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Advancements in Fuel Lubricity Studies



PCS Instruments’ state of the art HFRR, USV, EHD, and MTM have been used to develop a critical understanding in fuel lubricity research across automotive and aviation sectors. Through advanced instrumentation, insights into the differences in lubricity, wear and fuel-lubricant interactions among diesels, gasolines and jet fuels can be realised. This has facilitated the optimisation of lubricant formulations and engine designs for improved efficiency and sustainability. The select studies highlight the use of PCS instruments in aiding researchers in exposing the complex interplay between fuels, lubricants, and engine components, paving the way for optimised lubricant formulations and enhanced engine designs for the future.


 

Gasoline Lubricity


Gasoline, commonly used as a fuel for internal combustion engines, has drawn attention for its lubricating properties and interaction with engine lubricants. Two significant papers shed light on this topic.


Fuel dilution (also known as crankcase dilution) is a perennial problem that occurs in many internal combustion engines. It happens when fuel leaks past the engine piston rings and drives its way into the crankcase engine oil, contributing significantly to the wear of the engine components. Published in Tribology International, Zhang et al. [1] investigates the complex interactions between water and fuel dilution in engine oil, particularly in hybrid electric vehicle operating conditions. The study explores gasoline’s lubricating properties using the HFRR ASTM wear test method which is the industry standard for diesel fuel lubricity assessment and conforms to ASTM D6079 regulatory framework.


Through experimentation using PCS instruments—the HFRR wear testing rig, MTM, USV, and EHD—the study aims to understand how these contaminants affect boundary film performance, lubricant rheology, and film thickness. It reveals that gasoline dilution lowers viscosity without chemical changes, while water contamination affects anti-wear film formation, leading to decreased friction and increased wear. Despite both contaminants being present, they act independently, suggesting minimal interaction. These insights are crucial for developing lubrication strategies, especially for hybrid electric vehicles, to enhance engine efficiency and reliability.



Similarly, The Lubricity of Gasoline [2] focuses on characterising the effects of simultaneous water and gasoline dilution on lubricant performance. Through use of PCS instruments—the HFRR and MTM—the study provides valuable insights into gasoline’s role as a lubricant in engines. Contrary to diesel fuel, gasoline generally exhibits inferior lubricating capabilities, as evidenced by wear measurements ranging from 700 to 900 micrometers at 25°C.


Furthermore, the study sheds light on the nuanced effects of various additives on gasoline lubricity, emphasising the significance of additive interactions in influencing lubrication performance. The experiments reveal that gasoline dilution reduces viscosity predictably but has negligible chemical effects, while water affects low-shear viscosity and anti-wear film formation by competing for the surface, resulting in thinner and smoother anti-wear films.


These findings offer significant insights into gasoline lubricity and its implications for engine performance, guiding efforts to develop more efficient and sustainable engines. By identifying the effects of specific fuel components and additives, these studies contribute to optimising fuel and lubricant formulations, improving engine efficiency, durability, and sustainability.


 

Diesel Lubricity


Further insight into diesel fuel properties by Yu et al. [3], showed the chemical factors influencing diesel fuel lubricity through wear tests on diesel fuel fractions and model fuels, highlighting the significant role of polyaromatics and oxygen-containing polar impurities in enhancing diesel fuel lubricity. By employing rigorous testing methods, the study sheds light on the chemical factors influencing diesel fuel lubricity, offering valuable insights for optimising fuel and lubricant formulations.


The paper [3] discusses various tribological tests, such as the Stabinger viscometer, USV, UTFI, MTM, HFRR, and Stribeck curve analysis, to understand the impact of diesel contamination on lubricant performance across different lubrication regimes. The experimental results obtained from these tests provide insights into changes in viscosity, shear thinning behaviour, EHD film thickness, friction coefficients, and wear scar under diesel dilution conditions. Notably, the Stabinger viscometer showcases a significant reduction in viscosity due to diesel dilution, affirming the study’s focus on the rheological impact of diesel contamination.


USV data reveals how shear thinning behaviour varies between neat and diesel-diluted lubricants, highlighting the complex interplay between viscosity modifiers and diesel contaminants under different shear rates. Additionally, PCS’ MTM results demonstrate that diesel dilution affects friction coefficients in the mixed lubrication regime, suggesting an inhibition of friction-reducing boundary film formation.



