Theoretical question(RickB???)...engine rating.

[Baker]

We will use electronic common rail engines just for simplicity. It is my understanding that on some of these model engines, all that is done to make them "commercial grade" or rating is electronically turn down the governor basically "de-rating" the engine. Is this a true statement(I realize not all encompassing")?

Okay....I did a quick search and Cummins has a QSB 4.5L rated at 99-160hp at 2000-2500RPM with "numerous rating available".

I would assume this is the exact same motor with just an electronic adjustment to the governor which limits RPM/power and then they attach a "rating" to it?

NOW... what Eric was talking about on the other thread. If this engine is in my boat in it's higher HP rating, I would have the prop pitched to achieve that RPM....2500 in this case. And if it is in my boat in the lowest rating, I would increase the pitch on my prop to achieve the lower RPM...2000 in this case. I am assuming these statements to be correct.

NOW...let's simply switch propellers. Why all of the sudden am I overpropped on a boat with the exact same engine??? And what if I "promise" not to run the higher power engine over 2000RPMs...would that be the same.

I realize I am making assumptions here that may not be true and then asking questions based on the assumptions(ie are these two examples of engines EXACTLY the same other than the governor setting). If they are not exactly the same...can we assume they are for theoretical purposes? But I do believe them to be the same.

DISCUSS!!!!

PS...likely a question for the folks at boat diesel but thought I would put it on here so we can all learn!

 

[Baker]

OOOOORRRRRRR....................................

As I think longer and harder......

IS THE PROP PITCH FOR BOTH ENGINES EXACTLY THE SAME????!!!!!!!!!...in theory

 

[DavidM]

I had a conversation with Tony Athens (the Cummins guru on boatdiesel) some years ago after the QSB 5.9 had been in production for a few years.

He told me that the 230 up to 380 hp versions of this engine were exactly the same, other than the computer chip that manages fuel, etc. In fact he said that I could buy a 230 hp model and if later I wanted more hp I could buy (for about the difference in the cost of the new engines) a chip upgrade and data plate that would upgrade the 230 to a 380 engine.

Higher hp models that came out later (480 hp I think) do have internal differences, maybe turbocharger, injectors, etc.

To your question about propping, yes technically you are correct. A 230 hp QSB 5.9 propped to reach 100 rpm over rated is still pretty lightly loaded and could tolerate a bit of overpropping.

I am sure that all of the above applies to the QSB 4.5 series.

David

 

[Northern Spy]

Good question. Derating a diesel is a function of both rpm and cylinder pressure. This means changing both the governor settings and injection timing to reduce the specific fuel consumption. Certainly coolant flow would be considered too.

 

[Baker]

David. And I am sure I am missing something. Are the props the same...in MY example????? Or is the 2000RPM(max) prop a higher pitch propeller??? And if it is higher pitched(for lower RPM)....why am I "Properly propped" on the lower RPM engine and overpropped on the higher powered engine.....WHEN THEY ARE THE SAME ENGINE!!!!

 

[Nomad Willy]

John,

With the 2500rpm engine talked about if we had it propped WOT at 2500 and then changed the pitch or dia of the prop OR gear ratio to run the engine at 2000rpm then yes it would be overloaded.

But that is w/o changing the amount of fuel injected. Full throttle at 2000rpm is too much fuel. Now I'm getting above my knowledge and barking around the governor tree you mentioned on the other thread. I've been thinking the governor is like the gov on our lawnmowers. I'm quite sure my Mitsu has such a governor. Just an engine speed limiter. But I'm beginning to think the gov may be connected to fuel delivery and rating as you alluded to on the other thread. I need to go back and read djmarch and see if what he has to say sheds some light on this.

John wrote; "NOW... what Eric was talking about on the other thread. If this engine is in my boat in it's higher HP rating, I would have the prop pitched to achieve that RPM....2500 in this case. And if it is in my boat in the lowest rating, I would increase the pitch on my prop to achieve the lower RPM...2000 in this case. I am assuming these statements to be correct.

NOW...let's simply switch propellers. Why all of the sudden am I overpropped on a boat with the exact same engine???"

John if you "just simple switched" props you would be injecting too much fuel and would be seeing black smoke in your exhaust. Pure and simple you're just talking about over propping. But in the computer controlled engine it may be able to (through sensors) tell what rpm you're running and not allow more fuel to be injected than the engine can properly use. With a feature like this you'd have an engine that you couldn't overload. You could still over prop an engine but it would be "hobbled" and would have considerably less power than it's rated amount.

March,

It sounds like you're talking about electronically "hopping up" an engine. Fuel amount and timing is all that I can see that the computer would be able or likely to change. Different engines w different outputs may have different valve timing, numerical compression, valve and/or port size, low friction rings, turbo pressure relief gate changes and whatever else. Probably none of the latter would be changeable w the computer alone.

 

[RickB]

We will use electronic common rail engines just for simplicity. It is my understanding that on some of these model engines, all that is done to make them "commercial grade" or rating is electronically turn down the governor basically "de-rating" the engine. Is this a true statement(I realize not all encompassing")?!

