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The potential of using a KERS on a bicycle to store hydraulic energy can be achieved using a device such as a hydraulic accumulator. In a hydraulic accumulator the potential energy is stored in the form of a compressed gas or spring, which is used to exert a force against a relatively incompressible fluid.
Accumulators store energy when the hydraulic system pressure is greater than the accumulator pressure and releases hydraulic energy in the opposite case. By storing and providing hydraulic energy, accumulators can be used as a primary power source for a KERS.
Accumulators are naturally dynamic devices; meaning they function when configuration changes, for example, valves opening and closing. Accumulators respond rapidly to configuration changes, and nearly instantaneously for gas accumulators. They are usually used in conjunction with a pump/motor in a hydraulic circuit. A hydraulic system utilizing an accumulator can use a smaller fluid pump since the accumulator stores energy from the pump during low demand periods. The pump doesn't need to be so large to cope with extremes of demand, therefore the supply circuit can respond more quickly to any temporary demand and to smooth pulsations.
There are four types of accumulator: bladder, diaphragm bladder, piston (spring or gas controlled), and metal bellows. Depending on the application, the choice of most suited is based on the required speed of accumulator response, weight, reliability and cost. Compressed gas accumulators are the most commonly used type since they generally have the faster dynamic response and are most reliable. Accumulators with seals will generally have the lowest reliability as there is the potential for leaks.
Pressurised gas accumulators take advantage of the fact, that the gas is compressible. The potential to store energy and the affect of the accumulator is dictated by its overall volume and pre-charge of the gas. The pre-charge is the pressure of the gas in the accumulator when there is no hydraulic fluid within the accumulator. Too high of a pre-charge pressure, reduces the fluid volume capacity, and limits the maximum amount of hydraulic energy that will be available to the system.
A gas accumulator has a gas pre-charge, which is less than the nominal hydraulic system pressure. When hydraulic fluid enters the accumulator, the gas is compressed to the nominal system pressure, which is in an equilibrium position and corresponds to the maximum amount of energy that can be stored. As system hydraulic pressure drops, the gas will expand forcing hydraulic fluid back into the system.
Most gas accumulators are bladder type, made up of a vessel divided into two volumes, by a flexible membrane. Within the bladder, nitrogen is stored under high pressure, which is an efficient and safe medium since the ability of gas to store energy increases exponentially as pressure rises and because of its inert properties. As fluid from the hydraulic circuit, enters the vessel (under system pressure) and impinges against the bladder, the gas is compressed allowing energy to be stored. The release of energy when required is achieved via conventional valve arrangement.
Figure Bladder Accumulator  and 
Use of hydraulic KERS commercially
There are, a number of emerging systems that allow the operators of vehicles to reduce both fuel consumption and unwanted emissions, specifically to vehicles that are subject to constant stop-start operations, like for example buses, refuse collection vehicles (RCV).
Constant stop-start operations, such as braking in large vehicles, produce considerable kinetic energy, which is wasted as heat. Capturing this energy using conventional hydraulic technology enables it to be stored and then returned to the vehicle systems. The potential uses are not just limited to aid subsequent acceleration (reducing the energy required from the engine), but can potentially power ancillary equipment. For example, RCVs can use stored energy to drive the hydraulic refuse compacting and packing mechanisms. This enables a significant reduction of engine speeds and operating noise .
Hydraulic Power Train Technology
Hybrid hydraulic power-train technology normally incorporates a hydraulic system operating parallel to the IC engine to share the task of powering the vehicle. Although other arrangements are possible (in series), the simplest is where the conventional vehicle transmission and driveline components are replaced by a hydro-mechanical transmission, a system that works similarly to a hydrostatic CVT. In which the output shaft from the vehicle's engine is used to drive a hydraulic pump that in turn supplies pressure to hydrostatic motors; these are then connected via a gearing mechanism to the vehicle power-train to drive the wheels . These motors then, under braking, act as pumps to charge accumulators, where energy is stored before being released back to the power-train, transmitting torque to the driveshaft and propelling the vehicle. Fig depicts the capturing and releasing of energy in a hydraulic circuit.
Low Pressure Accumulator
High Pressure Accumulator
Figure Hydraulic Power-train
Examples of Commercial Hydraulic KERS
There are two commercial products of hydraulic hybrid KERS on today's market and both are implemented on delivery vehicles and refuse truck applications. These are Parker Energy Recovery System , and Eaton Hydraulic Launch Assistâ„¢ (HLA®) [7 ].Prototype testing proposes typically regenerative braking capability captures about 70% of the KE produced during braking, minimizing the load on the engine, and helping to reduce fuel consumption . The hydrostatic motors, when acting as pumps during vehicle braking, also help to slow the vehicle down by inducing drag on the rotating drive-train; a feature that helps to reduce brake wear  by more than 50% . Generally these systems operate at a maximum pressure of 5,000 PSI .
The hybrid technologies are controlled by specialized systems that are activated upon braking. The controls prevent service brake application until just before a complete stop. They also monitor if the energy stored in the accumulator falls below a predetermined level, upon which the vehicle engine can be used to provide supplementary power. However, on vehicles with frequent stop-start cycles, this is seldom required as even gentle braking is sufficient to maintain the stored energy at high levels.
