Squishing
Art of Squishing
by Dale Alexander
Note:
This series originally appeared in a series of articles in the US-based
Antique Air-Cooled Yamaha Two-Strokes club newsletter. It has been
translated into HTML with the permission of the author and original
publisher.
In
the past couple of issues, Doug has been doing a credible job of
explaining heat, race fuel and the likes. I've enjoyed what I've read
and this has compelled me to add my two cents based on my past racing
experience with RD-350's.
When
I started racing RD's, the fast guys on the west coast were Alan and
Dain Gingerelli, Dick Fuller, Scott Clough and Bob Tigert. They were
fast at tracks like Sears Point, Riverside and Ontario. I was just a wee
pup of eighteen and had a lot to learn. By the time I left racing, it
took Yamaha's then latest creation, the FZR 400, to make my fifteen year
old RD-375 obsolete.
What
I mean by obsolete was that I had been relegated to finishing 7th with
six FZR 400's in front of me. Not wishing to spend $5000 to ''purchase'
trophies, I hung it up. What's important here is the fact that I would
very much like to pass on the knowledge the I have accumulated during
all those years and as Doug has more or less started the ball rolling
with his articles on heat, I see no reason to break stride.
Stone Knives and Bear Claws
Way
back when Mt. Everest was a foothill and two-strokes only came in one,
two or three cylinders, companies like Denco Engineering and Hot Bike
Engineering were running Kawasaki triples regularly at Fremont
Drag-strip. Tony Nicosia would run the Kaws all the way down to the end
on the back wheel, severely stressing the wheelie bars. Quite a sight to
behold. And when the bikes were brought back to the pre-staging area, a
curious ritual would begin. Out came the pressurized water sprayers to
hose the cylinders, heads and cases off. Some wise-ass would invariably
make a comment like 'It don't matter how much they water them things,
they ain't gonna grow any faster'.
My
own experience with road racing was just beginning to develop. I was
soon to observe that the RD would run pretty strong in 6th gear out of
one corner, only to be reduced to 5th gear and finally 4th gear by the
end of the race. The engine was consistently losing power as the race
progressed. Something was overheating, BUT WHAT? Suzuki had Ram-Air
heads on their 380 and 550 triples, after-market water-cooled heads were
available but even the factory water pumpers were having a like problem
so what was the point?
To make matters really confusing, all manner of porting ideas
were being tried, but everyone was going pretty much the same speed or
slower. So it would seem the whatever the problem was, it was related to
heat, the development of power and the amount of time that the power
was being used.
So, What IS Going On ?
At
this time, the mid to late 70's, state of the art for porting and
compression was pretty much in it's infancy. Raise the exhaust port to
28 m.m., trim .020" off the head, add some richer jetting and let's go
racin'. This yielded a moderate increase in power and compression which,
if checked by a gauge, was about 150 p.s.i.. As my quest for knowledge
grew and my desire to extract reliable power increased, I started
looking down other avenues to expand my understanding of what was needed
to make heaps and heaps of consistent power. A not so obvious place to
look turned out to be car racing. Even though one might at first think
the four-stroke engines have little in common with two-strokes, I was
soon to prove myself wrong. With the exception of how gases move in and
out of cylinders, both designs are plagued with much the same problems
and the first lesson I learned about heat and how to control it led me
to investigate quench bands or squish bands as they are known to us.
The
squish band is the area along the outside edge of the head that is more
or less flat or matches the angle of the crown of the piston closely.
Its purpose is two fold: 1) it acts to create a mixing of the charge as
it is compressed by the piston. This helps to make a more homogeneous
mixture that burns faster with less ignition advance. And 2) when
properly set up, the squish band acts to cool the charge and the end
gases to help eliminate detonation. THIS is the really important aspect
of the squish band as it relates to a two-stroke.
It
acts to cool the charge. Weren't we just wondering where all this
damaging heat was coming from? I've looked at a ton of pistons and noted
early on that a lot about heat can be learned by turning the piston
upside down and looking at the area under the crown on the inside of the
piston. Good running bikes had a very light brown color that was
glossy. Better running bikes had a much larger area that covered the
entire underside of the crown and was much darker in color, but still
glossy. On bikes that didn't run that hard, this area had turned flat
black.
