Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems
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FUEL CELLS AND ADVANCED TECHNOLOGIES 411
FUEL-CELL CHALLENGES While major automobile man-
ufacturers continue to build demonstration vehicles and work
on improving fuel-cell system design, no vehicle powered by
a fuel cell has been placed into mass production. There are a
number of reasons for this, including the following:
High cost
Lack of refueling infrastructure
Safety perception
Insufficient vehicle range
Lack of durability
Freeze starting problems
Insufficient power density
All of these problems are being actively addressed by
researchers, and significant improvements are being made. Once
cost and performance levels meet that of current vehicles, fuel cells
will be adopted as a mainstream technology.
SEE CHART 32–1.
electrical system. Since they are powered by hydrogen and
oxygen, fuel cells by themselves do not generate carbon emis-
sions such as CO
2
. Instead, their only emissions are water
vapor and heat, and this makes the fuel cell an ideal candidate
for a zero-emission vehicle (ZEV).
A fuel cell is also much more energy efficient than a typi-
cal internal combustion engine. While a vehicle powered by an
internal combustion engine (ICE) is anywhere from 15% to 20%
efficient, a fuel-cell vehicle can achieve efficiencies upwards of
40%. Another major benefit of fuel cells is that they have very
few moving parts and have the potential to be very reliable.
Anumber of OEMs have spent many years and millions of dol-
lars in order to develop a low-cost, durable, and compact fuel
cell that will operate satisfactorily under all driving conditions.
SEE FIGURE 32–3 .
A fuel-cell vehicle (FCV) uses the fuel cell as its only
source of power, whereas a fuel-cell hybrid vehicle (FCHV)
would also have an electrical storage device that can be used
to power the vehicle. Most new designs of fuel-cell vehicles are
now based on a hybrid configuration because of the significant
increase in efficiency and driveability that can be achieved with
this approach.
SEE FIGURE 32–4 .
ZERO EMISSION
F Cell
FIGURE 32–3 The Mercedes-Benz B-Class fuel-cell car was
introduced in 2005.
FIGURE 32–4 The Toyota FCHV is based on the Highlander
platform and uses much of Toyota’s Hybrid Synergy Drive
(HSD) technology in its design.
PAFC (Phosphoric Acid
Fuel Cell)
PEM (Polymer
Electrolyte Membrane)
MCFC (Molten
Carbonate Fuel Cell)
SOFC (Solid
Oxide Fuel Cell)
Electrolyte Orthophosphoric acid Sulfonic acid in polymer Li and K carbonates Yttrium-stabilized
zirconia
Fuel Natural gas, hydrogen Natural gas, hydrogen, methanol Natural gas, synthetic gas Natural gas, synthetic gas
Operating Temp (F) (C ) 360–410°F 176–212°F 1,100–1,300°F 1,200–3,300°F
180–210°C 80–100°C 600–700°C 650–1,800°C
Electric Efficiency 40% 30–40% 43–44% 50–60%
Manufacturers ONSI Corp. Avista, Ballard, Energy Part-
ners, H-Power, International,
Plug Power
Fuel Cell Energy, IHI, Hitachi,
Siemens
Honeywell, Siemens-West-
inghouse, Ceramic
Applications Stationary power Vehicles, portable power, small
stationary power
Industrial and institutional
power
Stationary power,
military vehicles
CHART 32–1
Fuel cell types and their temperature operating range.
412 CHAPTER 32
TYPES OF FUEL CELLS There are a number of differ-
ent types of fuel cells, and these are differentiated by the type
of electrolyte that is used in their design. Some electrolytes
operate best at room temperature, whereas others are made to
operate at up to 1,800°F. See the accompanying chart showing
the various fuel-cell types and applications.
The fuel-cell design that is best suited for automotive appli-
cations is the proton exchange membrane (PEM). A PEM fuel
cell must have hydrogen for it to operate, and this may be stored
on the vehicle or generated as needed from another type of fuel.
ELECTRICITY
AIR
(O
2
)
WATER
(H
2
0)
HYDROGEN
(H)
NEGATIVE
ELECTRODE
POSITIVE
ELECTRODE
LOAD
ELECTRON
FLOW
FIGURE 32–5 The polymer electrolyte membrane allows only
H
ions (protons) to pass through it. This means that electrons
must follow the external circuit and pass through the load to
perform work.
FIGURE 32–6 A fuel-cell stack is made up of hundreds of
individual cells connected in series.
