Does engine RPM affect gas mileage?

[skeptic2]

The drag at low speeds is very small.

I thought we were talking about comparing gas mileage while going the same speed but in different gears.

 

[PA32R]

The drag at low speeds is very small.

A plot of the comparative contribution of rolling resistance and aerodynamic drag at 0 to 40 meters/second (about 90 m.p.h.) for my vehicle. Note that this only shows the percentage of total external resistive force contributed by each component, i.e., it doesn't show absolute numbers. But it's easily seen that at low speeds almost all resistance is from the tires.

For my vehicle, the two resistive forces are equal (i.e., the curves cross) at about 50 m.p.h.

 

[Cyrus]

A plot of the comparative contribution of rolling resistance and aerodynamic drag at 0 to 40 meters/second (about 90 m.p.h.) for my vehicle. Note that this only shows the percentage of total external resistive force contributed by each component, i.e., it doesn't show absolute numbers. But it's easily seen that at low speeds almost all resistance is from the tires.
For my vehicle, the two resistive forces are equal (i.e., the curves cross) at about 50 m.p.h.

Yup.

 

[Cyrus]

I thought we were talking about comparing gas mileage while going the same speed but in different gears.

I have no idea, I just gave a graph to curb speculation.

 

[PA32R]

I thought we were talking about comparing gas mileage while going the same speed but in different gears.

That was how it started back in 2007, but Integral brought in the idea of how rate of acceleration affects gas mileage.

 

[russ_watters]

A plot of the comparative contribution of rolling resistance and aerodynamic drag at 0 to 40 meters/second (about 90 m.p.h.) for my vehicle.

Just to clarify, when you say "rolling resistance", is that where the tires meet the ground only or does it include all drive losses?

 

[Pythagorean]

It's amazing what's contained inside of books...

http://img8.imageshack.us/img8/396/curvem.png

Every one done speculating? Scanned for his and her pleasure.

does this just apply to an non-computerized standard gasoline engine or what?

 

[Cyrus]

does this just apply to an non-computerized standard gasoline engine or what?

Why would it be any different?

 

[turbo]

Why would it be any different?

It could be very different. Modern computer-controlled engines can be programmed to do all kinds of neat tricks. Shortly after I bought my Softail, I ordered a custom exhaust (better scavenging, but not excessively loud), a custom air filter (less restriction) AND I ordered a Power Commander. That's a programmable plug-in computer that can tweak all kinds of things for you. You can tweak the bike for optimum mileage, optimum torque under load, and all kinds of other things that you want. You specify the custom equipment that you have, and choose from a variety of maps. You might want maximum torque under load for 2-up riding, you might want a broader power-band, or perhaps the best fuel economy. That last one is a big deal when you are riding a Sportster through long rural rides with no gas stations. Their fuel tanks are kind of puny.

 

[Cyrus]

It could be very different. Modern computer-controlled engines can be programmed to do all kinds of neat tricks. Shortly after I bought my Softail, I ordered a custom exhaust (better scavenging, but not excessively loud), a custom air filter (less restriction) AND I ordered a Power Commander. That's a programmable plug-in computer that can tweak all kinds of things for you. You can tweak the bike for optimum mileage, optimum torque under load, and all kinds of other things that you want. You specify the custom equipment that you have, and choose from a variety of maps. You might want maximum torque under load for 2-up riding, you might want a broader power-band, or perhaps the best fuel economy. That last one is a big deal when you are riding a Sportster through long rural rides with no gas stations. Their fuel tanks are kind of puny.

I don't see how reporgramming your engine fuel module changes the chart one bit. All you do is choose to follow a different curve from the one that says 'optimal'.

That chart says for a given pressure and RPM, the engine needs x amount of fuel to operate. Reprogramming it won't change that fact.

I could be reading that chart wrong, but I don't think I am. I'm no auto expert though.

 

[turbo]

I don't see how reporgramming your engine fuel module changes the chart one bit. All you do is choose to follow a different curve from the one that says 'optimal'.

