Why diesels are most efficient around 1,800 rpm

That reminds me. Emissions are something else to take into consideration on stock trucks. Load at low rpms create high cylinder temps(and EGTs) which create Nox. The EGR will open to introduce cooled exhaust gas in order to lower those temps to reduce Nox which reduces efficiency. Something that doesn't happen at higher rpms under the same amount of load. I don't believe these BSFC maps take emissions devices into consideration in these numbers.

Someone forgetting my truck makes an additional 300-350 lb-ft at the crank than stock...... :W

And it makes considerably more at lower rpms under peak than stock since i don't have certain emissions devices coming into play.

RoyJ wrote:
I'll add something, based on the map above, if we set 400 lb-ft, or roughly 40% torque put, as the "lightly loaded" driving situation. i.e. cruising in city traffic. Then it's definitely better (strictly in the mpg sense) to "lug" the engine between 1000 - 1400 rpm.

If we let the engine creep up to even 2000 rpm, fuel efficiency can drop well into the yellow / red zone.

But OEM probably factors in many other parameters, DPF life can be one of them, or "passing power" / "driver satisfaction". Hence they'll program shift points higher than 1400 rpm.

Funny how times change, we now call 400 lb-ft light load, whereas for the first ISB that was foot to the floor...

Just for clarification. Are you meaning to say 400 Nm or 400 lb-ft? The maps are in Nm.

ShinerBock wrote:

4x4ord wrote:


What mileage do you get in 5th vs 6th towing your 13 k trailer?

:S

I don't know why your truck's fuel economy is such a secret but anyway here's a map giving you all the benefit I can give you. The yellow and red dots represent 8 mpg.


According to your data a 6.7 Cummins pulling about as hard as it possibly can in 5th puts it at 832 Nm of torque output at 65 mph and 1375 rpm. The red dot is at the exact same power output. This power output represents 8mpg at 65 mph. Notice that according to the graph both gears are getting similar fuel economy with a slight favour possibly going to 5th gear.

As the hills get steeper 6th does not offer enough torque and the transmission absolutely has to shift to 5th so the green line represents shared output during heavier parts of the pull.

As the load gets lighter the bottom of the yellow line corresponds to the same power output as the bottom of the red line... Notice how the 6th gear is clearly getting better fuel economy during these stages of the journey.

There is absolutely no way that this BSFC map supports any claim that 5th gear will produce better fuel economy towing a 13k trailer than 6th gear.

Even if you slow down and increase the load by going up a steeper incline so that 5th gear is in the dark blue sweet spot a corresponding power output @ the same mph for 6th gear does not exist on the BSFC map. IE under heavy loads the transmission absolutely must downshift for the truck to maintain speed.

I'll add something, based on the map above, if we set 400 lb-ft, or roughly 40% torque put, as the "lightly loaded" driving situation. i.e. cruising in city traffic. Then it's definitely better (strictly in the mpg sense) to "lug" the engine between 1000 - 1400 rpm.

If we let the engine creep up to even 2000 rpm, fuel efficiency can drop well into the yellow / red zone.

But OEM probably factors in many other parameters, DPF life can be one of them, or "passing power" / "driver satisfaction". Hence they'll program shift points higher than 1400 rpm.

Funny how times change, we now call 400 lb-ft light load, whereas for the first ISB that was foot to the floor...

RoyJ wrote:
Shiner, as far as I can tell, BFSC is proportional to thermal efficiency.

An IC engine turns fuel into 3 components: mechanical power, exhaust, and radiator heat rejection. Both of the latter is wasted heat (I know, some is captured by the exhaust turbine, even more so if it's a compound turbo like Detroit DD15).

Since BSFC = fuel consumption / power. Therefore the better the BSFC, then the better mechanical power to wasted heat ratio. Thus the higher thermal efficiency.

So I believe we can safely state: the better the BSFC, the closer an engine is to max thermal efficiency.

You are correct. I agree.

Shiner, as far as I can tell, BFSC is proportional to thermal efficiency.

