Big HP vs Small HP

[Don L]

Here is a nice practical example,
The first engine in our ship was a 6 cylinder 60 hp Nanni.
The consumption was 4 liters per hour at a speed of 6 knots at 1400 prm
After 15 years, a new engine has been installed.
The second engine a 4 cylinder 100 hp Nanni
The consumption now is 4 liters per hour at a speed of 6.5 knots at 1400 prm.
The ship is a real displacer so the length of the boat ultimately determines the speed.
The 40 hp more power compared to the first engine results in a slightly higher top speed of 0.4 knots.

what is the boat?

 

[Pascall]

what is the boat?

https://www.trawlerforum.com/forums/s53/discover-ms-passi-65784.html

 

[North Baltic sea]

Hi, I think the difference isn't as big as you might think from a quick thought.

I am comparing a sailboat of the same length vs my nt37 with Cummins qsb 5.9L 380hp. I have often reached the trip computer with a measurement of 7.6 nm/gal with a speed of about 5.8 knots.

The consumption is about the same on a sailboat with a much smaller engine.

As the speed increases, consumption naturally increases, or wind, current, big waves.

another example 106 GT, L22.56m, B6.10m and D 2.8m vessel, engine Wärtsilä Vasa 514T 510 hp marine engine. At a speed of 7 knots consumption is 4nm/gal and this engine is really big in size and displacement.

https://fi.m.wikipedia.org/wiki/M/S_Famnen


NBs

 

[Greg QS]

So I have done a lot of reading on the trawler fuel use verse HP question. The articles have said that by far the biggest difference in fuel use/economy is speed. They say that if you run at displacement speed or less that the size of the engine doesn't matter all that much. So if motoring along at 7 kn on dual 135 HP engines you will use about the same at if you motored along at 7 kn on dual 350 HP engines.

Now part of me says that makes perfect sense as the required power is the required power and all that changes is the efficient operating point of the engines.

But the other part of me says ............BS bigger engines use more fuel PERIOD!

So what have the kind boaters here on TF with trawlers with big engines have to say on the matter?

At trawler speed no a significant difference.

advantages of larger basically More speed/ power when needed.
The ability to handle running currents
The ability to piggy back on heavy seas or inlet waves.
The ability to maneuver out of the way .

Primary disadvantage Larger engines normally come with
Higher maintenance costs
higher repair bills
every thing is just more expensive for a larger engine
especially if you consider a 150 hp to a 660 hp

 

[twistedtree]

I think there are two distinct situations that need to be considered.


First is same displacement engine, one with a low HP rating vs one with a higher HP rating. In this case, the two engines, producing equal HP and pushing the boat at the same speed will use the same fuel. An example is a Deere 4045AFM85, one with 168hp rating and the other with 225hp rating. Both burn 6gph when producing 100 hp.


The other situation is comparing a smaller displacement engine with a larger displacement engine. In this case I think the results are a bit counterintuitive. You would think the parasitic loads on a larger engine, especially if it has more cylinders, would be greater causing it to need more fuel to produce the same output. But in general larger displacement engines are MORE efficient, not less efficient. Here's an example comparing a 168hp Deere 4 cylinder engine (4045AFM85) to a Deere 6 cylinder 400hp engine (6135SFM85). When both are producing 100hp, the smaller engine consumes just under 6gph, where the larger engine only consumes 5gph.

 

[O C Diver]

The other situation is comparing a smaller displacement engine with a larger displacement engine. In this case I think the results are a bit counterintuitive. You would think the parasitic loads on a larger engine, especially if it has more cylinders, would be greater causing it to need more fuel to produce the same output. But in general larger displacement engines are MORE efficient, not less efficient. Here's an example comparing a 168hp Deere 4 cylinder engine (4045AFM85) to a Deere 6 cylinder 400hp engine (6135SFM85). When both are producing 100hp, the smaller engine consumes just under 6gph, where the larger engine only consumes 5gph.

That may be true at 100 HP. The more common scenario on this forum is a large engine running at 10% of its capacity near the bottom of its torque curve. I would imagine the results would be different with the 6 cylinder engine making 40 HP at 1,000 to 1,2000 RPM.

Ted

 

[Hippocampus]

TT that’s counterintuitive. Accept your wisdom and knowledge in these things although it makes my prior post untrue. So……… why is that?

