Optimization of tribological behavior of Pongamia oil blends as an engine lubricant additive

2.1 Experimental apparatus

The tribological behavior of Pongamia oil (Expo Essential Oil, Delhi, India) blend variants was investigated using a Ducom Macro Pad pin on disc tribometer (Bangalore, Karnataka, India), according to standard test methods of ASTM G99, which was connected with a personal computer with a data acquisition system. Linear variable differential transformer (LVDT) is used for determination of the wear and sensors are mounted to sense the changes in the coefficient of friction. The weight of the pin was determined before the test was conducted and after the tested result. Weight loss of the pin was determined as a function of different loads applied and sliding distances. Weighing was performed with an analytic balance Shimadzu AX 200 machine with a sensitivity of 0.1 mg. Specifications of the pin on disc tribometer are: maximum pin diameter=12 mm; maximum load=200 N; maximum disc rotating speed=2000 rpm; maximum wear measurement range=2000 μm.

2.2 Sample preparation

The specimens which were used for the experiment are aluminum silicon alloy with 7% silicon and EN31 steel with 60 hardness of 60 HRC (Rockwell C Hardness). The chemical composition of the Al-7 Si alloy is as follows: Si 7.39, Mg 0.356, Fe 0.116, B 0.0011, Sn 0.0027, Ti 0.115 and Al balanced and chemical composition of EN31 steel is C 1, Si 0.35, Mn 0.5, S 0.05, P 0.05, Cr 1.3 and Fe balanced. Hemispherical aluminum silicon alloy was used as the pin and the material used for the disc specimen was EN31 steel with a maximum diameter of 165 mm. The pin and disc specifications were: length of pin=30 mm; pin diameter=8 mm; hemispherical radius of pin=4 mm; disc diameter=165 mm; thickness of disc=8 mm; limit of disc track diameter=145 mm. Before conducting each experiment, ethyl alcohol was used to ensure that the surfaces are cleaned properly.

2.3 Test method

The details were as follows: load=50 N, 100 N or 150 N; ambient temperature was taken; track diameter=80 mm (for each experiment); sliding velocity=1.3 m/s, 2.5 m/s or 3.8 m/s; sliding distance=3000 m. For each test, the same track diameter was used and emery paper A350 was used for polishing the disc after each experiment. After completion of each test, the pin and disc specimen was cleaned ultrasonically with ethyl alcohol and stored in a vacuum oven furnace to avoid corrosion of the material. For examination of the worn surfaces, a trinocular stereo zoom microscope was used. The mean average value was used after completing each experiment three times to maintain accuracy in the results.

2.4 Lubricants used

In this investigation Jatropha oil was mixed with a conventional lubricant (SAE 20W40) in the ratios: 20W40 (PB 0); 10 (PB 15); 30 (PB 30) (% by volume). The homogeneous mixing was done by a magnetic stirrer. Table 1 shows the physiochemical properties of mineral and Pongamia oil-based blended lubricants.

2.5 Viscosity and total acid number test

An Anton paar viscosity meter and total acid number test (TAN)/total base number (TBN) analyzers were used for investigating degradation of the lubricant. The kinematic viscosity was measured at 40°C and 100°C according to ASTM D445 standard. For the TAN analysis, c(KOH)=0.2 mol/l in isopropanol as the titrant was used according to the ASTM D664-81 standard.

2.6 Wear scar diameter

The trinocular stereo zoom microscope was used to calculate the wear scar diameter (WSD) of the pin. A suitable magnification lens was chosen and the focus was adjusted until a clear image was shown on the computer screen. After that, view 7 software available in the computer was used for the measurement of WSD.

2.7 Flash temperature parameter

The flash temperature parameter (FTP) is a single number used to express the critical flash temperature above which given a lubricant will fail under given conditions [31]. The following formula has been used for FTP analysis:

where W=load in kg and d=mean WSD in mm at this load.

2.8 Design of experiments employing the Taguchi method

The Taguchi method developed an orthogonal array to study the effect of control factors involved and to minimize the number of experiments. The experimental results obtained are then converted to signal to noise (S/N) ratio which measures quality characteristics to understand that the results are deviating from or are nearer to the obtained results. There are three categories involved in quality characteristics to analyze S/N ratio. These are: the smaller the better; the larger the better; the nominal the better. These can be calculated according to the below mentioned equations.

The smaller the better:

where “n” indicates the number of replications and “y” indicates the observed response value. It is employed where a smaller value is desired.

The nominal the better:

where “μ” indicates mean and “σ” indicates the variance. It is employed where nominal or variation is minimum.

The higher the better:

where “n” indicates number of replications and “y” indicates the observed response value.

2.9 Process parameters

In this experiment three control factors with three levels were selected. The details of the factors to be considered and the level assigned with them are designated in Table 2. The level indicates the values taken during plan of the experiments.

Minitab 16 software was used for the selection of the orthogonal array. L9 orthogonal array was selected and details of the experiments to be investigated are shown in Table 3. Three columns were considered and each consists of three levels.

