(This point came up in a previous thread but thought this deserved a new thread since there were no takers in the original - Mazda rx8 wankel efficiency etc)
GraviMan said:-
"Having seen carbon fibre reinforced aluminium pistons (I think they were cast in an oxygen free environment), I feel that auto engines materials could do with another look. Aluminium and magnesium alloys are just the start. Ford were looking at composite 2-strokes, at one point. Rover (pre BMW) developed an adiabatic ceramic coated engine, which needed no cooling water!"
and patprimer replied
"To be really effective, ceramic coatings need to be thick, to be durable, they need to be thin. If new ceramics are developed that will allow adhesion without de-lamination over a wide temperature range with a thick coating, a lot of problems will be solved, and all engines, wither rotary or piston will have substantial increases in their thermodynamic efficiency."
to which I commented
" 2) If you could raise the thermal efficiency using insulating materials surely you could use atleast say solid ceramic on the piston crown in a conventional engine. I thought I read somewhere that this doesn't work because with a high surface temperature the boudary layer becomes turbulent leading (counter-intuitively) to higher heat transfer in the piston crown. (Analagous perhaps to how snow can keep you relativeley warm if its -40 outside. Or the fact that double glazing is no more effective with separations greater than a certain critical dimension since convection takes over as the dominant heat transport mechanism."
Not sure in retrospect whether the analogies are particularly apposite but you get the idea - not everything in this world is linear and pridictable and counterintuitive effects are everywhere
Actually I've read a number of posts (not necessarily on this site) recently suggesting that if suitable insulating materials were available it would inevitable make higher efficiency engines possible.
I'm not convinced. To react to cyclical changes in temperature (thoughout say 4 stroke cycle) any insulating material would also need to have tiny thermal mass otherwise there's no advantage over an engine that simply runs at a higher average temperature. On the other hand, steel construction engines probably could be run hotter than they are in practice, which kind of suggests that thereis no advantage and probably disadvantage in having higher surface temperature. Also what about detonation. Would this be encouraged be higher surface temperature?
Any takers?
GraviMan said:-
"Having seen carbon fibre reinforced aluminium pistons (I think they were cast in an oxygen free environment), I feel that auto engines materials could do with another look. Aluminium and magnesium alloys are just the start. Ford were looking at composite 2-strokes, at one point. Rover (pre BMW) developed an adiabatic ceramic coated engine, which needed no cooling water!"
and patprimer replied
"To be really effective, ceramic coatings need to be thick, to be durable, they need to be thin. If new ceramics are developed that will allow adhesion without de-lamination over a wide temperature range with a thick coating, a lot of problems will be solved, and all engines, wither rotary or piston will have substantial increases in their thermodynamic efficiency."
to which I commented
" 2) If you could raise the thermal efficiency using insulating materials surely you could use atleast say solid ceramic on the piston crown in a conventional engine. I thought I read somewhere that this doesn't work because with a high surface temperature the boudary layer becomes turbulent leading (counter-intuitively) to higher heat transfer in the piston crown. (Analagous perhaps to how snow can keep you relativeley warm if its -40 outside. Or the fact that double glazing is no more effective with separations greater than a certain critical dimension since convection takes over as the dominant heat transport mechanism."
Not sure in retrospect whether the analogies are particularly apposite but you get the idea - not everything in this world is linear and pridictable and counterintuitive effects are everywhere
Actually I've read a number of posts (not necessarily on this site) recently suggesting that if suitable insulating materials were available it would inevitable make higher efficiency engines possible.
I'm not convinced. To react to cyclical changes in temperature (thoughout say 4 stroke cycle) any insulating material would also need to have tiny thermal mass otherwise there's no advantage over an engine that simply runs at a higher average temperature. On the other hand, steel construction engines probably could be run hotter than they are in practice, which kind of suggests that thereis no advantage and probably disadvantage in having higher surface temperature. Also what about detonation. Would this be encouraged be higher surface temperature?
Any takers?
