Date: Sun, 08 Mar 1998 09:11:25 -0800
From: Robert Harris 
Subject: Flathead Obscuria
To: Dave Williams 

By now you have tore down the flathead in its entirety.  You remember me
talking about "two cycling" an engine.  This involves cutting a small port
near BDC and running it out its own exhaust pipe.  Obscurely done on
flatheads to greatly improve the exhaust flow without too badly screwing up
intake.  Haven't seen it in print since the late fifties as that's about
when people gave up on flatties - course haven't seen anyone other than
Jenkins and Vizard even mention anti-reversion valves.

Skip to present. Picture a small port on the outer cylinder wall near bottom
dead center.  It opens to its own exhaust manifold.  As the piston clears
the port on power stroke it opens the port.  Blowdown commences.  In fact,
thru BDC and somewhat up the exhaust stroke we have a dual exhaust valve
system in operation.  It does not take much of a port to double low/mid lift
flow of the poppet valve.  Properly placed, it can allow significantly delay
in the exhaust valve opening, thereby increasing effective expansion ratio
(good for both power and economy). Venting as much of the exhaust by
blowdown prior to pumping also reduces the pumping friction.

On the intake stroke, the port is not opened to nearly bottom dead center.
This is the ideal point to introduce EGR to the mixture.  It does not dilute
or heat the intake charge nor occupy manifold volume or port flow - and no
latent intake system heat.  My first order guess is that it would in fact
stratify the charge - the mixture separate and above the incoming EGR.  Part
of the limits of effective EGR is that charge dilution effect that inhibits
the flame spread due to loss's to the "dead" gas. If most of the "dead" EGR
is concentrated near the bottom of the cylinder, I should think that this
would allow more EGR with less unpleasant side effects. If the charge does
indeed stratify, this could allow massive EGR at light throttle (low
cylinder pressure) self tuning to modest EGR at full load (high cylinder
pressure - relative to the exhaust) with nothing but pipes. And since the
"ram" air is piling into the "top" of the cylinder through the poppet valve,
the port introduced EGR is coming into the bottom half of the charge without
having any effect on the top half (air is compressible).  So I think we get
some  full throttle anti-detonate without a total mixture charge penalty.

The port would be effectively "sealed" by the close proximity of the piston
skirt to it.  The leakage past the skirt would be "cold" exhaust gas because
of the extremely close clearance - particularly if hypereutic pistons were
used.

Now to comptemplate if any scavenging could be included - but thats for a
latter day.



Date: Sun, 15 Mar 1998 08:12:11 -0800
From: Robert Harris 
Subject: RE: Flathead Obscuria
To: Dave Williams 

Thoughts on timing.  The port and valve should open as close to BDC as
possible. Delay the valve opening such that it is at maximum lift before the
port closes - but count on the port to do most of the blowdown.  One or two
wider ports are preferable to a taller port of the same area and flow.  The
longer we can delay blowdown, the more effective expansion ratio of the
gasses and the more power and economy.

Problems with width are ring capture and skirt width.  We need a tight
fighting skirt to sorta seal the port when its not open to the chamber.
This is why I am devouring the two cycle book and going to scan relevant
portions and return this next weekend.  A good two cycle exhaust port would
work wonders on the V-6, but on the flattie - wow.

Think just a good cleanup on the existing ports.  Think about firing the
blowdown ports into the poppet ports manifold at the collector.  If the
blowdown port can siphon off 50% of the gas (pressure volume thing) and
delay the exhaust opening by 30 degrees or more, there will be very little
interference in the siamesed port - not enough to get excited about enough
to clean up with welded into splitters etc.  The blowdown accomplishes much
more than any clean up ever could.

Since we are dealing with pressure/volume relationships - remember the
blowdown port will flow orders of magnitude more out than in no matter what,
and keeping it near BDC means more controlled re-admission of EGR - which in
a blown flattie might not be a bad thing - specially since it aint pumped in
by the paxton.