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Old 08-24-2012, 02:53 PM   #1
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Default Fuel Economy: AFR vs. RPM?

Situation: (numbers rounded) While I am cruising around town at 40mph: In 4th gear I am pulling ~3000 RPMs averaging a 15.5 AFR. In 5th gear I am pulling ~2500 RPMs averaging a 14.5 AFR.

Question: Which scenario will yield better fuel economy and why? Will a 1 point leaner AFR yield better fuel economy compensate for a 500 RPM increase?

Also, yes I know I could simply tune my VE table, but I'm curious.
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Old 08-24-2012, 02:56 PM   #2
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I do not know the answer to your question but your avatar infuriates me. You should change it.
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Old 08-24-2012, 02:57 PM   #3
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Why? My special juice is gonna help me wiiiiin!
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Old 08-25-2012, 11:49 AM   #4
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Originally Posted by palmtree View Post
Situation: (numbers rounded) While I am cruising around town at 40mph: In 4th gear I am pulling ~3000 RPMs averaging a 15.5 AFR. In 5th gear I am pulling ~2500 RPMs averaging a 14.5 AFR.

Question: Which scenario will yield better fuel economy and why? Will a 1 point leaner AFR yield better fuel economy compensate for a 500 RPM increase?

Also, yes I know I could simply tune my VE table, but I'm curious.
Why not tune it for 15.5 at 2500 rpms and win at MPG/life?
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Old 08-25-2012, 12:36 PM   #5
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Why tune for AFR vs RPM? I tune for AFR vs Kilopascals...and I taper it depending on RPM.
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Old 08-27-2012, 03:00 AM   #6
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Why not tune it for 15.5 at 2500 rpms and win at MPG/life?
Like I said in the OP, I know you can just tune fuel, but I'm just curious.
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Old 08-27-2012, 03:04 AM   #7
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I do not know the answer to your question but your avatar infuriates me. You should change it.
Eff this guy. Your avatar is one of my favorites on this forum!
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Old 08-27-2012, 03:04 AM   #8
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The latest update on the Enhanced MS2 ECUs will spit out MPG readings thanks to vehicle speed, so someone may answer that question soon!
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Old 08-27-2012, 03:42 AM   #9
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Going leaner, even with spark advance, doesn't always mean better fuel economy. I played with this for a few hundred miles and found that 15.4 and 34* of spark was the happy place for my daily.
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Old 08-27-2012, 08:10 AM   #10
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look at the problem more like a fuel usage rate than an MPG and you'll answer your own question.

you know the size of the injector
you know the squirt length
you know the rpm

you can calculate the duty cycle and estimate the rate of fuel use.
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Old 08-30-2012, 03:50 PM   #11
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Originally Posted by hustler View Post
Going leaner, even with spark advance, doesn't always mean better fuel economy. I played with this for a few hundred miles and found that 15.4 and 34* of spark was the happy place for my daily.
Same thing with my car. Iím using E85 and didnít find any difference between 15.4 and 16.5. But in the dyno we see torque go down after 15.4 so I leave it to that and 36-38* ign. Advance.
With E85 23 MPG is best I can get with my 9.5:1 pistons. I still save a lot if you consider that 98RON gasoline cost here 6.8$/gallon and E85 = 3.7$
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Old 08-31-2012, 05:51 PM   #12
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Quote:
Originally Posted by palmtree View Post
Will a 1 point leaner AFR yield better fuel economy compensate for a 500 RPM increase?
Neither.

Both.

Either.

Insufficiant data is available to make this determination.


In an "ideal" world (eg, one populated by frictionless, spherical cows radiating milk isotropically in a vacuum) the engine operating at the slightly lower RPM would run more efficiently, but only by the extremely trivial amount attributable to internal friction and pumping losses.


All else being equal, the amount of power required to propel the car down the road is a constant for any given vehicle speed. And the amount of power being generated by the engine is a constant for any given mass of fuel flow per second. If it takes 30 HP to propel the car at 65 MPH, then you're going to be flowing exactly enough fuel (in lbs/hr) to produce 30 HP.

The factor which is unaccounted for in the OP is manifold pressure, or in layman's terms, how hard you're pushing down on the throttle.

At lower RPM, you're going to have your foot further into it, flowing more air and injecting more fuel PER ENGINE REVOLUTION than you would be at higher RPM. And at a leaner mixture, you'll be flowing more units of air PER UNIT OF FUEL than you would at a richer mixture.


But in either case, if we assume that complete combustion is occurring and the engine is operating with equal efficiency in both conditions, neither RPM nor AFR will meaningfully affect fuel economy for a given load.


Of course, in the real world, cows are neither spherical nor frictionless, and they do not radiate milk isotropically.
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Old 09-01-2012, 04:05 PM   #13
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Originally Posted by Joe Perez View Post
Neither.

