Little higher rpm = better gas mileage

[eljefino]

An engine's at its best in its BSFC curve, eg peak torque. I found a curve for my saturn 1.9; its sweet spot was 2300 RPM. Amazingly, that's what it turned in 5th at 65.

With VTEC etc the curve's flattened out, in a good way. Don't know why we need 9 speed trannys.

 

[Vikas]

Do you have a graph of showing your fuel consumption for last few years?

 

[Bgallagher]

Originally Posted By: abycat
Originally Posted By: tomcat27
optimal rpm is very engine-specific;


I strongly agree. my truck obviously 2800 is just screaming and my car at 3000 the cam profile changes and it has way more power but uses a ton of fuel. I'd say under 2800 for most engines though.


+1 I feel like all three of my vehicles have smooth and quicker acceleration just shy of 3k. I am not jackrabbiting but it is comfortable.

 

[IndyIan]

Originally Posted By: eljefino
An engine's at its best in its BSFC curve, eg peak torque. I found a curve for my saturn 1.9; its sweet spot was 2300 RPM. Amazingly, that's what it turned in 5th at 65.

With VTEC etc the curve's flattened out, in a good way. Don't know why we need 9 speed trannys.

I'd like to see an engineering analysis on the efficiency difference between 6-8-9 speed autos, with the same VVT engine. The average range of rpm running during normal acceleration must be with 100-200 rpm, and the final overdrive ratio could be made the same.
With a 5 spd I can see that you can't make 5th too tall as the drop to 4th will raise the rpms up out of the best BSFC range to often, but once you get 6 speeds, I can't see the difference in efficiency adding up to more than 2-3% if you have the same top gear ratio when you add more gears.

 

[Gokhan]

When will people learn not to report gas-mileage observations without doing several controlled runs? It's a rhetorical question. No one has the means to do such controlled runs and no one should report gas mileage and make precise claims. Gas mileage depends wildly on many factors. You miss the green light -- that could subtract 1 MPG. You miss another one, you idle more, different temperature, etc., etc. On top of that, there are measurement errors. Reading the fuel gauge for mileage calculations is absurd. Even the fuel pump readings are unreliable because how much fuel you put depends on the incline and pump.

Next time you say 22 MPG, note that it means 17 - 27 MPG.

 

[kschachn]

This is exactly correct, except that even with several runs there is no such thing as a real world "controlled" run. Each is an independent event.

Originally Posted By: Gokhan
When will people learn not to report gas-mileage observations without doing several controlled runs? It's a rhetorical question. No one has the means to do such controlled runs and no one should report gas mileage and make precise claims. Gas mileage depends wildly on many factors. You miss the green light -- that could subtract 1 MPG. You miss another one, you idle more, different temperature, etc., etc. On top of that, there are measurement errors. Reading the fuel gauge for mileage calculations is absurd. Even the fuel pump readings are unreliable because how much fuel you put depends on the incline and pump.

Next time you say 22 MPG, note that it means 17 - 27 MPG.

 

[willbur]

Originally Posted By: Trav
BMW did testing years ago and claimed 2800 RPM was optimal. If that's correct or applicable today I have no idea this was before the internet and google, it was a paper document.
I do know German manufacturers geared their US export cars lower to raise the RPM due to the 55 MPH speed limit at the time.


Interesting. Toyota conducted some research on this topic and concluded the 4 stroke engine is most efficient at 40-45 % of red line. This seems to be on line what some of the posters are observing...

 

[demarpaint]

Originally Posted By: willbur
Originally Posted By: Trav
BMW did testing years ago and claimed 2800 RPM was optimal. If that's correct or applicable today I have no idea this was before the internet and google, it was a paper document.
I do know German manufacturers geared their US export cars lower to raise the RPM due to the 55 MPH speed limit at the time.


Interesting. Toyota conducted some research on this topic and concluded the 4 stroke engine is most efficient at 40-45 % of red line. This seems to be on line what some of the posters are observing...


I think the rings would be less likely to stick over time too. A gas engine putting along all day long in the area of 1,500 rpms or so might not be as good for the engine as many of us may think.

 

[Duffyjr]

Originally Posted By: Gokhan
When will people learn not to report gas-mileage observations without doing several controlled runs? It's a rhetorical question. No one has the means to do such controlled runs and no one should report gas mileage and make precise claims. Gas mileage depends wildly on many factors. You miss the green light -- that could subtract 1 MPG. You miss another one, you idle more, different temperature, etc., etc. On top of that, there are measurement errors. Reading the fuel gauge for mileage calculations is absurd. Even the fuel pump readings are unreliable because how much fuel you put depends on the incline and pump.

Next time you say 22 MPG, note that it means 17 - 27 MPG.


I'm a creature of habit and I live in small town, I may be closer to controlled runs than you think. Traffic is never a variable in fact I usually don't get out of work until 5:10 and by that time I've missed rush hour and there is only one stop sign and two lights in between work and home. If it so absurd why do I consistently see the same readings. Using my Buick as and example since I've owned it longer, when the light comes on the gas gauge my trip meter is usually between 190 and 210 miles, very rarely does it waiver. These are all city miles of course, it would be more if I made a highway run. With a 16 gallon tank that is only a little over 1 mpg difference. These are non winter readings in the winter you can drop that 20 or 30 or more miles since it idles more its not as consistent.

With my truck that 1/8 of a tank works out to four gallons. That means my truck used four gallons of gas less in 150 miles than usual, even it its off by a couple gallons its still a nice jump.

This was only an observation and wondered if any one else had noticed something similar, I wasn't looking to publish my observations.

 

[circuitsmith]

According to this paper engine efficiency in my Matrix peaks at 3100rpm.
But that's at wide open throttle. At part throttle it's going to be lower due to pumping losses.
I typically up shift at 2000 rpm when "moseying along".
I'll try shifting at 2500-3000 for a couple tanks and see if there's a difference.

Efficiency is reduced at low rpms mainly because of the longer time for heat lost to the combustion chamber surfaces during the power stroke.
That reduces pressure and thus torque output.

 

[another Todd]

Originally Posted By: abycat
Originally Posted By: tomcat27
optimal rpm is very engine-specific;


I strongly agree. my truck obviously 2800 is just screaming and my car at 3000 the cam profile changes and it has way more power but uses a ton of fuel. I'd say under 2800 for most engines though.


Optimal RPM is also load specific