# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 1 Introduction  
Lubricating oil in an internal diesel engine is exposed to various strains that depend on operating conditions, fuel quality, ambient conditions, and operating parameters. The rate of deterioration strongly depends on these influences. To avoid engine failure, the oil must be changed before it loses its protective properties. However, an unnecessary oil change should be avoided for environmental and economic reasons. To reliably determine the optimum oil change interval, it is necessary to monitor the actual physical and chemical conditions of the oil [1, 2]. The aging process of the oil is very much influenced by, among other things, fuel quality, because of the blow-by gases of the combustion process [3, 4]. Therefore, especially for gas engines fueled with biogas of a priori unknown and fluctuating fuel quality, direct monitoring of the oil condition is essential [5]. According to the current state-of-the-art for automotive applications, the condition of engine oil is estimated by means of indirect methods that mainly rely on operating parameters, such as engine speed, performance, and temperature [6]. For large-scale stationary engines, a more reliable, but also more elaborate approach is currently used, in which oil samples are taken at regular intervals for off-line laboratory analysis to determine the optimal oil change interval [7, 8]. The disadvantages of this method are high fixed costs and the delay between sample drawing and return of analysis results. Currently developed alternative approaches are aimed at utilizing sensors to provide input for on-board diagnosis systems that determine the current oil condition inside an engine. The sensor signals should provide information on the actual physical and chemical oil quality to provide the information needed for the prediction algorithms, as shown in Figure 1. Moreover, the sensors should exhibit a long lifetime, even though they are continuously exposed to an aggressive environment inside an operating combustion engine [9; 10]. The entire system should give a reliable indication of the current oil condition and estimate when the oil will reach the end of its lifetime and an oil change will be necessary. Previous attempts in the automotive field were based on measuring the permittivity or conductivity of the oil [11]. Although these parameters are measured directly in the oil, they are of limited use because they are influenced by many different oil wear mechanisms; this results in a signal that is of only limited utility. There have also been attempts to sense a representative chemical parameter directly, but chemical sensors commonly exhibit a limited lifetime and show adverse properties with respect to aging and drift [12; 13; 14]. Monitoring of internal engine oil condition with a fluid property sensor definitively simplifies oil quality management by providing a higher capacity for detection of slow and rapid changes in oil contamination or quality levels, thereby enabling economical and environmentally friendly optimization of oil change intervals [15; 16]. [PERSON] et al. [17] adopted the low frequency commercial piezoelectric tuning forks (QIFs) to the measurement of physicochemical properties of liquids, and the result showed that the applied measurement technique based on the analysis of the QTF equivalent electric circuit proved its usefulness for determining liquid viscosity in the range up to 16 cP. [PERSON] et al. [18] researched a novel sensor system for downhole measurements of viscosity, density, and other fluid properties. [PERSON] et al. [19] demonstrated the use of a tuning fork sensor for fast viscosity and density measurements that would enable the characterization of transient fluid properties, such as in multiphase flows of liquids. The result showed that by monitoring the frequency and the decay rate during ring-down,  
Figure 1: Structure of a sensor-assisted algorithm for a lubrication-monitoring system.