Both.

Either.

Insufficiant data is available to make this determination.


In an "ideal" world (eg, one populated by frictionless, spherical cows radiating milk isotropically in a vacuum) the engine operating at the slightly lower RPM would run more efficiently, but only by the extremely trivial amount attributable to internal friction and pumping losses.


All else being equal, the amount of power required to propel the car down the road is a constant for any given vehicle speed. And the amount of power being generated by the engine is a constant for any given mass of fuel flow per second. If it takes 30 HP to propel the car at 65 MPH, then you're going to be flowing exactly enough fuel (in lbs/hr) to produce 30 HP.

The factor which is unaccounted for in the OP is manifold pressure, or in layman's terms, how hard you're pushing down on the throttle.

At lower RPM, you're going to have your foot further into it, flowing more air and injecting more fuel PER ENGINE REVOLUTION than you would be at higher RPM. And at a leaner mixture, you'll be flowing more units of air PER UNIT OF FUEL than you would at a richer mixture.


But in either case, if we assume that complete combustion is occurring and the engine is operating with equal efficiency in both conditions, neither RPM nor AFR will meaningfully affect fuel economy for a given load.


Of course, in the real world, cows are neither spherical nor frictionless, and they do not radiate milk isotropically.
This is what I was looking for. I knew that in some way engine speed is associated with fuel consumption, but I wasn't sure about the intricacies of it. Thank you for a detailed explanation
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Old 09-01-2012, 04:56 PM   #14
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The key point to take away from this is simply that, for any given load condition, running at a leaner AFR does NOT automatically mean that you are burning less fuel. It may simply mean that you are flowing more air than is necessary.


As an aside, I hate your avatar. It simultaneously freaks me out and diminishes my overall optimism towards the future of humanity. And while I see that Ryan_G has already expressed a similar sentiment, the difference between the two of us is that I have the power to change your avatar to something really embarrassing.
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Old 09-01-2012, 05:00 PM   #15
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lower PW = better fuel economy. /thread.
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Old 09-01-2012, 05:06 PM   #16
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As an aside, I hate your avatar. It simultaneously freaks me out and diminishes my overall optimism towards the future of humanity. And while I see that Ryan_G has already expressed a similar sentiment, the difference between the two of us is that I have the power to change your avatar to something really embarrassing.
Well since I respect your authoritaaay and under threat of an even more embarrassing avatar, I will find something else.
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Old 09-01-2012, 05:17 PM   #17
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Quote:
Originally Posted by hustler View Post
lower PW = better fuel economy.
Not when RPM is a variable.

Consider an engine turning at 3,000 RPM with a fuel pw of 14ms per injector per cycle. (We'll handwave over injector latency and dead-time.) Total combined injector duration will be 21,000 ms per injector per minute.

Now, decrease the fuel pw to 11ms, but also increase the engine speed to 4,000 RPM. Total combined injector duration is now 22,000 ms per injector per cycle.

PW went down, but total fuel consumption went up.


If you want a quick-n-dirty metric, injector duty cycle is what you need to look at. In the first example, the duty cycle is 35%, whereas in the second (the one with the lower PW) duty cycle increased to 36.7%.

I'm just making these numbers up, but they illustrate how PW alone is not the sole determining factor in total fuel flow, as it fails to take into account the total number of injector events per unit time.
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Old 09-01-2012, 05:19 PM   #18
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Quote:
Originally Posted by Joe Perez View Post
Not when RPM is a variable.

Consider an engine turning at 3,000 RPM with a fuel pw of 14ms per injector per cycle. (We'll handwave over injector latency and dead-time.) Total combined injector duration will be 21,000 ms per injector per minute.

Now, decrease the fuel pw to 11ms, but also increase the engine speed to 4,000 RPM. Total combined injector duration is now 22,000 ms per injector per cycle.

PW went down, but total fuel consumption went up.


If you want a quick-n-dirty metric, injector duty cycle is what you need to look at. In the first example, the duty cycle is 35%, whereas in the second (the one with the lower PW) duty cycle increased to 36.7%.

I'm just making these numbers up, but they illustrate how PW alone is not the sole determining factor in total fuel flow, as it fails to take into account the total number of injector events per unit time.
I always do this type of tuning on cruise control so RPM is static, this makes sense though.
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Old 09-01-2012, 05:20 PM   #19
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Done
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Old 09-01-2012, 05:22 PM   #20
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I always do this type of tuning on cruise control so RPM is static, this makes sense though.
Yeah, I follow. I just brought it up since Tina Turner (or whoever the hell that is in Palmtree's new "let's taunt the angry, drunk moderator by changing our avatar from something really disturbing to something even more disturbing" avatar) had specifically mentioned operating at different RPMs in the original post.
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