Accumulators Primary and Secondary functions
“The only thing I could add would be that the accumulators don't serve as shock absorbers in any meaningful sense. The purpose of the two large ones is to maintain the system pressure when the suspension plungers are quickly extended (and the pump can't keep up). The smaller one is present on the rear axle to offset the increased flow resistance caused by the longer return pipe to the oil cooler.”
Dear Forum Member, I partially agree with your comment (above) but here are some points that do not all together agree with your specific phrase “
meaningful sense”.
It is common knowledge that when
we experience a very hard suspension and additionally the ABC-oil level in the reservoir does not sink to the lower value on engine start, then the two or either of the accumulators (front and rear axle) can be defective. With strong bumps a brief red ABC message comes on the screen, even though the pressure of the tandem pump is still about 190 bar. As it gets worse, this message will come on even with small bumps. The diagnosis then gives the error C1525-016: level calibration has not been performed successfully. New accumulators are the solution to the problem.
Because ride becomes hard and bouncy due to accumulator sphere damage, this shows the (partial) shock absorption property of the compressed nitrogen and diaphragm setup of accumulator spheres.
In my post I talked about shock absorption with regards to compression and rebound and these effects can be smoothened out with springs, air or any decompressable material. There was a time when bicycles only had the air in their tyres as a means of absorbing shocks. Then springs in the riders’ seat were introduced to further enhance a smoother ride. With purpose built cylinder shock absorbers, the spring eventually gave way to advanced systems.
The Mercedes W215 W220 vehicles using ABC Hydraulic systems use several components (all working in tandem) to absorb shock.
These include: 1. The Air in tyres 2. Rubber in bushes 3. Gas filled Shock absorbers 4. Pressure accumulators and 5. Foam in the seats of the car.
The hydraulic accumulators store fluid under pressure and serve a number of functions within a hydraulic system. With a specific amount of fluid stored under pressure it is released when required to perform a specific task in the hydraulic system. The accumulators provide several functions, which are:
• Energy storage
• Compensation of temperature fluctuations
• Cushioning of pressure shocks
• Reduction of Fluid Hammer during sudden switching of valves
• Dampening vibrations
• Leakage compensation of internal seals and valves
Applications that utilise large flows at high speeds primarily use accumulators for energy storage. When required, the accumulator pushes fluid into the hydraulic circuit to add to the pump flow. When the actuators in the hydraulic system are not in motion, the accumulator will refill. Accumulators can also operate as energy sources during normal operation of the system (i.e. cushioning).
Gas-pressurised accumulators use nitrogen both because it is relatively inert. This prevents danger of explosion in case of leakage of the bladder, diaphragm or piston. Also, nitrogen is relatively cheap because it is the most abundant gas in our atmosphere.
Hydraulic fluid is not very compressible and in fact, hydraulic oil will compress less than 0.5% per 1,000 psi. So at an astounding pressure of 10,000 psi, oil will be compressed by a measly 4%. In actual hydraulic systems, the compression can actually be higher due to entrained air within the oil.
Decompression of high pressure fluids are a definite concern, as a lot of energy can be released, that energy release typically happens in fractions of a second. High pressure systems, require sub circuits such as (gas filled) Accumulators to control decompression. Gases are highly compressible.
The short explanation of accumulator operation is this: Air bag is filled with gas, hydraulic fluid is squeezed into the space taken up by the gas, gas tries to push out the hydraulic fluid, and opening a downstream valve allows the gas to push out the hydraulic fluid. This is done to store energy, to compensate for leakage or to reduce shock or vibration.
Because a small pump can be used with an accumulator to provide high flow in hydraulic systems, size and cost are saved on the pump which is the prime oil mover making this combination method of storing energy, economical and efficient.
Energy storage doesn’t have to be for continuous cycling, and the accumulators are used for emergency energy during pump failure. The pressurized fluid in the accumulator (with the shut-off solenoids) can be used to retain pressure in the system to move the vehicle to a safe position where it can stay until power is restored or the malfunction corrected.
For use as leakage compensation, an accumulator can last for extended periods of time. An accumulator can provide constant pressure, even while flow is slowly lost to leakage through piston seals or control valve clearances. When accumulator pressure drops to a critical point, a pressure switch will tell the pump to come on for only as long as it takes to refill the accumulator.