Accepting your post then why not put in the highest HP M2 or M1 whose weight and size the boat could tolerate. Was the 6 cylinder the M2 rated version ? I’ve assumed wear relates to the amount of fuel that goes through an engine not hours. Assumed that’s due to heat and friction. I’ve assumed the lower M number has better cooling and lube so could tolerate being at maximal load mor 80% load for a longer uninterrupted time and with common rail no issues at 20-40% load. Is this irrelevant as much wear occurs at startup and shut down and little while running or is it something else?

Would a bigger lower M number engine asked to give the same amount of different HPs in the same range as a smaller higher M number have less wear and a longer service life before rebuild? Most recreational boats have high M numbers. Many long range or commercial boats have low.

I very rarely use the 540hp I have access to. Might need 14-15 kts once in a blue moon but 17-18kts I don’t think I’d miss. Do admit exceeding hull speed at times is quite helpful as is the ability to overcome current or seas but would think most SD trawlers are overpowered for how they are used. So other than marketing what’s the advantage of overpowering recreational trawlers?

 

[twistedtree]

That may be true at 100 HP. The more common scenario on this forum is a large engine running at 10% of its capacity near the bottom of its torque curve. I would imagine the results would be different with the 6 cylinder engine making 40 HP at 1,000 to 1,2000 RPM.

Ted

Well, let's take a look and see.


The 6 cyl Deere at 1000 rpm produces 73hp and consumes 3.7 gph


The 4 cyl Deere at 1800 rpm produces 76hp and consumes 4.3 gph.


This is perhaps the more telling summary, looking at the BSFC (fuel consumption per hp produced). It's quoted in grams per KWh.


Across the prop loading power range from 1000 rpm to 2300 rpm, the Deere 4 cylinder consumes a high of 279 at 1000 rpm, and a low of 237 at 2300 rpm. From 1200-2000 rpm it ranges from 244 to 258.


The 6 cylinder Deere that is nearly 3x the displacement consumes 219 at 1000 rpm, 213 at 1800 rpm (WOT), and from 1100 to 1700 rpm ranges from 212 to 221.


So the 13L is more fuel efficient at every operating point compared to the 4.5L.


Granted, this is data from two engines where I have ready access to the data sheets. If someone has the data, it would be interesting to see how this plays out for something like a Cummins 5.6 or 6.7 compared to an 8.3L or 11L


My 13L Scania runs 190 to 200 across its operating range, but I haven't compared it to the 9L or the 16L.

 

[FWT]

It is a fascinating question.

When solving for fuel use ONLY, comparing high to low HP engines, its a hard comparison when the data sets are different hulls, different make engines, etc.

Which is where, to me, looking at the Cummins QSB line comes in handy. There is a broad range of HP offered from the same manufacturer, same engine block, and decent data provided by the manufacturer. The means of testing or estimating would be the same, and wash out differences in assumptions and testing gear from manufacturer to manufacturer

Cummins does offer different configurations within the same block, same HP. High Output is designed for recreational use, and ANY commercial use voids the warranty. Intermittent Use is designed for commercial sport fishing, and other commercial use. And so forth.

https://www.sbmar.com/technical-information/cummins-diesel-ratings-definitions/

To keep the answers to the narrow question of fuel use by max HP of the engine, its a good idea to be sure to compare engines within the same class.

Here are 3 data sheets for the QSB, at 250HP, 380HP, and 480HP. All High Output (recreational use.)

250
https://mart.cummins.com/imagelibrary/data/assetfiles/0056067.pdf

380
https://mart.cummins.com/imagelibrary/data/assetfiles/0056005.pdf

480
http://https://mart.cummins.com/imagelibrary/data/assetfiles/0055949.pdf

One of the things one discovers is the importance of viewing at least 3 different engines. There are some wiggles in the data, where results from 250-380-480 go up then back down. Obsess on a difference between only 2 and I think some wrong conclusions can be drawn.

So there's the data.

My conclusions:
1) At 1400 RPM, which in a Helmsman 38 would give a cruise speed in the area of about 7.5 knots, a 250 burns 2.4 gph, a 380 burns 2.6 gph, and a 480 2.6 gph. A material difference? Not to me.

