With stringent fuel economy and CO2 regulations on the horizon, powertrain designers are increasingly investigating new ways to improve engine efficiency through friction and mass reduction by using high-strength materials and advanced design methods. One key contributor to the friction of an engine is the mass of its power cell unit (PCU), which is comprised of the connecting rod, conrod bearings, piston, piston pin, and piston rings. By reducing the mass of these components, the mechanical efficiency of the engine can be improved.
The connecting rod is a key component that affects the mass of the rest of the cranktrain; a lighter connecting rod would allow for reduced ancillary component mass. While the basic component design has remained the same for over a century, new materials and advanced calculation methods are now allowing for significant reductions in mass with no degradation in durability.
The challenge is to reduce connecting rod mass, which, in turn, improves engine performance and fuel economy in the more highly loaded downsized engines of the future. PCU supplier Mahle recently completed a project aimed at demonstrating the advantages of an ultra-lightweight connecting rod (ULWC). The goal was to engineer a fracture-split forged-steel conrod that is as light as possible, adheres to modern durability standards, and is feasible for volume production.
By reducing mass, the conrod would also be reduced in cost through decreased material usage.
The main design considerations of the ULWC include the bearing housing (big end) integrity, small-end hydrodynamic conditions, shank robustness against buckling and fatigue, and structural integrity. The platform chosen for the conrod design optimization was the Chrysler 3.6-L Pentastar V6. This modern and highly loaded naturally aspirated engine is rated at 218 kW (292 hp) at 6200 rpm and 353 N·m (479 lb·ft) at 4000 rpm.
Design yields a 27% mass reduction
The original production conrod is a Mahle 36MnVS4 micro alloy, forged-steel unit designed to have a minimum fatigue safety factor of 1.6 in the shank. This conrod is already lightweight by industry standards at 548 g (19.3 oz). Four iteration steps were performed and resulted in a connecting rod 27% lighter than the original.
To achieve such a dramatic weight reduction while maintaining production durability, an exact understanding of both the material properties and advanced conrod design was required. The material used for the connecting rod concept needed to possess exceptional material properties and good machinability without the addition of any secondary operations such as heat treatment.
The strongest material suitable for a fracture-split volume-production forged light-vehicle connecting rod was chosen. The 46MnVS6 material is a fine-grained ferritic-pearlitic micro alloy forged steel that has a mean fatigue strength of 496 MPa (71.9 ksi) at R = -2.5. This is about 20% higher in strength than premium 3% Cu powder forged alloys.
The ULWC retains all characteristics of the original, while adding several advanced design features. At the small end, a bushingless, stepped design is used with optimized pin bore profile, clearance, and surface finish. The pin bore features hydrodynamically optimized forged-in oil pockets (patent pending). The pockets serve to both introduce oil to the joint through the capillary effect and provide structural flexibility during severe engine operation.
In the shank, an I-beam design is used with a 4:1 ratio between the oscillating and cantilever moduli of inertia. By optimizing the cross section of the beam in this way, the buckling stress (and therefore critical buckling force) of each plane during engine operation is made equivalent to significantly reduce the risk of buckling.
The web thickness of the shank beam is only 2.3 mm (0.09 in) and the edge radii are 1.5 mm (0.06 in), which are the absolute minimum according to current forging process limitations.
The beam section transitions into the big end’s closed bolt holes to maximize bore housing integrity, reduce bore distortion, and eliminate the notch factor of a through hole. The big end of the conrod features a fracture-split design, high-strength torque-to-yield fasteners, and intelligent distribution of material that provides maximum stiffness and minimum mass.
The conrod was first hydrodynamically analyzed. The profile of the pin bore and the stiffness of the small end were optimized to distribute the surface pressure evenly while maintaining a lubrication boundary between the piston pin and conrod small end.
To calculate stresses for use in the fatigue analysis, the maximum operating conditions of the engine were considered. As a result of a finite-element analysis (FEA), in the optimized design the fatigue factor is at its minimum in a large portion of the I-beam, as the loads are evenly distributed over the beam section without any stress concentrations. The absolute minimum safety factor of 1.20 occurs in this large-portion I-beam section during the maximum gas pressure case.
