Electric cars have been around
since the inception of the automobile. But in the early race for
dominance, the internal combustion engine (ICE) quickly won out as
the best power system for cars. Although the electric power train was superior in many respects, as a source of
energy, the battery was no match for the high energy content, ease
of handling, and cheap and abundant supplies of petroleum motor
fuel. Today, nearly a century after the electric vehicle (EV) was
forced into near oblivion, EVs may actually become the
ultimate winner. As easily-recoverable petroleum deposits dwindle,
automobile populations soar, and cities become choked with
combustion by-products, the ICE is increasingly becoming the victim
of its own success. Automobiles must become cleaner and more
energy efficient. This document explores the benefits and challenges
of clean and efficient electric powered automobiles.
EV Energy Efficiency
Most researchers agree that a
switch to EVs would reduce the total primary energy consumed for
personal transportation. However, many do not agree on the precise
amount of energy that might be saved. The divergence in estimations
is mainly due to the fact that energy use comparisons between
battery-electric vehicles (BEVs), hybrid-electric vehicles (HEVs),
and conventional vehicles (CVs) are affected by a number of
variables and necessary assumptions. Vehicle mass, performance,
range requirements, system configuration, operating schedule, and
the upstream losses of converting source fuels into useable energy
and delivering it to the end user all affect system-wide energy use.
And a realistic baseline EV performance profile is difficult to
define, primarily because the technology is relatively undeveloped
and rapidly changing.
Considering only the vehicle
itself, EVs are more energy efficient than CVs. A BEV operates
at roughly 46% efficiency, whereas a CV operates at about 18%
efficiency. In other words, approximately 46% of the electrical
energy taken from the wall plug to charge EV propulsion batteries is
delivered to the drive wheels as useful work. In contrast, only
about 18% of the energy dispensed into the fuel tank as liquid motor
fuel ends up at the drive wheels of a CV. In order to determine
system-wide energy efficiency (from source fuel to drive wheels), the
upstream losses of refining and delivering motor fuel and the losses
of generating and delivering electricity must be factored in.
The losses of converting source
fuels into electrical energy (conversion losses) and delivering the
energy to a local electrical outlet are far greater than the losses
of extracting, refining, and delivering petroleum motor fuel.
However, petroleum fuel-chain efficiency does not include conversion
losses, as does the electrical energy chain. Conversion of liquid
motor fuel into useable power takes place in the vehicle and is
therefore considered a component of CV energy efficiency.
Specifically, about 83% of the energy contained in crude oil arrives
at the service station as gasoline, whereas only 20% to 27% of the
primary energy used to generate electricity (depending on the source
fuel and conversion efficiency) arrives at the electrical outlet
ready to charge EV batteries. When the entire energy chain is
considered, studies generally conclude that battery-electric cars
are roughly 10% - 30% more energy efficient than conventional
gasoline cars, depending on the particular assumptions of vehicle
energy use and energy chain efficiency. Comparisons between HEVs and
CVs are more diverse because of the many design variables of the
hybrid power system. HEVs are generally considered slightly more
efficient to significantly more efficient than CVs - again,
depending on the assumptions used in the comparison.
Due to the variables and the
significant differences between electric and conventionally powered
vehicles, precise energy use comparisons are difficult to achieve,
and conclusions are often open to debate. Comparisons may not fully
account for the differences in engineering and performance between
baseline EVs and CVs. For example, comparison EVs may be better
engineered or more poorly engineered than their gasoline-fueled
counterparts. And studies often include an indirect energy penalty
for EVs in the form of greater vehicle mass. Looking to the future,
however, improved batteries and hybrid systems will likely reduce
the mass disadvantage that exists today with EVs.
It is
important to csider that EV/CV comparisons are comparisons between
a highly developed power system and a new power system in the early
stages of development. Significant improvements can be expected as
EV technology evolves. Regardless
of whether EV energy efficiency, based on today's technology, would
save 10-, 20-, or 30-percent, an electric powered personal
transportation system will become more efficient over time. In
addition, generating plant conversion efficiency is steadily
improving, and the total primary energy consumed by EVs will decline
in step with more efficient plants. Moreover, the benefits of
electric powered transportation extend far beyond the prospect of
saving energy. Electrical generation plants can use a number of
alternative fuels that are not easily adaptable to mobile power
systems, and emissions are more easily and effectively controlled at
the relatively fewer fixed sites than with millions of individual
systems on independent vehicles.
