Low compression pressure in diesel engines 1

[jeromero]

In 3200HP diesel engine we have a particular thermodynamic behavior in a particular cylinder, monitored buy a pressure sensor that relates pressure with crankshaft position and comparing with other cylinders: there are low compression pressure, low peak pressure and a big delay in peak angle. If there are leaks in the combustion chamber (by rings or valves), what would be the cause of a low compression pressure. We think that maybe there is a difference in volume between chambers (short connnecting rod, etc).

Thanks,

 

[ivymike]

If there are leaks in the combustion chamber (by rings or valves), what would be the cause of a low compression pressure

The leaks?

 

[jeromero]

Sorry, there are NO leaks in combustion chamber (typing mistake). We check rings and valves many times and everithing is OK, the same with the injection.

 

[PJGD]

Consider checking the valve timing of the inlet valves. If the inlet valve closing point relative to crank angle is different across the cylinders, then you may have a lower trapped mass of air in some cylinders. Probably most likely if the inlet valve is closing later than the other cylinders on a slow speed engine like this.

PJGD

 

[Metalguy]

If the inlet valve (s) for that cyl. ARE closing on time, you may well have a bent con rod. We had one on a 20 cyl. 8,000 HP diesel. How it got bent is another story.

 

[willeng]

jeromero:

Have you checked:
The lobe & valve lift of intake & exhaust,
Intake runner for possible differences,
Exhaust port restriction.
Exhaust manifold restriction.
Or like you said different volume.
Different stroke-- check crank stroke.
Pushrod--bent.

Is it the rear cylinder?
Low peak?
Is the cylinder getting the same amount of fuel as the rest?

 

[jeromero]

Thank you for your soon answers guys. We checked there are no leaks in combustion chamber nor problems with time in valves, they (intake and exhaust) are closing properly. We are using computer aided reciprocating machinery analysis: pressure, vibration and ultrasound signals taken in the cylinder head and related with the crankshaft position.

There is not restriction in intake and exhaust port.

We have checked stroke and is OK. This is an 8-in line engine and the problem is in cylinder 3.

The fuel injection amount can modify the behavior of the pressure after TDC so by that way you can "adjust" this part of the pressure vs. angle curve. But the question is why there is low compression pressure, the other phenomena (low peak pressure and peak pressure delay) can be consequence of the air amount, the ignition pressure is reached later.

 

[jeromero]

About con rod bent. Actually we thought about that but we have no tools for measuring it. Even we are not sure of the con rod length. Metalguy, how did you detect the bent?

We think in some reasons of changin volume in the chamber:

Because of the age of the con rods they may well are deforming in operation.

The big end diameter is not uniform and we have ovality in this. This can be related with problems in the con rod dowels.

Some engine data:

bore: 12.8 in, stroke 14.57 in, con rod length: 34.06 in

The big question is what amount of volume will reduce the compression pressure in 250 psi, the normal pressures at TDC is around 1100 psi, this cylinder have 850. The deformation of a con rod could not be more than 50 or 100 mils, this deformation can change the pressure in 250 psi?

 

[willeng]

jeromero:

Sounds like, if you can't find it with the equipment you have at your disposal, you should try it the good old way.

Rip the sucker down & have a look, Blueprint the entire engine & CC everything, this way you'll know exactly what you've got & go from there. saves a lot of time thinking about it.

 

[jeromero]

willeng, could you explain more the procedure, I do not understand completely

 

[willeng]

Have you checked your testing equipment.

Can you run a diagnostic check on it, if not or even if you can swap some sensors around & see if it is the same cylinder all the time.

 

[willeng]

Basically blue printing means to make each cylinder the same as the next, stroke, rod length, piston to deck height,
clearances, swept volume etc etc.

CCing basically means to measure the volume of the chambers, compressed head gasket bore appeture, volume above the piston at TDC depending on piston shape of course,
so you can equalise the area above each piston at TDC & so you can set the compression as you want.

Just one other thought maybe the compression height of the piston in the troubled cylinder is different.
It is the distance between the gudgeon pin centre & the top of the piston.

 

[Metalguy]

We noticed one cyl. had low comp. pressure and firing pressure. First thought was late closing of the inlet valves-the cam lobes are pressed onto the camshaft, and have been known to slip. That wasn't the problem. That cyl. had previously had a stuck-open air starting valve, which "supercharged" the cyl. to a very high degree. When the injector sprayed on that 1st stroke, the cyl. pressure went sky-high. Believe it or not, but it took something like 9 months to actually find the rod was bent. Besides being bent, it had extruded some of the lower bushing in the master rod (the bent rod was the articulating one). The engine still ran fine, since it had 19 other cyls. and we only use 7,000 HP from it. But my job was to convince our "regulator" that the engine could have produced 7,000 HP for a full week had it been needed-the engine is in standby mode for most of it's life. Fortunately I succeeded, and saved my employer a possible $12 million fine.

 

[jeromero]

How did you measure the con rod bending?. I guess you change both con rods, after that the problem disappear?

 

[aviat]

A bent rod will generally show up as having two steps where the piston reverses at TDC. The top one would be normal, but because a bent rod is storter there will be another step below it. If you can use a borescope you should be able to see it without removing head.

Or you may be able to make up a fixture and measure with a dial guage, comparing the length of rod with other rods.

 

[Metalguy]

In our case there were no top-ring ridges, because of the very low operating hours. When we finally crawled into the crankcase the bend in the rod was easily seen by eye!

The engine was fine after we replaced both rods, and I had them change the piston, liner and head, considering how much stress they saw on that one cycle.

 

[enginesrus]

Can you remove the fuel injectors easily? Remove one from a good cylinder, and the one from the bad cylinder. Bring the pistons to TDC use a calibrated beaker or burret and fill the combustion space with fluid, like real light motor oil. Of course you will want to use something like a sovent gun, wet dry vaccum cleaner etc. to remove the excess when done, and maybe even crank it or fire it, with out those injectors in, to make sure you don't have enough to cause a problem. If this engine has individual heads on it, you may have a head that is incorrect as far a combustion space goes. You could have one head meant for natural gas or ??
Hey I'm looking for a job. LOL

 

[patprimmer]

You will probably have a lot of leakbown with a very light oil. I would be inclined to use a heavier oil, then flush it with a quick drying solvent like acetone so as to prevent risk of hydraulic lock on starting.

Regards
pat

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[76GMC1500]

I haven't done much work on large diesels yet, I am about to start by installing pistons in a 900kw generator on Wednesday. Anyways, to test for a bent connecting rod, pull the head of the number 3 cylinder and pull the head of any other cylinder head you feel like pulling. Then, take a dial guage and run each of the cylinders up to TDC and measure the height of the piston in the cylinder. If one is significantly lower, you've got a bent rod. I also saw in another post you were asking about cracked sleeves in the same cylinder. The bent rod is could be causing uneven loading of the cylinder wall and causing uneven heat buildup which could crack the cylinder.

 

[dwedel]

I suppose you have already found the problem. In a spark ignited engine I would be checking ignition timeing and fuel delivery again, before I would pull a piston and con-rod.

Low fuel and lack of ignition would cause a low peak firing pressure.

Just a question, can you see the cylinder pressures when the engine is rolling over with the fuel turned off? This would maybe help differentiate between the crank/rod/valve problems and ignition problems. I realize that diesels don't have ignition, but perhaps a fuel injector is having problems.

I guess I am all for checking the little stuff before pulling appart the big stuff. I'm sure you have done this already.