Friction Reduction and Reliability for Engines Bearings (HEIG-VD in collaboration with Renault. Lubricants 2015)

May 25, 2024
I found another interesting open access article you guys might find interesting: Friction Reduction and Reliability for Engines Bearings by the Swiss University HEIG-VD in collaboration with Renault. Lubricants 2015, 3(3), 569-596;

To ensure long engine life, it's essential to address the reliability and wear resistance of engine bearings. Low-viscosity oils, while beneficial for reducing friction, can lead to increased wear if not paired with advanced bearing materials. Bearings must withstand high temperatures, pressures, and cyclic loads without degrading.

Wear Mechanisms:

Adhesive Wear: Occurs when two surfaces slide over each other, causing material transfer from one surface to the other. This can lead to surface degradation and increased friction. Advanced coatings like DLC (Diamond-Like Carbon) minimize this by providing a hard, low-friction surface.

Abrasive Wear: Results from hard particles or asperities between the sliding surfaces causing material removal. This can be mitigated by using coatings with high hardness and smooth surface finishes to reduce the impact of abrasive particles.

Fatigue Wear: Arises from repeated loading cycles leading to the initiation and propagation of cracks. This is particularly relevant in high-load applications where bearings are subject to significant cyclic stresses.

Wear in bearings occurs primarily through adhesive, abrasive, and fatigue mechanisms. Advanced materials like DLC (Diamond-Like Carbon) coatings or composite overlays can significantly enhance wear resistance. These materials reduce direct contact between asperities on opposing surfaces, minimizing abrasive wear and extending bearing life. The Stribeck curve, which plots friction coefficient against the Hersey number, shifts to lower friction values with these advanced materials, indicating more efficient lubrication.

Fatigue damage in bearings is a critical concern, particularly in high-performance and downsized engines that operate under more severe conditions. Bearings must resist fatigue caused by repeated loading and unloading cycles, which can lead to crack initiation and propagation.

Fatigue resistance in bearings is influenced by the material properties and the stress distribution within the bearing. Thermo-Elasto-Hydro-Dynamic (TEHD) simulations help predict realistic pressure and temperature distributions, ensuring that bearing materials can handle the expected loads without failure. TEHD simulations integrate thermal effects, elastic deformation, and hydrodynamic lubrication to provide a comprehensive understanding of bearing behavior under operational conditions. By improving the bonding strength between bearing layers and optimizing the antifriction layer thickness, the fatigue life of bearings can be significantly extended.

TEHD simulations are crucial in understanding and optimizing bearing performance. They consider:

Thermal Effects: Heat generation due to friction and its dissipation through the bearing material and lubricant.

Elastic Deformation: The elastic response of bearing materials to loads, which affects the pressure distribution and oil film thickness.

Hydrodynamic Lubrication: The formation of a lubricant film that separates the bearing surfaces, reducing friction and wear.

These simulations help in predicting the temperature rise, pressure distribution, and potential points of failure, enabling engineers to design bearings that can withstand operational stresses and extend engine life.

Bearing seizure, a catastrophic failure mode, can be prevented through careful thermal management and optimal clearance design. Ensuring that the bearing operates within the correct temperature range and maintaining appropriate clearances are crucial to avoiding seizure.

Seizure occurs when the heat generated by friction exceeds the bearing's ability to dissipate it. Proper lubrication and material selection can enhance heat dissipation and prevent excessive temperature rise. The clearance between the bearing and shaft must be carefully controlled to avoid excessive contact pressure and thermal expansion issues. By using materials with high thermal conductivity and optimizing the oil flow, the risk of seizure can be minimized.

Implementing advanced lubrication strategies and coatings can significantly enhance bearing performance. Low-friction coatings like MoS₂, WS₂, and polyimide can provide superior protection under mixed lubrication conditions, where traditional oils may fail.

Coatings reduce the friction coefficient by providing a solid lubricant layer that supports the load during boundary lubrication conditions. These coatings, such as DLC (Diamond-Like Carbon) and MoS₂, are designed to reduce wear and friction even when the lubricant film is thin or temporarily absent. They achieve this through high hardness, low friction coefficient, and high thermal stability at high temperatures.

