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Open Roads Forum  >  Tow Vehicles

 > Why diesels are most efficient around 1,800 rpm

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ShinerBock

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Posted: 10/20/20 01:34pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

ShinerBock wrote:

4x4ord wrote:

Shiner, you are applying a little bit of truth improperly to come up with a stupid recommendation.

I couldn't read your post without at least doing a simple test to give some real somewhat scientific numbers so:

I drove my truck in 5th gear at 1800 rpm/92 kph up a long incline and measured the fuel economy on the DIC .... 13mpg (18.6 liters per 100). I turned around, went back and conducted the same test at 92 km per hour in 6th gear .... 13.6 mpg (17.8 liters per 100). I then did a similar experiment over a stretch of highway going down a grade for the duration of the test... 43 mpg in 6th at 92 kph and 33 mpg in 5th at 1800 rpm/92kph. So although a diesel engine may be most efficient at 1800 rpm it is best to let the engineers worry about what gear and rpm your truck should run at. Put it in "D" and go. And choose the highest speed rear end recommended for the load you expect to tow.


Yeah, you are right and Cummins engineers are wrong. What are they thinking? Also, the algorithms the truck computers uses is not even close to being 100% accurate. It does not measure the actual amount of fuel used and instead relies on various sensors and PID's to come up with it's number.

BTW, I am not making a recommendation here. I am just posting information I have that explains why every diesel BSFC map I have seen shows that it's peak efficiency under load is around 1,800 rpm.

I am also not sure you know how transmission tuning works. It is a vanilla tuning that relies on pressure, rpms, and throttle input to know when to shift. It does not have the AI capabilities to know what kind of load you are carrying, the drag resistance, the weight, or if you are about to come up to a hill so it will know to downshift. The only transmissions that I know can do that are the Eaton Endurant transmissions which they(Eaton) teamed up with Cummins to program the transmission to shift according to GPS data and grade sensors inside the trans. This GPS data is constantly being updated. You are giving your truck's TCM more credit than it is due. It is not that smart.


If the Ram/Cummins engineers believed they could increase the fuel economy of their trucks by holding the truck in a lower gear at 60 mph to keep the engine at 1800 rpm instead of shifting into overdrive do you honestly think they wouldn't do it? Try it on your Ram .... drive at 1800 rpm in 4th gear and measure your fuel economy and mph. Now drive the exact same test in drive (presumably 6th gear)at the same mph ... there is no way you are going to measure better fuel economy at 1800 rpm and 4th than what you will measure in the gear the truck wants to run in.


I have! I have driven the same 160 mile route to the cost towing my 5ver in 6th and in 5th many times. Filled up before and after to get calculated numbers instead of computer. Averaging out each time I towed in 6th and in 5th on my app, I got better fuel mileage in 5th. Unloaded or very light loads below 5k or 7k with low drag, being at the lowest rpm possible is best for fuel economy according to my calculated numbers.

My truck does not know whether I am towing 6k or 12k to know which gear is the best one to stay in. All it knows is pressure, load(throttle input/torque), and rpm to dictate which gear to be in. It is like fuel tables. If it sees X pressure, Y load, and Z rpm, then it looks up this point on the table to see if it should up shift or or not. It knows nothing about the trailer you are towing or if you are about to come up on an incline to downshift. It is not that smart.

* This post was edited 10/20/20 01:44pm by ShinerBock *


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BobsYourUncle

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Posted: 10/20/20 01:35pm Link  |  Quote  |  Print  |  Notify Moderator

The solution is very simple:

Just do like we used to do with our cars in the early 70's.

Jack the back end up with big shackles so it is way up in the air, put on some big fat tires, (Deep Purple Highway Star) and then you are always traveling downhill! Everybody knows you burn less fuel going downhill!

You're welcome... [emoticon][emoticon]


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4x4ord

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Posted: 10/20/20 01:48pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

4x4ord wrote:

ShinerBock wrote:

4x4ord wrote:

Shiner, you are applying a little bit of truth improperly to come up with a stupid recommendation.

