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How Series Hydraulic Hybrid Vehicles Work

In a regular vehicle, power is provided to the wheels by the engine through the transmission and drive shaft. In a parallel hydraulic hybrid there is a conventional engine and drive train system with hydraulic hybrid technology attached to the drive shaft (to learn more visit How Parallel HHVs Works). In a series HHV, the engine is not directly connected to the wheels. Instead a pump/motor, acting as a motor, uses high pressure fluid from an accumulator to propel the vehicle (for more on accumulators see the Accumulators page).

Here is how a series hydraulic hybrid words during its three modes of operation:

Light Acceleration/Short Cruising

When the accelerator pedal is pressed, the drive pump/motor uses high pressure fluid from the high pressure accumulator to rotate the wheels. The fluid that has been used to rotate the wheels is then at a lower pressure and is transferred into the low pressure reservoir.

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Download Video: MP4(13 seconds, 4,674kb), WebM (13 seconds, 938kb), Ogg (13 seconds, 983kb)

Silent animation of a series hydraulic hybrid vehicle accelerating.

Extended Cruising/Heavy Acceleration

When the pressure level drops to a certain point the engine will turn on and begin to take fluid from the reservoir, pressurize it and transfer the fluid to the drive pump/motor. Any excess high pressure fluid from the engine pump not needed by the drive pump/motor will be stored in the accumulator. Since it is not connected to the wheels, the engine will operate at its "sweet spot" where it runs most efficiently when it is on. The engine will shut off when it is no longer needed.

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Download Video: MP4(13 seconds, 4,695kb), WebM (13 seconds, 991kb), Ogg (13 seconds, 985kb)

Silent animation of a series hydraulic hybrid vehicle cruising.

Regenerative Braking

In city stop-and-go traffic a key way fluid is pressurized is the braking process. When the vehicle starts braking, the pump/motor uses the momentum of the vehicle to pressurize fluid from the reservoir and stores it in the accumulator. Later when the vehicle accelerates only this newly pressurized fluid is used. This process captures and reuses over 70% of the energy normally lost during braking.

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Download Video: MP4(12 seconds, 4,346kb), WebM (12 seconds, 802kb), Ogg (12 seconds, 916kb)

Silent animation of a series hydraulic hybrid vehicle braking.

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