AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Hybrid Vehicle engines

A hybrid electric vehicle (HEV) is a type of hybrid vehicle and electric vehicle which combines
a conventional internal combustion engine (ICE) propulsion system with an electric propulsion
system. The presence of the electric powertrain is intended to achieve either better fuel
economy than a conventional vehicle, or better performance. A variety of types of HEV exist,
and the degree to which they function as EVs varies as well. The most common form of HEV is
the hybrid electric car, although hybrid electric trucks (pickups and tractors) and buses also
exist. Modern HEVs make use of efficiency-improving technologies such as regenerative
braking, which converts the vehicle's kinetic energy into battery-replenishing electric energy,
rather than wasting it as heat energy as conventional brakes do. Some varieties of HEVs use
their internal combustion engine to generate electricity by spinning an electrical generator (this
combination is known as a motor-generator), to either recharge their batteries or to directly
power the electric drive motors. Many HEVs reduce idle emissions by shutting down the ICE at
idle and restarting it when needed; this is known as a start-stop system. A hybrid-electric
produces less emissions from its ICE than a comparably-sized gasoline car, since an HEV's
gasoline engine is usually smaller than a comparably-sized pure gasoline-burning vehicle
(natural gas and propane fuels produce lower emissions) and if not used to directly drive the
car, can be geared to run at maximum efficiency, further improving fuel economy.

Types of powertrain
Hybrid electric vehicles can be classified according to the way in which power is supplied to the
drivetrain:

 In parallel hybrids, the ICE and the electric motor are both connected to the mechanical
transmission and can simultaneously transmit power to drive the wheels, usually through
a conventional transmission. Honda's Integrated Motor Assist (IMA) system as found in
the Insight, Civic, Accord, as well as the GM Belted Alternator/Starter (BAS Hybrid)
system found in the Chevrolet Malibu hybrids are examples of production parallel
hybrids. Current, commercialized parallel hybrids use a single, small (<20 kW) electric
motor and small battery pack as the electric motor is not designed to be the sole source
of motive power from launch. Parallel hybrids are also capable of regenerative braking
and the internal combustion engine can also act a generator for supplemental
recharging. Parallel hybrids are more efficient than comparable non-hybrid vehicles
especially during urban stop-and-go conditions and at times during highway operation
where the electric motor is permitted to contribute.

 In series hybrids, only the electric motor drives the drivetrain, and the ICE works as a
generator to power the electric motor or to recharge the batteries. The battery pack can
be recharged through regenerative braking or by the ICE. Series hybrids usually have a
smaller combustion engine but a larger battery pack as compared to parallel hybrids,
which makes them more expensive than parallels. This configuration makes series
hybrids more efficient in city driving. The Chevrolet Volt is a series plug-in hybrid,
although GM prefers to describe the Volt as an electric vehicle equipped with a "range
extending" gasoline powered ICE as a generator and therefore dubbed an "Extended
Range Electric Vehicle" or E-REV.

 Power-split hybrids have the benefits of a combination of series and parallel

characteristics. As a result, they are more efficient overall, because series hybrids tend
to be more efficient at lower speeds and parallel tend to be more efficient at high
speeds; however, the power-split hybrid is higher than a pure parallel. Examples of

Unit II: Power System and New Generation Vehicles Page 1 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

power-split (referred to by some as "series-parallel") hybrid powertrains include current

models of Ford, General Motors, Lexus, Nissan, and Toyota.

Types by degree of hybridization

 Full hybrid, sometimes also called a strong hybrid, is a vehicle that can run on just the
engine, just the batteries, or a combination of both. Ford's hybrid system, Toyota's
Hybrid Synergy Drive and General Motors/Chrysler's Two-Mode Hybrid technologies are
full hybrid systems.[18] The Toyota Prius, Ford Escape Hybrid, and Ford Fusion Hybrid
are examples of full hybrids, as these cars can be moved forward on battery power
alone. A large, high-capacity battery pack is needed for battery-only operation. These
vehicles have a split power path allowing greater flexibility in the drivetrain by
interconverting mechanical and electrical power, at some cost in complexity.

