Stribeck curve in Electric vehicle

Introduction

Lubricants are integral to perform various functions in the vehicle components, significantly impacting overall performance. Electric vehicles (EVs) are mainly equipped with electric motors that typically exhibits higher acceleration rates compared to vehicles with mechanical engines. Consequently, lubricants in EVs endure higher shear rates. It is presumed that the vehicles under consideration are adequately lubricated to maintain optimal performance.

The Stribeck curve is a widely accepted performance parameter with respect to lubrication-based studies. It was employed to understand the difference between the internal combustion engine (ICEs) based and electric vehicle-based lubrication. Figure 1A illustrates the estimated coefficient of friction (CoF) against time, with blue denoting ICE and red representing EV. Figure 1B displays the plotted Stribeck curve alongside the Sommerfeld number (ηV/P, where η = fluid viscosity, V = speed, P = load), assuming the hydrodynamic lubrication regime is achieved in the best-case scenario for EVs, resulting in a friction coefficient as low as that of ICE vehicles. Initial conditions stipulate that lubricants have a finite CoF at time = 0, gradually decreasing to a minimum value over time. Subsequently, in the hydrodynamic lubrication regime, the CoF rises from its minimum value. Each letter denotes a point, with lowercase letters indicating EV lubricants and uppercase letters denoting ICE vehicle lubricants. BDL, ML, and HDL represent boundary lubrication, mixed lubrication, and hydrodynamic lubrication regimes, respectively, for ICE vehicle lubricants, while bdl, ml, and hdl denote the corresponding regimes for EV lubricants. The parameter λ, where λ = t/r (t = film thickness and r = surface roughness), determines the lubrication regimes.

 Figure 1A illustrates the estimated coefficient of friction (CoF) against time, with blue denoting ICE and red representing EV. Figure 1B displays the plotted Stribeck curve alongside the Sommerfeld number

Detailed Explanations

1. In EV transmissions, lubricants with low viscosity are preferred. At the outset (time = 0), the low viscosity of the lubricant leads to increased metal-to-metal contact compared to ICE transmissions, which typically use higher viscosity oil. Consequently, the initial CoF would be higher in EV transmissions due to the heightened metal-to-metal contact. As a result, the initial CoF at the start (point p) for EV transmissions would either overlap with or slightly exceed that of ICE transmissions (point A).
2. For BDL, λ is less than 1.2; for ML, λ ranges from 1.2 to 3; for, λ is greater than 3. Due to the lower viscosity of lubricants in EVs compared to ICE vehicles, the t is lower in EVs (i.e., tEV < tICE), as fluid thickness is directly proportional to fluid viscosity. Consequently, λEV < λICE. Therefore, achieving a value of 1.2 (the threshold for the transition from boundary lubrication to mixed lubrication) necessitates a higher corresponding increase in the Sommerfeld number (and thus speed, V) for EV lubricants compared to ICE lubricants. In other words, a greater speed increment (1V) is required for EV lubricants to reach the transition to the mixed layer (ML) lubrication regime (i.e., 1VEV > 1VICE), resulting in a longer boundary layer regime (BDL) in EVs compared to ICE vehicles.
3. The mixed lubrication regime is denoted by region ML for ICE lubricants and ml for EV lubricants. The slopes of the curves qr (for EV lubricants) and BC (for ICE lubricants) are of interest. Due to the lower viscosity of EV lubricants, a greater speed increment (1V) is required to transition to the HD lubrication regime compared to ICE lubricants. Consequently, the slope of qr is gentler compared to BC. Moreover, EV lubricants experience higher thermal loads due to large currents and fluctuating electric and magnetic fields, leading to slower declines in CoF with increasing lubricant temperature. This is indicated by the gentler slope of qr compared to BC.
4. In the hydrodynamic lubrication regime, the consistently high film thickness at very high speeds in EV transmissions prevents metal-to-metal contact, delaying the rise of CoF due to thermal and electrical degradation of the lubricant. Consequently, the rs segment of the EV curve is more elongated than that of ICE vehicles (point C). Beyond point s, further speed increases lead to a steeper rise in CoF (st) compared to CD in ICE lubricants. This is attributed to high heat generation at high speeds, resulting in thermal degradation of the lubricant and reduced film thickness, as well as electric field-induced film deformation, such as electrowetting and micro-bubbling, causing increased metal-to-metal contact and thus a steeper CoF rise.

Reference

[1] Chen, Y., Jha, S., Raut, A., Zhang, W. and Liang, H., 2020. Performance characteristics of lubricants in electric and hybrid vehicles: a review of current and future needs. Frontiers in Mechanical Engineering, 6, p.571464.

[2] https://www.lifewire.com/do-electric-vehicles-use-oil-6543300