2) The 250 hp max continuous RPM is 2400. So to stick with that Higher RPM across the engine sizes: the 250 burns 10.1 gph, the 380 burns 10.6 gph, and 480 burns 10.1 gph. It would appear the 380 hp data is some anomaly. Again, no difference

And if you pull a 4th data set, to the 550 Government Service configuration there is a big jump in burn. Why?

If you want to dig deeper looking at different models, go here, and search under Engines / Marine / QSB
http://https://www.cummins.com/brochures

The data seems clear enough. Solving as best one can for HP difference alone, there is no material difference in fuel burn.

Just looking at the Cummins lineup, there is a smaller marine engine I know nothing about. The B5.4L 230hp. At 2600 RPM it burns 12.3 gph compared to the QSB 380 at 13.1 gph. Heavens knows if the two represent a decent comparison, because at 2600 RPM the 380 produces 255 HP compared to 230 HP for the B5.4L.

Might there be a different make of smaller HP engines with more efficient fuel burn? Sure. But that would be a different question.

 

[KnotYet]

I think there are two distinct situations that need to be considered.


First is same displacement engine, one with a low HP rating vs one with a higher HP rating. In this case, the two engines, producing equal HP and pushing the boat at the same speed will use the same fuel. An example is a Deere 4045AFM85, one with 168hp rating and the other with 225hp rating. Both burn 6gph when producing 100 hp.


The other situation is comparing a smaller displacement engine with a larger displacement engine. In this case I think the results are a bit counterintuitive. You would think the parasitic loads on a larger engine, especially if it has more cylinders, would be greater causing it to need more fuel to produce the same output. But in general larger displacement engines are MORE efficient, not less efficient. Here's an example comparing a 168hp Deere 4 cylinder engine (4045AFM85) to a Deere 6 cylinder 400hp engine (6135SFM85). When both are producing 100hp, the smaller engine consumes just under 6gph, where the larger engine only consumes 5gph.

There are several factors at work in your specific examples. The published fuel
consumption curves that I can find give calculated propeller horsepower figures.
At 100hp, the 4045's values are in the 'heart' of that engine's power range, but,
at 100hp, the 6135's values are down on the 'toe' of the measurement range. On
some of the data charts the curves omit that first few hundred rpm around 100hp.
Since it is a calculated hp value, not measured, it might be an approximation.

Another reason a much larger engine could post better fuel consumption figures
is that it may be operating in an ultra-lean condition at such a low power output.
I will agree that, whatever the case, the 6135sfm85 is a really efficient engine!

 

[O C Diver]

Well, let's take a look and see.


The 6 cyl Deere at 1000 rpm produces 73hp and consumes 3.7 gph


The 4 cyl Deere at 1800 rpm produces 76hp and consumes 4.3 gph.


This is perhaps the more telling summary, looking at the BSFC (fuel consumption per hp produced). It's quoted in grams per KWh.


Across the prop loading power range from 1000 rpm to 2300 rpm, the Deere 4 cylinder consumes a high of 279 at 1000 rpm, and a low of 237 at 2300 rpm. From 1200-2000 rpm it ranges from 244 to 258.


The 6 cylinder Deere that is nearly 3x the displacement consumes 219 at 1000 rpm, 213 at 1800 rpm (WOT), and from 1100 to 1700 rpm ranges from 212 to 221.


So the 13L is more fuel efficient at every operating point compared to the 4.5L.


Granted, this is data from two engines where I have ready access to the data sheets. If someone has the data, it would be interesting to see how this plays out for something like a Cummins 5.6 or 6.7 compared to an 8.3L or 11L


My 13L Scania runs 190 to 200 across its operating range, but I haven't compared it to the 9L or the 16L.

Ok, so how are you measuring HP?

I'm having a hard time buying that there is a 10% efficiency difference in the production of fuel to HP.

Ted

 

[Hippocampus]

Excuse my ignorance. Still don’t get it.

Why would a higher displacement engine be more efficient in producing the same HP?

If repowering or doing a new build how should you go about picking an engine? Largest displacement that weight and space allows producing an adequate range of HP? Lightest smallest engine regardless of M rating?

What’s the downside of being “overpowered” given modern aftercooled, turbocharged, common rail engines?

