CONNECTING ROD MULTI-OBJECTIVE OPTIMIZATION

The OM606 connecting rod must survive three distinct loading scenarios. First: Peak Gas Load during combustion, where 130 bar cylinder pressure on an 87mm bore creates a compressive force of F_gas ≈ 77.3 kN — this drives the buckling constraint. Second: Top Dead Center Inertia at the end of the exhaust stroke, where the piston reverses direction and puts the rod in tension. The inertial force scales with ω², which is why I designed for 5500 RPM redline conditions. Third: Transverse Inertia (Whipping) at 90° crank angle, where lateral acceleration causes the rod to bow outward. Each load case produces different stress distributions and failure modes.

The objective was simple: minimize shank mass f(x) = Σρ A_i L_i while satisfying four constraints. Buckling stability required P_cr/(F_gas × 2.5) ≥ 1, where critical buckling load follows the Euler formula P_cr = π²EI/L². Fatigue life under tensile inertial loading needed SF ≥ 1.3. Static yield under compressive gas loading also required SF ≥ 1.3. Finally, a monotonicity constraint ensured the geometry tapered smoothly from small end to big end — no undercuts that would be impossible to forge. I solved this constrained NLP using SLSQP in scipy.optimize.

Going into this project, I assumed buckling would be the active constraint — that's what most textbooks emphasize for slender columns under compression. The post-optimization analysis told a different story. The optimized I-beam geometry achieved a Buckling Safety Factor of 7.3, nearly triple the requirement. Meanwhile, Static Yield converged to exactly 1.3 — it was the active constraint. This means the rod will crush under compressive stress long before it ever buckles. The I-beam flanges are so efficient at maximizing area moment of inertia that buckling became non-critical.

To validate the I-beam topology selection, I ran a comparative optimization using an H-beam cross-section under identical constraints. The H-beam configuration, with its vertical side walls instead of horizontal flanges, is geometrically less efficient at resisting in-plane buckling within the 35mm width constraint. The result: the H-beam converged to 103.8 g — a 17.5% mass penalty compared to the I-beam's 88.34 g. This performance gap conclusively demonstrated why I-beam sections dominate high-performance connecting rod design.