23rd International Colloquium Tribology - January 2022 229 Influence of the rotation direction of the cam on the friction losses of a cam/ finger follower contact Johnny Dufils Institut de Recherche En Ingénierie des Surfaces (IREIS), HEF Group, Andrézieux-Bouthéon, France Corresponding author: jdufils@hef.group Christophe Héau Institut de Recherche En Ingénierie des Surfaces (IREIS), HEF Group, Andrézieux-Bouthéon, France Etienne Macron Institut de Recherche En Ingénierie des Surfaces (IREIS), HEF Group, Andrézieux-Bouthéon, France Philippe Maurin-Perrier Institut de Recherche En Ingénierie des Surfaces (IREIS), HEF Group, Andrézieux-Bouthéon, France 1. Introduction Mechanical friction losses in the cylinder head of internal combustion engines stand for 7 to 15% of the total mechanical losses in a fired engine [1]. Replacing roller finger followers in valvetrains by sliding finger followers has many advantages, among others it enables to reduce the weight of the valvetrain and the resulting dynamic forces. In addition, using DLC-coated sliding finger followers instead of roller finger followers allows to reduce friction power losses up to 50%. In the following, the influence of the rotation direction of the cam on the friction losses of a DLC-coated cam/ DLC-coated sliding finger follower contact is assessed. 2. Materials & Methods 2.1 Testbench In order to assess the influence of the rotation direction of the cam on the friction losses, a dedicated single cam test bench operating real automotive engine components was used (Figure 1). The testbench is equipped with a dynamic torque-meter connected to the camshaft and a high-sample frequency acquisition system so that the instantaneous torque applied on the camshaft is measured. Figure 1: Geometry of the tested valvetrain system and positioning of the oil spray lubricating the cam/ finger follower contact. 2.2 Testing conditions Both the cam and the sliding finger follower were DLC-coated (a-C: H). The main test parameter was the rotation direction of the cam with respect to the oil spray lubricating the cam/ finer follower contact. The other test parameters are presented in Table 1. Table 1: Testing conditions Oil viscosity grade 5W30 Oil temperature Room temp. & 80°C Camshaft speed From 350 to 2500 rpm 230 23rd International Colloquium Tribology - January 2022 Influence of the rotation direction of the cam on the friction losses of a cam/ finger follower contact 3. Results With an oil at 80°C, a reduction of the average friction torque is observed in the counterclockwise direction compared to the clockwise direction (Figure 2). This friction torque reduction leads to a reduction of friction power losses up to 30%. No difference in friction is observed when the oil is at room temperature. Figure 2: Comparison of the average friction torque as a function of the camshaft rotation speed in the clockwise and counterclockwise rotation directions. The evolution of the average friction torque for the reference roller finger follower is also plotted for comparison The instantaneous torque enables to identify how friction losses are distributed in the cam/ finger follower contact as presented in Figure 3(a). By calculating the mechanical torque needed to open and close the valve if there was no friction in the system, it is possible to calculate the contribution of friction in the measured instantaneous torque. The contribution of friction in the instantaneous torque is the friction torque that is presented in Figure 4(a). For each cam revolution, the pad of the sliding finger follower is rubbed twice as illustrated in Figure 3(b) thus there are two turnaround points on the pad. The two vertical dashed red lines on Figure 3 show that most of the friction power losses occur when the contact point on the pad moves from one turnaround point to the other. Figure 3: (a) Comparison of the instantaneous torque as a function of the cam rotation angle in the clockwise and counterclockwise directions. The dashed black line is the torque needed to open and close the valve if there was no friction in the system. (b) Evolution of the contact position on the pad of the sliding finger follower as a function of the cam rotation angle in the clockwise and counterclockwise directions Figure 4: (a) Evolution of the friction torque and oil film thickness as functions of the cam rotation angle in clockwise and counterclockwise directions at identical contact positions on the pad (b) Evolution of the contact position on the pad as a function of the cam rotation angle. Circled numbers are the number of times a given area of the pad is rubbed in the clockwise and counterclockwise directions per revolution. In Figure 4, the instantaneous friction torque is plotted as a function of the cam angle at identical contact positions on the pad. It shows that, even though oil film thickness calculations [2] give identical values, instantaneous fric- 23rd International Colloquium Tribology - January 2022 231 Influence of the rotation direction of the cam on the friction losses of a cam/ finger follower contact tion losses are dependent on the cam rotation direction and that, after the turnaround points, friction is affected by the cam having already wiped the pad surface. 4. Discussion Thermal and transient effects controlling the oil film thickness in the cam/ finger follower contact may explain some of the differences seen in the instantaneous friction torque. The differences in instantaneous friction after the turnaround point suggest an oil starvation effect due to the surface of the pad having been rubbed by the cam combined to a screening effect of the oil spray by the cam. The intensity of the starvation seems to be dependent on the cam rotation direction. Indeed, in the counterclockwise direction, the cam is oiled by the spray just before entering the contact whereas in the clockwise direction, the cam is oiled by the spray just after leaving the contact and thus can be removed from the cam by its rotation. 5. Conclusion With an oil at 80°C, a reduction up to 30% of the friction power losses is observed in the counterclockwise rotation direction compared to the clockwise rotation direction. The lifetime of the a-C: H coating is also affected by the rotation direction: wear is lower in the counterclockwise direction compared to the clockwise direction. Using DLC-coated sliding finger followers associated with an adequate positioning of the follower with respect to the rotation direction of the cam, it is possible to reduce the friction power losses of 65% compared with the roller finger follower reference. References [1] Wong, V.W. & Tung, S.C., “Overview of automotive engine friction and reduction trends-Effects of surface, material, and lubricant-additive technologies” Friction 4, 2016, 1-28. [2] Grubin, A.N., “Fundamentals of the Hydrodynamic Theory of Lubrication of Heavily Loaded Cylindrical Surfaces” Central Scientific Research Institute for Technology and Mechanical Engineering, Book n°30, 1949, 115-166.