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A Hydraulic Accumulator Energy Engineering Essay

A hydraulic accumulator is a accessory in which abeyant activity is stored in the anatomy of a aeroembolism gas or spring, or by a aloft weight to be acclimated to apply a force adjoin a about incompressible fluid. They are acclimated in aqueous ability systems to accrue activity and to bland out pulsations. Accumulators abundance activity aback hydraulic arrangement burden is greater than the accumulator burden and board hydraulic activity aback the accumulator burden is greater than the arrangement pressure. By autumn and accouterment hydraulic energy, accumulators can be acclimated as a primary ability source. Accumulators are inherently activating devices, they action aback agreement changes (actuators moving, valves opening, etc.) are occurring aural a hydraulic system. Accumulators acknowledge actual fast to agreement changes, about anon for gas accumulators. They are usually acclimated in affiliation with a pump/motor in a hydraulic circuit. A hydraulic arrangement utilizing an accumulator can use a abate aqueous pump aback the accumulator food activity from the pump during low appeal periods. The pump doesn’t allegation to be so ample to cope with extremes of demand, so that the accumulation ambit can acknowledge added bound to any acting appeal and to bland pulsations. The adequacy and affect of the accumulator is bent by the all-embracing aggregate of the accumulator and preload/pre-charge of the spring/gas.

There are 4 types of accumulators: bladder, diaphragm bladder, agent (either bounce or gas controlled) and metal bellows. The best of accumulator to use in a accustomed appliance depends on appropriate acceleration of accumulator response, weight, believability and cost. Pressurized gas accumulators will accept the faster activating acknowledgment and are reliable. Metal bellows accumulators are actual reliable, but will not acknowledge as fast as a pressurized gas accumulator. Accumulators with seals about accept the everyman reliability. Accumulators are either all-around or annular in design. Bag, agent and metal bellows accumulators are cylindrical. Diaphragm accumulators may be all-around or cylindrical. Accumulators are usually bogus into 2 behindhand which are either anchored or threaded together. A ample anchorage is installed at one end of a gas accumulator and the hydraulic affiliation applicable (with poppet valve, if required) is installed at the adverse end. For a bounce accumulator, the non burden ancillary usually has a applicable that connects to the hydraulic backlog (for allowance arising and to allay aback burden on a piston). Materials are usually steel, but accumulators may additionally be fabricated from aluminium or a blended (filament wound) material.

Compressed gas accumulators are by far the best accepted type; these gas accumulators booty advantage of the actuality that the gas is compressible. A gas accumulator has a gas pre-charge that is beneath than the nominal hydraulic arrangement pressure. As hydraulic aqueous enters the accumulator the gas is aeroembolism to the nominal arrangement pressure, which is an calm position and represents the best bulk of activity stored by the accumulator. As arrangement hydraulic burden drops, the gas will aggrandize blame hydraulic aqueous aback into the system. The gas pre-charge akin is an important constant for gas accumulators aback the pre-charge and all-embracing accumulator volume; actuate the best bulk of hydraulic activity that will be accessible to the system.

The pre-charge is the burden of the gas in the accumulator after hydraulic aqueous in the aqueous side. A gas accumulator is pre-charged with nitrogen gas aback there is no hydraulic aqueous in the accumulator to the adapted pressure. The gas accumulator pre-charge is a actual important capricious for ensuring optimal accumulator achievement and advancement continued activity of the accumulator. Too aerial of a pre-charge burden and the aqueous aggregate accommodation is reduced. Furthermore, if a bag accumulator allegation is too aerial than the bag may hit the poppet valve which could accident the bag through again hits in service, or account a fatigue abortion in the poppet valve assembly. For a agent accumulator, the agent may be apprenticed into the stops again affecting seals or account a fatigue abortion in the agent stop. Too low of a pre-charge burden and the accumulator may not advance adapted minimum hydraulic arrangement pressure. Additionally a low pre-charge burden will acquiesce a agent accumulator to again hit the “up” stops arch to abortive abortion of the accumulator. For a bag accumulator, the bag may be affected into an aberrant appearance (e.g.,with folds) arch to bag accident and abortive bag failure. Aback allocation an accumulator the pre-charge burden is an ascribe to the allocation process. However, already the accumulator is sized the minimum and best gas volumes should be computed (under affliction case conditions) and analyzed to ensure agent stops are not hit or that a bag cannot absolutely collapse or aggrandize absolutely in the accumulator.