Because of the physical properties of hydraulic fluid, it is easy to transmit shock and vibration through the pipes, tubes and hoses of the system. Some pumps, for example, create pulses of pressure when the pistons or gears reach their outlet port. Accumulators built into the system absorb these pulses and shocks like the struts on your cars suspension, absorbing bumps in the road and providing smoother operation.
Sometimes pressure spikes are quite large, but by adding an accumulator into the return line, decompression shocks can be absorbed and prevent damage to the downstream components, which in a return line, are often not rated for high pressure.
History
The first Hydraulic accumulator was made by William Armstrong in 1846 where he built a crane powered by water of the town mains at Newcastle, United Kingdom.
Accumulators are devices used to store fluid power to do the following:
• Dampen pulsations and shocks of a periodic nature
• Increase the speed of the operational circuit.
• Standby power supply circuits.
• Surge reduction circuits
• Compensate for internal leakage
Accumulators
• A Hydraulic Accumulator is energy storage device.
• It is pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source.
• The external source used can be a spring, a raised weight, or a compressed gas.
• The main reasons that an accumulator is used in a hydraulic system, is that the pump doesn’t need to be so large to cope with extremes of demand and supply circuit can respond quickly to any temporary demand and to smooth pulsation.
Benefits of the Accumulator
• Lower installed system costs, accumulator assisted hydraulics can reduce the size of the pump and electric motor which results in a smaller amount of oil used, a smaller reservoir and reduced equipment costs.
• Less leakage and maintenance costs, the ability to reduce system shocks will prolong component life, reduce leakage from pipe joints and minimize hydraulic system maintenance costs.
• Improved performance, low inertia bladder accumulators can provide instantaneous response time to meet peak flow requirements. They can also help to achieve constant pressure in systems using variable displacement pumps for improved productivity and quality.
• Reduced noise levels, reduced pump and motor size couple with system shock absorption overall machine sound levels and results in higher operator productivity.
• Flexible design approaches. A wide range of accumulator types and sizes, including accessory items, provides a versatile and easy to apply design approach.
• Reduced energy costs, cost savings of up to 33% are achievable in high performance industrial machinery using accumulators.
Diaphragm Accumulators used in the W215 and W220 are similar to bladder type, expect an elastomeric diaphragm is used in place of a bag.
• This would typically reduce the usable volume of the accumulator, so the diaphragm accumulator may not have volume capacity of a bladder accumulator.
• Diaphragm accumulator may be spherical or cylindrical.
• The main difference with bladder accumulator is an increased maximum compression ratio of approximately 8:1
• It is low weight, compact design and good for shock applications (good response characteristics)
•
Their main Advantage: Highest efficiency with tests showing 97 percent energy retainment.
•
Their main Disadvantage: Nitrogen will permeate the foam bladder material over time and need to be periodically recharged.
W215 / W220 ABC Hydraulic System
The hydraulic system in the Mercedes type 215 and 220 contains about 4 litres of CHF11s fully synthetic oil.
The pressure supply operates in normal driving operation with an ABC-oil pressure between 180 and 200 bar.
Pressure peaks in the system are intercepted in the front valve unit pressure supply (52) by a pulsation damper (52a). This unit prevents and relieves pressures over 200 bar through a pressure relief valve. Also in the return manifold at the front valve block unit Y36 / 1 and in the pressure valve unit 52 along with its pulsation damper (52a) are back pressure valves running a minimum pressure of 10 bar in the return line as a system safely.
In the main steel-pressure accumulators, a membrane separates the oil chamber from the gas chamber with a gas volume of 650 cc, gas pressure is 100 bar, but imprinted outside the Pulsation sphere is value of 200 bar.
Working pressure: red, control pressure: blue, return: green
1 tandem pump, 2a oil filter, 9 oil radiator, 9 oil cooler, 53 accumulator return, B4 / 5 pressure sensor, y1 control valve, y2 check valve , 14 accumulators front, 4 axles accumulators rear axles, B40 / 1 temperature sensor, 52 pulsation dampener, 52b pressure relief valve 56/57 vent valves
Citroen Pressurized Spheres
A fundamental component of the Citroën hydraulic system is the pressurized sphere. They are used for a variety of purposes, Pressure maintenance, Height levelling, Load bearing and Shock absorption; but each one functions in exactly the same way.
