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???

 

[Fabrico]

It would be most helpful to all if you could explain more about when it smokes and what is meant by "experimental".

 

[mattsooty]

Norcape I would guess that your engine is running to little boost or the injection pressure is to low.

From the fact that you are also seeing mega pressure rises I would also say that you may be to advanced on your injection timing - however increasing the boost will compensate for this.

In effect you are injecting the fuel into the cylinder before combustion can occur, then when conditions are right for combustion the whole charge is burning in one go - I bet the engine is vey noisy as well.

My recommendation would be to increase the boost as much as possible, this will reduce the smoke. Then adjust the rail pressure & timing to bring back SFC & Emissions.

Thanks

MS

 

[CapeNor]

They always smoke. Manouevring in port or at full load at sea, there's always a big black cloud comming out of the funnel. It has always been there! from newbuilding!

The engines are experimental. They are built by a big japanese shipyard/steel company. They thought it was a must to have there own engine design. They built six of these engines, and afterwards cancelled the whole project.

2 prototypes ashore, 2 in our vessel and 2 in the sistership. They al have the same smoke problem.

So there has never been any follow up on the engines. It stayed an experiment. The shipyard company nowadays makes engines under license MAN's they told me.

 

[WGJ]

I've seen these triple peaks on gasoline engines where cylinder ionisation has been datalogged in attempst to use ionization currents as an analogue of cylinder pressure.
The triple pressure-pulse plots I saw were associated with knocking due to over-advance. Does that tie up with over-advanced fuel injection in a big diesel?



Bill

 

[Fabrico]

Norcape, how is the engine being throttled? Is it by means other than control of fuel delivery?

 

[21121956]

Hello everybody:

According to the post, I can understand that the cylinder pressure graph shows "after firing" phenomena, with pressure increases during the expansion stroke.

If this is it, let me tell you that we have to stand up to a similar situation in our medium speed Diesel engines, 514 RPM, above 88% of maximum load. The manufacturer when asked about this subject said that, the reason for the "after firing" indication is a pressure oscillation in the measuring channel between the combustion space and the measuring point.

The amplitude of the oscillations may vary depending on engine load, speed, volume in the bore (between combustion space and measuring device, etc.).

They handed over a graph with lab tests at full load , showing the measurement of the firing pressure, simultaneously in the combustion chamber and from the "indicator valve" on top of the cylinder head (where normally is connected the measuring device).

The two curves are really quite different. The curve of the "in cylinder" measure is certainly smooth all along the combustion arc; but the second one, shows those peaks.

In our engines, with loads around aprox. 88%, those peaks do not appear.

Hoping this can help you

 

[CapeNor]

I have been thinking about bad measurements. But the measurements with the same equipement on the auxiliary engines (2x 8 cylinder diesels (750 rpm) shows a clean nice graph. Both measuring points are at the indicator valvese. Both have a valve, and a length of pipe.

But the main engines have 2 bends and the lenght of the pipe is almost the double.

It is a possible. Big question?? How can we be sure??

 

[stenis]

When measuring the cylinder pressure at the indicator valve you will get a totally incorrect pressure curve. The compression may be fairly good but the combustion will (seem to) be delayed and the start of combustion too steep. There will also be oscillations in the pipe from the combustion chamber to the indicator valve. Three pressure peaks sounds like oscillations. There may of course be something fundamentally wrong with the design of the engine but that is impossible to analyse without thorough investigations. If you need to analyse the combustion you need to put the pressure transducer into the combustion chamber. The quality of the pressure curve can be analysed by plotting a pV diagram with logarithmic scales on the x- and y-axis. You should get a straight compression line and almost straight expansion line. I think Heywood, Internal Combustion Engine Fundamentals has something about that. If you want to see the influence of the pipe you can put a piece of pipe and two bends on the indicator valve of one cylinder on you auxiliary engines. You will get a different pressure curve.

