Continuously Variable Transmissions

[Yellowjacket]

I'm not sure where your logic comes from but..... that pretty much is the point of the advantages of a CPP.

Teddy,

You need to look carefully at the chart and the scales involved. What this chart shows is that SFC from 30% to max power is only changing by about 5 g/kw-h, which, if this engine is typical of most diesels, best SFC is somewhere around 200 -220 g/kw-h. This shows an swing in SFC that is just about 2.5%. If you go down to really low power (like 20%) you will see a kick up in the SFC curve, but Matt is correct in that in the range from 30 to 100% SFC for diesels is very flat.

 

[apex1]

No offense, but I think I'm a little more studied on the topic than you.

There I have my doubts! Seriously!

this shows very clear you do´nt have the slightest clue what you are talking!

CPP is typically used to maintain a constant engine RPM for generators, hydraulic pumps, refridgeration plants,

And the table you posted is from a very large crude oil engine and has nothing in common with a yacht Diesel.

Teddy was right, sorry! Study before you contradict.

Richard

 

[MattZ]

Similar behavior is basically true of all turbocharged diesels.

I just took a look at the C7 non-ACERT C-rating (fairly typical yacht engine, no?), and between 181kW and 46.6kW, BSFC varies by 10%.

So you want to save at best 10% on fuel when you're only burning ~14L/hour anyways? It's going to take many hours before that CPP has paid for itself.

The efficiency of a CPP at other than design pitch is abysmal. Look at what happens when you set the pitch to zero. You can't. Basically the outer part has negative pitch and the inner has positive pitch, and you're burning up fuel to spin water in circles and not go anywhere.

When you're not at optimum pitch one part of the propeller is capturing energy from the water and converting it into rotation, and another part is capturing using that water to push the boat. It's basically a little perpetual motion machine, and you're burning up fuel to make up for the losses in that process.

You can minimize this by using a large hub and short blades, but then you're losing efficiency too.

Within a narrow range of angle variation you can increase efficiency, but it's not wide, and the gains are small.

 

[baeckmo]

The fuel consumption curves shown previously in this thread are not relevant for a discussion on continuously variable transmissions, of which the CPP is one. They are either showing fuel consumption (FC) at full brake power, or, at best at some "propeller curve" setting. To fully understand the situation, a complete fuel map diagram must be available. For all those, who are familiar with the process, sorry to keep teaching grandma how to make babies, but I hope to make the CPP less diffuse for others.......!

For engine development, the format below (mep as a function of rpm, with constant power and constant FC added) is mostly in use. Mean effective pressure is directly related to torque. The matching process starts in the max power/max speed operating point ("A"), where vessel drag times speed divided with propeller efficiency, represents total power required (let's forget about transmission losses for the sake of clarity).

In this example, we have a 6 cyl 5.5 l direct injected diesel engine, 155 hp intermittent power, and a typical displacement hull. Selecting a 3 bladed Gawn-Burril propeller, BAR 0.5, P/D 0.8, operating at "A" with an advance ratio J ~0.58, we get a prop eta of ~60 %. The effective propeller power at 2200 rpm is then 90 hp. The specific fuel consumption (SFC) is ~167 g/hph, giving a total fuel consumption of 25.9 kg/h. By iteration combining propeller data and hull resistance, a curve for effective propeller power asf of rpm can be drawn; the continuous blue line in the diagram.

The continuous red line above the eff prop pwr (EPP) line is showing constant J (~constant relative slip). As the difference between the red and blue lines increase (for this hull characteristic), the propeller is working at higher advance ratio when the speed is reduced. If we use 2000 rpm for cruising, the EPP will be 66 hp, point "C", and the advance ratio will jump over the top of the propeller efficiency curve to a J of ~0.72, with a prop efficiency of ~0.59 %. The engine power (point "B") will be 112 hp with a SFC of 167 g/hph; total 18.7 kg/h.

Now to the CPP. The hull is requiring 66 effective hp at cruise. Follow the constant hp line for 66 hp (thin blue line) to point "E" at 1400 rpm. Here the propeller is heavier loaded at a J of about 1.0 (=higher slip). This may be achieved by increasing pitch. The optimum pitch for this advance is ~1.2; fixed pitch efficiency 73%. The efficiency of a CPP at non-optimum setting is about 2 % lower. So, with the CPP we have an engine working at point "F" with 93 hp at SFC 159 g/hph; total 14.8 kg/h for the same job!

The example is a "simple" cruising application; if anything like increased resistance (tug, trawling, station holding et c.) were involved, the difference in favour of the CPP would increase dramatically!

Increasing the propeller rpm's (with the original propeller) by changing gear ratio in order to reach the fuel efficient engine range for cruising is not possible, since that prop is operating with ever decreasing J, where the propeller efficiency is falling off, which is one of the reasons CVT's are out of play.

