A Comprehensive Numerical Study on Friction Reduction and Wear Resistance by Surface Coating on Cam/Tappet Pairs under Different Conditions

Abstract

As a vital component in the valve train of internal combustion engines (ICEs), the cam/tappet pair undergoes high mechanical and thermal loads and usually works in a mixed and boundary lubrication regime. This leads to considerable friction loss and severe surface wear. Currently, the applications of diamond-like carbon (DLC) coatings for automotive components are becoming a promising strategy to reduce the friction and lower the wear. However, the practical performance of the coating is related to many factors, including friction coefficient, thermal properties, load conditions, and surface topography. In order to investigate these factors and successively improve the fuel efficiency and durability of the cam/tappet pair, a comprehensive multi-physics analytical model considering the mechanical, thermal and tribological properties of DLC coatings is established in this paper. Simulations are carried out for the coated as well as the uncoated cam/tappet conjunctions with different roughness at various ambient temperatures. The results show that both the fluid and asperity contact friction for the coated cam/tappet conjunction are significantly reduced due to their favourable characteristics. As a result, the friction loss of the coated cam/tappet pair is noticeably lower by almost 40% than that of the uncoated, despite a slightly higher asperity contact. In addition, the wear resistance of DLC coatings is also impressive, although the wear condition becomes progressively more severe with the increasing ambient temperature. Moreover, the roughness has complex effects on the friction and wear under different conditions.

Understanding Friction in Cam–Tappet Contacts—An Application-Oriented Time-Dependent Simulation Approach Considering Surface Asperities and Edge Effects

“…As already described in other papers on cam-tappet contact, there are pronounced mixed friction conditions over the entire cycle. The resulting friction force results mainly from the solid contact for all load cases considered, as was also the case in the studies of LYU [32] for similar surface properties. The majority of the friction is accounted for by the cam tip contact.…”

“…When investigating the surface roughness, it becomes obvious that even small deviations have a tremendous effect on the tribological behavior in the contact. The high influence of the surface asperities on the tribological behavior was also presented by LYU [32]. The consideration of the roughness in the simulation is thus essential, as, otherwise, the real mixed friction conditions are not represented correctly.…”

“…The author described that the friction losses during a cold start-up are significantly higher than the losses during a warm startup, and that a coating generally reduces the friction losses due to thermal insulation. LYU [32] considered the surface asperities of the contact partners for the investigation of coated cam-tappet contacts and evaluated the resulting fluid friction and solid friction for different rough surfaces, as well as coated and uncoated contacts. Depending on the roughness and coating, the resulting friction forces differed significantly, with the influence of solid-state friction clearly exceeding that of fluid friction in all cases.…”

A Review of Energy Loss Reduction Technologies for Internal Combustion Engines to Improve Brake Thermal Efficiency

“…A reduction of 45% in friction torque was observed when using a ta-C coating compared with the result for a conventional phosphate coating on carburized steel at an engine speed of 2000 rpm [46]. A novel multiphysics analytical model considering the mechanical, thermal, and tribological properties of DLC coatings was proposed by Lyu et al [52], and their simulation results predicted a reduction in friction loss of up to 45% within a cam cycle of the DLC-coated (a-C:H) cam/tappet under a heavy engine load. Even though the wear condition became progressively more severe as the ambient temperature increased, the wear resistance of the DLC coatings was still impressive.…”

Predictions of friction and temperature at marine cam-tappet interface based on mixed lubrication analysis with real surface roughness

“…10 Lyu et al used the micro-convex model 11 to describe the roughness and examined the tribological characteristics of the coated cam-tappet. 12 Their results indicated that the coating may lessen friction loss.…”