Tribological Assessment of Bio-Lubricants Influenced by Cylinder Liners and Piston Rings

Abstract

This study presents a comprehensive evaluation of the tribological behavior of cylinder liners and piston rings—key components in internal combustion engines (ICEs). Experiments were conducted using a pin-on-disc wear tester under varying loads (50–100 N) and speeds (175–350 rpm) to determine the coefficient of friction (μ) and wear rate. The selected pin and disc materials represent real engine components to ensure realistic operating conditions. Before and after each experiment, the cylinder liner-piston ring pair was cleaned with acetone to ensure accurate measurement of mass loss. Surface roughness (Ra, Rq, Rz, µm) was assessed using a Mahr M-1 profilometer, and Brinell hardness tests were carried out using a digital optical Brinell hardness testing machine to determine the mechanical properties of the contact surfaces. The results revealed that safflower oil achieved the lowest coefficient of friction at higher speeds, with an 18% reduction compared with conventional 20W-50 engine oil. Camelina oil, camelina biodiesel and safflower biodiesel each exhibited a reduction of approximately 12.5% in friction, highlighting their potential as viable alternatives to petroleum-based lubricants.

1. Introduction

Rising global concerns over energy sustainability and environmental protection have heightened interest in reducing frictional losses in internal combustion engines (ICEs). Friction and wear account for nearly one-third of the mechanical energy losses in ICEs [1,2]. The piston ring-cylinder liner (PR-CL) interface is recognized as one of the major contributors to mechanical losses [3]. Bio-lubricants and biodiesel possess superior properties, making them promising lubricant alternatives. The presence of polar functional groups optimizes their ability to form stable lubricating films. Biodiesel has emerged as a promising, eco-friendly lubricant due to its renewable nature, biodegradability, and low toxicity. Structurally, bio-lubricants and biodiesel comprise long-chain fatty acid esters, which inherently improve lubricity and help form stable boundary films under sliding contact conditions [4].

Previous tribological evaluations, such as those by Salimon et al. [5], reported that bio-lubricants exhibit a high viscosity index, oxidative stability, and thermal resistance. These features make them viable alternatives to mineral oils. Malik et al. [6] reported that bio-lubricants are biodegradable, renewable, and non-toxic. R. Song et al. [7] reported that rapeseed oil-based bio-lubricants and biodiesel blends reduce friction and mass losses in PR-CL tribo-pairs compared to conventional lubricants. Babu et al. [8] demonstrated, using a pin-on-disc tribometer, that biodiesel derived from sunflower oil blended with ZnO nanoparticles reduced both the coefficient of friction and wear volume under high load. Hamdan et al. [9] reported that biodiesel dilution in SAE engine oils improved boundary lubrication and reduced wear in the PR-CL contact at lower sliding speed. Guzler et al. [10] confirmed that biodiesel blends mitigate lubricant degradation and reduce energy losses in the piston assembly, which is consistent with the friction reduction trend reported in our tests. Dia et al. [11] provide an in-depth review of gallium-based liquid metals (GLM), highlighting their exceptional ability to form tribo-chemical layers (e.g., Ga-rich and FeGa₃ films) that significantly enhance wear resistance and reduce friction. These findings reinforce the importance of surface-lubricant interactions. These studies support the hypothesis that the fatty acid ester structures of biodiesel enhance surface film formation, improving its tribological performance compared with mineral oil [12].

The viscous resistance between the piston ring and cylinder liner interfaces plays a vital role in evaluating the overall tribological performance of IC engines. A lubricant with appropriate viscosity can establish a hydrodynamic film that effectively separates the sliding surfaces, thereby minimizing tribological losses. The viscosity of lubricants has a direct effect on the formation of the lubricating film. Higher viscosity enhances load-carrying capacity and reduces metal-to-metal contact. However, excessive viscosity may increase drag and energy losses. Conversely, lower viscosity lubricants may not provide sufficient film thickness under high load, leading to increased friction and wear. Therefore, achieving an optimal balance in the viscosity of lubricants is essential for ensuring reliable lubrication. Petroleum-based lubricants increase environmental concerns due to their high volatility, low biodegradability, harmful emissions, and non-renewability.

