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:
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.
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.
Silent animation of a series hydraulic hybrid vehicle cruising.
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.