 Mild hybrid, is a vehicle that can not be driven solely on its electric motor, because the
electric motor does not have enough power to propel the vehicle on its own. Mild hybrids
only include some of the features found in hybrid technology, and usually achieve limited
fuel consumption savings, up to 15 percent in urban driving and 8 to 10 percent overall
cycle. A mild hybrid is essentially a conventional vehicle with oversize starter motor,
allowing the engine to be turned off whenever the car is coasting, braking, or stopped,
yet restart quickly and cleanly. The motor is often mounted between the engine and
transmission, taking the place of the torque converter, and is used to supply additional
propulsion energy when accelerating. Accessories can continue to run on electrical
power while the gasoline engine is off, and as in other hybrid designs, the motor is used
for regenerative braking to recapture energy. As compared to full hybrids, mild hybrids
have smaller batteries and a smaller, weaker motor/generator, which allows
manufacturers to reduce cost and weight.

Stratified charge engines

In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This
allows for higher compression ratios without "knock," and leaner air/fuel mixtures than in
conventional internal combustion engines. Conventionally, a four-stroke (petrol or gasoline) Otto
cycle engine is fuelled by drawing a mixture of air and fuel into the combustion chamber during
the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and
fuel, which is ignited by a spark plug at a predetermined moment near the top of the
compression stroke.

In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A
stoichiometric mixture contains the exact amount of air necessary for a complete combustion of
the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any
attempt to improve fuel economy by running a lean mixture with a homogeneous charge results
in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides or NOx.
If the Otto cycle is abandoned, however, and fuel is injected directly into the combustion-
chamber during the compression stroke, the petrol engine is liberated from a number of its
limitations. First, a higher mechanical compression ratio (or, with supercharged engines,
maximum combustion pressure) may be used for better thermodynamic efficiency. Since fuel is
not present in the combustion chamber until virtually the point at which combustion is required
to begin, there is no risk of pre-ignition or engine knock. The engine may also run on a much
leaner overall air/fuel ratio, using stratified charge.

Unit II: Power System and New Generation Vehicles Page 2 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling
a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in
the combustion-chamber. This results in a stratified charge: one in which the air/fuel ratio is not
homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially
quite complex) way across the volume of the cylinder.

A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This
mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in a
high-quality combustion of the much weaker mixture elsewhere in the cylinder. Direct fuelling of
petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-
fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge),
with no real drawbacks. Powerful electronic management systems mean that there is not even a
significant cost penalty. With the further impetus of tightening emissions legislation, the motor
industry in Europe and north America has now switched completely to direct fuelling for the new
petrol engines it is introducing.

It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels.

Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus
greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher
energy density, and in combination with higher combustion pressures can deliver very strong
torque and high thermodynamic efficiency for more 'normal' road vehicles.

Learn burn engines: Principle

A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for
providing the power desired for acceleration, but must operate well below that point in normal
steady-speed operation. Ordinarily, the power is cut by partially closing a throttle. However, the
extra work done in pumping air through the throttle reduces efficiency. If the fuel/air ratio is
reduced, then lower power can be achieved with the throttle closer to fully open, and the
efficiency during normal driving (below the maximum torque capability of the engine) can be
higher.

The engines designed for lean burning can employ higher compression ratios and thus provide
better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found
in conventional petrol engines. Ultra lean mixtures with very high air-fuel ratios can only be
achieved by direct injection engines. The main drawback of lean burning is that a complex
catalytic converter system is required to reduce NOx emissions. Lean burn engines do not work
well with modern 3-way catalytic converter—which require a pollutant balance at the exhaust
port so they can carry out oxidation and reduction reactions—so most modern engines run at or
near the stoichiometric point. Alternatively, ultra-lean ratios can reduce NOx emissions.

