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Open Roads Forum  >  Tow Vehicles

 > Why diesels are most efficient around 1,800 rpm

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ShinerBock

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Posted: 10/20/20 04:47pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:



I guess we agree then.

When you look at the BSFC map that you posted you can see that the rpm and load you selected are for one very specific circumstance. If you start pulling a little harder the transmission is going to automatically shift down a gear. If you start pulling slightly lighter you will be getting better fuel economy in the higher gear. Towing a trailer down the road is a constantly changing load so the actual load might seldom be at the average. Even if you have a BSFC map for your engine, unless you are constantly watching your instantaneous fuel consumption and are able to calculate in your head how your mpg converts to engine torque and rpm, you will have an impossible task of knowing which gear is most fuel efficient. I'm saying it is best to keep it simple and just let "Drive" with "tow haul" do the thinking for you. The basic principle is throttle down and gear up for better fuel economy. If we find ourselves on a road where the transmission is constantly up shifting only to end up in too high of gear where it needs to recover with a downshift then I know we would agree it's time to lock out 6th.


I was able to find out which gear was most efficient without a map by using the averages of multiple trips of pulling my trailer to the coast at 6th gear and 5th. It really wasn't that hard to do and if I would had let the transmission do all the work then I would be getting less fuel economy even in tow/haul mode.

My father in law will be testing this theory in his 2019 F350 Fx4 3.55 in the coming weeks. He will tow four 20 ft shipping containers one at a time back and forth from out deer lease(130 miles). He said he was going to try towing two with 6th unlocked and 2 with 6th locked out.

Below where my truck would be at towing my 13k 5ver. Up/down is the amount of torque needed for each grade and right/left is rpm at each speed. Yellow is 6th gear and red is 5th. There are definitely certain speeds(especially 60-65 mph) and loads when my truck is more efficient in 5th than 6th. Of course these torque requirements are based on weight and do not calculate for a lot of drag so I likely need more torque pulling my 5ver at each grade.

[image]

* This post was edited 10/20/20 04:59pm by ShinerBock *


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ShinerBock

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Posted: 10/20/20 04:57pm Link  |  Quote  |  Print  |  Notify Moderator

On the right is the BSFC of the new 2019 standard output Cummins with a higher compression ratio. Its sweet spot is much larger than the 2018 and it would be even more advantageous to tow in 5th at more speeds and loads than 6th if you had a 3.42 axle.

[image]

valhalla360

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Posted: 10/20/20 05:08pm Link  |  Quote  |  Print  |  Notify Moderator

Actually, the bore and stroke work with the speed of fuel burn so that for our typical truck diesels, 1800 rpm is the sweet spot.

If you used a shorter stroke with a wider bore, so that you maintain the same compression ratio, the ideal RPM would in theory change.

Lots of small boat diesels are optimized for higher RPM. Yanmar LY hit peak torque around 2500 RPM.


Tammy & Mike
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StirCrazy

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Posted: 10/20/20 06:51pm Link  |  Quote  |  Print  |  Notify Moderator

every engine will have a different sweet spot, 1800 is not a best fuel milage point but rather a max power point, but this is even changing with new tech in diesel engins. this was originaly to do with power (as in electricity) direct drive generators need to run at 1800 rpm to make ac power, and there are exceptions to this now also.

you max fuel economy will be the lowest rpm you can do the work with out getting into the turbo to much. for my 5th wheel and my 14 ford 6.7 going down the highway at 95kph at just over 1400 rpm and 4 to 5 psi on the turbo gives me my best milage but my torque hits 800ftlb at 1600 rpm and stays there to 2000 rpm so new technology is taking that 1800 power spot and making it higher lower or wider as in my truck.

Steve


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azrving

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Posted: 10/20/20 07:16pm Link  |  Quote  |  Print  |  Notify Moderator

I knew from the beginning that this had pissing contest written all over it. Carry on lol

Grit dog

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Posted: 10/20/20 07:16pm Link  |  Quote  |  Print  |  Notify Moderator

Bird Freak wrote:

I think you are stretching to far with your ALL DIESEL'S remark. When I was the lead mechanic at Saunders Leasing all of our class 8 trucks had governors set at 1800 rpm. You think they had to run wide open to get max mpg?


I think many can't actually read objective, helpful or interesting information on this forum without resisting the urge to automatically dispute it and start an argument....basically acting like a small child.
They say as people age, they regress back to their childhood tendencies and abilities.....maybe some truth to it?

I mean, use some anticipated context for cripes sakes. There are diesels that don't even reach 1800rpms. The worlds largest diesel engine redlines at 102 RPMS!
There Shiner, what a dumb @ss....1800rpms....you're an idiot....stupid false claims again! (obviously kidding)

But are we talking about class 8s? What does the vast majority of discussion revolve around here?

I'll let you answer that......
Or if you'd rather, you can google some other useless info like I did and tell me I'm wrong because you found something that says the worlds largest diesel redlines at 101 RPMS.....


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Grit dog

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Posted: 10/20/20 07:17pm Link  |  Quote  |  Print  |  Notify Moderator

azrving wrote:

I knew from the beginning that this had pissing contest written all over it. Carry on lol


Everything here does....
See my post above!

4x4ord

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Posted: 10/21/20 04:14am Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

On the right is the BSFC of the new 2019 standard output Cummins with a higher compression ratio. Its sweet spot is much larger than the 2018 and it would be even more advantageous to tow in 5th at more speeds and loads than 6th if you had a 3.42 axle.

[image]


When that engine is working at 1800 rpm and just up to the dark blue sweet spot as you call it it is putting out a minimum of 150 horsepower. (440 lb ft of torque) According to the graph it would be burning 7 gallons of diesel per hour. Assuming you’re towing at 65 mph you’d be getting 9.3 mpg. So in order to be in the dark blue you’re working that truck . The dark blue represents 6.2 to 9.8 mpg at 1800 rpm and 65 mph. I would expect when you’re down at that kind of mileage the automatic is going to be selecting 1800 rpm over 1360 rpm.

Edit: I didn’t do the math on whether or not you can get a tire size and final drive ratio to do 65 mph at 1800 rpm. If you’re only doing 60 mph at 1800 your fuel economy is going to be about 10% worse to get into the dark blue of the graph.


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Groover

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Posted: 10/21/20 06:29am Link  |  Quote  |  Print  |  Notify Moderator

The advantages to lower rpm are earlier and more complete fuel burn plus less friction loss. The drawbacks are cylinder leakage and heat loss. The gains improve with lower rpm but so do the losses. However, the losses decline with cylinder size and can be reduced further by eliminating valves. So it is common for large diesel engines where weight isn't an issue to run a much lower rpm and be 2-stroke. This is epitomized by the world's largest ship engine that runs at 15-102rpm and has over 50% thermal efficiency. It weighs 2300 tons so don't plan on putting one in your pickup.

Wartsila Sulzer RTA96-C / Engine

The engine is so big that it has staircases built inside the lower part of the engine block to facilitate work on the crankcase bearings.

4x4ord

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Posted: 10/21/20 07:20am Link  |  Quote  |  Print  |  Notify Moderator

^^^^ According to the graph the 6.7 Cummins running in the dark blue is running at 41% efficient.

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