What is Squish?

mdavlee said:

The space between the top of the piston and the area around the outside on the top of the cylinder. It is not very wide on the cylinder.

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Adirondack said:

OK so what makes squish so important? Obviously I see if you have too little you will hit the piston on the top of the cylinder. But I assume there is a perfect tollerence. Why?

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tdi-rick said:

The idea of the squish band has nothing to do with increasing compression, and everything to do with increasing combustion efficiency as Terry alluded to, and reducing detonation potential (it's primary reason for initial development, admittedly unlikely on a saw which have relatively low compression compared to high performance two strokes)

A good discussion here http://www.arboristsite.com/showthread.php?t=114728&highlight=squish&page=2
and here http://www.arboristsite.com/showthread.php?t=114728&highlight=squish&page=3

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logging22 said:

Little pieces of soft solder on top of the piston. I use a little vasoline to stick them to the piston. Run it around a few times. Take it back apart and measure with a mike. Put the pieces accross from each other along the piston pin. Works for me.

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Analyst Man said:

To avoid confusion we need to talk apples to apples. The purpose of the squish area is different than the results desired by reducing its thickness. Any reduction to the volume of the combustion chamber will increase compression. If the squish band is factory set at .040, and we reduce it by .005 or .010 there will be an increase in compression. Auto tuners regularly shave cylinder heads .020 to .030 to increase compression. I don’t know what the compression gain is in a 350 CI V8 Chevy engine, but with the smaller diameter pistons like we have in chainsaws .005 to .010 would be notable. It might even require the use of a higher octane fuel.

Other factors that may be even more important to a tuner (which I’m not) is the piston and combustion chamber design, i.e., flat top piston vs. dome shaped piston, flat head vs hemi head, etc.

The important thing for Adirondack is to know is the design of the combustion components in your particular saw, and what you hope to accomplish by tinkering with that design. Try to find someone who made similar changes to your model saw successfully and learn from them. Unless money and time is no object.

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Researchers have found that it is the gases at the very outer limits of the
combustion chamber, called the 'end gases', that self-ignite to cause detonation. These
end gases are heated by the surrounding metal of the piston crown and combustion
chamber, and also by the heat radiating from the advancing spark-ignited flame. If the
spark flame reaches the outer edges of the combustion chamber quickly enough, these
end gases will not have time to heat up sufficiently to self-ignite and precipitate
detonation. Herein lies the key to prevent detonation — keep the end gases cool and
reduce the time required for the combustion flame to reach the end gases.
The most obvious step that would satisfy the second requirement is to make the
combustion chamber as small as possible, and then place the spark plug in the centre of
the chamber. Naturally the combustion flame will reach the end gases in a small
combustion space more quickly than if the chamber were twice as wide. Additionally, a
central spark plug reduces flame travel to a minimum. (FIGURE 2.1)
In meeting the second requirement, the need to keep the end gases cool can also be
accommodated. If we move the combustion chamber down as close to the piston crown
as possible, no combustion will occur around the edges of the chamber until the piston
has travelled well past TDC. This large surface area acts as a heat sink and conducts
heat away from the end gases, preventing self-ignition.
The chamber just described is called a squish-type combustion chamber because of
the squish band around its edge. Originally, the squish band was designed to squish the
fuel/air charge from the edges of the cylinder toward the spark plug which, of course,
it still does. The fast moving gases meet the spark plug and quickly carry the
combustion flame to the extremity of the combustion chamber, thus preventing
detonation.
Since that time, more benefits of the squish chamber have come to light. The
mixture being purged across the combustion chamber from the squish band
homogenises the fuel/air mixture more thoroughly and also mixes any residual exhaust
gas still present with the fuel charge. This serves to speed up combustion by preventing
stale gas pockets from forming. Such pockets slow down, and in some instances can
prevent, flame propogation.
Turbulence caused by the squish band also serves to enhance heat transfer at the
spark-initiated flame front. Without proper heat transfer, jets of flame would tend to
shoot out toward the edges of the combustion chamber, prematurely heating the
surrounding gases to start off the cycle leading to detonation.
Rapid combustion has other advantages besides controlling detonation. With an
increase in combustion speed there is, of necessity, a corresponding decrease in spark
advance. The closer to TDC we can ignite the charge, the less negative work we have to
do compressing a burning charge that is endeavouring to expand. Also there is less
energy loss in the form of heat being transferred to the cylinder head and piston crown.
When less heat is conducted to the head and piston, the engine runs cooler and
makes more power. A side benefit resulting from the cooler piston also enhances the
power output. A cool piston does not heat the charge trapped in the crankcase as
much, therefore a cooler, denser fuel/air charge enters the cylinder each cycle, to make
more power.
If you think about it, you will see that the compact squish type combustion chamber also contributes to a cool piston by confining the very intense combustion
flame to about 50% of the piston crown just before and after TDC.

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However, if you want top power and no risk of detonation, the squish clearance
must be closed up. A squish band that isn't working is worse than no squish band at all
as it wastes part of your fuel/air charge. Wasted fuel charge spells less horsepower.
To give you an idea of how much horsepower you could be losing it would be good
to consider the example of a TZ250 Yamaha road racer. These engines have a bore
54mm in diameter and an offset squish chamber. The compression ratio uncorrected is
about 15:1, meaning that the trapped charge is compressed into a space 8.8cc in
volume. If the squish clearance is 1.7mm (lots of motors come from the factory like
that) 1.94cc of the trapped charge will not be burned until well past TDC, too late to
produce any power. 1.94cc represents 22% of the inlet charge lost. When the squish
clearance is reduced to 0.8mm the charge loss is reduced to 0.92cc or 10.5%. On paper
it would seem an easy way to pick up 11.5% more power, but losses reduce this increase
to about 5-6% on the dyno. Therefore maximum power goes up from 52 to 55 hp. Midrange
power can rise as much as 10%, so the bike is easier to ride and it doesn't
detonate.

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