25-08-2014, 10:24 AM
HYDRAULIC REGENERATIVE SYSTEM FOR A LIGHT VECHILE
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ABSTRACT
The thesis is based in a constructed light vehicle that must be improved by adding a hydraulic energy recovery system. This system consists in two hydraulic motors hub mounted which are used to store fluid at high pressure in an accumulator when braking. Through a valve the pressure will flow from the high pressure accumulator to the low pressure one, and consequently the vehicle will get extra acceleration.
This thesis consists in finishing the assembly and testing it, as the main idea was already thought and some of the necessary parts were acquired before. Firstly, a quick overview of the bike is done and the current state of it at the end of the thesis is discussed. After that, the mechanism used to actuate the system is developed and explained, with which some CAD software was used to design and make some FEA. Straight afterwards the work focused on the tests and its development. A quick discussion about what tests should be done, the preparations and also the way that some measurements were done is commented. In order to do these measurements a data acquisition device and some software to deal with it was used. Thereupon, calculations to know if the system auto-compensates the weight added, causing more rolling resistance, and the oil frictions are done. In this part the performance and reliability of the system is discussed, as well as the feelings of the driver. Finally, improvements and possible modifications are listed with the aim of upgrade the vehicle, the system and the way of work.
INTRODUCTION
In this thesis is presented the methodology used to implement the hydraulic regenerative system in a light vehicle. The chapters of this thesis are chronologically ordered as they were carried out.
Background:
To achieve speed, we were thinking that in a system that would increase the autonomy and could be innovative in this kind of vehicle. This way, they would get more points to win. So they came with the idea of an energy recovering system when braking. In this field, many solutions could be found since it is not a novel idea. Firstly they thought in a system which would recharge the batteries, using the motor as a generator when braking. The problem is that with this system, even including capacitors, charging this kind of batteries takes a long time, more than stop the vehicle.
Another kinetic regenerative system then was looked for. Some garbage or delivery trucks are using hydraulic regenerative systems to save energy and reduce the environmental impact. This system has a good advantage when is used in vehicles that must operate with lots of stop-and-go. This way, an amount of energy can be stored. It is also easier because they have place for a system like this since is implemented in the powertrain. In the end, the fuel consumption is reduced and also the maintenance in brakes.
Inspired in the system explained above, the same kind would be implemented in the vehicle. However, the regenerative system would not be mounted in the power train. Since the vehicle has three brake friction discs, one in each wheel, a secondary hydraulic regenerative system will be added. Two hydraulic motors would be hub mounted in the
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front wheels. The motors should be connected through a valve to the accumulators. When braking is needed, the valve’s lever is positioned to one side and the pump/motors work pumping oil to the HPA (High Pressure Accumulator). When the HPA is full, if acceleration is needed, the valve’s lever is positioned opposite than before and the apump/motors work as motors. Then, oil from the HPA flows through the motors to the LPA (Low Pressure Accumulator); the motors now get the energy and give an extra acceleration to the vehicle. Thanks to this system, electric energy should be saved in the end.
Aim of Project:
Sometimes inventions of the past can be beyond belief such as a point that electric vehicles are not new. As early as 1900, Ferdinand Porsche invented an electric car with motors in both front wheels. Nowadays novel electric vehicles are in every motor show, showing its improvements and innovations. This hydraulic system was thought with the purpose of save energy. Not as the point of view of replacing petrol powered cars, but regenerate some of the energy that is being wasted while braking so as to decrease the total energy used.
It is always in mind the amount of pollution that mankind produces every day. When thi4nking in save energy it is also a way to reduce pollution. A hybrid vehicle can be 44considered as a really good sustainable vehicle, depending on where the electric energy used is coming from.
In other respects, a similar hydraulic linkage is being used in heavy garbage trucks (a Parker application can be seen in (Gannon, 2009) or more information in (Ogando, 2007)). Testing this technology in light vehicles is a way to learn how it works there and make new research and developments. Going further with that, light vehicles are being developed to use less energy, as mass is an important factor. But the higher the mass is, the better the hydraulic system works, so this is a contradiction that can affect the development of this technology in light vehicles.
The vehicle is equipped with an electric DC motor powered by batteries. For more information about the vehicle, is recommended. On it, how the vehicle was built and also many more particular specifications can be found.
Hydraulics:
Hydraulics is the science of transmitting force and/or motion through the medium of aconfined liquid. In a hydraulic device, power is transmitted by pushing on a confined liquid. Figure shows a simple hydraulic device. The transfer of energy takes place because a
quantity of liquid is subject to pressure. To operate liquid-powered systems, the operator
should have a knowledge of the basic nature of liquids. This chapter covers the properties of
Liquids and how they act under different conditions.
The word “hydraulics” generally refers to power produced by moving liquids. Modern hydraulics is defined as the use of confined liquid to transmit power, multiply force, or produce motion. Though hydraulic power in the form of water wheels and other simple devices has been in use for centuries, the principles of hydraulics weren’t formulated into scientific law until the 17th century. It was then that French philosopherBlaise Pascal discovered that liquids cannot be compressed. He discovered law which states: Pressure applied on a confined fluid is transmitted in all directions with equal force on equal areas.
Hydraulics is a topic in applied science and engineering dealing with mechanical properties of liquid. At a very basic level hydraulics is the liquid version of pneumatics.
Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control, and transmission of power by the use of pressurizedliquids.
