The Aerodynamics of Future Electric Cars - Research Paper Example

?The Aerodynamics of Future Electric Cars Automobiles are an integral part of human lives and support transportation of goods and services alike. Conventional automobiles have had a dependence on fossil fuels including petroleum and gas derivatives such as gasoline, diesel and CNG (compressed natural gas), LPG (liquefied petroleum gas) respectively. Changes in global demand patterns, market forces, environmental concerns and the ever present danger of fleeting fossil fuel reserves are forcing automobile manufacturers and research scientists to look for alternative means to fuel automobiles. One facet of these developments has been electric vehicles that are powered exclusively through electric batteries. The emergence of electric vehicles has put in motion a number of new design challenges such as the aerodynamics of such vehicles because aerodynamics represent a large loss during normal functioning of all kinds of automobiles. 1. Introduction Drag created by a moving vehicle represents one of the largest losses of energy created by an automobile’s engine. Conventional automobiles may lose as much as 40% of the total power to air drag. (TUM, 2011) As with other conventional bodies, the aerodynamic drag exerted by air on a vehicle is directly proportional to the square of the velocity of the vehicle. Mathematically this can be expressed as: where: is the total drag force is the density of the air is the velocity of the vehicle is the coefficient of drag is the area subject to the drag For typically aerodynamic automobiles the coefficient of drag and the accompanying area need to be as small as possible in terms of design considerations in order to minimize the drag encountered. There has been an ongoing struggle to create vehicles with as low a coefficient of drag as possible. Typically well designed vehicles display coefficients of drag of the order of 0.13 while a coefficient of drag of 0.1 is achievable through special design considerations. (Roche et al., 2006) The other major design variable that is area is also minimized through the use of design techniques. Typical frontal exposure areas for well designed vehicles range between 0.75 m2 and 1.3 m2. (Roche et al., 2006) Reductions and compromises of this scale demand that automobile design be optimized for considerations such as component placement and packaging. Moreover considerations of an acoustical nature are also reduced through the use of electric engines that produce far less noise than conventional engines. However this has been criticized for increasing danger to blind people because the incoming vehicle would not possess a sound. (The Week, 2010) Based on these considerations it can be seen that the design of electric vehicles is an altogether different domain from conventional automobile design. The inclusion of new components such as the electric engines places new constrains on design that require solutions through out of the box thinking. This paper will attempt to describe the various major challenges being posed in terms of design and their current solutions along with their future outlook. 2. Conventional Automobile Packaging and Acoustics Conventional automobiles have been built and packaged in nearly the same way for decades. The early pioneering research into automobiles has created a stable platform that is dogmatically used as per vehicle class and usage. For example most passenger cars created along conventional design philosophy house the engine in the front and use a front wheel drive system while load carriers such as trucks use front mounted engines with rear wheel drive. Moreover recent advances in computational fields have allowed designers to create more light weight and singular construction frames better known as monocques. While some of these design elements such as a light weight bodies, four wheels and singular construction have been applied to building electrical vehicles but other packaging constraints have changed altogether. The design configuration and considerations for conventional automobiles are being discussed before hand to form a baseline for comparison with emerging ideas in the design of electric vehicles. Typically an automobile’s structure consists of the following major parts (Hilier, 1976): The basic body shell that serves as the vehicular skeleton; A single engine (which serves as the power plant of the vehicle); A cooling system (typically water or coolant cooled with the use of a radiator); A transmission system that transfers the engine’s power to the wheels based on driving concerns; A suspension system to dampen road based shocks; Brakes to retard the vehicle as required; Lighting systems (front and back); Electrical wiring. A large number of these components and their packaging is centered on the propulsion system for typical automobiles which are generally internal combustion engines. An internal combustion engine requires that a singular power plant be used from which power is transmitted to wheels as required using a transmission system. (Judge, 1939) However these concepts are being redefined by electric vehicles where there is no compulsion to use a singular power plant and there is no compulsion to use a transmission system. Similarly the major precepts of braking used in conventional automobiles is also being redefined as electric motors can be used to control braking motion in electric vehicles. Another major area of concern in electric vehicles is the method used to transport the electrical power that serves the same purpose as fossil fuels in a conventional automobile. While fuel tank sizes and designs have been configured for conventional automobiles, there is still much ground to be covered when it comes to batteries for electrical vehicles. (The Boston Consulting Group, 2010) All of these challenges necessitate that new packaging schemes be introduced in order to find commercially fitting solutions. 