Low compression pressure in diesel engines
Low compression pressure in diesel engines
(OP)
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,
Thanks,





RE: Low compression pressure in diesel engines
The leaks?
RE: Low compression pressure in diesel engines
RE: Low compression pressure in diesel engines
PJGD
RE: Low compression pressure in diesel engines
RE: Low compression pressure in diesel engines
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?
RE: Low compression pressure in diesel engines
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.
RE: Low compression pressure in diesel engines
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?
RE: Low compression pressure in diesel engines
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.
RE: Low compression pressure in diesel engines
RE: Low compression pressure in diesel engines
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.
RE: Low compression pressure in diesel engines
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.
RE: Low compression pressure in diesel engines
RE: Low compression pressure in diesel engines
RE: Low compression pressure in diesel engines
Or you may be able to make up a fixture and measure with a dial guage, comparing the length of rod with other rods.
RE: Low compression pressure in diesel engines
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.
RE: Low compression pressure in diesel engines
Hey I'm looking for a job. LOL
RE: Low compression pressure in diesel engines
Regards
pat
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RE: Low compression pressure in diesel engines
RE: Low compression pressure in diesel engines
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.