The smoke problem sounds like bad injection. At high loads you should have some kind of boost pressure and if the engines are still smoking the problem should be elsewhere. I assume the engines are quite old and then new more efficient turbochargers and higher boost would help but if the injection is bad no turbocharger can cure the problem.

The injection nozzles are dimensioned for 100% (or 110%) load and if you only run 70% load you could decrease the injection nozzle hole area by 30%. Do not decrease the area too much since this will increase the injection pressure and increase the load on the pump cam and roller. Smaller holes should improve combustion and decrease smoke. You can also have a look at the injection nozzle hole angles. Look at the piston where the fuel sprays are hitting and make a drawing of the combustion chamber and see if it seems logical. Your guess will be as good as the original guess the Japanese made. As always, only change one parameter at the time.

For this you would need new nozzles and there may be companies interested in making nozzles for 48 cylinder burning HFO if the OEM is not interested. You can also write “Powered by XXX” with 5 m high letters on the side of the ship if the engine maker is not interested in helping you

 

[MikeHalloran]

I think the conglomerate that owns the shipyard and built the engines also builds some decent motorcycles, but this may have been their first and last foray into big Diesels.

I wouldn't paint their name on the boat, or threaten to do so, yet. Instead, find their engine guy, ask what he would do differently if he had the chance, and solicit his help in figuring out how these engines are different from the MAN engines that superseded them. That might give you a finite chance of success at improving their performance wrt smoke.



Mike Halloran
Pembroke Pines, FL, USA

 

[CapeNor]

Hello, I'm back from the ship again. Thanx for all your responses.

I have been checking some things out, on your advices.

1. Is there a way so I can add a picture or a graphic to this board???

2. Pressure ossicillations in the measuring tube:

I've been checking this out. There are 3 curves of 90 degrees in the pipe and 2 curves of 45 degrees. I have been studying the pulses and there are much more peaks than the three I mentioned before. It seem like we're having some kind off a harmonic. Different sinusus with different wavelength causing a mix off positive and negative resonances.

I contacted the manufacturer of the measuring device, he said it's impossible. But I did the same test, with full trottle and with this only one cilinder at minimum fuel injection (while the other 11's are at full load.) And guess what, the pulses and peaks are almost the same. Even if there is no combustion any more.

So maybe the measuring fault is indeed the cause of the "tripple peaks"

3. Smoke: The 2 turbo's of one engine has been fitted with smaller nozzle rings, we have more boost air pressure, the exhaust temperature is lower, but... still smokes like hell, no improvement visual.

When we changed a cilinder head last weak, I noticed there' s a burn pattern on the piston. It's a print of the 8 injector conusses. I checked it on the injector test bench, and downward angle is nearly 25 degrees. I have no idee what influences the swirll of the compression have on this conus, but it looks like the injector is just spraying its fuel on the piston instead of in the air??????

I have asked the chief engineer if he could look the details of the injector up for me. Maybe the downward angle is not correct. I will try to convince the chief to ask the question to the Bossch representative. (it are there injector tips!)

4. A big question remains. What causes the problem, bad injection, to low a compression, not enough air??
The bosses are convinced it is the last option, but I start to have second thoughts. So I begged the chief engineer for an exhaust gas analysis. That will tell us if there is yes or no enough air inside the cilinder.

One thing for sure, this story will be continued!!!!!!!

And if I find a way to put pictures on the forum, I will show you the details.

 

[MikeHalloran]

My _limited_ understanding says:

- Diesels always run with excess air.

- Low-smoke Diesels use higher injection pressure or other means to get a more finely atomized injection spray.


Maybe your chief can get the Bosch rep to take on your boat as a personal project; he could get some good PR out of it.



Mike Halloran
Pembroke Pines, FL, USA

 

[CapeNor]

I finally discovered it, it was here al the time, right before my Eyes,

Picture of the ship with the obvious big black cloud, both at sea at full load and in port at part load.

