Efficiency of modern diesels at low power

[DavidM]

We have debated around this point on several threads: what is the efficiency of diesels at low power settings. One might think that efficiency drops off dramatically at low power, low rpm settings. Not so, it seems.

Eric R. a mechanical engineer turned diesel mechanic posted this data on a recent boatdiesel thread. It compared fuel consumption at a high cruise setting- 2,600 rpm with fuel consumption at very low settings- 1,200 rpm for two engines, the Cummins 370 hp 6BTA and the Cummins 380 hp QSB. The latter is a totally modern common rail engine and the former is a very well designed mechanical engine.

Here are the numbers:

6BTA, 2,600 rpm
QSB 2,600 rpm 285 hp 14.6 gph 19.5 hp/gph

6BTA 1,200 rpm 31 hp 1.9 gph 16.4 hp/gph
QSB 1,200 rpm 32 hp 1.9 gph 16.9 hp/gph

He didn't post the 6BTA at 2,600 rpm numbers but they should be pretty near the QSB value.

So the fuel efficiency drop was about 15% from high to low rpms/power but also surprisingly the common rail injection made no real difference at low (or high) rpms.

BTW the boatdiesel thread was about the wisdom of repowering with the Cummins Q engine vs the C engine (the B's big brother). Eric showed that fuel consumption between the two engines which spans a 20 year design period was not significant.

David

 

[Baker]

And to further a point...or maybe a related point...a beleaguered one at that. Many people always talk about the "proper sizing" of an engine to a boat. IOW, compare a smaller engine developing that 31hp and you end up with only about a 5% difference at most. I realize there is no need to put big engines in a displacement hull that does not benefit from it. But running big engines on a big planing boat at low power is not really inefficient when compared to smaller engines moving the same boat making the same power....as most people think!

 

[RickB]

One might think that efficiency drops off dramatically at low power, low rpm settings. Not so, it seems.

Considering that engine builders, naval architects, and commercial operators might consider selling their daughters to increase fuel efficiency by 2 or 3 percent, 15 percent is a huge number. However, the place where that 15 percent lives makes it all but irrelevant.

You avoided mentioning the elephant in the stateroom. The fuel burn at reduced power is reduced so dramatically that the loss of efficiency is moved into the meaningless category. If I can save a $1000 in fuel each day and still get the job done by reducing power why should I be upset that I am not saving $1150?

 

[Codger2]

But running big engines on a big planing boat at low power is not really inefficient when compared to smaller engines moving the same boat making the same power....as most people think!

I've found this to be true when running my Cummins 330B @ 2000rpm & below.

 

[Baker]

I've found this to be true when running my Cummins 330B @ 2000rpm & below.

I hope to be running a couple of 330Bs at 2400 by the end of the week!!!

 

[Brooksie]

Considering that engine builders, naval architects, and commercial operators might consider selling their daughters to increase fuel efficiency by 2 or 3 percent, 15 percent is a huge number. However, the place where that 15 percent lives makes it all but irrelevant.

If I were building from scratch (maybe in my next life) I would do this with a 2 speed transmission to be sure I am "where that 15% lives" at speeds I use most frequently.

 

[Nomad Willy]

Diesel engines are heat engines and any loss of heat that doesn't do work is just lost.

Bigger engines have bigger piston crowns, combustion chambers, greater surface area of their cylinders and bigger heat exchangers. All of this is wonderful paths for heat loss. Pumping heat from burning fuel into the atmosphere and the sea should be a very good path to inefficiency.

Comparing two Lehmans w 760 cu in putting 35hp to her props to two 20hp Yanmars at 80% load one can see an extreme difference in potential heat loss. But the fuel burn difference I'm sure would not be that great. I'm sure a lot of the reason for that is that the temps in the big engines would be FAR lower than the tamps in the little Yanmars. But both would probably burn about 2gph pushing the boat about 6 knots.

I see some very big old diesel engines that their owners claim very low fuel burn rates. It seems to me the reason is that big engines loafing generate so little heat there's not much heat to loose.

That's the only explanation that generally works for me but there's much more to it like friction and auxiliary systems power loss.

