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 > Why diesels are most efficient around 1,800 rpm

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Turtle n Peeps

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Posted: 10/26/20 11:51am Link  |  Quote  |  Print  |  Notify Moderator

Shiner have you checked out the TPS % between 5th and 6th?

If not please do so and report back. I bet for what you're doing 5th will be lower.


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ShinerBock

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Posted: 10/26/20 12:20pm Link  |  Quote  |  Print  |  Notify Moderator

Turtle n Peeps wrote:

Shiner have you checked out the TPS % between 5th and 6th?

If not please do so and report back. I bet for what you're doing 5th will be lower.


Yes, the TPS % is lower in 5th and so is engine load % according to my Edge CTS2 which reads from my ECU.

ShinerBock

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Posted: 10/26/20 12:49pm Link  |  Quote  |  Print  |  Notify Moderator

I think part of the case here is that people are applying gasoline engine physics to a diesel engine. For one, the parasitic losses between the rpms of 5th gear and 6th gear in my truck is minimal. Diesels have significantly less pumping losses versus a gas engine. Diesel is not like a gasoline engine that has to stay around a 14.7:1 air/fuel mixture. With a gas engine, the more air added due to higher rpms pumping more air through a gas engine, the more fuel that needs to be added to keep from going too lean.

Diesels don't operate that way. They are regulated by fuel and can lean out utilizing more air and less fuel. Most modern diesels operate from 20:1 to as high as 60:1 air fuel ratio depending many variables. Especially where the turbo compressor map is most efficient meaning it is utilizing more air per drop of fuel to make power. At low rpms, depending on your turbo, your engine may not be able to spool enough air for the amount of fuel being injected therefor it's air/fuel ratio is on the rich side.

This is why EGT's climb much higher at low rpms because turbo cannot spin fast enough so the engine does not have enough air along with less air from lower rpms for the amount of fuel being injected to make the amount of power that is demanded from the driver. Basically, mid-high load at lower rpm below the compressors efficiency range means more fuel is needed to make power. At mid-high load at higher rpms around the compressors efficiency range, the engine needs less fuel to make power since it has more air available and can lean out it's air/fuel ratio to make power.

A diesel will just utilize the amount of air available for the amount of fuel being injected. Hence the reason why my engine is under more of a load in 6th than 5th when towing my trailer.

* This post was last edited 10/26/20 02:03pm by ShinerBock *   View edit history

noteven

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Posted: 10/26/20 01:02pm Link  |  Quote  |  Print  |  Notify Moderator

The "most efficient gear" to pull in the transmission is whatever number "direct" 1:1 is.

My old irrelevant Cummins with a 6 spd manual will drop 100deg off the EGT shifting out of OD 6th to direct 5th at the same road speed.

Does 100deg of exhaust temp mean more or less fuel?

My 6spd auto F-350 heads for 4th (direct) asap it has to lean into the harnesses'z.

Neither of the above have 1000 claimed lbs ft of torques or 500 horsepowers. They need to rely on their gearing once in a while.

SweetLou

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Posted: 10/26/20 01:03pm Link  |  Quote  |  Print  |  Notify Moderator

Cummins12V98 wrote:

otrfun wrote:

We always run in 5th when towing a large 5th wheel, TT, or hauling a large, in-bed camper with our '16 Ram Cummins. Made a number of MPG calculations running in 5th and 6th while under heavy load. Very close, if not identical MPG's. By running in 5th, our RPM's hover around 1,800 RPM at 60-65 mpg. The engine (and engine brake) is much more responsive in 5th and there's no downshifting on mountain passes. If for no other reason, we'd keep it in 5th just to reduce wear and tear to the tranny (no downshifts).


You understand the reason 3.42's are golden!

Exactly right again!


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noteven

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Posted: 10/26/20 01:06pm Link  |  Quote  |  Print  |  Notify Moderator

BobsYourUncle wrote:

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]


hahaahahahahahaha...

A guy in my little home town made big shackles out of combine rub bars for his Chevy II with a 327 / 4 speed....

Turtle n Peeps

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Posted: 10/26/20 01:52pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

Turtle n Peeps wrote:

Shiner have you checked out the TPS % between 5th and 6th?

If not please do so and report back. I bet for what you're doing 5th will be lower.


Yes, the TPS % is lower in 5th and so is engine load % according to my Edge CTS2 which reads from my ECU.


Boom. That's all anybody needs to know. TPS % lower= less fuel. TPS higher= more fuel.

IMHO I also think your correct about people applying gasoline engine knowledge to diesel engines.

ShinerBock

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Posted: 10/26/20 02:24pm Link  |  Quote  |  Print  |  Notify Moderator

For those who are applying gas engine physics to a diesel. Here is a great article from Gale Banks explaining it. It is a little long, but worth the read.


UNDERSTANDING TODAY’S DIESEL

TomG2

Central Illinois

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Posted: 10/26/20 02:27pm Link  |  Quote  |  Print  |  Notify Moderator

What rpm are the big trucks cruising at? It sounds like they are almost idling under 1,000 rpm's.

4x4ord

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Posted: 10/26/20 02:46pm Link  |  Quote  |  Print  |  Notify Moderator

Turtle n Peeps wrote:

ShinerBock wrote:

Turtle n Peeps wrote:

Shiner have you checked out the TPS % between 5th and 6th?

If not please do so and report back. I bet for what you're doing 5th will be lower.


Yes, the TPS % is lower in 5th and so is engine load % according to my Edge CTS2 which reads from my ECU.


Boom. That's all anybody needs to know. TPS % lower= less fuel. TPS higher= more fuel.

IMHO I also think your correct about people applying gasoline engine knowledge to diesel engines.


You could just say MPG lower more fuel; MPG higher less fuel.


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