I couldn't read your post without at least doing a simple test to give some real somewhat scientific numbers so:

I drove my truck in 5th gear at 1800 rpm/92 kph up a long incline and measured the fuel economy on the DIC .... 13mpg (18.6 liters per 100). I turned around, went back and conducted the same test at 92 km per hour in 6th gear .... 13.6 mpg (17.8 liters per 100). I then did a similar experiment over a stretch of highway going down a grade for the duration of the test... 43 mpg in 6th at 92 kph and 33 mpg in 5th at 1800 rpm/92kph. So although a diesel engine may be most efficient at 1800 rpm it is best to let the engineers worry about what gear and rpm your truck should run at. Put it in "D" and go. And choose the highest speed rear end recommended for the load you expect to tow.


Yeah, you are right and Cummins engineers are wrong. What are they thinking? Also, the algorithms the truck computers uses is not even close to being 100% accurate. It does not measure the actual amount of fuel used and instead relies on various sensors and PID's to come up with it's number.

BTW, I am not making a recommendation here. I am just posting information I have that explains why every diesel BSFC map I have seen shows that it's peak efficiency under load is around 1,800 rpm.

I am also not sure you know how transmission tuning works. It is a vanilla tuning that relies on pressure, rpms, and throttle input to know when to shift. It does not have the AI capabilities to know what kind of load you are carrying, the drag resistance, the weight, or if you are about to come up to a hill so it will know to downshift. The only transmissions that I know can do that are the Eaton Endurant transmissions which they(Eaton) teamed up with Cummins to program the transmission to shift according to GPS data and grade sensors inside the trans. This GPS data is constantly being updated. You are giving your truck's TCM more credit than it is due. It is not that smart.


If the Ram/Cummins engineers believed they could increase the fuel economy of their trucks by holding the truck in a lower gear at 60 mph to keep the engine at 1800 rpm instead of shifting into overdrive do you honestly think they wouldn't do it? Try it on your Ram .... drive at 1800 rpm in 4th gear and measure your fuel economy and mph. Now drive the exact same test in drive (presumably 6th gear)at the same mph ... there is no way you are going to measure better fuel economy at 1800 rpm and 4th than what you will measure in the gear the truck wants to run in.


I have! I have driven the same 160 mile route to the cost towing my 5ver in 6th and in 5th many times. Filled up before and after to get calculated numbers instead of computer. Averaging out each time I towed in 6th and in 5th on my app, I got better fuel mileage in 5th. Unloaded or very light loads below 5k or 7k with low drag, being at the lowest rpm possible is best for fuel economy according to my calculated numbers.

My truck does not know whether I am towing 6k or 12k to know which gear is the best one to stay in. All it knows is pressure, load(throttle input), and rpm to dictate which gear to be in. It is like fuel tables. If it sees X pressure, Y load, and Z rpm, then it looks up this point on the table to see if it should up shift or or not. It knows nothing about the trailer you are towing or if you are about to come up on an incline to downshift. It is not that smart.


Unless the Ram/Cummins engineers are idiots which I am certain they are not your transmission will do better at selecting the appropriate gear for fuel economy than you can. I just did a quick search to see what Cummins says about running the 6.7 at 1800 rpm:

Here


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ShinerBock

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Posted: 10/20/20 01:58pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:



Unless the Ram/Cummins engineers are idiots which I am certain they are not your transmission will do better at selecting the appropriate gear for fuel economy than you can. I just did a quick search to see what Cummins says about running the 6.7 at 1800 rpm:

Here


I never said they were idiots. They just don't have all of the data and computing power to know how am using my truck at the moment. They can only make a vanilla tune that will be okay for most situations. They don't know the terrain I will be driving in, the weight I will be carrying and so on.

Making a custom transmission tune based on different weights, grades, and driving conditions would take A LOT of computing power and space which would likely increase the cost exponentially. This is probably the reason why Ford and Ram give you the ability to take out gears since you know how much weight you are carrying and what kind of terrain you will be towing in, not them.

And Cummins is correct, between 1,300 and 1,500 is the sweet spot unloaded or requiring a light load from the engine. However, as engine load increases, that sweet spot changes.

