Main Content

Spring-Loaded Accumulator (IL)

Accumulator with a spring used for energy storage in an isothermal liquid system

  • Library:
  • Simscape / Fluids / Isothermal Liquid / Tanks & Accumulators

  • Spring-Loaded Accumulator (IL) block


This block models a spring-charged accumulator in an isothermal liquid network. The accumulator consists of a preloaded spring and a liquid chamber.

As the liquid pressure at the accumulator inlet becomes greater than the preload pressure, liquid enters the accumulator and compresses the spring. A decrease in the liquid pressure causes the spring to decompress and discharge stored liquid into the system. The separator motion is restricted by a hard stop when the liquid volume is zero and when the liquid volume is at the liquid chamber capacity.

The inlet liquid resistance and spring properties, such as the spring inertia and damping, are not modeled. The spring pressure is assumed to be linear with respect to the liquid volume. The flow rate is positive if liquid flows into the accumulator.

This diagram represents a spring-loaded accumulator.


Volume of the liquid in the accumulator.


Liquid chamber capacity


Pressure at full capacity


Liquid pressure in the liquid chamber, which is equal to the pressure at the accumulator inlet.


Hard-stop contact pressure.

KstiffHard-stop stiffness coefficient.
KspringSpring gain.

Mass flow rate of liquid coming into port A.


Density of the liquid in the liquid chamber.

Conservation of Mass

Conservation of mass is represented by the following equation.

{p˙IdρIdpIVL+ρIV˙L=m˙A,     compressibility onρIV˙L=m˙A,                       compressibility off   


V˙L={p˙IKspring,            if 0 <VL<VCp˙IKspring+Kstiff,                  else

Conservation of Momentum

Conservation of momentum is represented by the following equation.




pHS={(VLVC)Kstiff,    if VLVCVLKstiff,                          if VL  00,                                    else


Use the Variables tab to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables.



expand all

Hydraulic conserving port associated with the accumulator inlet. The flow rate is positive if liquid flows into the accumulator.


expand all

Amount of liquid that the accumulator can hold.

Spring pressure when the liquid chamber is empty.

Spring pressure when the liquid chamber is at capacity.

Proportionality constant of the hard-stop contact pressure with respect to the liquid volume penetrated into the hard stop. The hard stops are used to restrict the liquid volume between zero and liquid chamber capacity.

Whether to model any change in fluid density due to fluid compressibility. When Fluid compressibility is set to On, changes due to the mass flow rate into the block are calculated in addition to density changes due to changes in pressure. In the Isothermal Liquid Library, all blocks calculate density as a function of pressure.

Introduced in R2020a