Variable Compression, Variable Displacement you decide

[Roger Heflin]

> Gary Derian wrote:
> 	A supercharged engine has two stages of compression and only one stage of
> expansion.  This is not efficient under boost.  The displacement of the
> supercharger determines the actual engine size (in theory).  Therefore a
> supercharged engine is like a non-supercharged engine with higher
> displacement and compression but no higher expansion.  These engines are not
> efficient while running under boost but they make lots of power.  If the
> superchargers are bypassed during cruise, then you do have, in effect, a
> variable displacement engine.  While bypassed, this engine gains back its
> efficiency.  This works well if most of the time, the engine operates at
> part throttle and only uses boost for quick passing maneuvers.
> 
> 	A turbocharged engine has two stages of compression and two stages of
> expansion.  This is efficient under boost and turbocharged engines can make
> lots more power than supercharged engines.  Again, boosting pressure has the
> same effect as increasing engine displacement.
> 

I will disagree what what you said.  Having seen a small engine with large
boost compared to a large engine, the engines have vastly different
torque corves, therefore they aren't equivalent.    Also the number of
compression stages and the number of expansions stages has nothing
to do with the efficiency.  Thermodynamics says more stages don't
increase efficiency.  The reason a supercharge is generally
less efficient is that they compress more air than is needed, and substantially
increase the pressure in the intake more than is necessary.  If there
were a way to make the boost constant over rpm things would be better.
The way a turbo handles this is when a certain pressure is reached 
some of the exhaust bypasses the turbo keeping the pressure lower.  This
allows a turbo to have high boost at lower rpm where a superchargers
boost curve goes up with rpm until at close to redline it is at
close to the detonation limit.  With real world examples if you look 
both a turbo and super engines, same displacement, and same boost
makes close to the same peak power.  The curves are somewhat different
because of the boost characteristics of each, but fuel efficiency 
is very close.   The power the turbo uses is not free.  If a super
engine dumps its exaust at x psi into the exhaust, and the turbo
dumps it at the same x psi into the exhaust (after the turbo), they
can have the same efficiency if they are both designed correctly.

			Roger Heflin
			rah at horizon.hit.net