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How Accumulators Work
The accumulators use nitrogen to keep the hydraulic fluid pressurized. When the fluid is pumped into an accumulator the nitrogen (N2) inside the accumulator is compressed. When all the hydraulic fluid is in an accumulator designed for high pressure side of an HHV, the pressure of the nitrogen reaches 5000 pounds per square inch (psi). If empty of fluid, the pressure of the nitrogen is about 2000 psi. The pressure of the nitrogen in the low pressure reservoir will vary from 60 psi when empty to 200 psi when full.
EPA has used two types of accumulators in their hydraulic hybrids:
Bladder Accumulator Type
In this type of accumulator hydraulic fluid compresses a nitrogen-filled bladder to create pressure. In HHVs, high pressure accumulators can operate between 2000 and 7000 pounds-per-square inch (psi).
Advantage: Highest efficiency with tests showing 97 percent energy retainment.
Disadvantage:Nitrogen will permeate the bladder material over time and need to be periodically recharged.
Piston Accumulator Type
In this type of accumulator, pressure is created when nitrogen is compressed in a thin-walled metal cylinder shell by the hydraulic fluid pushing on a metal piston.
Advantage: Virtually no nitrogen escapes so they will not have to be recharged.
Disadvantage: A bit heavier, and less efficient than the bladder model.