Design Approach

Flat silicon nitride ceramic pads were designed for two components in the valve train. The first of these was a wear pad for the 'valve crosshead'. The crosshead, a flat beam transfers the downward force from the nose of the rocker lever to either two intake or two exhaust valves. The rocker lever nose directly bears on the contact pad of the crosshead in an oscillatory combined rolling and sliding motion. The second component for which a ceramic pad was designed was the 'cam follower tappet'. The cam follower tappet rides directly against the camshaft lobe creating a rotational sliding motion at the contact interface between the flat face of the tappet and the cam lobe. On these simple geometries, closed-form analytical approaches were used to optimize the pad design and minimize their size and cost. Both brazing and adhesive attachment processes are being developed that will.be compatible with high volume (fast, cost-effective) production.

Ball and socket joints for the ends of the links and pushrods were designed using finite element modelling routines. These analyses assured that the final designs did not exceed safe maximum operating stress levels. Two areas of these components were modelled: 1) the contact interface between the ball and socket, and 2) the press-fit attachment between the rod and ceramic tip. The contact interface analysis confirmed that the anticipated component loads would not exceed material capabilities to transmit these loads when the ball-socket geometries were carefully controlled. Likewise, the press-fit analysis identified a range of interferences which would prevent overstressing the ceramic during assembly and engine operation.

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