[C4ENG]

Originally Posted by goplay

I wonder then, if the issue is incomplete burning of fuel, an oil cleaner system can be created.

Then again, it might just be simpler to change the oil after a long (many 100 mile) low speed run.

That raises the question of how much it takes to cause the pitting. Is it 100's of engine hours at low rpm or a dozen hours, and over what time period. Is it the engine sitting around for days afterwards?

You can never seperate the fuel from oil becuase they are both oils.

How much does it take for the pitting? Way to many varibles to answer. But from what I gather from Cat..basically your overhaul of engine can be due at a few thousand hours shorter than normal. Example..instead of 10 thousand hours over haul, maybe 7 or 8 thousand hours. But still so many varibles.
Sport fishing boats can troll at low loads which can be thousands of hours. They are made to do so with the appropiate rated engines. Yet they still have there issues..
Have your oil anylsed at oil change and if you are having corrision issues you will know from the test. Then you might catch some damage before it get's to out of hand..

[TK-F430]

Originally Posted by C4ENG

Here is straight from Cat..
At low load you have low cylinder temps (not to be confused with water jacket temps) which then causes there to be un burned fuel. Un burned fuel will then enter the oil through blow by which every engine has and even more so at low load becuase the metals or not at there expanded rate for best seal. Fuel in oil now.. besides thining out the oil.. fuel has sulfur in it. When sulfur meets water it creates sulfuric acid (there will always be some condensation taking place in your engine for there to be water inside) Now you are throwing sulfuric acid all over the inside of your engine to create corrision and pitting. That's when they showed pics of pitted cylinder linings and bearings of low load running engines.
But then there is the additive in oil to combat and nuturalize the acid which is called TBN (total base number). Cat sells kits to test your oil TBN becuase when that aditive is used up it is no longer combating the acid. You may need oil change sooner than recomended according to usage but who ever wants to change sooner? That's when the problems arrive.
At rated suggested loads the cylinders are up to temp and almost completly burning the all the fuel.

Great info.......good to know.

[Marmot]

There are a few bits of the low-load puzzle missing in this thread.

One, the ratings go from commercial continuous duty “A” rating for vessels operating at 100 percent rated load 100 percent of the time or cycling service at 80 to 100 percent load factors to “E” rating which is for vessels operating at rated load up to 8 percent of the time or up to 30 percent load factor.

If it helps to clear this up a bit, the 3412 is rated at 431 hp @ 1200 rpm for an “A” rating and 1420 hp @ 2300 rpm for the “E” rated version.

If an engine is “broken in” and the rings are properly seated at the beginning of its life there is little risk from operating at low loads for extended periods, particularly in the case of electronically controlled engines. The only way raw fuel can enter the lube oil is through poor injection quality and poor ring sealing. Low load operation does not introduce large quantities of fuel in the cylinder in any event so fuel dilution of lube oil is minimal if the injectors are in good condition.

Blowby will introduce exhaust gases into the crankcase and if you are using a high sulfur fuel and oil temperature is low, condensation can occur and produce sulfuric acid. Where we are now using low(er) sulfur fuels the potential for ring and exhaust path deposits along with increased emissions from too high a TBN may be greater than the risk from sulfuric acid.

Operating the engine at high power for part of each period will heat the oil and reduce the risk of condensation. Either test the oil yourself or send in samples for analysis. Change oil regularly and you won’t go wrong, broke maybe but not wrong.

Generators tend to have issues related to low power operation. Part of the reason for this is that they rarely operate at high loads and many have not been properly run-in when installed. They may see many hours of very low load operation before the boat even leaves the yard and the liners are glazed from the beginning.

Low power operation of mechanically injected engines can lead to poor combustion and soot deposits on turbocharger turbines and fouling of exhaust paths. This can lead to poor scavenging, even poorer combustion and short exhaust valve life.

If the engine is routinely operated across its rated power range there should be little risk of the dreaded low load disease that seems to be scaring so many people recently.

[TSI AV]

Hi everyone,


First of all, my answer to the question posted - YES. Running at lower rate can cause the damage.

All correct. TBN, "A"/"E" categories, sulphuric acid, thermical expansion of parts, etc.

Additionally, I'd like to divide engines into groups, differentely affected by "low load":
1. Running at constant speed, as gensets, CPP applications.
2. Electronic engines.
3. Running at variable RPM's.

