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| Fig. 1: Electric car charging in Amsterdam. (Courtesy of L. Hirlimann. Source: Wikimedia Commons.) |
Escalating fuel prices and the increasing alertness of high fuel emissions and its impact on our environment have gravitated attention toward electric and hybrid cars. Toyota's Prius and Honda's Insight vehicles are examples of the first hybrid vehicles commercialized in response to the escalating problem of personal vehicle fuel consumption. Nonetheless, this idea is far from new. As early as 1839, Sir William Grove (known as the "Father of the Fuel Cell") discovered that it is possible to generate electricity via reversing the electrolysis of water. Furthermore, in the late 1950s, NASA (National Aeronautics and Space Administration) began building compact electric generators for use on space missions. In the last few decades, the hype on the development of energy efficient vehicle technology has increased dramatically, causing numerous manufacturers and various federal agencies to support ongoing research into its development. [1]
The primary energy conversion devices in an electric or hybrid vehicle are the (1) IC engine, (2) electric machine, and (3) energy storage device. The IC engine converts chemical energy to mechanical energy. The electric machine is used as either a motor or a generator to convert the mechanical power to electric power, or from electric power to mechanical power. Energy storage devices and electrical to electrical conversion devices are key in understanding power and energy transfer in electric and hybrid vehicles. [2]
Battery development has moved the status of electric vehicles (EVs) forward with the lithium-ion, lithium polymer, and NiHM; innovation that resulted in increased storage capacity and less product weight. However, it was the promotion of clean energy sources in order to halt global warming that really gave EVs a lot of attention. For example, in 2005, about 20 percent of the oil used by the United States was used solely for transportation. EVs have proved to have a lot of advantages - first, they have about 70 percent fewer moving parts than an ICE, meaning "less maintenance - no oil changes or replacement of items such as filters, fuel pumps, alternators, and the like." Additional qualities such as less pollution, cheaper to run, more conserving of energy are also factors that make such cars popular among the consumers. [3]
Why do hybrids get good mileage? Features unique to hybrid cars, such as regenerative braking, a downsized engine, electric-only propulsion, and motor assist all contribute to major miles per gallon benefits. Such contributions can add up to over 30 to 40 percent gain in mpg. In hybrid cars, the energy of motion, or kinetic energy, is recovered by electrical braking during stops. This, in contrast, to friction braking, helps recover the kinetic energy. This technology alone yields 10%-15% additional fuel savings. [4]
The lowest cost hybrid technology is known as a "soft hybrid." Soft hybrid vehicles use a 6-10 kW integrated starter generator (ISG), which can also be known as a combined starter or alternator. The starter generator is mounted on the front of the engine and uses the front engine accessory drive belt to start the engine. The time it takes the re-start the engine is much lower than a vehicle with a conventional mounted started because the integrated starter generator assumes 3 to 8 times as much power, resulting in a silent and smooth start. [5]
Plug-in hybrid vehicles (PHEVs) are hybrid electric vehicles that consume and store energy from the electric grid to support tractive power. This functional change provides the additional benefits of reducing vehicle greenhouse gas emissions and reducing vehicle petroleum consumption. Such cars can be made with parallel or series PHEV architecture - the parallel systems have fewer components than series, as a parallel requires one engine and one motor instead of an engine, generator, and motor as in a series system. While both designs can be successful, the size of the traction motor must be large enough to provide the necessary speed and torque to propel the vehicle. However, there are some constraints on a vehicle-level design based on the various architecture characteristics. For example, series hybrid vehicle require larger electronic components such as the motor and the generator. Such larger components result in a heavier and more costly system. Parallel systems, on the other hand, are composed of fewer power electronic components, resulting in a less expensive and lighter car. [6]
While PHEVs are known to play a role in reducing greenhouse gas emissions, a study by Samaras and Meisterling has found that meaningful greenhouse gas emissions reductions with such vehicles are conditional on low-carbon electricity sources. [7] A critical component of PHEVs are the batteries, and decreased greenhouse gas emissions associated with lithium-ion battery materials and production comprise 2-5% of these vehicles' life cycle emissions.
Environmental and economical factors provide compelling reasons for the development of clean and efficient vehicles for urban transportation, and undoubtedly, we will continue to see more advancements in the development of this technology.
© Danielle Rasooly. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Ehsani, Y. Gao, and A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design, 2nd Ed. (CRC Press, 2009).
[2] I. Husain, Electrical and Hybrid Vehicles: Design Fundamentals, 2nd Ed. (CRC Press, 2011).
[3] C. D. Anderson and J. Anderson, Electric and Hybrid Cars: A History, 2nd Ed. (McFarland, 2010).
[4] A. Fuhs, Hybrid Vehicles and the Future of Personal Transportation, 1st Ed. (CRC Press, 2008).
[5] A. Emadi, M. Ehsani, and J. M. Miller, Vehicular Electric Power Systems: Land, Sea, Air and Space Vehicles (CRC Press, 2003).
[6] G. Pistoia, Electric and Hybrid Vehicles (Elsevier, 2010).
[7] C. Samaras and K. Meisterling, "Life Cycle Assessment of Greenhouse Gas Emissions from Plug-in Hybrid Vehicles: Implications for Policy," Environ. Sci. Technol. 42, 3170 (2008).