Bio-lubricants form boundary or mixed lubrication regimes depending on viscosity, load, and speed [13]. The performance of bio-lubricants at the piston ring–cylinder liner interface under real engine conditions remains underexplored. This experiment addresses this gap by evaluating real piston ring–cylinder liner materials under two-load and two-speed conditions, using various bio-lubricants.

2. Materials and Methods

This study presents an in-depth investigation into the tribological behavior of the piston ring–cylinder liner (PR-CL) interface, which plays a critical role in the efficiency, durability, and performance of internal combustion (IC) engines. Excessive friction and wear in these interfaces can significantly affect the fuel economy, increase maintenance costs, and reduce engine lifespan. Safflower (Carthamus tinctorius) and camelina (Camelina sativa) seeds were selected as feedstock due to their favorable oil yields, low cultivation requirements, and availability.

These seeds were obtained from the Faculty of Agricultural Sciences at Selçuk University, Konya, Turkey. The 20W-50 engine oil and diesel fuel used in this test were obtained from the industrial zone in Konya, Turkey. Biofuel (biodiesel) was prepared from safflower oil and camelina seed oil using the esterification method. The trans-esterification process involved six steps: mixing methanol with NaOH as a catalyst, conducting chemical reactions to separate glycerol, removing impurities, washing with water, drying at 100 °C, and finally storing the biodiesel in containers. Two chemical reactions were required to separate glycerol, which also involved the separation of impurities, water washing, drying at 100 °C, and finally storage.

Table 1. Analysis results of tested bio-lubricants and biofuel (biodiesel).

Properties	Safflower Oil	Camelina Oil	Safflower Biodiesel	Camelina Biodiesel	20W-50 Engine Oil	Diesel Fuel
Density (g/cm3)	0.915	0.918	0.88371	0.8959	0.8267	0.860
Kinematic viscosity 40 (°C) (mm2/s)	29.5133	30.30	5.0071	8.3	177	4.5
Flash point (°C)	200	150	150	105	60	55
Water content (mg/kg)	82.415	330.15	176.39	458.18	8.7931	5.68
Heating value (MJ/kg)	38.935	38.803	39.639	39.168	47.5	44

presents the physicochemical properties of camelina oil, safflower oil, their corresponding biodiesels, diesel fuel, and engine oil 20W-50. Physicochemical properties of tested biodiesels—including kinematic viscosity, density, calorific value, flash point, and water content—were determined using standard laboratory equipment at the Biodiesel Laboratory, Faculty of Agriculture, Selçuk University. Measurements were performed with a Köhler K23377 viscometer, Kem Kyoto densitometer, oxygen bomb calorimeter, Köhler K16270 flash point tester, and Karl Fischer titration, respectively.

2.1. Tribological Test Specimens

The tribological test specimens, namely pin and disc, were fabricated from GG25 grade cast iron, a material commonly used in automotive engine components due to its excellent wear resistance, good thermal conductivity, and damping characteristic. The cast iron alloy was sourced from the industrial zone in Konya, Turkey. The chemical composition of GG25 cast iron alloy material was presented in

Table 2. Chemical composition of pin and disc used in the experiments.

Chemical Composition	Selected Values
Carbon (C)	3.10–3.30
Silicon (Si)	2.30–1.90
Manganese (Mn)	0.90–0.60
Sulfur (S)	0.12
Iron (Fe)	Balance
Phosphorus (P)	0.15

.

2.2. Surface Preparation

To ensure consistency and repeatability in the contact conditions during the wear experiments, the specimens’ surfaces were prepared with care. Both the pin and disc specimens were mechanically polished using silicon carbide (SiC) sandpaper with finer grit sizes (600, 800, 1200 grit) until a smooth surface was achieved. After polishing, the samples were thoroughly cleaned with acetone and alcohol to remove any impurities that could affect the test outcomes.

2.3. Experimental Conditions

The operational parameters used in tribological tests are summarized in

Table 3. Triboloigical test operation conditions [14].

