Does engine RPM affect gas mileage?


by ShawnD
Tags: affect, engine, mileage
Cyrus
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#55
Mar28-09, 07:40 PM
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The chart I gave is only valid for the engine that made it. But it gives you a general idea of whats going on.
turbo
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#56
Mar28-09, 07:53 PM
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Quote Quote by Pythagorean View Post
Cy, turbo,

I was also asking about the gasoline and standard aspect.

For instance, would you use the same chart for a diesel or rotary engine? It says in the subscript that it's for a V-8 engine. Is it much different for a V-6 or a straight six?

my intuition tells me it should be different for a diesel engine, but I really have no idea.
It can be very different, Pythagorean. The firing angle can affect the efficiency tremendously, and that depends greatly on the physical configuration of the engine. The vibration and long-term wear posed by unbalanced firing angles can be quite detrimental to engine life.

Ranger Mike addressed some of these variations here:

http://www.physicsforums.com/showthread.php?t=292286

If you have a well-balanced engine firing at at well-controlled intervals, you can make them lighter and more powerful than competing designs, and they will last longer. I have a lot of experience with H-D engines and since they are staggered 2-cylinders, they tend to lope at some points. Still, they are fun to tweak. I could never launch off the line like the crotch-rockets, but when I could sell a 10-year-old bike for several thousand more than I paid for it after making years of incremental improvements, it was pretty nice.
Pythagorean
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Mar28-09, 07:59 PM
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Quote Quote by turbo-1 View Post
It can be very different, Pythagorean. The firing angle can affect the efficiency tremendously, and that depends greatly on the physical configuration of the engine. The vibration and long-term wear posed by unbalanced firing angles can be quite detrimental to engine life.

Ranger Mike addressed some of these variations here:

http://www.physicsforums.com/showthread.php?t=292286
wow, that's a pretty intense packet of information!
turbo
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#58
Mar28-09, 08:13 PM
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Quote Quote by Pythagorean View Post
wow, that's a pretty intense packet of information!
He's got some practical application knowledge under his belt, and is a pretty reliable source. I have tweaked bikes much more than autos, and I gladly accept his advice on the latter. The principles are the same - the applications differ little in practice, though sometimes it's nice to have a lower-mass vehicle to tweak.

I have a friend (local guy) who has captured the US drag-racing title in his category. Darn! Who'd have thought that a 340 Duster could show you its oil pan off the line and beat out the competitors to the traps?
PA32R
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#59
Mar29-09, 11:34 AM
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Quote Quote by russ_watters View Post
Just to clarify, when you say "rolling resistance", is that where the tires meet the ground only or does it include all drive losses?
Only tires meeting pavement. It treats the vehicle as a "black box."
physical1
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#60
May29-09, 11:22 PM
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Quote Quote by PA32R View Post
Yes, I'm sure I re-opened it. I stumbled on the thread by querying something or other in Google, and when I read the reply from Integral, I couldn't leave it alone. The conclusions (all else being equal, lower rpm's and relatively slow acceleration are better for fuel economy) were generally correct but his rationale for both cases was flawed.

Sorry if I've transgressed.


A lot of people get testy when old threads are re-ignited and I have no idea why. It would be like claiming that Newton or Faraday is obsolete and should not be brought up any more.
physical1
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#61
May29-09, 11:25 PM
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At certain RPM's if the car is tuned well there is an effect from the exhaust which improves the mileage/performance of the car similar to a turbo charger. It is called the scavenging effect where a pulse wave sucks air into the engine on the intake side, all the way from the exhaust side while the valves are open at the right positions. Engine tuning books have information on this. Therefore, RPM is not always directly related to mileage.

That being said, I have found with the majority of cars that keeping my foot on the gas peddle as absolutely little as possible, accelerating really slowly, and keeping RPM's at about 1800-2100 on highway (85-95km/h) does improve mileage significantly. Going below 1800 RPM usually lugs the engine and I find my foot becoming heavier to maintain speed.

I have read going above 85-95km/h starts to cost more mileage due to air resistance also.
ShawnD
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#62
May30-09, 02:09 AM
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Quote Quote by physical1 View Post
I have read going above 85-95km/h starts to cost more mileage due to air resistance also.
Rule of thumb given in driving class was about 10% drop every 10km/h.


It's nice to see people are still thinking about this kind of thing. I still own the car I had when I started this thread and it still seems to get very consistent mileage regardless of what I'm doing. Last fillup was roughly 48mpg UK (40mpg US). All city miles during off hours (no rush hour driving). Pedal right to the floor until I get up to speed, then set cruise control
Buzzworks
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#63
Jan10-11, 09:33 PM
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I drive a '97 Toyota Starlet automatic(with a home-brewed cold air intake, anyway..). Even though it's automatic, you are actually still much in control of engine rpms by how you use the throttle and switching overdrive on/off.

