Exhaust Valve Sticky Problem for Diesel Engine Running in HFO




The exhaust valves move inside the valve guides in reciprocation motion. One end of the valve and guide is exposed to exhaust gasses (Down Side), while the other end exposed to relatively cool lubrication oil (Top Side). The valve stem fits securely into the valve guide. If enough deposit form on the valve stem or in the guide, the valve stem sticks in the guide.

Sticky valve after force removing
Sticky valve after force removing

Fuel play a vital role for deposit formation. Fuel contains high sulphur, Sodium and Vanadium causes cold and hot corrosion and deposit formation.

Why exhaust valve sticky problem happens?

The main reason of exhaust valve sticky is deposit formation in the middle of valve guide and valve.

The valves designed to slowly rotate along their longitudinal axis when the engine is running. This allows valve seats to have even wear. If the valves touch their seats in the same position every time the valve will lose its roundness/sealing very quickly.

Cross section of cylinder head showing exhaust and inlet valve
Cross section of cylinder head showing exhaust and inlet valve



With the longitudinal movement, the valve also moves little radially. Valve rides in the valve guide so the radial movement occur at the bottom end of the valve (Just after the opening of valve guide down side). For this radial movement proper clearance between the valve and guide always required. If the clearance decreased, it tends to valve stickiness.

Sticky Exhaust Valve
Sticky Exhaust Valve

During engine running condition lubrication oil flow from the top side of valve guide. An O-ring control the flow of the lubricating oil. At the same time from the bottom side of the valve guide exhaust gas tends to flow into the gap. As a result carbonization of lubrication oil occurs and from thick hard deposit. If the oil flow is not sufficient to clear the carbon deposit, clearance gradually decreased. This gradual clearance decrease tends to sticky valve.

 

We can show the system step by step from both ends (Lubrication oil flow side and Exhaust gas flow side):





Exhaust valve scenarios for two condition
Exhaust valve scenarios for two condition

Exhaust gas flow side (Bottom side/Hot side):

“Considering too little radial valve dynamics”

Step 1: Lubrication oil supply limited into the valve guide, so too little oil flow in the valve guide.

Step 2: Exhaust gas gets into valve guide

Step 3: Temperature under dew point (acid forming) and cold corrosion

Cold Corrosion of exhaust valve stem
Cold Corrosion of exhaust valve stem

Step 4: Volume of corrosion product much higher

Step 5: No possibility of eject corrosion product

Step 6: Loss of valve guide clearance in the middle of the valve guide

Lubrication oil flow side (Top side/Cool side):




“Considering too little radial valve dynamics”

Step 1: Loss of valve guide clearance for entry of exhaust port

Step 2: Lube oil stays in valve guide (no flow at dead end)

Step 3: Carbonization of lubrication oil

Hard deposit formation
Hard deposit formation

Step 4: Increase in volume

Step 5: No possibility of eject corrosion product

Step 6: Loss of valve guide clearance in the middle of the valve guide

And the final result is “STICKY VALVE”.

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