Low RPM cruise

[Dockhead]

Originally Posted by Delfin

I wasn't really questioning his data, I just found it remarkable given the head wind and current. That said, even my boat, which weighs 65 tons but is essentially the same length as yours has dramatically increased fuel economy at very low speeds. At 1,000 rpm, she only uses a gph or so, and at that power level I think she's doing around 5 knots. That translates to only about 18 hp. Which, now that I think about it, was the size engine in my 12 ton Cape George, and motoring she would do around 6.5 knots in a flat sea. I guess the point is that when you are at well under hull speed, the amount of energy to move a displacement hull through the water is astonishing low.

Well, your numbers line up pretty well with Ken's, then -- discounting the headwind, though.


I think the main takeaway from this for many people will be that although everyone knows that fuel consumption goes way up around hull speed, a lot of people are not aware of how steep the fuel consumption curve is several knots below hull speed. It's hard (or impossible) to know this without a fuel flow meter. But big ship designers know about it.

[Singularity]

Originally Posted by Dockhead

I think the main takeaway from this for many people will be that although everyone knows that fuel consumption goes way up around hull speed, a lot of people are not aware of how steep the fuel consumption curve is several knots below hull speed. It's hard (or impossible) to know this without a fuel flow meter.

The other main takeaway, I think, is to appreciate the profound decrease in MPG realized with any headwind or wave action.

I suspect that the "your numbers are wrong / I'm not lying" argument came about when someone is recording MPG in flat/quiet conditions that were not. If you look at MPG with a headwind, for example, the "doesn't make sense" assertions in a lot of posts (claiming to be recorded in calm conditions) do start to make sense.

[a64pilot]

Originally Posted by Dockhead

This is true, but I'm not sure it makes a huge difference. Not having the Autoprop just means you have to use more RPM. Diesel engines are remarkably consistent in fuel burned per horsepower/hour -- all efficiency improvements, even with turbochargers, are relatively small. I think the only really game-changing difference in diesels is common rail.


One should not confuse diesels with spark-ignition engines in this regard. Spark-ignition engines have very significant efficiency losses at partial load because of pumping losses around the throttle plate. So you can save a lot of fuel by getting them into "overdrive" and getting the throttle plate open more and reducing manifold vacuum. There is no such effect with diesels, which have no throttle plates and don't pull a vacuum to modulate their power output.

Common rail makes very little efficiency difference, it’s more about emission, NVH, and power, but I suppose it could likely be optimized for efficiency. One thing common rail allows is easy manipulation through software, witness all the “chips”.

Spark ignition pumping losses are nearly insignificant. I know because I have tested for that in aircraft. You can get identical power output within a range of course at different RPM’s with different throttle openings. In other words you can be wide open, but at a lower RPM and make the identical power you can at a higher RPM and a lower throttle opening. In theory the low RPM and wide open throttle should result in lower fuel consumption from both lower friction of the lower RPM and little pumping losses from a wide open throttle plate.
Guess what? It was all within the noise band, meaning even though I have very accurate NIST calibrated engine instrumentation, altimeter and airspeed, the difference if it existed couldn’t be broken out for certain.
It’s another one of those theories that don’t translate to actual use, you can “prove” it on paper, but it’s pretty insignificant.
The power to fuel consumption curves your most likely looking at are assuming a correct fixed pitch prop. Any engine if properly loaded will be more efficient than one either under or over loaded.

However the Toyota Prius has an exceptionally variable, transmission and Toyota under normal conditions operates the engine as much as possible at a wide open throttle setting, yet drags the RPM down to power required is matched by power output, so it’s real, but Toyota has optimized that thing as much as is financially possible, picking up one or two percent here and there.

[Dockhead]

Originally Posted by Delfin

Surprisingly, that isn't true, at least in terms of fuel efficiency at less than full power. The Cummins QSB7 common rail engine uses 181 grams of fuel per hour to generate 1 hp at about 1/4 power, which is right in the range of a mechanically injected engine. They shine in terms of emission reduction and are more efficient at max power, which boaters in displacement hulls use very rarely.

Originally Posted by a64pilot

Common rail makes very little efficiency difference, it’s more about emission, NVH, and power, but I suppose it could likely be optimized for efficiency. One thing common rail allows is easy manipulation through software, witness all the “chips”.

I stand corrected -- I thought the difference was greater than that. Thanks both of you.

[Dockhead]

Originally Posted by a64pilot

. .. Spark ignition pumping losses are nearly insignificant. I know because I have tested for that in aircraft. You can get identical power output within a range of course at different RPM’s with different throttle openings. In other words you can be wide open, but at a lower RPM and make the identical power you can at a higher RPM and a lower throttle opening. In theory the low RPM and wide open throttle should result in lower fuel consumption from both lower friction of the lower RPM and little pumping losses from a wide open throttle plate.
Guess what? It was all within the noise band, meaning even though I have very accurate NIST calibrated engine instrumentation, altimeter and airspeed, the difference if it existed couldn’t be broken out for certain.
It’s another one of those theories that don’t translate to actual use, you can “prove” it on paper, but it’s pretty insignificant.
The power to fuel consumption curves your most likely looking at are assuming a correct fixed pitch prop. Any engine if properly loaded will be more efficient than one either under or over loaded.. . .

