Nipping valve-train noise in the virtual bud

Software roots out design flaws before building pricey prototypes. The PV6 valve-train (right engine bank) simulation models 38 separate components. A simulation snapshot reveals valve-stem deformation — in reality, too small to see — exaggerated for clarity. Load dropping to zero indicates socket hop. The valve train is probably one of the most complex and critical assemblies in a modern automobile engine. Its operation directly impacts engine efficiency and emissions. Engineers with the General Motors Power Train Group, Pontiac, Mich., used to model valve-train kinematics and dynamics with homegrown, 2D programs, which was OK for simple, planar motion. Obviously, such programs won't work for 3D lateral forces associated with more advanced designs, nor can they predict nonlinear behavior in helical springs. Overcoming these limitations is the 3D multibody dynamic code, LMS DADS from LMS International, Leuven, Belgium ( ). GM engineers used the program to eliminate socket hop in a concept V6 engine valve train before building prototype engines, saving considerable development time and the associated costs. Socket hop is defined here as a momentary loss of contact between a rocker arm and lash adjuster. Such disturbances generate noise and vibration and can rob power, especially at high engine speeds. The concept-turned-production 3.5-liter Premium V6 (PV6) 24-valve, dual-overhead cam engine powers the Oldsmobile Intrigue. Valves actuate with separate finger follower rocker arm assemblies. Each finger follower has a socket at one end mated to a spherical head on a self-adjusting hydraulic lash adjuster. The socket also acts as a pivot point. A roller shaft/needle bearing assembly located about midspan rolls on the cam lobe surface. On the opposite end of the follower is a valve pallet. It contacts the stem top of an intake or exhaust valve. The valve pallet is radiused to prevent edge contact with the valve tip. The