Diesel RPM?

[Bowdrie]

Once we use solenoid valves to start/stop fuel delivery we can use a computer to monitor all kinds of things; temp, barometric air pressure, load, rpm, crankshaft position, etc.
The computer takes all these inputs and controls not only amount of fuel delivered but precise timing of, and even sequence the fuel delivery in pulses over time.
There is a common-rail injection system that uses low pressure fuel and mechanical injection, Detroit Diesel being a well-known example.

[Lee Jerry]

Originally Posted by malbert73

“ Yes, I generally accept this analogy. But the load/power is the same in both gears (determined by the speed / drag), only the efficiency might change. And the "less efficient" isn't as much as you think. (Especially for the boat side of the analogy, anyway.)”

Really? Load on the engine is the same regardless of gear ratio (cars) or prop pitch (boats)?

Yes, really.

I'm not an automotive engineer, but let's use the Saturn SL sedan and the engine that Adelie presented, and give it a go. I looked up some data (tires and gearing).

Tire size is P185/65R14. This would give a diameter of 23.5”, but car tire pressure usually isn’t high enough for to get the full circle; let’s call it 23”. Therefore, the effective circumference is:
C = pi*D = 72.26” = 6.02’

The gear ratios from 1st to 5th are: 3.250 / 1.950 / 1.172 / 0.811 / 0.605. The final axle ratio is 4.060.

Let’s look at traveling at a constant speed of 75 mph:
75 mi/hr * 5280 ft/mi * hr/60 min = 6600 ft/min

And the tire rotational speed will be:
6600 ft/min / 6.02 ft = 1100 1/min

Let’s assume that the torque required at the wheels is 200 lb-ft. (This is a bit of a WAG, but it doesn’t change the “the results.”)

Now to look at the engine performance. In 4th gear, the engine speed will be:
1100 1/min * (4.060 * 0.811) = 3622 1/min

And the torque @ the engine will be:
200 lb-ft / (4.06 * 0.811) = 60.7 lb-ft

Therefore, engine power is:
60.7 lb-ft * 3622 1/min / 5252 = 41.9 hp

(Yes, I’ve ignored some mechanical losses between the tires and the engine. You can include or correct for that 2-5% if you want. Again, it doesn’t change “the results.”)

When you then shift to 5th gear, while maintaining the same 75 mph speed, the drag is the same and therefore the torque demanded at the tires is the same (200 lb-ft).

Now in 5th gear, the engine speed will be:
1100 1/min * (4.060 * 0.605) = 2702 1/min

And the torque @ the engine will be:
200 lb-ft / (4.06 * 0.605) = 81.4 lb-ft

Therefore, engine power is:
81.4 lb-ft * 2702 1/min / 5252 = 41.9 hp

So to review, that was 41.9 hp in 4th gear and 41.9 hp in 5th gear. That sure seems like the same load to me. Is this not the case? (You can plot the points on the engine map to see fuel con and other info.)


For the boat, the analysis is very similar, but does have one added variable, which is propeller slip. (Tires don’t generally slip in normal operation.) However, rather than repeating the above, let me try a different tack (see what I did there?) with this descriptive analysis:

So at any given boat speed, say cruise speed, a vessel has a given resistance. This can be converted to a power value, call it effective power. The prop thrust/power then has to match this value of resistance/power to maintain that speed. It doesn't do this at 100% efficiency, let's call that efficiency propulsive coefficient (PC). (PC can be broken down into multiple different components, but that’s not necessary here.) So divide effective power by PC to get the power needed by the prop, call that thrust power or delivered power. There are then mechanical losses between the prop and engine, and you get to brake or shaft power provided by the engine. (I've simplified / combined some things here, but you get the idea.)

So consider two identical boats except for the props. Boat A is “properly” propped and Boat B has a similar prop except for the higher pitch (i.e. over-prop). They are moving at the same cruise speed, so resistance and the associated effective power are the same. The mechanicals on the upstream side of the prop are the same, so the losses there are the same. That leaves the PC to account for any differences going from A to B. I see three options: the PC increases, the PC decreases, or the PC stays the same.

If the PC increases, then the required brake power will decrease. This would not help (increase) with engine loading (but would be good for fuel consumption). If the PC decreases, then the required brake power will increase. This would increase engine loading (but would not help with fuel consumption). If the PC stays the same, then the required brake power will stays the same.

I contend it is that last one - the PC and therefore power (and loading) are the same (at least to the fidelity of this discussion). The only way loading increases is if PC goes down - why would you want that?

Ok, like Boatpoker up thread, I’m out.

Yeah, that's a common response when one realizes that they're wrong or at least have no case.

[Lee Jerry]

Originally Posted by Adelie

As you say diesel and gas engine fuel maps are similar which is my opinion from looking at a bunch of them. The overprop shows consistent fuel efficiency gains except at 20-22hp where the lines are about even. The little bubble is specific to this engine and there might or might not be something similar on a different map, diesel or gas. Regardless it is gas not diesel, they are similar.

Below are some more fuel maps, both diesel & gas.

Which ones are diesel?

