Term Paper, 2009
I Proj ect statement
II Executive Summary
III Table of contents
IV Working papers
1 Hybrid Cars
2 Optimization and control of hybrid cars
3 The electrical drive system
4 Costs and Benefits of Hybrid Cars
V Team chief’s report
Introduction It is very unusual in today’s world that an evolutionary development step turns out to be revolutionary. Some experts claim, hybrid cars can revolutionize the car industry, but will they? Due to the immense public interest in possible solutions against climate change, the issue turns more and more into a political debate. Because of the major impact a success of these new concepts will have on national industries and global markets, different interest groups try to influence public opinion.
In order to come to an objective conclusion, the following analysis deals primarily with the technical improvements of hybrid cars and compares key aspects to conventional car design.
The overall problem is efficiency. When resources like coal or gas were cheap, there was little reason to care about their consumption. It was the easiest way to increase sales numbers and profit by simply raising the amount of production. However, situation has changed. Today we are facing a limit to all of our currently known resources and we can already see prices rising steadily. To develop revolutionary new technology takes a lot of time and money, therefore the rational approach is to make more out of the same resources. Rising efficiency still allows for economic growth while at the same time slowing down the vicious circle of exploding consumptions.
All western economies depend massively on car traffic. Despite all the disadvantages, cars are still the easiest and most comfortable way for most people for going to work and shopping. Due to the relatively high fuel consumption and poisonous greenhouse gas emissions, improved efficiencies will have a significant effect.
Hybrid car concepts, deemed as the future in car technology by some experts, are, however, not a definite answer to the question. This is because complex questions do not allow for simple answers but a differentiate analysis of all relevant aspects.
Structure is one of the first and most crucial decisions to be made. Since hybrid cars are in a relatively early stage of development, there is no commonly established concept. Mistakes made at this point can hardly be corrected later by changing single components. Evaluating different structures is thus an important aspect of an overall analysis.
The relatively complex situation of having two engines combined leads to the question of optimized controlling strategies. Because two entirely different engines offer a massive potential for adjusting their operation characteristics to points of best efficiency, optimal controlling devices have to be discussed and their use under different conditions evaluated.
After the advantages and disadvantages of different theoretical concepts are analyzed, it has to be focused on how the actual implementation works. Since combustion engines are already very well known today, the electrical subsystem deserves special attention. Most important are the rechargeable batteries that are responsible for storing the electric energy.
With these aspects in mind, a detailed conclusion on costs and benefits is possible. This should allow for a differentiate view on where hybrid cars are indeed a revolution and where other approaches may give better results.
Our market today offers quite a few different hybrid cars. Some of them are supposed to be more efficient than cars with a combustion engine, others not really.
The evaluation of different hybrid concepts makes it not as easy as it could be. Not one concept is the best hybrid and not every hybrid is better than a conventional car!
But for different fields of applications, there is a different concept of a hybrid car.
Short distances and urban areas with a lot of stopping and accelerating are the best fields of application for a series hybrid, due to its higher efficiency in accelerating than any other vehicle (combustion or other hybrid concept).
Driving long distances, no hybrid concept shows a clear advantage over the conventional car. They are heavier and use the combustion engine to drive, due to the low energy (compared to fuel) that can be stored in a battery.
To ensure that a new hybrid car is as efficient as it can be, a lot of research has to be conducted to the optimization and controlling process.
Due to changing driving styles, changing traffic situations and changing roadway types, a computer controlled mechanism needs to adapt the engines to ensure the best efficiency possible. This is one of the most challenging and research intensive topics.
A new Lithium Ion technology promises lower weights of the battery packs and therefore helps reducing the fuel consumption. On the negative side this technology still involves risks due to the high energy density of the batteries and a number of technology specific failure modes with the possibility that battery starts burning. These risks however can - at least partially - be eliminated by new materials that prevent the critical chemical processes.
The term hybrid car is a known one. Most people think of a Toyota Prius, as it was one of the first hybrid cars available to everyone. But what is its scientific definition?
“A hybrid car is a car which can run on two or more fuel sources.”
http://www.greenfo otsteps.com/hvbrid- Our research will only be done on hybrid electric cars. car-definition.htmi 07-15-09 /11.18
“A hybrid electric vehicle is a hybrid vehicle that combines a conventional propulsion system with a rechargeable energy storage http://en-wikiPedia.o rg/wiki/Hvbrid electr system.” ic vehicle 07-15-09/11.25
To simplify that, a car only needs a combustion engine, a battery and an electric engine.
Whenever the term hybrid comes up, it will be referred to the hybrid electrical car.
These three mandatory parts have to be in a car to call it a hybrid electric car. The way they are put together is not specified. But how many ways can you put these three parts together and for what use of a car are they good for?