That chart says for a given pressure and RPM, the engine needs x amount of fuel to operate. Reprogramming it won't change that fact.

It does change the chart, though. You can increase or reduce fuel consumption at a given RPM by loading the map you want. The module also controls the advance of the electronic ignition. You have to be a good troubleshooter to use them properly, including knowing how to properly shut down at speed, coast to a stop and "read" your plugs. It's easy with fuel injection to create either excessively rich or lean conditions, neither of which are desirable. Of course, you could do that with carbs, too, with improper jetting and adjustment, but these plug-in modules make it easy for newbies to screw up their bikes.

 

[Cyrus]

It does change the chart, though. You can increase or reduce fuel consumption at a given RPM by loading the map you want. The module also controls the advance of the electronic ignition. You have to be a good troubleshooter to use them properly, including knowing how to properly shut down at speed, coast to a stop and "read" your plugs. It's easy with fuel injection to create either excessively rich or lean conditions, neither of which are desirable. Of course, you could do that with carbs, too, with improper jetting and adjustment, but these plug-in modules make it easy for newbies to screw up their bikes.

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. You can't simply 'load a new map' and magically change this value. It's inherent to the engine itself. The only way to change the map would be to physically modify your engine.

Why don't I simply load a map that has no fuel flow ever and be done with it? -because that's nonphysical.

 

[russ_watters]

I would think that variable ignition timing would be a good way to improve performance and change that map. Still, I don't think it is something you would want to turn on and off. If you can get better performance, why turn it off?

 

[turbo]

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. You can't simply 'load a new map' and magically change this value. It's inherent to the engine itself. The only way to change the map would be to physically modify your engine.

Why don't I simply load a map that has no fuel flow ever and be done with it? -because that's nonphysical.

Ask a mechanic, Cy. The maps are available to change all kinds of performance/efficiency parameters. I bought the module because I understand ICE technology and how to troubleshoot them, and I didn't want to pay the H-D place hundreds to reprogram the stock module every time I wanted to try something different. You can change a LOT of stuff within the design parameters of the engine. Change fuel flow at certain RPMs and you have changed consumption. Change EI advance, and you have changed some more parameters. Did you know that modern cars are fitted with throttle-position sensors? That's so the computer will know the difference between the acceleration you are demanding (with the accelerator pedal) and what you are getting, so it will (according to the map) adjust fuel flow and ignition timing accordingly. For instance, your ignition timing might be advanced during hard acceleration and retarded somewhat when you are cruising at-speed. There's nothing sacred about that chart, though it is a nice graphic on how your automatic transmission should shift to optimize fuel economy.

 

[Cyrus]

Ask a mechanic, Cy. The maps are available to change all kinds of performance/efficiency parameters. I bought the module because I understand ICE technology and how to troubleshoot them, and I didn't want to pay the H-D place hundreds to reprogram the stock module every time I wanted to try something different. You can change a LOT of stuff within the design parameters of the engine. Change fuel flow at certain RPMs and you have changed consumption. Change EI advance, and you have changed some more parameters. Did you know that modern cars are fitted with throttle-position sensors? That's so the computer will know the difference between the acceleration you are demanding (with the accelerator pedal) and what you are getting, so it will (according to the map) adjust fuel flow and ignition timing accordingly. For instance, your ignition timing might be advanced during hard acceleration and retarded somewhat when you are cruising at-speed. There's nothing sacred about that chart, though it is a nice graphic on how your automatic transmission should shift to optimize fuel economy.

This is exactly what I'm talking about. If you change the fuel flow at a given RPM, you will change the PSI on the y-axis. Once you pick an RPM and PSI value, you are stuck with the fuel flow rate. What you are doing is when you reporgram it is to use a non-optimal curve along the map of the engine. I don't see how you are changing the map itself.