 

[Codger2]

I hope to be running a couple of 330Bs at 2400 by the end of the week!!!

Max cruise,Huh? I guess that's why you jet jockeys get the big money!

 

[Nomad Willy]

March,
Based on the assumption that the newer of the engines you posted on (the QSB) has variable fuel injection timing I wonder if that feature would have an effect on fuel economy while running slow. It's been established on another thread that the variable timing makes engines much more tolerant re under loading. But what of the economy?

 

[DavidM]

I can draw a number of inferences from Eric's data, most of which have been noted by other posters to this thread:

1. The fuel consumption penalty of running a high output engine in a fast trawler slow, is minimal- maybe a few tenths of a gph, vs having an engine designed closer to the slow speed power requirement- see more below.

2. The benefits of overpropping any engine to move it higher and to the left on the power/rpm/fuel curve is also minimal, again maybe a few tenths of a gph.

3. And as RickB noted, the numbers are so small anyway, that any differences are mostly meaningless.

To expand a bit on #1:

A moderate size, full displacement trawler like a Willard 36 will require about 30 hp to push it to hull speed. A reasonable engine sized for this requirement is the NA Yanmar 4JH5E. At 30 prop horsepower this engine burns (to the best of my reading Yanmar's curve) about 1.6 gph.

A huge QSBxxx, about 3 times the displacement of the Yanmar, running at a bit less than 1,200 rpm will make that same hp and burn 1.8 gph. Now no one should run a QSB that slow, but it illustrates the point. That extra 0.2 gph goes to turn over the bigger iron- bearing area, ring area, etc and dump a bit of extra heat (but not much as someone noted that this engine will run much cooler at 1,200 rpm, too cool probably) due to its greater surface area.

The Yanmar will have to run at 2,500 rpm which is probably at its lowest BSFC point on the curve so it isn't so surprising that it is efficient.

And FWIW the fuel consumption penalty of a semi-displacement hull running at displacement speed is much more significant than any engine inefficiency in the big engine that powers it.

So lets talk more about hull shapes ;-).

David

 

[DavidM]

March,
Based on the assumption that the newer of the engines you posted on (the QSB) has variable fuel injection timing I wonder if that feature would have an effect on fuel economy while running slow. It's been established on another thread that the variable timing makes engines much more tolerant re under loading. But what of the economy?

I don't know if the QSB implements variable injection timing, but I suspect that it does.

But I also suspect that that feature was implemented to reduce emissions, not to improve fuel economy. In fact the data shows that common rail injection, as implemented, has had no meaningful effect on fuel economy.

Smoothness, quiet running and low smoke- yes but not fuel economy.

David

 

[Brooksie]

Not to belabor the point but 1.6 vs: 1.8 GPH is a 15% savings which is a huge gain from a designer's point of view so RickB's point seems well taken.
A properly designed 2 speed system could retain these savings at the most commonly used speeds. I don't know what else could accomplish this.
CPP's have an inherent inefficiency that would cancel out some of the gain but the gain that remained would be available at many speeds. Tried & true but complex & expensive.
Two engines on one shaft, as discussed in detail in Beebe's first edition, might accomplish the same savings at 2 speeds but expensive, untried, & complicated.

 

[RickB]

Two engines on one shaft, as discussed in detail in Beebe's first edition, might accomplish the same savings at 2 speeds but expensive, untried, & complicated.

Not really, look up lohmann and stolter or "twin in single out gearbox." There are many in use, most with CP propellers and not really very complicated. They are sturdy enough to work for years in commercial service without problems.

You might be amazed at the kind of marine machinery that is available if you look around.

 

[capt jerry]

years ago the LST of the navy had 4 engines on each shaft

 

[FF]

If I were building from scratch (maybe in my next life) I would do this with a 2 speed transmission to be sure I am "where that 15% lives" at speeds I use most frequently.

This would be the easiest way to go efficient.

The LST trannys can be found used , last I saw (boats & Harbors) a US NAVY rebuild of two 6-71 with tranny was about $6,000 on a shipping pallet.

For most replacing one 6-71 with a 2-71 or 3-71 would solve the efficiency hassle, at really low bucks , BUT the 71 series is very heavy compared to todays flyweight engines.

