Investigation of friction and wear characteristics of palm biodiesel

Investigation of friction and wear characteristics of palm biodiesel

Energy Conversion and Management 67 (2013) 251–256 Contents lists available at SciVerse ScienceDirect Energy Conversion and Management journal homep...

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Energy Conversion and Management 67 (2013) 251–256

Contents lists available at SciVerse ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Investigation of friction and wear characteristics of palm biodiesel M.A. Fazal ⇑, A.S.M.A. Haseeb, H.H. Masjuki Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

Article history: Received 18 August 2010 Received in revised form 22 November 2012 Accepted 2 December 2012 Available online 2 January 2013 Keywords: Four-ball tests Wear and friction B100 Palm methyl esters

a b s t r a c t Use of biodiesel in automobile engine is creating tribology related new challenges. The present study aims to assess the friction and wear characteristics of palm biodiesel at different concentration level by using four-ball wear machine. The investigated fuels were biodiesel (B100), diesel (B0) and three different biodiesel blends such as B10 (10% biodiesel in diesel), B20, B50. Tests were conducted at 75 °C under a normal load of 40 kg for 1 h at four different speeds viz, 600, 900, 1200 and 1500 rpm. Worn surfaces of the balls were examined by SEM. Results showed that wear and friction decreased with the increase of biodiesel concentration. The wear of steel ball in B100 was appeared to be 20% lower than that in diesel (B0). Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The quality of fuels is considerably affected by their level of lubricity. Lubricity of engine fuel is essential to prolong the machine life. It provides protection to the moving surfaces against wear. Good lubricity is also important to cut down the energy consumption by reducing friction of automotive parts [1]. For few components of automobile such as fuel injectors and pumps, the lubricity issue is very important as they are lubricated by the fuel itself [2,3]. In automobile engine, the temperature of the fuel inlets is appeared to be above 60 °C [4,5] which also can influence the lubricity. Especially, lubricity of the fuel at fuel injectors and high-pressure fuel pumps could be greatly changed because of existing higher temperature. In fact, the lubricity of conventional diesel fuel is poor. Additionally, rapid growth of modernization and technological developments are leading towards the depletion of limited fossil fuel (diesel) resources of the world. Therefore, it is very important to find such an alternative fuel which can meet these growing concerns. Biodiesel is one of the most promising alternative fuels to meet these problems. It is renewable and can be produced from different vegetable oils and animal fats [6–8]. Biodiesel offers some technical advantages as compared to conventional petroleum diesel. The most common advantages of biodiesel over petroleum diesel are biodegradability, higher flash point, improved citane number, reduced exhaust emissions, etc. [9–11]. It has been reported that the neat biodiesel provides inherently greater lubricity than petroleum diesel [12]. Although biodiesel is getting more popularity because of these positive aspects, there exist some significant drawbacks which have limited its commercial usage. The major ⇑ Corresponding author. Tel.: +60 3 79675212; fax: +60 3 79675317. E-mail addresses: [email protected] (M.A. Fazal), [email protected] (A.S.M.A. Haseeb), [email protected] (H.H. Masjuki). 0196-8904/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2012.12.002

concerns include injector coking, filter plugging [9], deposition of carbon or dense slurry rich in fatty esters [13], auto-oxidation [14,15], corrosive nature [16–18], reactivity of unsaturated hydrocarbon chains [18–20], etc. Many of these problems may also influence the friction and wear of different engine components that come in contact with biodiesel under sliding condition. Wear properties of biodiesels have recently been studied by several researchers using different tribometer techniques such as high frequency reciprocating rig (HFRR), four-ball testing. Sulek et al. [19] investigated the tribological properties of rapeseed biodiesel by HFRR tribometer. They observed that B5 (5% rapeseed biodiesel in diesel) and B100 produced 20% and 30% respectively lower friction coefficient than that of petroleum diesel no. 2. Improvement of lubricity has been reported for even lower (<1% biodiesel) blend levels [20]. However, adverse effect of biodiesel on tribological properties has also been reported by several researchers. In a recent four-ball wear test [12], both wear and friction were found to be increased slightly with the increase of temperature. It was found that wear rate was highly influenced by absorption of moisture as well as oxidation. Maleque et al. [21] investigated the tribological performance of 5% POME (palm oil methyl ester) blended lubricant by using a steel–cast iron pair. They found that the corrosive wear and pits on the damaged surface were the dominant wear mode. In another study [22], it was reported that more than 5% palm oil methyl ester (POME) in lubricant caused oxidation and corrosion. These observations demonstrate that lubrication properties of biodiesel are influenced by temperature, oxidation, moisture absorption, etc. Despite the fact that a number of studies are available in this field, there is a need to conduct further systematic research confirming tribological behavior of different biodiesel blends. The present study aims to characterize the lubricity in terms of friction and wear for different

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Table 1 Conditions for the four-ball wear test.

