Variable Compression, Variable Displacement you decide

[Clare Snyder]

Roger Heflin wrote:
> 
> 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
A case of poor engineering. General practice is to gear the supercharger
so that at full speed, full boost is attained. If overboost occurs, you
are over-driving the supercharger. If full boost cannot be reached, the
supercharger is under-driven. A plenum reduces the spike effect
considerably, but also raises the "hood line" This is why the long
ram-type manifolds are used on the funnies and pro street cars with
huffers. Ram tuning is only a silly dream when pressurised induction is
used. Runner length is almost irelevant, but plenum volume CAN be
extremely beneficial.
With a turbo, a waste gate widens the effective range of the turbo, and
prevents overboost.
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