Group Nr 3 is of a "higher risk".
The reasons:
1. Everything, what was already mentioned by other forum members.
2. Low piston's speed allows piston rings getting stucked in grooves.
3. Injection's timing instability.

And some other factors abt running at low load, what were not mentioned yet:
1. Charge air temperature.
Of course it's low at low load... and this affects combustion dramatically for all groups.
There are a lot of marine applications, where charge air is even preheated up to 60-70 deg C during running at lower rate.
This improves quality of combustion.

2. Injection timing.
Running at different RPM, timing may shift to "EARLY" or "LATE" side of diagram.
Of course, depends on factors like a plunger configuration and electronic timing (if any).
But this can cause an improper combustion as well.

3. Fuel atomization.
If it's bad, don't be impressed of having problems.

P.S. If interested, I can post some of my recent diesel engine's diagrams,
performed with digital engine's analyzer.

Regards,

Andrei

[Kniffin]

Given todays computerized fuel injection, I would not think that the injectors would "slobber" fuel at any point.

I also wonder, that if you can test oil for proper additives, if you can also have field tests for fuel in oil.

Is added filtration going to filter out the blowby byproducts such as fuel?

If the tests show some additive levels to be low, can some be added? Are some oils, such as synthetic oils preferable in cases such as this?

My CAT guy thinks that having pyrometers installed will help determine the best rpm's at which engines are operating well and burning the fuel efficiently, so as to prevent glazing and fuel blowby. Maybe it's not idle speeds but upwards 2-400 rpm (just speculating) and not near the rpm when the turbo wants to kick in.

I also wonder why some charter boats go 10k hours between overhauls with big engines and tons of trolling.

[Marmot]

“Low piston's speed allows piston rings getting stucked in grooves.”

Piston speed is not one of the reasons rings get stuck. Blowby, poor combustion, failure of scraper rings, excessive cylinder lube, and faulty injection can all contribute to sticking rings but not piston speed.

”Injection's timing instability.”

You’ll have to explain that one in a bit more detail.

” Charge air temperature. Of course it's low at low load... and this affects combustion dramatically for all groups. There are a lot of marine applications, where charge air is even preheated up to 60-70 deg C during running at lower rate.”

Please list the engines and applications where that is the case.

The only reason to ever heat charge air is to aid in starting of small high-speed engines (and very few do that), or in the case of extremely low inlet air conditions where condensation might become an issue. In the latter case, coolant flow through the charge air cooler is reduced to maintain temperatures that do no exceed around 40 degrees C.

You will find that most engines with turbochargers and aftercoolers aim for a charge air temperature of about 12 degrees C above the temperature of the cooling medium. In the case of seawater cooled aftercoolers that is around 38 C. No engine manufacturer looks for charge air temps much above 50 at maximum and most are rated at under 40. Lowering charge air temperature increases charge density and increases power, it also decreases NOx production and allows engines to meet the EPA and IMO emissions limits. For EPA rated engines you must refer to the engine manufacturer’s technical file, as approved by class to determine the maximum charge air temperature.

“This improves quality of combustion.”

In some cases higher charge air temperatures may reduce soot production, but, at the cost of increased NOx and higher fuel consumption, both of which are not acceptable and may be illegal. No good comes from having high charge air temperatures and the marine propulsion industry is working overtime to reduce them on all engines.

”Injection timing.”

This is not a problem with modern engines, even mechanical engines do not suffer from this problem as at low rpm there is more time available over a given crank angle. Electronically controlled engines, especially common rail completely eliminates any of the problems we normally associate with antique engines.

”Fuel atomization.”

See above

[Marmot]

"I also wonder, that if you can test oil for proper additives, if you can also have field tests for fuel in oil. "

Yes, but it takes a bit of practice and is not particularly accurate. Here is a link to an explanation of how to do it.

http://www.worldlube.com/self-test.html

"Is added filtration going to filter out the blowby byproducts such as fuel?"

No, nothing aside from distillation will remove fuel from the oil. If you have fuel dilution, change the oil.

"If the tests show some additive levels to be low, can some be added?"

On larger engines like the slow speed or medium speed engines used on larger ships, a partial oil change is done to restore the TBN to within limits. It is not worth bothering with for the smaller engines used on yachts. Just change the oil according to the engine manufacturers reccommendation.

