Step-type exhaust valve seat

[Pengze]

Hi Everyone!
For a medium speed diesel engine, what advantage do the step-type exhaust valve seat (nimonic & stellited) has over the plain-type (no step) valve seat?
Any specific reason for the design of step-type exhaust valve seat ? Thank you in advance.

[Sébastien]

The engine I currently sail with is a Burmeister & Wain 8K42EF. The original design of the exhaust valve seat did not have a step but it was added later on by the manufacturer. The idea is that a seat with a step or groove will allow the valve to crush and cut off any accumulation of carbon on the seat. This allows any carbon accumulation to be blown off in miniscule chunks that do no harm on their way out. With the old design any accumulaton of carbon on the exhaust valve seat would have been hammered into a large chunk that would eventually lead the valve to not seat properly and burn out. It should be mentioned that the step is a preventive measure only, under normal circumstances there should not be any accumulation of carbon deposits on the engine exhaust valve seats. Hope this helps, Sébastien

[Pengze]

Hi Sebastian!
Thks for yr comment.
But I am sorry that you could be getting my message wrong. Anyway this is interesting.
What I mean by step-type valve seat is that the seat has a long skirt (sorry I didn't have the picture).
Usually you can find in Mirrlees Blackstone medium speed engine.
Previously, the seat has a short skirt whereby resulting in high exhaust temps and seat dropping off frequently.
After upgrading to step-type exhaust valve seat, the problem is solved. What I would like to know is the logic behind this upgrade?
Any significant comment?