In boundary lubrication, the lubricant film is not sufficient to completely separate the surfaces, leading to increased asperity contact. Advanced coatings help by providing a protective layer that minimizes direct metal-to-metal contact.This layer ensures the coating remains intact under high loads, reduces shear stress by lowering friction by allowing easier sliding at the interface, and enhances durability.

Designing these coatings involves selecting materials that provide the desired hardness, adhesion, and friction properties. Techniques such as Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are used to apply these coatings uniformly and with strong adhesion to the substrate.

Cavitation is a phenomenon where vapor bubbles form in a liquid due to localized low pressures, typically occurring in high-speed fluid flows. When these bubbles collapse, they generate shock waves and micro-jets that can cause significant surface damage.

To resist cavitation damage, bearings must possess a high level of hardness. This hardness helps withstand the micro-impacts generated by collapsing bubbles, thereby prolonging the bearing's operational life. However, this property is somewhat antagonistic to other desirable properties such as embeddability and conformability, making it a challenge to balance these characteristics in bearing design.

Rapid changes in fluid velocity can lead to pressure drops below the vapor pressure of the fluid, causing bubble formation. Sharp changes in the flow path, such as in narrow passages or around obstacles, can induce cavitation. Movements of the shaft centerline in hydrodynamic bearings can create regions of low pressure, leading to cavitation.

Cavitation can cause pitting and erosion on bearing surfaces, leading to a reduction in bearing life and reliability. The severity of cavitation damage depends on the frequency and intensity of bubble collapses, as well as the material properties of the bearing surface.

The paper highlights several design optimizations to mitigate the effects of cavitation in hydrodynamic bearings. Designing bearings and lubrication systems with gradual transitions in cross-sectional areas to minimize sudden pressure drops. Incorporating features like anti-cavitation grooves or relief holes to manage pressure distribution and reduce the likelihood of bubble formation. Ensuring appropriate clearances to maintain steady fluid flow and prevent localized low-pressure zones.

Movements of the shaft centerline within hydrodynamic bearings can cause regions of low pressure, leading to cavitation. This phenomenon is critical to understand because it can significantly impact the bearing's longevity and performance. Changes in load during operation can shift the shaft within its bearing. Rotational imbalances can cause the shaft to deviate from its intended path. Poor alignment of the bearing and shaft can lead to uneven pressure distribution.

When the shaft moves, the pressure distribution within the lubricant film changes. Regions of low pressure can fall below the vapor pressure of the lubricant, causing cavitation. The resultant vapor bubbles, upon collapse, generate micro-jets and shock waves that erode the bearing surface, leading to wear and potential failure.

The paper discusses the phenomenon of cavitation resistance in hydrodynamic bearings, where sudden depressions create vapor-filled bubbles in the flowing liquid. When these bubbles collapse, they generate shock waves and micro-jets at the bearing surface, producing surface damage characterized by circular craters of approximately 10 µm diameter. Over time, these impacts can destroy the bearing surface and lead to bearing failure. To resist cavitation damage, bearings must have a high level of hardness, which is often in opposition to the need for softness in other properties like embeddability.

High hardness is required to resist the micro-impacts caused by collapsing cavitation bubbles. Hard bearing surfaces are less likely to suffer from pitting and erosion, thereby extending the bearing's life. While hardness improves resistance to cavitation, it can reduce the material's ability to embed hard particles, which is essential for preventing surface damage and maintaining smooth operation.

Bearings need to embed small hard particles that are not filtered out by the oil system. This property helps in avoiding surface damage and scratches that can increase friction and wear. Materials with good embeddability typically have lower hardness but higher ductility and elastic modulus. This combination allows the surface to deform slightly and trap hard particles without significant damage. The paper emphasizes the challenge of designing bearing materials that strike a balance between these properties. While high hardness is critical for specific applications requiring long life and resistance to cavitation, embeddability remains crucial for general engine longevity and reliability.

