A picture-fuzzy MULTIMOORA-based multi-criterion decision-making (MCDM) approach for optimal fuel selection of plastic oil in compression ignition engine

The sustainable alternative fuels show strong potential to replace fossil fuels in compression ignition (CI) engines. In this study, the feasibility of plastic pyrolysis oil as a CI engine fuel was evaluated through detailed engine tests, focusing on key performance and emission characteristics. Identifying optimal fuel parameters is complex due to the large number of variables and outputs involved. To support effective fuel selection, a fuzzy multi-criteria decision-making (MCDM) model was proposed. To more accurately represent decision preferences, picture fuzzy sets were incorporated. A knowledge-based quantitative technique was then applied to determine criterion weights using a knowledge measure for fuzzy numbers. Furthermore, an enhanced multi-objective optimization by ratio analysis plus the full multiplicative form (MULTIMOORA) method-supported by a unique score function and the Borda rule-was used to rank fuel candidates objectively. Experiments were conducted on a 3.5 kW CI engine operating at 1500 rpm to compare various blends: PO25/D75, PO50/D50, PO75/D25, and PO100. Results showed that the PO25/D75 blend achieved the highest overall performance index, demonstrating superior efficiency and balanced behaviour among the tested fuels. The sensitivity analysis shows that the top-ranked alternative, PO25D75 remained unchanged, confirming the robustness of the proposed model in identifying the optimal fuel. The comparative analysis with established MCDM methods i.e., SAW, TOPSIS, and COPRAS methods was found to be identical (Spearman's rank correlation coefficient ρ = 1.00), and highly consistent with VIKOR (ρ = 0.90), with only a minor exchange between the ranked alternatives. Importantly, all four methods identified PO25D75 as the best-performing fuel and Diesel as the second-best, confirming that the optimal fuel selection obtained by the proposed framework. This study highlights the promising potential of plastic oil blends, particularly PO25/D75, as sustainable and effective alternatives for CI engine applications.