Load

Test parameters Applied load (kg) Rotation (rpm) Fuel temperature (°C) Test duration (s)

40 600, 900, 1200, 1500 75 3600

Test ball Materials Composition Diameter (mm) Hardness (HRc) Surface roughness Ra (lm)

Chrome alloy steel C: (0.95–1.10)%, Cr: (1.3–1.6)%, Fe: balance 12.7 62 0.040

Rotating ball

Rotating gripper

Test fuel Stationary ball

concentration of biodiesel in diesel blends as compared to that of petroleum diesel no. 2.

Steel Cup

Fig. 1. Schematic diagram of four ball wear geometry [Adapted from Ref. 12].

2. Experimental details Friction and wear characteristics of palm oil methyl ester were investigated by four ball wear machine, IP 239/85. The test parameters in the present study were regulated by ASTM D4172 standard except the speed which was 600–1500 rpm instead of 1200 rpm. Details of the test conditions are described in Table 1. Palm oil methyl ester used in this study meets EN14214 specifications and was supplied by Golden Hope Biodiesel Sdn Bhd, Malaysia. The analysis report given by the supplier showed 97.01% ester content. Main impurities were monoglyceride (0.45%), diglyceride (0.05%), triglyceride (0.01%) and methanol content (0.01%). The acid value, viscosity and moisture content of the investigated biodiesel were 0.57 mgKOH/g, 4.63 mm2/s and 500 mg/kg respectively. In addition to petroleum diesel no. 2 (B0) and biodiesel (B100), three different blends such as B10 (10% biodiesel in diesel), B20, B50 were made on volume basis for investigating the lubricity. The schematic diagram of four-ball wear machine is seen in Fig. 1. Among the four balls, the lower three were held in fixed position in a steel cup and the rest ball into the upper chuck was rotating one. Sufficient amount (approx. 10 ml) of test fuel was poured into the steel cup to cover the lower three balls to a depth of at least 3 mm. The four-ball wear machine was connected with computer in order to record the friction torque. Later, friction torque for each test was converted into friction coefficient by following the Eq. (1). The wear scar diameters of the steel balls were measured before removing those from the cup. The results reported here are mean wear scar diameter of the lower three balls. For removing the worn products, the worn surfaces were scrubbed lightly in a stream of water with polymer brush so as not to

B0 B50

B10 B100

B20

Coefficient of friction ðlÞ ¼

The variation of friction coefficient, calculated form recorded torque has two parts. One part is run-in period and another one is steady state. Fig. 2 shows that at the very beginning of each test (during run-in period), the friction coefficient was unstable with time and few minutes later, it reached to a stable condition. With the increase of time, it is believed that the contact surfaces of the test balls become smoother and more prominent asperities are lost or flattened. This may result the transition of unsteady state friction coefficient to a steady state level. It is noted from Fig. 2 that petroleum diesel (B0) shows higher unsteady state friction coefficient with longer duration of time as compared to that of other fuels. This demonstrates that the presence of biodiesel can promptly change the unsteady state friction coefficient into steady state condition. In general, esters biodiesel are considered to show better scuffing protection behavior as compared to hydrocarbons of petroleum diesel [23]. This could be attributed to the absorbed

0.19

0.11

0.07 0

300

600

900

Time (s)

1200

1500

ð1Þ

3. Results and discussion

B0 B50

(a) 600 rpm

0.15

pffiffiffi T 6 3Wr

where T is friction torque (kg/mm), W is applied load (kg), r is the distance from the centre of the contact surfaces on the lower balls to the axis of the rotation (which is 3.67 mm).

Friction Coefficient

Friction Coefficient

0.19

mechanically abrade the original surface. These were then degreased with acetone. The cleaned worn surfaces of balls were then investigated by using scanning electron microscopy.

B10 B100

B20

(b) 1500 rpm

0.15

0.11

0.07

0

300

600

900

1200

Time (s)

Fig. 2. Variation of friction coefficient with respect to time (for first 1500 s) at speeds (a) 600 rpm and (b) 1500 rpm.

1500

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600 rpm

900 rpm

1200 rpm

1500 rpm

0.09

B0 B50

B10 B100

B20

900

Friction Coefficient

Run-in Perriod (s)

1200

600

300

0.0885

0.087

0.0855

0 0

20

40

60

80

100

0.084 600

% of biodiesel Fig. 3. Fall in run-in period with the increase of biodiesel concentration in different blends for different rotating speeds.

1500

Fig. 5. Effect of speed on average friction coefficient in different fuels under a fixed load of 40 kg at temperature 75 °C for 1 h.

