Search

One route to reducing global CO2 emissions is to improve the energy efficiency of machines. Even small improvements in efficiency can be valuable, especially in cases where an efficiency improvement can be realized over many millions of newly produced machines. For example, conventional passenger car combustion engines are being downsized (and also downspeeded). Increasingly, they are running on lower viscosity engine lubricants (such as SAE 0W-20 or lower viscosity grades) and often also have stop-start systems fitted (to prevent engine idling when the vehicle is stopped). Some of these changes result in higher levels of mixed and boundary friction, and so accurate estimation of mixed/boundary friction losses is becoming of increased importance, both for estimating friction losses and wear volumes. Traditional approaches to estimating mixed/boundary friction, which employ real area of contact modelling, and assumptions such as elastic deformation of asperities, are widely used, but recent experimental data suggests that some of these approaches underestimate mixed/boundary friction losses. In this paper, a discussion of the issues involved in reliably estimating mixed/boundary friction losses in machine elements is undertaken, highlighting where the key uncertainties lie. Mixed/boundary lubrication losses in passenger car and heavy-duty internal combustion engines are then estimated and compared with published data, and a detailed description of how friction is related to fuel consumption in these vehicles, on standard fuel economy driving cycles, is given. Knowing the amount of fuel needed to overcome mixed/boundary friction in these vehicles enables reliable estimates to be made of both the financial costs of mixed/boundary lubrication for today’s vehicles, and their associated CO2 emissions, and annual estimates are reported to be approximately $290 billion dollars with CO2 emissions of 480 million tonnes. This paper is an expanded version of a conference paper

Lubricants are used to reduce friction and wear in machines, saving billions of dollars worldwide in energy and breakdown costs and lowering CO2 emissions. Today, most lubricants are made using hydrocarbons derived from crude oil, which is a finite resource, although alternative bio-based lubricants are also being investigated, as is the re-refining of used lubricants to make new base oil. The CO2 emissions from current lubricants (due to their manufacture and waste disposal) are estimated and it is found that CO2 emissions from the production and disposal of lubricants are very much lower than the CO2 emissions associated with the energy used by those machines. It is also shown that an effective way to make lubricants more sustainable is to extend lubricant oil drain intervals and collect used oil and re-refine it to make base oil for re-use. The role of bio-based lubricants, and their benefits and disadvantages are discussed. Other aspects in which lubricants can be made more sustainable are also briefly covered, such as lubricant packaging, the removal of toxic additives via improved regulatory chemistry, and the use of renewable electricity in blending plants.

One route to reducing CO2 emissions is to improve the energy efficiency of machines. For example, conventional combustion engines are being downsized (and also down-speeded), and are now running on lower viscosity lubricants (such as 0W-20 or lower viscosity grade lubricants) and often also have stop-start systems fitted. Some of these changes may result in higher levels of mixed and boundary friction, and so estimating the friction losses due to mixed/boundary friction, and the corresponding wear levels, is becoming of increasing importance. There is recent experimental evidence that traditional approaches (such as the Greenwood & Tripp model [1]) to predicting friction in mixed and boundary friction tend to underestimate these losses [2-5]. A new model is described, based on experimental data, that estimates the proportion of mixed/boundary lubrication, X, as a function of the lambda value (where lambda is the ratio of the oil film thickness separating the surfaces to the combined root mean square roughness of the surfaces). The precise equation that describes the way in which X varies with lambda takes the form of a “reverse S-curve” which makes sense physically since S-curves arise naturally in growth processes and the real area of contact of rough lubricated surfaces grows as 1/lambda increases. Numerical estimates of the amount of mixed/boundary lubrication losses in internal combustion engines are made and compared with recently published experimental data [2, 6]. In addition, these improved calculations are used to estimate both the financial cost of mixed/boundary lubrication for today’s vehicle fleet, and the CO2 emissions associated with these losses.

