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, Wed, 10 Sep 2003 16:24:11
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During combusion, as the flame front spreads and the pressures greatly increaase, some of the AF mixture is forced into the piston wall gap above the top compression ring. And some of that gets past the rings, but that is another issue. The AF mixture in that gap will not burn very well or not fully. As the piston moves downward and the pressures drop, that gas expands out of that gap. Some will burn, some will not.
To reduce HC emissions, the top compression rings have been moved closer to the top of the piston to reduce the amount of gap volume above the ring. This has happened to all modern engines. The top piston ring groove is now running hotter than ever before. This greatly increases the tendancy for oil in that groove to break down, oxidize, gum and coke. Many of the new engine oil specifation changes have been driven by things like this. New oils are formulated to not have a certain degree of oil breakdown in the top piston ring groove in a test engine under a specified load and duration etc. This does not mean that no deposits are forming, but that they are limited. The oil in the rings does not get changed much. It is sort of trapped. There is oil breakdown there. All modern oils will meet the spec for such things, but only a true synthetic will provide a superior result. The problem is probably worse for high compression N/A engines than for turbos with lower compression ratios.
So the top ring groove is where oil will breakdown these days. Stuck rings are a real risk. You will hear stories about old engines that ran forever on dyno oil. But the top rings were located lower and ran cooler than in a modern engine. This is one case where its clear that an emissions related change is clearly creating a dangerous situation. Another is exhast gas recirculation in Diesel engines. That can foul intake manifolds and ports with carbon deposits that can choke off the air flow over time. The PCV gasses combine with the exhaust soot and a very hard tough deposit is created. NOX probably helps it harden. New diesel oil specs exist to address that problem as well, but the problem is not eliminated. Heavy over the road truck operators may be having fits over this one and there may be law suits over this. They are been forced to change over to these new engine technologies.
posted by 208.24.179...
, Wed, 10 Sep 2003 16:24:11
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