Variable Compression, Variable Displacement you decide

[David Piper]

The energy to spin the turbine is derived from the thermal expansion of the
exhaust gas in the turbine diffuser.  This recovered thermal energy
contributes greatly to the thermal effy of the turbo engine when under
boost, especially at max power, if designed to take advantage of it, of
course.  

I think that the heat wasted in the exhaust can contribute up to 40% of the
lost effy, so there is a tremendous resource there.  A high output turbo
motor can approach 38% thermal effy whereas a NA is more like 28%.

The ultimate in SI engine design for aircrafts after WWII was the 3500 hp
turbo compounded motor developed just when jets became feasible, hence it
was stillborn.  The design used an additional blowdown stage of expansion
after the turbo to extract additional energy from the expanding exhaust.
The shaft was geared to the crank to assist in the power output.

TurboDave (aka Dynodave)