Monthly Technical Notes
A Supplement to "The Engine Analysis Software"

Combustion Chamber (Cylinder Heads):

The Squish Band

I have long known that the Two Cycle Engine's combustion chamber is one of the most important items contributing to outstanding performance. One of my close friends, Steve O'Donnell and his father Jack have been competing at the highest level in individual RC Sports, e.g., Tether Race Cars, Straight-away RC Boats, etc. They spend a lot of time in finding the correct head design to maximize performance. I am not telling you here what they do, because I don't know. I do know some of the basic principles they use to make their engines a "cut above" the typical RC Modeler.

I recently had the opportunity to spend a couple of days talking engines, props and boats with John and Andy Brown, of Mongoose, Eagle, SG fame. Andy really lives two cycle engines and he found a book laying on my reading table which he asked about. I told him that I had a friend tell me about a recent book done by some researchers in Belfast Northern Ireland. They had just published this book in conjunction with the Society of Automotive Engineers and I had a copy. The also came up with some VERY SOPHISTICATED simulation software which would be available later. This book can be purchased from SAE (Click Here to Go to Order Page of SAE). If you would like to go to the SAE Home Page and look at what they have - Click Here!

Enough about the books, let's talk about general head design for High Performance two cycle RC Engines.

Squish Band:

One of the most important items in the design of a good combustion chamber is the squish band. I believe that a flat squish band produces much more power than an angled squish band. The flat squish band head has a flat area (squish band) around the perimeter of the head which comes in close proximity to the piston at top dead center. This squish band is designed to keep the layer of combustion mist very thin, in order to let heat travel quickly from a hot piston to a cooler combustion chamber (head). The thinner this layer (the closer the head clearance), the better this heat transfer is accomplished. If your head has its squish band to far away from the piston at TDC and the compression ratio is high, you will get pre-detonation (knock). You can tell if this is happening by looking at the squish band. If it looks like it has been lightly sand blasted, it is pre-detonating. Most people when they see this pre-detonation automatically raise their squish band piston clearance. That is the WRONG WAY!

Is the flat or angled squish band better? If you can run your engine with a flat squish band, you will make more power. With anything which produces more power there is a downside. With the flat squish band it is in the form of a "shaking effect" The engine will seem to run roughly and wont transition smoothly on the tuned pipe. It will act as if someone turned on a switch and more power came on abruptly. If this doesn't seem to hurt your application, I would stick with the flat squish band. If not you should put a 3 degree angle on the squish band starting in .100" or so from the outer edge of the squish band and proceeding inward to the combustion chamber. Be sure to slightly round the edges at this point.

Next time I will talk about the shape and proportion of the combustion chamber………….

ENGINE Analysis Software for the Serious RC Competitor



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