Case Study: Neutron tomography of pistons at IMAT, ISIS Neutron and Muon Source
Finden Ltd, ISIS Neutron and Muon Source, Infineum UK Ltd
Coking of engine components can reduce efficiency and lifetime of engines (automotive, marine, aviation) through increased wear. Lubricants not only act to reduce friction of such components, key for wear and fuel economy but they also play a key role in engine thermal management and cleanliness. The amount of deposit varies based on the engine, its application, and is limited by additives in the lubricating oil, for instance antioxidants limit the oxidative degradation of the oil and dispersants keep coked material colloidally stable.
The engine component temperature and lubricant residence time unsurprisingly correlate to coke formation, which is affected by the engine cooling strategy, fuelling and load. Coked components have poorer thermal conductivity and act to insulate the engine components, this can lead to localised increases in temperature, which in extreme cases can lead to physical failure, e.g. cracking of the component. The coked material (typically black, hard, porous, brittle and mainly carbonaceous) if shed can also travel around the engine, blocking filters and narrow regions away from their point of formation.
Visual appearance is often used to assess the performance of lubricating oils on components during engine testing, however deposit formation within components is hidden from this type of assessment, and so vital information may be missed. Current methods for assessing internal deposits are destructive, as often the component must be cut open; not only does this prevent continued use, but it can also damage brittle coked deposits, potentially distorting the interpretation.
A key challenge in studying carbonaceous deposits on metal components is achieving appropriate contrast of a low-density material (the carbonaceous deposit) on top of a high-density substrate (the metal surface). Whilst X-ray imaging is able to achieve this with small components, the large size of engine components such as pistons or turbochargers strongly attenuate X-rays, meaning that although external surface deposits can be characterised, those on inner surfaces are obscured.
Car engine piston
In the proof-of-concept measurements performed at IMAT, the unique properties of neutrons allowed for the lighter carbonaceous deposits to be clearly imaged on both external and internal surfaces of the metal engine components, and quantified e.g. in terms of thickness, volume, area covered. The upcoming diffraction capabilities in IMAT will complement the imaging by revealing if any of the deposits have crystallised, and identifying their chemical composition.
These results mean that we can now quantify where and when coking occurs within engines – even looking at the same component before, during, and after an engine test, and identify how this is affected by different conditions and lubricants to help build a model of deposit formation and aging.
This article first appeared on the ISIS Neutron and Muon Source website. For more information on this work and ISIS Neutron and Muon Source visit https://www.isis.stfc.ac.uk/Pages/IMAT-Coking-of-engine-components.aspx