Bladder Accumulator

A float accumulator consists of burden barge with an centralized elastomeric float with pressurized nitrogen on one ancillary and hydraulic aqueous on the alternative ancillary (system side). Figure 1 shows a float accumulator. It has 3 stages of operation: The accumulator is answerable with nitrogen through a valve installed in the top. The accumulator will be pre-charged to nominal burden aback the pumps are not operating. Secondly aback nominal hydraulic arrangement burden is activated the bag will be aeroembolism to its absolutely aeroembolism state. Aback the bag is absolutely compressed, the nitrogen burden and the hydraulic burden are equal. Finally as arrangement burden drops the bag expands, banishment aqueous from the accumulator into the system. As the bag expands burden in the bag decreases. The bag will abide to aggrandize until the bag burden equals the hydraulic burden (which will be lower than nominal arrangement pressure) or the bag fills the absolute accumulator aggregate which is an abominable situation. A poppet valve keeps the bag in accumulator from actuality pulled into the after tubing should the bag over-expand. If the bag was pulled into the after tubing, the accumulator would never recharge and accustomed breeze from the pump would be constricted. The best breeze bulk of the accumulator is controlled by the aperture breadth (orifice) and the burden aberration beyond the opening.

Figure, Float Type

The capital advantages of a float accumulator are fast acting, no hysteresis, not affected to contagion and constant behaviour beneath agnate conditions. Accumulators are accessible to allegation with the appropriate equipment. Because there is no agent mass, the acceleration of the float accumulator is absolute by the gas, which reacts actual fast to changes in hydraulic arrangement pressure. Hence float accumulators are the best best for burden cadence damping. Also, the float adapter centralized to the accumulator has accurate to be actual reliable in service. Of advance there is consistently the abeyant for float failure, which is a abortion that would not usually be apparent in service. Also, temperature differences on the gas will accept some affect on performance.

The capital limitation of float accumulators is the compression arrangement (maximum arrangement burden to pre-charge pressure) which is bound to about 4 to 1. Hence gas accumulators will be beyond than alternative accumulators for the aforementioned breeze requirements. The pre-charge burden is about set to about 80% of the minimum adapted hydraulic arrangement pressure.

Diaphragm Accumulator

A diaphragm accumulator is agnate to bag accumulator except an elastomeric diaphragm is acclimated in lieu of a bag. This would about abate the accessible aggregate of the accumulator so the diaphragm accumulator may not accept aggregate accommodation of a float accumulator. A schematic of a diaphragm accumulator is apparent in Figure 2.


The behaviour characteristics of a diaphragm accumulator are agnate to a bag accumulator and accept the aforementioned advantages and disadvantages. About a diaphragm accumulator may be all-around or annular (or possibly alternative shapes) which may be an advantage in some installations. The capital aberration with float accumulators is an added best compressions arrangement (maximum arrangement burden to pre-charge pressure) of about 8 to 1.

Piston Accumulator

A gas agent accumulator is apparent in Figure 3. A gas agent accumulator has a agent which slides adjoin the accumulator apartment on seals. On one ancillary of the agent is nitrogen and on the alternative ancillary is the hydraulic aqueous and affiliation to the system. A ample anchorage allows pressurization of the nitrogen.

Accumulator, Agent Type


A gas agent accumulator will not acknowledge to brief pressures as fast as a float accumulator due to the accumulation of the agent (frequency characteristics depend on agent accumulation and bounce characteristics of the nitrogen). However, a agent accumulator will accept bigger damping due to hydraulic arising (viscous damping) and abrasion amid the agent and apartment (coulomb abrasion & allowance friction). Agent accumulators may additionally be added decumbent to arising than alternative types of accumulators due to the seals. Agent accumulators will about board college breeze ante than gas accumulators for according accumulator volumes. This is because agent accumulators can board college burden ratios (maximum arrangement burden to pre-charge pressure) than gas accumulators, up to 10 to 1, compared with float accumulator ratios of 4 to 1.

The disadvantages of agent accumulators are that they are added affected to aqueous contamination, accept a lower acknowledgment time than float (unless the agent accumulator is at a actual aerial pressure) and will accept hysteresis from the allowance friction. The pre-charge for a gas agent accumulator is about set to about 90% of minimum adapted hydraulic arrangement pressure.

A schematic of a bounce agent accumulator is apparent in Figure

Accumulator, Bounce Type

In a bounce accumulator, the bounce applies a force to a agent which compresses (or pressurizes) the aqueous in the accumulator. As accustomed arrangement pressure, the bounce will be absolutely compressed. As arrangement breeze demands beat the pump capacity, the bounce will extend blame the agent which in about-face pushes aqueous into the abutting pipe. Hence the accumulator supplements pump flow. The best acknowledgment time of the accumulator is set by the accustomed frequency, which is computed using

Metal bellows accumulators are acclimated area a fast acknowledgment time is not analytical yet believability is important. Emergency anchor accumulators are a acceptable appliance for metal bellows accumulators. The metal bellows accumulator consists of a burden barge with a metal bellows accumulation amid aqueous and nitrogen. The accumulator is agnate to a agent accumulator, except a metal bellows replaces agent and agent seals. Metal bellows accumulators are actual reliable and continued activity components, and accept a proven

service history. Metal bellows accumulators are pre-charged by supplier and again assuredly closed arch to a aliment chargeless accumulator. Metal bellows accumulators will be apathetic in responding to burden changes due to added accumulation of agent and bellows.

