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 > Why diesels are most efficient around 1,800 rpm

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ShinerBock

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Posted: 10/21/20 07:56am Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:


When that engine is working at 1800 rpm and just up to the dark blue sweet spot as you call it it is putting out a minimum of 150 horsepower. (440 lb ft of torque) According to the graph it would be burning 7 gallons of diesel per hour. Assuming you’re towing at 65 mph you’d be getting 9.3 mpg. So in order to be in the dark blue you’re working that truck . The dark blue represents 6.2 to 9.8 mpg at 1800 rpm and 65 mph. I would expect when you’re down at that kind of mileage the automatic is going to be selecting 1800 rpm over 1360 rpm.

Edit: I didn’t do the math on whether or not you can get a tire size and final drive ratio to do 65 mph at 1800 rpm. If you’re only doing 60 mph at 1800 your fuel economy is going to be about 10% worse to get into the dark blue of the graph.



The 1,800 rpm is not the thermal efficiency sweet spot because I call it, it is the sweet spot due to the burn rate of diesel fuel. It is not like it is an opinion that I can change at will. Doing some more research on it, there have been many studies proving this. Of course thermal effiecny of diesel fuel is a little different than the point at where a certain diesel is most efficient due to rev limits and so on. However, most diesels that can rev that high, then 1,800 is generally their sweet spot because of this.

Take this slide from a study Caterpillar did for the DOE showing that their C15 engine having much higher thermal efficiency at 1,800 over 1,200 rpm. Peak torque of the C15 is at 1,200 rpm for most if not all applications yet it has a higher thermal efficiency at 1,800 rpm.

[image]

Just to tow a combined weight of 21,000 lbs(sometimes 22k) at 65 mph up a slight 1% grade requires 150 hp. Of course this does not account for a lot of wind drag which gets worse the closer I get to the coast. The automatic will try to hold 6th for as long at it can and will for most slight hills, but the boost will be high, fuel pump will be close to max, and my EGTs will be 1-200 F higher than what it would if it were in 5th. My brothers 2012 F350 PSD 3.55 did the same towing his 5ver and he locked out 5th as well.

My whole family camps out at the same RV park at the coast and we live within 5 miles of each other so we generally follow each other down when we are towing the RV's.

* This post was last edited 10/21/20 09:16am by ShinerBock *   View edit history

FlatBroke

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Posted: 10/21/20 07:58am Link  |  Quote  |  Print  |  Notify Moderator

azrving wrote:

I knew from the beginning that this had pissing contest written all over it. Carry on lol

Yeah turned into a boring slug fest. I’m out.



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4x4ord

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Posted: 10/21/20 09:53am Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

4x4ord wrote:


When that engine is working at 1800 rpm and just up to the dark blue sweet spot as you call it it is putting out a minimum of 150 horsepower. (440 lb ft of torque) According to the graph it would be burning 7 gallons of diesel per hour. Assuming you’re towing at 65 mph you’d be getting 9.3 mpg. So in order to be in the dark blue you’re working that truck . The dark blue represents 6.2 to 9.8 mpg at 1800 rpm and 65 mph. I would expect when you’re down at that kind of mileage the automatic is going to be selecting 1800 rpm over 1360 rpm.

Edit: I didn’t do the math on whether or not you can get a tire size and final drive ratio to do 65 mph at 1800 rpm. If you’re only doing 60 mph at 1800 your fuel economy is going to be about 10% worse to get into the dark blue of the graph.



The 1,800 rpm is not the thermal efficiency sweet spot because I call it, it is the sweet spot due to the burn rate of diesel fuel. It is not like it is an opinion that I can change at will. Doing some more research on it, there have been many studies proving this. Of course thermal effiecny of diesel fuel is a little different than the point at where a certain diesel is most efficient due to rev limits and so on. However, most diesels that can rev that high, then 1,800 is generally their sweet spot because of this.

Take this slide from a study Caterpillar did for the DOE showing that their C15 engine having much higher thermal efficiency at 1,800 over 1,200 rpm. Peak torque of the C15 is at 1,200 rpm for most if not all applications yet it has a higher thermal efficiency at 1,800 rpm.

[image]

Just to tow a combined weight of 21,000 lbs(sometimes 22k) at 65 mph up a slight 1% grade requires 150 hp. Of course this does not account for a lot of wind drag which gets worse the closer I get to the coast. The automatic will try to hold 6th for as long at it can and will for most slight hills, but the boost will be high, fuel pump will be close to max, and my EGTs will be 1-200 F higher than what it would if it were in 5th. My brothers 2012 F350 PSD 3.55 did the same towing his 5ver and he locked out 5th as well.

My whole family camps out at the same RV park at the coast and we live within 5 miles of each other so we generally follow each other down when we are towing the RV's.


The rear wheel HP required to tow 22k combined up a 1% grade at 65 mph is 38 hp plus the power required to overcome drag and rolling resistance.... So 38 more ho is required on a 1% grade vs level ground.

It makes total sense that the EGTs are going to be higher doing the same work at lower rpm.... so what? Are you saying the EGTs are too hot? What kind of fuel economy are you getting in 5th vs 6th? Maybe this is a result of after market tuning?

* This post was edited 10/21/20 10:00am by 4x4ord *


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ShinerBock

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Posted: 10/21/20 10:18am Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:


It makes total sense that the EGTs are going to be higher doing the same work at lower rpm.... so what? Are you saying the EGTs are too hot? What kind of fuel economy are you getting in 5th vs 6th? Maybe this is a result of after market tuning?