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ShinerBock

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Posted: 10/20/20 02:37pm Link  |  Quote  |  Print  |  Notify Moderator

Even Ford talks about locking out higher gears when the truck is heavily loaded or towing on grades in their Selectshift tutorial on how to use the feature. So that tells me that not even the Ford trans doesn't know how much you are towing or at what grade/terrain you are towing in to always be in the right gear. Hence the reason why they leave it to you to choose what is best for your truck given the conditions. As is said before, you are giving your transmission too much credit. It is not that smart.

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RoyF

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Posted: 10/20/20 02:57pm Link  |  Quote  |  Print  |  Notify Moderator

Miles per gallon depends on more than engine efficience. Wind resistance is a big factor at higher speeds.

4x4ord

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Posted: 10/20/20 02:57pm Link  |  Quote  |  Print  |  Notify Moderator

There are times it would be advisable to manually shift or lock out gears I’m not arguing that. I am saying that revving an engine higher under light loads with the idea of increasing fuel economy is foolish. No one in their right mind is going to make it a habit of locking out overdrive gears to increase their fuel economy,

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Posted: 10/20/20 03:13pm Link  |  Quote  |  Print  |  Notify Moderator

I think you are stretching to far with your ALL DIESEL'S remark. When I was the lead mechanic at Saunders Leasing all of our class 8 trucks had governors set at 1800 rpm. You think they had to run wide open to get max mpg?


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ShinerBock

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Posted: 10/20/20 03:19pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

There are times it would be advisable to manually shift or lock out gears I’m not arguing that. I am saying that revving an engine higher under light loads with the idea of increasing fuel economy is foolish. No one in their right mind is going to make it a habit of locking out overdrive gears to increase their fuel economy,


I never said to revving to 1,800 rpm is a good idea under light load. In fact I said the opposite that under light loads it is best to be at the lowest rpm and higher gear possible. The 1,800 sweet spot only comes into play when you are moderately to heavily loaded, towing something with high wind resistance(like an RV), and/or going up grades.

Also, 5th gear is still overdrive on both my Ram and your Ford so you are not taking out all of your overdrive gears, just one of them. I also have 3.42 gears which puts me exactly at 1,800 in 5th at 65 mph while 6th puts me below 1,400 at the same speed.

* This post was edited 10/20/20 04:05pm by ShinerBock *

4x4ord

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Posted: 10/20/20 04:23pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

4x4ord wrote:

There are times it would be advisable to manually shift or lock out gears I’m not arguing that. I am saying that revving an engine higher under light loads with the idea of increasing fuel economy is foolish. No one in their right mind is going to make it a habit of locking out overdrive gears to increase their fuel economy,


I never said to revving to 1,800 rpm is a good idea under light load. In fact I said the opposite that under light loads it is best to be at the lowest rpm and higher gear possible. The 1,800 sweet spot only comes into play when you are moderately to heavily loaded, towing something with high wind resistance(like an RV), and/or going up grades.

Also, 5th gear is still overdrive on both my Ram and your Ford so you are not taking out all of your overdrive gears, just one of them. I also have 3.42 gears which puts me exactly at 1,800 in 5th at 65 mph while 6th puts me below 1,400 at the same speed.


I guess we agree then.

When you look at the BSFC map that you posted you can see that the rpm and load you selected are for one very specific circumstance. If you start pulling a little harder the transmission is going to automatically shift down a gear. If you start pulling slightly lighter you will be getting better fuel economy in the higher gear. Towing a trailer down the road is a constantly changing load so the actual load might seldom be at the average. Even if you have a BSFC map for your engine, unless you are constantly watching your instantaneous fuel consumption and are able to calculate in your head how your mpg converts to engine torque and rpm, you will have an impossible task of knowing which gear is most fuel efficient. I'm saying it is best to keep it simple and just let "Drive" with "tow haul" do the thinking for you. The basic principle is throttle down and gear up for better fuel economy. If we find ourselves on a road where the transmission is constantly up shifting only to end up in too high of gear where it needs to recover with a downshift then I know we would agree it's time to lock out 6th.

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