Shanghai Siken Industries Inc.

Web:  www.siken-inc.com

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Hydraulic accumulators rely on nitrogen (N₂) gas to keep the hydraulic fluid under pressure. When fluid is pumped into the accumulator, the nitrogen inside is compressed. On the high-pressure side of a Hydraulic Hybrid Vehicle (HHV), the nitrogen pressure reaches approximately 5,000 psi when the accumulator is full of fluid and drops to about 2,000 psi when empty. In the low-pressure reservoir, nitrogen pressure varies from roughly 60 psi (empty) to 200 psi (full).

Bladder Accumulator

In a bladder-type accumulator, hydraulic fluid compresses a nitrogen-filled elastomer bladder to generate pressure. High-pressure versions used in HHVs typically operate in the 2,000–7,000 psi range.

Advantage: Highest efficiency — laboratory tests have shown energy retention of up to 97%.

Disadvantage: Nitrogen slowly permeates the bladder material over time, requiring periodic recharging.

Piston Accumulator

In a piston-type accumulator, hydraulic fluid pushes a metal piston that compresses nitrogen inside a thin-walled metal cylinder.

Advantage: Virtually no nitrogen loss, so recharging is normally unnecessary.

Disadvantage: Slightly heavier and somewhat less efficient than bladder designs.