Quote:
Originally Posted by jt20
I can't speak for oil consumption.
But it is quite possible that accelerated valve guide wear could be avoided (primarily on the exhaust valves) by not allowing oil to burn on the valve shaft after the exhaust stroke.
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This is a very true statement but as you will see there are many design compromises and the net result is less that great.
If clearance is insufficient oil will not penetrate the interstitial zone between the valve guide and stem. The friction which results will cause wear and heat. This heat will cause the stem to expand making the clearance even tighter. Valve sticking may now occur which will result in abrasion and scoring. This is referred to as 'galling' which is a bonding and breaking loose process. This can procreate more damage as new high spots are created which give rise to more galling as the valve oscillates up and down.
So with this in mind it is a good thing to have sufficient clearance....but how much is sufficient. The valve stem has a significant temperature gradient and the sodium filled valves do try to equalize this some, or so I think. All in all it is a very complex set of conflicting forces. Clearance will give way to oil consumption since a differential pressure exists along the guide and a supply is present. Some deflector type methods have been used to reduce the supply of oil to the top of the guide. All in all the reciprocating piston engine is an abominable design but it was the first and so we have made it good enough after one hundred years of engineering and billions of dollars of R and D. I wish that investment had been made in rotary designs. Incumbency is not a great thing when it comes to politicians and poor designs.