Heavy-duty gas engines

Lean burn concepts are often used for the design of heavy-duty natural gas, biogas, and
liquefied petroleum gas (LPG) fuelled engines. These engines can either be full-time lean burn,
where the engine runs with a weak air-fuel mixture regardless of load and engine speed, or part-
time lean burn (also known as "lean mix" or "mixed lean"), where the engine runs lean only
during low load and at high engine speeds, reverting to a stoichiometric air-fuel mixture in other
cases.

Unit II: Power System and New Generation Vehicles Page 3 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Heavy-duty lean burn gas engines admit as much as 75% more air than theoretically needed for
complete combustion into the combustion chambers. The extremely weak air-fuel mixtures lead
to lower combustion temperatures and therefore lower NOx formation. While lean-burn gas
engines offer higher theoretical thermal efficiencies, transient response and performance may
be compromised in certain situations. Lean burn gas engines are almost always turbocharged,
resulting high power and torque figures not achieveable with stoichiometric engines due to high
combustion temperatures. Heavy duty gas engines may employ precombustion chambers in the
cylinder head. A lean gas and air mixture is first highly compressed in the main chamber by the
piston. A much richer, though much lesser volume gas/air mixture is introduced to the
precombustion chamber and ignited by spark plug. The flame front spreads to the lean gas air
mixture in the cylinder. This two stage lean burn combustion produces low NOx and no
particulate emissions. Thermal efficiency is better as higher compression ratios are achieved.

Manufacturers of heavy-duty lean burn gas engines include GE Jenbacher, MAN Diesel &
Turbo, Wärtsilä, Mitsubishi Heavy Industries and Rolls-Royce plc.

Honda lean burn systems

One of the newest lean-burn technologies available in automobiles currently in production uses
very precise control of fuel injection, a strong air-fuel swirl created in the combustion chamber, a
new linear air-fuel sensor (LAF type O2 sensor) and a lean-burn NOx catalyst to further reduce
the resulting NOx emissions that increase under "lean-burn" conditions and meet NOx
emissions requirements. This stratified-charge approach to lean-burn combustion means that
the air-fuel ratio isn't equal throughout the cylinder. Instead, precise control over fuel injection
and intake flow dynamics allows a greater concentration of fuel closer to the spark plug tip
(richer), which is required for successful ignition and flame spread for complete combustion. The
remainder of the cylinders' intake charge is progressively leaner with an overall average air:fuel
ratio falling into the lean-burn category of up to 22:1.

The older Honda engines that used lean burn (not all did) accomplished this by having a parallel
fuel and intake system that fed a pre-chamber the "ideal" ratio for initial combustion. This
burning mixture was then opened to the main chamber where a much larger and leaner mix
then ignited to provide sufficient power. During the time this design was in production this
system (CVCC, Compound Vortex Controlled Combustion) primarily allowed lower emissions
without the need for a catalytic converter. These were carbureted engines and the relative
"imprecise" nature of such limited the MPG abilities of the concept that now under MPI (Multi-
Port fuel Injection) allows for higher MPG too. The newer Honda stratified charge (lean burn
engines) operate on air-fuel ratios as high as 22:1. The amount of fuel drawn into the engine is
much lower than a typical gasoline engine, which operates at 14.7:1—the chemical
stoichiometric ideal for complete combustion when averaging gasoline to the petrochemical
industries' accepted standard of C6H8. This lean-burn ability by the necessity of the limits of
physics, and the chemistry of combustion as it applies to a current gasoline engine must be
limited to light load and lower RPM conditions. A "top" speed cut-off point is required since
leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be
"complete" by the time the exhaust valve open

Hydrogen engines
A hydrogen internal combustion engine vehicle (HICEV) is a type of hydrogen vehicle using
an internal combustion engine. Hydrogen internal combustion engine vehicles are different from
hydrogen fuel cell vehicles (which use hydrogen + oxygen rather than hydrogen + air); the

Unit II: Power System and New Generation Vehicles Page 4 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

hydrogen internal combustion engine is simply a modified version of the traditional gasoline-
powered internal combustion engine.[

Low Emissions
The combustion of hydrogen with oxygen produces water as its only product:
2H2 + O2 → 2H2O

The combustion of hydrogen with air can also produce oxides of nitrogen, though at negligibly
small amounts. Tuning a hydrogen engine to create the most amount of emissions as possible
results in emissions comparable with consumer operated gasoline engines from 1976.