"Are some oils, such as synthetic oils preferable in cases such as this?"

Synthetic oils are excellent for high temperature applications. They may not be economical for "normal" propulsion applications unless you are running E ratings for extended periods at high temperature and then this is the wrong thread for that discussion.

"My CAT guy thinks that having pyrometers installed will help determine the best rpm's at which engines are operating well and burning the fuel efficiently, so as to prevent glazing and fuel blowby."

Pyrometers measure exhaust gas temperature at the exhaust valve. Low loads equate to lower exhaust temperatures than high loads. Pyrometers provide useful information for determining overload conditions or poor performance on a cylinder and for balancing cylinders but are not very useful for tweaking low power settings.

What he might be talking about is the condition where at lower loads the exhaust gas temperature might be very slightly higher than it would be at a slightly higher load. This is due to the fact that turbocharged engines use a technique called valve overlap to provide better scavenging. The exhaust and inlet valves are open at the same time for a period and this allows complete removal of exhaust gases and the cooler scavenging air blows over the exhaust valves and cools them and the pyrometer sensor. At low power there isn't as much scavenging air pressure developed by the turbo so the exhaust may actually be a few degrees higher. It's hard to see on an analog guage though.

"I also wonder why some charter boats go 10k hours between overhauls with big engines and tons of trolling."

Because this isn't really much of an issue except on a few boating bulletin boards. That and the fact that unlike generators which operate at constant speeds regardless of loads, propulsion engines operate at varying loads and speeds which is a very good way to avoid the problems generator engines suffer from when operated at very low loads for most of their lives.

[goplay]

Marmot, thanks for the reply... the most rational and technically based set of responses I've seen yet on this "bulletin board issue"!

[Kniffin]

Thanks Marmot for your response to my questions. The oil test website is very interesting.

[K1W1]

Originally Posted by Marmot

[i]Pyrometers measure exhaust gas temperature at the exhaust valve.

Hi,

As you seem to like to use CAT as you reference point it should be drawn to your attention that the larger engines (3500's) the standard Pyro offering is one per bank and that's it. The smaller one give one per engine usually on the outlet of the Turbo combiner if you are lucky or it's nothing. The 3500 Family have facility to fit and it can be done at the customers cost individual probes, the accuracy of these for day to day fault finding leaves something to be desired as somethine a healthy engine has a couple of low temps. Stopping the engine swapping the probes and wires often leads to a completely different reading.

[TSI AV]

Hi everyone,

1."Piston's speed". Oops, sorry for misleading. This is not the reason, but additional (proven) factor.

2.Yes, electronic engines do not have injection problems vs RPM. It's clear, :-)
I already mentioned that in my previous post.

3.Here is one of diesels, where preheating is used - Wartsila. However, not solely.
Very modern models and 20 years old as well. Running on MGO / MDO / HFO. Small and big ones. High speed and medium.
Temperature controller is operated by pressostat, which is installed on air inlet manifold.
So, when engine is running below ca 40 %, the temperature controller keeps charge air appx 60 deg C.
If load is higher than 40 %, then controller keep the charge air appx 40 deg C.
*I hope not to be asked to post instruction manuals here...

Besides, I've never stated, that charge air temperature is kept high at nominal load.
Please read my previous post thoroughly.
And, there is nothing illegal, this design is accepted by all Class. Societies.


4. About "injection timing" and "bad atomization". Please have a look at diagrams below. All 3 are taken from one cylinder. This is my explanation.

P.S. Exhaust temperaures can not be used as reference for cylinder balancing solely. They are only relative indication of process.
Well, it's possible to have them all equally adjusted, however, this will not mean that all peak pressures (Pz) are equal.
This is the pressure force, what makes pistons moving, not temperatures.


rgds,

Andrei

[Marmot]

"As you seem to like to use CAT ..."

It was a CAT mechanic that kniffin mentioned, might as well use the engine that has been used as a reference. CAT was also the engine discussed in the "load factor" thread and this one closely parallels the issues raised last week so we may as well use what has become the local currency. An engine is an engine, substitute MTU, Wartsila, or MAN if you prefer.

There wasn't much point in writing that most little engines don't have a pyrometer (or even an indicator port) for each cylinder, if you have one you know what you have and whether or not my description applies.