The plan of experiments includes nine row experiments in which the first column was assigned to the Jatropha oil blended lubricants (%), second to the sliding velocity (m/s) and the third to the load (N). Responses taken during the experiment were pin weight loss, friction force (N) and friction coefficient (μ).

2.10 ANOVA

ANOVA stands for analysis of variance. It is a statistical technique which control factors involved and is used to determine the percentage contribution of each control factor to reveal the effect on the quality characteristics. The increase in S/N ratio determines the increase in control factor. It can be used to investigate the different factors including degree of freedom, sequential sum of square, adjusted sum of square, sequential mean square and the last column indicates the p value for each control parameter. The control factor having a minimum p≥F had significant influence on the responses involved and the control factor having maximum p≤F value contributed less.

3.1 Control factor effects

Table 4 shows the experimental orthogonal array with determined values for pin weight loss, friction coefficient and wear rate. Responses of each control factor on pin weight loss, friction coefficient and wear rate were determined and analyzed using an S/N ratio table.

Figure 1A–C show the main effect plot for S/N ratios. Among the PB 0, PB 15 and PB 30, PB 15 (15% blend with conventional lubricant) shows the minimum pin weight loss, friction coefficient and wear rate as it provides a better protective film between metal to metal contact in comparison to PB 0 and PB 30. This could be attributed to the fatty acid composition of the Pongamia oil-based lubricants. These fatty acid compositions consists of molecules which form a long chain covalently bonded hydrocarbon chain and act as an efficient barrier for protecting sliding surfaces contact; they provide better wear protection than conventional hydrocarbon-based lubricants. Esters have polar functional groups which provide better affinity to metal surface and contributed towards formation of a protective layer between metal surfaces. Minimum pin weight loss, friction coefficient and wear rate were observed at a higher sliding velocity and lower load. In comparison to sliding velocity, load had significant effects on the responses considered during the analysis. With increase of load, friction force increases which results in more pin weight loss, friction coefficient and wear rate. PB 15 shows better results with increase of sliding velocity due to higher viscosity of Pongamia oil, which provides a better protective layer with increase of sliding velocity and cannot provide better lubricity at a lower sliding velocity. Another reason behind the better results at higher sliding velocity is the increase of temperature, which reduces viscosity, making it responsible for the formation of an excellent tribo layer. Aluminum has an inherent property of forming an oxide layer on its outer periphery. When sliding at high velocity, the temperature increases over the contact surface, making the material oxidize. This phenomenon leads to the transferring of materials, forming a mechanically mixed layer, also called a tribo layer. As the velocity increases, this tribo layer will act as a barrier or lubricant between the two surfaces, decreasing the coefficient of friction, wear rate and pin weight loss [32]. Optimum results for the control factors considered were determined from the main effect plot. The optimum result for all the responses, i.e. pin weight loss, friction coefficient and wear rate, was A2B3C1. This means that the second level of Pongamia oil blends (%), i.e. PB 15, the third level of sliding velocity, i.e. 3.8, and the first level of load applied, i.e. 50 N, were considered as better control factors.

Figure 1:

(A) Main effect plot for pin weight loss (mg), (B) main effect plot for friction coefficient, (C) main effect plot for specific wear rate.

3.2 ANOVA analysis

Tables 5–7 show the ANOVA table for pin weight loss, friction coefficient and specific wear rate. From Table 5, the Pongamia oil percentage (43.93%) and load (42.5%) had significant influence on the pin weight loss and the contribution of sliding velocity (4.46%) was least as compared to the other two control factors. The reason behind significant influences of the Pongamia oil percentage and load was stated earlier. Among the interactions, sliding velocity with load (5.71%) and Pongamia oil (%) with sliding velocity had significant influences on pin weight loss. It can be observed from Table 6 that sliding velocity (66.42%) and load (17.64%) have a greater contribution on the friction coefficient as compared to Pongamia oil (2.98%). Frictional forces had significant influence on the friction coefficient according to the below formula:

where F=frictional force in Newton and N is the applied load (Newton).

Among the interactions, Pongamia oil with load has the greatest influence followed by Pongamia oil with sliding velocity and sliding velocity with load.

From Table 7, the Pongamia oil percentage (58.7%) and load (31%) had significant influence on the specific wear rate and the contribution of sliding velocity (4.8%) was least as compared to the other two control factors. The contribution of sliding velocity was least as the wear rate depends on the load applied and volume loss according to the below mentioned formula:

Among the interactions, Pongamia oil percentage with load had the greatest influence followed by sliding velocity with load (5.71%) and Pongamia oil (%) with sliding velocity.

3.3 Validation of optimized results

The last step was to confirm the validity of the optimized results with the experimental results for the quality characteristics. With a new set of control factors A2B3C1, the experiment was performed for the pin weight loss, friction coefficient and wear rate. The predicted experiment for the pin weight loss can be performed according to the following equation:

where η=estimated average, T=overall experimental average and A2, B3, C1 are the mean responses for the control factors.

