The Effects of Engine Oil Rheology on the Oil Film Thickness Between a Cam and Rocker Follower

The widespread use of overhead camshaft (OHC) rocker-follower valve-train configurations in current automotive engines allows a more compact cylinder head design and improved valve operation. Unfortunately, this valve train configuration can be difficult to lubricate, as evidenced by a number of wear problems occurring in service. As a consequence, there have been a proliferation of industry standard wear tests. Little work has been published on the rheological behaviour of the lubricant in these severe non-conformal contacts.

A motored cylinder head utilising a cam and rocker-follower-valve train configuration has been instrumented in order to measure the oil film thickness (OFT) in an exhaust valve contact by means of an electrical capacitance technique. The experimental apparatus and data acquisition system are described, together with the subsequent data processing. Oil film thickness measurements over the whole of the active part of the cam cycle, using a range of model Newtonian base oils, are presented to illustrate the capability of the measurement system and to show the reproducibility of the data.