Valvetrain Friction - Modeling, Analysis and Measurement of a High Performance Engine Valvetrain System 2010-01-1492

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.