 

[CapeNor]

OK Pictures of the piston with the combustion marks. The marks shows the spot where the injector puts the fuel.

Is it correct to state that there may not be a visual spot like this on the piston??

Is it correct to state that the marks are to close to the center of the piston??

Does anybody recognise such pattern??

 

[CapeNor]

I scanned a drawning of the engine. In the close up of the cilinder top, I've drawn in red what I GUESS is the injection at this moment. In Green I have drawn what I THINK it should be.

The red patern is drawn, based upon measuring the mark on the piston, and the estimated angle of injection (visual on the test bench). I haven't found anny official information about the nozzle.

Is there any nozzle specialist who can confirm this?? Or are there any other options, remarks, warnings???

I have to convince the bosses that we need to try another nozzle tip!!

 

[jbthiel]

I am not a diesel expert, but info I have seen on diesel combustion (including a couple dozen research papers) would indicate that you want the green angle.

I also notice that there is no swirl to the pattern. Most modern diesels induce swirl to get more complete combustion. You only get the proper air/fuel mixture on the outside of the fuel stream from each nozzle hole. As the fuel enters the cylinder the outside particles start shearing off and atomizing into small enough droplets to combust cleanly. By inducing swirl you spread the stream out, help atomiz the fuel faster, and creat more area with the proper ratio. You can get too much swirl (which obviously isn't your problem ), which causes the tail of one stream to run into the one next to it which no longer helps.

Along with changing the angle you may want ask Bosch about changing the number of holes to 9 or 10. This will affect your atomization characteristics as well. More holes should improve the situation, but you also have to consider how much of the injector tip is left.

Have you measured the injection pressure? Low pressure would limit the fuel atomization. What is you timing set to? Do you know what the injector duration is?

 

[CapeNor]

Hello jbthiel, thanx for your reply

There is a swirl in the cilinder. After some years, Mitsui changed the cilinder heads and installed swirl plates at the inlet valves.
This was done to force the inlet air in the cilinder under a certain angle, and so introduce a swirl trough the cilinder. But maybe the fuel doesn't get to the swirl pattern area"??
These swirlplates have been taken out for several years because they didn't improve combustion, and they get dirty very soon. We got new ones now, but we have to clean them every month!! (2x2x12 swirl plates = a hell of a job).
The design of the piston doesn't improve swirl. There are 4 area's sunk in the piston for the valves. So you got some "onstructions" standing just in the flow of the swirl.

The Fuel injectors are set to open at 400 bar. After some running time, that pressure drops. The spring in the injector get worn out or something. It happens that we take out an injector and the opening pressure has dropped as low as 260 bars???? (after 2500 running hours)

What actual pressure the fuel pump reaches during injection, I don't know. I haven't measured it, I don't have the tools to do so.

The pump starts the compression at 14,5 degrees before TDC. I do not know the duration of the injection.

 

[CapeNor]

History

The ship in her early days, under the name Tipperary as she was built... Smoking!!!

 

[Fabrico]

The 4 valve notches are almost universal on Diesel engines.

Compared to a truck engine, the spray pattern shown looks similar but more intense. The aim looks about right but I have not seen the pattern extend up onto the flat part of the piston. Each area is also rather wide. It's almost as if the injector is designed to work with a larger piston.

 

[jbthiel]

Here is a picture of a CAT medium size engine (8-11L) spray pattern. A Cummins picture a saw seemed to indicate the future injectors might get even flatter in the coming years. There is a lot of work going on right now to analyse the actual atimization in a running engine - this allows much more accurate computer models which should improve the engines quicker and cheaper.

I also found some info that the small-med sized engines are going to increase the number of injector holes to 8+ for emissions, power, and efficiency. They want to reduce the holes from the current minimum of 125um to ~50um. (or 0.005" to 0.002") Some research by GE on locomotive engines was looking at 10 holes nozzles.

That injection pressures sounds kind of low to me, but I have no experience with engines this big. Sounds like you are "pouring" the fuel in.