layback40 wrote:
geordi wrote:
WWdiesel - if there is any soot scouring or exfoliation of the exhaust valves, it would have to be seen on an electron microscope. I don't think there is an argument to be made for that weakening the material strength of the stem by simply reducing the diameter, as that would have to be a VISIBLY narrower result to weaken it enough to separate in the way they do.
Geordi,
The reason I advanced the idea of soot sticking to the valve was that if it did & then under lean conditions, it burned, this could provide the sort of temperature necessary to cause the grain size increase that was seen. I would have expected that with the EGT's seen on these engines there would be enough heat transfer (cooling) between the stem & the valve guide to keep the lower alloy part of the stem cool enough to prevent grain enlargement.
I agree in order to see microscopic exfoliation of the exposed valve stem area, it would have to be seen under an electron microscope?
But another question I have pondered is what are the chemical affects the exposure to the high carbon content of the combustion gases may have on the exposed valve stem metallurgical composition over enons of cyclic life of the valve. I am not a chemical engineer, but when the high soot loading is exposed to the high pressures of the combustion process which includes high EGT's, what does it become chemically and can it have any possible effects on the exposed valve stem area and maybe contribute to grain elongation as shown in the report?
I agree with your synopsis on the valves making contact with the piston that if so visible indication would be very prevalent in the piston top.
And further agree that any EGR system on a diesel engine is detrimental to their longevity and their overall efficient operation. That is why it should be totally disabled or removed on this engine ASAP if you still have one in operation. It is also the reason I hang on to my noisy 98 Dodge Cummins that came from the factory with no EGR, CAT, or CCV system of any kind from the factory.
But we know emission control proponents care little about engine longevity or efficiency!
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