Will slowing down damage larger diesel engines

“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

"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.

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

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.

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

"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.