Thick film sensors for engine oil acidity detection

Abstract

Engine oil condition monitoring has attracted considerable interests from industries and general public over the years due to its critical role in maintaining the performance and longevity of cars and industrial engines. Lubricants degrade during the course of operation and can be costly or detrimental to the engine if oil change intervals are not optimised. However, on-line robust monitoring for oils has been very challenging since oil degradation process is often complicated and influenced by a number of parameters, such as the operating temperature and contamination.
Due to the complexity of the oil chemistry and their degradation processes, there have not been any commercially available on-line sensors for oil chemical property monitoring reported. Oil acidity is traditionally measured through potentiometric or photometric/colorimetric titration methods. Recent attempts at miniaturising infrared (IR) and chronopotentiometric (CP) sensors based on solid-state devices for acidity/alkalinity determination of oil have not been very successful since the CP technology suffers from sensitivity and stability issues and IR sensors are still bulky and adversely susceptible to the engine?s harsh environment.
This project, sponsored by Shell Global Solutions, aims to develop robust chemical sensors that can detect oil acidity due to oil degradation. The initial comprehensive literature review has identified that thick film (TF) sensor technology offers compact and low cost mass production solutions and have been proved to be robust with good reproducibility for aqueous solutions acidity measurements. Their feasibility in detecting oil acidity was thus investigated in this study and experimental work has been carried out to fabricate TF electrodes and evaluate them in a range of oils to explain their performance in detecting acid content. Based on their performance in aqueous solutions in previous studies, this study has investigated the performance of one type of TF working electrode (Ruthenium Oxide (RuO2)) combined with various TF reference electrodes in order to develop the most suitable electrodes for oils. To simulate oil ageing, a fully formulated engine oil and a base oil were oxidised under controlled conditions. Also, different amount of nitric acid was added to a fully formulated oil to simulate the oil acidity changes. Acid number (AN) of the oil samples was obtained using conventional titration methods and viscosity and conductivity of the oil samples were measured using laboratory-based equipment in order to validate TF sensor measurements and establish a relationship between different properties of oil samples during their degradation process. Temperature effects on thick film electrodes as well as their long-term stability and repeatability were also investigated.
The results show that, for the first time, TF sensors respond to the acidity changes in all oil types tested and linear correlation between the TF responses and the AN was found in the oxidised oils within certain ranges at the tested temperatures (50 °C and 80 °C). TF sensors can detect oil acidity up to AN of 28 mgKOH/g. Although oil conductivity and viscosity were affected by the oil oxidation process, but no direct relationship was found between them and the TF responses. Based on the experimental results, sensing mechanisms of the TF electrodes in oils are proposed.

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ORCID for Ling Wang: orcid.org/0000-0002-2894-6784

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