# Piston diameter size, fricition, mathematical proof

by gloo
Tags: diameter, fricition, mathematical, piston, proof, size
 PF Patron Sci Advisor P: 2,792 You're nearly there. The parameter commonly used is volume to surface area ratio. The main source of loss in a combustion chamber isn't due to piston ring friction (which you're correct in saying is a function of circumference) but due to heat loss (which is a function of surface area, both of the piston crown which transfers heat from the combustion gas, of the piston skirts which dissipate heat to the liner and coolant, and of the undercrosn which dissipates heat to the lube oil). Then (and only then!) do you consider ring friction. I don't know what you mean about using water instead of oil. Theoretically yes, but practically water is hopeless at lubricating, sealing and cleaning engine components.
 P: 75 My device is not really a piston with gas combustion as much as it is a hydrualic (water) as the main substance being sealed. The device basically works in this manner: A chamber has an outside water seal and a seal inside. Both these seals are used to seal the passage of water and they are dynamic in nature (like a piston ring). Basically i want the chamber to be moved by water pressure from the water surrounding the chamber. The chamber will move back and forth at low speed past the water seals. The pressure driving the chamber is between 500 to 1000 Kpa. The water seal outside prevents water from passing through between itself and a certain point on the outside of the chamber wall. The water seal inside prevents water from passing between itself and the inside chamber walls. I figured that a larger chamber with a larger area exposure to the water pressure, even though the circumference of the o ring is also larger, would be more likely to have a tendency to move better b/c proportionately, the larger surface area more than compensates for the larger diameter (o ring friction). Thus for every larger radius chamber, the pressure pushing on the larger surface area of the bottom of the chamber, is proportionately greater than for that of a smaller radius chamber and smaller circumference o ring. Would the math example i used be correct in this scenario then? thanks
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