The Cylinder Head part.1 (squish chamber)

THE TWO-STROKE cylinder head certainly doesn't look very exciting but its
design has a large bearing on how well your engine will run. Manufacturers use various
external shapes and cooling fin patterns but the main requirement here is that the
cooling area be large enough to adequately cool the engine. Some people feel that the
head must have radial fins to be any good, but I disagree. Conventional finning is
entirely adequate. It is the surface area which counts, not the fin pattern.

What is more important is the shape of the combustion chamber and the location
of the spark plug. Over the years many combustion chamber designs have been tried,
but only a couple are conducive to a reliable, high horsepower engine. The one thing a
powerful two-stroke doesn't need is a combustion chamber that promotes detonation,
the killer scourge of all racing two-strokes.
To understand the type of combustion chamber you need it is necessary to
appreciate just what detonation is and what can be done to be rid of the problem.
Detonation occurs when a portion of the fuel/air change begins to burn spontaneously
after normal ignition takes place. The flame front created by this condition ultimately
collides with the flame initiated by the spark plug. This causes a rapid and violent
pressure build-up, and the resulting explosion hammers the engine's internal
components.
Detonation leaves many tell-tale signs for which the two-stroke tuner should have
an ever-wary eye. The most obvious sign is a piston crown peppered around the edge as
though it has been sand blasted. Bikes with plated aluminium cylinders will usually
show the same sand blasted effect around the top lip of the bore. A cracked (not
molten) spark plug insulator also indicates detonation. If kept running, a detonating
engine will eventually seize and/or have a hole punched right through the top of the
piston.
The conditions leading to detonation are high fuel/air mixture density, high
compression, high charge temperature and excessive spark advance. A high piston
crown or combustion chamber temperature can also lead to this condition. In a racingtwo-stroke all of these detonation triggers are virtually unavoidable, with the exception
of excessive spark lead.
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 combustionchamber 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.Engine designers have known about these things for a considerable time. This is

why you will find the best racing engines follow the squish design. Also you will notice

that these engines have a very small bore in relation to their stroke, as this too cuts

down the size of the combustion chamber and reduces the area of piston crown exposed

to the combustion flame.

In an effort to minimise cylinder and piston distortion, some manufacturers have

chosen to use an offset squish type combustion chamber (FIGURE 2.2). The exhaust

side of a two-stroke cylinder and piston is always the hottest, even though cooling air

flow is much better here than on the back (inlet side) of the engine. There are several

reasons for this, all associated with the passage of very hot (630°C) exhaust gas

through the exhaust port. The escaping gas heats the exhaust port and cylinder wall as

well as the side of the piston. This can cause the piston to expand abnormally and in

some circumstances to seize. To take care of this possibility, the manufacturer may

choose to increase piston to cylinder clearance, but this may not be desirable as extra

clearance can increase leakage past the rings and usually results in high piston wear. A

safer step is to move the combustion chamber to the rear of the head. If this is done,

the front of the piston crown is shielded from the combustion flame by the squish

surface. Then, when the front of the piston is heated during the exhaust stroke, it will

not expand so far due to its being initially much cooler.

Several two-stroke engines are produced with squish and offset squish chambers,

but unfortunately mass production usually reduces their effectiveness. It is a very

difficult task to keep tolerances of closer than about 0.2mm in production. Therefore

you find many engines with a squish clearance of 1.3-1.8mm instead of the 0.6-0.8mm

clearance that is required.