H2 + O2 + N2 → H2O + N2 + NOx

Adaptation of Existing Engines

Difference between a hydrogen ICE from a traditional gasoline engine could include hardened
valves and valve seats, stronger connecting rods, non-platinum tipped spark plugs, higher
voltage ignition coil, fuel injectors designed for a gas instead of a liquid, larger crankshaft
damper, stronger head gasket material, modified (for supercharger) intake manifold, positive
pressure supercharger, and a high temperature engine oil. All modifications would amount to
about one point five times (1.5) the current cost of a gasoline engine. These hydrogen engines
burn fuel in the same manner that gasoline engines do. The power output of a direct injected
hydrogen engine vehicle is 20% more than for a gasoline engine vehicle and 42% more than a
hydrogen engine vehicle using a carburetor.
HCCI engine

Homogeneous charge compression ignition (HCCI)

It is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are
compressed to the point of auto-ignition. As in other forms of combustion, this exothermic
reaction releases chemical energy into a sensible form that can be transformed in an engine
into work and heat.

Introduction

HCCI has characteristics of the two most popular forms of combustion used in SI engines:
homogeneous charge spark ignition (gasoline engines) and CI engines: stratified charge
compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and
oxidizer are mixed together. However, rather than using an electric discharge to ignite a portion
of the mixture, the density and temperature of the mixture are raised by compression until the
entire mixture reacts spontaneously. Stratified charge compression ignition also relies on
temperature and density increase resulting from compression, but combustion occurs at the
boundary of fuel-air mixing, caused by an injection event, to initiate combustion.

The defining characteristic of HCCI is that the ignition occurs at several places at a time which
makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion.
This makes the process inherently challenging to control. However, with advances in
microprocessors and a physical understanding of the ignition process, HCCI can be controlled
to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI
engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NO x)
without an aftertreatment catalytic converter. The unburned hydrocarbon and carbon monoxide

Unit II: Power System and New Generation Vehicles Page 5 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still
be treated to meet automotive emission regulations.

Recent research has shown that the use of two fuels with different reactivities (such as gasoline
and diesel) can help solve some of the difficulties of controlling HCCI ignition and burn rates.
RCCI or Reactivity Controlled Compression Ignition has been demonstrated to provide highly
efficient, low emissions operation over wide load and speed ranges *.

HCCI engines have a long history, even though HCCI has not been as widely implemented as
spark ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was
popular before electronic spark ignition was used. One example is the hot-bulb engine which
used a hot vaporization chamber to help mix fuel with air. The extra heat combined with
compression induced the conditions for combustion to occur. Another example is the "diesel"
model aircraft engine.

Operation: Methods
A mixture of fuel and air will ignite when the concentration and temperature of reactants is
sufficiently high. The concentration and/or temperature can be increased by several different
ways:

 High compression ratio

 Pre-heating of induction gases
 Forced induction
 Retained or re-inducted exhaust gases

Once ignited, combustion occurs very quickly. When auto-ignition occurs too early or with too
much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an
engine. For this reason, HCCI is typically operated at lean overall fuel mixtures.

Advantages
 HCCI provides up to a 30-percent fuel savings, while meeting current emissions
standards.
 Since HCCI engines are fuel-lean, they can operate at a Diesel-like compression ratios
(>15), thus achieving higher efficiencies than conventional spark-ignited gasoline
engines.
 Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions.
Actually, because peak temperatures are significantly lower than in typical spark ignited
engines, NOx levels are almost negligible. Additionally, the premixed lean mixture does
not produce soot.
 HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.
 In regards to gasoline engines, the omission of throttle losses improves HCCI efficiency.