When it comes to non-displacement hulls, the situation becomes completely different, more on that later.

The fuel maps are highly individual for different engines, see examples. Gas engines are just as varying as diesels in this respect; gas turbines a little more "conformal to the type". So, mattZ, you are in deep water where it is wise not to open the mouth too much.......

 

[apex1]

I'm afraid that I'm right and you're wrong.

Ah, then............. here we go.

So, mattZ, you are in deep water where it is wise not to open the mouth too much...

 

[alan white]

Wait! Diesels taste good PLUS they are good for you!

 

[Yellowjacket]

Baeckmo's third chart directly supports what both Matt and I have said, that the fuel consumption curve for a diesel being relatively flat, and specifically that the ability to vary speed for a given load via a CVT or a CPP isn't going to buy you more than 10% in fuel consumption at the engine (in the very best case). Prop efficiency is an altogether different matter. I'm just talking about the effect of varing engine speed for a given power on SFC here.

This can be conclusively demonstrated as seen in Baeckmo's third chart, if you look at the 100 hp line (30% of full power) in the SFC map, the SFC can vary from .37 (best) to a max of close to 0.39, (only a 5.1% possible improvement) based on engine speed, and the full power SFC varies from 0.39 to 0.365, again depending on engine speed. Consequently, the absolute maximum that could be gained by a variable pitch prop or a cvt at those speeds is 10% or less. The swing in SFC at any given power curve is less than 10% if you consider a typical propeller demand curve. The need for increasing power at higher engine speed is going to drive a fixed pitch prop up a curve not far from the optimum power line, a bit flatter, but still following the general trend. Between a full power point at 2200 rpm and 350 hp, you are looking at an SFC of 0.39 at max speed and power, and at 1200 rpm at 100 hp the SFC is going to be 0.37. That is a swing of only a bit over 5% and the total swing in SFC over the entire operating range of the engine is 10% (from a best at .352 if you could hit the best op point dead center to .0.39 at max speed and power).

In short, the statement that diesel SFC varies greatly at lower power is simply not supported by the facts. Quit berating the kid when he is right. Matt made a statement that within the range of 30% to 100% power that diesel SFC typically does not vary much more than 10%. I have just shown you that, for the engine 350 hp engine that Beckmo posted the SFC map for, that this statement is absolutely correct. As Matt posted earlier, if you look at the propeller demand curves for the diesels on the CAT site, you will see numerous SFC curves with a kick upwards below 30%. All can agree that is the case, but it only makes a difference at very low speeds and power settings, not where you will expect to operate in the real world.

Bottom line is that a variation of engine speed for a given power will typically only gain you about 5% in fuel consumption, and can't get you much more than 10%, it simply isn't possilbe. That said, propeller efficiency (as Baeckmo) aptly noted) is another story, but if you want to make the case for a CPP or CVT, the gains have to come from the prop and not the engine.

 

[baeckmo]

Just for the sake of clarity, all three maps shown are for diesel engines, showing typical variation in characteristics between engine individuals. Please note that the subject of the thread is "Continuously variable transmissions". The fixed pitch propeller has a fuel consumption in the example, that is 26 % higher than the CPP in cruising mode!

The diesel MAY have a flat characteristics, but the locus of the min SFC can be far away from a FP propeller operating line, or very close as in the third example. My point here is that you should not use the simple SFC versus rpm diagrams for propeller optimizing.

 

[TeddyDiver]

Prop efficiency is an altogether different matter. I'm just talking about the effect of varing engine speed for a given power on SFC here.

No you aren't....

At really low speeds boat props have high slip. A prop that is sized to work effectively at high speeds is smaller than what would be optimum for very low speed work. They act basically like a torque converter, and provide higher thrust. In essence props act much like a transmission in that they have effectively a torque converter effect that allows them to slip and generate thrust. They are not highly efficient out of their primary operating range but there is a substantial thrust multipication effect and that works to get the boat moving and up into a range where they are efficient.

Aside from the fact that cvt's aren't all that efficient and can't (because they rely on hertzian contact to transmit force thru friction) transmit big torque loads, they really aren't necessary for a boat.

If you really want to turn a big prop (for low speed grunt) and jump out of the hole, a lot of drag boats have used two speed (and as many as four speed) transmissions, but part of that is the larger speed range (from zero to 100+) and the fact that they want big torque at lower speed to help them leap out of the hole.

While a variable prop can improve efficiency in some cases, unfortuately the larger hub and higher underwater drag tends to limit their use to larger slower applications.

..and taking account the subject of this thread you shouldn't either. And besides how can you seriously compare the difference of CPP and FP by load/rpm/power combination that neitherone hardly meets (in the case they do meet it's an intersection of curves).

 

[Yellowjacket]

No you aren't...