PEM FUEL CELLS
DESCRIPTION AND OPERATION The Proton Exchange
Membrane fuel cell is also known as a polymer electrolyte
fuel cell (PEFC). The PEM fuel cell is known for its lightweight
and compact design as well as its ability to operate at ambi-
ent temperatures. This means that a PEM fuel cell can start
quickly and produce full power without an extensive warm-up
period. The PEM is a simple design based on a membrane
that is coated on both sides with a catalyst such as platinum
or palladium. There are two electrodes, one located on each
side of the membrane. These are responsible for distributing
hydrogen and oxygen over the membrane surface, removing
waste heat, and providing a path for electrical current flow.
The part of the PEM fuel cell that contains the membrane,
catalyst coatings, and electrodes is known as the membrane
electrode assembly (MEA).
The negative electrode (anode) has hydrogen gas directed
to it, while oxygen is sent to the positive electrode (cathode).
Hydrogen is sent to the negative electrode as H 2 molecules,
which break apart into H
ions (protons) in the presence of the
catalyst. The electrons (e
) from the hydrogen atoms are sent
through the external circuit, generating electricity that can be
utilized to perform work. These same electrons are then sent to
the positive electrode, where they rejoin the H
ions that have
passed through the membrane and have reacted with oxygen
in the presence of the catalyst. This creates H
2
O and waste
heat, which are the only emissions from a PEM fuel cell.
SEE
FIGURE 32–5 .
NOTE: It is important to remember that a fuel cell gener-
ates direct current (DC) electricity as electrons flow in
only one direction (from the anode to the cathode).
FUEL-CELL STACKS A single fuel cell by itself is not par-
ticularly useful, as it will generate less than 1 volt of electrical
potential. It is more common for hundreds of fuel cells to be
built together in a fuel-cell stack. In this arrangement, the fuel
cells are connected in series so that total voltage of the stack is
the sum of the individual cell voltages. The fuel cells are placed
end to end in the stack, much like slices in a loaf of bread.
Automotive fuel-cell stacks contain upwards of 400 cells in their
construction.
SEE FIGURE 32–6 .
CO Poisons the PEM Fuel-Cell Catalyst
Purity of the fuel gas is critical with PEM fuel cells.
If more than 10 parts per million (ppm) of carbon
monoxide is present in the hydrogen stream being
fed to the PEM anode, the catalyst will be gradually
poisoned, and the fuel cell will eventually be disa-
bled. This means that the purity must be “five nines”
(99.999% pure). This is a major concern in vehicles
where hydrogen is generated by reforming hydro-
carbons such as gasoline because it is difficult to
remove all CO from the hydrogen during the reform-
ing process. In these applications, some means of
hydrogen purification must be used to prevent CO
poisoning of the catalyst.
TECH TIP
The total voltage of the fuel-cell stack is determined by the
number of individual cells incorporated into the assembly. The
current-producing ability of the stack, however, is dependent on
the surface area of the electrodes. Since output of the fuel-cell
FUEL CELLS AND ADVANCED TECHNOLOGIES 413
CO
+
6H
3H O
6H
+
3/2 O
2
H O
+
CH CH
2
2
2
2
H
H
H
H
H
H
H
H
H
H
H O AIR
(UNUSED O )
2
2
2
OXIDANT
AIR (O )
CO UNUSED
METHANOL/WATER
FUEL
METHANOL/WATER
ANODE (-)
ELECTRODE
CATHODE (+)
ELECTRODE
PROTON EXCHANGE
MEMBRANE (PEM)
LOAD
6e- 6e-
2
FIGURE 32–7 A direct methanol fuel cell uses a methanol/
water solution for fuel instead of hydrogen gas.
M
E
THA
NOL
FIGURE 32–8 A direct methanol fuel cell can be refueled
similar to a gasoline-powered vehicle.
which methanol makes its way across the membrane assembly
and diminishes performance of the cell. Direct methanol fuel
cells also require much greater amounts of catalyst in their con-
struction, which leads to higher costs. These challenges are
leading researchers to look for alternative electrolyte materials
and catalysts to lower cost and improve cell performance.
NOTE: Direct methanol fuel cells are not likely to see
service in automotive applications. However, they are
well suited for low-power applications, such as cell
phones or laptop computers.
stack is related to both voltage and current (voltage current
power), increasing the number of cells or increasing the surface
area of the cells will increase power output. Some fuel-cell ve-
hicles will use more than one stack, depending on power output
requirements and space limitations.
DIRECT METHANOL FUEL CELLS High-pressure cyl-
inders are one method of storing hydrogen onboard a vehicle
for use in a fuel cell. This is a simple and lightweight storage
method but often does not provide sufficient vehicle driving
range. Another approach has been to fuel a modified PEM
fuel cell with liquid methanol instead of hydrogen gas.