This ULWC was validated in the same manner as a production conrod design. One of the primary concerns of a conrod with such an aggressive design is the integrity of both the small and big ends. The optimization of the big end is based primarily on deformation rather than fatigue, as the integrity and stiffness of the bearing housing is an integral factor in the life of the bearings. With the mass reduction of the shoulders as well as the cap rib, the big end of the optimized design was more susceptible to deformation than the current Mahle design.
For the small end, a targeted deformation is part of the design. The deformation is not large enough to induce fatigue failure but is of sufficient magnitude to improve pin bore hydrodynamic conditions by introducing oil to the joint. This is accomplished in tandem with the oil pockets, which effectively draw in oil that is splashed up to the joint.
Even with the significant reduction in bore housing material, both the big- and small-end deformation are within acceptable limits. They are not much changed from the current series design due to the 14% reduction in inertia force from the reduced component mass.
Another primary concern was the buckling strength of the I-beam. Although calculation showed the safety factor to be well above 1.2, further testing was required to validate the calculation. Two conrods were loaded up to a maximum of 80 kN (18,000 lb) in compression (45% above the nominal load), and no buckling occurred.
Engine validation testing
To validate the strength of the ULWC, a final “split-test” of three lightweight conrods vs. three current production conrods was conducted.
A 250-h modified durability test cycle, with the engine cycling between 4000 and 6000 rpm wide-open throttle (WOT) was performed. The engine and conrods completed the 250-h test with no issues. Pre- and post-test inspections were conducted using a coordinate measuring machine to evaluate the dimensional integrity of the conrods. All critical dimensions were within tolerance both before and after the test.
A major concern with the optimized conrod was the integrity of the small-end and crank-end housings. If the bores of the optimized design suffered excessive deformation, scuffing in the small end would result, and the crank-end bearings would be prematurely worn and be at greater risk of failure. The post-test traces of the small and crank ends of the conrods after the durability testing resulted in almost perfectly round housings, exceeding the production specification for new Mahle conrods.
Visually, all three conrods completed the testing in excellent condition. Although the Pentastar engine is highly loaded, it appeared as though lubrication was sufficient in the pin bore to maintain a hydrodynamic boundary between the wrist pin and bore. This is likely due in part to the oil pockets in the small end.
The ULWC design for the Chrysler 3.6-L V6 achieved a significant 27% mass reduction, lowering the assembly mass from 548 to 400 g (19.3 to 14.1 oz). Advanced calculation and numerical simulation demonstrated that this design has a minimum fatigue factor of 1.2, while the bolted joint integrity remains intact and the pin bore is not under severe risk of wear.
Durability testing has shown that the design exceeds expectations in terms of fatigue strength, bearing wear, and hydrodynamic performance. Most importantly for Mahle, this design meets the company's manufacturing requirements for volume production forging and machining.
This ULWC was benchmarked against the industry, including a variety of contemporary passenger car gasoline production conrods. The fully optimized design is significantly lighter than designs found in similar engines.
The 148-g (5.2-oz) mass reduction from each conrod has significant effects on reciprocating mass and reliability. As the connecting rods have been reduced in mass, the counterweights, crankshaft, bearings, piston pins, and other critical engine components can also be downsized.
The effect of this conrod mass reduction in a six-cylinder engine can result in a total engine reciprocating mass reduction of up to 2 kg (4.4 lb). This mass reduction can help improve reliability and decrease CO2 emissions and fuel consumption. Mahle engineers believe the conrod design demonstrated by the ULWC project make it ideal for the engines of the future such as downsized aggregates or range extenders.
This article was written for Automotive Engineering by Dipl. Ing. Michael T. Lapp, Head of Engine Component Development, and Chris C. Hall, Mechanical Engineer, R&D department of connecting rods, both of Mahle Industries Inc.