Diesel efficency vs. rpm discussion

[DRU]

In researcing a jeep diesel swap I have learned alot. I want to move 3500 pounds at 70 mph. To do so with my current gas set up I turn 3000rpm. Now If I was going to use a diesel and want to run it efficently would I gear it for as low an rpm as I could get or does each engine have a ''sweet spot''? Its a 2. 8L power stroke made by international. I think it red lines at 4500 rpm. Any insight about this would be of help.

 

[EKlem]

Most engines have a sweet spot. Lower rpms are better to a point. On many engines, the sweet spot occurs at the torque peak or right around it. I have heard people referance a cummins sheet with power vs. rpm vs. fuel consumption but I don't know anything more about it. I am sure that some more knowledgeable people will chime in.

 

[CumminsBu]

I think 1800 rpms is the sweet spot for just about all diesel engines, it works for me.

 

[TSpecht]

After reading that article from Cummins about squeezing out efficiency, it mentioned to use the low end power the engine has. So this tank I've decided to see if that'll help me eek out extra mileage. So far this tank after having been topped off, I've not exceeded 1800 RPM's, and been trying to keep boost under 5lbs and EGT under about 650°, as well as coasting up to intersections, and some of the other tips. Of course I've not tryed towing this way, and I'm sure it'll require a slightly different driving style to keep the engine happy.


THe article from Cummins basically says for best efficiency keep RPM's as low as possible if you can do so without lugging the engine. From what I've seen the newer 5. 9's have their peak torque at about 1600 RPM's. Of course the peak HP is at 2900RPM, and heavier loads and/or grades will make it more likely you'll need to tap that power by downshifting.

 

[ezcurra]

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[DonTX]

Neat Chart

I have been trying to get that chart for the 6BT, this is for the 4 cyl model. Does anybody have one for the 6BT?

 

[rbattelle]

I have been trying to get that chart for the 6BT, this is for the 4 cyl model. Does anybody have one for the 6BT?

Have you asked Cummins for one?



-Ryan

 

[MMiller]

I gear it to run a little higher then peak torque at the speed you want to run. This way on a windy day, or pulling a grade, you'll have a little torque left and still pull the gear you want. If you go too high and run it at peak torque, then I think you will be down shifting at every hill, and have to run it harder on windy days. I'm sure we all have driven vehicles that need to be downshifted all the time, that sure does not make for good economy. Lugging won't make good economy either.



Michael

 

[ezcurra]

I suppose that while HP/Tq/Fuel consumption values would vary from a 4BT to a 6BT, the RPM ranges at which peaks occur shouldn't be too different.

 

[RustyJC]

Generally speaking, best brake specific fuel consumption (BSFC) will occur at or close to peak torque RPM.



Rusty

 

[WContreras]

What we want power or economy. I power then in what I see 4BT in marine aplication can get more power 224hp or even 300 to do that you have to check the compresion ration of you engine this marine engines have lower compression ratio so it can go higer RPM to reach more power, but cant keep on top RMP or the engine will destroy.



This comment is my pure evaluation analisis after reading a lot of groups similar of this.



Then How to reduce compresion ratio is the question to do that I see for experience in Detroit Diesel Series 60 they reduce the lengt of the connecting rod (BIELA is spanish)

 

[jwgary]

Remember that the charts posted above (if I understand them correctly) are made at WOT. You start at idle at the start of the dyno run and slam the fuel rack all the way open. The dyno measures the torque output and multiplies by the RPM to get HP. Then another sensor correlates BSFC to the same RPM. I would assume that the chart would be different if you were only at half rack. That chart can also be misleading since it looks like you are using less fuel at the higher RPM (the dip in the chart), but that is brake specific meaning that it's fuel consumption per HP. Of course as HP increases fuel consumption increases as well. What you would really need to see is a chart that was at constant HP (the road load HP required to drive at 70) while you vary the RPM. If you had a fuel consumption chart for that setup, you could see where the lowest fuel consumption was and set your gearing accordingly.

 

[Mhannink]

the best fuel econemy is gonna be just over the peak torque so the cummins peak is 1600 for all but a couple years then that means 18 to 1900 is the best place for fuel econemy ive tried it in my truck and it works consistantly better when i run up past 2000 is seems to go to 17 and stay there and between 17 and 19 it runs 20+ me i would go 200 rpms over peak so that you have a little power left for wind and hills seems to work on my truck like that very well when im running in those rpm's. need a grear vendor overdrive so i can run 75 and 1800

 

[Jahcad]

I do not agree with that at all. When I had my 3. 54s I would get the best MPG at 55mph (hwy); which put my rpm at around 1500. Which netted 20. 5mpg; and on my dyno sheet it says my peak torque was at 2250rpm. My van aaken lowered my peak tq. to 2150rpm. Unless I'm not getting something.

 

[RustyJC]

I do not agree with that at all. When I had my 3. 54s I would get the best MPG at 55mph (hwy); which put my rpm at around 1500.

You're putting aero drag (speed) into the picture. We've been discussing BSFC versus RPM, which is a pure engine function. In your example, could your mileage at 55 MPH have been better in another gear that would have put your RPM closer to your torque peak? That's the jist of this discussion.



Rusty

 

[nps]

In your example, could your mileage at 55 MPH have been better in another gear that would have put your RPM closer to your torque peak? That's the jist of this discussion.



Rusty

Nope - you raise the RPMs by downshifting and it is going to burn more fuel providing you weren't lugging it before the downshift.



Maybe this efficiency plays into a stationary industrial application where the PTO can be adapted to the application to work the engine near its theoretical 'most efficient' point.



However if true in an automotive application, then I guess that I need to replace my 3. 54s with 4. 10s to get better mileage.

 

[RustyJC]

Nope - you raise the RPMs by downshifting and it is going to burn more fuel providing you weren't lugging it before the downshift.

I'm not saying it will or it won't. Ultimately, the BSFC curve for that given HP requirement will decide.



Rusty

 

[Jahcad]

That makes sents; If your unloaded cruising at moderate speed your bound to only be at 20-30% engine load. Fuel and power demands are low. Now if your loaded and in need of 80-90% engine loads; Fuel and power are high; And so it would be best to be just above peak tq.



So if I got this right the BSFC curve applys only to full engine load.

 

[biggy238]

I've also found that the best mileage can be had between 1700-2000 rpm... on my truck this means 3rd for speeds below 60mph and 4th for 60+



At 1500 RPM it takes more fuel to adjust to terrain changes than it does when the engine is reving higher... this plays a big part in my driving because nothing is relatively flat for even a mile around here... .





Am I correct in thinking that Cummins cams these engines to peak torque where they do? Most of their road engines torque curves graph the same way with respect to RPM.





I would gear the cherokee so that the largest part of your average commute speeds fall into the 1500-2000 rpm range.

 

[WContreras]

Is not that simple, 15 year ago get graduate as mechanical Eng. As I can remember because now Im more practical than theretical, the fuel consumption in the chart for example will be lower at 2000 Rpm spendinge 0. 400 grams for each KW(hp) during one hour of work. That mean you have to multiply at 2000rpm produce this engine 100HP(134kw), each US gallon weight 7. 1Lb then you can make the conversion considering the diferential and tires size you can figure your mpg teoretical.



Simple talking mpg will depende on GVW and maintening this particular engine from 1900 to 2200 rmp will get the best mpg or fuel economy