SEE
FIGURE 32–7 .
Methanol is most often produced from natural gas and has
a chemical symbol of CH
3
OH. It has a higher energy density
than gaseous hydrogen because it exists in a liquid state at nor-
mal temperatures and is easier to handle since no compressors
or other high-pressure equipment is needed. This means that
a fuel-cell vehicle can be refueled with a liquid instead of high-
pressure gas, which makes the refueling process simpler and
produces a greater vehicle driving range.
SEE FIGURE 32–8 .
Unfortunately, direct methanol fuel cells suffer from a num-
ber of problems, not the least of which is the corrosive nature
of methanol itself. This means that methanol cannot be stored
in existing tanks and thus requires a separate infrastructure for
handling and storage. Another problem is “fuel crossover,” in
What Is the Role of the Humidifier
in a PEM Fuel Cell?
The polymer electrolyte membrane assembly in a
PEM fuel cell acts as conductor of positive ions and
as a gas separator. However, it can perform these
functions effectively only if it is kept moist. A fuel-
cell vehicle uses an air compressor to supply air to
the positive electrodes of each cell, and this air is
sometimes sent through a humidifier first to increase
its moisture content. The humid air then comes in
contact with the membrane assembly and keeps the
electrolyte damp and functioning correctly.
?
FREQUENTLY ASKED QUESTION
FUEL-CELL VEHICLE
SYSTEMS
HUMIDIFIERS Water management inside a PEM fuel cell
is critical. Too much water can prevent oxygen from making
contact with the positive electrode; too little water can allow
the electrolyte to dry out and lower its conductivity. The amount
of water and where it resides in the fuel cell is also critical in
determining at how low a temperature the fuel cell will start
because water freezing in the fuel cell can prevent it from start-
ing. The role of the humidifier is to achieve a balance where it is
providing sufficient moisture to the fuel cell by recycling water
that is evaporating at the cathode. The humidifier is located in
the air line leading to the cathode of the fuel-cell stack.
SEE
FIGURE 32–9 .
Some newer PEM designs manage the water in the cells
in such a way that there is no need to prehumidify the incom-
ing reactant gases. This eliminates the need for the humidifier
assembly and makes the system simpler overall.
FUEL-CELL COOLING SYSTEMS Heat is generated by
the fuel cell during normal operation. Excess heat can lead to
a breakdown of the polymer electrolyte membrane, so a liquid
cooling system must be utilized to remove waste heat from the
414 CHAPTER 32
other support devices use electrical power that is generated by
the fuel cell and therefore tend to decrease the overall efficiency
of the vehicle.
AIR SUPPLY PUMPS Air must be supplied to the fuel-cell
stack at the proper pressure and flow rate to enable proper per-
formance under all driving conditions. This function is performed
by an onboard air supply pump that compresses atmospheric
air and supplies it to the fuel cell’s positive electrode (cathode).
This pump is often driven by a high-voltage electric drive motor.
FUEL-CELL HYBRID VEHICLES Hybridization tends to
increase efficiency in vehicles with conventional drive trains, as
energy that was once lost during braking and otherwise normal
operation is instead stored for later use in a high-voltage bat-
tery or ultracapacitor. This same advantage can be gained
fuel-cell stack. One of the major challenges for engineers in this
regard is the fact that the heat generated by the fuel cell is clas-
sified as low-grade heat. This means that there is only a small
difference between the temperature of the coolant and that of
the ambient air. Heat transfers very slowly under these condi-
tions, so heat exchangers with a much larger surface area must
be utilized.
SEE FIGURE 32–10 .
In some cases, heat exchangers may be placed in other
areas of the vehicle when available space at the front of the
engine compartment is insufficient. In the case of the Toyota
FCHV, an auxiliary heat exchanger is located underneath the
vehicle to increase the cooling system heat-rejection capacity.
SEE FIGURE 32–11 .
An electric water pump and a fan drive motor are used to
enable operation of the fuel cell’s cooling system. These and
DRIVE TRAIN
RADIATOR
(SMALL)
FUEL CELL
RADIATOR
(LARGE)
HIGH PRESSURE
HYDROGEN TANKS
DC BRUSHLESS MOTOR
AND TRANSMISSION
POWER CONTROL
UNIT (PCU)
FUEL CELL
SYSTEM BOX
FUEL CELL
STACK
HUMIDIFIER
ULTRA-
CAPACITOR
FIGURE 32–9 Powertrain layout in a Honda FCX fuel-cell ve-
hicle. Note the use of a humidifier behind the fuel-cell stack to
maintain moisture levels in the membrane electrode assemblies.