A very modern light boat , light engine would be better with the 2 speed tranny.

QSB 2,600 rpm 285 hp 14.6 gph 19.5 hp/gph

6BTA 1,200 rpm 31 hp 1.9 gph 16.4 hp/gph

19.5 minus 16.4 is 3.1 ,

I believe 3.1 is a higher improvement percentage of 16.4 than 15%.(no computer)

The improvement is also in far lower noise at much lower cruise RPM , and fewer piston miles per boat mile , so longer engine life.

 

[Brooksie]

If I were building from scratch (maybe in my next life) I would do this with a 2 speed transmission to be sure I am "where that 15% lives" at speeds I use most frequently.

This would be the easiest way to go efficient.

QSB 2,600 rpm 285 hp 14.6 gph 19.5 hp/gph

6BTA 1,200 rpm 31 hp 1.9 gph 16.4 hp/gph

19.5 minus 16.4 is 3.1 ,

I believe 3.1 is a higher improvement percentage of 16.4 than 15%.(no computer)

.

The 15% RickB and I were talking about was for running any diesel only at its peak hp/gph by using CCP or multi speed transmission.
Others were talking about sending a large engine to do a small job.
I think your 20% savings figure includes both...

 

[FF]

That 20% + difference is the Holey Grail for the electric propulsion folks.

All their schemes and big bucks complexity are geared to capturing that 20% and eventually using it.

I would prefer to use a cruising prop, no complexity , or the 2 speed gear box and have no huge expensive heavy battery sets to drag out and down the dock.

A ZF 2 speed gearbox is OTS and should last the life of the boat , unlike a batt set.

 

[freshalaska]

3 speed gear box

My boat in the Philippines has an automotive type transmission. 3 forward gears. I have to clutch with my foot to shift. Why, because it is cheap set up. With a variable pitch prop I might really have a good set up. It does work however. I never use first gear as it basically only makes noise and reverse isn't too good either, wrong gearing. I usually run in 2nd gear but with the wind pushing me have used 3rd gear.
My engine 140hp Isuzu 6 cylinder old style, newly rebuilt diesel, with gear box, shafting, pillow block, stuffing box, wooden shaft bushing, wet exhaust, bilge cooler, the works installed would run less than $3000 US if done in the Philippines. As reverse doesn't do much we had to throw out a stern anchor once to stop the boat when we ran up on some corals.

 

[Ted]

In times past automotive transmissions were modified to make 2nd gear drive the reverse gear. Shifted using the clutch but the synchro still worked. Reverse was then at the 2nd gear reduction, not the deep reduction of the standard auto trans. reverse.
Ted

 

[freshalaska]

When I get back over to PI I will look into that. I've heard of doing that before but sort of got busy and forgot about it. Having a reverse that really worked would be nice. Thanks for the tip.

 

[FF]

You are a prime candidate for an EGT gauge.

That would allow the engine to not lug at the highest gear you select.

 

[freshalaska]

egt gauge

What is and how does a EGT gauge work?

 

[Nomad Willy]

It's a probe in the exhaust pipe to measure the temp of the exhaust.

To be accurate it should be located exactly where the manufacturer specifies.

If one props the engine for rated rpm at WOT no exhaust gas gauge will be needed. You won't be able to overload your engine except at specified engine speeds (almost at WOT) and for specified periods of time.

But if you over prop your engine you should have the EGT to know when you are overloading the engine. That point may be several hundred rpm below the engine's normal over loading limit conditions.

 

[FF]

"If one props the engine for rated rpm at WOT no exhaust gas gauge will be needed. You won't be able to overload your engine except at specified engine speeds (almost at WOT) and for specified periods of time."


IF the engine is being operated at max power (the reason you chose to prop it at max rpm) each and every time a displacement boat is stopped or really slowed by a big wave the engine will be in overload.

Only for a part of a min , just as sport fish are overloaded when accelerating from slow speeds.

Overload does not mean the engine will break , it means more fuel is being delivered than there is air to burn it ,, hence big black smoke .

Underload means there is less load on the engine than the fuel mix can deliver , which is a waste of fuel , and MAY harm the engine thru wet stacking , oil dilution from blowby , or cylinder burnishing .

