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 > Why diesels are most efficient around 1,800 rpm

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ShinerBock

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Posted: 10/20/20 10:25am Link  |  Quote  |  Print  |  Notify Moderator

Finally got more of an answer to why all diesels are most efficient around 1,800 rpm. In comparison to what I thought, it has more to do with how diesel burns than other things like compression ratio, bore/stroke ratio, and turbo map efficiencies. While those are still a factor in how wide/narrow the window of efficiency is around 1,800 rpm and how efficient it is, the fuel burn rate of diesel fuel itself plays a bigger role as to why all diesels are most efficient around 1,800 rpm.


"The answer to your question involves fuel chemistry, engine geometry and thermodynamics. You can look at the power, torque and fuel consumption graphs for a wide variety of consumer turbocharged (and most non-turbo) diesels going back decades, and you’ll see a common theme in the data. Whether you’re driving a new Chevy Cruze or a Dodge ¾-ton pickup equipped with a 6.7L Cummins, all produce the best fuel economy near to 1,800 rpm, and all produce the maximum engine torque somewhere near to 1,800 rpm.

A big factor in the magic of 1,800 rpm is the volatility/combustibility of diesel fuel and how it affects flame propagation during the combustion process. Diesel fuel, even when finely atomized, doesn’t combust as quickly as gasoline. It takes time for the fuel charge inside the cylinder to completely burn. As it turns out, the burn time during combustion is nearly ideal with an engine turning 1,800 rpm. This longer burn time is partly why diesel engines produce more torque than a gas engine—diesel combustion results in more push on the piston over a longer period of time. To take advantage of this longer burn time, modern diesel engines can have a bore/stroke ratio that’s square or under square, meaning a diesel engine generally has a longer stroke than piston diameter. On the other hand, gas engines generally have an over-square piston/stroke geometry (larger piston diameter than stroke) because of the higher volatility of gasoline.

Knowing all this, manufacturers of diesel engines, turbochargers, fuel injection systems and engine control systems have all directed their efforts at producing engines that make the most of this 1,800-rpm phenomenon. So, drive your diesel at or near to 1,800 rpm if you’re looking for the very best in both engine torque and fuel economy."


* This post was edited 10/20/20 10:40am by ShinerBock *

Lwiddis

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Posted: 10/20/20 10:54am Link  |  Quote  |  Print  |  Notify Moderator

Interesting. TY for posting.


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BenK

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Posted: 10/20/20 11:28am Link  |  Quote  |  Print  |  Notify Moderator

Another potential factor is multi-squirt during the combustion stroke

With today’s computer and injector’s there is that potential.

First squirt to get it going and is normally a bigger or more fuel injected

Subsequent squirts will have much lower amounts of fuel

Found this looking at gasoline ICE’s with no spark plug. Direct injection, combustion ignition (DICI) gassers

The early models had projections of higher MPG in Diesel range

Bottom line will always have diesel provide higher MPG because it has more BTU’s per measure compared to gasoline

IIrc, Nox was troublesome and the thought at that time to leave it to downstream components/system to manage (such a poor choice IMHO)

Forgot about that and will look into that again


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patperry2766

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Posted: 10/20/20 11:45am Link  |  Quote  |  Print  |  Notify Moderator

I know on my 2018 Ram I get better MPG at 77-80 MPH which puts me in the targeted RPM range. Seems like lower lugs the engine


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4x4ord

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Posted: 10/20/20 11:51am Link  |  Quote  |  Print  |  Notify Moderator

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 (1414 rpm) and measured 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 (1414 rpm) 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 (lowest numerical) rear end recommended for the load you expect to tow.

* This post was edited 10/20/20 12:44pm by 4x4ord *


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Old-Biscuit

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Posted: 10/20/20 12:48pm Link  |  Quote  |  Print  |  Notify Moderator

MPG

Wasn't that the big push YEARS ago for the '55' mph speed limits


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schlep1967

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Posted: 10/20/20 12:54pm Link  |  Quote  |  Print  |  Notify Moderator

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 (1414 rpm) and measured 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 (1414 rpm) 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 (lowest numerical) rear end recommended for the load you expect to tow.

So you're saying is all we have to do is drive away from school to get better fuel mileage. All us old folks know it is uphill both ways to school.
So if we go the other direction camping... it will be downhill both ways! I should have thought of that sooner..


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4x4ord

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Posted: 10/20/20 12:55pm Link  |  Quote  |  Print  |  Notify Moderator

patperry2766 wrote:

I know on my 2018 Ram I get better MPG at 77-80 MPH which puts me in the targeted RPM range. Seems like lower lugs the engine


I certainly believe you get better fuel economy at 80 mph and 1800 rpm than you would at 90 mph and 2025 rpm. If you found your Ram got better fuel economy at 80 mph and 1800 rpm than it did at 70 mph and 1575 rpm than I'd be willing to bet there is more to your story than what you are reporting.

ShinerBock

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Posted: 10/20/20 12:59pm Link  |  Quote  |  Print  |  Notify Moderator

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.

4x4ord

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Posted: 10/20/20 01:13pm Link  |  Quote  |  Print  |  Notify Moderator

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.

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