The Main Accumulator
The first use of a pressurized sphere is as the main accumulator. It collects high-pressure fluid from the pump and distributes it to the subsystems. It is connected to an electronic pressure regulator switch. When pressure is low, e.g. at start-up time, the switch is tripped, engaging the pump, filling the pressurized sphere with fluid. When the pressure inside the sphere reaches a certain point the switch is cut and the supply of high-pressure fluid stops.
Each of the subsystems draws more or less directly from this sphere as pressure is needed to perform its specific function. Even if there is a constant draw on the accumulated high-pressure fluid, the pump will only be engaged intermittently to ensure that the pressure in the accumulator never gets too low.
The "Load-bearing" Shocks
Each load-bearing shock is a simple piston with a pressurized sphere on top of it. Hydraulic fluid can pass back and forth between the piston and the sphere. The pressure of the compressed nitrogen in the sphere counteracts the force of the weight of the body. In this way the spheres function as springs or torsion bars would in conventional cars. An iris in the orifice between the piston and the sphere produces a dampening effect.
Pressure flows from the hydraulic circuit to the suspension cylinders, pressurizing the bottom part of the spheres and suspension cylinders. Suspension works by means of a piston forcing hydraulic fluid into the sphere, compacting the nitrogen in the upper part of the sphere; damping is provided by a two-way 'leaf valve' in the opening of the sphere. Hydraulic fluid has to squeeze back and forth through this valve which causes resistance and controls the suspension movements. It is the simplest damper and one of the most efficient.
The Mercedes W215 / W220 hydraulic suspension
41
Rear suspension strut
a - Rubber mount jounce stop n - Suspension strut support bearing
b - Top wiper o - Hydraulic pipe
c - Top spring plate p - Top guide sleeve
d - Piston rod tube q - High-pressure seal
e - Piston rod r - Steel spring
f - Position magnet s - Protective boot
g - Hydraulic cylinder t - Hydraulic cylinder bottom stop
h - Bottom guide sleeve u - Shock absorber piston rod
i - Bottom wiper v - Shock absorber
j - Inner bump stop w - Lower mount
k - Outer bump stop x - High-pressure seal
l - Bottom spring plate
m - Rubber mount rebound stop
B22/1 - Left rear suspension strut motion sensor
822/ - Right rear suspension strut motion sensor
The suspension strut forms a unit comprising hydraulic cylinder, steel spring and shock absorber. The suspension strut motion sensor is integrated in the hydraulic cylinder. The shock absorber is a twin-tube shock absorber with gas preload.
Function
The weight of the vehicle is supported by the oil in the hydraulic cylinder and by a coil spring connected in series. The coil spring is supported at the wheel on a spring plate rigidly connected to the shock absorber tube. The spring is supported at the body on the single-acting hydraulic cylinder (plunger).
By adjusting the hydraulic cylinder in the direction of the suspension strut and the resultant change in spring length, additional forces are produced which influence the suspension and damping in the frequency range up to 5 Hertz.
The vehicle level is raised and lowered by filling and emptying the oil chamber which reduces or increases the length of the suspension strut as a result.
To move the hydraulic cylinder out or in, oil is delivered to the suspension strut or oil flows out of the suspension strut into the return flow circuit.
A passive twin-tube gas pressure shock absorber connected in parallel with the hydraulic cylinder and suspension strut is responsible for damping the higher frequency wheel vibrations. The hydraulic cylinder takes over the active damping of the lower frequency body vibrations.
A Pressurised Condition
B Depressurised Condition
g- Hydraulic cylinder y- oil chamber r- steel spring v- shock absorber
In my conclusion, the accumulators’
Primary purpose is to store energy and maintain pulse free pressure in the system during operation. However (as in the Citroen system) the accumulators also have a
Secondary purpose of assisting the system (
of included gas shock absorbers) in the dampening of compression, rebound and liner shocks occurring in both static and dynamic use.