To some limited extent that may be the case. However, when you see a large range of power delivery across the ratings it usually means there are mechanical differences as well. To learn which differences you will have to contact the manufacturer. Generally speaking, some of the changes may include cam profiles, turbocharger sizing, injection timing, injection volume and duration, compression ratio, and in the case of the electronic engines, the control laws with regard to mean effective pressures at rated loads.

To an even more general degree, the power output is related to the life expectancy of the engine and what the manufacturer is willing to warranty under what conditions and maintenance regimes.

If I had to simplify the propeller loading vs rpm issue to its most basic, I have to say that it is imporant that those discussing the subject have a good understanding of how a diesel engine mechanical governor works, what parameters it controls, when and why, and what rules apply to the electronic control of power by a modern engine's electronic control system.

The bottom line, your throttle is not a throttle.

 

[BobH]

>>>The bottom line, your throttle is not a throttle.<<<

This is an issue I have wondered about. What does the throttle lever actually control? Does it provide input to a governor to control to a set speed with the governor controlling the amount of fuel injected OR does it control the amount of fuel injected and engine speed depends on loading?

For example, my engine manual has a graph that shows that at 1500 RPM fuel consumption is 2.64 gph. So does that mean that with the transmission in neutral and the engine at 1500 RPM I'm using 2.64 gph? Or is 2.64 gph at full load?

I've argued this back and forth with myself, really wasted effort, but have come to the conclusion that the throttle lever controls the amount of fuel injected for each injection cycle and the engine speed at a given throttle setting is a function of the load on the engine.

So which of me is correct?

Bob

 

[Nomad Willy]

Rick,

I realize there's no throttle on a diesel but what else is ther'e to call it that most will understand.

I will take your advise and bail on this discussion as I am one that does not understand fully the workings of the governor.

 

[Codger2]

>>>The bottom line, your throttle is not a throttle.<<<

I've argued this back and forth with myself, really wasted effort, but have come to the conclusion that the throttle lever controls the amount of fuel injected for each injection cycle and the engine speed at a given throttle setting is a function of the load on the engine.

Admitting, the answer to the question is certainly not within my pay grade, I tend to agree with Bob's conclusion. If that's not the case, I'll crawl back in my hole and lurk for awhile.

 

[RickB]

What does the throttle lever actually control?

This explanation is for a variable speed mechanical governor as fitted to most small boat diesels. There are many variations on governor function depending on the application so the following does not apply to all installations.

I don't know which engine you have but for the sake of discussion let me assume it is a good old fashioned mechanically governed diesel.

When you move the throttle you are increasing the tension on a "speeder spring" inside the governor. That spring resists the movement of a control that is moved by centrifugal force acting on a set of "flyballs." As the engine speeds up the balls (weights these days) move outward and that motion sets the position of the mechanism that controls the actual amount of fuel supplied to the injectors.

As engine speed increases above the desired level determined by the tension you set on the speeder spring it overcomes that spring pressure and slows the engine. As the rpm decreases below the set speed, the spring moves the control to supply more fuel.

A govern has a setting for maximum rpm that over rides the throttle setting, and it has a setting that limits the amount of fuel that can be delivered at "full throttle"

When you set full throttle you set a certain speed. With no load it might take (for the sake of discussion only) about 10 percent of injector pump capacity to reach high idle or maximum permissable rpm. At full rated rpm with a full rated load you might be delivering 95 percent of capacity.

If you overload the engine and try to take more power out of it than it can deliver, the governor will deliver the full amount of fuel up to the fuel limit stop. If the load still increases beyond this point and slows down you will see black smoke. Adjusting the fuel stop to attempt to get more power will work to a point but you will be working on the razor edge of overheating because the system is not designed to reject the amount of heat produced beyond a certain amount of fuel burn. That is the risk associated with "over propping."

The short version is that you select a desired rpm with the throttle, the governor decides how much fuel it needs to deliver in order to provide what you ask for.


An electronic governor references a lookup table of rpm/fuel flow/manifold pressure/temperature and in some cases, load to control how much fuel is delivered when. The throttle merely delivers an electrical signal to the computer that matches your desired outcome to what the engine can actually deliver safely.

 

[BobH]

RickB, thanks for the explanation. So in my example, the 2.64 gph at 1500 RPM would be with the engine loaded, not in neutral with no load. Talking about a FL, definitely old fashioned.

Bob

 

[RickB]

So in my example, the 2.64 gph at 1500 RPM would be with the engine loaded, not in neutral with no load.

Yeah, most likely. If you look at the curves on your performance graph you will see one (the lower of two) is called the propeller curve and represents the power required to turn a "well matched" (to boat and engine) fixed pitch propeller at the rpm indicated. The upper curve shows the brake power the engine can produce at a given rpm.