Not all engines, especially diesels, have the minimum SFC occur at the low RPMs shown in your figures. Perhaps many do, but not all.

For example, if you look at this Cat diesel, and the ratings table:


you will see that the specific fuel consumption is lower at 2600 rpm than it is at both 2500 and 2300 rpm (not same power though). While not a full fuel map (by a long shot, but I can only work with the data I have), it suggests to me that the minimum SFC is well to the right within the map.

The discussion of Controllable Pitch Props (CPPs) are germane to the discussion since this discussion has touched on why Overpropping might be desirable. At any desired horsepower/speed the pitch can be adjusted (overpropped) to minimize fuel use. I added a LAVENDER line to the previous fuel map to show the difference. At 40hp & 30% there is a 25-30% fuel efficiency advantage, At 20hp it's about 25%, at 10hp its 20%.

An overpropped Fixed Pitch Prop (FPP) will have more modest gains, but still consistent across the most of the range of powers.

My position is that the loss of operational range is not worth the modest (potential) increase in fuel efficiency associated with over-propping. Since CPP do not suffer from that same reduction in operation, it does not seem germane to me.

[malbert73]

Originally Posted by Lee Jerry

Yes, really.

I'm not an automotive engineer, but let's use the Saturn SL sedan and the engine that Adelie presented, and give it a go. I looked up some data (tires and gearing).

Tire size is P185/65R14. This would give a diameter of 23.5”, but car tire pressure usually isn’t high enough for to get the full circle; let’s call it 23”. Therefore, the effective circumference is:
C = pi*D = 72.26” = 6.02’

The gear ratios from 1st to 5th are: 3.250 / 1.950 / 1.172 / 0.811 / 0.605. The final axle ratio is 4.060.

Let’s look at traveling at a constant speed of 75 mph:
75 mi/hr * 5280 ft/mi * hr/60 min = 6600 ft/min

And the tire rotational speed will be:
6600 ft/min / 6.02 ft = 1100 1/min

Let’s assume that the torque required at the wheels is 200 lb-ft. (This is a bit of a WAG, but it doesn’t change the “the results.”)

Now to look at the engine performance. In 4th gear, the engine speed will be:
1100 1/min * (4.060 * 0.811) = 3622 1/min

And the torque @ the engine will be:
200 lb-ft / (4.06 * 0.811) = 60.7 lb-ft

Therefore, engine power is:
60.7 lb-ft * 3622 1/min / 5252 = 41.9 hp

(Yes, I’ve ignored some mechanical losses between the tires and the engine. You can include or correct for that 2-5% if you want. Again, it doesn’t change “the results.”)

When you then shift to 5th gear, while maintaining the same 75 mph speed, the drag is the same and therefore the torque demanded at the tires is the same (200 lb-ft).

Now in 5th gear, the engine speed will be:
1100 1/min * (4.060 * 0.605) = 2702 1/min

And the torque @ the engine will be:
200 lb-ft / (4.06 * 0.605) = 81.4 lb-ft

Therefore, engine power is:
81.4 lb-ft * 2702 1/min / 5252 = 41.9 hp

So to review, that was 41.9 hp in 4th gear and 41.9 hp in 5th gear. That sure seems like the same load to me. Is this not the case? (You can plot the points on the engine map to see fuel con and other info.)


For the boat, the analysis is very similar, but does have one added variable, which is propeller slip. (Tires don’t generally slip in normal operation.) However, rather than repeating the above, let me try a different tack (see what I did there?) with this descriptive analysis:

So at any given boat speed, say cruise speed, a vessel has a given resistance. This can be converted to a power value, call it effective power. The prop thrust/power then has to match this value of resistance/power to maintain that speed. It doesn't do this at 100% efficiency, let's call that efficiency propulsive coefficient (PC). (PC can be broken down into multiple different components, but that’s not necessary here.) So divide effective power by PC to get the power needed by the prop, call that thrust power or delivered power. There are then mechanical losses between the prop and engine, and you get to brake or shaft power provided by the engine. (I've simplified / combined some things here, but you get the idea.)

So consider two identical boats except for the props. Boat A is “properly” propped and Boat B has a similar prop except for the higher pitch (i.e. over-prop). They are moving at the same cruise speed, so resistance and the associated effective power are the same. The mechanicals on the upstream side of the prop are the same, so the losses there are the same. That leaves the PC to account for any differences going from A to B. I see three options: the PC increases, the PC decreases, or the PC stays the same.

If the PC increases, then the required brake power will decrease. This would not help (increase) with engine loading (but would be good for fuel consumption). If the PC decreases, then the required brake power will increase. This would increase engine loading (but would not help with fuel consumption). If the PC stays the same, then the required brake power will stays the same.

I contend it is that last one - the PC and therefore power (and loading) are the same (at least to the fidelity of this discussion). The only way loading increases is if PC goes down - why would you want that?



Yeah, that's a common response when one realizes that they're wrong or at least have no case.