Source Fuels Flexibility
In essence, BEVs are the ultimate
alternative fuel vehicles because their energy comes from the source
fuels used to generate electricity. In the U.S., which gets 55% of
its electrical energy from coal, battery-electric cars are
predominately coal powered cars. About a third of the energy used by
BEVs in the U.S. would come from clean-burning natural gas. In
Canada, which relies heavily on hydroelectric power,
battery-electric cars are powered mainly by the natural energy of
water seeking its own level. Over half of the electrical energy in
France comes from nuclear plants, which makes French BEVs
predominantly nuclear powered cars. In addition, BEVs make it
possible to meet transportation energy needs with solar, wind, and
geothermal energy, which are already viable options for fixed
generation sites, but are not well suited to mobile applications.
Source fuel flexibility alone offers significant practical and
economic benefits, especially in view of the diversity of regional
energy resources. And EVs, both battery-electric and hybrid-electric
configurations, are inherently cleaner.
Environmental Benefits
A BEV produces zero vehicular
emissions. However, emissions are produced at the generation site
when the source fuel is converted into electrical power. The
emissions of electric cars therefore depend on the emissions profile
of regional generating plants.
Some researchers conclude that,
in regions serviced by coal-fired plants, a switch to EVs may
actually increase emissions of sulfur oxides (SOx) and particulate
matter (PM), and perhaps increase emissions of carbon dioxide (CO2).
Conclusions, however, are usually based on the existing mix of
coal-fired plants, and often, they do not consider the effect of
newer and cleaner plant designs. Studies generally conclude that
emissions of SOx, PM, and CO2 are reduced in regions that rely on
natural gas, and virtually eliminated in regions supplied by
hydroelectric and nuclear power. According to Electric Power
Research Institute (EPRI), substituting EVs for CVs would reduce
urban emissions of non-methane organic gases (NMOG) by 98%, lower
nitrogen oxide (NOx) emissions by 92%, and cut carbon monoxide (CO)
emissions by 99%. In addition, EPRI estimates that, on a nationwide
basis, EVs in the U.S. will produce only half the CO2 of
conventional vehicles.
In another study of six driving
cycles in four U.S. cities, BEVs reduced HC and CO emissions by
approximately 97%, regardless of the regional source fuels mix. In
comparison to large generating plants, conventional cars produce
large amounts of HC and CO emissions, mainly because of cold starts
and short trips that do not allow vehicles to become fully warmed
up.
The environmental benefits of a
hybrid-electric vehicle depend on the design of the hybrid power
system. Some studies show that optimized hybrid vehicles may be
nearly as clean as battery-electric vehicles. Designs using a
combustion engine for onboard electrical generation and an operating
schedule that is heavily biased toward the engine/generator system
(genset) produce the greatest amount of harmful emissions. But even
in this worst-case scenario, emission levels are lower than those of
a typical CV. This is due to the fact that a hybrid vehicle genset
is either switched off, and therefore producing zero emissions, or
it is operating at predetermine output where it produces the fewest
emissions and achieves the best fuel economy per unit of output (the
region of lowest bsfc).
Typically, a hybrid genset is not throttled for variable output,
as is the engine in a conventional vehicle. This leads to more
effective emission controls because it is technically easier to
control combustion-engine emissions when the engine runs
continuously and at a constant output. When the hybrid operating
schedule is biased more toward the energy storage system (relies
more on the battery, rather than the genset), emission levels become
more like those of a BEV. And with fuel-cell hybrids, vehicular
emissions are virtually eliminated. Table T1 provides an energy
efficiency and emissions comparison
between electric vehicles and ICE vehicles running on a variety of
fuels.
