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A friend who is an engineer and a smart one told me during a discussion on a now forgotten topic about fluid behavior during a pressure raising event such as the compression stroke in an engine cylinder. As the pressure goes up, the fluid gases will separate. As they separate, they tend to condense in the boundary layer of the cylinder, one liquid on one side and another liquid on another side. This could explain why the films of engine combustion look so erratic and not the uniform event expected. This could also explain why there are lean and rich areas of the cylinder. The crevices that you talked of would have a larger boundary layer and collect more condensed fuel than the smoother surfaces elsewhere in the cylinder. This also applies to oxygen and nitrogen in the air as well as the gasoline in the mixture. How to combat these?
Minimize the crevices by radiusing (?) all sharp corners and edges? Minimize the size of the boundary layer with lots of motion, swirl and tumble? A more homogeneous mixture entering the cylinder? This last question brings me to a design question.
Why are cylinder inlet ports designed more like pipes uniform in size and not more nozzle shaped? The increased motion to crowd the incoming charge in a nozzle type design would certainly cause a lot of mixing and velocity increase just before the entry into the cylinder. This increase in velocity would also help the inertial charging at high speed to offset the loss of flow due to friction. This increase in velocity would make the mixture more homogeneous and the boundary layer would be minimized. At low speed the loss due to friction would not be a factor as there would be far less demand through the ports, but velocity would be higher and mixing better while at part throttle or low speed.
Just a few thoughts and questions
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