Fuel consumption vs. RPM

[ScottI2R]

Hi all,
Is it posible that an engine would get better fuel economy at a higher rpm as opposed to lower?

My question, although it seems simpleton, is based upon my 95 eclipse. 85% highway driving for work. This vehicle appears to obtain better fuel economy in the 75-80mph range than the 60-65 mph range. Rpm's are 2700-3000 vs 2100-2400. It is an auto trans, non turbo, non all wheel drive. 2.0l.
It seems that aerodrag alone would hinder better mileage.

I personally like operation @ 80 as opposed to 60. 60 seems to take forever to get to work.

Thanks for any considerations of this question,

Scott

In a hundred years, it isn't going to matter anyway.

 

[reidh]

It depends on many factors. Of course higher vehicle speed equates to higher drag. That aside, there are many engine issues working both for and against you.

Working against you, higher engine speed means more friction.

Working for you, a higher engine speed usually equates to a greater load (throttle more open), which reduces your pumping loss. A higher engine speed will often mean hotter cylinder head temperature, which reduces oil viscosity, and increases your thermodynamic efficiency.

That is just a few factors that influence fuel economy. You must also remember that how the transmission is geared will also play an important role. I remember a very useful thread on this topic a few months ago, but the search function is down on this site (at least on my computer). Greglocock had some useful insight if my memory serves me correct, so possibly he can chime in on this topic again.

Reidh

 

[TStaples]

This general topic was discussed in another thread, which you might want to peruse:

thread71-178400

-Tony Staples

http://www.tscombustion.com

 

[EdDanzer]

All engines have a brake specific fuel consumption map.

http://www-personal.engin.umich.edu/~amaliko/Research Projects_files/image016.jpg

If you look at this image you can see how a vehicle could get better fuel economy at different speeds and loads.

 

[Tmoose]

http://www-personal.engin.umich.edu/~amaliko/Research Projects_files/image014.jpg

image014.jpg offers better resolution, but does not identify the "island" units. I think they are grams/HP/hr.
Smaller numbers are less fuel per HP, which is better gas mileage at equivalent HP.
The Y axis could as easily be BMEP as torque, or generally small throttle openings = lower torque (and more grams of fuel for each horse pressure).

 

[globi5]

Aren't these curves usually taken at full throttle?
And, aren't the cars at these highway-speeds usually driven at partial throttle?

 

[Lou Scannon]

A BSFC "map" implies all speeds & loads.

Generally speaking, it would have to be a significant "fluke" for a conventional passenger car to get better steady state fuel economy at 75-80mph than 65-70 mph. All the friction terms are working against this, especially aerodynamic drag. I'll believe it when I see statistically valid data from a properly controlled experiment.
Supposing for a moment that Scott's observation is accurate, I would have to say there is something seriously wrong with the way the engine is running at the lower speed.

 

[GregLocock]

The usual trick is to plot the steady state torque requirement for the car, in each gear, onto that graph - easy if you have a manual trans. You can then read off the sfc, torque and engine speed, and so work back to a fuel consumption. I'd guess they are g/kWh

Nowadays of course that is easily done in a spreadsheet, 28 years ago my first serious job was to run fuel consumption estimates for a car by cranking through that lot by hand for different gear ratios. That was just tedious. Estimating 0-60 times from the same information was much more difficult, in response to which I wrote what was probably the first serious performance prediction software in the UK.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RossABQ]

What is the terrain like on your commute? If it is hilly, it is possible you eliminate some low-vacuum conditions climbing the hill with a higher initial speed. More dramatic if you compare 40 mph speed with 60; you may even have to downshift to maintain 40 mph up the hill.

On the other hand, I notice you aren't quoting actual mileage numbers. Unless you calculate your mileage, such discussions are meaningless. I've had people swear to me they get 19 mpg in their SUV and when we look at the "trip mileage" function in their computer, it says 13 - 14. They just "felt" they were doing well, because the gas gauge in their SUV (with 35 gallon tank) was moving slower than in their previous vehicle.

 

[al1]

I know it is counterintuitive to think that more fuel efficiency can be had with higher RPM when all things are equal via increase fiction and aero. However, using two examples of 2,000 to 3,000 engine RPM at the same cruising speed, there are significant food for thought to be considered.
Since it’s all about heat energy being converted to work the following facts are real. An engine at cruise (example only) is at best about 15 to 20% efficient which means that up to 80% of the heat energy goes out tail pipe radiator. At 2,000 RPM on a 180-degree power stroke there is around 15 milliseconds worth of time to lose the heat. At 3,000 RPM it’s 10 milliseconds or 50% less real time to lose heat energy. Also at the same cursing speed (all things being equal again) the engine should not have to have the throttle opened as much to make the same power so it uses less fuel per cycle even though it is doing it 50% more often. The net results in theory, there could be a fuel saving.
al1

 

[dcasto]

al1, try 30 to 45% efficient. Heat tranfer doesn't know about time per se. Resistance to heat transfer and temperature gradiant are what dictate loss of heat from an engine to the outside. So a hot engine with no cooling system would be more efficient than one circulating 100 gpm of collant with a .001 inch wall thickness, at any rpm would be the least efficient.