Table T1 Energy Efficiency and Emissions for
Mid-Size Automobile
VEHICLE TYPE / FUEL
|
Efficiency Over Fuel Chain (%) |
Net Emissions Over Fuel Chain (1) in g/mile (2)
|
SO2 |
NOx |
CO |
HC |
CO2 |
ICE Vehicle
Gasoline
Methanol
Ethanol
CNG
Hydrogen |
10.2
8.5
8.1
10.8
9.4 |
0.20
-----
0.04
-----
----- |
0.63
0.86
0.52
0.40
0.61 |
3.43
1.71
1.90
1.70
0.02 |
0.35
0.35
0.13
0.16
0.75 |
444
408
44(3)
337
388(4) |
BEV by Source Fuel
Coal
Natural Gas
Petroleum
Nuclear
Adv. NG |
16.5
15.1
14.6
14.4
20.0 |
1.73
----
0.93
0.10
---- |
0.81
0.52
0.52
0.05
0.36 |
0.07
0.09
0.08
----
0.20 |
0.01
0.01
0.02
----
0.07 |
485
302
459
25
229 |
Fuel Cell Vehicle
Methanol
Ethanol
Natural Gas
Hydrogen |
17.6
15.1
21.7
21.0 |
----
0.02
----
---- |
0.27
0.08
----
0.11 |
0.01
0.13
----
0.01 |
----
0.02
----
---- |
236
28
196
197 |
(1) From
primary resource extraction through vehicle end-use, except for SO2,
NOx, CO2, and HC emissions, which are estimated for fuel/electricity
production and vehicle tailpipe only.
(2) g/mile x 0.621 = g/km.
(3) Assumes ethanol-derived farm and conversion energy, and a zero
net CO2 release from biomass conversion due to the carbon content of
the biomass having been adsorbed from the environment during crop
growing.
(4) Assumes hydrogen from natural gas, which releases CO2 during
reforming.
* Condensed from "Diverse Choices for Electric and Hybrid Motor
Vehicles," OECD paper by John J. Brogan, et al, Director, Office of
Propulsion Systems,
U.S.
Department of Energy (1992).
Electric cars hold the promise of transforming personal
transportation into a far more environmentally benign commodity. And
by transferring the job of power generation to a more centralized
and specialized sector, emission controls become more effective and
economical, and source fuel options broaden and become less
technically challenging.
Technical Overview of Electric Vehicles
Electric vehicles are divided
into two general categories: battery-electric vehicles and
hybrid-electric vehicles, which represent the design orientation of
the vehicles' power system. Battery-electric vehicles, or BEVs, are
vehicles that use secondary batteries (rechargeable batteries,
normally called storage batteries) as their only source of energy. A
hybrid-electric vehicle, or HEV, combines an electrical energy
storage system with an onboard means of generating electricity or
augmenting the energy stores of the battery,
normally through the consumption of some type of fuel. Each type of
EV has its own operating characteristics and preferred design
practices, as well as advantages and disadvantages.
A primary technical advantage
with EVs of either category is the inherent bi-directionality of
their energy/work loop. An EV power train can convert energy stores
into vehicle motion, just like a conventional vehicle, and it can
also reverse direction and convert vehicle motion (kinetic energy)
back into energy stores through regenerative braking. In contrast,
combustion engine vehicles cannot reverse the direction of the
onboard energy flow and convert vehicle motion back into fuel. The
significance of regeneration becomes apparent when one considers
that approximately 60 percent of the total energy spent in urban
driving goes to overcoming the effects of inertia, and
theoretically, up to half of this energy can be reclaimed on
deceleration.
Other technical advantages center
on the superiority of the EV's electro-mechanical power train. In
comparison to the internal combustion engine, an electric motor is a
relatively simple and far more efficient machine. Moving parts
consist primarily of the armature (dc motors) or rotor (ac motors)
and bearings, and motoring efficiency is typical on the order of 70-
to 85-percent. In addition, electric motor torque characteristics
are much more suited to the torque demand curve of a vehicle.
A vehicle needs high torque at
low speeds for acceleration, then demands less torque as cruising
speed is approached. An electric motor develops maximum torque at
low rpm, then torque declines with speed, mostly in step with a
vehicle's natural demand. In contrast, an ICE develops very little
torque at low rpm, and must accelerate through nearly three-quarters
of its rpm band before it can deliver maximum torque. A multi-ratio
transmission is therefore necessary in order to correctly match ICE
output characteristics to the vehicle demand curve. Due to the more
favorable output curve of the electric motor, an EV drive train
usually does not require more than two gear ratios, and often needs
only one. Moreover, a reverse gear is unnecessary because the
rotational direction of the motor itself can be reversed simply by
reversing the electrical input polarity. These advantages lead to a
far less complex and more efficient power train, at least on a
mechanical level.
