08/23/2003

Boost Regions

Recently, based on discussion with Dave, tons of technical rapport, and bruces
experience, the operation regions of a supercharged engine were dropped into 4
parts - each very different with very different rules.

Region 1 - NA - boost ineffective - compressor acts as a restricter.
Region 2 - Transitional.  Boost is beginning to be effective
              Lower Region - Pressure does not go positive during cylce
              Upper Region - Pressure does go positive during cycle
Region 3 - "Full Boost" where the manifold pressure exceeds the pressure that
               can reasonably be associated with acoustical Tuning.
Region 4-  Choked Flow - where the air flow exceeds the compressors ability.

Kompressor Regions something was the titles.  And a major caution that what
works in one region will not be optimal in another.

In the regions of 1 and 2, acoustical tuning is very effective and highly
useful.  Region 3 is for full on boost - 5-7 psi upwards.  Region 4 is a too
small compressor.

There is a ton of data on truck engines with helmholtz resonators and there
effectiveness.  Good Data - caveat - the manifold pressure peaks at about 5
psi.  Hint?  You bet - helmholtz works - but what about at 20 psi.  Nothing
special - but SAE/other seldom goes above 5-7 psi.  Flow dynamics and fluid
mechanics show that the chamber does not do a thing ABOVE about 5 psi.

There is a battle between kinetic ram tuning and acoustical ram tuning.  Be
extremely careful here.  Take kinetic ram tuning.  Lets put a three couple at
one end ( 3 cylinders with 240 degree firing order ) and the throttle at the
other end.  As the length increases, the pressure will tend to increase up to
a certain subsonic velocity - but longer will make more power from idle to top
end no exception.  This btw is where the tuned pipe length data originates
from with slow speed diesels.

Now, go independent runner.  Now, we can no longer use just the velocity, but
we must get the phase arrival correct to build excess charge at the last point
we can use it.  IR works very nicely - at below a certain level of boost -
again around 5-7 psi.

In fact, the vast majority of data around piping resolves around NA to very
low boost.  Historically, superchargers were too expensive to deal with and
little past certain research has been published.  Consider also, that by 5 psi
boost, the engines power can be increase from 50% to 75% or more.

And all this acoustical tuning works very well up to about 5-7 psi.  That's
not disputable.

But Bruce is running 30 psi.  Bruce can get 5-7 psi with just putting his foot
down.  

The question goes back to where do you want to run the engine.  Transitional -
do the things in the transitional region.  Full boost - do full boost.

Bruce and myself among others, are shortchanging the "low" horsepower
transitional region to get as fast as possible into the full boost region.
The entire helmholtz/accoustical tuning will make little difference as the
compressor spools into the high boost region where the driving is fun.  Course
not enough so that the tires are no longer fuses.

A different engine, a different set of gears, and rethink it thru again and
optimize it again - maybe with a six speed and lower pressure and combined
ram/helholtz tuning.

That was the purpose of splitting things into regions because what works on
place does not work the other.  Know what you are doing and make intelligent
decisions.  Not to make blanket pronouncements on how it works.

---------------------------

And once again, after the smoke, it appears we are both on the same channel -
cept I'm cranky and obnoxious and getting deep into not caring.

That's about what I have been saying - and why I divided things into regions.
So that when I talk about x and supercharge, we all could know where and how
and why it works and independently decide if its a benefit to our engine and
driving patterns.

And the absolute concern about the buchi and the birmann exhausts with turbos
and the need for significant overlap especially with these exhausts on turbos.

And why I stated that below about 5-7 psi that kinetic tuning, helmholz tuning
and acoustical tuning is highly beneficial.

And with Delta P to think up, follow this:

The atmos is at say 15 psia ( nice day at the beach in californicate would be
close ).  Air equalizes pressure at the local speed of sound.  

When the intake opens, and the cylinder expands downward, a depression is
created.  The atmo pushes the air molecules towards the depression at the
local speed of sound.  The atmo pressure ( not the wave pressure ) moves
molecules to fill the depression at approaching the rate of volumetric
expansion of the cylinder x the valve area vs cylinder diameter rule.

This flow of air is not due to the cylinder "pumping", it is do to the
cylinder getting out of the way and the atmo pushing.  As the cylinder pass's
maximum lever arm, the expansion velocity of the cylinder will peak and begin
to fall off.  This point is approximately the point of maximum air velocity
and again - is not determined by the speed of sound - but the Delta P of the
atmo shoving air into the depression.  