FUEL CELL
RADIATOR
POWER TRAIN
RADIATORS
HONDA
FCX
FIGURE 32–10 The Honda FCX uses one large radiator for
cooling the fuel cell and two smaller ones on either side for
cooling drive train components.
When Is Methanol Considered to Be a
“Carbon-Neutral” Fuel?
Most of the methanol in the world is produced by
reforming natural gas. Natural gas is a hydrocarbon
but does not increase the carbon content of our
atmosphere as long as it remains in reservoirs below
the earth’s surface. However, natural gas that is used
as a fuel causes extra carbon to be released into the
atmosphere, which is said to contribute to global
warming. Natural gas is not a carbon-neutral fuel,
and neither is methanol, which is made from
natural gas.
Fortunately, it is possible to generate metha-
nol from biomass and wood waste. Methanol made
from renewable resources is carbon neutral because
no extra carbon is being released into the earth’s
atmosphere than what was originally absorbed by
the plants used to make the methanol.
?
FREQUENTLY ASKED QUESTION
FIGURE 32–11 Space is limited at the front of the Toyota
FCHV engine compartment, so an auxiliary heat exchanger is
located under the vehicle to help cool the fuel-cell stack.
FUEL CELLS AND ADVANCED TECHNOLOGIES 415
ULTRACAPACITOR
MODULE
CELL
POSITIVE POLE
COLLECTOR PLATE
NEGATIVE POLE
COLLECTOR PLATE
ELECTRODE BODY
ELECTROLYTE
ALUMINUM CASE
FIGURE 32–13 The Honda ultracapacitor module and con-
struction of the individual cells.
ULTRACAPACITOR
CAN ARRAY
(EXPOSED)
FIGURE 32–14 An ultracapacitor can be used in place of a
high-voltage battery in a hybrid electric vehicle. This example
is from the Honda FCX fuel-cell hybrid vehicle.
ions move within the electrolyte, but no chemical reaction takes
place. Ultracapacitors can charge and discharge quickly and
efficiently, making them especially suited for electric assist ap-
plications in fuel-cell hybrid vehicles.
Ultracapacitors that are used in fuel-cell hybrid vehicles
are made up of multiple cylindrical cells connected in paral-
lel.
SEE FIGURE 32–14 . This results in the total capaci-
tance being the sum of the values of each individual cell. For
example, 10 1.0- farad capacitors connected in parallel will
have a total capacitance of 10.0 farads. Greater capacitance
means greater electrical storage ability, and this contributes
to greater assist for the electric motors in a fuel-cell hybrid
vehicle.
Ultracapacitors have excellent cycle life, meaning that
they can be fully charged and discharged many times without
degrading their performance. They are also able to operate over
a wide temperature range and are not affected by low tem-
peratures to the same degree as many battery technologies.
The one major downside of ultracapacitors is a lack of specific
energy, which means that they are best suited for sudden bursts
of energy as opposed to prolonged discharge cycles. Research
is being conducted to improve this and other aspects of ultra-
capacitor performance.
FUEL-CELL TRACTION MOTORS Much of the technology behind
the electric drive motors being used in fuel-cell vehicles was
developed during the early days of the California ZEV mandate.
This was a period when battery-powered electric vehicles were
being built by the major vehicle manufacturers in an effort
to meet a legislated quota in the state of California. The ZEV
mandate rules were eventually relaxed to allow other types of
vehicles to be substituted for credit, but the technology that had
been developed for pure electric vehicles was now put to work
in these other vehicle designs.
The electric traction motors used in fuel-cell hybrid ve-
hicles are very similar to those being used in current hybrid
electric vehicles. The typical drive motor is based on an AC
synchronous design, which is sometimes referred to as a DC
brushless motor. This design is very reliable, as it does not use
a commutator or brushes but instead has a three-phase stator
by applying the hybrid design concept to fuel-cell vehicles.
Whereas the fuel cell is the only power source in a fuel-cell vehi-
cle, the fuel-cell hybrid vehicle (FCHV) relies on both the fuel cell
and an electrical storage device for motive power. Driveability is
also enhanced with this design, as the electrical storage device
is able to supply energy immediately to the drive motors and
overcome any “throttle lag” on the part of the fuel cell.