An experiment was conducted for the new set of the control factors and the output was compared with the output obtained from the predicted equation as shown in Table 8.

The following equation was used to obtain the confidence interval for the predicted mean of the confirmation test [33]:

where CI=confidence interval, F(1,η₂)=F value at required confidence interval at DOF 1, V_e=error variation from ANOVA and N_e=number of replications. The calculated confidence level is ±1.23 dB. The 95% confidence interval for the predicted mean of the confirmation test is shown in Table 9.

3.4 WSD analysis

Figure 2 shows the WSD of Al-7% Si alloy pin with Pongamia oil blends percentage at different load and sliding velocity. WSD is the formation of worn surface on the hemispherical pin in contact with the disc. WSD increases with increase of load and maximum WSD was observed at 150 N load for each blend considered. This is due to the increase in frictional forces applied with increase of load. The minimum WSD was found at 3.8 m/s sliding velocity and the maximum at 1.3 m/s sliding velocity. It reduces with increase of sliding velocity due to formation of a tribo layer acting as a lubricant, which was formed due to the increase of temperature with increase of sliding velocity. PB 15 shows the minimum WSD among all the Pongamia blends as it provides a better protective layer between metal to metal contacts.

Figure 2:

Wear scar diameter at various loads and sliding velocities.

3.5 FTP analysis

Figure 3 shows the effect of Pongamia oil blend on FTP at different load and sliding velocity. Generally by observation, the trend of FTP is increased when the load is increased from 50 N to 150 N, due to an increase in frictional force with increase of load. For 1.3 m/s sliding velocity, the lubricant performance was improved by 15% contamination of Pongamia oil with conventional lubricant, as viscosity provides a protective film layer with increase of sliding velocity. PB 15 shows a higher FTP compared to other percentages of contamination. It proves that PB 15% was a potential antiwear additive for conventional lubricant. The higher flash temperature ability of PB 15 makes this blend more compatible at high temperature. The lowest value of FTP was found to be at PB 30.

Figure 3:

Variation of flash temperature parameter at different loads and sliding velocities.

For 3.8 m/s sliding velocity test, the figure shows 15% of Pongamia oil was the highest value of FTP. Pongamia oil 15% was the best additive for fresh lube oil at the temperature of 40°C, in order to reduce wear phenomena. The lowest value occurred at 30% of Pongamia oil. That means, 30% contamination of Pongamia with lube oil was making much wear on metal surface in contact at 40°C.

3.6 Degradation of lubricant

Viscosity is a significant factor in determining the degradation of lubricant, as it provides a protective film thickness between the surfaces in contact and protects wear of metal surfaces during sliding. It is also contributes towards identification of oil grades and for monitoring the change in range of viscosity while the vehicle is in service. The deterioration of used oil can be due to oxidation or contamination, which indicates the increase in viscosity, while dilution of lower viscosity oil or fuel contributed towards a decrease in viscosity. Figure 4 shows the variation of viscosity with Pongamia oil percentages at different loads. It can be revealed from Figure 4 that the viscosity increased with increase in load, due to the oxidation process which results in the sludge formation or contamination of insoluble particles. This contributes towards increment of the length of molecular chain which results in increased viscosity of the used oil. From the figure, it can be seen that for the contamination of Pongamia oil with lube oil, the highest value was stated for 15%. Pongamia oil 15% was the best contamination with lube oil in order to maintain the antiwear characteristic such as kinematic viscosity. The figure also showed that normally the values of kinematic viscosity for all samples at 40°C were higher than 100 centistokes (cSt). It stated that basically the kinematic viscosity of these samples was lower than the samples at 40°C which is below 20 cSt. The lower value of kinematic viscosity was affected by the temperature. With the higher temperature, the kinematic viscosity will be lower due to the liquidity of the samples lubricant.

Figure 4:

Variation of kinematic viscosity with Pongamia oil blends at 40°C and 100°C.

Figure 5 shows the variation of Pongamia oil blended lubricant at different loads. It can be observed from the figure with increase of load total acid number increases due to increase in friction with applied load. PB 15 shows better results as compared to PB 0 and PB 30. This reveals that PB 15 provides better antiwear characteristics in comparison to other blends.

Figure 5:

Total acid number before and after test of different Pongamia oil blends.

3.7 Wear mechanism

Worn surface image of the optimum result A2B3C1 is shown in Figure 6. From the left to the right side of the figure, wear surfaces were for the PB 0, PB 30 and PB 15 at 50 N load and 3.8 m/s sliding velocity. Minimum wear surface was observed for Pongamia oil at 15% contamination (PB 15). The PB 15 makes a thick film between the sliding surfaces in contact and protects from wear in comparison to other contaminated Pongamia oil, i.e. PB 30 and PB 0.

Figure 6:

Worn surface images of the pin for optimum result A2B3C1.