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From: Walter Atkinson <walter@advancedpilot.com>
Subject: Re: [LML] Re: Where has all the power gone?
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Rob:

For starters, let's confirm some common understanding of some=20
definitions:
1) ICP - internal cylinder pressure.
2) thetaPP - the angle in degrees from TDC of the crank when the peak=20
pressure of the combustion event is realized.  Theta is negative before=20=

TDC and positive after TDC.  For example, timing is set at negative=20
thetas and the peak pressure is realized a positive thetas.   TDC =3D=20
thetaPP zero
3) Peak pressure - the point at which the combustion chamber is under=20
the highest pressure from the burning charge and/or compression.  This=20=

is at the point where one half of the charge has been burned (according=20=

to the accepted, authoritative engineering texts).

In response:

> **Consider a perfect ottocycle engine with perfect fuel. Mechanically=20=

> the engine can survive whatever pressure the fuel can generate and the=20=

> fuel burns infinitely fast. In this perfect world, with no cylinder=20
> leakage and no heat transfer, the cylinder pressure at any theta=A0after=
=20
> ignition on the power stroke is always the same, no matter where=20
> ignition takes place.**

I'll agree with you.  That doesn't exist.   <vbg>  I have no data on or=20=

knowledge of that engine, so whatever you say is unarguable.

> **=A0If the fuel is ignited at TDC, then the entire charge is =
combusted=20
> at TDC and Theta PP is also at TDC. Now, the piston sees the maximum=20=

> possible pressure force=A0at every inch of downward travel. Energy is=20=

> force times distance so this has to extract the most shaft power from=20=

> the fuel charge.**

There's one BIG problem with that statement.  If that case were to=20
exist, NO WORK would be accomplished.  You would have an infinite ICP=20
with a thetaPP of Zero and NO piston movement at all.  The engine would=20=

not run.  This is what happens in pre-ignition to destroy the=20
engine--the thetaPP approaches TDC until the increasing ICPs exceed the=20=

strength of the combustion chamber and it fails.
=A0
**If we ignite the fuel prior to TDC, then theta PP still occurrs at=20
TDC. We still see the same=A0energy extracted from the fuel charge as we=20=

did at TDC, however we now have to do additional work on the=20
compression side due to the higher presure of the precombusted fuel.=20
Thus the net=A0shaft power of the engine is reduced.**

Nope, NO work would be done if the thetaPP is at TDC.  The conrod angle=20=

is zero and the piston will not go down--the head will come off!  In=20
your mythical engine above, there will be a BIG explosion, NO piston=20
movement and it will get HOT--unless Sir Isaac Newton was wrong about=20
the Second Law.
=A0
**Next,=A0consider the condition where=A0ignition and theta PP are =
delayed=20
until ATDC. During=A0the period of crank rotation between TDC and=20
ignition the piston=A0is driven with only compression pressure. As this=20=

pressure, and the resulting piston force, is much lower than the=20
combustion pressure would have been, less energy is imparted to the=20
piston over this distance=A0and shaft power is reduced accordingly.**

There seems to be a misunderstanding of the effects of mechanical=20
advantage of crank-conrod geometry.  Either I do not understand your=20
premise , or you're missing something very basic.
=A0
**To extract the most energy from the combusted fuel during the power=20
stroke, theta PP should still be at TDC. Unfortunately, nearly all of=20
the fuel would have to be burned BTDC and the addtional compression=20
pressure would=A0absorb substantially more energy on the compression=20
stroke, greatly reducing the net shaft power. Accordingly, ignition is=20=

timed to provide for a minimal amount of combustion BTDC to mimimize=20
the compression stroke power requirement=A0and a minimal theta PP to=20
maximize power stroke power. The faster the engine turns, the bigger=20
the compromise.**

With all due respect, no.  The timing is set to optimize the thetaPP at=20=

as close to 16dATDC as possible, not as close to TDC as possible.  In=20
fixed timing applications the timing is a compromise to result in a=20
thetaPP that is closer to TDC at takeoff than optimal and further form=20=

