Concerns over greenhouse gas emissions are driving governments
and the automotive industry to seek out ways of reducing vehicle
CO₂ emissions. Engine friction reduction is one means of reducing
CO₂ emissions, through fuel consumption improvements. One area
where it is felt that friction reduction is possible is in
connection with the camshaft bearings.
Mahle has conducted experimental evaluation of rolling element
bearings used to support camshafts, replacing the standard plain
journal bearings. The aim of the testing was to gain an
understanding of the durability of rolling element bearings, tested
in a range of different operating conditions. The controlled test
conditions included variations to: - Camshaft speed, - Oil
temperature, - Oil age/specification, - Oil supply method/flowrate,
- Bearing journal line bore misalignment tolerance, and - Bearing
journal diametrical tolerance.
A Design of Experiments software package was used to generate a
reduced test matrix, with the aim of allowing a full understanding
of the effects of each of the test variables, whilst vastly
reducing the number of tests required. The testing required the
creation of a rig that could test multiple bearings simultaneously
in all of the different operating conditions required. This enabled
a large number of tests to be accomplished within the required
timescale.
The bearings were measured in a number of ways before and after
testing to ascertain wear mechanisms attributable to each of the
test variables. The results indicated that although there was a
measurable reduction in bearing assembly weight for all bearings,
there was very little measurable wear on any of the bearing contact
surfaces, and therefore no operating condition can be outlined as
detrimental to the bearing life. The conclusion therefore is that
rolling element bearings would be suitable to support camshafts in
production engines. This paper details the approach to the testing,
and discussion of the results.