That however contradicts everything I have read. I thought the difference in pumping losses was one of the key advantages of diesels.


If you've actually measured it, however, I am not going to argue with you!

[a64pilot]

Originally Posted by Dockhead

That however contradicts everything I have read. I thought the difference in pumping losses was one of the key advantages of diesels.


If you've actually measured it, however, I am not going to argue with you!

It is, it’s just by itself it’s not much of a difference.
Some of a Diesels advantages are.
Higher compression, you can extract more heat energy from a fuel with a higher compression engine. I found out in aircraft engines, specifically the Lycomong IO-540 that comes as both a high and a low compression engine that the EGT was higher on the low compression engine, that confused me until I realized the high compression motor was extracting more heat energy and the exhaust was cooler. Just like a Turbo, the exhaust exiting a turbo motor is much cooler than a non turbo motor, cause the turbo is extracting energy from the exhaust stream, and that’s heat, not pressure.

The fuel itself per weight or volume contains more energy, and that isn’t insignificant.

The lower RPM they are usually run at, a Diesel’s RPM is constrained by the time it takes to inject fuel, a gas motor isn’t.

Then of course a Diesel at part throttle is a “lean burn” or stratospherically charged motor, where most gas motors are not.

It all adds up, but it’s mostly the fuel, compression ratio and lean burn.

However we are seeing gas motors becoming more and more efficient as Diesels who have to meet emissions are actually becoming less efficient, pretty soon I expect they will meet, and likely the gas motor will squeeze ahead as we will have a form of Diesel that isn’t a true Diesel, and burns gasoline, and is spark ignited.
Direct injected gas motors for instance

[Dockhead]

Originally Posted by a64pilot

It is, it’s just by itself it’s not much of a difference.
Some of a Diesels advantages are.
Higher compression, you can extract more heat energy from a fuel with a higher compression engine. I found out in aircraft engines, specifically the Lycomong IO-540 that comes as both a high and a low compression engine that the EGT was higher on the low compression engine, that confused me until I realized the high compression motor was extracting more heat energy and the exhaust was cooler. Just like a Turbo, the exhaust exiting a turbo motor is much cooler than a non turbo motor, cause the turbo is extracting energy from the exhaust stream, and that’s heat, not pressure.

The fuel itself per weight or volume contains more energy, and that isn’t insignificant.

The lower RPM they are usually run at, a Diesel’s RPM is constrained by the time it takes to inject fuel, a gas motor isn’t.

Then of course a Diesel at part throttle is a “lean burn” or stratospherically charged motor, where most gas motors are not.

It all adds up, but it’s mostly the fuel, compression ratio and lean burn.

However we are seeing gas motors becoming more and more efficient as Diesels who have to meet emissions are actually becoming less efficient, pretty soon I expect they will meet, and likely the gas motor will squeeze ahead as we will have a form of Diesel that isn’t a true Diesel, and burns gasoline, and is spark ignited.
Direct injected gas motors for instance

Well, I always read that the Holy Trinity of efficiency advantages of diesels are (1) compression ratio; (2) abundance of air; (3) no pumping losses, and always heard that these three were roughly similar in magnitude.


Direct inject petrol engines can help catch up, but can't equal diesel efficiency, can they? You still need a throttle plate, don't you? And CR is still limited, isn't it?

[a64pilot]

This can explain better than I can
https://www.popularmechanics.com/car...t-than-diesel/

[a64pilot]

Comparison of BTU of some common fuels, look at how energy dense fuel oil is.

https://www.engineeringtoolbox.com/e...ent-d_868.html

[Ericson38]

Originally Posted by a64pilot

It is, it’s just by itself it’s not much of a difference.
Some of a Diesels advantages are.
Higher compression, you can extract more heat energy from a fuel with a higher compression engine. I found out in aircraft engines, specifically the Lycomong IO-540 that comes as both a high and a low compression engine that the EGT was higher on the low compression engine, that confused me until I realized the high compression motor was extracting more heat energy and the exhaust was cooler. Just like a Turbo, the exhaust exiting a turbo motor is much cooler than a non turbo motor, cause the turbo is extracting energy from the exhaust stream, and that’s heat, not pressure.

The fuel itself per weight or volume contains more energy, and that isn’t insignificant.

The lower RPM they are usually run at, a Diesel’s RPM is constrained by the time it takes to inject fuel, a gas motor isn’t.

Then of course a Diesel at part throttle is a “lean burn” or stratospherically charged motor, where most gas motors are not.

It all adds up, but it’s mostly the fuel, compression ratio and lean burn.