Disadvantages
 High in-cylinder peak pressures may cause damage to the engine.
 High heat release and pressure rise rates contribute to engine wear.
 The autoignition event is difficult to control, unlike the ignition event in spark ignition (SI)
and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors,
respectively.

Unit II: Power System and New Generation Vehicles Page 6 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

 HCCI engines have a small power range, constrained at low loads by lean flammability
limits and high loads by in-cylinder pressure restrictions.
 Carbon monoxide (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a
typical spark ignition engine, caused by incomplete oxidation (due to the rapid
combustion event and low in-cylinder temperatures) and trapped crevice gases,
respectively.

Control
Controlling HCCI is a major hurdle to more widespread commercialization. HCCI is more difficult
to control than other popular modern combustion engines, such as Spark Ignition (SI) and
Diesel. In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In Diesel
engines, combustion begins when the fuel is injected into compressed air. In both cases, the
timing of combustion is explicitly controlled. In an HCCI engine, however, the homogeneous
mixture of fuel and air is compressed and combustion begins whenever the appropriate
conditions are reached. This means that there is no well-defined combustion initiator that can be
directly controlled. Engines can be designed so that the ignition conditions occur at a desirable
timing. To achieve dynamic operation in an HCCI engine, the control system must change the
conditions that induce combustion. Thus, the engine must control either the compression ratio,
inducted gas temperature, inducted gas pressure, fuel-air ratio, or quantity of retained or re-
inducted exhaust. Several control approaches are discussed below.

Variable compression ratio

There are several methods for modulating both the geometric and effective compression ratio.
The geometric compression ratio can be changed with a movable plunger at the top of the
cylinder head. This is the system used in "diesel" model aircraft engines. The effective
compression ratio can be reduced from the geometric ratio by closing the intake valve either
very late or very early with some form of variable valve actuation (i.e. variable valve timing
permitting Miller cycle). Both of the approaches mentioned above require some amounts of
energy to achieve fast responses. Additionally, implementation is expensive. Control of an HCCI
engine using variable compression ratio strategies has been shown effective. The effect of
compression ratio on HCCI combustion has also been studied extensively.

Variable induction temperature

In HCCI engines, the auto ignition event is highly sensitive to temperature. Various methods
have been developed which use temperature to control combustion timing. The simplest method
uses resistance heaters to vary the inlet temperature, but this approach is slow (cannot change
on a cycle-to-cycle basis). Another technique is known as fast thermal management (FTM). It is
accomplished by rapidly varying the cycle to cycle intake charge temperature by rapidly mixing
hot and cold air streams. It is also expensive to implement and has limited bandwidth
associated with actuator energy.

Variable exhaust gas percentage

Exhaust gas can be very hot if retained or re-inducted from the previous combustion cycle or
cool if re circulated through the intake as in conventional EGR systems. The exhaust has dual
effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the
chemical energy and engine work. Hot combustion products conversely will increase the
temperature of the gases in the cylinder and advance ignition. Control of combustion timing
HCCI engines using EGR has been shown experimentally.

Unit II: Power System and New Generation Vehicles Page 7 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Variable valve actuation

Variable valve actuation (VVA) has been proven to extend the HCCI operating region by giving
finer control over the temperature-pressure-time history within the combustion chamber. VVA
can achieve this via two distinct methods:

 Controlling the effective compression ratio: A variable duration VVA system on intake
can control the point at which the intake valve closes. If this is retarded past bottom dead
center (BDC), then the compression ratio will change, altering the in-cylinder pressure-
time history prior to combustion.
 Controlling the amount of hot exhaust gas retained in the combustion chamber: A VVA
system can be used to control the amount of hot internal exhaust gas recirculation
(EGR) within the combustion chamber. This can be achieved with several methods,
including valve re-opening and changes in valve overlap. By balancing the percentage of
cooled external EGR with the hot internal EGR generated by a VVA system, it may be
possible to control the in-cylinder temperature.