Furthermore, HFRR tests indicate minimal differences in wear scar and friction coefficients among the three lubricants, neat 0W-30 engine oil, 15% diesel-diluted 0W-30 (0W-30D), and a specially blended 0W-16 to match the viscosity of 0W-30D, indicating negligible impact on anti-wear additives under harsh rubbing conditions. By comparing the Stribeck curves obtained from a modified journal bearing machine, the study understands the effects of diesel dilution on lubrication performance across different lubrication regimes. The findings suggest that while diesel dilution primarily reduces lubricant viscosity in the hydrodynamic regime, it hinders the functionality of viscosity modifier additives and influences friction behaviour in mixed and boundary lubrication regimes.


These studies contribute significant insights into diesel fuel lubricity and its implications for engine performance, informing efforts to develop more efficient and sustainable diesel engines. By identifying beneficial and detrimental effects of specific fuel components and additives, these findings aid in optimising fuel and lubricant formulations, contributing to ongoing efforts to improve the efficiency, durability, and sustainability of diesel engines, benefiting consumers, manufacturers, and the environment.


 

Jet Fuels

Jet fuels play a pivotal role in powering aircraft engines and facilitating air travel globally. Understanding the nuances of jet fuel properties and formulations is crucial for ensuring safe, efficient, and sustainable aviation operations. In recent years, significant attention has been directed towards advancing jet fuel technology, particularly in the realm of sustainable aviation fuels (SAFs) and their integration into the aviation fuel supply chain.


A critical aspect of recent research endeavours is the exploration of alternative fuel sources and formulations, as highlighted in studies like Agrawal et al. [4]. This study [4] investigated the production and performance characteristics of bio-derived jet fuels, highlighting their potential as viable alternatives to conventional petroleum-based jet fuels which, for the time being, are far cheaper than their sustainable counterparts. These efforts are driven by the aviation industry’s increasing focus on sustainable aviation fuels (SAFs) derived from renewable feedstocks such as biomass, algae, and waste oils. Bio-based jet fuels offer promising alternatives to conventional petroleum-based fuels, contributing to reduced greenhouse gas emissions and environmental sustainability.



The potential of SAFs to mitigate the environmental impact of aviation is underscored by findings in reports by the Air Transport Action Group [5]. These reports emphasise SAFs’ role in achieving carbon-neutral growth and long-term decarbonisation in aviation. Studies like those conducted by Basu et al. [4] delve into the rheological properties of jet fuels and their influence on engine lubrication and wear, shedding light on critical factors affecting engine performance and durability.


In addition to alternative fuels, research in jet fuel technology aims to enhance fuel efficiency, engine performance, and durability through advanced formulations and additives. Investigations into fuel properties such as viscosity, thermal stability, and lubricity provide valuable insights into optimising engine operation. Collaborative initiatives among industry stakeholders, governments, and research institutions are driving innovation in fuel technology and infrastructure development to facilitate the widespread adoption of SAFs and other alternative fuels.


Regulatory frameworks established by organisations like ASTM International and the International Civil Aviation Organisation (ICAO) play a pivotal role in ensuring the safety, quality, and compatibility of jet fuels. These standards are essential for maintaining the reliability and safety of aviation operations while transitioning towards sustainable fuel options. Ongoing research in jet fuels is essential for addressing environmental challenges and achieving sustainable growth in the aviation sector. Through collaboration and innovation, the industry can pave the way for a cleaner, greener future in air transportation.

 

This review underscores the critical role of fuel lubricity research in both automotive and aviation industries. By examining the differences in lubricity and wear characteristics among diesel, gasoline, and jet fuels, and highlighting their implications for engine performance and lubricant formulation, we shed light on key areas for future research and development. Advanced instrumentation facilitates precise analysis of fuel-lubricant interactions, driving optimisation efforts for enhanced engine efficiency and sustainability. Through collaborative endeavours, the industry can leverage these insights to address environmental challenges.


 

References


[1] Zhang, J., Yu, M., Joedicke, A., & Reddyhoff, T. (2022). Characterising the effects of simultaneous water and gasoline dilution on lubricant performance. Tribology International.

[2]  The Lubricity of Gasoline. Wei, D P; Spikes, H A; Korcek, S. Tribology Transactions; Philadelphia Vol. 42, Iss. 4, (Oct 1999): 813

[3] Yu, M., Shang, J., Joedicke, A., & Reddyhoff, T. (2020). Experimental investigation into the effects of diesel dilution on engine lubrication. Tribology International.

[4] Ababneh, H., Mohammad, N., Choudhury, H. A., Shang, L., Gani, R., McKay, G., & Elbashir, N. (2020). Enhancing the lubricity of gas-to-liquid (GTL) paraffinic kerosene: impact of additives. BMC Chemical Engineering, 2(9), Article 9.

[5] Air Transport Action Group. (2023). Beginner’s Guide to Sustainable Aviation Fuel, Edition 2023.






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