 

[DDW]

There is a big divide between mechanically controlled and common rail electronically controlled engines using piezo injectors. The OP clarified that he is considering mechanically controlled engines, and those will have a much more peaked specific fuel consumption map, the peak usually occurring near peak torque or between peak torque and peak power. A modern common rail electronically controlled engine has a much flatter specific fuel consumption map, because the injection spray efficiency is unaffected by rpm, and can be tailored at each running condition to peak efficiency. This is not a small difference, when run at fractional loads. In that case, a larger engine, or two engines, are likely to be less efficient than a small one run near its peak efficiency.

For the naysayers about the SD Nordic Tug, my SD American Tug does similar numbers. Around 3.7 nm/gal at 7.2 knots, from a 380 QSB running at about 40 hp output. We typically run at 1200 or 1250 rpm, 1.9 g/hr. resulting in 7 knots full fuel load and 7.3 knots near empty. I've run it several thousand miles that way.

But that is a common rail engine. Injection pressure is 20 Kpsi regardless of rpm, and there may be several injection events per stroke. Engines more modern than the QSB run at 30 Kpsi pressure and can have 7 or 8 injection events per stroke. That doesn't happen in a mechanical engine.

 

[DDW]

Another reason a much larger engine could post better fuel consumption figures is that it may be operating in an ultra-lean condition at such a low power output.

Ultra-lean isn't really a Thing in diesels. The fuel injected is burned, up until there is no oxygen left - and that is max torque. At any condition less than full torque, it is lean (if we define that as unburned oxygen in the exhaust), as you reduce the load on the engine leaner and leaner until the only load is the parasitic loads of keeping the engine turning at that rpm. This is true of any diesel.

Large and small engines will have different combustion chamber shapes, surface to volume ratios, and flame front characteristics. Small non-common rail engines also are often indirect injection engines. These things have an effect on efficiency.

 

[mvweebles]

To bring this full circle with respect to the original post. Recall, the exam question (well, statement for comment) was: "if you run at displacement speed or less that the size of the engine doesn't matter all that much."

Answer is yes, most definitely. There are shades of grey, but within a range of +/- 10%-15% or so, true statement.

Peter

 

[twistedtree]

TT that’s counterintuitive. Accept your wisdom and knowledge in these things although it makes my prior post untrue. So……… why is that?

Accepting your post then why not put in the highest HP M2 or M1 whose weight and size the boat could tolerate. Was the 6 cylinder the M2 rated version ? I’ve assumed wear relates to the amount of fuel that goes through an engine not hours. Assumed that’s due to heat and friction. I’ve assumed the lower M number has better cooling and lube so could tolerate being at maximal load mor 80% load for a longer uninterrupted time and with common rail no issues at 20-40% load. Is this irrelevant as much wear occurs at startup and shut down and little while running or is it something else?

Would a bigger lower M number engine asked to give the same amount of different HPs in the same range as a smaller higher M number have less wear and a longer service life before rebuild? Most recreational boats have high M numbers. Many long range or commercial boats have low.

I very rarely use the 540hp I have access to. Might need 14-15 kts once in a blue moon but 17-18kts I don’t think I’d miss. Do admit exceeding hull speed at times is quite helpful as is the ability to overcome current or seas but would think most SD trawlers are overpowered for how they are used. So other than marketing what’s the advantage of overpowering recreational trawlers?

Don’t take it as knowledge or wisdom, it’s just what the engine data sheets say which is all carefully measured to international standards.

When I compared lower rated and higher rated versions of the same underlying engine, they do indeed correspond to different M ratings which I think is to be expected.

Longevity I think is a whole other discussion, and I think favors slower turning engines, which generally translates into higher displacement to get the same power.

 

[twistedtree]

Ok, so how are you measuring HP?

I'm having a hard time buying that there is a 10% efficiency difference in the production of fuel to HP.

Ted

I expect the numbers are all off a dynamometer, but don't know for certain. They data sheet does list the test conditions (temps, fuel density, air density, etc.)

 

[KnotYet]

Ultra-lean isn't really a Thing in diesels. The fuel injected is burned, up until there is no oxygen left - and that is max torque. At any condition less than full torque, it is lean (if we define that as unburned oxygen in the exhaust), as you reduce the load on the engine leaner and leaner until the only load is the parasitic loads of keeping the engine turning at that rpm. This is true of any diesel.