Parameters	Selected Values
Normal load (N)	50, 100
Angular velocity (rpm)	175, 350
Distance traveled (m)	1000
Diameter of the contact area (mm)	10
Pin diameter (mm)	10
Disc diameter (mm)	55
Lubricants	20W-50 engine oil, safflower oil, safflower biodiesel, camelina oil, camelina biodiesel, diesel fuel
Lubricants volume (mL)	1000
Lubrication method	Oil bath
Temperature	Ambient

. These parameters were carefully selected based on the comprehensive review of previously published tribological studies. The chosen loads, sliding speeds, and durations were intended to simulate mechanical stress conditions encountered in real engine operation. This approach optimizes the practical relevance of the study and enables more meaningful evaluation of the performance of the tested bio-lubricants under simulated engine conditions.

2.4. Experimental Set-Up

Tribological experiments were carried out using a pin-on-disc tribometer, following the ASTM G99 standard procedure [15]. The operation conditions are summarized in Table 3. These conditions were carefully selected based on a comprehensive review of previously published studies involving the pin-on-disc test under tribological environments relevant to internal combustion engine components. To provide a better understanding of the experimental setup, a three-dimensional schematic representation of the pin-on-disc wear tester is illustrated in Figure 1. This illustration highlights the critical components involved in the tribological interaction, including the pin holder, the rotating disc, and the normal loading mechanism.

Each experimental trial was repeated three times under identical operating parameters to ensure data reproducibility and reduce experimental error. The coefficient of friction values recorded for each trial were averaged, and average values for each lubricant condition are presented in Table S1. Specific wear rate SWR values were calculated using the present formula in Equation (2).

Δm = m₂ − m₁,

(1)

$$\mathit{SWR}={\displaystyle \frac{\mathsf{\Delta }\mathrm{m}}{\mathsf{\rho }\mathrm{F}\mathrm{n}\mathrm{L}}}$$

(2)

where Δm refers to mass losses; $\mathsf{\rho }$, density of samples’ material; Fn, applied load; L, sliding distance. Mass losses (Δm) were calculated using Equation (1), where (m₂ − m₁) are weights of the samples before and after each test.

$$A=\pi (\frac{d}{2}{)}^2$$

(3)

$$P = \frac{\mathrm{F}}{\mathrm{A}}$$

(4)

The normal contact pressure P was evaluated using the basic pressure formula, where F is the applied normal load, and A is the cross-sectional area between the pin and disc. As illustrated in Equations (3) and (4). The applied loads of 50 and 100 N correspond to contact pressures of 0.63 MPa and 1.27 MPa, respectively. These values were used to simulate mild and severe contact conditions commonly encountered in tribological interfaces of internal combustion engine components.

3. Results

3.1. Coefficient of Friction (μ)

3.1.1. Dry Friction

Under dry sliding conditions, the tribological performance exhibited sensitivity to changes in both applied load and speed. The highest coefficient of friction (μ = 0.325) was observed at 100 N and 175 rpm, indicating severe interfacial resistance due to the absence of lubrication. However, at a lower load of 50 N, the coefficient of friction decreased to μ = 0.250, suggesting reduced contact stress and minimized adhesive interaction at the tribo-interfaces. A clear inverse relationship between speed and coefficient of friction was also observed. At a load of 50 N, increasing speed from 175 rpm to 350 rpm results in a reduction in the friction value from μ = 0.272 to μ = 0.250, as illustrated in Figure 2a,c. This behavior is attributed to the formation of solid wear particles on the contact surface. These particles contribute to the development of a tribo-layer that acts as a solid lubricating film, reducing adhesive interaction and stabilizing frictional performance under higher speeds. These findings are consistent with those of Sánchez-Islas et al. [16], who reported that the formation of oxide layers significantly contributed to a reduction in the coefficient of friction by acting as a barrier and solid lubricating film. Similarly, in the current study, the decrease in the friction at lower load and high speeds can be attributed to the formation of a three-body tribo-layer composed of wear debris, which effectively reduced adhesive interaction.