While electro-hydraulic and camless VVA systems can be used to give a great deal of control
over the valve event, the componentry for such systems is currently complicated and expensive.
Mechanical variable lift and duration systems, however, although still being more complex than
a standard valvetrain, are far cheaper and less complicated. If the desired VVA characteristic is
known, then it is relatively simple to configure such systems to achieve the necessary control
over the valve lift curve. Also see variable valve timing.

Variable fuel ignition quality

Another means to extend the operating range is to control the onset of ignition and the heat
release rate is by manipulating fuel itself. This is usually carried out by adopting multiple fuels
and blending them "on the fly" for the same engine . Examples could be blending of commercial
gasoline and diesel fuels , adopting natural gas or ethanol ". This can be achieved in a number
of ways;

 Blending fuels upstream of the engine: Two fuels are mixed in the liquid phase, one with
low resistance to ignition (such as diesel fuel) and a second with a greater resistance
(gasoline), the timing of ignition is controlled by varying the compositional ratio of these
fuels. Fuel is then delivered using either a port or direct injection event.
 Having two fuel circuits: Fuel A can be injected in the intake duct (port injection) and
Fuel B using a direct injection (in-cylinder) event, the proportion of these fuels can be
used to control ignition, heat release rate as well as exhaust gas emissions.

Direct Injection: PCCI or PPCI Combustion

Compression Ignition Direct Injection (CIDI) combustion is a well established means of
controlling ignition timing and heat release rate and is adopted in Diesel engines combustion.
Partially Pre-mixed Charge Compression Ignition (PPCI) also known as Premixed Charge
Compression Ignition (PCCI) is a compromise between achieving the control of CIDI
combustion but with the exhaust gas emissions of HCCI, specifically soot . On a fundamental
level, this means that the heat release rate is controlled preparing the combustible mixture in
such a way that combustion occurs over a longer time duration and is less prone to knocking.
This is done by timing the injection event such that the combustible mixture has a wider range of
air/fuel ratios at the point of ignition, thus ignition occurs in different regions of the combustion
chamber at different times - slowing the heat release rate. Furthermore this mixture is prepared
such that when combustion occurs there are fewer rich pockets thus reducing the tendency for

Unit II: Power System and New Generation Vehicles Page 8 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

soot formation . The adoption of high EGR and adoption of diesel fuels with a greater resistance
to ignition (more "gasoline like") enables longer mixing times prior to ignition and thus fewer rich
pockets thus resulting in the possibility of both lower soot emissions and NOx.
High peak pressures and heat release rates

In a typical gasoline or diesel engine, combustion occurs via a flame. Hence at any point in time,
only a fraction of the total fuel is burning. This results in low peak pressures and low energy
release rates. In HCCI, however, the entire fuel/air mixture ignites and burns nearly
simultaneously resulting in high peak pressures and high energy release rates. To withstand the
higher pressures, the engine has to be structurally stronger and therefore heavier. Several
strategies have been proposed to lower the rate of combustion. Two different fuels, with
different autoignition properties, can be used to lower the combustion speed. However, this
requires significant infrastructure to implement. Another approach uses dilution (i.e. with
exhaust gases) to reduce the pressure and combustion rates at the cost of work production.

Power
In both a spark ignition engine and diesel engine, power can be increased by introducing more
fuel into the combustion chamber. These engines can withstand a boost in power because the
heat release rate in these engines is slow. However, in HCCI engines the entire mixture burns
nearly simultaneously. Increasing the fuel/air ratio will result in even higher peak pressures and
heat release rates. In addition, many of the viable control strategies for HCCI require thermal
preheating of the charge which reduces the density and hence the mass of the air/fuel charge in
the combustion chamber, reducing power. These factors make increasing the power in HCCI
engines challenging.