The propeller is considered well matched if it absorbs the full rated power at the rpm that power is available i.e. both curves meet at the upper right hand corner of the envelope. If you have a "cruise prop" it might be capable of absorbing more power than the engine can produce at a given rpm and create an overloading condition. The propeller curve in that case would rise sharply above the rated power curve before maximum brake power is developed.

If there is a third curve it will show torque and you will see the maximum is at a lower rpm than you would normally operate.

 

[sunchaser]

RickB is indispensable for oh so many TF issues. Rick, in other applications rotating ball tension controls are called a Lilly Control - is this the case in a mechanical diesel engine governor?

 

[RickB]

The principles are identical. The old fashioned elevator controls used actual flyball actuators as well.

Here is a link to a super simple graphical explanation of the mechanical diesel governor. Trying to visualize the operation of a modern rotary injector pump and its governor by looking at a drawing gives me headaches so this site is like an aspirin.

MECHANICAL GOVERNORS | AIMES, Srinivas Integrated Campus,Mukka

 

[marine nut]

You might read through these (any a few others on the web site) to some different insight on your question.


Continuous Duty - A Different Perspective


Propellers Move Boats, Engines Just Turn Them


Wes

 

[RickB]

What is so different? He seems to have a problem understanding the concept of max continuous power as the power output at which an engine can be operated 24/7 until it wears out. Of course it will be less than the maximum that can be pulled out of it by running every parameter at redline ... duh.

Many, if not nearly all, engines have a maximum output that is limited by the manufacturer or physics for one reason or another and that output is higher than max continuous ... it ain't rocket science.

And what is the mystery about propellers? They absorb power to produce thrust and even a "well matched" propeller can overload an engine in some operational conditions. If you had a torquemeter on each shaft of a twin screw boat you would be able to see how the power required to maintain rpm of the inboard wheel during a sharp turn climbs very rapidly, for example.

If driving a propeller complicates things too much, look at generator ratings. They use engines that are identical in every respect to drive a range of generator output ratings (standby, prime, or continuous depending on the reason the generator is installed.

 

[marine nut]

QSB 4.5 160 HP at 2500 RPM

I found these on another forum.. A commercial marine QSB 4.5

Wes

 

[FF]

For the simple minded an easy rule is 3 cubic inches equals 1 hp IN OPERATION , and service life is great.

2 CI per HP , maybe OK, if the engine was industrial, not lawn implement , or pickup truck takeout .

At 1 hp per CI , 1000 hours may be hard to get.

 

[Baker]

FF, the Mainship I just sold was 213CID Yanmar rated at 240hp for an hour and 190hp continuous. It had 1000 hours on it when I sold it and it was showing absolutely no evidence of giving up anytime soon. I understand what you're saying. General rule of thumb. But there are some good engines out there. BTW....it was well under propped. It could reach 3600rpm on a 3300rpm engine.

I still haven't got the answer. Are the props the same on the two differently rated engines or is the lesser rated engine have a higher pitched prop. Remember we are talking theory here. The engines are the same. So don't dodge the question by saying they aren't.

 

[THD]

Baker-all you need to do is check out a prop calculator. If I take my boat and do the calculation as is, with continuous duty rated JD's, then change only the rating on the engine (raising HP and raising Max RPM to JD's max rating on our engines), there is no appreciable change in either the diameter (about 0.1") or pitch (about 0.3"). On a full displacement hull, this should not be surprising. It takes a calculated 209.5 HP to move our hull at hull speed, it does not matter whether that HP is produced by a 154 HP engine or a 225 HP engine (the lowest and highest ratings on our JDs). Since all else remained the same in the above, including the reduction ratio, it should take the same size prop at the same prop rpms to move the boat. One interesting note, the 209.5 HP is 72% of our HP available at the shaft. With the higher rated engine, the 209.5 HP is less than 50% of the HP available at the shaft-a lot of wasted power.

 

[Baker]

That is what I suspected!

 

[FF]

One interesting note, the 209.5 HP is 72% of our HP available at the shaft. With the higher rated engine, the 209.5 HP is less than 50% of the HP available at the shaft-a lot of wasted power.

Its not "wasted power" , the reduction is what is required to allow a longer power on rating.

Big difference between 24/7/365 and a picnic boat rating for a couple of hours a month .

 

[RickB]

209 is less than half of 225?

It doesn't matter if 209 HP is delivered by a 154 HP engine?

What am I missing?

Do you guys know the difference between brake horsepower and a propeller curve?

If you have a boat that takes 209 HP to move it at (for example) 10 knots and then repower it with a 150 HP engine (and keep the same prop) you are not going to see 10 knots again. Thottle your existing 209 HP engine back to the 150 HP mark on the propeller curve and let us know what your boat speed is.

 

[Capthead]

I think it depends on the oil. Synthetic or fossil?

 

[THD]

RickB-I was less than clear-we have twins. The calculations were done on that basis. Thus, 308 total HP (and 450 for the alternative calculation). The prop calculator I used calculated a 5.5% mechanical HP loss from engine to shaft, so SHP of 291. It was interesting to me that the continuous duty engine uses a higher % of available SHP than the higher rated engine.