3.1.2. Camelina Oil

Under a load of 100 N and a speed of 175 rpm, a low friction value (μ = 0.151) was recorded, indicating the effectiveness of the lubricating film formation between the contacting surfaces. However, when the speed increased to 350 rpm, the friction value rose to 0.257, suggesting partial thinning of the lubricating layers at higher speeds. At a lower load of 50 N and 175 rpm, μ was measured at μ = 0.148, with a decrease at 350 rpm, as shown in Figure 2b,d, suggesting enhanced hydrodynamic and mixed lubrication effects under reduced contact pressure and higher speed. Such conditions promote the formation of more stable tribo-films, minimizing asperity interaction and reducing friction. In contrast, under high load and low speed, the lubricant film may collapse, increasing direct metal-to-metal contact and resulting in higher friction. These findings are consistent with those of Kumar et al. [17], who highlighted the ability of vegetable oils to transition smoothly through boundaries to mixed regimes. This transition contributes to friction reduction under moderate speed and loads, whereas partial film breakdown may lead to friction escalation.

3.1.3. Safflower Oil

At 50 N and 350 rpm, the lowest coefficient of friction (μ = 0.132) among all tested conditions was recorded. When the speed decreased to 175 rpm under the same load, the coefficient slightly increased to 0.148, suggesting a transition toward boundary or mixed lubrication regimes as the speed decreased. Under a higher load of 100 N and 175 rpm, the coefficient of friction increased significantly to μ = 0.283, indicating asperity interaction and lubricating film degradation under higher contact pressure, as shown in Figure 2c. These results align well with the findings of Özçelik [18], who demonstrated that safflower oil offers superior friction-reducing performance under lower load and higher speed conditions. The observed reduction in friction was attributed to the formation of a stable lubricating film, which effectively reduces metal-to-metal contact. This finding supports the hypothesis that safflower oil as a bio-lubricant provides efficient lubrication through the development of a thin film.

3.1.4. Engine Oil (20W-50)

The tribological behavior of the commercial engine oil (20W-50) was investigated under various operating conditions. At 50 N and 350 rpm, a coefficient of friction of μ = 0.146 was recorded, indicating moderate lubrication efficiency under low-load and high-speed conditions. Reducing the speed to 175 rpm caused the coefficient to increase to μ = 0.200, reflecting a shift from hydrodynamic to boundary lubrication due to insufficient film formation. Under higher loads of 100 N at the same speed, the coefficient peaked at μ = 0.356 (Figure 2b). This increase is attributed to a thinning lubricating film leading to dominance adhesive wear mechanisms. These results are consistent with findings of Zhang et al. [19], who reported that increasing the load leads to film breakdown and elevated friction due to intensified contact pressure. Conversely, at higher speeds, a reduction in the coefficient of friction was observed, which was attributed to improved lubricant entrainment and optimized film stability. This behavior aligns well with the trend observed in the current study.

3.1.5. Camelina Biodiesel

Under tribological testing, camelina biodiesel exhibited a noticeable sensitivity to changes in both load and speed. At a high load of 100 N and a speed of 175 rpm, it recorded a relatively high coefficient of friction (μ = 0.326), indicating limited film strength and increased asperity interaction under these conditions. However, when the speed increased to 350 rpm, under the same load, a significant reduction in friction was observed, as illustrated in Figure 2b,d, suggesting improved lubrication through optimized film formation. At a lower load of 50 N and a speed of 175 rpm, the coefficient of friction value was measured at (μ = 0.148), with a slight decrease to (μ = 0.145) at 350 rpm (Figure 2a). These findings align with those reported by Rahman et al. [20], who determined the tribological behavior of first and second-generation biodiesel fuels using a pin-on-disc setup. Their study highlighted that higher speed facilitated hydrodynamic lubrication, leading to reduced friction.