Large and small engines will have different combustion chamber shapes, surface to volume ratios, and flame front characteristics. Small non-common rail engines also are often indirect injection engines. These things have an effect on efficiency.

To clarify my post, the 4045 engine is using more fuel than stoichiometrically
needed to produce 100hp. I wouldn't call this a lean burn condition. There can
be any number of reasons for this but that is the example. It would help my
understanding to know more about how the 'propeller' power and fuel curves
are calculated.

 

[twistedtree]

Excuse my ignorance. Still don’t get it.

Why would a higher displacement engine be more efficient in producing the same HP?

If repowering or doing a new build how should you go about picking an engine? Largest displacement that weight and space allows producing an adequate range of HP? Lightest smallest engine regardless of M rating?

What’s the downside of being “overpowered” given modern aftercooled, turbocharged, common rail engines?

I don't know why it's more efficient - it would probably take a diesel engine engineer to explain. But I'll mention another data point which are the giant diesels in container ships. Wartsila boasts 169g/kwh for their 5-10mw engines.

 

[O C Diver]

I expect the numbers are all off a dynamometer, but don't know for certain. They data sheet does list the test conditions (temps, fuel density, air density, etc.)

So is this a graph of maximum HP for a given RPM and the corresponding fuel consumption?

Or is it a fuel map that shows for a given RPM a number of HP values and corresponding fuel consumption from actual testing?

Ted

 

[twistedtree]

It would help my
understanding to know more about how the 'propeller' power and fuel curves
are calculated.

Here's what my interpretation would be, but I don't know for sure.


The calculated prop load comes up with a curve plotting rpm vs hp. That's the load the theoretical prop will present at each rpm point. Different props may present different loads at each rpm point, but you need to pick an example curve so they have. The performance at each of those data points I expect is from a dynamometer, so as accurate as it could possibly be.


So if the chart shows 100hp @1500 rpm with fuel consumption of 5gph, then if you load up the engine to 100hp at 1500 rpm, that's what the fuel burn will be. If an actual prop presents more or less load at 1500 rpm, consumption will go up or down correspondingly, and BSFC may or may not change. You would need the full 3d BSFC map (BSFC (z) plotted across RPM (x) and load to know more, or to see what value there might be in operating the engine at different rpm/load points. But these don't seem easy to come by for modern engines.

 

[twistedtree]

So is this a graph of maximum HP for a given RPM and the corresponding fuel consumption?

Or is it a fuel map that shows for a given RPM a number of HP values and corresponding fuel consumption from actual testing?

Ted

Attached are the datasheets that I have been referring to.


The consumption and BSFC numbers are off the prop load curve. Max power is largely irrelevant for a propulsion engine.


So the BSFC numbers are a single line through the full BSFC map, following the RPM/load points from the prop curve. It would be great to see the whole map, but they seem to be closely guarded. The only ones I have ever seen are either very old full maps, or only show one line through the map, typically full power or the prop curve.

 

[Don L]

To bring this full circle with respect to the original post. Recall, the exam question (well, statement for comment) was: "if you run at displacement speed or less that the size of the engine doesn't matter all that much."

Answer is yes, most definitely. There are shades of grey, but within a range of +/- 10%-15% or so, true statement.

Peter

Thanks I was hoping maybe the thread would come back to the asked question.

 

[DDW]

To clarify my post, the 4045 engine is using more fuel than stoichiometrically
needed to produce 100hp. I wouldn't call this a lean burn condition. There can
be any number of reasons for this but that is the example. It would help my
understanding to know more about how the 'propeller' power and fuel curves
are calculated.

I don't think stoichiometry enters into it. Diesel run at a very wide range of stoichiometric ratios efficiently, unlike gas. The question is how much energy is extracted as power and how much as wasted heat. A large engine may have an advantage there, because the power density is higher. Against this is extra friction, but friction (bearing and ring area and travel etc.) goes up as size ^2 while displacement goes up as size ^3.

 

[O C Diver]

Attached are the datasheets that I have been referring to.


The consumption and BSFC numbers are off the prop load curve. Max power is largely irrelevant for a propulsion engine.


