saviour stivala wrote: ↑
Tue Nov 27, 2018 7:36 pm
.......But you are right, a forced induction/turbo engine does not need as extreme a valve lift as an NA engine and also much less need of valve overlap to avoid turbo boost going right through the exhaust. It is valve overlap that mostly necessitates valve pockets in pistons. In turn valve pockets in pistons is driven by the need to keep combustion chamber space above the pistons as small as possible to obtain high compression ratios.
Gilles Simon wrote that there would be a conflict between adequate valve size/lift and adequate compression ratio
and boost isn't free in a turbocompound engine - pressure lost at the valves is work recovery denied to the turbine
we might also think that overlap would be needed to cool the exhaust valves
the exhaust mass-specific heat is unusually low but it's volume-specific heat is unusually high
and in these engines overlap loses no fuel and most of its flow work is recovered
could there even be some weird valve timing to use exhaust backpressure variations ?
sometimes overlap overscavenge and at other times underscavenge for internal exhaust retention and super-lean mixture