This paper describes the design of a test apparatus which simulates the lubrication of large, slow, two-stroke marine engines in which the ring pack is lubricated by means of injectors supplying lubricant above the piston. The equipment is able to control lubricant injection parameters (volume, frequency, etc.) and employs capacitance based lubricant film thickness transducers to allow instantaneous oil film thickness and film extent around the compression ring to be investigated on a stroke-by-stroke basis. It is demonstrated that the equipment can be used to study the development of lubricating films on successive strokes under differing injection strategies. Time varying changes in lubricating film thickness and film extent have been measured and the rate at which the lubricant spreads across the cylinder wall has also been investigated. It has been observed that increases in oil-film thickness are strongly linked to the transition from starved to fully-flooded inlet conditions and that net lubricant transport rates along different parts of the cylinder can be evaluated from measured data.

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, for both estimating friction losses and wear volumes. Traditional approaches to estimating mixed/boundary friction, which employ real area of contact modelling, and assumptions such as the elastic deformation of asperities, are widely used, but recent experimental data suggest 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 USD 290 billion with CO₂ emissions of 480 million tonnes.

Oil film thickness measurement is a vital experimental technique which has been widely applied to investigate lubrication in a range of tribological components. This article focuses on its development and use as a tool to study the hydrodynamic lubrication of piston-rings in internal combustion engines. This includes the development of experimental sensors to measure lubricant film thickness and a review of phenomena related to the hydrodynamic lubrication of piston-ring packs. Future applications of oil film thickness measurement as part of a feedback-based system to control the lubrication of ring packs are also discussed.

Several approaches have been developed to measure instantaneous friction between the piston assembly and cylinder in internal combustion (IC) engines, such as floating-liner, reciprocating liner, instantaneous mean effective pressure (IMEP), fixed sleeve, and (P-ω) method and tribological bench tests. However, the “floating-liner method” and the “(IMEP) method” are the most common methods used to measure instantaneous friction between the piston assembly and IC engines. This paper critically evaluates different approaches to the design of the “floating-liner”. The paper begins by discussing piston assembly frictional losses and their significance and then discuss the development of instantaneous piston-friction measurements. After that, it reviews the main design challenges in the floating-liner approach. “Methods of cylinder sealing” and “force balancing methods” are also reviewed. Design challenges associated with firing operation were presented. Floating-liner designs were classified into different categories with a detailed presentation of the features of each. The paper ends by presenting a range of broad recommendations for further work which would benefit future designs.

This paper explores the development of a subject called “tribotronics”, the science of actively controlled tribological elements and considers the potential of tribotronics as part of a revolution machine element design for the future. It presents a “state of the art” assessment of some devices that can be considered to be “tribotronic” which are in common use now, as well as reviewing examples of recent progress in research in tribotronics. It also presents a “perspective” on the future challenges and likely developments in the subject, including the integration of tribotronics with developments in other fields of digital technology, such as Industry 4.0, the “Internet of Things” and machine learning, potentially leading to “cyber tribotronic systems”. It is concluded that it is inevitable that tribotronics will contribute significantly to transforming the design of the components and machines of the present day into the cyber-physical systems in the future.