3.1.6. Safflower Biodiesel

At 50 N and 175 rpm, the coefficient of friction was measured at μ = 0.145, which slightly decreased to 0.140 at 350 rpm, as illustrated in Figure 2a,c. This behavior suggests that increasing speed facilitates the transition from the boundary to mixed and hydrodynamic lubrication regimes. These findings align with the recent study by J. M. Encinar et al. [21], who reported that safflower biodiesel, derived from vegetable oil rich in long-chain fatty acids, promotes the formation of a protective oil film. The study also highlighted that bio-lubricants develop thicker lubricating films, demonstrated by high contact angles—which effectively reduce metal-to-metal contact. These properties explain the observed reduction in the coefficient of friction of safflower biodiesel at high speeds and low loads in the current study.

3.1.7. Diesel Fuel

Under the operating condition of 50 N and 175 rpm, diesel fuel exhibited a relatively low coefficient of friction (μ) reaching 0.151. However, when the applied load was increased to 100 N at the same rotational speed (175 rpm), the coefficient of friction significantly increased, peaking at 0.315. When the speed was further increased to 350 rpm under the same 100 N load, μ slightly increased to 0.307. This behavior suggests that the influence of speed on frictional performance is more prominent under low-load conditions, where higher speeds promote hydrodynamic lubrication and reduce metal-to-metal contact. Conversely, under high load conditions, the formation and stability of the lubricating film may deteriorate due to elevated contact pressures, thereby diminishing the beneficial effects of increased speed. These findings are illustrated in Figure 2a,b,d. The current study agrees well with previous studies, especially with Mujtaba et al. [22], who observed an increase in frictional resistance of diesel under elevated load conditions, attributed to its low ester content and limited film-forming capabilities. Moreover, they reported a stabilization of the coefficient of friction with increasing sliding distance, a behavior similarly observed in the current study. These correlations reinforce the notion that the incorporation of suitable additives in diesel fuel can optimize film stability and reduce friction under tribological conditions. A line graph of the coefficients of friction comparison of the tested bio-lubricants is presented in Figure 3.

3.2. Specific Wear Rate (SWR)

The specific wear rate (SWR) is a critical parameter for evaluating the tribological performance of bio-lubricants under varying operating conditions. In the present study, under the load of 100 N and a speed of 175 rpm, the highest wear was observed under dry conditions, indicating severe material degradation in the absence of lubrication. Furthermore, high wear values were also recorded for camilina oil, safflower biodiesel, and camelina biodiesel, although both exhibited improved performance compared with dry conditions, as shown in Figure 4c. When the load was reduced to 50 N and the same speed of 175 rpm, the trend remained similar, with the dry conditions resulting in high wear, followed by diesel fuel and 20W-50 engine oil, as illustrated in Figure 4a.

Conversely, increasing the speed to 350 rpm at both load levels (50 and 100 N) resulted in a significant reduction in the wear observed across all lubricant types. This behavior is attributed to optimized hydrodynamic film formation, which effectively separates the contact surfaces and reduces direct metal-to-metal interaction, thereby minimizing wear. This inverse relationship between speed and wear rate is presented in Figure 4b,d. Regarding the wear mechanisms, the experimental results show a transition from adhesive to abrasive wear depending on the test conditions. Adhesive wear dominated under dry and low-speed conditions and was characterized by material transfer, surface welding, and plastic deformation. In contrast, abrasive wear became more prominent at higher speeds and loads, where wear debris plowed into softer surfaces, resulting in material removal.

These findings demonstrate the dynamic nature of tribological interaction and emphasize the importance of the lubricating film’s stability in controlling wear behavior. The specific wear rate (SWR) findings in the present study are further supported by previous studies. For instance, Peng [23] reported that increasing biodiesel concentration in petro-diesel blend causes a reduction in friction and wear rates, which can be attributed to the lubricating effects of fatty acid methyl esters and polar components. Similarly, a study on the spheroidal gravity of cast iron subjected to pin-on-disc testing with a palm biodiesel blend found that wear resistance significantly improved as biodiesel percentage increased [24]. These outcomes reinforce the trend observed in this work: biodiesel-based lubricants (safflower and camelina biodiesels) exhibited lower wear compared to dry conditions and improved performance with increasing speed. The presence of ester-based molecules and high polarity facilitates hydrodynamic film formation and limits asperity interactions, thereby minimizing wear.

