strange combustion in 510rpm 4stroke marine diesel engine 2

[CapeNor]

Very strange,

We have experimental v12 engines, 9000hp each, but they smoke like hell. People think we're burning cokes or something.

If we look at the P-a diagram of the combustion (pressure vs crank angle)We see that the ignition delay is rather long. The initial ignition is very "agressive". It rises very fast, more an explosion than a combustion. And most strange of all, the rest of the cycle, the controlled combustion, has always a tripple peak curve. Somehow the pressure in the cylinder goes up and down in peaks, instead of just going up or down. If I look at it, I think there are 3 explosions, a big one followed by 2 small ones instead of one smooth combustion. Does anybody has ever seen anything like this before?? Does anybody knows a web adress where combustion problems are explained by P-V or P-a diagrams.

I will try to upload a graph later.

I must admit that the engines are running at 70% max, because of crankpinbearing problems. Maybe caused by the same strange combustion????

And also I will admit, because the turbochargers are surging a lot, we reduced the scavenge air temperature to a very low 30°C only. We use 180 cSt HFO, but even on MDO, she smokes like hell.

The TDC pressure is 60 bars, pressure peaks during combustion go up to 95 bars.

I hope anyone has any idee???

 

[al1]

I read this post with great interest and my opinion on the possible cause is based on the following assumptions; the stroke is in the 375 to 400 MM range and the fuel injection on time is in the 8 to 12 millisecond time frame. The great photos show the injection pattern on the top of the piston and the injectors are identified as on a 124-degree angle.

However, my rough calculations show that at even at the low 510 RPM the piston would move far enough down the bore (over 20 MM) for the fuel injectors to continually spray ATDC on the bare cylinder walls. I also assume the engine has relatively cooler cylinder walls (due to unlimited cooling source) and would condense the injected fuel thus always causing smoking even at low power.

Since this smoking has happened when it was new, I assume that the inherent design is basically wrong. The main area of the problem is the lack of a true deep bowl in the top of the piston and the angle of attack of the injectors that correspond with this deep piston bowl design that would prevent this from happening.

Again, just some food for thought.
al1

 

[Fabrico]

Smoking all the time is certainly supportive of poor engine design. However, fuel delivery is about equally capable of creating the same problem.

Some engines, such as the Detroit 71 Series 2-strokes, are very sensitive to injector parameters. Even with the ability to fully adjust injector timing on the engine, there are still nearly a dozen different injectors available. Most all in the name of smoke.

 

[FJTOM]

You indicated that you had some nozzles with a larger center hole. If the outside diameter is the same, then this would reduce the l/d ratio resulting in less penetration. This would reduce the possibility of impingement on the cylinder wall. By the looks of the pistons, the carbon buildup, impingement is probably happening. Try the other nozzles.

Also, it is unlikely that the spray is impinging at low load or idle. The marks on the psiton would indeed be from full(70%) load.

Even though the angle is 124 degrees, you may still have the wrong injector gasket, so that the spray pattern is not positioned in the cylinder correctly. If you have excess inpingment on the piston, add another injector gasket or use a thicker one, to raise the nozzle. This may reduce the smoke.
Are there any impingement marks on the head?

 

[TJNBKA]

Hello,

Hot exhaust & low combustion pressure, I would again suggest to check properly the injection timing.
As I already wrote last time, the announced 14° timing seems very late for me! A timing of 24 - 25° looks more usual for this size & speed of engine as I checked on deutz and Pielstick.
Has somebody other values to compare with? What's captured on the original test report?

Regards!

 

[CapeNor]

Hello guys, I'm back from the ship.

We had some more problems, a crankpin bearing has gone bad and damaged the shaft. So there was little time to do some experiments.

But as an answer about injection timing:

We take P/V diagrams and very clear P/crankangle diagrams. These shows us that the combustion starts at about TDC. For the current conditions, the timing is correct. If we can shorten the ignition delay period, we will have to retard the timing. I will try to bring forward such a graph so you will see it.

We have tried to advance the timing to 16.5° , but this gave us such a hard sound that we put it back to 14.5° immediatly

Greetings

 

[21121956]

Hello everybody:

Hi Norcape. Please take a look to this document, I know it is not the source of solution for your problem, but certainly it can help in understanding possible causes of the problem.

http://www.wartsila.com/Wartsila/gl...nical_papers/sulzer/paper_lowsfuels_welsh.pdf

Go ahead and do not give up, all in the forum, I am sure, are supporting you the best we can.

 

[dicer]

What is the latest on this project?

 

[CapeNor]

An update on the project...

Because of a bad bearing shell, they had to refinish one of the crankpins, so everything has been a little bit to busy.

BUT: The office has agreed. Next month a specialist will come to measure the complete engine. They say he will need 5 days to do all the test he needs.

I hope he will come up with something.µ

 

[Lookin4a6FigSalary]

Norcape,

Here are some thoughts:

1) Check your lubrication oil system for adequate volume and pressure.....bearings need oil.....better too much than not enough, especially on an engine with crankshaft problems.

2) I doubt the swirl devices are doing much. The extreme inward air movement from the swish area of the piston at TDC most likely cancels out the little rotation air movement generated by the swirl devices. Of greater concern is the possibility of one of those little fins breaking off and wiping out the entire cylinder and causing other major engine damage.

3) The fact that swirl devices are getting dirty indicates some reverse flow of burnt exhaust gases. I wonder about the timing of the intake valves on these experimental engines. Or maybe the intake charge pressure isn't high enough or maybe the exhaust back pressure is too high. I wonder if the turbo is properly sized.

4) The injector pressure dropping so much over time concerns me. Yes, it should drop a little, but I am not sure if it should drop that much. I wonder if the spring is of adequate strength for the task because it seems to get excessively weaker during use.

5) The fuel impingment pattern on the piston seems a little aggressive. All that fuel hitting that relatively cool piston does not make for a clean burn. As a jet of fuel travels through the highly pressurized combustion chamber, the fuel molecules on the outside edge of the jet atomize first. So as the jet continues it travel it actually becomes smaller in diameter until there is no more jet remaining. Hence a jet of fuel will only travel so far. Knowing this and the obvious large diameter of your cylinder bore, I doubt that raw fuel is reaching the cylinder walls. Instead the fuel most likely is not atomising well because it comes into contact with the relatively cool piston. So to improve the fuel atomizing, you could try a new injector nozzle design. I would suggest more injector holes but much smaller in diameter. Currently you have 8 holes of a certain diameter which have a total sum area. And you are only running your engines at 70% of max power. And due to the smoke problem, you are probably over-fueling your engines. And the diameter of you holes are probably 110% of what is needed (assuming they were even designed). Therefore maybe Bosch or some aftermarket custom supplier could make custom nozzle tips for you. Maybe 12 or more lazer cut or EDM machined holes with a total area of about 60% of your current situation. Also, reducing the angle of the holes will help by not allowing the fuel to hit the piston.

In summary, more injector holes will help spread the fuel more evenly in the combustion volume. An improved angle of the holes (like your green spray pattern) will help. And smaller holes will make small jets of fuel which will travel shorter distances before becoming totally atomized.

I hope some of these ideas will help. Be sure to tell us what the engine specialist reports.