Ultra lightweight conrod concept from MAHLE

MAHLE has concluded the study of ultra light weight connecting rod (ULWC) sometimes in 3rd quarter last year. The aim of the research was to demonstrate the advantage of minimization of the cranktrain mass in optimizing the engine efficiency.  With ever tightening and stringent regulation in fuel economy and CO2 emission reduction, it is a paramount important for each of the vehicle subsystems to find their own means to contribute towards the new target. For powertrain engineer, the aim is to increase the engine efficiency through the reduction of friction and mass by capitalizing on new advanced materials as well as more superior predictive design tool.

One of the main contributor to the engine friction is the cranktrain mass. The cranktrain system consists of connecting rod (conrod), conrod bearings, piston (including pin and ring) and the crankshaft itself. The reduction of conrod mass is important, as it drives further mass reduction for the rest of the cranktrain system.

The figure above shows the evolution of the conrod design as Mahle iterated the design towards the most optimum mass vs strength criteria. The base design parameters were derived from Chrysler 3.6L Pentastar engine that produced 292bhp@6200rpm and 353Nm@4000rpm. The original conrod in this case is Mahle 36MnVS4 forged steel. This conrod is already among the class leader in terms of weight, 548g and has a minimum fatigue safety factor of 1.6 (!) in the shank. The main design consideration of ULWC is to optimize the big end integrity, small end hydrodynamics and shank robustness against buckling and fatigue.


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Four iterations of  Finite Element Analysis (FEA) were performed and Mahle achieved a staggering 27% weight reduction! The base material used was 46MnVS6 which is the strongest volume production material suited for fracture split construction, which met the criteria for exceptional properties and good machinability without secondary operation such as heat treatment. It has  a mean fatigue strength of 496MPa which is 20% higher than  the premium 3% Cu powder forged alloys. As for the new design, it adopted a bushless stepped small end and the pin bore itself featured forged-in oil pocket which was hydrodynamically optimized. As for the shank, the I-beam with optimized cross-section with 4:1 ratio between oscillating and cantilever moduli of inertia was developed. This way, the buckling stress of each plane during engine operation is made equivalent to reduce the risk of buckling. The analysis optimization results in 2.3mm of web thickness for the shank beam and the edge radii was used 1.5mm. The blend of the shank to the big end is with the closed bolt holes to improve structural integrity by eliminating the notch factor of a through hole. More details after the jump.

In overall, the design achieved an evenly distributed load across the beam without any glaring stress concentration region. The minimum fatigue safety factor during the maximum cylinder pressure scenario is 1.2, lower than the the baseline figure but accepted regardless. The conrod was validated by initially loading to a maximum of 80kN in compression (45% beyond nominal load) where no buckling was observed. The conrod was then tested with the running engine, where it was subjected to a modified 250 hours durability test cycle where the engine was cycled between 4000 to 6000rpm wide-open-throttle. All the critical  dimension are within tolerance after the completion of the test cycle.

The success of the ULWC concept in this case has resulted in 27% weight reduction (now stands at 400g) with structural integrity of FSF 1.2 by analysis and  validated by the durability test. The reduction in conrod has a significant effect in reciprocating mass whereby the conterweights, crankshaft, bearings, piston pins and some other cranktrain components can be downsized subsequently. For Chrysler 3.6L Pentastar, this brought a 2kg of reciprocating mass reduction.