Eliminating friction in engines could save up to nine per cent of global fuel consumption. Currently, friction is controlled using specialist additives in engine oil, which create ultra-thin films between the moving parts. However, these additives cause harmful emissions and they are due to be phased out within the next five years. So, the challenge is on for engineers to find a way to replicate their protective effect without the environmental impact.
Mechanical engineers from the University of Leeds have, for the first time, been able to accurately quantify the viscosity – or resistance to deformation - of these ultra-thin films that protect our engines’ moving parts. And they discovered something surprising – the films are continuously moving like molten glass.
Dr Abdel Dorgham explains: “The assumption has always been that these engine oil additives form a solid layer on the parts in contact and that this is sacrificial – it was thought to be worn away as the parts move together, protecting the material underneath. But in fact, this isn’t the case. We found that this layer moves like molten glass when it’s sheared or compressed, and it is this property that provides the protection.”
Analysis at the nanoscale
Analysing and measuring these films in situ, under comparable conditions to the inside of a combustion engine, is no small feat. The films are just 100 nanometres thick; as a comparison, a human hair is around 100,000 nanometres in diameter. The team used an atomic force microscope to watch these films forming, measure their thickness and quantify their shape and behaviour – the first time such equipment has been used to gather this data.
The findings provide a huge leap forward in the race to find new ways of reducing friction, says Dr Dorgham.
“Because we now understand how these films are protecting the engine and can put numbers on that, we’re in a better position to engineer alternatives,” he explains. “Rather than replicating additives in the oil, we believe it should be possible to add a coating to the engine parts themselves, which will behave like molten glass at the right temperatures. This is what we’re looking at now.”
From engines to hearts
The primary application is the internal combustion engine, which will still be needed within hybrids as we transition to fully electric vehicles, but the technology has other potential uses as well, says Dr Dorgham.
“The same technology could be used to coat the massive bearings that keep wind turbines moving, protecting them from wear. Not only would this reduce maintenance time and cost, but if it reduces friction as well as extending the lifespan of the turbine, it could easily double their efficiency.”
Another application is in tribo-electric nanogenerators or TENGs, which use small movements between surfaces to harness energy. TENGs are already being developed for use in heart pacemakers, or for use in vast networks to harvest energy from waves.
With up to 1.5 per cent of global Gross National Product estimated to be wasted in energy lost through friction, any technology that can reduce friction is guaranteed to have a major impact.
The research is part of the Friction Project, a collaboration between the University of Leeds and University of Sheffield, funded by the Engineering and Physical Sciences Research Council.