Yes I am and I am doing so to directly differentiate what the effects of a variable transmission are on the engine and what factors contribute to any gain that is or isn't realized. A variable transmission has a potential effect on both the engine and the propeller, and those effects can be broken down and quantified and that is what I was doing.

...and taking account the subject of this thread you shouldn't either. And besides how can you seriously compare the difference of CPP and FP by load/rpm/power combination that neitherone hardly meets (in the case they do meet it's an intersection of curves).

In my earlier post that you have quoted I was addressing the specific reason why a variable transmission isn't required from a torque demand standpoint on higher speed craft. Propeller slip makes any transmission, to a large extent unnecessary. You will note that I specifically said in the post that this was not a particularly efficient way of transmitting power. But in the real world it does't matter since you really don't spend any measurable time at that condition. I don't think anyone on here will disagree with that line of reasoning. What I was simply trying to do was explain why props are different than wheels on a car in terms of the need for gearing.

The thread is exactly about continuously variable transmissions, of which CPP's are a subset, and the discussion has moved on from a discussion as to why do you need a transmission at all, to one of addressing the potential effects of CVT's on overall propulsion system efficiency. One of the express reasons for a variable transmission is to optimize the engine and attempt to achieve an improvement in engine SFC over the range of operating conditions. While that type of logic is very applicable to spark ignition engines in land vehicles, which operate at very low power percentages at cruising speeds, and have very poor part power SFC, it does not make much sense in a marine environment, where the best speed/efficiency characteristic of a diesel is more closely related to a prop demand curve.

Consequently you cannot achieve a significant gain in fuel consumption from the engine standpoint with any kind of variable transmission. As clearly pointed out, the effects of a CVT on engine efficiency is typically less than 5% and is no more than 10% under all conditions for the example noted. The specific fuel consumption of the engine isn't the factor here, in a nutshell, it is the prop.

I wasn't attempting to compare CPP and FP systems, they are clearly different and each has advantages and disadvantages depending on the needs of user, the speeds/loads and power requirements. I was simply addressing the potential effects of efficiency of a diesel at off design conditions that seemed to be a point of contention in a couple of previous posts so that we can get on to more substantive discussion.

 

[TeddyDiver]

it does not make much sense in a marine environment, where the best speed/efficiency characteristic of a diesel is more closely related to a prop demand curve.

This is exactly the point where you lose the track.. With CPP (and with any true variable transmission) that curve is not where you think it is.. becouse there isn't any curve instead it's wide field where the best possible combination can be selected. Some prefixed limitations there's, blade area for instance, but thats a characteristic belonging more in the same category with the hullform you playing with.
I think you have more experience of planning speeds over displacement so it's understandable to have somewhat different conception of this matter, but the world is still in displacement with knots not planning at mph

 

[wardd]

Wait! Diesels taste good PLUS they are good for you!

and they're fun to paint and listen to

 

[Yellowjacket]

This is exactly the point where you lose the track.. With CPP (and with any true variable transmission) that curve is not where you think it is.. becouse there isn't any curve instead it's wide field where the best possible combination can be selected. Some prefixed limitations there's, blade area for instance, but thats a characteristic belonging more in the same category with the hullform you playing with.
I think you have more experience of planning speeds over displacement so it's understandable to have somewhat different conception of this matter, but the world is still in displacement with knots not planning at mph

I doesn't matter where you go on the map, the gain can't be any more than 10% in engine efficiency with any kind of CVT. That's all there is worst case.

In the efficiency assessment I was making and comparing the percentage gain, you can go anywhere on the map and at best get a 10% difference between best SFC and worst SFC. While a planing hull will have a parabolic (with a hump) load/speed curve, and a displacement hull will have a hocky stick curve, it doesn't matter.

The effect I was describing, while more applicable to planing speeds I've tried to sketch in both a displacement (red lines) and a planing (blue lines)example on Baeckmos chart. The solid lines would be a worst case, optimized for maximum power at higher speed and might be more applicable to the planing hull, where you would prop the boat to have a higher engine speed to help you get up on a plane faster. The other (dashed) lines are optimized for a lower engine speed and power as might be applicable to a displacement hull. These aren't by any means difinitive, but you get the basic idea and it will become apparent that the gains in engine efficiency are small because the hull loading, even for a displacement hull is basically going to ride along the efficiency islands and not cut across them. Like I said, worst case it is 10%. Real world, look at the difference at the displacement hull at a 140 hp crusie and you are talking the difference between a .36 and a .375 SFC, or more like 2.5%. If you optimize at a lower speed the differences are even smaller.

You have to trade cost versus performance benefits, and, as far as the engine is concerned there isn't much reason for a CVT in either displacement or planing craft.

 

[Fanie]

Originally Posted by alan white
Wait! Diesels taste good PLUS they are good for you!

and they're fun to paint and listen to

If you plant them they also grow faster than petrols

 

[wardd]

If you plant them they also grow faster than petrols

and rabbits dont eat them