 

[turbo]

You're missing the point, Cy. You can easily run engines over-rich or over-lean at any RPM (with possible detrimental effects to fuel-consumption and/or engine life). The idea that an engine MUST consume a certain amount of fuel at a certain RPM regardless of load, ignition timing, etc is a gross over-simplification. I've been tweaking ICEs for about 40 years now. If you don't believe me, ask a mechanic.

When I bought my old Wide-Glide, I was getting about 40-45 mpg. After fitting it with better-scavenging pipes, low-restriction air filter, high-flow petcock and filter, and rebuilding the S&S Super E racing carb with a Yost Power-tube for improved atomization, I could get 50+ mpg riding through the mountains 2-up, and the bike ran like a scalded cat. Many of the parameters that I had to adjust manually on that bike can be tweaked through mapping control modules these days. BTW, getting reasonable low-speed performance out of a large-bore butterfly carb is somewhat of an art, which is why you see many older modded H-Ds smoking on acceleration at low RPM. Air-speed across the main-jet venturi is insufficient to atomize the fuel properly, so it is not burned completely. Nothing wrong with the design - just poor skills on the part of the person who tuned the bike and thinks that oversized jets are the answer for performance gain.

 

[Cyrus]

You're missing the point, Cy. You can easily run engines over-rich or over-lean at any RPM (with possible detrimental effects to fuel-consumption and/or engine life). The idea that an engine MUST consume a certain amount of fuel at a certain RPM regardless of load, ignition timing, etc is a gross over-simplification. I've been tweaking ICEs for about 40 years now. If you don't believe me, ask a mechanic.

When I bought my old Wide-Glide, I was getting about 40-45 mpg. After fitting it with better-scavenging pipes, low-restriction air filter, high-flow petcock and filter, and rebuilding the S&S Super E racing carb with a Yost Power-tube for improved atomization, I could get 50+ mpg riding through the mountains 2-up, and the bike ran like a scalded cat. Many of the parameters that I had to adjust manually on that bike can be tweaked through mapping control modules these days. BTW, getting reasonable low-speed performance out of a large-bore butterfly carb is somewhat of an art, which is why you see many older modded H-Ds smoking on acceleration at low RPM. Air-speed across the main-jet venturi is insufficient to atomize the fuel properly, so it is not burned completely. Nothing wrong with the design - just poor skills on the part of the person who tuned the bike and thinks that oversized jets are the answer for performance gain.

Turbo, I thought the y-axis of the map (PSI) was for a given load. The map tells you for a given load and RPM you need z- amount of fuel.

I agree with what you said about leaning or making it rich. But that's really not where the engine was designed to operate at.

I assume the PSI is manifold pressure like in an airplane.

 

[turbo]

I would think that variable ignition timing would be a good way to improve performance and change that map. Still, I don't think it is something you would want to turn on and off. If you can get better performance, why turn it off?

It is NOT something you would want to turn off, Russ. You still want variable ignition timing (not the old vacuum advance type), but the sweet part of the Power Commander is that you can change the slope of that timing advance to optimize acceleration or perhaps to sacrifice some performance for fuel efficiency. Unless you really want to dig into the maps, and figure out what's going on, the tweaking of the ignition timing is hidden from the user. You are faced with choices like what kinds of performance, economy, etc you want at what loads and rpms, and the web-site suggests maps that might help you get there, based on what kinds of modifications you might have made to the bike. There are tons of maps, tailored to specific combinations of exhausts, intakes, etc. It might take a few tries to get the responsiveness, torque, etc you're looking for and it's a hell of a lot cheaper to program your own module than to run back to H-D and fork over $$$ to remap your stock module.

 

[Pythagorean]

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.

 

[Cyrus]

The chart I gave is only valid for the engine that made it. But it gives you a general idea of what's going on.

 

[turbo]

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:

https://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]

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:

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

wow, that's a pretty intense packet of information!

 

[turbo]

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]

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]

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]

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]

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]

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]

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]

US gallons are smaller.

 

[FlexGunship]

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]

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]

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]

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]

...keeping the pyrometer...

"pyrometer?"

Sorry, but that's not in the SAE manual.