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Old 05-15-2013, 01:20 AM   #21
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Peter, you left out a choice. And that is to run the engines in gear under moderate load (1,200 rpm) until the coolant temperatures get to their normal reading at which point the engines continue to be run at that power and load for at least 30 minutes. This is what we do if we are unable to take the boat out for four to six weeks due to weather, my travel schedule, etc.

Don't know if it's good, bad, or indifferent for the engines but it was what was suggested we do by our diesel shop and our acquaintances at Northern Lights/Lugger when we bought the boat, so we've done it for the last fifteen years. It's pretty rare we have to do this, however, as we are almost always able to take the boat out at least once every four to six weeks.

Regarding THD's question, is that not the function of the governor? The governor-- flyball as you once told me in the case of the FL120--- slows down if the load increases which therefore causes the injection pump to send more fuel to the injectors until the rpm is back where the throttle has set it to be and the governor is spinning at that speed again thus "locking" the fuel being sent to the injectors at that volume until the load changes again and the governor again adjusts and "locks" the amount of fuel being pumped. Yes, no, maybe so?
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Old 05-15-2013, 06:24 AM   #22
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"Unless you idle your engine for 8 hours a day the likelihood of this condition occuring is hardly worth thinking about."

As the oxymoronic "Fast Trawler " with a 300hp engine putting cheap at 7K, 3gph ,is usually almost at idle.

"The cruising load matches the rpm of a fixed pitch propeller all by itself ... it doesn't need any help from us."

To load an engine a good percentage of the engines available power at the reduced RPM must be used.

Look at a HP vs prop load graph and notice the prop load at low speeds , say 1500rpm will be but a small fraction of the factory rated power available at 1500.

Using more power at the low RPM will give better efficiency , a CPP or cruise prop is the usual choice.

Read the PBB ,Calder article on electric boat propulsion , and notice his comments on Volvo overproping factory recommendation.
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Old 05-15-2013, 09:50 AM   #23
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... how does the injection system sense the engine load?

Marin pretty much described it.

A diesel doesn't have a throttle that works the way a gas engine does, it doesn't vary the amount of air admitted to the cylinder as a means to control the weight of fuel in the mixture. A diesel always (though that is a highly nuanced "always" for reasons we needn't go into here) admits the same amount of air regardless of the speed or load. It is the amount of fuel that determines how fast the engine turns or how much load it can handle. The governor controls how much fuel is injected.

Moving the throttle on a mechanically injected engine does not open or close a fuel valve or throttle plate, it changes the pressure on a spring in the governor. That spring resists the force created by a spinning weight that is hinged so that increasing speed and resultant centrifual force trys to fling the weight outwards. As it moves outwards against the spring it moves a lever that, in the short version, opens or closes a valve that regulates how much fuel is above the injector pump plunger and sent to the injector itself.

The force on the flyballs is the same at a given rpm regardless of the load on the engine so we use the throttle lever (speed lever) to change the force on the spring (depending on the governor, it may increase the force or decrease it) so that the position of the valve that controls the fuel matches the amount of fuel required to oppose the spring and keep the engine turning at the rpm we command.

At no load, it requires very little fuel to reach max rpm, at high load, it takes a lot. With a properly matched fixed pitch propeller the maximum load possible matches the rated output of the engine at the "speed stop" rpm set by those screws on the side of the governor that you are not supposed to touch. The governor is set so that the range of fuel flow matches the range of power and all we have to do is move the throttle to obtain the speed we want.

If we had a controllable pitch propeller, we could "fool" the governor and overload the engine very easily. If the engine control system did not have a "combinator" or a more sophisticated governor to match pitch with rpm automatically, we could set a low rpm and a high pitch and quickly overheat and destroy the engine by overloading. Sort of like driving around in 4th gear all the time. In an overload condition more fuel is delivered than the engine can burn and all that excess fuel turns to heat and black smoke.

