2nd Gen Non-Engine/Transmission "Fuel Consumption vs. RPM" question

[Kurz82]

A quick question that I was thinking about while warming up my truck a little bit ago and trying to conserve fuel. Is the relationship between fuel consumption and the rpm of the engine a one-to-one relationship? By that, I mean is the amount of fuel consumed with the truck working at 800 rpm half of the amount when it is operating at 1600 rpm (assuming all other things to be equal)? TIA for any light that people can shed on this subject...



Rich

 

[The patriot]

The truck shouldnt be under any significant load at 800 RPM.

It would be shown in a chart that has BSFC in it.

Does anyone have one of these they can post?

I'd be interested in seeing one for a 12 valve.

All other things equal, your best MPG's should be where you get maximum torque.

HTH

Eric

 

[jponder]

Your max mileage is going to come at the lowest RPM that will carry the speed. There is all this talk about A "sweet Spot" and that the engine "Likes " to run at 2-2. 2K and it is true that there is a range there where the engine produces the maximum HP per gas burned. It is its volumetric most efficient point. What does this tell you about mileage there? ZERO,NOTHING, NADA. If you are producing HP you dont need thats not helping you. What is better than runing 1500 answer 1400 what is better than 1400 answer 1300 what is the limiting value??? At some point your engine will lug. Engine off coasting down a hill will yeild the max mileage . next would be engine idling and going down a hill in high gear. You cant always do this because road conditions are changing. When people talk about where to run the engine for max mileage and they say at the peak volumetric efficiency well thats bull. Peak torque is bull also. Its the point where you are running minimum RPM and producing just enough torque to carry your speed and its a fine line.

 

[pwr2tow]

I agree with jponder, all this modern day data, charts, graphs, and the like are just someones way to gain noteriaty, Alot of times they have nothing to do with real world situations. Too many variables to come right out and say situation 1 will enhance situation 2 so there-for situation 3 will be such and such. This is my opinion and I'am sticking to it.



Ron

 

[jponder]

Originally posted by Kurz82

A quick question that I was thinking about while warming up my truck a little bit ago and trying to conserve fuel. Is the relationship between fuel consumption and the rpm of the engine a one-to-one relationship? By that, I mean is the amount of fuel consumed with the truck working at 800 rpm half of the amount when it is operating at 1600 rpm (assuming all other things to be equal)? TIA for any light that people can shed on this subject...



Rich

Okay Kurz I finally found some performance curves in issue29 page 48 I calculated lbs/hour from the chart and here they are

(1400/41. 69)(1600/47. 18)(1800/53. 08)(2000/58. 45)(2300/69. 14)(2500/77. 52). This is for the ISB-235 I know there are more charts in there for 12 valve and the ETH but I cant find them. Speed is a big factor. The ISB-235 produces the exact same torque from 1400 to2500. I just cant see how you could pick any rpm other than 1400! But there is at LEAST one RPM that will beat 1400 and I bet you geussed what it was SHOCKER Its 1300!!!!!!!!!!

 

[Kurz82]

Thanks guys. All of this stuff makes a lot of sense, but I still do not know this: Does the fuel consumption double when the engine speed doubles? Or is the relationship not linear?



Rich

 

[klenger]

The answer is no, fuel consumption is not 1/2 at 1/2 RPM.

 

[jponder]

Originally posted by Kurz82

Thanks guys. All of this stuff makes a lot of sense, but I still do not know this: Does the fuel consumption double when the engine speed doubles? Or is the relationship not linear?



Rich

Look at Gallons per Hour per RPM starting at 1400 and going to2700 (. 0298,. 0295,. 0295,. 0292,. 0301,. 0310,. 0318) That looks pretty darn close to a line to me. I'd say its linear and it doubles

 

[jponder]

Originally posted by klenger

The answer is no, fuel consumption is not 1/2 at 1/2 RPM.

YES IT DOES IN FACT ITS less THAN 1/2

 

[Rman]

kind of a loaded question. The fuel consumption is FAR from linear if your taking into account the acceleration period. It will have spikes all over. If your talking constant rpm's constant load (all kinds of constants that are not possible) then yes it is somewhat linear. Close enough to call it linear. The ratio i couldn't be sure on. I kind of doubt that it is 1 to 1. Meaning that fuel consumption doubles as rpm does. Again this all would be heavily effected on outside "constants" that would be useless to just assume.



To me the question's answer is really worthless because the result would be a situation that could never be reproduced.

 

[illflem]

A sort of similar recent thread asked if folks using add-on overdrive gearboxes such as US Gear and Gear Vendors were getting better mpgs with the lower rpms. The overwhelming answer was no, mpgs were the same. At some speed aerodynamics takes over to lower mpgs no matter what the rpm.