This
is perhaps one of the best areas to keep an eye on the heat health on
an engine, so it's important to understand what information has been
given to us here. Oil, whether in a four-stroke or two-stroke, is being
churned up by the crankshaft and is being thrown against the underside
of the piston crown. The heat of combustion moves from the chamber side
of the piston crown to the underside as the piston tries to rid itself
of the heat before melting. It is the presence of this heat the bakes
(or burns) the oil onto the underside of the piston. A little heat, a
small light colored area. Too much heat and the oil burns carbon black. A
very useful indicator indeed.
But
a function of the heat that is unique to a two-stroke is that the worst
of its effects is yet to be felt. When gases are heated they expand,
and if the container that they are expanding into happens to be sealed,
pressure rises. Well, isn't the crankcase of a two-stroke sealed for the
time needed to build pressure to start the scavenging cycle? Yes and
here's the rub. As the piston crown grows hotter, the underside radiates
this heat into the crankcase, increasing the pressure to such an extent
that when the intake port opens, the pressure inside the case is
momentarily higher than that of the incoming charge and everything
stalls for a brief moment: brief for all things but the engine. It WILL
NOT fully charge the case and as a result the next scavenge event will
not fully charge the combustion chamber and the engine is now not
developing the power it did when things were cooler. Hence, Tony Nicasia
waters his bikes to try and battle this problem externally.
What
can be done to take care of this problem internally can best be summed
up be understanding some of the nature of combustion and the physical
properties of the engine. This would be a good time to glance at Figure 1 .
This drawing represents to the best of my memory a cross section of an
RD-350. It could be any engine actually. It should be fairly easy to
make out the cylinder, head, piston, gasket, bolts, etc. The boxed in
area is the area that I wish to spend some time talking about 'cause
this is where all the problems regarding heat begin. Figure 2
is a blow-up of this area so move along and be quick about it! Along
the left side of Figure 2 there is a darker area that corresponds to the
head gasket. On an RD-350, the gasket is .040"; thick. The step just
above the gasket represents the .020" step that one will find in a
stock, unmodified head. Together, these two figures add up to a value of
.060". Keep this in mind, because these very small values will become
VERY important in a moment. For future reference, this .060" is properly
known as piston/head clearance and will be called such.
Figure 3
shows an additional dark area that encircles the combustion chamber.
This shaded area represents all the area of the head, piston crown and
cylinder wall that is exposed to the heat of combustion at T.D.C.. I
like to call this area the "boundary cooling layer" area. Please note as
well that I have given a value of 1000 degrees to this layer. For sake
of argument, let's say that the fuel gets into trouble (detonation) at
any value greater than 1000 degrees. This is not the true temperature
involved here, but for ease of arithmetic, let's keep the numbers round.
The real numbers aren't important, just the concept. This boundary
layer depicts the physical effect that occurs when a hot gas is in
proximity to a cooler object: the combustion gas is cooled by the
presence of the cooler head, piston, etc. By experimentation, I feel
comfortable saying that this layer is usually no thicker that .020". As
the piston has a boundary area that is .020" thick and the head is .020"
thick as well, it doesn't take a rocket scientist to see that the area
between the two cooled surfaces is .020" thick AND is uncooled by the
boundary effect!!! This is the area where the problems with heat start.
The combustible gases all the way out to the left side of this area are
known as "end gases". When the gases in the main portion of the chamber
are ignited, several things happen at once:
1) the spark starts the actual chemical reaction that is combustion
2) the temperatures and pressures build quickly
3) the flame front moves rapidly away from the spark plug.
As
the flame front moves to the end gas area, pressures rise quickly even
though the piston is descending. At some point, the pressures and
temperatures are great enough that the end gases will spontaneously
ignite. This is known as detonation. When the end gases ignite in this
fashion, the pressures in this area grow to tremendous values leading to
piston fractures, hydraulic type stress failure of small and big end
needle roller bearings and other not so nice things. Detonation can be
inaudible what all with the racket that the intake, exhaust, piston slap
and ring flutter can make, so damage can be occurring and pistons
overheating without any warning to the rider. Assuming that the ignition
is timed to a reasonable value and the octane rating of the fuel is
sufficient for the use the engine is seeing, one of the only other
things that can be done to reduce the possibility of detonation is to
reduce the piston/head clearance to .035". That way, the boundary
cooling layers overlap and all end gases in this problem-prone area
should be reduced in temperature to a level below the "auto-ignition"
point. When this happens, the piston crown is no longer heated to such
an extreme extent, the charge in the crankcase is reduced in temperature
reducing the pressurization and allowing a more complete filling and
power goes up and stays longer as a result.
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