So the BSFC numbers are a single line through the full BSFC map, following the RPM/load points from the prop curve. It would be great to see the whole map, but they seem to be closely guarded. The only ones I have ever seen are either very old full maps, or only show one line through the map, typically full power or the prop curve.

Not sure I'm buying their numbers.

For the 4045 there are 4 power ratings from 160 to 225 HP. All 4 have prop power ratings of 75 or 76 HP. The 160 HP version burns 4.5 GPH. All the others burn the same at 4.0 GPH. That's a 12% difference!

My engine (4045TFM75 ) had 3 power ratings from 104 to 132 HP. You could switch between the power ratings by only changing the software. I don't know if this engine is the same in that regard. If so, it's hard to fathom a 12% efficiency difference switching software.

Ted

 

[twistedtree]

Not sure I'm buying their numbers.

For the 4045 there are 4 power ratings from 160 to 225 HP. All 4 have prop power ratings of 75 or 76 HP. The 160 HP version burns 4.5 GPH. All the others burn the same at 4.0 GPH. That's a 12% difference!

My engine (4045TFM75 ) had 3 power ratings from 104 to 132 HP. You could switch between the power ratings by only changing the software. I don't know if this engine is the same in that regard. If so, it's hard to fathom a 12% efficiency difference switching software.

Ted

One thing I notice is that gph is only give in whole gallons. lph has a bit more precision, and even then, BSFC seems to show the most detail between data points. Looking at that, the variation seems to be about 7%, so a bit more understandable. But they are still pretty close at 256, 248, 239, and 241. across the different power ratings.

 

[MacPhid]

Not sure I'm buying their numbers.



For the 4045 there are 4 power ratings from 160 to 225 HP. All 4 have prop power ratings of 75 or 76 HP. The 160 HP version burns 4.5 GPH. All the others burn the same at 4.0 GPH. That's a 12% difference!



My engine (4045TFM75 ) had 3 power ratings from 104 to 132 HP. You could switch between the power ratings by only changing the software. I don't know if this engine is the same in that regard. If so, it's hard to fathom a 12% efficiency difference switching software.



Ted

The power ratings appear to be identical on the prop curve for the same rpm. Higher HP comes with the higher rpm rating. If you ran the M4 rated engine at 2300 rpm it would produce the same HP as the M1 rating.

James

 

[O C Diver]

One thing I notice is that gph is only give in whole gallons. lph has a bit more precision, and even then, BSFC seems to show the most detail between data points. Looking at that, the variation seems to be about 7%, so a bit more understandable. But they are still pretty close at 256, 248, 239, and 241. across the different power ratings.

Still find 7% hard to swallow for the same engine, but getting back to the original point from post #66:

That may be true at 100 HP. The more common scenario on this forum is a large engine running at 10% of its capacity near the bottom of its torque curve. I would imagine the results would be different with the 6 cylinder engine making 40 HP at 1,000 to 1,2000 RPM.

Ted

The question focuses on high HP engines running at much lower RPMs than what they were designed for and producing low HP efficiently. The point of the thread was that the 6 cylinder wouldn't be as good as the 4 cylinder at 40 HP, as far as efficiency. There is a point below which an engine no longer produces prop HP efficiently.

I found that at 40 HP with my Cummins 6CTA 450 HP.

Ted

 

[mvweebles]

Thanks I was hoping maybe the thread would come back to the asked question.

I will caution that while fuel economy may not differ much, the total cost of ownership includes much more. My tiny 75hp Perkins 4.236 natutal is pretty affordable to maintain and repair. I replaced all pumps a couple years ago - coolant circulation pump, raw water pump, and lift pump. All three pumps with gaskets were around $600. Was not worth it to rebuild. I replaced my wet exhaust elbow with new which was around $650 for a stainless one. Different conversation with large hp engines where turbos alone can run several thousand dollars.

There's a lot more to the equation than fuel.

Peter

 

[DDW]

Cummins 6CTA is a mechanically injected engine. 4045 is electronically controlled (though I don't think common rail?). Among things you can change with software programming is the fuel injection timing, which can have a large effect on both power and efficiency. With a mechanical engine, you have to pick a fixed injection timing that is inevitably a compromise at some corner of the operating envelop. That can easily make 10% difference in power or efficiency. With electronic controls, you can set the timing arbitrarily to achieve what you want, at any operating point.