SECONDARY BATTERIES All hybrid vehicle designs require a
means of storing electrical energy that is generated during regen-
erative braking and other applications. In most FCHV designs, a
high-voltage nickel-metal hydride (NiMH) battery pack is used
as a secondary battery. This is most often located near the back
of the vehicle, either under or behind the rear passenger seat.
SEE FIGURE 32–12 . The secondary battery is built similar to
a fuel-cell stack because it is made up of many low-voltage cells
connected in series to build a high-voltage battery.
ULTRACAPACITORS An alternative to storing electrical energy
in batteries is to use ultracapacitors. A capacitor is best known
as an electrical device that will block DC current but allow AC
to pass. However, a capacitor can also be used to store electri-
cal energy, and it is able to do this without a chemical reaction.
Instead, a capacitor stores electrical energy using the principle of
electrostatic attraction between positive and negative charges.
Ultracapacitors are built very different from conventional
capacitors. Ultracapacitor cells are based on double-layer
technology, in which two activated-carbon electrodes are im-
mersed in an organic electrolyte. The electrodes have a very
large surface area and are separated by a membrane that al-
lows ions to migrate but prevents the electrodes from touching.
SEE FIGURE 32–13 . Charging and discharging occurs as
FIGURE 32–12 The secondary battery in a fuel-cell hybrid
vehicle is made up of many individual cells connected in
series, much like a fuel-cell stack.
416 CHAPTER 32
TRANSAXLES Aside from the hydrogen fueling system, fuel-
cell hybrid vehicles are effectively pure electric vehicles in that
their drive train is electrically driven. Electric motors work very
well for automotive applications because they produce high
torque at low RPMs and are able to maintain a consistent power
output throughout their entire RPM range. This is in contrast to
vehicles powered by internal combustion engines, which pro-
duce very little torque at low RPMs and have a narrow range
where significant horsepower is produced.
ICE-powered vehicles require complex transmissions with
multiple speed ranges in order to accelerate the vehicle quickly and
maximize the efficiency of the ICE. Fuel-cell hybrid vehicles use
electric drive motors that require only a simple reduction in their
final drive and a differential to send power to the drive wheels. No
gear shifting is required, and mechanisms such as torque convert-
ers and clutches are done away with completely. A reverse gear is
not required either, as the electric drive motor is simply powered
in the opposite direction. The transaxles used in fuel-cell hybrid
vehicles are extremely simple with few moving parts, making them
extremely durable, quiet, and reliable.
SEE FIGURE 32–17 .
POWER CONTROL UNITS The drive train of a fuel-cell hybrid
vehicle is controlled by a power control unit (PCU), which con-
trols fuel-cell output and directs the flow of electricity between
the various components. One of the functions of the PCU is
to act as an inverter, which changes direct current from the
fuel-cell stack into three-phase alternating current for use in the
vehicle drive motor(s).
SEE FIGURE 32–18 .
and a permanent magnet rotor.
SEE FIGURE 32–15 . An elec-
tronic controller (inverter) is used to generate the three-phase
high-voltage AC current required by the motor. While the mo-
tor itself is very simple, the electronics required to power and
control it are complex.
Some fuel-cell hybrid vehicles use a single electric drive
motor and a transaxle to direct power to the vehicle’s wheels. It
is also possible to use wheel motors to drive individual wheels.
While this approach adds a significant amount of unsprung
weight to the chassis, it allows for greater control of the torque
being applied to each individual wheel.
SEE FIGURE 32–16 .
FOUR WHEEL MOTORS
PROPEL THE VEHICLE
HEAT VENTS TO DISSIPATE
HEAT GENERATED BY THE
FUEL CELL AND ELECTRONICS
FUEL CELL STACKS AND
HYDROGEN STORAGE TANKS
UNIVERSAL DOCKING CONNECTION
CONNECTS TO BODY CONTROL SYSTEMS
MECHANICAL LOCKS
SECURE THE BODY
TO THE SKATEBOARD
FIGURE 32–16 The General Motors “Skateboard” concept
uses a fuel-cell propulsion system with wheel motors at all
four corners.
FIGURE 32–17 The electric drive motor and transaxle assem-
bly from a Toyota FCHV. Note the three orange cables, indicat-
ing that this motor is powered by high-voltage three-phase
alternating current.
HONDA
FCX
POWER CONTROL
UNIT (PCU)
FIGURE 32–18 The power control unit (PCU) on a Honda
FCX fuel-cell hybrid vehicle is located under the hood.
FIGURE 32–15 Drive motors in fuel-cell hybrid vehicles often
use stator assemblies similar to ones found in Toyota hybrid
electric vehicles. The rotor turns inside the stator and has per-
manent magnets on its outer circumference.
