to pee all over the feature, but it aint that bad. Its nice to have, use it when you want to and save a bit of pollution and turn it off when it gets on your nerves.

Last edited by jwalz1; 06-09-2013 at 08:22 AM.
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Old 06-09-2013, 08:26 AM
HokieXDriver HokieXDriver is offline
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Quote:
Originally Posted by jwalz1 View Post
I still plan to use the feature. I would probably turn it off in stop and go traffic, but for long lights it does not bother me at all, like that it saves gas just for the sake of burning it with no forward motion does nothing for me. I think people like to nitpick it and find reasons to pee all over the feature, but it aint that bad. Its nice to have, use it when you want to and save a bit of pollution and turn it off when it gets on your nerves.
I agree. Couple of points.

First, I and a few other forum members discussed having adaptive ASS that would turn itself off if it detected stop and go traffic. I'm finding that it seems to do that anyway. If you restart the car within a second or so of it turning off, ASS turns off and the Efficient Dynamics screen says, "Auto Start/Stop off - driver request". It then seems to turn back on after a time interval.

Second, I'm going even further and using Eco Pro more often. Metro DC has terrible traffic and I frequently find myself in slow moving traffic. I just switch to Eco Pro. It seems just fine, especially since my car has the M Performance Power Kit. It feels like a Nissan Sentra. Slow, but quite adequate for that kind of traffic.

I just had my ASS recoded to OFF by default. I figure if I need it I will switch to Eco Pro (which turns ASS on no matter what) and save even more gas than just ASS alone.

I like having all these choices. Dynamic Drive Control is great.
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  #17  
Old 06-28-2020, 03:33 PM
Autoputzer Autoputzer is online now
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You can measure fuel consumption at idle by resetting the MPG and trip odometer at fill-up. From the MPG and trip miles, you can calculate the amount of fuel used since fill-up. Then measure the time it takes the MPG to drop. Assume the MPG is rounded, e.g. the instant MPG goes from 25.4 to 25.3 MPG, the actual MPG is 25.35. From the fuel used since fill-up at the beginning of an idle period and the fuel used since fill-up at the end of the idle period, you can calculate the fuel used when idling. From the elapsed time, you can calculate the fuel burn rate at idle.

My data's similar to the OP's:
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Old 06-28-2020, 05:22 PM
Michael Schott Michael Schott is online now
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You do realize you bumped a 7 year old thread.
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