A series hybrid consists of a combustion engine which only powers a generator. The generated power is than used by one or more electrical engines. If not all the power is needed to drive the car, the remaining power is stored in a battery. That allows the car to turn off the combustion engine if the battery is full or to use more power than the combustion engine can generate (to accelerate faster).
One of the main problems is the power storage. Batteries are known for their low efficiency as well as they need a DC current.
Generators that produce DC currents are too expensive and too big to produce the needed amount of power, which only leaves AC generators.
The AC current needs to be changed to DC bye a rectifier and changed again to be used bye the electrical engine(s).
This results in a lower efficiency of about:
93% for the DC generator 92% for the rectifier 92% for the second rectifier 90% for the electrical engine(s)
70% of the energy coming from the combustion engine
But why are they used?
Since the engines only propels the generator and does not need to
compensate any peaks or lows of the power needed, it can be used at the optimal engine operating point.
While driving, the efficiency of a combustion engine changes and will not be for long at the optimal engine operating point. Resulting in a 50% higher efficiency (for the series Hybrid compared to a usual combustion engine) when used for short distances and 2030% lower efficiency for long distances.
The electrical engine that is used to propel the car can be used, while breaking, to generate electricity instead of wasting the energy and turning it into heat, also making the car more efficient.
The following image shows how a series hybrid might be set up:
Abbildung in dieser Leseprobe nicht enthalten
http://upload.wikime dia.org/wikipedia/co mmons/l/lO/Hvbrid peak.pne O7-l5-O9/l2.lO
A parallel hybrid uses a combustion engine a battery and an electrical engine.
The difference to the series hybrid is, that both engines are used to propel the car, if necessary.
The result is a car with a lot of power, if the engines are not downsized. Due to that fact, parallel hybrid cars usually have 2 smaller engines (than a series hybrid), which are used together, if a lot of power is needed.
A parallel hybrid is not able to recharge his battery, because the combustion engine is only used to propel the car.
If you are out of electrical energy, you need to stop the car and recharge it or only use the combustion engine (you do not have a hybrid anymore - only a very slow car).
Due to the long time needed to recharge a car, the parallel hybrid is not used for series production.
When accelerating from standstill the electrical engine has high torque, while the combustion engines torque is minimal. Both engines can be used together to enhance the overall efficiency.
The following image shows how a parallel hybrid might be set up:
As seen above, both systems have their own advantages, but the series hybrid seems to be quite in favor (as well as the new Chevrolet Volt will be a series hybrid).
Should all new cars be a series hybrid? What, if a car for long distances is needed?
A series-parallel hybrid uses a combustion engine, a battery and two electrical engines (one functions as engine, one as generator). The setup is more complex than either the parallel or series hybrid. Therefore a picture to show a possible set-up:
The engine is able to propel the car, as well to propel the generator.
This allows the car to be more flexible and a smart computer system can figure out the most efficient way to use the energy.
The electrical engine is used to to accelerate (from 0 mph) until the combustion engine reaches a higher efficiency than the electric motor. That is when the electric motor (and generator) [depending on the battery, if it needs to be charged the generator will charge the battery] are turned off (disconnected bye a clutch) and the car only runs on the combustion engine (as long as that is more efficient).
A series-parallel hybrid (as the Toyota Prius) have a high cost price, due to the fact, that they need a big battery, a big generator, a big electric motor and a big combustion engine (because all parts need to be able to propel the car on their own). Therefore the efficiency on medium to long distances is better than any of the other two, only beaten at short distances from the series hybrid (who has a lower weight and therefore a better efficiency).
Heading Analysis References
The main component of a hybrid vehicle is its combination of a conventional diesel engine and a powerful electric motor. These two forms of torque generation have entirely different operating characteristics and their points of best efficiency are not the same. By intelligent co-action ofboth engines, the overall efficiency can be improved. However, this requires for a sophisticated control device.
Abbildung in dieser Leseprobe nicht enthalten
The task of moving a technical process - for example a car - from one state (e. g. zero speed) to another (accelerated to 50 mph) with minimum use of energy is called an optimal control problem. The solutions found during the last century can be applied to various technical issues, for example hybrid vehicles.
A simple example is the one of a road leading through a hilly region. How does the driver have to shift gears at each part of the road in order to minimize overall travelling time? A completely different solution to the same problem would be reached when choosing the fuel consumption optimality criterion.
The first step is to find a special model of the system. It consists Finding the of state equations and further equation constraints. The dynamic right model behavior has to be correctly represented but minor aspects left out in order to simplify the process. This is a major problem because a significant number of physical relations have to be found and their parameters measured. Some of these are rolling resistance, air drag and flywheel effect in relation to the torque applied to the wheels. Other aspects, for example the engine’s torque-speed characteristic are non-linear and can hardly be calculated.
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