Simulation of Medium-Duty Truck Equipped with an Air-Assisted Hydraulic Hybrid Drive System

Abstract

Heavy-duty commercial vehicles such as refuse trucks, urban delivery vans, transit buses and construction machinery are characterized by frequent stop-and-go duty cycles in which a significant share of the kinetic energy delivered by the prime mover is dissipated as heat at the friction brakes. The Hydraulic Hybrid Vehicle (HHV) architecture stores this otherwise-wasted braking energy in a hydro-pneumatic accumulator and releases it during the subsequent acceleration phase, exploiting the very high-power density of fluid-power components compared with electrochemical batteries. This paper presents an updated review of HHV powertrain architectures, including parallel, series and power-split configurations, together with the dynamic models of their principal sub-systems: the diesel engine, the bent-axis variable-displacement pump/motor, the hydro-pneumatic accumulator, the air tank with pressure exchanger, and the multi-stage reciprocating compressor. A forward-facing closed-loop simulation environment developed in MATLAB/Simulink is used to evaluate the theoretical and real-world energy-recovery efficiency of a Class VI medium-duty truck (gross mass 7,340 kg) equipped with an air-assisted series hydraulic hybrid drive over the Federal Urban Driving Schedule (FUDS). Theoretical recovery efficiencies of up to 0.89 and real (component-loss-corrected) values of up to 0.61 are obtained, with both metrics decreasing as the share of cruising relative to total cycle time increases. Recent peer-reviewed and demonstration data are reviewed: real-driving emissions tests reported in 2024 confirm fuel-consumption reductions of approximately 17% on urban routes, while Electro-Hydraulic Hybrid Vehicles (EHHV) configurations combining a battery, an electric machine and a hydraulic accumulator are emerging as a promising response to the European Union 2024/1257 (Euro VII) and 2024 CO₂ standards that mandate a 90% CO₂ reduction for new heavy-duty vehicles by 2040. The discussion concludes that hydraulic hybridization remains the most cost-effective and durable pathway for vocational duty cycles dominated by high-power, short-duration transients and that further work should focus on intelligent supervisory control, plug-in air/oil pre-charging, and integration of the accumulator with battery-electric and fuel-cell powertrains.