An engine is an explosion-driven air pump and at its heart pistons, operating within ring-sealed cylinders, endure the blast, converting the effects of the expanding explosive mixture into kinetic energy—the energy of motion.
The motion is evidenced by the speeding pistons and the blast is delivered by an electrical firing cycle that occurs at a specific instance in time and crank angle. As a result, high cylinder pressures are exacted upon piston crowns and top ring grooves. But the piston is also exposed to additional loads exerted upon it due to connecting rod swing and crank pin rotation.
Loads exerted on one of the piston’s skirts, referred to as the “major thrust side” (on the right side when viewing the engine from the flywheel end with the crankshaft rotating counterclockwise) can reach 10 times higher than the load exerted on the opposing skirt, referred to as the “minor thrust side.” The disparity in skirt loading varies in magnitude depending on key variables, including the engine’s stroke length, rod length, and peak cylinder pressure.
To address skirt load disparities asymmetrical pistons were devised. Common place in the leading auto racing arenas, including F1, Le Mans Sportscars and NASCAR, JE Pistons was first to introduce them for operation in GM LS engines. Created with major and minor thrust faces these new asymmetrical pistons are now available for all LS variants.
WHY PLACE THE PISTON PIN OFF CENTER?
In traditional symmetrical piston designs the piston pin journal that secures the pin that joins the piston to the connecting rod is located at the piston’s center. As
a rule, symmetrical pistons are just easier to design and manufacture and they work acceptably well in most applications. However, though adequate in coping with the stresses, symmetrical pin placement usually isn’t the ideal formula.
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