PREDICTION of POWER LOSSES in an AUTOMOTIVE GEARBOX INCORPORATING a THERMALLY COUPLED LUBRICATION MODEL’ and the Work Presented in It Are My Own
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PREDICTION OF POWER LOSSES IN AN AUTOMOTIVE GEARBOX INCORPORATING A THERMALLY COUPLED LUBRICATION MODEL by Athanasios Christodoulias This dissertation is submitted in fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) and Diploma of Imperial College (DIC) in the Tribology group Department of Mechanical Engineering Imperial College London June 2017 ABSTRACT The continuous tightening of environmental standards, and in particular the stricter vehicle CO2 emission regulations, have put increased pressure on passenger vehicle manufacturers to develop more efficient components. One way of improving fuel consumption and reducing emissions in a vehicle is by reducing the friction-related power loss in major components such as the engine and the transmission. Between 5% and 6% of the total fuel energy that the car converts is lost through friction in the transmission and ends up as heat being absorbed by the transmission lubricant. The principal aim of this project was developing a method to accurately predict power losses in the gearbox -the main transmission component- while at the same time having the ability to assess the influence of lubricant properties on the gearbox efficiency. For this purpose, a thermally coupled numerical model was developed, incorporating lubricant parameters extracted from rheological tests. The model calculates the friction coefficient in gear elastohydrodynamic (EHL) contacts and then uses it in an iterative scheme to predict in-contact and bulk temperatures as well as power losses. Finally, the model’s predictions for gears are coupled to current experimentally derived models for bearing and churning losses. The method has been applied to a single speed gearbox and a manual six speed automotive gearbox. Two fully formulated lubricants of nominally same specification were tested and comparisons were made for different input conditions and gear geometry changes. In the second case, a light duty truck fitted with the simulated gearbox has been instrumented and tested under different conditions. Temperature predictions from experimental drive cycles have been compared to model predictions and standardised drive cycles have also been simulated. Results show that the overall gear power loss is heavily dependent on the input torque and that the lambda ratio of the contact affects each component differently. In addition, the distribution of losses between gear friction, bearings and gear churning is not constant but depends on the selected components, the gear geometry, the surface roughness and the specific lubricant rheology. Furthermore, significant power loss differences of up to 11.7% and sump temperature differences of up to 3.1 °C respectively have been predicted between the two oils. Simulations of the six speed gearbox have shown that the model is able to predict bulk oil sump temperatures within 5-10% of measured values and can effectively be used to compare and rank different lubricants in terms of overall efficiency for a given gearbox. ii ACKNOWLEDGMENTS First and foremost, I would like to thank Professor Andy Olver for giving me the opportunity to embark on this journey and helping me in every way possible throughout my first year. I will always be grateful. I would like to thank Dr Amir Kadiric for his supervision and his valuable and continuous support throughout the remainder of this project. A special thanks to all the people at Valvoline USA, who sponsored this work, for providing me with their support and technical expertise. I would like to acknowledge all my colleagues in the Tribology Group, and all of my friends and family for their support, be it professional or personal. Finally, a very special thanks to my parents; simply put, I wouldn’t have done this without their support. This thesis is dedicated to them. iii DECLARATION OF AUTHORSHIP I, Athanasios Christodoulias, declare that this thesis titled, ‘PREDICTION OF POWER LOSSES IN AN AUTOMOTIVE GEARBOX INCORPORATING A THERMALLY COUPLED LUBRICATION MODEL’ and the work presented in it are my own. I confirm that: This work was done wholly or mainly while in candidature for a research degree at the Imperial College of London. Where I have consulted the published work of others, this is always clearly attributed. Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. I have acknowledged all main sources of help. Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself. Signed: ATHANASIOS CHRISTODOULIAS Date: 26th June 2017 iv COPYRIGHT DECLARATION The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. v CONTENTS ABSTRACT ............................................................................................................................. II ACKNOWLEDGMENTS .................................................................................................... III DECLARATION OF AUTHORSHIP ................................................................................ IV COPYRIGHT DECLARATION ........................................................................................... V CONTENTS........................................................................................................................... VI LIST OF FIGURES ................................................................................................................ X NOMENCLATURE .......................................................................................................... XVII 1. INTRODUCTION............................................................................................................ 1 1.1BACKGROUND .................................................................................................................... 2 1.2 RESEARCH GOALS ............................................................................................................. 6 1.3 THESIS OUTLINE ................................................................................................................ 8 2. LITERATURE REVIEW ............................................................................................. 10 2.1 INTRODUCTION................................................................................................................ 11 2.2 BACKGROUND ................................................................................................................. 11 2.2.1 Automotive drivetrain ............................................................................................. 11 2.2.2 Gearboxes ............................................................................................................... 12 2.2.3 Gears ....................................................................................................................... 13 2.2.4 Bearings .................................................................................................................. 16 2.3 LUBRICATION .................................................................................................................. 17 2.3.1 The importance of lubrication ................................................................................ 17 2.3.2 Lubricant composition ............................................................................................ 18 2.3.3 Lubrication methods ............................................................................................... 19 2.3.4 Lubricant viscosity and rheology ............................................................................ 21 2.3.5 Film thickness and lubrication regimes .................................................................. 25 2.4 LOSSES IN TRANSMISSION SYSTEMS ................................................................................ 30 2.4.1 Gear EHL losses ..................................................................................................... 30 vi 2.4.2 Churning losses ....................................................................................................... 34 2.4.3 Bearing and seal losses ........................................................................................... 35 2.4.4 Other losses ............................................................................................................. 38 2.4.5 Lubricant selection in transmission systems ........................................................... 39 2.4.6 Gearbox efficiency .................................................................................................. 40 3. METHODOLOGY FOR THE PREDICTION OF POWER LOSSES IN A GEARBOX ............................................................................................................................. 44 3.1 INTRODUCTION................................................................................................................ 45 3.2 EHL LOSS PREDICTION .................................................................................................... 45 3.2.1 General approach