1.1 600 rpm

900 rpm

1200 rpm

1500 rpm

B0 B50

B10 B100

B20

0.95

WSD (mm)

0.086

Steady State FC

1200

Speed (rpm)

0.089

0.083

0.8

0.65

0.08

0.077

900

0

20

40

60

80

100

% of biodiesel

0.5 600

900

1200

1500

Speed (rpm)

Fig. 4. Change in steady state friction coefficient (FC) in different fuels for last 1000 s of each test.

Fig. 6. Effect of speed on wear scar diameter (WSD) in different fuels under a fixed load of 40 kg at temperature 75 °C for 1 h.

ester molecules from biodiesel which may act as surfactants for the metal surface. Fig. 3 shows that the run-in period decreases with the increase of biodiesel concentration. Decreasing trend of run-in period is faster for the blend containing up to 20% biodiesel and beyond that concentration, it becomes very slow. The decrease of run-in period with the increase of biodiesel concentration suggests that biodiesel can successfully reduce the scuffing period and thereby reduce the friction coefficient. It is also observed that the run-in period decreases slightly with decreasing the speed. The steady state friction coefficient for the last 1000 s of each test has been presented in Fig. 4. It is seen in Fig. 4 that at higher percentage of biodiesel (above 20%), the steady state friction coefficients are almost similar (around 0.0835) for each rotating speed. This seems to suggest that minimum 20% biodiesel is needed to reach at the state friction level. Fig. 5 shows that the average friction coefficient decreases with the increase of biodiesel concentration. For petroleum diesel (B0) and biodiesel (B100), the increasing rates of average friction coefficient with the rotating speeds are almost linear while for blends are not. It is noted from Fig. 5 that the average friction coefficient in diesel is found to be higher than that in biodiesel. Though the variation of friction coefficients for different fuels is in small scale, it is found that the increasing trends for friction coefficient with speed are almost linear for B0 and B100 fuels while

for blends are not. This is more likely attributed to the non-uniform availability of the concentration of heteroatom (e.g. oxygen, sulfur) in different blends which plays important role in creating the lubricating film. It is found that under a rotating speed of 1500 rpm, the average friction coefficients of diesel and biodiesel are 0.0857 and 0.0889 respectively. This suggests that the films provided by biodiesel at the contact surface are more effective than that of diesel to reduce the friction. According to Wain et al. [24], biodiesel containing more oxygen can reduce friction as compared to diesel fuel. In another study [25], it was reported that the formation of several compounds on the frictional surface of steel such as FeS, FeSO4 as well as organic compounds with C–C, COH and COOH groups could be the main mechanism in reducing friction. It is seen in Fig. 6 that the wear scar diameter (WSD) increases with the increase of rotating speed. On the other hand, the wear scar diameter is decreased with the increase of biodiesel concentration. The increasing trends of wear scar diameter (WSD) almost follow the increasing trends of friction coefficient as shown in Fig. 5. Under 1500 rpm rotating speed, the wear of steel ball is appeared to drop by 10% for B20. A further increase in the concentration of biodiesel leads to a consistent reduction in wear whose value for B100 is by about 20% lower than that for petroleum diesel. It is observed for each fuel that the higher wear occurs for higher speeds and the increasing trends of wear are almost similar to the friction. The increased wear with the increase of speed may

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be attributed to the generation of higher heat between the contact surfaces as a consequence of higher speed. It is found that pure biodiesel or higher biodiesel blends caused less wear than that in lower biodiesel blends or diesel. This indicates that the reduced wear of steel ball was due to presence of biodiesel. A pronounced reduction of wear seemed to be observed in the concentration range of 10–20%. Friction and wear in higher biodiesel blends are more likely to be reduced by having a comparatively more stable film to separate the adjacent surfaces and limit metal to metal contact. This can be attributed to the presence of aliphatic fatty acid of general formula CnH2n+1COOH, such as stearic acid in POME which can enhance the lubricating property by controlling friction and wear between the contact surfaces through developing of lubricating films [25]. The protective films can reduce thermal energy in sliding contact and thereby improve lubricity [26,27]. According to Sharma et al. [28], the ester ends of the fatty acid chain are absorbed to metal surfaces and thus permit monolayer film formation with the hydrocarbon end of fatty acids oriented away from the metal surface. The fatty acid chain thereby offers a sliding surface that prevents the direct metal-to-metal contact.

