Trends in Electric Propulsion

Tomado de: http://electricvehicle.ieee.org/2013/09/29/trends-electric-propulsion/

By Kaushik Rajashekara, Fellow IEEE, Department of Electrical Engineering, The University of Texas at
Dallas, Richardson, TX
Electric vehicles (EV) have been around since late 1800s. However, in the past, EV development activities
were discontinued because of low cost of gasoline and advancement of internal combustion engines. In the
last decade, Electric vehicles (EV) and Plug-in Hybrid Electric Vehicles (PHEV) are gaining increasing
interest in North America and in other countries due to rising fuel prices, concern for the environment and
the sustainability of fossil fuel based transportation.
The critical subsystem required in an electric vehicle is the propulsion system, which provides the tractive
force to propel the vehicle. This propulsion system consists of an energy storage system, the power
converter, the propulsion motor and associated controllers as shown in Figure 1.

Figure
1:
Typical
Click Image to Enlarge

propulsion

System

components

of

EV

Power-train

Electric motor(s) converts the energy supplied by the battery into mechanical energy to provide traction
power to the wheels. Today, interior permanent magnet (IPM) synchronous motor widely used in
automotive propulsion system because of its high efficiency, high torque, high power density and relative
ease of field weakening operation. Toyota Prius, Ford Escape, Chevy Volt are some of the vehicles that use
IPM machine. However, there are rising concerns about the availability of rare-earth based magnets and
their increasing costs. A number of companies and researchers are working on the development of motors
that do not use permanent magnets, but achieve the same performance as IPM motors. These include
Induction, Switched Reluctance, Synchronous Reluctance, and PM-assist Synchronous Reluctance Motors.
In the near future, the interior PM motor will likely continue to dominate the market.
In the area of power electronics, presently Insulated Gate Bipolar Transistor (IGBT) devices are being used
in almost all commercially available EVs, HEVs, and PHEVs. The IGBTs will continue to be the
technology of choice until the silicon carbide (SiC) and gallium nitride (GaN) based devices are
commercially available at a cost similar to that of silicon IGBTs. Various properties of silicon carbide such
as; wider band gap, larger critical electric field, and higher thermal conductivity enables the SiC devices to
operate at higher temperatures and higher voltages offering higher power density and higher current density
than the pure Si devices. Gallium devices are projected to have significantly higher performance over
silicon-based devices, and much better performance than SiC devices, due to their excellent material

properties such as high electron mobility, high breakdown field, and high electron velocity. GaN-based
power electronics also feature both low on-resistance and fast switching, leading to substantial reduction in
both conduction and switching losses. Achieving the highest power density in a compact package
(considering the thermal aspects and reliability) is critical for successful deployment of power electronics
systems in electric and hybrid vehicles.

Table
I:
Characteristics
Click Image to Enlarge

of

commonly

used

batteries

in

EVs

Lithium-based energy storage technologies (ie. lithium-ion batteries) are leading the way to meet the storage
requirements of EV/HEVs. In the past, Lead-acid batteries and Nickel-Metal-Hydride (NiMH) batteries
were popular in EV/HEVs. The Tesla Roadster was the one of first production automobile to use lithiumion battery cells to travel more than 200 miles per charge. Presently, the Nissan Leaf (BEV) and the GM
Chevy Volt (PHEV) also use lithium-ion based batteries [1-2]. Typical values of energy, power and cycle
life for lead acid, NiMH and lithium-ion batteries are shown in Table I and Figure 2. Lithium-ion presents a
higher-density and more efficient way to power modern hybrids and EVs. The future of EV batteries could
be based on lithium-air technology. The energy density of lithium-air batteries theoretically is equivalent to
the energy density of gasoline. This is because it has an air cathode made of a porous materials that draw
in oxygen from the surrounding air. When the lithium combines with the oxygen, it forms lithium oxide and
releases energy. Since the oxygen doesnt need to be stored in the battery, the cathode is much lighter than
that of a lithium-ion battery, which gives lithium-air batteries their higher energy density. Toyota Motor
Corp and BMW have announced a joint research program on lithium-air battery that will be expected to be
more energy-dense than the lithium-ion batteries of today [3]. This technology is also being studied by
other researchers, including IBM, working to develop a lithium-air battery that would enable electric
vehicles a range of 500 miles on a single charge [4]. Researchers have demonstrated coin-sized,
rechargeable lithium-air batteries with a current density of 600 mAh/g (much higher than lithium-ion
batteries at 100 to 150 mAh/g). However, lithium-air batteries are still experiencing challenges, such as
limited charge/discharge cycles and a relative slow charging process.

Figure
2:
Power
Click Image to Enlarge

(acceleration)

and

energy

(range)

by

battery

type

The U.S. governments current rules for the Corporate Average Fuel Economy (or CAF) standards
mandates an average of about 29 miles per gallon gradually increasing to 35.5 mpg by 2016 and 54.5 miles
per gallon by 2025. In order to meet these standards, automakers will gradually switch from the current
pure internal combustion engine based vehicles to various forms of plug-in-hybrid and battery-electric
vehicles. Once battery technology and costs are achieved to provide about 300 miles per charge, the electric
vehicles will be more prevalent than the PHEVs. Another significant change to look for is the advancement
of the clean diesel, diesel hybrids, diesel engine based plug-in hybrids, and liquid natural gas based
vehicles. Clean diesel based hybrids may make it possible for automakers to stretch towards the 100 mpg
mark in coming years. Although fuel cell technology had shown a great promise, the full fuel cell vehicle
continues to remain only as demonstration vehicles. Issues related to cost of manufacturing, robustness of
the technology, hydrogen production, and the hydrogen distribution infrastructure will limit adoption.
However, with the advancement of Polymer Electrolyte Membrane (PEM) and Solid oxide Fuel Cell
(SOFC) technologies, the fuel cells could be used as range extenders in place of internal combustion enginegenerators in series hybrid vehicles. These plug-in, fuel-cell hybrid vehicles (PFCV) consisting of a smaller
fuel cell and a larger battery (battery dominant) could be a future direction for automobiles.
Continuing worldwide R&D by industry, academia, and government will advance the propulsion
technologies further, making EVs more viable with longer range, higher performance, and lower cost. All
electric vehicles will be joined by PHEVs and PFCVs to serve broader segment of the transportation market.
The acceptance these vehicles will be judged based on their performance, reliability, lifespan, and cost.
References
1.

Nissan Leaf Overview, March 2010.

http://www.reuters.com/article/2013/01/24/us-toyota-bmw-fuelcell-idUSBRE90N0L020130124

4.

http://www.gizmag.com/ibm-lithium-air-battery/22310/

Dr. Kaushik Rajashekara, Professor of Electrical Engineering and