Turbochargers recover heat from the exhaust, so that adds to effiency as would some form of preheating air and pre vaporizing the fuel.

 

[Tmoose]

http://www-personal.engin.umich.edu/~amaliko/Research Projects_files/image014.jpg

looks like 780 NM or 575 lb-ft at 2100 rpm.
Moderately big diesel engine.

Let's say I need to make 40 HP for cruising at a particular speed. At 2000 rpm we need 105 lb-ft, and 3000 70 lb-ft. If I scale the engine and chart by dividing all the torques by an all purpose "correction" factor of 3 the specific fuel at 3000 rpm and 210 is about 297 g/hp/hr. 2000 rpm and 315 is about 235 g/hp/hr. About 20% less fuel to do the same work at the same road speed. For most charts I've played with it sure looks like driving in the high gear is the way to go. If the engine was so tiny it was cruising at full throttle at the torque peak rpm the gas mileage might be optimized. Think original VW beetle with out the huge benefit of FI and electronic ignition control.

 

[globi5]

Not sure whether this has been covered before:
But specific fuel consumption and mileage don't necessarily generate comparable data. At least theoretically specific fuel consumption could go up and mileage could still improve.

Example: If one drives 10% faster and specific fuel consumption per hour goes up by only 9%, than the amount of fuel consumed on a given distance would still be 1% less.
(I don't think though that this will likely be the case.)

On a side note: I'd be curious why there are very few cars that do actually have a very high (economy) gear?

 

[GregLocock]

Because people who buy new cars are not prepared to make the sacrifices necessary to have an ultra high top gear.

And even when they are given one they don't use it.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ScottI2R]

Hi everyone,
Thanks for all the replys. They give a lot of food for thought. I have to say what I really need is what Hemi states. " I'll believe it when I see statistically valid data from a properly controlled experiment."
In day 2 day driving, I just cannot do that. Maybe a long (tank full ) trip would do it. But the differences I am seeing may be accounted for strictly in the 15% of non-highway driving I do. Stopping at a light more often , got caught by a train this time, etc.

Ross, you are right, I didnt quote actual mpg. Difference was 30 mpg vs 28.8. Not much, but enough to make me wonder.
I suppose it could actually vary because of how much I topped off when I fueled.
Too many uncontrolled parameters. I'll have to work on that when the weather stays warm.

Thanks,
Scott

In a hundred years, it isn't going to matter anyway.

 

[malak2]

I would think at a lower rpm the load would be higher, so the fuel requirement should be a bit higher with a higher load.

I personnally get better milage per gallon running my bike at 90 vs 65 (tho the rpm's are higher (and fuel consumption) the distance covered offset the increased fuel used).

 

[Lou Scannon]

What controlled experiment did you do to measure your mileage?

 

[dicer]

If that is true then the auto manufactures don't need to install overdrive ratios.

More rpms= more air and I'm sure it still needs fuel to burn also. Higher revs should take more fuel.


someone mentions the load being higher at a low rpm?
The load that the engine sees is the same, we just have one running faster and the other running slower. Anyway that is how I read these posts. Then again it is not a fair comparison in a vehicle. Only would be a true test on a dynamometer. Cause there would be a variance of gear ratios to maintain a certain speed. Or a difference of speeds and air resistance.

A simple non scientific test:
Let the engine idle for a specific time record fuel consumption and adjust data for mixture differences. Then run at 3000 plus rpms for same time. Unloaded tests.
I know there are losses that need to be addressed.

 

[malak2]

"A simple non scientific test:
Let the engine idle for a specific time record fuel consumption and adjust data for mixture differences. Then run at 3000 plus rpms for same time. Unloaded tests.
I know there are losses that need to be addressed."

So your saying if my car idles at 800rpm in park, and in gear it drops to 500 but i hold the gas pedal to maintain 800 rpm (in gear) the fuel usege would be the same ?

I find that hard to believe but I am working up the math to backup my previous assumption (my dyno run charts as well as the published fuel usage curves from my service manual) surprisingly my manuals charts indicate the optimum fuel efficiency for my motor peaks at about 4700rpm but as my dyno charts were compiled in 5th gear (1:1 ratio ) and not in top gear (.86:1 if I remember correctly) they may need some adjustment.

 

[patprimmer]

More rpm does not mean more air if the throttle is closed tighter.

The amount of air will control the rpm at a given load, not the rpm control the air.

Higher rpm does increase load due to increased pumping and inertia and friction losses.

I would think that at very low speed with the engine lugging and hitting back, fuel efficiency might fall of a lot due to reversion in the intake upsetting fuel distribution, and reversion in the exhaust at TDC overlap allowing exhaust back into the inlet and upsetting ignition timing and fuel distribution. These factors will only offset the factors mentioned in the previous paragraph if the engine speed is very low.

Generally very tall gears do increase fuel efficiency.

From a drivers point of view, keeping the car in the tallest gear possible while maintaining smooth running of the engine will give the best economy.

That may, under some circumstances, translate to a slightly higher road speed to allow smooth use of the highest gear.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.