The kinetic pipe effect is that objects once in motion tend to stay in motion.
The cylinder depression has forced the atmo to shove molecules into the
cylinder at close to the expansion rate of the cylinder.  The rate or velocity
is determined by the diameter of the manifold and the expansion.

This column of air, once in motion, will continue to stay in motion - even
when the piston has stopped expanding - provided the column is long enough and
the valve stays open.  This leads to the standard hold the valve open and get
the last gasp charge.  Why - because the motion was maximized at mid stroke
depression and the column continues to move air and with the right length
column, the air will still be moving even at valve closure.

The longer we hold the valve open and the piston close to still, the more air
from the column that can be added to engine.  This can be combined with
excessive compression and appropriate length ( the Lyscholm cycle differing
from the miller cycle only in the mechanism ) such that the pipe fills the
cylinder at a non-detonating rpm and doesn't at detonating rpm.

It is assumed that the cylinder will be about atmospheric at intake opening.
If so, then the only mechanism for creating the depression and the delta p is
the piston.  But, suppose it is not.  Suppose there is a vacuum caused by the
exhaust having a depression near tdc.  Then, the atmosphere is pushing air
into a combined vacuum of both the cylinder and the exhaust, and the velocity
of the charge movement becomes higher.  This increases the air flow total into
the cylinder.

Vizard has bluntly stated that this exhaust depression is the most important
part of the tuning.  Because it has the most effect.

Note that by Schweitzer - "Scavenging of Two Stroke Cycle Diesel Engine" - an
obsolete useless old book that thoughly deals with it and is a standard text
reference by Heywood etc, both the effects of scavenge and the long pipe
effect first demonstrated by Crossley at the turn of the century 19 are forms
of supercharge and engines using them are in fact correctly described as super
atmospheric.

The effect of long pipe can be greatly increased by the pre "Kill the Krauts
II" Wibu system.  Silly Polish Dude.  Probably gassed by hitlers heros.  

Any way, this system used very low initial lift such that a high velocity
across the valve ( combination scavenge and piston depression ) until the
cylinder was depressed to about 10 to 11 psia, at which point the intake was
snapped full open.  This created a very large massive airflow with few
limitations and supercharged the chamber.  Because this is a kinetic effect,
it worked well and complemented buchi exhaust.  Silly diesel stuff.

Note that when the charge particles in motion come to rest in the vicinity of
the intake at the end of the stroke, that this are move kinetically rammed
particles here than in the same space at atmospheric.  Thus the pressure is
higher - Supercharged or more correctly Super Atmospheric.  When the valve is
closed, this excess pressure must go somewhere - and it does.  It reverts
backwards down the intake tract until it exists into the atmosphere.  This is
called reversion.  It occurs every time the valve closes and a bunch of other
things.

If we put a one way valve - say a reed valve - on the intake in series with
the valve, when the intake closes, the reed locks the excess charge into the
cylinder and we start with a higher charge pressure - again another form of
super-atmospheric pressure for the engine.

Note to this point, even though we are able to raise the charge pressure at
some points above atmosphere, since there is no compressor - we can not be
supercharged.  And that all of this is due to the energy in the atmosphere
surrounding the engine and nothing from the engine.  The engine creates a
depression and the atmo fills in.

Pumping work by the engine is the work needed to physically move the
reciprocating assembly against the resistance of the air on the bottom side of
the piston.  

Acoustical tuning is taking the pressure pulse generated at the valve by the
pressure difference between atmo and cylinder/scavenging, allowing it to
travel up the up until it meets a major discontinuous and reverting back down
the pipe until it hits the valve again.  This positive pressure will assist
the kinetic motion in cramming more charge into the cylinder.

Think of a line of people waiting to pee on Davis.  Everyone is standing very
close together in this line.  The last man stumbles into the line and a line
of bumping each others bunns goes down the line.  That's the pressure wave -
contact to contact at the speed of sound.  How fast they can do it and zip up
and get out of the way - that's the velocity of molecule movement.  Trust me -
it works this way and thousands of texts agree and several million of us want
to confirm it works.

This returning acoustical waves pressure raises the delta P at the valve and
aids in filling the cylinder.  Another form of super atmospheric.