I drive to and from school a total of 40 miles a day and here's my observation: It doesn't matter to the mpg whether I'm driving like a hormone-crazed teen(keeping high rpms by flooring it) or driving like an old lady(the previous owner of my car is one!).

What does seem to matter to mpg is the speed I chose to cruise at the motorway(freeway, Interstate, w/e). Some of your got this correct!

Also keep in mind, that car engines are usually most efficient at their peak torques when the Helmholtz Resonance is in full effect (in a beneficial manner). Peak torques of gas engines for cars are by no means low rpm. They are usually 4000 rpm or higher.

Helmholtz Resonance is btw, like a 'free boost' to your engine without added fuel consumption so it improves efficiency. It is good for efficiency, however, since it's at a relatively high rpm, it can increase wear on the engine if you always to try to run at peak torque rpm.
Office_Shredder
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#64
Jan10-11, 11:23 PM
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Quote Quote by ShawnD View Post
Last fillup was roughly 48mpg UK (40mpg US).
Is this some exotic relativity effect that I'm unaware of, or are gallons just smaller in the US?
Kurdt
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Jan11-11, 07:13 AM
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US gallons are smaller.
FlexGunship
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#66
Jan11-11, 08:33 AM
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Quote Quote by ShawnD View Post
That's not true at all. In third gear I can go 50km/h when barely touching the gas pedal. To maintain a speed of 50km/h in fifth gear, I need to floor it. lower rpm * more fuel per ignition = similar amount of fuel being burned.
Man, what car do you drive?! From 1st to 6th, my car is always trying to hit 120mph.



I get significantly better mileage by modulating pedal use in higher gears. I have an engine-tuner that feeds back realtime injector pulse-width data. By looking at the PWM signal and figuring in the RPM at the time it can also display fuel consumption on a "per mile" basis.

To my knowledge, this is the most accurate possible way to find fuel mileage and I see a difference between 55mph in 5th and 55mph in 6th. I can't possibly call it a scientific test, but I get about 16mpg in 5th and 19-10 in 6th. That's NOT trivial.

Furthermore, I also know at which speed my car is most fuel efficient. Sadly, it turns out to be about 75-80mph.
FlexGunship
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Jan11-11, 08:35 AM
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Quote Quote by Cyrus View Post
I don't understand what you mean by 'loading the map you want'. The map decribes a physical system. For x RPM and x PSI you NEED y fuel flow.
Engines often run in non-stoichiometric configurations. Check the voltage of your O2 sensor and you'll find that... WOAH... it's not always giving you a 14.7:1 ratio!! Honda is famous for running their engines super lean at low RPM and low load. This allows them to get crazy gas mileage numbers during highway driving.

EDIT: and I totally load different maps. At the track I have one that keeps the engine rich and ready for hard driving. For daily driving I switch back to something closer to stoichiometric, and for winter (i.e. now) I have a fuel-miser map (because I don't do a lot of hard driving in the winter).
1MileCrash
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#68
Jan11-11, 11:06 AM
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I get up to 29 MPG average in my 'vette by using cruise control. If I make a say, 30 mile drive, very robustly, I'll be lucky if my average doesn't fall below 17. It absolutely makes a difference.
Jasongreat
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#69
Jan11-11, 02:30 PM
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The easiest way, ime, to get the best mileage in a gasser is to install a vacuum gauge, the lower you can keep the vacuum the better the mileage. In a diesel, keeping the pyrometer at the lowest number you can, will do the same thing.
mugaliens
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#70
Jan11-11, 02:50 PM
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Quote Quote by Jasongreat View Post
...keeping the pyrometer...
"pyrometer?"

Sorry, but that's not in the SAE manual.
mugaliens
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#71
Jan11-11, 03:00 PM
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Quote Quote by 1MileCrash View Post
I get up to 29 MPG average in my 'vette by using cruise control.
And I averaged 47 mpg in my 1979 VW diesel. Back in 1979. That was more than thirty years ago for those who can't count.

We're fooling ourselves if we think we're somehow making "progress" on this front. Please stop fooling yourselves. That milage is better than what you'll enjoy from a Toyota Prius.

If you want to make progress, step into this.

Grow a brain, folks! Here on PF, we actually HAVE brains, so please use them, and let's begin with 100 mpg or better, ok? Thanks. We owe it to both ourselves as well as the other several billion.
Jasongreat
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#72
Jan11-11, 03:01 PM
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I guess I will have to go take the one out my truck cause its not in the SAE manual then. On a diesel, measuring the heat of the exhaust gasses tells you how much fuel is being injected. Higher the temp, the more fuel you are burning.


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