3.3. Hardness and Surface Roughness Measurement Results

The hardness of both pin and disc specimens was evaluated using the Brinell hardness test, and a hardness value of 153 HB was observed for both compartments, indicating comparable mechanical resistance to deformation. This hardness value was maintained as the baseline parameter for determining the influence of lubrication and operating conditions on the surface topography. Surface roughness of samples was measured for (pin = 0.5, disc = 0.65).

Regarding surface roughness, safflower biodiesel exhibited a significant increase in the average roughness (Ra) by 61.03% at 350 rpm and 14.55% at 175 rpm, which can be attributed to the partial breakdown of the lubricating film at elevated speed and under high-load conditions. This breakdown results in an increase in the asperity contact formation of micro-ploughing marks on the surface. In contrast, the lowest Ra value was observed under 50 N and 350 rpm, showing only a 6.57% increase, suggesting optimal hydrodynamic film formation that effectively reduced asperity interactions and preserved the surface integrity (Figure 5a).

In terms of root average square roughness (Rq), significant increases of 36.87%, 37.74%, and 30.07% were recorded for safflower oil, camelina oil, and safflower biodiesel, respectively, as illustrated in Figure 5b. This behavior indicates that the presence of bio-lubricants improves the formation of protective film at moderate speeds and loads, but at higher speeds, the shear stress acting on the lubricating film may cause partial degradation, leading to increased surface roughness values. The maximum peak-to-valley roughness (Rz) was the most substantial for safflower oil, which was linked to the formation of larger abrasive grooves and micro-cutting effects on a wear track (Figure 6). This behavior suggests that while bio-lubricants such as safflower and camelina oils provide improved boundary and mixed lubrication performance, their stability under high-speed conditions may be compromised, resulting in localized abrasive wear.

These results further confirm that surface roughness parameters (Ra, Rq, Rz) are strongly influenced by the interplay between speed, load, and the ability of the lubricant to maintain a stable protective layer. These findings strongly align with previous studies such as Jikol et al. [25], who evaluated palm oil biodiesel lubricants using a ball-on-disc tribometer under varying loads (30–90 N) and speed (150 rpm). They reported that increasing biodiesel content reduced surface roughness and wear due to enhanced hydrodynamic film formation, but beyond certain speeds, surface shear stress initiated film breakdown and surface damage—mirroring the behavior observed in this work.

4. Conclusions

This study comprehensively evaluated the tribological performance of six bio-lubricants under varying operation conditions using a pin-on-disc tribometer. The results demonstrated that friction and wear behavior were significantly influenced by viscosity and physicochemical properties of the tested bio-lubricants.

Among the tested lubricants, camelina biodiesel and safflower oil exhibited superior tribological performance across all test conditions, as evidenced by their lower coefficient of friction and reduced wear rate. This enhanced performance is attributed to their higher viscosities, the presence of polar functional groups, and ability to form a stable lubricating film.

In contrast, diesel fuel and safflower biodiesel, which possess lower kinematic viscosities, showed weaker film formation capabilities, resulting in higher wear and surface roughness. The dominant wear mechanisms observed were a combination of adhesive and abrasive wear.

Under dry and low-speed conditions, adhesive wear prevailed due to direct metal-to-metal contact, resulting in material transfer and surface damage. However, as the speed increases, lubricant film formation improved, transitioning lubrication regimes from boundary to mixed or hydrodynamic, which in turn mitigated adhesive wear and limited abrasive interactions.

Coefficient of friction at various speeds is influenced by the combined effects of lubrication transitions from boundary to mixed or hydrodynamic lubrication regimes, the viscosity of tested oils, and surface interactions and asperity contact. As illustrated by the Stribeck curve, engine oil (20W-50) demonstrated the lowest friction at higher Stribeck numbers, indicating excellent lubrication under hydrodynamic conditions. Camelina oil started with higher friction, showed significant improvement as speed or viscosity increased. Diesel fuel and safflower biodiesel provided moderate lubrication performance—not as efficient as heavier oils, but still acceptable for certain conditions.