Hydraulic Fluid:
Hydraulic fluid must be essentially non-compressible to be able to transmit power instantaneously from one part of the system to another. At the same time, it should lubricate the moving parts to reduce friction loss and cool the components so that the heat generated does not lead to fire hazards. It also helps in removing the contaminants to filter. The most common liquid used in hydraulic systems is petroleum oil because it is only very slightly compressible. The other desirable property of oil is its lubricating ability. Finally, often, the fluid also acts as a seal against leakage inside a hydraulic component. The degree of closeness of the mechanical fit and the oil viscosity determines leakage rate. Figure below shows the role played by hydraulic
Fittings and Seals:
Various additional components are needed to join pipe or tube sections, create bends and also to prevent internal and external leakage in hydraulic systems. Although some amount of internal leakage is built-in, to provide lubrication, excessive internal leakage causes loss of pump power since high pressure fluid returns to the tank, without doing useful work. External leakage, on the other hand, causes loss of fluid and can create fire hazards, as well as fluid contamination. Various kinds of sealing components are employed in hydraulic systems to prevent leakage. A typical such component, known as the O-ring is shown below in Figure.
LIMITATIONS
The regenerative braking effect drops off at lower speeds, therefore the friction brake is still required in order to bring the vehicle to a complete halt.
The friction brake is a necessary back-up in the event of failure of the regenerative brake.
Most road vehicles with regenerative braking only have power on some wheels (as in a 2WD car) and regenerative braking power only applies to such wheels, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels
Compressed gas:
A compressed gas accumulator consists of a cylinder with two chambers that are separated by an elastic diaphragm, a totally enclosed bladder, or a floating piston. One chamber contains hydraulic fluid and is connected to the hydraulic line. The other chamber contains an inert gas under pressure (typically nitrogen) that provides the compressive force on the hydraulic fluid. Inert gas is used because oxygen and oil can form an explosive mixture when combined under high pressure. As the volume of the compressed gas changes, the pressure of the gas (and the pressure on the fluid) changes inversely. Existing hydraulic accumulator designs are large and heavy due to the need for two storage tanks and do not have the high energy density needed for many applications.
It is possible to increase the gas volume of the accumulator by coupling a gas bottle to the gas side of the accumulator. This is mainly done since a gas bottle normally is cheaper to produce than an accumulator
Linkage:
A mechanical linkage is an assembly of bodies connected to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for example, and are called joints. A linkage modeled as a network of rigid links and ideal joints is called a kinematic chain.
Linkages may be constructed from open chains, closed chains, or a combination of open and closed chains. Each link in a chain is connected by a joint to one or more other links. Thus, a kinematic chain can be modeled as a graph in which the links are paths and the joints are vertices, which is called a linkage graph
The movement of an ideal joint is generally associated with a subgroup of the group of Euclidean displacements. The number of parameters in the subgroup is called the degrees of freedom (DOF) of the joint.
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Mechanical linkages are usually designed to transform a given input force and movement into a desired output force and movement. The ratio of the output force to the input force is known as the mechanical advantage of the linkage, while the ratio of the input speed to the output speed is known as the speed ratio. The speed ratio and mechanical advantage are defined so they yield the same number in an ideal linkage.
A kinematic chain, in which one link is fixed or stationary, is called a mechanism, and a linkage designed to be stationary is called a structure.
Conclusion:
This hydraulic system was developed with the purpose of saving energy. Due to regeneration in the braking, the driver should be able to get extra acceleration. And it was like that.
The first thing that must be said is the loss of maneuverability of the vehicle when the system is pressurized. The hoses are quite stiffer and the tilting and the steering are getting worse. Some improvements must be done if hydraulic systems are going to be used in steering wheels or tilting vehicles.
Although the feeling between the system and the driver using the footlever were quite good, in future applications this lever should be suppressed. To be ergonomic and used to its maximum, the system must be actuated electronically so the driver will not have to think which brake or throttle is going to use. Furthermore, the lever can be released on time, to avoid a reversion of the working cycle or to high pressures in the low pressure side of the system.
The behavior of the system was really soft and pleasant to be used. The braking and the acceleration were changing progressively when the pressure was increasing or decreasing but always smooth. Because of this, an alternative braking system needed for harder or emergency brakes.
Through the plots of the pressure during the tests, it was found that when the HPA is full, there is some constant leakage in the valve. The constant flow into the valve causes a decrease of pressure that means losses of energy. The more pressure the HPA has, the more powerful the system is. Thus, it is necessary to have a valve as leakproof as possible.
Losses in friction and rolling caused a decrease in the energy stored. Therefore, the efficiency was reduced to such a point that it cannot be ensured their auto-compensating. When the speeds were too low, the friction forces were too high, and the energy could not be used properly. Nevertheless, the tests determined that when the HPA was full-filled and this energy used, the HERS was auto-compensating its own weight. This efficiency is pretty low and when the system is not used near to its maximum then it does not compensate the weight.
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In order to make use of this system in future applications, the stored energy should be used when the vehicle is already moving and the driver wants to speed it more up. Otherwise the vast majority of the energy is wasted for the work of the non-conservative forces.
Regarding to the efficiency of the different parts of the system, the vast majority of the losses are in the gerotors. This kind of rotors/pumps has a low performance and some research in this field must bedone. Future applications will be centered in variable motors that will be able to adjust the power needed. Thus, the acceleration will be controlled but also the deceleration, since braking hard will be possible. Otherwise, an alternative braking is needed.
Some research in a device to control the lever and the pressures should be done. It is an important part of the system that will avoid some losses, worries of the driver and also will take care of the maximum and minimum pressure. This way the system can be used properly and the driver will be focused on the track and not on the vehicle.
For the simulations that were done before the tests, the later comparisons said that they were quite good. The values found through the simulations did not have the losses of rolling resistance, but pressures and speeds were close to the acquired in the real tests.
Finally, this system perhaps is not the best solution for recovering energy. It has low efficiency and cannot work in high angular speeds. The power is not controllable and the hoses need to be softer. But this system is a step for future applications, for next improvements and researches.