Restriction to inlet/outlet flow

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Inlet flow

•              Once the inlet valve becomes fouled, friction and restriction will limit the rate of air flow across the valve. Hence although the boost or scavenge pressure is nominal, the scavenging efficiency will be reduced, due to the pressure drop across the valve. The same problems will be experienced for reduced valve lift and heavily shrouded valves.

Consequences:

•              Reduced pressure in the cylinder when the air inlet valve closes. Results in reduced oxygen in cylinder (slower burning and high thermal loading), producing fouling in exhaust and possible combustion space component cracking.

•              Higher air speeds can occur if fouling is close to the valve itself. This will lead to a faster air flow cutting into the exhaust gas within the cylinder, promoting mixing, rather than replacement. Hence the air purity of the final cylinder contents will be reduced.

Exhaust flow

Fouling of the exhaust passages (exhaust valve passage, nozzle ring, blades, EGB, etc) will also reduce the gas flow through the combustion chamber. This will also reduce the efficiency of the scavenge process.

Consequences:

•              Increased resistance to exhaust flow will cause a back pressure in the exhaust line. This will increase the work required by the piston to push the cylinder contents out of the cylinder. Hence engine power output will reduce.

•              Reduced air flow through the cylinder will also increase thermal loading, and reduce the margin against turbocharger surge.