The effect of soot and diesel contamination on wear and friction of engine oil pump
Introduction
In recent years, there has been an increasing demand in the automotive industry to improve fuel economy. Poor fuel economy is closely linked to high friction of tribological components and can also lead to high wear. Designing fuel efficient vehicles is arguably the primary focus for all automotive industry [1], [2].
A Variable Displacement Vane Pump (hereafter denoted VDVP) is an innovative type of oil pump that has been introduced to the automotive industry to improve the efficiency of engines. The VDVP is mainly composed of a slide ring with a circular inner bore, a rotor with several radially disposed vanes, a shaft, a spring and a casing. Fig. 1 shows a schematic representative of the VDVP. The VDVP has a variable capacity that is adjustable to the engine demand, i.e. it only generates the flow rate required at any one time [3], [4]. Despite high efficiency and reduced power consumption, the VDVP has its own disadvantages in terms of high wear and friction in the components of the pump which can lead to failure of the pump.
It has been reported that the wear occurring in vane-slide ring and vane-rotor contact is a critical aspect of the VDVP [4], [6]. Fig. 2 shows the contact between the vane and rotor, which is designed to be a flat-on-flat reciprocating sliding contact, however when the vane tilts during the operation of VDVP, this contact will change to a line contact which causes severe wear on the vane faces. In this paper, the tribological behaviour of the vane-rotor contact is considered.
Many studies have been conducted to improve the efficiency of VDVP with the specific focus on factors such as design parameters and operating conditions [7], [8], [9]. One source of failure in VDVP is improper lubrication, a factor that has not been studied in detail. The contacts in the VDVP are sensitive to contamination due to the small clearances between the components [10]. Therefore it is essential to understand how oil contamination affects the performance of the VDVP. Table 1 indicates the possible range of oil contaminants in diesel engines which can potentially influence the tribological performance of the system [3], [4].
Oil contamination such as that affected by soot and diesel influence the physical and chemical properties of the oil thus changing the tribological performance of VDVP. Soot is also known to induce high wear in engine components. The effect of soot contamination in engine oils on wear of engine components have been investigated in many studies [11], [12], [13], [14], [15], [16], [17], [18], [19]. Various mechanisms have been proposed by which soot induces high wear.
Abrasion is the most accepted mechanism [13], [15], [16], [17], [18], [19]. It has been reported [20], [21], [22], [23] that soot particles are hard enough to abrade both tribofilms formed on the surfaces and metallic engine components. Torrance [13] and Cadman et al. [16] postulated that soot removes the antiwear tribofilm formed on the surfaces and exposes a fresh reaction underlying metallic surface. Some other studies [23], [24], [25] suggested that soot in the oil can accumulate at the contact inlet and causes high wear due to the starvation of oil in the contact. It has also been reported [26] that soot competes with antiwear additives in adsorbing on the surface. The adsorption of soot on the surface prevents the adsorption of antiwear additives and their subsequent decomposition to form antiwear films. Additive adsorption on soot particles is another possible mechanism. It has been reported [11], [23], [27] that soot adsorbs antiwear additives in the oil phase reducing the concentration of the additives in the oil. Consequently, less antiwear additives adsorb at the contact interface to form antiwear films. A corrosive-abrasive mechanism has been recently reported by several studies [19], [27]. Olomolehin et al. [19] demonstrated that the interaction of CB and ZDDP in the oil can lead to aggressive wear. Although, there has been many studies on soot contamination, the actual mechanisms of wear caused by soot and its interaction with additives are not clearly understood.
Diesel fuel contamination can also cause starvation and deposits which reduce the functionality of the oil [28]. The effect of fuel contamination on the performance of oil is normally underestimated compared to the other contaminants such as soot and water. Fuel normally enters the engine oil through internal leakage of the injector and contaminates the oil [28].
Oil ageing is a destructive mechanism that can affect the physical and chemical properties of an oil and reduce its performance. Some factors such as temperature, metal content, type of base oil and additives, contamination content and the rate of air circulation influence the ageing rate of oil [29]. Kreuz et al. [30] showed that the high engine temperature significantly affected the ageing process of the oil. Zhang et al. [31] showed that the ageing process resulted in the formation of decomposition products and acids. It is thus necessary to understand the mechanisms of degradation and its impact on the oil properties and performance. This study aims to investigate the effect of oil contamination (soot and diesel) and oil degradation on wear and friction of the vane-rotor contact in VDVP.
Section snippets
Test lubricants
In order to have reliable and repeatable results, oil with consistent properties was needed for the experiments. Therefore, used engine oil extracted from an operational vehicle could not be used in the experiments since it would likely vary from car to car. An alternative approach was taken which was based on degrading the engine oil artificially in the lab in accordance to a novel ageing procedure [32], [33].
A commercially available synthetic fully-formulated engine oil with a viscosity grade
Physical properties of lubricants
Fig. 6(a) and (b) show the dynamic viscosity of oils containing various contaminants at 40 °C and 100 °C respectively. It can be observed that the viscosity of the oils increased during the ageing process by the oxidation process and the addition of CB contamination. This increase was expected due to the oxidation of the oil [37]. Diesel contamination slightly reduced viscosity of the oil samples. The combination of diesel and CB contamination also reduced the viscosity of the oil samples.
Conclusions
In this study friction and wear behaviour of vane components in VDVP were studied using various contaminants and varying ageing times. The results from FTIR, SEM/EDX and Raman analysis together provided a consistent explanation of the wear mechanism and surface chemistry. The results presented in this paper indicate that the ageing process on its own did not have a significant effect on the performance of FFO used in this study. However, when CB was present in FFO during the ageing process it
Acknowledgment
The authors would like to thank Magna Powertrain for providing funding for this research.
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