I like to keep EGT's as cool as I can because it keeps the rest of the engine temps cool as well. In 5th, they are in the 800-900F range and in sixth they are in the 1,000-1,100F range for most of my drive. Not too hot, but the cooler the better for the longevity of parts. Other temps that are lower are trans temps since it is not shifting between 5th and 6th under load.

My fuel economy is roughly .5-1 mpg on average. Not much, but considering it also allows me to run at cooler EGT's and lower trans temps, it is a win win. This is after tuning. However, my brothers 2014 Ram was stock until recently and he recorded the same as me.

I have provided more than my fair share of data to back up what I am saying. And I am talking about actual data from manufacturers, not data from one trip going off of my truck computer. All have backed up what I am saying that A)my truck is more efficient in 5th between 60-70mph pulling my RV than 6th and B) the automatic transmissions in these truck is not smart enough to put you in the right gear in all conditions hence the reason why the truck makes offer a gear select option.

You, on the other hand, have fought me in this and other threads without providing much data to back up what your are saying while asking me for more even though you refuse to believe the data I am providing because it does not back up your beliefs. At this point I would have to say put up or shut up. Provide some actual data like BSFC maps, thermal efficiency numbers at different rpms, and so on or you really don't have a leg to stand on in this debate.

* This post was edited 10/21/20 10:28am by ShinerBock *

SweetLou

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Posted: 10/21/20 11:15am Link  |  Quote  |  Print  |  Notify Moderator

Not to take away from this informative debate, but I have been curious on my 2013 6.7, the RPM in 6th seems low at 1200-1300 unloaded. Just by my ear it sounds as if it needs to be higher so I manually shift it into 5th to be around 1500-1600. Do I have an issue, and need to get something reprogrammed?


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ShinerBock

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Posted: 10/21/20 12:08pm Link  |  Quote  |  Print  |  Notify Moderator

SweetLou wrote:

Not to take away from this informative debate, but I have been curious on my 2013 6.7, the RPM in 6th seems low at 1200-1300 unloaded. Just by my ear it sounds as if it needs to be higher so I manually shift it into 5th to be around 1500-1600. Do I have an issue, and need to get something reprogrammed?


That is just how the stock 68RFE with 3.42 gears is with stock programming. Nothing wrong with being that low unloaded and the Cummins will easily lug even lower than that. I would not want to be towing 5-7k at that low of rpms though and the stock trans tuning will put you there sometimes.

I had an old trans tune on my truck that allowed it to be at 45 mph in 6th(975 rpm) and it still had enough power to slowly accelerate up hills unloaded. Only had it in my truck for about a week and switched to a better tune that kept rpms above 1,400.

* This post was edited 10/21/20 12:15pm by ShinerBock *

SweetLou

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Posted: 10/21/20 01:50pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

SweetLou wrote:

Not to take away from this informative debate, but I have been curious on my 2013 6.7, the RPM in 6th seems low at 1200-1300 unloaded. Just by my ear it sounds as if it needs to be higher so I manually shift it into 5th to be around 1500-1600. Do I have an issue, and need to get something reprogrammed?


That is just how the stock 68RFE with 3.42 gears is with stock programming. Nothing wrong with being that low unloaded and the Cummins will easily lug even lower than that. I would not want to be towing 5-7k at that low of rpms though and the stock trans tuning will put you there sometimes.

I had an old trans tune on my truck that allowed it to be at 45 mph in 6th(975 rpm) and it still had enough power to slowly accelerate up hills unloaded. Only had it in my truck for about a week and switched to a better tune that kept rpms above 1,400.

Just didn't know if it was normal and good to be lugging that low. Towing I understand, its just when everyday driving to town.

4x4ord

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Posted: 10/21/20 05:34pm Link  |  Quote  |  Print  |  Notify Moderator

^^^ if the engine is not loaded heavy it won’t be lugging. Check your boost pressure. When a turbo diesel is pulling hard at a low rpm the turbo will be producing high boost pressure.

4x4ord

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Posted: 10/21/20 08:40pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:



My fuel economy is roughly .5-1 mpg on average..


I would say a 1 mpg savings is significant if you’re actually getting 6 mpg in 5th and 5 mpg in 6th. If on the other hand you’re getting 13.5 in 5th and 14 in 6 th .... it’s still something but I know I find I can get a difference of more than that between two trips leaving the transmission in drive both times. Anyway, what you actually measured for fuel economy in each gear really is relevant to our discussion. Did you set the cruise at 65 mph each time? Was the wind each time from the same direction and speed?

* This post was edited 10/22/20 02:12am by 4x4ord *

ShinerBock

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Posted: 10/21/20 09:35pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

ShinerBock wrote:



My fuel economy is roughly .5-1 mpg on average..


I would say a 1 mpg savings is significant if you’re actually getting 6 mpg in 5th and 5 mpg in 6th. If on the other hand you’re getting 13.5 in 5th and 14 in 6 th .... it’s still something but I know I find I can get a difference of more than that between two trips leaving the transmission in drive both times. Anyway, what you actually measured for fuel economy in each gear really is relevant to our discussion.


At this point, nothing I can say will change a thing. If data from the very manufacturers who makes my diesel engine and from other diesel engine manufacturers will not change your opinions and beliefs, then there is nothing I can tell you that will either. You are going to believe what you want to believe regardless of facts.

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