Comparison Between Journal and Rolling Element Bearings in a Camshaft Application 2012-01-1324
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.
The use of rolling element bearings is generally considered to
provide friction reductions by two means: 1. As a direct
substitution of the journal bearings by rolling element bearings
and 2. As an enabling opportunity to reduce the oil flow
requirement of the engine.
MAHLE has undertaken a motored friction-testing program on a
2.5-liter gasoline engine, comparing the drive torques associated
with the standard camshaft bearings and also with camshafts
supported by rolling element bearings.
The test engine incorporated a direct-acting valve train design.
For manufacturing reasons, the camshaft supported in rolling
element bearings incorporated sintered camshaft lobes whilst the
standard engine employed a chill cast camshaft supported in
conventional journal bearings.
Contrary to previously reported results, the direct substitution
tests demonstrated that the camshaft supported on journal bearings
had a lower level of friction than the camshaft supported by
rolling element bearings. The test engine configuration fitted with
rolling element bearings showed a lack of sensitivity to variations
in oil flowrates applied both to the rolling element bearings and
also to the tappet/cam lobe interface
This paper considers the approach to the testing, the test
results obtained and some further discussion, including a possible
basis for the results obtained.