FUEL CELLS AND ADVANCED TECHNOLOGIES 417
BATTERY
DRIVE
WHEELS
FINAL DRIVE
MOTOR
POWER
CONTROL
UNIT
FUEL CELL
FIGURE 32–19 Toyota’s FCHV uses a power control unit
that directs electrical energy flow between the fuel cell,
battery, and drive motor.
A number of methods of hydrogen storage are being
considered for use in fuel-cell hybrid vehicles. These include
high-pressure compressed gas, liquefied hydrogen, and solid
storage in metal hydrides. Efficient hydrogen storage is one
of the technical issues that must be solved in order for fuel
cells to be adopted for vehicle applications. Much research
is being conducted to solve the issue of onboard hydrogen
storage.
HIGH-PRESSURE COMPRESSED GAS Most current fuel-cell
hybrid vehicles use compressed hydrogen that is stored in
tanks as a high-pressure gas. This approach is the least
complex of all the storage possibilities but also has the least
energy density. Multiple small storage tanks are often used
rather than one large one in order to fit them into unused
areas of the vehicle. One drawback with this approach is
that only cylinders can be used to store gases at the required
pressures. This creates a good deal of unused space around
the outside of the cylinders and leads to further reductions
in hydrogen storage capacity. It is common for a pressure
of 5,000 PSI (350 bar) to be used, but technology is avail-
able tostore hydrogen at up to 10,000 PSI (700 bar).
SEE
FIGURE 32–21 .
Power to and from the secondary battery is directed
through the power control unit, which is also responsible for
maintaining the battery pack’s state of charge and for con-
trolling and directing the output of the fuel-cell stack.
SEE
FIGURE 32–19 .
During regenerative braking, the electric drive motor acts
as a generator and converts kinetic (moving) energy of the vehi-
cle into electricity for recharging the high-voltage battery pack.
The PCU must take the three-phase power from the motor (gen-
erator) and convert (or rectify ) this into DC voltage to be sent to
the battery. DC power from the fuel cell will also be processed
through the PCU for recharging the battery pack.
A DC-to-DC converter is used in hybrid-electric vehicles
for converting the high voltage from the secondary battery
pack into the 12 volts required for the remainder of the ve-
hicle’s electrical system. Depending on the vehicle, there may
also be 42 volts required to operate accessories such as the
electric-assist power steering. In fuel-cell hybrid vehicles,
the DC-to-DC converter function may be built into the power
control unit, giving it full responsibility for the vehicle’s power
distribution.
HYDROGEN STORAGE One of the pivotal design issues
with fuel-cell hybrid vehicles is how to store sufficient hydrogen
onboard to allow for reasonable vehicle range. Modern drivers
have grown accustomed to having a minimum of 300 miles
between refueling stops, a goal that is extremely difficult to
achieve when fueling the vehicle with hydrogen. Hydrogen has
a very high energy content on a pound-for-pound basis, but
its energy density is less than that of conventional liquid fuels.
This is because gaseous hydrogen, even at high pressure, has
a very low physical density (mass per unit volume).
SEE
FIGURE 32–20 .
THREE HIGH PRESSURE
HYDROGEN STORAGE
TANKS
FIGURE 32–20 This GM fuel-cell vehicle uses compressed
hydrogen in three high-pressure storage tanks.
G
LASS FIBER
CARBON FIBER
ALUMINUM
FIGURE 32–21 The Toyota FCHV uses high-pressure
storage tanks that are rated at 350 bar. This is the equivalent
of 5,000 pounds per square inch.
418 CHAPTER 32
The tanks used for compressed hydrogen storage are typi-
cally made with an aluminum liner wrapped in several layers of
carbon fiber and an external coating of fiberglass. In order to
refuel the compressed hydrogen storage tanks, a special high-
pressure fitting is installed in place of the filler neck used for
conventional vehicles.
SEE FIGURE 32–22 . There is also a
special electrical connector that is used to enable communica-
tion between the vehicle and the filling station during the refuel-
ing process.
SEE FIGURE 32–23 .
The filling station utilizes a special coupler to connect
to the vehicle’s high-pressure refueling fitting. The coupler is
placed on the vehicle fitting, and a lever on the coupler is ro-
tated to seal and lock it into place.
LIQUID HYDROGEN Hydrogen can be liquefied in an effort to
increase its energy density, but this requires that it be stored
in cryogenic tanks at 423°F (253°C). This increases vehicle
range but impacts overall efficiency, as a great deal of energy
is required to liquefy the hydrogen, and a certain amount of the
liquid hydrogen will “boil off” while in storage.