However we are seeing gas motors becoming more and more efficient as Diesels who have to meet emissions are actually becoming less efficient, pretty soon I expect they will meet, and likely the gas motor will squeeze ahead as we will have a form of Diesel that isn’t a true Diesel, and burns gasoline, and is spark ignited.
Direct injected gas motors for instance

They do not have throttle plates and don't develop measurable or useful intake manifold vacuum as a result, so from that standpoint, the work it takes to create a continual pressure drop across the throttle plate for a given mass flow rate per unit time (work) is not expended. So the pumping losses are less as a result. It's fuel has about 12% more expendable energy per unit volume than gasoline.

[Dockhead]

Originally Posted by Singularity

The other main takeaway, I think, is to appreciate the profound decrease in MPG realized with any headwind or wave action.

I suspect that the "your numbers are wrong / I'm not lying" argument came about when someone is recording MPG in flat/quiet conditions that were not. If you look at MPG with a headwind, for example, the "doesn't make sense" assertions in a lot of posts (claiming to be recorded in calm conditions) do start to make sense.

Yes, absolutely right. With a fuel flow meter, you can see all this instantaneously.

[Simi 60]

Originally Posted by Dockhead

Yes, absolutely right. With a fuel flow meter, you can see all this instantaneously.

As long as its calibrated accurately

Saying that it would still indicate increased fuel usage, just the actual numbers will be wrong.

[saghost]

Originally Posted by Dockhead

Diesel engines are remarkably consistent in fuel burned per horsepower/hour.

This isn't really true in the general case - running a diesel at fairly low speeds and near full fuel is much more efficient than running it at high speeds with low torque for the same output.

It does seem to be true for some (many?) boats, though - because the propeller power/rpm curve lines up to one of the lower bsfc curves, as noted here:

https://www.morganscloud.com/2015/06...gine-fuel-map/

Hand plotting the data Yanmar provides on the 3YM30AE as an example, it seems the entire propeller curve runs between 25 and 30% thermal efficiency (the engine is probably capable of 35% or better in the middle speed range at higher fuel flows - but a single speed ship propeller can't operate it there without sacrificing the top end power output.)

Polar power quotes 35% efficiency *to electricity* (thus including electric conversion losses as well) for the same size Volvo Penta linked to a modern permanent magnet generator.

[Dockhead]

Originally Posted by saghost

This isn't really true in the general case - running a diesel at fairly low speeds and near full fuel is much more efficient than running it at high speeds with low torque for the same output.

It does seem to be true for some (many?) boats, though - because the propeller power/rpm curve lines up to one of the lower bsfc curves, as noted here:

https://www.morganscloud.com/2015/06...gine-fuel-map/

Hand plotting the data Yanmar provides on the 3YM30AE as an example, it seems the entire propeller curve runs between 25 and 30% thermal efficiency (the engine is probably capable of 35% or better in the middle speed range at higher fuel flows - but a single speed ship propeller can't operate it there without sacrificing the top end power output.)

Polar power quotes 35% efficiency *to electricity* (thus including electric conversion losses as well) for the same size Volvo Penta linked to a modern permanent magnet generator.

We discussed this in another thread. What it boils down to is this -- what are the numbers? Where is the "much more efficient"?



If you look at the prop curve superimposed on fuel map, in the article you linked:





The engine running a regular dumb prop is still working at the edge of the optimum part of the fuel curve.



Here is the fuel map which was used:





Every line on the fuel map represents 20g/kWh. At all times, the ordinary dumb fixed prop, not overpropped, is within 60g/kWh of maximum efficiency. That's a 25% difference -- worst case.



Your hybrid drive generator is not going to keep the engine right in the middle of the sweet spot, either. Because it is small relative to total power demands, it will spend a lot of its time running flat out, which will have worse efficiency than any regime used by a conventional drive.



I don't see any way that a hybrid drive, in any regime, can save more fuel with a better operating regime, than it loses conversion losses, and in the most usual regime with the generator running flat out, it will have worse efficiency at the engine level, than a conventional drive, then PLUS the efficiency losses.




There is a much better way to get a diesel engine into a better operating regime -- that is a variable pitch propeller. Note what the fuel curve looks like for an over-propped situation:





Now THAT is where you get, in the middle range, with a variable pitch prop -- right in the middle of the sweet spot, and no conversion losses!


I actually have one of these. The fuel curve looks better than this, because the prop pitches up to maintain almost constant loading at the optimum level.



But it's still not a huge difference. Improved specific fuel consumption (fuel burned per kWh produced) is drowned out in the differences of absolute quantity of power required, at different speeds and different conditions -- what we were discussing in this thread.


So I like the variable pitch prop more for getting an improved noise situation by being able to cut revs, especially in low load situations like motorsailing, than for saving fuel. Noise is the great benefit of a variable pitch prop. A hybrid drive where you are trying to produce the same amount of power with an engine half the size, would be the opposite of this -- the generator would be screaming its lungs out flat out most of the time. Not only would that be inefficient, but it would be unpleasant, and the engine would last much less long.



If you can plug in, and do a good part of your motoring on electric alone, this is a use case where this type of drive has a big advantage. But once you start running a generator to produce power, I think these advantages evaporate.

[907Juice]

Also In case anyone is wondering I pronounce potato with a long A. Any well respected person knows there is only one true way to say it: Po Tay to.