Why diesels are most efficient around 1,800 rpm

[ShinerBock]

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."

[ShinerBock]

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.

[ShinerBock]

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.

[ShinerBock]

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.

[4x4ord]

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.

[RoyJ]

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...

[ShinerBock]

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.

[RoyJ]

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.

[RoyJ]

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.

[ShinerBock]

4x4ord wrote:


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

:S

[4x4ord]

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?

[ShinerBock]

Although, they already came up with a way to lock out 6th and I don't even have to be in tow/haul mode. It is that little button on the shifter with a "-" on it. I believe your truck has one too!!!!

It would not be that smart to lock out 6th in tow/haul for the times I am towing lighter 7k loads or towing on flat land with little wind drag.

[ShinerBock]

4x4ord wrote:


I'm not accusing them of being clueless you are. I'll bet you could even wire your truck to lock out 6th automatically every time you select tow haul. Do you think a Ram engineer can't even come up with that?

I am not saying they are clueless. You say it is simple. So I am just saying to go enlighten the engineers at Ram, Ford, and other makes.

[4x4ord]

ShinerBock wrote:

4x4ord wrote:

ShinerBock wrote:

4x4ord wrote:
I would guess that towing 13k down the highway at 65 mph takes about 140 hp. If an engine is designed to put out 300 hp at 1700 rpm and 400 hp at 2800 I’m thinking it is lightly loaded towing a 13k trailer. The best rpm to tow at for fuel economy is not mystical. If a transmission control module is loaded with a BSFC map it is very easy for it to “know” exactly and instantaneously where it is on the map at all times. The decision to shift or not should be very simple for a computer ..... “I’m at 600 Nm at 1270 rpm and after the shift I’ll be at 1700 rpm and 450 Nm. My BSFC is better after the shift hmmm I think I’ll shift.” We have only a seat of the pants feel for where our trucks are running in relation to their BSFC map ..... especially when we have never even seen a map for our engine.

1) Your 140 hp assumption is based on flat land and normal wind drag. I have told you multiple times that my route to the cost is not flat and the part that is flat has very high wind(next to a wind farm). The terrain is a constant up down with up to 3% grades. I am not sure why you keep neglecting this part. My truck will pull most hills in 6th, but I have to put my foot more to the floor to do so. 5th requires considerably less throttle.






Your truck has been tuned so it could be that even though it can make big torque at low rpm it might not get enough air to do it efficiently... I don't know. We agree that a stock Cummins 6.7 towing up a 3% grade should be in 5th. If conditions are such that the transmission is constantly and annoyingly shifting back and forth, lock out 6th. We agree on that as well. I'm pretty sure my Ford will downshift if it's pulling hard enough to be down at 8 mpg and 1400 rpm but when the road levels out it will upshift back to 6th .... it's an automatic.


2)Transmission control is not loaded with a BSFC maps. It is multiple charts that basically says at X rpm, Y load, and Z throttle position upshift or downshift(there are a few other parameters depending on the make). The way you think it works and the way it actually works are two different things my friend. All it knows is to shift when the preset parameters are met. If it was able to be tuned like you think it is, then there would be no need for tow/haul mode or select shift.

I tell you what, why don't you contact Ram and tell them your brilliant idea and how they should be tuning their transmissions from now on. Let us know what they tell you.

That sounds pretty simple to me .... x rpm, y load and z throttle position .... they don't need any more info than that. They can just take a look at a BSFC map to see what those parameters should be to initiate the shift points and program it in. Come on, it doesn't take a rocket scientist to understand this stuff.... If you really think the Ram engineers are that clued out don't you think they could at least wire the transmission so that when you click on the tow haul mode, it automatically locks out 6th... that would at least satisfy what you're saying?

Then why are you speaking to me about it? Go tell Ram and Ford what you think they should do. Then you can go tell ZF and BMW as well because there are times my car holds on to gears longer than it should too. Heck, why stop there. You might as well go enlighten all the makes about this awesome new discovery.

The only one you probably won't have to tell is Eaton although their way doing this is to spend years doing research with the engine manufacturer and deciding that using GPS and preloaded maps in the now very expensive TCM to tell the trans when to shift up hills. Maybe if you tell them how stupid they were and they could have just used engine BSFC maps that they already have available then they will give you some type of medal or post your picture on the employee of the month wall.

I'm not accusing them of being clueless you are. I'll bet you could even wire your truck to lock out 6th automatically every time you select tow haul. Do you think a Ram engineer can't even come up with that?

[ShinerBock]

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.