Saving weight saves fuel01 November 2011
Removing mass from engine components yields gains in fuel efficiency across the power range
With stringent fuel economy and CO2 regulations arriving, powertrain designers are investigating new ways to improve engine efficiency through friction and mass reduction. One key contributor to the friction of an engine is the mass of its connecting rods, conrod bearings, pistons, piston pins and piston rings. By reducing their mass, the engine's mechanical efficiency can be improved, says Mahle's Dipl. Ing. Michael T. Lapp, head of engine component development.
Mahle has recently completed a project to demonstrate the advantages of an ultra-lightweight connecting rod (ULWC).
The platform chosen for the conrod design optimisation was Chrysler's 3.6l Pentastar V6, rated at 218 kW at 6,200 rpm and 353 Nm at 4,000 rpm. The original conrod is a Mahle 36MnVS4 micro alloy, forged-steel unit, designed to have a minimum fatigue safety factor of 1.6 in the shank and, at 548g, is already lightweight. Four iteration steps resulted in a 27% weight saving over the original.
Lightweight 46MnVS6 is a fine-grained ferritic-pearlitic micro alloy forged steel, with a mean fatigue strength of 496 MPa at R = -2.5. This is about 20% higher in strength than premium 3% Cu powder forged alloys.
The ULWC has several advanced design features: at the small end, a bushing-free stepped design with optimised pin bore profile, clearance and surface finish is used. The pin bore features hydrodynamically optimised forged-in oil pockets (patent pending).
In the shank, an I-beam design is used, with a 4:1 ratio between the oscillating and cantilever moduli of inertia. By optimising the cross section of the beam, the buckling stress (and therefore critical buckling force) of each plane during operation is equivalent to reduce significantly the risk of buckling.
The web thickness of the shank beam is only 2.3 mm and the edge radii are 1.5 mm – the absolute minimum for current forging process limitations.
The beam section transitions into the big end's closed bolt holes to maximise 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, providing maximum stiffness and minimum mass.
The profile of the pin bore and the stiffness of the small end were optimised to distribute the surface pressure evenly, while maintaining a lubrication boundary between the piston pin and conrod small end.
As a result of finite-element analysis, the fatigue factor is minimised in a large portion of the I-beam, since the loads are evenly distributed over the beam section. The absolute minimum safety factor of 1.20 occurs in this large-portion I-beam section during the maximum gas pressure case.
The optimisation of the big end is based on deformation, rather than fatigue, as the integrity and stiffness of the bearing housing is an integral factor in the bearing's life. With the mass reduction of the shoulders, as well as the cap rib, the big end of the optimised design was more susceptible to deformation than the current design.
For the small end, a targeted deformation is part of the design. This is not 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. Another concern was the I-beam's buckling strength. Although calculations showed the safety factor to be well above 1.2, further testing was required for validation.
No buckling occurred when two conrods were loaded to a maximum of 80 kN in compression, 45% above the nominal load. A 250hrs modified durability test cycle, with the engine cycling between 4,000 and 6,000 rpm wide-open throttle, was performed, with no issues.
If the bores of the optimised design suffered excessive deformation, it would result in scuffing in the small end, prematurely wearing the crank-end bearings. However, post-test traces of the small and crank ends resulted in almost perfectly round housings, exceeding Mahle's production specification.
The ULWC design for the Chrysler 3.6-L V6 achieved a 27% mass reduction, lowering the assembly mass from 548 to 400 grams. Advanced calculation and numerical simulation demonstrated the 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.
The 148gram mass reduction from each conrod has significant effects on reciprocating mass and reliability. As the connecting rods mass has been reduced, the counterweights, crankshaft, bearings, piston pins and other 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 kgs.
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