One way to increase power is to use fuels with different autoignition properties. This will lower
the heat release rate and peak pressures and will make it possible to increase the equivalence
ratio. Another way is to thermally stratify the charge so that different points in the compressed
charge will have different temperatures and will burn at different times lowering the heat release
rate making it possible to increase power. A third way is to run the engine in HCCI mode only at
part load conditions and run it as a diesel or spark ignition engine at full or near full load
conditions. Since much more research is required to successfully implement thermal
stratification in the compressed charge, the last approach is being studied more intensively.

Emissions
Because HCCI operates on lean mixtures, the peak temperatures are lower in comparison to
spark ignition (SI) and Diesel engines. The low peak temperatures prevent the formation of NOx.
This leads to NOx emissions at levels far less than those found in traditional engines. However,
the low peak temperatures also lead to incomplete burning of fuel, especially near the walls of
the combustion chamber. This leads to high carbon monoxide and hydrocarbon emissions. An
oxidizing catalyst would be effective at removing the regulated species because the exhaust is
still oxygen rich.

Difference from Knock

Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in a spark
ignited engine spontaneously ignite. The unburnt gas ahead of the flame is compressed as the
flame propagates and the pressure in the combustion chamber rises. The high pressure and
corresponding high temperature of unburnt reactants can cause them to spontaneously ignite.
This causes a shock wave to traverse from the end gas region and an expansion wave to
traverse into the end gas region. The two waves reflect off the boundaries of the combustion

Unit II: Power System and New Generation Vehicles Page 9 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

chamber and interact to produce high amplitude standing waves. A similar ignition process
occurs in HCCI. However, rather than part of the reactant mixture being ignited by compression
ahead of a flame front, ignition in HCCI engines occurs due to piston compression. In HCCI, the
entire reactant mixture ignites (nearly) simultaneously. Since there are very little or no pressure
differences between the different regions of the gas, there is no shock wave propagation and
hence no knocking. However at high loads (i.e. high fuel/air ratios), knocking is a possibility
even in HCCI.
Simulation of HCCI Engines
The development of computational models for simulating combustion and heat release rates of
HCCI engines has required the advancement of detailed chemistry models. This is largely
because ignition is most sensitive to chemical kinetics rather than turbulence/spray or spark
processes as are typical in direct injection compression ignition or spark ignition engines.
Computational models have demonstrated the importance of accounting for the fact that the in-
cylinder mixture is actually in-homogeneous, particularly in terms of temperature. This in-
homogeneity is driven by turbulent mixing and heat transfer from the combustion chamber walls,
the amount of temperature stratification dictates the rate of heat release and thus tendency to
knock . This limits the applicability of considering the in-cylinder mixture as a single zone
resulting in the uptake of 3D computational fluid dynamics and faster solving probability density
function modelling codes.

Other Applications of HCCI Research

To date there have only been few prototype engines running in HCCI mode however the
research efforts invested into HCCI research have disseminated into/resulted in direct
advancements in fuel and engine development. Examples are;

 PCCI/PPCI combustion - A hybrid of HCCI and conventional diesel combustion offing

more control over ignition and heat release rates with lower soot and NOx emissions.
 Advancements in fuel modelling - HCCI combustion is driven mainly by chemical kinetics
rather than turbulent mixing or injection, this reduces the complexity of simulating the
chemistry which results in fuel oxidation and emissions formation. This has led to
increasing interest and development of chemical kinetics which describe hydrocarbon
oxidation.
 Fuel blending applications- Due to the advancements in fuel modelling, it is now possible
to carry out detailed simulations of hydrocarbon fuel oxidation, enabling simulations of
practical fuels such as gasoline/diesel and ethanol . Engineers can now blend fuels
virtually and determine how they will perform in an engine context.

VCR engine: Variable Compression Ratio (VCR) engines

Because cylinder bore diameter, piston stroke length and combustion chamber volume are
almost always constant, the compression ratio for a given engine is almost always constant,
until engine wear takes its toll.

One exception is the experimental Saab Variable Compression engine (SVC). This engine,
designed by Saab Automobile, uses a technique that dynamically alters the volume of the
combustion chamber (Vc), which, via the above equation, changes the compression ratio (CR).