One liter of liquid hydrogen only has one-fourth the en-
ergy content of 1 liter of gasoline.
SEE FIGURES 32–24
AND 32–25 .
SOLID STORAGE OF HYDROGEN One method discovered to
store hydrogen in solid form is as a metal hydride, similar to
how a nickel-metal hydride (NiMH) battery works.
A demonstration vehicle features a lightweight fiber-
wrapped storage tank under the body that stores 3 kg (about
6.6 pounds) of hydrogen as a metal hydride at low pressure.
The vehicle can travel almost 200 miles with this amount
of fuel. One kilogram of hydrogen is equal to 1 gallon of
gasoline. Three gallons of water will generate 1 kilogram of
hydrogen.
A metal hydride is formed when gaseous hydrogen
molecules disassociate into individual hydrogen atoms and
bond with the metal atoms in the storage tank. This process
uses powdered metallic alloys capable of rapidly absorbing
hydrogen to make this occur.
FIGURE 32–22 The high-pressure fitting used to refuel a
fuel-cell hybrid vehicle.
LIQUID
HYDROGEN
STORAGE
FIGURE 32–24 GM’s Hydrogen3 has a range of 249 miles
when using liquid hydrogen.
FIGURE 32–23 Note that high-pressure hydrogen storage
tanks must be replaced in 2020.
LOCKING
MECHANISM
STAINLESS-STEEL
BRAIDING
FILLER LID
FIGURE 32–25 Refueling a vehicle with liquid hydrogen.
FUEL CELLS AND ADVANCED TECHNOLOGIES 419
to send that high-pressure fluid back to the pump, which then
acts as a hydraulic motor and adds torque to the drive line. The
system can be used to launch the vehicle from a stop and/or
add torque for accelerating from any speed.
While the concept is simple, the system itself is very
complicated. Additional components include the following:
Pulse suppressors
Filters
An electric circulator pump for cooling the main pump/
motor
Potential problems with this system include leakage
problems with seals and valves, getting air out of the hydrau-
lic fluid system, and noise. In prototype stages the system
demands different driving techniques. Still, this system was
built to prove the concept, and the engineers believe that
these problems can be solved and that a control system can
be developed that will make HPA transparent to the driver.
A 23% improvement in fuel economy and improvements in
emissions reduction were achieved using a dynamometer
set for a 7,000-pound vehicle. While the HPA system could
bedeveloped for any type of vehicle with any type of drive
train, it does add weight and complexity, which would add
to the cost.
Hydrogen Fuel ⫽ No Carbon
Most fuels contain hydrocarbons or molecules that
contain both hydrogen and carbon. During combus-
tion, the first element that is burned is the hydrogen. If
combustion is complete, then all of the carbon is con-
verted to carbon dioxide gas and exits the engine in
the exhaust. However, if combustion is not complete,
carbon monoxide is formed, leaving some unburned
carbon to accumulate in the combustion chamber.
SEE FIGURE 32–26 .
TECH TIP
FIGURE 32–26 Carbon deposits, such as these, are created
by incomplete combustion of a hydrocarbon fuel.
Ford Motor Co. is experimenting with a system it calls hydrau-
lic power assist (HPA). This system converts kinetic energy
to hydraulic pressure and then uses that pressure to help ac-
celerate the vehicle. It is currently being tested on a four-wheel-
drive (4WD) Lincoln Navigator with a 4.0-L V-8 engine in place
of thestandard 5.4-L engine.
A variable-displacement hydraulic pump/motor is mounted
on the transfer case and connected to the output shaft that
powers the front drive shaft. The HPA system works with or
without 4WD engaged. A valve block mounted on the pump
contains solenoid valves to control the flow of hydraulic fluid.
A 14-gallon, high-pressure accumulator is mounted behind the
rear axle, with a low-pressure accumulator right behind it to
store hydraulic fluid. The master cylinder has a “deadband,”
meaning the first few fractions of an inch of travel do not pres-
surize the brake system. When the driver depresses the brake
pedal, a pedal movement sensor signals the control unit, which
then operates solenoid valves to send hydraulic fluid from the
low-pressure reservoir to the pump. The pumping action slows
the vehicle, similar to engine compression braking, and the
fluid is pumped into the high-pressure reservoir. Releasing the
brake and pressing on the accelerator signals the control unit
HYDRAULIC HYBRID
STORAGE SYSTEM
HCCI
Homogeneous charge compression ignition (HCCI) is a
combustion process. HCCI is the combustion of a very lean
gasoline air–fuel mixture without the use of a spark ignition. It is
a low-temperature, chemically controlled (flameless) combus-
tion process.