Wait, now you are trying to say load on an engine is the same regardless of gear? That’s crazy. Wonder why car transmissions even are installed according to your contention. Do you think cars should just drive at max rpm when they want to go faster and only be equipped with a 1 speed transmission?
And based on your above logic, overpropping wont even overload an engine since the load is the same regardless of pitch?

Never mind- I don’t even remember your angle except to parrot the engine manufacturer dogma about “correct” propping - which is irrefutably “wrong” for every RPM exact max.

[Lee Jerry]

Originally Posted by malbert73

Wait, now you are trying to say load on an engine is the same regardless of gear?

Yes. But I did more than that, I demonstrated it. But please show where it was flawed, I'd love to learn something.

That’s crazy.

Why? Please explain why/how, in your opinion, it changes. (As opposed to just saying that it does.)

Wonder why car transmissions even are installed according to your contention.

Torque.

(And fuel economy.)

Do you think cars should just drive at max rpm when they want to go faster and only be equipped with a 1 speed transmission?

No, that would be stupid.

And based on your above logic, overpropping wont even overload an engine since the load is the same regardless of pitch?

Correct. Load is determined by resistance (i.e. boat speed, including environmental factors).

(And to be thorough, any connected loads, like an alternator.)

Never mind- I don’t even remember your angle except to parrot the engine manufacturer dogma about “correct” propping -

That's it, you got me. I'm part of the vast right-wing conspiracy trying to get everyone to properly match their prop to their engine so that ... errrr ... ummmm ... uh ... remind me why we do that; what's in it for the engine manufacturers? (Other than assuring their customers get the best out of their product?)

...which is irrefutably “wrong” for every RPM exact max.

This statement, which you've said before, indicates a fundamental misunderstanding about the whole process. "Correct" propping is not about any single RPM or point. As I previously said, it's about maximizing performance across the largest range of operational conditions.

Further, at any given RPM (where you say it is "wrong"), there is a range of performance (i.e. match) available from the prop. (IOW, again as I previously said, it's not just about the calm water condition.)

Furthermore, the "match" is "wrong" (or should be) at max RPM if the prop is properly matched - see the graph in Post #90.

[malbert73]

�� now I’m truly out. I’ll leave others to point out your own self-contradictions— or not as likely they have given up long ago

[Lee Jerry]

Originally Posted by malbert73

�� now I’m truly out. I’ll leave others to point out your own self-contradictions— or not as likely they have given up long ago

Please show me one.

[boatpoker]

Do not feed.

[Adelie]

Originally Posted by Lee Jerry

....
My position is that the loss of operational range is not worth the modest (potential) increase in fuel efficiency associated with over-propping. Since CPP do not suffer from that same reduction in operation, it does not seem germane to me.

That is a perfectly valid decision based on your values and preferences.

Others prefer to increase fuel economy at the expense of max power and speed.

Neither is right or wrong, just different.

[Lee Jerry]

Originally Posted by Adelie

That is a perfectly valid decision based on your values and preferences.

Others prefer to increase fuel economy at the expense of max power and speed.

Neither is right or wrong, just different.

I have no problem with people doing whatever they want to their boat. It's their boat after all.

However, I do think what they do should be based on an informed opinion. There are many here for whom that does not appear to be the case. And I have a bigger problem with those same under-informed people giving recommendations based on their under-informed decisions.

[malbert73]

Originally Posted by Lee Jerry

I have no problem with people doing whatever they want to their boat. It's their boat after all.

However, I do think what they do should be based on an informed opinion. There are many here for whom that does not appear to be the case. And I have a bigger problem with those same under-informed people giving recommendations based on their under-informed decisions.

This coming from the “expert” who many posts ago decided that my references to Calder, Dashew, and Harries were “too much homework” despite their expertise suggesting some very valid use cases for overproppng.

[boatpoker]

Originally Posted by malbert73

This coming from the “expert” who many posts ago decided that my references to Calder, Dashew, and Harries were “too much homework” despite their expertise suggesting some very valid use cases for overproppng.

Malbert ... You may find this interesting ...
https://www.fao.org/3/i2461e/i2461e.pdf

I can't find it online or in my library but worth a read if you can find a copy of Professional Boatbuilder Magazine #150 with Nigel's "Praise of Big Props" article.

[boatpoker]

Malbert ... I've never tried uploading a pdf. here but if it works, here is the Professional Boatbuilder Calder article ...

[barnakiel]

Revving the engine up does NOT add load.


You want to LOAD the engine, not rev it !


b.

[skipperpete]

Originally Posted by boatpoker

Malbert ... I've never tried uploading a pdf. here but if it works, here is the Professional Boatbuilder Calder article ...

Go straight to the end of the Boatpoker attachment and read the “I’m not a propeller expert” and particularly where he mentions overpropping on boats that are NOT used at WOT. First part of the article is about larger diameter propellers and low shaft speeds more appropriate to power boats and not in contention , few cruising yachts use very large diameter props unless it’s a folding,feathering or CPP and few yacht gearboxes are much above 2.5:1 ( yanmar have a 3:1) while trawlers are able to use deeper reductions, ( 3.5:1 up to a hefty 7:1 Twindisc MG 520) and large propellers.