Engine efficiency is one of the key aspects to reduce CO₂
emissions. Lamborghini S.p.A. has focused attention on the engine
friction modeling, analysis and measurement to understand and
control the phenomena. To reduce friction it is necessary to
improve understanding of the behavior of the engine components and
to pay attention to detail at every tribological contact. The valve
train can make a significant contribution to whole engine friction
especially at low engine speed and this is particularly true for a
high speed sports car engine. Direct acting valve trains are often
used for this type of engine to minimize the moved mass and so
enable high speed operation. However the sliding contact between
the cam and tappet results in higher friction loss than the roller
finger follower valve train used on many modern passenger car
engines. In addition, the high maximum engine speed demands a large
valve spring force to maintain contact between cam and tappet. The
large spring force can lead to increased valve train friction at
low engine speed when the inertia force is low. Thus the
development of calculation methods to quantify friction of direct
acting valve trains and support the design of components is
important.
This paper describes the use of advanced mathematical models to
quantify power loss at cam/tappet contact, tappet/bore contact and
camshaft bearings.
The mathematical models are sufficiently detailed to capture the
major influencing factors while being quick enough to use to enable
engine designers to make decisions in the required time frame. This
paper compares calculated and measured friction data for the valve
train of a high speed passenger car engine as tested on a motored
cylinder head test rig. The system friction was measured and
calculated across the operating speed range with different oil
supply temperature. The effect of diamond like carbon (DLC)
coatings on the tappets was quantified by measurement and
analysis.
