Variable Compression, Variable Displacement you decide

[Tony Cooper]

Roger - as promised, now that I am back from the dead I shall reply to
you. DO NOT think that because I used single word replies in a previous
corresponence, that that was all I understood on the subject. The reason
single word replies were used is because it physically HURT me to type -
so get off your high horse, calm down (Have a large drink) and read
wwhat I think.

Roger Heflin wrote:
> 
> >
> > Roger Heflin wrote:
> >
> > > 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.
> >
> > True.
> >
> > > 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
> >
> > False.
> >
> You say false.  Lets hear your argument and reasons.  And why anyone
> should believe your false.

Ok, what exactly are you saying in this passage? - Are you saying that
the super-charger is producing too much pressure? - or is compressing
the charge too much? - this passage just doesn't work - please clarify.
As to why anyone should believe my 'false' - well I will explain, just
as soon as you do.

> > > 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.
> >
> > Very false...
> >
> Again same as above.  The auto books I have read say the biggest problem
> with a supercharger is how much power the use at high rpms.  This they
> say is the only disadvantage of a supercharger over a turbo.
> 

Many problems here - I will go through each in turn.....
1.
	"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.". 

What is being said here???? A turbo's boost is directly proportional
(inertia/heat not included) to exhaust gas flow - therefore to achieve
higher boost with a turbo, the engine must first be doing some of its
own work. The exhaust gas pressure rises, and so the turbo spins faster
- etc etc. A supercharger will supply boost from tickover. The only
reason it doesn't produce boost all the time is the fact that you are
stopping it by keeping the throttle shut. Open the throttle, and it will
produce as much boost as it can NOW. The amount of boost it will produce
is directly proportional to its drive ratio (And of course how much air
it is trying to move against what sort of restirction etc etc ). Just
what has the detonation limit got to do with any of this I don't rightly
know - maybe you can explain.

2.
	"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."

Wrong. From what you are saying is that both the supercharged engine and
the turbo engine must therefore have the same losses??? - But arn't you
turning the supercharger round with some energy from the engine????
This fact alone could account for the supercharged engine being down by
10% at peak power. (Not peak torque through). The torque curves will be
radically different (Different charge delivery = different curve) -
true.

> > > 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.
> > >
> >
> > Sorry, but that last bit is pure rubbish as well...

The second half of this paragraph doesn't quite make sense, so I refer
to the bit that says "The power the turbo uses is not free". Well this
statement is true. It is not free - you had to pay for it by putting
fuel in first, then burning it, however, what were you going to do with
it then? Let me guess - make some noise, heat the atmos up a bit with
it????? - It may not be free but the fact that you are now going to use
some of that energy to turn a pump round can only be a good thing!!!

> >
> Again why is it rubbish?  Because you say so?  Lets hear reasons.  If
> it was rubbish there should be not production superchargers in vehicles
> requiring fuel efficiency, yet there are so the above statement most
> almost certainly be true.   Lets hear your reasons...
> 
> > I am sorry to be so curt, but have had flu since Sat morn, can still
> > only just walk - will reply in MUCH detailt when I can type again.
> > Tony
> 
> For that matter, lets hear what training allows you to be such an
> "expert".
> 
> Sorry to be so curt.
> 
>                         Roger

Expert? Me?, No I am no expert - far from it. Its just my life's work
thats all. I do not believe in experts - (A drip under pressure etc etc
etc) - However I do seem to know more about the subject than you do, so
if you have any further questions, please put on your cone shaped hat
(Yunno - the one with the 'D' on it), sit in the corner, put your hand
in the air, and I will see if I can help you.

Me - I just luv a bit of forced induction...

Tony

-- 
Sent By Tony Cooper.
email: tony.cooper at virgin.net
Allow at least 10 working minutes for reply. ;)