The role of lubricant viscosity in optimising electric vehicle transmission efficiency and reliability
File(s)
Author(s)
MacLaren, Alexander
Type
Thesis
Abstract
Electric Vehicles (EVs) are now the most promising solution for sustainable road transport. The rapid growth of this emerging technology means there is presently considerable scope to optimise drivetrain efficiency and reliability. EV transmissions must transmit high torques at low speeds, while also operating efficiently at very high speeds, and lubricant viscosity is crucial in balancing these competing requirements. Efficiency depends on both load-dependent (including traction) and load-independent (primarily oil churning) losses. A test rig is developed to measure churning losses in-situ in a popular EV drive-unit, and the influence of speed, oil volume and oil viscosity are examined. The relationship between churning loss and fluid viscosity in the range 4−200cSt is found to be nonmonotonic, and relatively weak compared to the influence of speed and oil fill. Measurements of total drive unit efficiency using a dynamometer are also presented. Elastohydrodynamic traction and film thickness, which influence both efficiency and reliability, are measured simultaneously at entrainment speeds up to 20 m/s using a new optical tribometer developed during this study. Measurements are obtained for 10 fluids representing 4 API groups, and compared to existing thermal EHD film thickness predictions. A novel approach to film thickness measurement using colorimetric interferometry is also proposed, using a context-aware colour matching procedure which addresses the fringe-order ambiguity problem. A selection of candidate transmission lubricants including a water-based formulation, are compared using a ball-on-disc contrarotation scuffing method, and a triple-disc micropitting rig. The formulated fluids are found to induce much more rapid micropitting than their unadditivised base oils, which instead rapidly fail by scuffing. In both tests with the water-based fluid, mild wear outcompetes scuffing and micropitting. These data elucidate the influence of lubricant formulation on these hard-to-model phenomena, contributing to future holistic optimisation of EV fluid formulation and transmission architecture for efficiency and reliability.
Version
Open Access
Date Issued
2024-06-27
Date Awarded
01/03/2025
Advisor
Kadiric, Amir
Spikes, Hugh
Sponsor
Valvoline
UK Research and Innovation
Grant Number
EP/T51780X/1
Publisher Department
Department of Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)