Eric, I've seen the BSFC chart for 12 valves, at no load the graph is flat until above 2000 rpm were it takes a slight rise.

 

[bighammer]

There is a certain efficient RPM for each (model of) engine where it operates most effectively. (is that volumetric efficiency?) If you are below that, the fuel is not burned in a way to give the most energy. Above the optimal RPM, friction takes a bite.



From what I've heard, most 12 valves like to run under 2k and the 24v engines seem to like 2k or a touch over.



Of course, I'm no authority on this stuff. Just my worthless $. 02

 

[Pit Bull]

Ouch, I read this thread and now my head hurts. My ETH/DEE runs best with the windows down and the radio off so I can hear the Cummins very efficient and linear

 

[jponder]

Originally posted by illflem

A sort of similar recent thread asked if folks using add-on overdrive gearboxes such as US Gear and Gear Vendors were getting better mpgs with the lower rpms. The overwhelming answer was no, mpgs were the same. At some speed aerodynamics takes over to lower mpgs no matter what the rpm.



Eric, I've seen the BSFC chart for 12 valves, at no load the graph is flat until above 2000 rpm were it takes a slight rise. [/QUOTE



Okay, I am using page 48 of TDR #issue 29. and it is very easy to calculate from theses charts that @ 1400 rpm the engine is using 41. 7 lbs of fuel/hr and at @2800 rpm the engine is using 84. 9 lbs/hr of fuel. If I understand these charts correctly that is under full load. Ifflem, he is asking if the engine is using 2X the fuel at 2X the RPM and the answer is Yes its using 2. 04 times as much fuel. that is loaded use. I dont know what it would be like under no load and going down the road would be somewhere in the middle. You and I have both explored the effect of constant throttle improving mileage and that always varies RPM. You did it first with a modified cruise and I think your best mileage was holding your foot constant. I use a throttle cable. If those people were using overdrives and cruise they would experience increased engine braking going down a hill, something you do not want for maximum MPG. If they are on level ground, Its kind of hard to find 600 miles of level ground where I am, for the duration then its all a moot point. It seems you are saying that because BSFC is constant then fuel is constant which is absolutely not the case. That is a measure of fuel/HP and that means that fuel does increase per rpm becuase HP rises. I stand by the statement that Fuel increases almost linearly with RPM from 1400 to 2800 RPM. I cant wait to get some tall tires and prove that using constant throttle will yeild some big numbers at low rpm's. I geuss what I am saying is if you have a choice of going 60 MPH at 1600 or at 1400 due to different gearing and if your engine doesnt lug then 1400 is going to whip the hell out of 1600 everytime.

 

[The patriot]

Originally posted by illflem





Eric, I've seen the BSFC chart for 12 valves, at no load the graph is flat until above 2000 rpm were it takes a slight rise.

Thanks Bill.

Guess I should have said the engine will be most efficient (BSFC)at max torque.



Eric

 

[DBCooper]

I know that I could go thru my magazines to find this out, but just what is the optimum rpm for the 12 valves?

 

[Silver]

Fuel consumption may be linear at a constant load but, in a vehicle, generally, as RPM increases, so does speed and wind resistance. Thus, as RPM increases, so does the load. So- Fuel consumption should not be linear while driving.



I'm guessing that the fuel consumption charts were made on an engine under constant load.



On a related note, I get good mileage between 58 and 66mph (1700 - 2000 RPM) then it drops. It rises again when I get between 78 and 82mph. (around 2400 RPM)





I hope I'm not rambling. My 2 cents

 

[geusterman]

My truck (with 245X19. 5's with 3:55's and Gear Vendor) runs 74 mph @ 1500 rpm. I have no cruise but have purchased (but not installed) an adjustable throttle cable which I hope is productive. I have 10K on the truck and mileage has been about 15 mpg which is sad but I am hopeful that those who talk about waiting for break-in about 20K to see mileage are right. Bill Heath, from Heath Performance Diesel in Ellensburg WA builds a nice 400hp that cruises @24mpg. I have an Edge EZ but mileage was about 14-15 before the EZ anyway. The throttle cable should be an improvement. geusterman

 

[LSMITH]

Lower RPM is going to help economy to a point, then it will be counterproductive. guesterman, try running around at 1700-1800 for a while, see what fuel economy is like for a couple tanks. I recently increased my RPM by 100 at cruise and it hurt by about 1 mpg, it is too early to tell exactly how much it hurt.

 

[The patriot]

Originally posted by DBCooper

I know that I could go thru my magazines to find this out, but just what is the optimum rpm for the 12 valves?

About 1500 RPM's. This is where the engine is most efficient.

Eric