Variable Compression, Variable Displacement you decide

[Roger Heflin]

Andris wrote:
> 
> 
> I'm going to disagree on this point (from a purely theoretical
> standpoint). If you have a cycle on the T-S chart (temp and entropy, I
> think) a multistage cycle can get you more efficiency than just one
> large single stage. 

Actually not.  All that matters from a therotical standpoint is
where you started and where you end up.  More stages is not necessarly
more efficiency.  If you do expansion in two stages it will still track
down the same line.  You would have to reheat things to change the
T-S chart, or do the expansion in a different way.   This was what my 
thermo book said.  It also said that a turbo or super may be more or l
ess efficient depending on your system setup.  Efficient being defined
as the standard forumla with heat in, heat out, and work.

> I don't understand this point. I thought more air (higher boost) was
> always better. You have more air, so you add more fuel to keep the right
> AF ratio, and vroom! More power :) If you are talking about this from an
> efficiency point of view, I understand then :)
> 
> Andris - mech E student struggling with Heat xfer at the moment
> 
>From an efficiency standpoint alot of the pressure is wasted in excessively
compressing the air.  I believe the problem is when there are not 
enough valves open the pressure spikes (wasting alot ofenergy) and
when the valves open it goes down, I am only guessing on this.  I was
told by someone that 10-15 years ago someone put a 2 speed tranny
on a blower (full drag car) and it boosted the power at the wheels
a fairly large amount.  The transmission was slowing the blower down
at higher rpms  They found the blower was using a large portion of 
the cars hp at high rpms.  This was after they measured the pressure
in the intake and figured out how much was wasted.  Also keep in
mind that if you cam overlaps some of the boost goes straight out
the exhaust doing no work.  The point was you could boost so much
that the blower was using more power than the extra air was making,
this takes high pressures, but can be noticed in supercharged
cars.  

					Roger