Thanks for all of the contributions...
The more I think about it, the more I reckon that gapless must logically be the way to go for small GDI-Turbo engines, for all sorts of reasons.
First there's the issue of dumping unburnt fuel into oil. I'm guessing this is most likely when the engine is cold and when the piston & its rings are in their least thermally expanded state (meaning bigger gapping). Going gapless takes away what it effectively a direct drain hole to the sump (the conventional ring end gap). This might be critical until everything has warmed up and properly expanded.
Second, by minimising blow-by, you unload the PCV system and significantly reduce the tendency of oil to migrate out of the crankcase and in to the air intake. It seems a very sensible fix for long term build-up of carbon on the backsides in the inlet valves. Given that turbochargers effectively increase compression ratio and for a given gap, blowby flowrate increases with differential pressure, then I would have thought gapless and turbos should logically go together?
Third, the OEMs have got their knickers in a twist about LSPI on GDI-Turbos, reputedly caused by 'throw off' of engine oil which accumulates in the top land space, into the combustion zone. I always have problems visualising how oil interacts with the ring pack but the way I see it, if the ring pack is doing its job properly, there should never be ANY oil in the top land space! The only logical path for oil to get there is via the top ring end gap. So if you close off this pathway with a gapless top ring, do you at a stroke eliminate LSPI? To my mind, this seems a more sensible fix than shifting the entire PCMO market over from calcium detergents to ones based on magnesium.
The more I think about it, the more I reckon that gapless must logically be the way to go for small GDI-Turbo engines, for all sorts of reasons.
First there's the issue of dumping unburnt fuel into oil. I'm guessing this is most likely when the engine is cold and when the piston & its rings are in their least thermally expanded state (meaning bigger gapping). Going gapless takes away what it effectively a direct drain hole to the sump (the conventional ring end gap). This might be critical until everything has warmed up and properly expanded.
Second, by minimising blow-by, you unload the PCV system and significantly reduce the tendency of oil to migrate out of the crankcase and in to the air intake. It seems a very sensible fix for long term build-up of carbon on the backsides in the inlet valves. Given that turbochargers effectively increase compression ratio and for a given gap, blowby flowrate increases with differential pressure, then I would have thought gapless and turbos should logically go together?
Third, the OEMs have got their knickers in a twist about LSPI on GDI-Turbos, reputedly caused by 'throw off' of engine oil which accumulates in the top land space, into the combustion zone. I always have problems visualising how oil interacts with the ring pack but the way I see it, if the ring pack is doing its job properly, there should never be ANY oil in the top land space! The only logical path for oil to get there is via the top ring end gap. So if you close off this pathway with a gapless top ring, do you at a stroke eliminate LSPI? To my mind, this seems a more sensible fix than shifting the entire PCMO market over from calcium detergents to ones based on magnesium.
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