Hydraulic hybrid vehicle

Hydraulic hybrid vehicles, or HHVs use a pressurized fluid power source, along with a conventional internal combustion engine (ICE), to achieve better fuel economy and reductions in harmful emissions. They capture and reuse 70%-80% of the vehicle's braking/decelerating energy compared to 25% for electric hybrids. For trucks and buses, this can also be less expensive than electric systems, due to the price of batteries required for the latter. Hydraulic hybrid vehicle systems can also weigh less than electric systems, due to the high weight of the batteries. This can lead to a lower impact on payload capacity, especially for heavy vehicle classes.

Hydraulic hybrid vehicle systems consists of four main components: the working fluid, reservoir, pump/motor (in parallel hybrid system) or in-wheel motors and pumps (in series hybrid system), and accumulator. In some systems, a hydraulic transformer is also installed for converting output flow at any pressure with a very low power loss. In an electric hybrid system, energy is stored in the battery and is delivered to the electric motor to power the vehicle. During braking the kinetic energy of the vehicle is used to charge the battery through the regenerative braking. In hydraulic hybrid system, the pump/motor extracts the kinetic energy during braking to pump the working fluid from the reservoir to the accumulator. Working fluid is thus pressurized, which leads to energy storage. When the vehicle accelerates, this pressurized working fluid provides energy to the pump/motor to power the vehicle. For a parallel hybrid system, fuel efficiency gains and emissions reductions result from reduced mechanical load on the internal combustion engine due to the torque provided by the hybrid system.

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