If we take the air side of several ( usually 3 or 4 ) runners and place them
into a common chamber, strange new kinetic things happen.  Assume the chamber
has a one way valve that only lets air in and blocks exits.  As the reverting
air from a valve closing enters the chamber, there may be a depression from
another valve opening and a delta p working.  This reverting air now raises
the pressure of the plenum and the delta p on the working cylinder is
increased and consequently the combined atmo forces more air into the cylinder
because of delta p.  

Now, if we replace the one way valve with an appropriate length and diameter
"tube", this tube will develop its own kinetic affects based on the plenums
draw.  When the charge reverts, this new kinetic tube's charge is slowed and
possibly stopped - but properly done - does not flow backwards.

The helmholtz chamber is actually a musical description of certain horn
instruments operation.  Each reverting pulse triggers a response and when
everything is right, not only is there a pressure peak for delta p in the
plenum, but there is air movement already in progress in the runner when the
valve opened.

This in general is how the acoustical/kinetic tuning creates above atmospheric
pressures.  The air is shoved into the cylinder by the increasing depression
and re-routed or kept from reverting.  It is well know from even the kinetic
effects only dating back to Crossley that the cylinders motion combined with
kinetics can move significantly more air than the geometric displacement.
Thus endeth the natural atmospheric pressure word.

All external compressors are one way fluidics devices - just like a reed valve
or the helmholtz ram pipe. 

Now the peak velocity of a cylinder induced depression ( the velocity at which
the volume expands ) is at about mid stroke.  This established a velocity in
the runner.  This is a fast as the cylinder can be filled and all other points
will be filled slower.

At 15 psia + 7.5 psig or 22.5 guess what?  We know is this caused by the delta
p of the air and the volume of the chamber.  At this point, we have doubled
the delta p at that point.  And the starting air flow is the same as the peak
air flow NA.  Starting.  The compressor has just down everything that the very
best of the best NA tuning could ever reach - by pumping it up to a 22.5 psia
"atmosphere".

Oh yeah?  Well, if you completely empty the cylinder and then completely fill
the cylinder exactly, you are at 100% ve.  The peak velocity occurs well
before half the cylinder is depressed. The air volume expansion will be
slowing by the time the cylinder is mid cylinder.  I'm being very generous
saying the depression peak velocity at half way down.  This means that at that
point, the cylinder can only be half filled. 

And of course, we have "forgotten" the effects of pressure on air flow ( fluid
dynamics ).  

Then of course, there is this little thing about critical pressure.  Going to
Schweitzer - Appendix to chapter 8, according to the testing of Nusselt (1932)
and the table of Nusselt Coefficients, we will reach critical pressure at
about 2:1 delta p absolute across the valve.  If the cylinder is pulled down
to about 15" at mid stroke and the charge is atmo - we are tapping at critical
pressure across the intake valve.  About 2:1.  With 7.5 psig, and 15" we are
at 3:1.  Best possible shape for Nusselt Coefficients is a 3:1 drop.  

If we are close to critical pressure, this artificially limits the air flow.

Based on the Delta P, at 15 psig, we will have tripled the air velocity and by
the time you are up to the black cars blipping pressure of 30 psig routinely
on romp - you have multiplied the air velocity by 5 over the maximum that an
NA can provide.  You will have reached critical pressure on the intake for a
significant percentage of time, and you will have had to increase the valve
opening time to overcome the massive back pressure the rammy whammys cause.

I hope this helps explain it.  I think I'm done.




On Fri, 22 Aug 2003 19:48:38 -0400, you wrote:

> >Its all about delta P across the engine.  An atmo engine always has a bit
> >more exhaust pressure than intake pressure.  Rammy whammy gives a few psi
> >boost with no resultant backpressure and no compressor drag.  This scavenges
> >the combustion chamber, stuffs a bit more air into the engine and adds
> >power.
> >
> >A turbo adds pressure to both the intake and exhaust.  If the exhaust
> >pressure is high no need for valve overlap or other tuning aids.  If the
> >intake to exhaust pressure difference is low enough for the rammy whammy to
> >work, then it works and power is increased.
> >
> >All these turbo regions are fine, and the 5 to 7 psi boost thing is fine,
> >but we need to talk in terms of delta P, not boost levels when determining
> >if rammy whammy can work.
> >
> >If delta P is say above 5 psi, then scavenging happens no matter what.  If
> >delta P is negative 5 psi or more, scavenging never happens.  If it is in
> >between, rammy whammy can be used to improve power at a given level of
> >boost.
> >
> >