3. Packaging Concerns One of the foremost challenges with developing electric vehicles is with reconfiguring existing packaging techniques. The inclusion of large battery packs requires that certain compromises be achieved in terms of passenger capacity, cargo volume, spare tire space etc. (Jones et al., 2009) Other than the main concern for battery packaging, there are a number of other concerns that arise due to the introduction of an all electric system. The packaging has to ensure that electrical leakages do not travel to the vehicle skin as much as possible or this could cause massive short circuits. Similarly electrical leakages are also far more damaging to vehicle bearings than concerns in conventional automobiles such as grit and dirt. Another major facet for packaging to deal with is the heat generated by the electronics employed in electric vehicles. Various designs have been employed with limited success such as the use of coolant or water cooled systems. However there is a continuous move towards air cooled systems. (Jones et al., 2009) These concerns and others are discussed in detail below to fully appreciate the need for innovative packaging techniques in electric vehicles. 3.1. Battery Packaging 3.1.1. Concerns Two kinds of batteries have found wide currency with electric vehicles as well as hybrid electric vehicles. These are the nickel metal hydride battery and the lithium ion battery. The nickel metal hydride battery poses similar performance characteristics to nickel cadmium batteries however it offers greater energy and power densities. Moreover nickel metal hydride batteries charge faster as well. The metals used along with cadmium to absorb the produced hydrogen are proprietary. However the battery’s cells must be totally sealed in order to avoid the reaction of air and the metal hydride. In terms of packaging challenge, nickel metal hydride batteries must be packaged such that their continual use in the vehicle along with vibration and shocks from the road do not cause the battery to puncture. A similar challenge must be dealt with in terms of accident because an accident could likely puncture the battery and cause harm to the passengers inside. Furthermore another associated challenge with nickel metal hydride batteries is the need to cool the batteries if they are charged too fast. The inability to cool these batteries may cause battery failure as well as fire damage. Current design trends are meeting these challenges by using rigid and soft packing techniques that tend to isolate the battery from vibration based damage while allowing it to “breath” in case of a heat up. The other kind of battery finding great appreciation is the lithium ion battery. One of the issues with the lithium ion batteries is the need for precise voltage control as high voltage levels can damage the battery while low voltage levels will keep the battery undercharged. The restriction on precise voltage control implies that additional electronics has to be employed in order to keep voltage levels totally stable. This in turn increases packaging requirements as per the available space as well as the thermal load due to additional electronics. On the other hand, lithium ion batteries have found large favor in electric vehicles because they are considerably lighter than other types of batteries. The lighter weight of these batteries means that they offer significantly greater range as more electrical power can be carried in the same battery weight. Moreover as the batteries are smaller in size, they offer a low cost during manufacturing as producing larger batteries is far more expensive. (Gover, 2011) 3.1.2. Solutions A primary solution to battery packaging has been the proposition of implementing batteries as an integral part of the chassis. Figure 1 - Battery as an Integral Part of the Monocque Previous solutions to battery placement insisted on placing batteries in proximity to the motor (if used singularly). However more recent solutions envision the placement of batteries under the passenger cabin in order to conserve hood space as well as cargo space. (Gage, 2003) Batteries are being created as single long cells that are placed on the bottom floor of the vehicle and are provided air access through ducting to ensure that the batteries remain cool during operation. (Gover, 2011) 3.2. Power Electronics Cooling When integrated circuits are used along with conventional packaging, the aim of the design is to limit heat generation to the top surface of the chip. A heat sink is generally deployed between the printed wiring board and the chip to remove heat faster. If such a scheme is employed in electric vehicles, there would be a need to develop a cooling system as well in order to keep the circuitry cool enough. These would give rise to new packaging problems such as water or coolant leakage, design complications, use of auxiliary components such as valves, tubing etc. However a new approach has been adapted in order to design circuitry that is adapted according to the particular mechanical and thermal needs as well as space requirements and the primary principles of electronics design. (Gover, 2011) The adoption of this new approach to power electronics in electric vehicles ensures that space is utilized as optimally as possible without presenting danger to the major components of power electronics. (Chen & Omura, 2007) The optimization of the cooling system ensures that extraneous weight is not added to the vehicle in the shape of a cooling system. 