Improving the operational efficiency of a heavy-duty diesel bus engine through parallel hybridization: A Tehran-specific case study

Tehran’s severe air pollution problem, mainly caused by heavy-duty diesel vehicles, calls for an urgent move toward cleaner and more sustainable transportation. This study uses simulation to explore converting a conventional diesel city bus into a parallel Hybrid Electric Vehicle (HEV), tailored to Tehran’s specific urban driving conditions. The original bus, equipped with a Mercedes-Benz OM457 LA engine, was modeled and validated with real-world data. Then, a parallel hybrid powertrain was designed and simulated using the ADVISOR software, calibrated to a digitized Tehran bus driving cycle. The results show that the hybrid setup cut fuel consumption from 43.2 L/100 km to 36.8 L/100 km, improving efficiency by 15.4%. It also boosted performance, reducing the 0–50 km/h acceleration time from 8.7 to 6.8 s (a 21.8% improvement), even with a slight 1.3% increase in overall vehicle weight. Although the hybrid system raises initial costs by 18.1%, the findings confirm its technical feasibility as an effective way to lower fuel use and emissions in Tehran’s bus fleet. This study offers valuable local insights to guide sustainable transport policies and highlights the need to develop domestic production of hybrid components to make the technology more economically viable.