godlameroso wrote: ↑
Sat Feb 08, 2020 6:03 pm
Mudflap wrote: ↑
Sat Feb 08, 2020 4:30 pm
Why would it?
Because the exhaust valves are partially held open during combustion, instead of the normal combustion cycle which has the exhaust valve fully closed during combustion. This allows the valve face to absorb the combustion heat and radiate it directly to the cylinder head. If you hold the valve open and allow combustion gas to escape you don't get this direct combustion heat convection to the head, forcing the valve guide to handle the task on its own. Perhaps with different materials and engineering this issue can be largely avoided.
I'm sorry to be blunt but that's complete and utter rubbish.
First of all Miller cycle is all about the inlet valve timing (late closing typically). Why would the exhaust valve even be open during combustion ? That is just plain nonsense.
Secondly, you don't really seem to understand what the different heat transfer mechanisms are and how they work.
First, you say "This allows the valve face to absorb the combustion heat and radiate it directly to the cylinder head."
There is no such thing as valve radiating heat to the head. The heat transfer mechanism between valve, valve seat and head is conduction.
The heat transfer mechanism between valve and guide is also conduction
Then you say " If you hold the valve open and allow combustion gas to escape you don't get this direct combustion heat convection to the head, forcing the valve guide to handle the task on its own."
The primary heat transfer mechanism between the combustion gases and any part they come into contact with is convection. If the exhaust valve would actually stay open during combustion (it doesn't though!) the convective heat transfer would actually increase (there is flow across the valve and the heat transfer coefficient is a function of Reynold's number). Either way, there is a convective heat load on any surface in direct contact with combustion gases, regardless of whether the exhaust valve is open or closed.
And finally, exhaust valves in these highly boosted engines are guaranteed to have sodium filled stems. In such case the heat transfer along the valve stem is greatly increased and hence heat input into the valve guide is also high compared to a conventional engine. Luckily water cooled valve guides have been in use since around WW2.