A "proper" load is where the engine is using most (80%) of the power it can produce at THAT RPM.

Since most folks have only 2 or 3 cruise RPM (some only one) the ability tp operate efficiently is a personal choice and skill.

 

[Brooksie]

That 20% + difference is the Holey Grail for the electric propulsion folks.

All their schemes and big bucks complexity are geared to capturing that 20% and eventually using it.

I would prefer to use a cruising prop, no complexity , or the 2 speed gear box and have no huge expensive heavy battery sets to drag out and down the dock.

A ZF 2 speed gearbox is OTS and should last the life of the boat , unlike a batt set.

I read the articles by Caulder in ProBoat re: electric propulsion. My letter in this month's issue says it all.

Re: ZF, I contacted them and they had no interest in using their 2 speed gearbox in that manner and would not even comment on my application. They said, as I recall, their gearbox is only to shifted "up" under load not "down" which is what I was proposing to them. Also it seemed that the ratios were all wrong. What I was proposing was starting off in "High" gear and running up to cruise speed at which point the engine would be fully loaded (or nearly so) then downshifting to "low" gear to continue up to flank speed. They dismissed the idea offhand... Their loss

 

[Baker]

Max cruise,Huh? I guess that's why you jet jockeys get the big money!

I don't know about MAX cruise...just cruise. These engines should be quite happy at 2400rpm. I would guess that to be right in the sweet spot of 70-80%. And I have since run the boat there....and I have to say that I am becoming a Cummins convert. They are quiet sweet running engines. Nothing but a low hum at that rpm and a solid 19 knots!!! Now I need to go to boatdiesel to check out the fuel/power curves to see what it is really doing.

 

[motion30]

when I bought my 44 foot send a cruiser with 320 hp. Cats. The po. Had really large props installed that would not let the motors ven approach the recommended the RPM I installed the correct props and really do not see much. difference in low speed cruise and fuel economy but the top speed is greatly increased

 

[Nomad Willy]

I said;

"If one props the engine for rated rpm at WOT no exhaust gas gauge will be needed. You won't be able to overload your engine except at specified engine speeds (almost at WOT) and for specified periods of time."

FF replies;
"IF the engine is being operated at max power (the reason you chose to prop it at max rpm) each and every time a displacement boat is stopped or really slowed by a big wave the engine will be in overload."

The reason to prop at wot at the rated rpm has nothing to do w the ability to run the boat at max speed or max engine output. Nothing to do w it at allll. It's a proper adjustment of the load the engine sees that matches the power output to the resistance of the propeller so the engine can be operated throughout it's range of power output and engine speeds so that all is balanced and working together.

If one is going to operate in very large seas at wot for any more than a few moments he's probably an absolute fool. A good skipper will quickly adjust throttle to match conditions. But an over propped boat will be over propped during much of it's upper throttle range and lack considerable power to climb the big seas. Under propping and a speed limiter set only 100rpm over rated rpm would probably be best for that condition.

The reason one props an engine at rated rpm is basically just a test to know the system is operating correctly. Then one can go to the book and see (for example) that one can operate said engine continuously at 150rpm below rated speed and rated speed is 2500 then the engine can be operated at 2350rpm all day long.

If you over prop an engine you may only be able to apply or generate 90hp w a 120hp engine and the engine may still be over loaded at 2000rpm. You may as well through away your 120hp engine and get a 60hp engine if 60hp is all you need.

But most here HAVE the 120hp engine that is putting out 60hp and about 5% more efficiently than it would if it were propped correctly so why do it right? If you NEVER use more than 60hp running it like that MAY not have a significant downside as long as you KNOW your'e not injecting more than the optimal amount of fuel. In other words not running too rich. If you absolutely know that and NEVER run harder than a 60hp load you probably can over prop for a long time but that's just my opinion and you'll need to live w your own opinion.

Motion 30,
Of course. You've opened the door for what the engine has to offer and you'll probably never notice the slight extra fuel you burn and the slightly higher burn rate will only be noticed at very low rpm. At higher speeds you will probably burn less not more. All around a better way to go.