The mechanical simplicity of the
EV power train is somewhat offset by increased complexity on an
electronic level. Electrical power is delivered to the wall outlet
in the form of alternating current, and must be converted into
direct current in order to charge EV batteries. In the case of EVs
powered by dc motors, electricity from the battery must then be
"chopped" into small bursts of variable duty cycle in order to
control the speed and torque of the motor. With EVs using ac motors,
the direct current from the battery must undergo complex power
condition in order to deliver alternating current and provide
control over motoring output. Power conditioning systems have
traditionally been large and expensive devices. In recent years,
however, electronic control technology has improved and costs and
size have declined. With increased demand, the technology should
continue to improve, and economies of scale will come into play.
The main
disadvantage of BEVs is limited energy stores due to the limitations
of the secondary battery, and HEVs tend to be plagued by increased
mass and costs due to the increased complexity of the power system.
EV Batteries
BEVs use rechargeable batteries as a
source of electrical energy. A BEV's batteries do not store
electrical energy in the same sense that a fuel tank stores liquid
fuel. Instead, they are essentially self-contained electrochemical
reactors in which the by-products are retained within the battery
housing. During recharge, these by-products are reconstituted into
their original state where they are ready for another
electrochemical reaction cycle.
Secondary batteries are limited
in their capacity to produce electrical energy by the accumulation
of by-products, and by the limited quantity of reactants they can
contain. And because the recharge cycle does not fully reverse the
changes that take place during discharge, waste products accumulate,
the reacting components degrade, and the battery's ability to
produce electrical energy steadily declines until it is no longer
serviceable. In comparison, a tank of gasoline contains roughly 100
times more energy than an equal mass of lead/acid batteries.
Moreover, part of the IC-engine's reactants are taken from the air,
by-products are continuously discharged, rather than retained and
reconstituted, and the storage and conversion system is largely
unaffected by the process. The task of designing a BEV that will
match the conventional vehicle's specific energy profile is
enormously challenging because of the inherent limitation of its
electrochemical energy system.
The most promising replacement for the lead/acid battery appears to
be the Lithium-based batteries. Lithium-based batteries can store four
or five times as much energy. However, these advanced batteries are
very costly. Table T2 provides a performance comparison between
different battery couples.
Table T2 Battery Comparison
BATTERY TYPE |
Specific Energy
W-h/kg |
Specific Power
W/kg |
Energy Efficiency
In Percent |
Lead/Acid |
40 |
130 |
65 |
Aluminum/Air |
200 |
150 |
35 |
Lithium/Iron-Disulfide |
>130 |
>120 |
---- |
Lithium/Polymer |
200 |
100 |
---- |
Nickel/Cadmium |
56 |
200 |
65 |
Nickel/Iron |
55 |
130 |
60 |
Nickel/Metal Hydride |
80 |
200 |
65 |
Nickel/Zinc |
80 |
150 |
65 |
Sodium/Sulfur |
100 |
120 |
85 |
Zinc/Air |
120 |
120 |
60 |
Zinc/Bromine |
70 |
100 |
65 |
Hybrid-Electric Cars
The idea of a hybrid-electric
vehicle naturally evolves from the inherent limitations of the
storage battery. As first conceived, a hybrid vehicle would employ
an onboard means of generating electricity in order to augment the
limited energy available from the battery. The vehicle might then
run on battery energy alone when range is within the capability of
the battery's energy stores, then use the heat-engine when range
requirements exceed the energy stores of the battery.
Although simple in concept, the
task of achieving significant improvements in energy efficiency
depends on the correct integration of subsystems within a
sophisticated control strategy that continuously monitors and
balances the energy flow onboard the vehicle. When approached as a
system, a hybrid power system is no longer a simple battery-electric
system augmented by a heat-engine. Instead it is an integrated,
self-adapting, propulsion system that may ultimately utilize
batteries (or ultra-capacitors) as an energy reservoir for load
leveling, rather than in their traditional role of supplying total
vehicle motive power.
Much of the research today is
oriented toward developing the most effective control strategy, the
best bias between subsystems, and the correct combination of
subsystem types needed to achieve maximum efficiency with a minimum
of hardware, mass, and manufacturing costs. The original concept of
a heat-engine-augmented BEV is not necessarily incorrect, but the shift
in perspective to a systems approach has opened new opportunities
for greater efficiency and performance.
Types of Hybrids
Hybrids are normally divided into
the subtypes of either series or parallel, which
refers to the way in which the engine supplies power to the
propulsion system. In the series hybrid, a heat engine powers a
generator which charges the battery or supplies power directly to
the propulsion circuit and thereby reduces demand on the battery. Town
Car (available in a DIY plans package on this site) is a
series hybrid. The XR3 Hybrid, also available on this site, is a ground-connected parallel hybrid. In a parallel hybrid, the heat engine delivers
mechanical power directly to the drive train, and the generator is
eliminated. With this type, the battery-electric system or the heat
engine may be used to propel the vehicle, or they may be used
simultaneously for maximum power.