TDC during cruise.  The solution to this is being able to move the=20
timing on the fly so the thetaPP is always occurring at the optimal=20
crank angle of 16dATDC....  (according to Taylor and Heywood, two of=20
the acknowledged experts).  For example, if the thetaPP is 10 and all=20
you do is alter the timing so it moves to 16, HP output will go UP. If=20=

the ThetaPP is 20 and you move the timing to make the thetaPP move to=20
16, HP will go up.  That's not just theory, it's measurable.  It's one=20=

of the biggest reasons to have moveable timing.  There are three ways I=20=

know of to do that. 1) measure many parameters and calculate the=20
estimated thetaPP, 2) estimate the timing needed, or 3) MEASURE the=20
thetaPP and adjust accordingly.   #1 is what the auto industry and=20
TCM's FADEC are attempting to do.  (requires a lot of sensors and data=20=

crunching.)  #2 offers some very engaging advantages over mags, but=20
isn't confirmable as optimal to an outside observer.  #3 is not yet=20
commercially available, but appears to be the holy grail--simply,=20
directly measure that which you need to know and adjust accordingly. =20
Easier said than done.

** Consequently, it should be clear that there is no=A0universal value=20=

for optimal theta PP or even the ratio of burned to unburned fuel at=20
theta PP. These values change for different engines as well as=20
different speeds on the same engine.**

Taylor and Heywood disagree with that.  They are two of the most=20
respected, authoritative combustion technology text authors out there.
=A0
Walter=

--Apple-Mail-12-808929481
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	charset=ISO-8859-1

Rob:


For starters, let's confirm some common understanding of some
definitions:

1) ICP - internal cylinder pressure.

2) thetaPP - the angle in degrees from TDC of the crank when the peak
pressure of the combustion event is realized.  Theta is negative
before TDC and positive after TDC.  For example, timing is set at
negative thetas and the peak pressure is realized a positive thetas. =20
TDC =3D thetaPP zero

3) Peak pressure - the point at which the combustion chamber is under
the highest pressure from the burning charge and/or compression.  This
is at the point where one half of the charge has been burned
(according to the accepted, authoritative engineering texts).


In response:


=
<excerpt><fontfamily><param>Arial</param><x-tad-bigger>**</x-tad-bigger><x=
-tad-bigger>Consider
a perfect ottocycle engine with perfect fuel. Mechanically the engine
can survive whatever pressure the fuel can generate and the fuel burns
infinitely fast. In this perfect world, with no cylinder leakage and
no heat transfer, the cylinder pressure at any theta=A0after ignition on
the power stroke is always the same, no matter where ignition takes
place.</x-tad-bigger><x-tad-bigger>**</x-tad-bigger><x-tad-bigger>

</x-tad-bigger></fontfamily></excerpt>

I'll agree with you.  That doesn't exist.   <<vbg>  I have no data on
or knowledge of that engine, so whatever you say is unarguable.


=
<excerpt><fontfamily><param>Arial</param><x-tad-bigger>**</x-tad-bigger><x=
-tad-bigger>=A0If
the fuel is ignited at TDC, then the entire charge is combusted at TDC
and Theta PP is also at TDC. Now, the piston sees the maximum possible
pressure force=A0at every inch of downward travel. Energy is force times
distance so this has to extract the most shaft power from the fuel
charge.</x-tad-bigger><x-tad-bigger>**</x-tad-bigger></fontfamily>

</excerpt>

There's one BIG problem with that statement.  If that case were to
exist, NO WORK would be accomplished.  You would have an infinite ICP
with a thetaPP of Zero and NO piston movement at all.  The engine
would not run.  This is what happens in pre-ignition to destroy the
engine--the thetaPP approaches TDC until the increasing ICPs exceed
the strength of the combustion chamber and it fails.

=
<fontfamily><param>Arial</param><x-tad-bigger>=A0</x-tad-bigger></fontfami=
ly>

=
<fontfamily><param>Arial</param><color><param>0000,0000,FFFF</param><x-tad=
-bigger>**</x-tad-bigger><x-tad-bigger>If
we ignite the fuel prior to TDC, then theta PP still occurrs at TDC.
We still see the same=A0energy extracted from the fuel charge as we did
at TDC, however we now have to do additional work on the compression
side due to the higher presure of the precombusted fuel. Thus the
net=A0shaft power of the engine is
reduced.</x-tad-bigger><x-tad-bigger>**

</x-tad-bigger></color><x-tad-bigger>

Nope, NO work would be done if the thetaPP is at TDC.  The conrod
angle is zero and the piston will not go down--the head will come off!=20=

