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Purpose: The purpose of this paper was to investigate the lubricity of palm biodiesel (PB)–diesel fuel with plastic pyrolysis oil (PPO) and waste cooking biodiesel (WCB). Design/methodology/approach: Three quaternary fuels were prepared by mechanical stirring. B10 (10% PB in diesel) fuel was blended with 5%, 10% and 15% of both PPO and WCB. The results were compared to B30 (30% PB in diesel) and B10. The lubricity of fuel samples was determined using high-frequency reciprocating rig in accordance with ASTM D6079. The tribological behavior of all fuels was assessed by using scanning electron microscopy on worn steel plates to determine wear scar diameter (WSD) and surface morphology. The reported WSD is the average of the major and minor axis of the wear scar. Findings: The addition of PPO and WCB to B10 had improved its lubricity while lowering wear and friction coefficients. Among the quaternary fuels, B40 showed the greatest reduction in coefficient of friction and WSD, with 7.63% and 44.5%, respectively, when compared to B10. When compared to B30a, the quaternary fuel mixes (B40, B30b and B20) exhibited significant reduction in WSD by 49.66%, 42.84% and 40.24%, respectively. Among the quaternary fuels, B40 exhibited the best overall lubricating performance, which was supported by surface morphology analysis. The evaluation of B40 indicated a reduced adhesive wear and tribo-oxidation, as well as a smoother metal surface, as compared to B20 and B30b. Originality/value: Incorporation of PPO and WCB in PB–diesel blend as a quaternary fuel blend in diesel engines has not been reported. Only a few researchers looked into the impact of PPO and WCB on the lubricity of the fuel.

Engine oil's lubricity and tribological properties are changed when it is diluted with unburned fuels. SAE40 lubricating oil (LO) samples were contaminated with known percentage (5%) of fuels (commercial diesel (B10), diesel-palm biodiesel (PB) blend (B30a), and diesel–PB–plastic pyrolysis oil (PPO)–waste cooking biodiesel (WCB) blends (B20, B30b, and B40)). Utilizing four-ball tester, the influence of each of these fuels on wear and frictional properties of LO was measured, and wear type on worn surfaces was examined. LO diluted by B10 had a high coefficient of friction (COF) (11.52%) with severe abrasive and adhesive wear than mineral lubricant followed by B30a (3.00%). LO mixed with quaternary fuels had a lower COF, wear scar diameter (WSD) and polishing wear. When compared to SAE40–B30a, SAE40–B30b had fewer frictional properties with adhesive wear. When compared to SAE–40 mineral lubricant, SAE40–B20 had the smallest increase in COF value (1.90%), followed by SAE40–B30b (2.17%), and SAE40–B40 (2.63%). While the WSD values for all tested samples are reduced by 3.12, 5.68, 6.33, 11.11, and 17.33%, respectively, when compared to pure lubricant for SAE40–B10, SAE40–B20, SAE40–B30a, SAE40–B30b, and SAE4–B40. PPO and WCB in B10 are found to reduce lubricant degradation when compared to diesel-PB blend and commercial diesel.

This research was aimed to examine the diesel engine's performance and emission of secondary fuels (SFs), comprising waste plastic oil (WPO) and palm oil biodiesel (POB), and to analyze their tribological properties. Their compositions were analyzed by gas chromatography-mass spectrometry (GC-MS). Five SFs (10-50% POB in WPO) were prepared by mechanical stirring. The results were compared to blank WPO (WPO100) and Malaysian commercial diesel (B10). WPO90 showed the maximum brake power (BP) and brake torque (BT) among the SFs, and their values were 0.52 and 0.59% higher compared to B10, respectively. The increase in POB ratio (20-50%) showed a negligible difference in BP and BT. WPO70 showed the lowest brake-specific fuel consumption among the SFs. The brake thermal efficiency (BTE) increased with POB composition. The maximum reductions in emission of hydrocarbon (HC, 37.21%) and carbon monoxide (CO, 27.10%) were achieved by WPO50 among the SFs. WPO90 showed the maximum reduction in CO2 emission (6.78%). Increasing the POB composition reduced the CO emissions and increased the CO2 emissions. All SFs showed a higher coefficient of friction (COF) than WPO100. WPO50 showed the minimal increase in COF of 2.45%. WPO90 showed the maximum reduction in wear scar diameter (WSD), by 10.34%, compared to B10. Among the secondary contaminated samples, SAE40-WPO90 showed the lowest COF, with 5.98% reduction compared to SAE40-WPO100. However, with increasing POB content in the secondary contaminated samples, the COF increased. The same trend was also observed in their WSD. Overall, WPO90 is the optimal SF with excellent potential for diesel engines.