SEE FIGURE 32–27 .
HCCI combustion is difficult to control and extremely sen-
sitive to changes in temperature, pressure, and fuel type. While
the challenges of HCCI are difficult, the advantages include
having a gasoline engine being able to deliver 80% of diesel ef-
ficiency (a 20% increase in fuel economy) for 50% of the cost.
A diesel engine using HCCI can deliver gasoline-like emissions.
Spark and injection timing are no longer a factor as they are in
a conventional port-fuel injection system.
While much research and development needs to be per-
formed using this combustion process, it has been shown to
give excellent performance from idle to midload and from ambi-
ent to warm operating temperatures as well as cold-start and
run capability. Because an engine only operates in HCCI mode
at light throttle settings, such as during cruise conditions at
highway speeds, engineers need to improve the transition in
and out of the HCCI mode. Work is also being done on pis-
ton and combustion chamber shape to reduce combustion
noise and vibration that is created during operation in the HCCI
operating mode.
Ongoing research is focusing on improving fuel economy
under real-world operating conditions as well as controlling
costs.
420 CHAPTER 32
as natural gas and hydroelectric power. Plug-in hybrids could
be recharged using rooftop photovoltaic systems, which would
create zero emissions.
DIESEL ENGINE
(COMPRESSION IGNITION)
GASOLINE ENGINE
(SPARK IGNITED)
HCCI ENGINE
(HOMOGENEOUS CHARGE
COMPRESSION IGNITION)
FUEL
INJECTOR
HOT REGIONS
CREATES NO AND
SOOT (PM) EMISSIONS
X
LOW-TEMPERATURE
COMBUSTION RESULTS
IN REDUCED EMISSIONS
HOT REGIONS
CREATES NO
EMISSIONS
X
FIGURE 32–27 Both diesel and conventional gasoline engines create exhaust emissions due to high peak temperatures created
in the combustion chamber. The lower combustion temperatures during HCCI operation result in high efficiency with reduced
emissions.
A plug-in hybrid electric vehicle (PHEV) is a hybrid electric
vehicle that is designed to be plugged into an electrical outlet
at night to charge the batteries. By charging the batteries in the
vehicle, it can operate using electric power alone (stealth mode)
for a longer time, thereby reducing the use of the internal com-
bustion engine (ICE). The less the ICE is operating, the less fuel
is consumed and the lower the emissions. At the present time,
plug-in hybrids are not offered by any major manufacturer but
many conventional HEVs are being converted to plug-in hybrids
by adding additional batteries.
If a production plug-in hybrid is built, it should be able to
get about twice the fuel economy of a conventional hybrid. The
extra weight of the batteries will be offset, somewhat, by the
reduced weight of a smaller ICE. At highway speeds, fuel ef-
ficiency is affected primarily by aerodynamics, and therefore
the added weight of the extra batteries is equal to one or two
additional passengers and thus would only slightly reduce fuel
economy. Recharging will normally take place at night during
cheaper off-peak hours. Counting fuel and service, the total
lifetime cost of ownership will be lower than a conventional
gasoline-powered vehicle.
There are emissions related to the production of the elec-
tricity, which are called well-to-wheel emissions . These emis-
sions, including greenhouse gases, are far lower than those of
gasoline, even for the national power grid, which is 50% coal.
Vehicles charging off-peak will use power from plants that can-
not be turned off at night and often use cleaner sources such
PLUG-IN HYBRID
ELECTRIC VEHICLES
The future of electric vehicles depends on many factors, including
the following:
1. The legislative and environmental incentives to overcome
the cost and research efforts to bring a usable electric
vehicle to the market.
2. The cost of alternative energy. If the cost of fossil fuels in-
creases to the point that the average consumer cannot afford
to drive a conventional vehicle, then electric vehicles (EVs)
may be a saleable alternative.
3. Advancement in battery technology that would allow the
use of lighter-weight and higher-energy batteries.
COLD-WEATHER CONCERNS Past models of electric
vehicles such as the General Motors electric vehicle (EV1) were
restricted to locations such as Arizona and southern California that
had a warm climate because cold weather is a major disadvantage
to the use of electric vehicles for the following reasons:
Cold temperatures reduce battery efficiency.
Additional electrical power from the batteries is needed
to heat the batteries themselves to be able to achieve
reasonable performance.
THE FUTURE FOR
ELECTRIC VEHICLES