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 center 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.

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 traveled well past TDC (Top Dead Center). 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 homogenizes 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 propagation.

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 (Top Dead Center) 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 (Top Dead Center).

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 minimize cylinder and piston distortion, some manufacturers have chosen to use an offset squish type combustion chamber (see pic below). 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.

Hoser,

Have You heard anything from the machine shop on the Heads.

TD

Hoser,

That looks Good,

Is that the dome from your coolhead,

TD

Any feed back on these heads from their owners?