In your mythical engine above, there will be a BIG explosion, NO
piston movement and it will get HOT--unless Sir Isaac Newton was wrong
about the Second Law.</x-tad-bigger></fontfamily>

=
<fontfamily><param>Arial</param><x-tad-bigger>=A0</x-tad-bigger></fontfami=
ly>

=
<fontfamily><param>Arial</param><color><param>0000,0000,FFFF</param><x-tad=
-bigger>**</x-tad-bigger><x-tad-bigger>Next,=A0consider
the condition where=A0ignition and theta PP are delayed until ATDC.
During=A0the period of crank rotation between TDC and ignition the
piston=A0is driven with only compression pressure. As this pressure, and
the resulting piston force, is much lower than the combustion pressure
would have been, less energy is imparted to the piston over this
distance=A0and shaft power is reduced
accordingly.</x-tad-bigger><x-tad-bigger>**

</x-tad-bigger></color><x-tad-bigger>

There seems to be a misunderstanding of the effects of mechanical
advantage of crank-conrod geometry.  Either I do not understand your
premise , or you're missing something very =
basic.</x-tad-bigger></fontfamily>

=
<fontfamily><param>Arial</param><x-tad-bigger>=A0</x-tad-bigger></fontfami=
ly>

=
<fontfamily><param>Arial</param><color><param>0000,0000,FFFF</param><x-tad=
-bigger>**</x-tad-bigger><x-tad-bigger>To
extract the most energy from the combusted fuel during the power
stroke, theta PP should still be at TDC. Unfortunately, nearly all of
the fuel would have to be burned BTDC and the addtional compression
pressure would=A0absorb substantially more energy on the compression
stroke, greatly reducing the net shaft power. Accordingly, ignition is
timed to provide for a minimal amount of combustion BTDC to mimimize
the compression stroke power requirement=A0and a minimal theta PP to
maximize power stroke power. The faster the engine turns, the bigger
the compromise.</x-tad-bigger><x-tad-bigger>**

</x-tad-bigger></color><x-tad-bigger>

With all due respect, no.  The timing is set to optimize the thetaPP
at as close to 16dATDC as possible, not as close to TDC as possible.=20
In fixed timing applications the timing is a compromise to result in a
thetaPP that is closer to TDC at takeoff than optimal and further form
TDC during cruise.  The solution to this is being able to move the
timing on the fly so the thetaPP is always occurring at the optimal
crank angle of 16dATDC....  (according to Taylor and Heywood, two of
the acknowledged experts).  For example, if the thetaPP is 10 and all
you do is alter the timing so it moves to 16, HP output will go UP. If
the ThetaPP is 20 and you move the timing to make the thetaPP move to
16, HP will go up.  That's not just theory, it's measurable.  It's one
of the biggest reasons to have moveable timing.  There are three ways
I know of to do that. 1) measure many parameters and calculate the
estimated thetaPP, 2) estimate the timing needed, or 3) MEASURE the
thetaPP and adjust accordingly.   #1 is what the auto industry and
TCM's FADEC are attempting to do.  (requires a lot of sensors and data
crunching.)  #2 offers some very engaging advantages over mags, but
isn't confirmable as optimal to an outside observer.  #3 is not yet
commercially available, but appears to be the holy grail--simply,
directly measure that which you need to know and adjust accordingly.=20
Easier said than done.

</x-tad-bigger></fontfamily>

=
<fontfamily><param>Arial</param><color><param>0000,0000,FFFF</param><x-tad=
-bigger>**</x-tad-bigger><x-tad-bigger>
Consequently, it should be clear that there is no=A0universal value for
optimal theta PP or even the ratio of burned to unburned fuel at theta
PP. These values change for different engines as well as different
speeds on the same engine.</x-tad-bigger><x-tad-bigger>**

</x-tad-bigger></color><x-tad-bigger>

Taylor and Heywood disagree with that.  They are two of the most
respected, authoritative combustion technology text authors out =
there.</x-tad-bigger></fontfamily>

=
<fontfamily><param>Arial</param><x-tad-bigger>=A0</x-tad-bigger></fontfami=
ly>

=
<fontfamily><param>Arial</param><x-tad-bigger>Walter</x-tad-bigger></fontf=
amily>=

--Apple-Mail-12-808929481--