However, the stability of these films also depends on operating conditions such as load, temperature, speed as well as fluid viscosity and composition [21]. The generated acids could also be corrosive for the automotive components [19,20] though they can improve lubricity in short term operation. In an another explanation, trace components found in biodiesel fuels including free fatty acids, monoglycerides, diglycerides are reported to improve the lubricity of biodiesel [29]. Oxygenated moieties, degree of unsaturation, chain length of molecules in biodiesel also play an important role in increasing lubricity. Geller and Goodrum [30] have observed that lubricity is increased with increasing of the degree of unsaturation. They found that methyl linolenate (C18:3) in the C18 series demonstrated the best performance as a lubricity enhancing component and methyl stearate (C18:0) was the least effective. Unhindered electrons in the form of free electron pairs or double-bond electrons toward the end of a chain of C atoms in unsaturated molecules such as methyl linolenate (C18:3) are likely to be effective in enhancing lubricity. Lubricity can also be influenced by oxidation process. This is because the oxidation process reconverts esters into different fatty

(a) B0/50X

(e) B0/2k X/Edge

(i) B0/2k X/Center

(b) B20/50X

(f) B20/2k X/Edge

(j) B20/2k X/Center

(c) B50/50X

(g) B50/2k X/Edge

(d) B100/50X

(h) B100/2k X/Edge

(k) B50/2k X/Center

(l) B100/2k X/Center

Fig. 7. Scanning electron micrographs of worn surfaces of used balls at speed 1500 rpm: (a)–(d) are magnified by 50 and (e)–(l) are magnified by 2000.

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acids including formic acid, acetic acid, propionic acid, caproic acid, etc. [31] which seems to cause improved lubricity [12]. The content of polyunsaturated components in palm biodiesel, such as oleate and linoleate esters could be the strongest factor in influencing the auto-oxidation. Apparently, in short term test, oxidation process can provide better lubricity but in case of long term application, it may cause degradation of fuel and therefore may result reduced lubricity [32], enhanced corrosion and degradation of materials [33–35]. The oxidation of biodiesel depends not only on its unsaturated components, but also on the concentration of impurities, natural antioxidants in biofuels and storage conditions (temperature, light, and humidity). It is believed that the effect of oxidation on lubricity is crucial. Further investigation should be conducted to explore the related mechanism. Fig. 7 shows the SEM micrographs of worn surfaces of the steel balls. Fig. 7a–d demonstrates that wear scar diameter decreases with the increase of biodiesel concentration. Deformation of surface is also decreased with increasing biodiesel. It seems that the sizes of the particles removed from cavities of the worn balls in B0–B50 (Fig. 7e–k) are much bigger than 20 lm (adhesive wear) while for B100 (Fig. 7h–l), the particle sizes are less than 20 lm (abrasive wear). Surface morphology of the worn scar shows that layers of debris from the surfaces have been extruded sideways, while flacks of debris are extruded out from the contact interface in the sliding direction of the rotating ball. It is seen that the edge of the surface at B0 has been plastically deformed and elongated with more cracks and wear debris. This deformation has been gradually decreased in the following samples in B20, B50. It seems that the cavities are much bigger than 20 lm and the particles removed from the cavities are also more likely bigger than 20 lm in size. According to the definition given by Robinowicz [36], removal of wear debris bigger than 20 lm results adhesive wear. This indicates that the wear for sample in B0, B20, B50 can be categorized as adhesive wear. But for the sample in B100, no such type of severe damage is seen. No crack is found like other samples and also the edges are not so elongated and irregular. Some thin, regular and radial scratches are seen on the surface which result abrasive wear. Removal of small particles (<20 lm) with a few surface features results abrasive wear [37]. This study shows that the severity of the wear is reduced with the increase of biodiesel concentration in blends and better surface is found for pure biodiesel.

4. Conclusions The following conclusions can be drawn from this study: 1. Both wear and friction decrease with the increase of biodiesel concentration. This is more likely attributed to the presence of oxygenated moieties, degree of unsaturated molecules, free fatty acid components, etc. in biodiesel. 2. The wear of steel ball is appeared to drop by 10% for B20 under 1500 rpm rotating speed. A further increase in the concentration of biodiesel leads to a consistent reduction in wear whose value for B100 is by about 20% lower than that for petroleum diesel. A pronounced reduction of wear was appeared to at the range of 10–20% biodiesel. 3. Lubricity in terms of wear and friction decreases with the increase of rotating speed. However, at the concentration of biodiesel above 20%, the steady state friction coefficients are almost similar in each speed. 4. Deformation of the worn surfaces decreases with increasing the concentration of biodiesel in blends. The steel ball in pure biodiesel is subjected to abrasive wear while in diesel it is subjected to adhesive wear.

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Acknowledgements The authors would like to acknowledge the financial support provided by the Ministry of Science, Technology & Innovation (MOSTI) under the science fund by Grant No.: 03-02-03-SF3073, University of Malaya Research Grant (UMRG) by Project No.: RG137-12AET and by the Institute of Research Management and Consultancy, University of Malaya (UM) under the IPPP Fund Project No.: PS093/2008B.

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