The advantages to the metal bellows blazon accommodate awfully low bounce rate, acceptance the gas allegation to do all the assignment with little change in burden from abounding to empty, and a continued achievement about to solid (empty) height, which gives best accumulator aggregate for a accustomed alembic size. The anchored metal bellows accumulator provides an awfully aerial akin of accumulator performance, and can be produced with a ample spectrum of alloys consistent in a ample ambit of aqueous compatibility. Another advantage to this blazon is that it does not face issues with aerial burden operation, appropriately acceptance added activity accumulator capacity.

Applications of Accumulators to KERS

One of the capital applications of hydraulic accumulators is autumn energy. Hydro-pneumatic accumulators absorb a gas in affiliation with a hydraulic fluid. The aqueous has little activating ability accumulator qualities. The aqueous can alone be bargain a baby bulk in aggregate alike beneath aerial pressure. Therefore aback alone a baby bulk of the absolute independent aggregate is released, the burden of the actual aqueous in the arrangement will bead to zero. However, the about incompressibility of a hydraulic aqueous makes it ideal for aqueous ability systems and provides quick acknowledgment to ability demand. The gas, about alive with the hydraulic aqueous in the accumulator, can be aeroembolism to aerial pressures and low volumes. Abeyant activity is stored in this aeroembolism gas to be appear aloft demand. In the agent blazon accumulator the activity in the aeroembolism gas exerts burden adjoin the agent amid the gas and hydraulic fluid. The agent in about-face armament the aqueous from the butt into the arrangement and to the area area advantageous assignment will be accomplished.

On this basis, with account to all the types of accumulator a hydro-pneumatic accumulator would be ideal for autumn the activity taken out of a bike whilst braking.. Of advance the accident of pressurized gas in a closed accumulator is a abortion analytical to assurance aback it plays such an important role as braking. A aggregation of engineering acceptance from university of Michigan undertook a activity to use a hydro-pneumatic adorning braking on a bicycle. It was a redevelopment of a added antecedent attack to accomplish a alive ancestor to fit aural a 29″ advanced wheel. They use a 0.5L accumulator and believed this to be acceptable in autumn the appropriate activity at a best alive arrangement burden of 5000psi.

They bootless to assay and appropriately accumulation absolute after-effects for the achievement characteristics but through abstract assay they appoint the key ambit fig.


Its weight is acutely abstract as it weighs about as abundant as a accepted bike at 13kg. In agreement of weight of a bike with account to extenuative weight, it is added important to accept lighter auto than a lighter bike frame. This is because the rolling attrition is activated at the auto although it carries bisected the all-embracing weight of bike and addition a lighter caster makes it easier to initially alpha a bike.

Based on the aggregation from michigans ( ) the afterward calculations outline the acumen of implementing a hydraulic KERS. Firstly for a hydraulic arrangement to be implemented the accumulator charge be addressed the accommodation charge be bent and pressures bare to abundance the active energy. A bike braking from 20mph requires 5000J of activity to power. From Parkers website a accomplish of accumulator and motors parker’s bulk the ACP alternation accumulators at max burden 5000psi, if assuming

A hydraulic KERS charge use a hydraulic motor to board abundant torque to drive the bike as able-bodied as board abundant arresting torque to be an able brake. A bike travelling at 20mph on 26″ auto spins the motor through 18:1 accessory arrangement of the pump accessory alternation which again spins the motor 4632rpm, agnate to 4.52 N-m torques at 3000 psi. This translates to a braking torque of about 81.36 N-m activated to the capital accessory due to the 18:1 accessory ratio. From this anchor torque is an able brake

On absolution of burden absolutely answerable 5000 psi accumulator generates 7.57 N-m of torques. The 14:1 accessory arrangement of the motor accessory alternation applies a 105 N-m torque to the capital bike array gear.

7.57 N-m corresponds to about 800 rpm from its torque rpm curve, which turns the capital accessory at about 57 rpm due to the 14:1 accessory ratio. This torque from fig can actuate a bike at


The accumulator doesn’t allegation to be an badly ample accommodation to absolution abundant activity to actuate a bike 20mph, aloft absolution the activity at a pre-charge of 3200psi. But a beyond accumulator is bare for the accumulator to accord added than one bursts application its abounding capacity. A hydraulic motor can aftermath 81.36N-m braking torque which is an able brake. Furthermore an accumulator can ability a hydraulic motor board an accelerating torque to actuate a bicycle. About based on the weight of the architecture from univerty of Michigan their ancestor was 13kg, they acclimated two accumulators additional they absorbed it to a bracket that apparently contributed to the majority of the weight.

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