The parallel hybrid is more efficient than the series hybrid. The
efficiency advantage comes from the fact that parallel hybrids
deliver heat-engine power (mechanical power) directly to the power
train, rather than converting the power into electricity.
Losses occur whenever power is
converted from one form to another. In the series hybrid, a
heat engine runs a generator to produce electricity. This conversion
(from mechanical to electrical power) results in a loss of about 20
percent. Electricity may be delivered to a battery or to a
motor to provide motive power. If it is delivered to a motor,
the motor converts electrical energy into mechanical power, but the
conversion results in a loss of about 20 percent. If electrical
energy is delivered to a battery, an additional 20 to 30 percent of
the energy will be lost in the conversion process (putting the
charge into the battery and then taking it out again).
Since the heat engine produces mechanical power in the first place,
it is more efficient to put it to work in its native form rather
than making a double conversion - from one form to another form and
then back to its original form. But decisions about when to convert
and how much to convert are engineering decisions that are made
within the context of the total design of the system.
The first hybrids introduced were “mild
hybrids”. With a mild hybrid a
downsized heat engine is installed to provide the primary motive
power. The electric power system is configured to augment the
shortfall in torque of the heat engine during periods of
acceleration. At cruising speeds the heat engine powers a
generator (normally the torque-assist motor is electronically switched into a generation mode) to replenish the electrical energy that was used for
acceleration. These types of hybrids are not designed to run on
battery power alone. So the battery pack is very small in
comparison to that of a conventional BEV – perhaps enough energy stores to drive
for two or three blocks.
The next generation of hybrids
will be built on the “plug-in” hybrid (PHEV) architecture. These
types of hybrids contain enough energy stores to drive significant
distances on battery power alone. Much of the plug-in hybrid’s
energy is taken from the grid system (wall-plug electricity) where
it is less expensive to produce. Electricity taken from the grid
system roughly equates to motor fuel at 50 to 75 cents per U.S.
gallon (depending on many variables).
Power System Architecture and Control Strategy
The best power system
architecture for HEVs is still the subject of ongoing investigation. There are many options, and there is no “right way” to design a
HEV.
Traditionally, hybrid control
strategy has been dependent on the mission of the vehicle and the
particular tradeoffs made by engineers when they define the
architecture of the power system. Before designers can proceed, the
vehicle mission and power and energy requirements are defined.
Typically, energy and power requirements are based on a multiple of
driving schedules that include power/duration plots of acceleration,
cruise, and total trip energy demands for each driving schedule. The
vehicle and its power system is then designed according to the
mission profile, which may be comprised of worst-case demand levels
from several different driving schedules. Control system strategy is
designed according to the characteristics of the subsystems and the
power demand curves of driving schedules.
In a system that is heavily
biased toward either on-board generation or direct mechanical power from a
heat engine, the battery is normally downsized and reconfigured for
maximum specific power. In a series hybrid, for example, average
power may be supplied by a genset with the battery serving as a
reservoir for regenerative braking energy, and to supply peak power
for acceleration and passing. Ultra-capacitors offer many advantages
with this type of system.
Plug-in hybrids offer the ability
to change the bias of the power system between an emphasis on
battery stores and an emphasis on heat-engine power. Flexible
biasing between the two power systems can offer some advantages; the XR3 Hybrid is built around this concept. For trips on the
order of 40 miles or less, the XR3 may be operated on battery power
alone. For greater range, including range equal to that of a
conventional vehicle, the XR3 may be operated on a combination of
electrical and heat-engine power, or on heat-engine power alone.
Widescale Use of Electric Cars
The roadblocks to widescale use of EVs have included technical,
economic, and perceptual disadvantages. Technical problems have
traditionally centered on the limitations of the storage battery,
which is responsible for today's emphasis on hybrid power systems.
But HEVs have also been plague by inherent disadvantages - primarily
greater vehicle mass and higher manufacturing costs, which are
natural by-products of their inherently greater mechanical and
electrical complexity. Industry has been hard at work
developing new designs that will reduce manufacturing costs and
provide environmentally benign personal transportation products.
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