3.3. Placement of Motors While conventional automobiles employ singular combustion engines in the front of the vehicle, there are no such stipulations for an electric vehicle. There are two major schemes employed in electric vehicles to place the power plant(s): either a single motor is placed in the hood or multiple motors are placed with the wheels. The second scheme for motor placement is preferred as it allows the recovery of energy through regenerative braking. Instead of losing braking force as friction and heat, this energy is converted to electrical energy that is used to charge the batteries. The use of regenerative braking allows increases in range between 5 and 10% of the original range. (AVT, 2009) Motors that are used on electric vehicles present a new dimension of packaging and mechanics based challenges that are absent in conventional automobiles although they offer significant advantages too. Using motors in proximity of the wheels allows the wheels to be used in both propulsion and braking as required. This in turn aids the traction of the vehicle. Moreover placing the motors near the wheels allows the center of gravity to be far lower than the center of gravity of conventional automobiles. (Evans, 2009) Moreover the placement and packaging of motors in the wheels allows the removal of several components such as axles, differentials, transmission etc. and this in turn promotes lower drive train rotational inertia. On the other hand placing the motors in the wheel tends to increase the un-sprung weight of electric vehicles which in turn affects the handling of the vehicle directly. Moreover the suspension system of the vehicle needs to be more robust to live up to a greater un-sprung mass which means adding larger suspension components. 3.4. Ball Bearings Behavior The impedance of ball bearings is an important consideration when it comes to alternating current drive trains. The upper and lower races of the ball bearings employed in the drive trains tend to display capacitive coupling. The capacitance is non linear in behavior and depends on the separation between balls. As the vehicle moves around, the changes in ball separation vary the overall capacitance. In case that electrical leakage develops and interacts with the bearing, the bearing will end up with pitted balls and races which indicate a significant failure in terms of the bearing’s life. Moreover as the current moves from the bearings to the motor itself, the development of stray capacitance cannot be denied. The creation of low impedance at a high frequency will in turn shorten the effective life of the insulation on the stator windings. (Zare, 2007) It is still a large challenge for packaging developers to create methods that would limit the amount of leakage to bearings. The nature of conducting materials used to create the ball bearing pose large problems in dealing with the electrical leakage. One proposed solution has been the implementation of non-metal bearings but this in turn requires large research and development efforts. 4. Acoustical Considerations The primary noise generated by a conventional automobile springs from the engine and is protected against through the use of special insulation and moldings. The absence of an engine in electric vehicles removes the possibility of such noise. However the presences of a large number of electronics that are planted and packaged around the cabin mean that their noise levels must be contained. The main noise sources in an electric vehicle are from the invertors and rectifiers. The use of specialized insulation materials and molding tends to dissipate such noise with relative ease and so this does not become a major problem. The removal of noise has been considered as dangerous as approaching vehicles would be hard to detect for blind pedestrians. Certain regions have stipulated electric vehicle manufacturers to use special noise producers on the outside of vehicles in order to alert people nearby. (The Week, 2010) 5. Conclusion There are a number of design challenges associated with creating electric vehicles but continuous research and development has the potential of solving these problems with the added use of innovative design techniques. The kinds of problems encountered in designing electric vehicles are unique and their solutions need to be unique as well in order to provide commercial solutions. 6. Bibliography AVT, 2009. EV Power Systems (Motors and Controllers). [Online] Available at: http://avt.inl.gov/pdf/fsev/power.pdf [Accessed 28 October 2011]. Chen, Z.J. & Omura, I., 2007. Power Semiconductor Devices for Hybrid, Electric and Fuel Cell Vehicles. In Proceedings of IEEE, April 2007., 2007. IEEE. Evans, P., 2009. UK team develops plug-in hybrid retrofit kit for ICE vehicle. [Online] Available at: http://www.gizmag.com/plug-in-hybrid-retrofit-kit-ice-vehicle/11631/ [Accessed 28 October 2011]. Gage, T.B., 2003. Development and Evaluation of a Plug-in HEV with Vehicle to Grid Power Flow. [Online] AC Propulsion Available at: http://www.acpropulsion.com/icat01-2_v2gplugin.pdf [Accessed 28 October 2011]. Gover, J., 2011. A tutorial on hybrid electric vehicles: EV, HEV, PHEV and FCEV. [Online] Available at: http://www.midmichigansae.org/documents/DrGoverPresentationSAEApril20.pdf [Accessed 28 October 2011]. Hilier, V.A.W., 1976. Motor Vehicle Basic Principles. 1st ed. London: Nelson Thornes. Jones, S., Mendoza, J., Frazier, G. & Khalil, G., 2009. Developing a Methodology for the Evaluation of Military based Hybrid Electric Vehicle Thermal Management Systems. In Ground Vehicle Systems Engineering and Technology Symposium., 2009. UNCLAS. Judge, A.W., 1939. The Mechanism of the Car, Its Principles, Design, Construction and Operation (Motor Manuals, a Series for All Motor Owners and Users, Volume III. 3rd ed. London: Chapman and Hall. Roche et al., 2006. Speed of Light. 073341527th ed. The Boston Consulting Group, 2010. Batteries for Electric Cars: Challenges, Opportunties, and the Outlook to 2020. Investigation. Boston: The Boston Consulting Group The Boston Consulting Group. The Week, 2010. The future of the electric car. [Online] Available at: http://theweek.com/article/index/206278/the-future-of-the-electric-car [Accessed 28 October 2011]. TUM, 2011. Aerodynamics. [Online] Available at: http://www.mute-automobile.de/en/technology/aerodynamic.html [Accessed 28 October 2011]. Zare, F., 2007. EMC and Modern Power Electronics. 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