Valve Train and Injection Train Tribological Analyses

Valve and injection train tribological analyses at Southwest Research Institute (SwRI) use computation techniques to analyze the cam/follower, tappet/bore, roller/roller pin, and bearing interfaces on engine valve and injection train systems for lubrication, friction, and wear performance.

Typical valve train configurations for gasoline engine

Illustration of injection train analysis results: elastohydrodynamic pressure, asperity contact pressure, subsurface stress contours, and surface crack growth pattern

Applications

SwRI's valve train and injection train tribological analyses have the following applications:

• To configure valve train and injection train layouts to optimize their tribological performance by increasing minimum film thickness, reducing cam follower friction, and reducing surface wear
  
  
• To conduct sensitivity analyses to study component design effects such as roller diameter, surface roughness, and roller pin clearance on valve train and injection train tribological performance
  
  
• To assess the durability of the cam follower design against surface failures initiated by either sliding wear or contact fatigue

SwRI's valve train code is applicable to all types of valve train and injection train configurations such as:

• Overhead cam with roller follower
• Overhead cam with sliding follower
• Direct acting system
• Finger follower system
• Pushrod system

Parameters

Through extensive simulations, the following mechanical and tribological parameters can be obtained to evaluate system performance:

• Cam follower kinematics
• Cam follower minimum film thickness, pressure, and friction
• Surface contact stress
• Subsurface stress
• Mid-film oil temperature
• Roller slippage
• Roller pin friction and secondary motion
• Roller pin film thickness and pressure
• Roller pin leakage flow rate

Capabilities

The advanced capabilities of the SwRI-developed tribological analysis tools include:

• Elastohydrodynamic lubrication model for cam follower interface
• Mixed lubrication to account for surface roughness effects
• Dynamic analysis for roller slippage prediction
• Component stress calculation within Hertzian region
• Surface sliding wear and contact fatigue wear rate analysis
• Friction prediction

The ultimate goal is to provide engineers with an in-depth understanding of the relevant engine design effects so that an optimal design can be achieved, in order to improve system performance and durability. SwRI has developed the following tribological analysis tools to help achieve this goal.

Kinematics Analysis

• Based on theory of differential geometry
• Provides information such as cam instantaneous radius, cam entrainment velocity, cam follower pressure angle, rocker motion, etc.

Cam Follower Interface Lubrication Analysis

• Based on mixed lubrication concept (integrated elastohydrodynamic/boundary lubrication model)
• Capable of analyzing both sliding and rolling followers
• Capable of solving both line contact and elliptical contact problems

Asperity Contact Analysis

• Greenwood-Williamson (GW) model applied for light to moderate load conditions
• Contact mechanics model (developed by SwRI) uses actual measured surface roughness profiles
• Contact mechanics model normally applied for greater accuracy and wider load range

Flow Rate Analysis

• Estimates oil consumption by cam bearings, rocker pivot bearings, and roller/roller pin interfaces
• Assures that sufficient oil flow is provided to valve train and injection train to prevent failures arising from starved lubrication
• Simultaneously solves journal lubrication and dynamics for each bearing interface

Cam Follower Friction Analysis

• Predicts evaluation of viscous friction due to shearing of oil film, boundary friction due to shearing of surface film, and rolling resistance
• Includes both shear thinning and thermal effects on viscous friction

Roller Slippage Analysis

• Based on torque balance and conservation of angular momentum
• Predicts instantaneous roller slippage to prevent excessive wear

Surface and Subsurface Stress Analysis

• Quantifies surface contact stress levels using SwRI-developed contact mechanics model
• Compares subsurface stresses to material strength to determine potential for damage
• Evaluates effects of material hardness and residual stresses arising from heat treatment

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