Thread: Squish Talk
05-12-2008, 05:40 PM #1
.The mechanism of a squish band is to push the gas as close to the spark as possible at the ignition phase. During the squish, the gas will also increase the vaporizing of the fuel and add kinetic energy which increases burn efficiency. This is one of the reasons why the ignition timing needs to be reduced with higher crank speed in a 2-stroke engine. The charging from the exhaust pipe is another.
Usually, squish velocities of 25-50 m/s are the upper limit dependent on design, materials, cooling, fuel, etc. Too high squish velocity will transfer too much heat from the gas to the surrounding metal and will make the gas self detonate due to the energy increase. The fuel usually burn with this mentioned speed and it will serve as a good limit for the squish velocity. Detonation is an explosive behavior with reaction velocities in the region of 6000 m/s. A bad designed squish band will cause such detonations which will destroy the surrounding metal and sometimes hammer the piston, making it plastic deformed over a big area and size due to the expansion.
When the gas at the squish band is moving into the center, it will have to increase its velocity due to the fact that the area is decreasing. The red length is shorter than the blue length in the picture and the gas must pass these gates.
To optimize the squish behavior we need to have a constant squish velocity over the squish band. This is achieved by tapering the squish band height with the corresponding area ratio, so that A is the Squish Gap and B is the reduced height found as Y(C4) in the coordinate table of the program. This height reduction also reduces the inefficient burned volume. The blue line shows the mathematical correct squish band shape. We can see that a strait line will approximate the shape perfect over the squish band width.
The squishtaper angle is not constant, it increases with increased squish gap A. The taper angle is tangent with the piston edge at B.
The Head geometry gets more important with high squish velocities, the surrounding surface needs to be protected from the heat transfer. This can be achieved by different thermal barriers (surface treatment) and with the gas itself. The secret key is to avoid the hot gas to transfer heat. This is why we like sharp inside corners from multistage heads. The two pictures below shows the difference.
In the upper right picture there is no barrier to the head surface, more heat transfer to expect due to high gas velocity next to the head surface.
In the picture below right , the small gas pocket above the red line forms a natural barrier to heat transfer with low gas velocity next to the surface.
The same principal can be used to minimize heat transfer at the squish band. The Yamaha TZ-head (right picture) uses both these ideas. The edges are marked.
The final conclusion from this discussion is that the squish band at all times will leak heat and transform useful piston work to kinetic gas energy, a power loss source we need to get as much out of the fuel as possible at combustion. If the squish band gets over dimensioned then there will be more energy loss but no improved combustion.
05-12-2008, 05:42 PM #2
Interesting read for some of you...something Thats not mentioned in that article is the effectiveness of duel squish angles which can be a good way to reduce heat transfer. We are currently experimenting with some different squish angles. In our test boat we have been able to run 170psi compression on cheap low octane pump gas with no detonation simply by cutting a different 2 staged squish angle with very little power loss. I havent had a chance to try this on my jetski, but the exciter is running well.
Last edited by jdog800; 05-12-2008 at 05:54 PM.
05-12-2008, 08:56 PM #3
- Join Date
- Nov 2006
Lowell needs to chime in here...
05-12-2008, 09:00 PM #4
- Join Date
- Nov 2006
Does this mean that the squish band should be close to parrallel to the piston crown angle? A stock GPR 1300 head seems to have a squish angle less than the crown angle of the piston.
05-12-2008, 09:09 PM #5
05-12-2008, 10:43 PM #6
different applications call for different angles depending on how modified your application is. Depending on what has been done to the motor should determine the degree of the squish angle. These angles should be tuned to the rest of the motor the same way the port angles are tuned for different applications(more bottom end powercurve...or more higher rpm power curve)
Different strokes for different folks
05-12-2008, 11:41 PM #7
Good thread Jason it was great talking to you tonight
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