Synthetic oils, especially those containing PAOs and esters, maintain a more consistent dynamic viscosity over a wide temperature range. This consistency ensures a stable Hersey number, maintaining effective lubrication regardless of operating conditions. Synthetic oils resist shear thinning better than conventional oils, which means their dynamic viscosity remains stable under high shear conditions, contributing to a stable Hersey number.

Synthetic oils have a higher viscosity index, meaning their viscosity changes less with temperature. This results in a more predictable Hersey number across different operating temperatures, ensuring the lubricant stays within the optimal lubrication regime. With a high VI, synthetic oils can maintain an appropriate film thickness across a broad temperature range, preventing shifts into the boundary lubrication regime (low Hersey number) or unnecessary thickening (high Hersey number).

Conventional oils have a lower viscosity index, leading to significant changes in viscosity with temperature. This variability affects the Hersey number, making it challenging to maintain optimal lubrication conditions. Conventional oils are more prone to shear thinning, reducing dynamic viscosity under high shear conditions. This can lead to a lower Hersey number and increased risk of boundary lubrication, where wear is more likely.

The advanced formulation of synthetic motor oils, particularly those containing PAOs and esters, ensures a more consistent dynamic viscosity and higher viscosity index, directly influencing the Hersey number. This stability allows synthetic oils to maintain optimal lubrication regimes across a wide range of temperatures and operating conditions, enhancing engine performance and longevity.
My personal opinions:

Engines oil grades such as 0w-16 are completely safe in engines approved to run them. Engines which spec these oils have superior lubrication system designs and state-of-the-art coatings which mitigate wear in ways unimaginable even ten years ago due to a superior ability to model wear charactersitcs nearly at an atomic level and offset them with superior designs and material combinations. Additionally, nanotechnology based coatings are now commercially viable and affordable.

Small wear particles that are not captured by oil filters get embedded into bearing surfaces often if they are larger than the minimum oil film thickness (MOFT). Elasto-hydrodynamic calculation can give some minimum oil film thickness around 0.25 µm for the most severe bearing application in downsized automotive engines which specify low oil viscosities. This value is arguable but in comparison with older calculations and older engine bearing applications, it indicates an extremely thin film.

0.25 µm corresponds to 250 nanometers. Particles above this size pose a risk to bearings in these engines. AMSOIL Ea Oil filters specify 99% filtration of particles under 20 microns, which corresponds to particles that are 20,000 nanometers wide. Such a particle is 80 times larger than the minimum oil film thickness of around 0.25 µm. Even if the oil film is initially sufficient, these particles can disrupt the protective lubricating film, leading to metal-to-metal contact and accelerated wear. AMSOIL Ea Bypass Filters remove 98.7% of all contaminants two microns or larger (ISO 4548-12), and many at less than one micron, which corresponds to 1000 nanometers. This is pretty good but the kit costs several hundred dollars plus the biannual cost of the filter media replacement. In my opinion there is no cost effective way to filter these particles between 250 and 20,000 nm out. An oil change is likely the most cost effective approach to getting these particles out of the engine for the vast majority of people. Therefore, as minimum oil film thickness drops in newer engines it is wise to focus on changing oil more frequently and using the best oil filter possible, replacing it often, probably every 5k along with the oil. This might be the death sentence for extended oil change intervals strictly because of the filtration challenge.

Particle size distribution in passenger car motors:

Nanometer Range (1-100 nm)
Relative Frequency: Very high; these particles are the most abundant.

Sub-Micrometer Range (100 nm - 1 µm)
Relative Frequency: High; these particles are common.

Micrometer Range (1-10 µm)
Relative Frequency: Moderate; less frequent but still common.

Large Particles (10-100 µm)
Relative Frequency: Low; less frequent but significant when present.

Very Large Particles (>100 µm)
Relative Frequency: Very low; rare but extremely impactful.

Finally, conventional motor oil should not be used in these engines for the reasons mentioned toward the end of the prior post.
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(More speculation) UOAs might become increasingly useless as bearing materials are engineered for wear causing particles to become embedded in the bearing surface. This could be a big problem for people who swear by UOA metal concentration as a proxy for engine wear. There may be a saturation point after which wear levels explode and it won't be preventable by UOA.