Figure Hybrid Hydraulic KERS implemented on large Vehicles 
The HLA® has two modes of operation, "Economy Mode" and "Performance Mode". When the operating in "Economy Mode", the energy stored in the accumulator during braking is used alone to initially accelerate the vehicle. Once the accumulator has emptied, the engine will begin to perform the acceleration. This process results in increased fuel economy of 30% and provides increased acceleration of 2% [7 ]. Economy mode allows for maximum fuel savings & maximum exhaust emission reductions of 20% to 30% .
In Performance Mode, acceleration is created by both the energy stored in the accumulator and the engine. Once the accumulator has emptied, the engine is completely responsible for acceleration. While a 17% increase in fuel economy is possible, the greatest benefit is an increased acceleration of 26% .
The benefits of hybrid solution are numerous; reduced emissions, increased brake life, and better fuel economy. The technology also allows the possibility to reduce the size of the vehicle engine as this can be sized for peak speeds, rather than for low-end torque.
Application of Hydraulic KERS to a Bicycle
A team of engineering students from the University of Michigan  undertook a project to use a hydro-pneumatic regenerative braking system on a bicycle. It was a redevelopment of a heavier previous attempt to make a working prototype to fit within a 29" front wheel (fig). They use a 0.5 litre accumulator and believed this to be sufficient in storing the required energy at a maximum working system pressure of 5000psi. It's weighed an impractical 13kg almost as much as a bike and is its major drawback, its weight can be accounted for by its separate high and low accumulators, separate hydraulic pump and motor and its relatively large mounting bracket (fig).
Figure Prototype Hydry-pneumatic KERS from the University of Michigan 
They failed to test and thus supply conclusive results for the performance characteristics of their prototype, but instead prescribed its key performance parameters via theoretical calculations. In the same way and based on the same calculations the following section outlines the performance of a hydro-pneumatic KERS.
Firstly for a hydraulic system to be implemented the storage of fluid must be addressed, the capacity must be determined and pressures needed to store the kinetic energy. The combined mass of cyclist and bicycle (90kg) braking from 32km/h (20mph) has 2880kJ of kinetic energy. Parker  (manufacture of accumulator and motors) rates the ACP series accumulators at max pressure 5000psi, if assuming ideal gas law:
Parker rates the ACP Series Accumulators 2" bore with capacities of 0.32l, 0.5l, 0.75l. If the accumulator has a pre-charge of P1 = 3200psi and max pressure is P2=5000psi; then rearranging gives:
Taking the capacity as: V1=0.5l gives V2= 0.32l.
Energy stored is:
Thus 0.5l capacity accumulator pre-charged to 3200psi provides more than 5kJ.
Hydraulic Motor/Pump Performance
Fig presents torque-rpm curve for the Parker 09 series hydraulic motor. It will be used to determine braking and launching performance of the hydraulic motor/pump.
Figure Torque-Rpm Curves for Parker 09 Series Hydraulic Motor 
A hydraulic KERS must use a hydraulic motor to provide enough torque to run the bicycle as well as providing enough resistive torque to be an effective brake. If the bicycle travelling at 32km/h (20mph) on 0.66m (26inch) diameter wheels, which spins the motor at 4632rpm through the 18:1 gear ratio of the pump gear train, then this corresponds to 4.52Nm of torque at 3000psi (fig ). This translates to a braking torque of about 81.36Nm applied to the main gear due to the 18:1 gear ratio.
On release of pressure, a fully charged 5000psi accumulator generates 7.57Nm of torques (fig). The 14:1 gear ratio of the motor gear train applies a 105 Nm torque to the main bicycle cluster gear. 7.57Nm corresponds to around 800rpm from motors torque rpm curve (fig), which turns the main gear at around 57rpm due to the 14:1 gear ratio. This is an initial speed of 8km/h (5mph) which will increase as pressure is expended.
In many applications, especially those where high power densities are required, hydro-pneumatic systems offer a more efficient alternative to system driven by electric motors. The technology can be used to capture and transfer high levels of energy extremely quickly compared with similarly sized electric systems, which generally require long periods over which batteries have to be charged. They are also likely to have a longer operating life than battery-powered systems.
The main disadvantage of a hydro-pneumatic KERS would be its weight, which is attributed to by weight of hydraulic fluid, accumulator material (steel), and the fact that in application it would be necessary to have separate high and low pressure accumulators. As well as potentially needing separate hydraulic pump and motor.
In hydro-pneumatic systems when the gas is not charged by the hydraulic fluid and thus not storing energy, the fluid can be considered dead weight. If implemented on a bicycle to be used as a KERS, this would be counterproductive.
Lastly hydro-pneumatic systems are limited where consistent levels of power are required for extended periods at near constant speeds, such as long-distance cruising.
The major consideration when using hydro-pneumatic accumulator for storing the energy whilst braking, is of course the loss of pressurized gas in a sealed accumulator. It is a failure critical to safety when it plays such an important role as braking.
It is apparent the hydraulic accumulator needed for a KERS, does not have an excessively large capacity (pre-charged to 3200psi), in order to release enough energy to propel a bike to 32km/h (20mph). Furthermore, a hydraulic motor can produce 81.36Nm braking torque which makes it an effective brake.
However based on the weight of the prototype (13kg) from the University of Michigan, it is impractical to use a hydro-pneumatic technology, as it stands currently, for a bicycle KERS.