That is the super simple description. If you want to really get a firm grounding on governors, take a look at the Woodward governor website. they have probably built more governors than anyone else on the planet and they have some excellent training materials online. If you are the least bit of a geek, the subject is fascinating.
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Old 05-15-2013, 05:07 PM   #24
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So do I get a more efficient "burn" at lower revs by reducing the rated hp of my engine? I am underpropped at the moment by 150 revs and can derate the motor. (mechanical fuel pump) I dont really need the 300 HP. She was a fishing trawler.

A mechanic told me we can reduce the fuel pressure to the fuel pump and "try it" (Adustable screw on the cummins fuel pump so easily undone) I have also been told to leave it all alone because it's running well but it seems logical and only provide benefits so I cant get it out of my mind. What do you think? Anyone been there?

Rick, I did have a look for info on the woodward governers but the only site I found seemed mainly about aircraft. Could you give a link?
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Old 05-15-2013, 05:24 PM   #25
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So do I get a more efficient "burn" at lower revs by reducing the rated hp of my engine?
the best way to do that is by pulling the throttle back. If you don't trust yourself to self limit, reset the speed lever stop on the governor so it doesn't move as far.


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A mechanic told me we can reduce the fuel pressure to the fuel pump and "try it"
You can't adjust the injection pump pressure ... the injectors "pop" at a certain pressure and anything less will create problems and anything more will create problems. The injection pump plunger is a very positive displacement device and there is no relief valve or adjustment on the outlet. Injection quantity is adjusted by changing the volume of fuel above the plunger at the start of injection, not the pressure.

I can't imagine what he was talking about. If he meant to reduce the pressure into the pump enough to starve the injector pump itself that would quickly destroy the pump by cavitation.




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Old 05-15-2013, 05:59 PM   #26
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RickB-thanks for the diesel lessons! I just learned more on this thread in an hour or so than I have learned in the last 25 years of playing around with them.
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Old 05-15-2013, 06:23 PM   #27
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Please keep in mind that all I have written about governors and injector pumps so far applies to good old fashioned mechanical injection systems only. Modern common rail systems and electronically controlled injectors operate much differently and are far more accurate..

But ... once the fuel is in the cylinder there isn't much difference.
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Old 05-16-2013, 06:42 AM   #28
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"Please keep in mind that all I have written about governors and injector pumps so far applies to good old fashioned mechanical injection systems only."

But not for Detroit Diesel , where engine power ratings can be changed quite a bit by changing injector tips.

Big difference between power and fuel burn with 135's or 45's or anywhere in between.

Sometimes the folks in the 1930 era had great engineering!

" (CPP) we could set a low rpm and a high pitch and quickly overheat and destroy the engine by overloading. Sort of like driving around in 4th gear all the time. In an overload condition more fuel is delivered than the engine can burn and all that excess fuel turns to heat and black smoke."

It is always assumed a CPP system has an operator with the intelligence of a car operator .

No high IQ is required , few cars are operated in 4th all the time .

An EGT gauge is required for the boat driver when setting efficient pitch , about 1 /10 of a boat buck.
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Old 05-16-2013, 07:31 AM   #29
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But not for Detroit Diesel , where engine power ratings can be changed quite a bit by changing injector tips.
No, it requires changing the injector which is a fuel pump in itself. Changing the tip alone would really screw things up badly.

The plunger and bushing that determine the maximum quantity of fuel that can be injected are different for different numbers and types of DD unit injectors.


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No high IQ is required , few cars are operated in 4th all the time .
Boats don't buck and jerk and rattle when severely "over-propped" either.

If there were any real benefits to putting a CP wheel on a small recreational boat there would be thousands of them in use. Just because you can tweak something doesn't mean it is worth tweaking. Reliability, low cost, and simplicity of use are the best attributes for a recreational vessel.

You could probably put an 18 speed two stick gearbox in your Toyota if you wanted but what would you really gain?
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