The main purposes of this research were to study the diesel engines' performance and emission characteristics of quaternary fuels, as well as to analyze their tribological properties. The quaternary comprised waste plastic pyrolysis oil, waste cooking oil biodiesel, palm oil biodiesel, and commercial diesel. Their compositions were analyzed by gas chromatography and mass spectrometry. By using mechanical stirring, four quaternary fuels with different compositions were prepared. Because Malaysia is expected to implement B30 (30% palm oil biodiesel content in diesel) in 2025, B30a (30% palm oil biodiesel and 70% commercial diesel) mixture was prepared as a reference fuel. In total, 5%, 10%, and 15% of each waste plastic pyrolysis oil and waste cooking oil biodiesel were mixed with palm oil biodiesel –commercial diesel mixture to improve fuel characteristics, engine performance, and emission parameters. The palm oil biodiesel of the quaternary fuel mixture was kept constant at 10%. The results were compared with B30a fuel and B10 (10% for palm oil biodiesel and 90% for diesel; commercial diesel). The findings indicated that compared with B30a fuel, the brake power and brake thermal efficiency of all quaternary fuel mixtures were increased by up to 2.78% and 9.81%, respectively. Compared with B30a, all quaternary fuels also showed up to a 6.31% reduction in brake-specific fuel consumption. Compared with B30a, the maximum carbon monoxide and carbon dioxide emissions of B40 (60% commercial diesel, 10% palm oil biodiesel, 15% waste plastic pyrolysis oil and 15% waste cooking oil biodiesel) quaternary fuel were reduced by 19.66% and 4.16%, respectively. The B20 (80% commercial diesel, 10% palm oil biodiesel, 5% waste plastic pyrolysis oil and 5% waste cooking oil biodiesel) quaternary blend showed a maximum reduction of 41.86% in hydrocarbon emissions collated to B30a. Compared with B10, the average coefficient of friction of the quaternary fuel mixture of B40, B30b (70% commercial diesel, 10% palm oil biodiesel, 10% waste plastic pyrolysis oil and 10% waste cooking oil biodiesel), and B20 were reduced by 3.01%, 1.20%, and 0.23%, respectively. Therefore, the quaternary blends show excellent utilization potential in diesel engine performance.
