Some info on "
Exhaust Blow Down" from one of my F1 friends that he posted on a F1 forum on how today's F1 turbo compound power units are now the most efficient F1 engine ever.
extracted energy does not necessarily mean back pressure
because there is lots of energy in blowdown (the exhaust 'pulses') ie between EVO and BDC
a very useful amount (eg 10-15% to crankshaft power) is extractable without any increase in exhaust pressure after BDC
as stated by Wright, who made 14000 such aircraft engines in the 1950s
so can be called free energy
why is it free energy ??
the turbine work is not seen by the crankshaft as an additional impediment
because blowdown is a choked process, with great loss of useful pressure/velocity energy
choked by the exhaust port, not by whether there is an exhaust turbine or not
the energy extracted by the turbine is simply energy that would otherwise have been dissipated by more supersonic (shock) flow
greater amounts can be extracted if exhaust pressure after BDC is allowed to increase
this again reducing blowdown losses
in the 1940s it was shown that BTE was improved this way (improving total power for a given fuel quantity in 2014 F1)
even if exhaust pressure exceeded induction pressure (as long as the EVC was suitable for this ie relatively early)
this can also be called free energy
these two routes seem in principle complementary (though realisation of this would be design dependent)
Tommy Cookers
LINK:
TERS : Thermal Energy Recovery System - Forum - F1technical.net