jaguar
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Anyone know of a small engine that has a head without a squish band? I need to know in order to have some evidence in an ongoing discussion I am in. In a forum I'm in I posted the following info and wound up getting flamed for it. Hey, don't shoot the messenger! Someone said that Honda ALWAYS uses squish bands. I can't imagine their CT90 had a squish band. help!
In the paper "Advances in the Design of Two-Stroke, High Speed, Compression Ignition Engines" it is written that with a squish band there is more heat loss due to swirl. [which brings more burning fuel in contact with metal.] Heat is what causes expansion of gases to push against the piston so it equates to say that heat loss, without regard to other factors, reduces power. The advantage of swirl is better mixing and flame propagation. But in a small engine there is usually already enough mixing due to its high intake/transfer turbulence. When I tested a head on my 55cc that had a squish band the power output was less. This most probably is only the case for small engines since larger engines need help with fuel/air mixing, and flame speed increase is helpful because of larger distances from spark plug to cylinder wall.
This is not to say that a good cylinder head with a squish band will never improve power. There are always many factors at play in 2 strokes. Increasing the compression can give as much as a 5% boost in power. But it is my opinion that these small engines will do better without a squish band. I don't remember where I read that small engines have more intake/transfer turbulence but I did read it.
Gordon Jennings - Two Stroke Tuners Handbook
"But if you want to use a true (measured from exhaust-closing) compression ratio much over 6.5:1, on a high-output engine, combustion control beyond that afforded by a non-squish cylinder head will be necessary." (he is referring to a head w/squish band being necessary for high compression high rpm engines)
What I recommend and use [for bicycle engines] is no more than a compression of around 130psi which is right around the mentioned 6.5:1 ratio. If you go over that and have ported the engine to rev up to at least 9,000 rpm then you should use a cylinder head with a squish band. The engine I used to test had a top rpm of around 8,000.
Since very few people with these engines run higher than 130psi and/or rev higher than 9000 I think my advice concerning squish bands is accurate for the majority.
In the paper "Advances in the Design of Two-Stroke, High Speed, Compression Ignition Engines" it is written that with a squish band there is more heat loss due to swirl. [which brings more burning fuel in contact with metal.] Heat is what causes expansion of gases to push against the piston so it equates to say that heat loss, without regard to other factors, reduces power. The advantage of swirl is better mixing and flame propagation. But in a small engine there is usually already enough mixing due to its high intake/transfer turbulence. When I tested a head on my 55cc that had a squish band the power output was less. This most probably is only the case for small engines since larger engines need help with fuel/air mixing, and flame speed increase is helpful because of larger distances from spark plug to cylinder wall.
This is not to say that a good cylinder head with a squish band will never improve power. There are always many factors at play in 2 strokes. Increasing the compression can give as much as a 5% boost in power. But it is my opinion that these small engines will do better without a squish band. I don't remember where I read that small engines have more intake/transfer turbulence but I did read it.
Gordon Jennings - Two Stroke Tuners Handbook
"But if you want to use a true (measured from exhaust-closing) compression ratio much over 6.5:1, on a high-output engine, combustion control beyond that afforded by a non-squish cylinder head will be necessary." (he is referring to a head w/squish band being necessary for high compression high rpm engines)
What I recommend and use [for bicycle engines] is no more than a compression of around 130psi which is right around the mentioned 6.5:1 ratio. If you go over that and have ported the engine to rev up to at least 9,000 rpm then you should use a cylinder head with a squish band. The engine I used to test had a top rpm of around 8,000.
Since very few people with these engines run higher than 130psi and/or rev higher than 9000 I think my advice concerning squish bands is accurate for the majority.