It has been shown experimentally that boundary friction is proportional to load (commonly known as Amontons’ law) for more than 500 years, and the fact that it holds true over many scales (from microns to kilometres, and from nano-Newtons to Mega-Newtons) and for materials which deform both elastically and plastically has been the subject of much research, in order to more fully understand its wide applicability (and also to find any deviations from the law). Attempts to explain and understand Amontons’ law recognise that real surfaces are rough; as such, many researchers have studied the contact of rough surfaces under both elastic and plastic deformation conditions. As the focus on energy efficiency is ever increasing, machines are now being used with lower-viscosity lubricants, operating at higher loads and temperatures, such that the oil films separating the moving surfaces are becoming thinner, and there is a greater chance of mixed/boundary lubrication occurring. Because mixed/boundary lubrication occurs when the two moving rough surfaces come into contact, it is thought timely to review this topic and the current state of the theoretical and experimental understanding of rough-surface contact for the prediction of friction in the mixed/boundary lubrication regime

A critical review of recent work on fuel lubricant interactions is undertaken. The work focusses on liquid fuels used in diesel and gasoline vehicles. The amount of fuel that contaminates the lubricant depends on driving conditions, engine design, fuel type, and lubricant type. When fuel contaminates a lubricant, the viscosity of the lubricant will change (it will usually decrease), the sump oil level may increase, there may be a tendency for more sludge formation, there may be an impact on friction and wear, and low speed pre-ignition could occur. The increased use of biofuels (particularly biodiesel) may require a reduction in oil drain intervals, and fuel borne additives could contaminate the lubricant. The move towards the active regeneration of particulate filters by delayed fuel post-injection and the move towards hybrid electric vehicles and vehicles equipped with stop-start systems will lead to increased fuel dilution. This will be of more concern in diesel engines, since significant fuel dilution could persist at sump oil temperatures in the range of 100–150 °C (whereas in gasoline engines the more volatile gasoline fuel will have substantially evaporated at these temperatures). It is anticipated that more research into fuel lubricant interactions, particularly for diesel engines, will be needed in the near future.

The motivations for the move to electrified vehicles are discussed with reference to their improved energy efficiency, their potential for lower CO2 emissions (if the electricity system is decarbonized), their lower (or zero) NOx/particulate matter (PM) tailpipe emissions, and the lower overall costs for owners. Some of the assumptions made in life-cycle CO2 emissions calculations are discussed and the effect of these assumptions on the CO2 benefits of electric vehicles are made clear. A number of new tribological challenges have emerged, particularly for hybrid vehicles that have both a conventional internal combustion engine and a battery, such as the need to protect against the much greater number of stop-starts that the engine will have during its lifetime. In addition, new lubricants are required for electric vehicle transmissions systems. Although full battery electric vehicles (BEVs) will not require engine oils (as there is no engine), they will require a system to cool the batteries—alternative cooling systems are discussed, and where these are fluid-based, the specific fluid requirements are outlined.

Professor Duncan Dowson had close connections to industrial partners and sponsors throughout his academic career, and I believe he viewed industrial tribological challenges as problems that could usefully be solved by academia, and as a source of ideas (and funding) for new projects. Professor Dowson’s famous work on a numerical solution for the lubrication of an elastohydrodynamic line contact, with Professor Higginson, was motivated by the need to better understand gear lubrication. These first calculations took 18 months to complete(!), and it was realized that for the theory to be of use to practicing engineers, simpler correlation functions fitted to numerical simulations would be much more convenient, and the Dowson-Higginson correlation functions for the prediction of elastohydrodynamic oil film thickness that were developed are still widely used today. Industrial partners such as Shell supplied high-pressure fluid properties required for the elastohydrodynamic calculations (such as the pressure coefficient of viscosity and the way in which lubricant density varies with pressure). Professor Dowson also famously served on the Jost Committee, which quantified, for the first time, the financial impact of tribology, and highlighted that investments in good tribological practices would pay for themselves many times over. It should be remembered that in setting up the Jost Committee, the UK Government specifically asked the committee to investigate the state of lubrication education and research and to establish the requirements of industry in this regard. This paper also includes personal memories of the significant collaborations that I was involved with, as an industrial research scientist, with Leeds University from the mid 1990’s to around 2013, which predominantly focused on piston ring tribology. Also included is a discussion of the Leeds-Lyon Symposia on Tribology, of which I attended at least 10 of these meetings from 1994 to 2018.