An IC engine turns fuel into 3 components: mechanical power, exhaust, and radiator heat rejection. Both of the latter is wasted heat (I know, some is captured by the exhaust turbine, even more so if it's a compound turbo like Detroit DD15).

Since BSFC = fuel consumption / power. Therefore the better the BSFC, then the better mechanical power to wasted heat ratio. Thus the higher thermal efficiency.

So I believe we can safely state: the better the BSFC, the closer an engine is to max thermal efficiency.

4x4ord wrote:
Yes. The current model Cummins makes 343 HP at 1800 rpm ..... At what HP or torque level would you say this engine is lightly loaded while turning 1800 rpm? What about the Powerstroke ... it makes about 360 HP at 1800 rpm. I might say under 50% load but could we agree that usually under 40% load at a given rpm would constitute a light load? That percentage would be significantly lower for many aftermarket tuned engines. Bottom line is we can only guess what gear our truck should be in at any given load and rpm. The engineers who design these trucks could very easily program them to pick the right gear for fuel economy .... If they don't take fuel economy into account when they program the shifting strategy I certainly don't know why they don't but it's not because it is too complicated for them.

"Light load" is indeed too arbitrary, I prefer to look at the lands on the BSFC map, and set a lower boundary as "light load". Here's a map Shiner posted on the other post:



If we use 200-210 as the lower fuel efficiency limit, then on the old engine 480 lb-ft is our light loaded limit, and the 2019 can reach down to 400 lb-ft at certain rpm, and still achieve a favourable efficiency.

At 300 lb-ft, the engine loading is too light, and we need an upshift to load it into a favourable map land.

Wish we can find a map for the post-2019 Powerstroke. But I wouldn't be surprised if OEM transmission logic takes these maps heavily into account. For "tow haul" map, they may assume you're heavily loading the engine, and thus mileage will suffer if you purposely use that mode under a light load.

4x4ord wrote:


What mileage do you get in 5th vs 6th towing your 13 k trailer?

:S

ShinerBock wrote:

4x4ord wrote:

ShinerBock wrote:

4x4ord wrote:
^^^^I don’t think I’m confused. I’m just asking some questions. If an engine is designed to produce 600 lb ft of torque at 1700 rpm and it is running at 1700 rpm putting out 375 lb ft of torque I think we agree the engine is not lightly loaded. If an engine is designed to produce 1000 lbft of torque and is putting out 375 lbft I would say it is lightly loaded. If an engine is designed for to put out 850 lb ft and is tuned to make 1300 lbft I‘m not sure how to determine how it is loaded? Maybe measure it’s fuel economy? If it’s burning more fuel in 6th than 5th towing 65 mph than it likely indicates it is not lightly loaded.

It is not about the load of the engine at peak, but rather the load at the rpm it is at due to its gearing. There is a difference between using 5% to 75% of an engines output at an rpm you are forced to be at due to gearing, and using a percentage of the peak hp of the engine. I am forced to be at a different rpm in 6th than a 4.10 geared truck with the same trans ratio. If we all drove CVT's then I can see your point, but we don't so you have to look at things differently.

Yes. The current model Cummins makes 343 HP at 1800 rpm ..... At what HP or torque level would you say this engine is lightly loaded while turning 1800 rpm? What about the Powerstroke ... it makes about 360 HP at 1800 rpm. I might say under 50% load but could we agree that usually under 40% load at a given rpm would constitute a light load? That percentage would be significantly lower for many aftermarket tuned engines. Bottom line is we can only guess what gear our truck should be in at any given load and rpm. The engineers who design these trucks could very easily program them to pick the right gear for fuel economy .... If they don't take fuel economy into account when they program the shifting strategy I certainly don't know why they don't but it's not because it is too complicated for them.

Again, take it up with Ram and Ford. Not sure the current Cummins or Powerstroke has to do with my truck getting better fuel mileage in 5th rather than 6th when towing my 13k trailer at 65 mph. Again, take it up with Ram, Ford and the other makes if you think you can tune a transmission better than they can.

What mileage do you get in 5th vs 6th towing your 13 k trailer?