RoyJ wrote:ShinerBock wrote:
The engine output torque curve of a Cummins 6.7 is MUCH steeper than that at the lower rpm range.
On the left is the curve of a 350hp/800lb-ft Cummins 6.7L from similar to what my truck was stock but with 20 less hp. Max rpm is 3,000.
6th gear at 65 mph in my truck is a 1,370 rpm. 5th gear at 65 mph is 1,790 rpm.
I know the torque curve of a turbo-diesel is much steeper, I was referring to the steepness of the BSFC ringlands (320, 365, 410, etc.) on my graph.
If the curve of the 410 and 365 bends up vertical, so that 6th gear (1370rpm) high load lands above 410, and when you shift to 5th (1790rpm) low load, it falls to the 365 ringland, then you could get better fuel economy.
On my graph, this only happens once, where the 200kW constant power curve intersects the 275 BSFC curve @ ~3250 rpm. So running that engine at 3200 rpm WOT lands you in the 320 BSFC zone, with a downshift to say 4200 rpm, and 70% load, you land just within the 275 zone. But we'll need to see an ISB map like that.
These are best I can get. It is on a new 2019 385 hp/850 lb-ft ISB, but I would wager that my 370 hp/800 lb-ft version is not too far off.
I estimated that it would take about 130 hp to tow what I tow with a mixture of flat land 1% grades at 65 mph. Although this does not count for a lot of wind drag which this route is notorious for having a lot of.
Using this, I calculated how much lb-ft it would take to make this amount of hp at each rpm and then converted that to N-m. Red is 6th gear and white is 5th. Ironically if I had 3.73 gears, I would be very close to the dark blue at 65 mph.
RoyJ wrote:4x4ord wrote:
This is very useful info but it needs to be interpreted properly. So the 5.9 was capable of achieving its best fuel economy at 2000 rpm BUT that is under heavy load. Under heavy load it was able to make 175 hp at 2000 rpm. If its fuel consumption was measured at 2000 rpm and it was only loaded to 50% of its capability it certainly would have been using more fuel than .334 lbs per hphr. I'm talking about slowing an engine down under light load conditions to improve fuel economy.
If that old Cummins had a demand on it of 80 hp, I would call that a light load, It could easily produce 80 hp at 1500 rpm and I would be willing to bet it would achieve better fuel economy producing 80 hp while running at 1500 rpm than it would running 2000 rpm with a 80 hp demand on it.
What I'd really like to see is the following BSFC map (key work being map, not curve) for an ISB or any other modern diesel:
This shows for a given power, a heavily load 5th gear, vs lighter loaded 4th gear, at constant road speed, constant power output (110 kW), you always land on a better BSFC map.
That absolute best for this NA engine is between 4000 - 4500 rpm, and between ~80% - 95% of max torque output at that given rpm range.
Unless a Cummins' curve is MUCH steeper at the lower rpm range, so that 6th gear lands on a lower BSFC "ring" than 5th at a higher rpm, Shiner's case of better fuel economy (in 5th @ lower load) is unlikely. But then again I haven't seen a Cummins BSFC map.
The engine output torque curve of a Cummins 6.7 is MUCH steeper than that at the lower rpm range.
On the left is the curve of a 350hp/800lb-ft Cummins 6.7L from similar to what my truck was stock but with 20 less hp. Max rpm is 3,000.
6th gear at 65 mph in my truck is a 1,370 rpm. 5th gear at 65 mph is 1,790 rpm.
Here is an actual dyno of my truck. Note that the rpms on the bottom are not exact because it is hard to get the torque converter to lock at such low rpms. The curve should start a few hundreds rpms less than it does here.
4x4ord wrote:
This is very useful info but it needs to be interpreted properly. So the 5.9 was capable of achieving its best fuel economy at 2000 rpm BUT that is under heavy load. Under heavy load it was able to make 175 hp at 2000 rpm. If its fuel consumption was measured at 2000 rpm and it was only loaded to 50% of its capability it certainly would have been using more fuel than .334 lbs per hphr. I'm talking about slowing an engine down under light load conditions to improve fuel economy.
If that old Cummins had a demand on it of 80 hp, I would call that a light load, It could easily produce 80 hp at 1500 rpm and I would be willing to bet it would achieve better fuel economy producing 80 hp while running at 1500 rpm than it would running 2000 rpm with a 80 hp demand on it.
What I'd really like to see is the following BSFC map (key work being map, not curve) for an ISB or any other modern diesel:
This shows for a given power, a heavily load 5th gear, vs lighter loaded 4th gear, at constant road speed, constant power output (110 kW), you always land on a better BSFC map.
That absolute best for this NA engine is between 4000 - 4500 rpm, and between ~80% - 95% of max torque output at that given rpm range.
Unless a Cummins' curve is MUCH steeper at the lower rpm range, so that 6th gear lands on a lower BSFC "ring" than 5th at a higher rpm, Shiner's case of better fuel economy (in 5th @ lower load) is unlikely. But then again I haven't seen a Cummins BSFC map.