The Atkinson cycle engine was one of the first attempts at variable compression. Since the
compression ratio is the ratio between dynamic and static volumes of the combustion chamber
the Atkinson cycle's method of increasing the length of the power stroke compared to the intake
stroke ultimately altered the compression ratio at different stages of the cycle.

Unit II: Power System and New Generation Vehicles Page 10 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Dynamic compression ratio

The calculated compression ratio, as given above, presumes that the cylinder is sealed at the
bottom of the stroke, and that the volume compressed is the actual volume.However: intake
valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the
intake charge to be compressed backwards out of the cylinder by the rising piston at very low
speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port
tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric
pressure) to be trapped in the cylinder than the static volume would suggest ( This "corrected"
compression ratio is commonly called the "dynamic compression ratio".

This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and
lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the
static or "nominal" compression ratio.

The actual position of the piston can be determined by trigonometry, using the stroke length and
the connecting rod length (measured between centers). The absolute cylinder pressure is the
result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for
the ratio of variable heats for air and similar gases at the temperatures present. This
compensates for the temperature rise caused by compression, as well as heat lost to the
cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value,
generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines
based on design, size and materials used, but provides useful results for purposes of
comparison. For example, if the static compression ratio is 10:1, and the dynamic compression
ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or
13.7 bar. The two corrections for dynamic compression ratio affect cylinder pressure in opposite
directions, but not in equal strength. An engine with high static compression ratio and late intake
valve closure will have a DCR similar to an engine with lower compression but earlier intake
valve closure. Additionally, the cylinder pressure developed when an engine is running will be
higher than that shown in a compression test for several reasons.

 The much higher velocity of a piston when an engine is running versus cranking allows
less time for pressure to bleed past the piston rings into the crankcase.

 a running engine is coating the cylinder walls with much more oil than an engine that is
being cranked at low RPM, which helps the seal.

 the higher temperature of the cylinder will create higher pressures when running vs. a
static test, even a test performed with the engine near operating temperature.

 A running engine does not stop taking air & fuel into the cylinder when the piston
reaches BDC; The mixture that is rushing into the cylinder during the downstroke
develops momentum and continues briefly after the vacuum ceases (in the same respect
that rapidly opening a door will create a draft that continues after movement of the door
ceases). This is called scavenging. Intake tuning, cylinder head design, valve timing and
exhaust tuning determine how effectively an engine scavenges.

Compression ratio versus overall pressure ratio

Compression ratio and overall pressure ratio are interrelated as follows:

Unit II: Power System and New Generation Vehicles Page 11 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Compression ratio 2:1 3:1 5:1 10:1 15:1 20:1 25:1 35:1
Pressure ratio 2.64:1 4.66:1 9.52:1 25.12:1 44.31:1 66.29:1 90.60:1 145.11:1

The reason for this difference is that compression ratio is defined via the volume reduction:

while pressure ratio is defined as the pressure increase:

In calculating the pressure ratio, we assume that an adiabatic compression is carried out (i.e.
that no heat energy is supplied to the gas being compressed, and that any temperature rise is
solely due to the compression). We also assume that air is a perfect gas. With those two
assumptions we can define the relationship between change of volume and change of pressure
as follows:

where γ is the ratio of specific heats for air (approximately 1.4). The values in the table above
are derived using this formula. Note that in reality the ratio of specific heats changes with
temperature and that significant deviations from adiabatic behavior will occur.

Surface ignition engines

Relative surface ignition resistance of fuels in engines could be predicted from data obtained in
the laboratory: heat of combustion of the fuel, heat capacities of the fuel and its products,
radiant energy of flames, ignition energies at the temperature and pressure existing in an engine
at the time surface ignition occurs, extent and nature of preflame reactions at these
temperatures and pressures, effect of fuel-air ratio on energy required for ignition at the
temperature and pressure existing in an engine, and the relative activity of the igniting surface
under the conditions existing in an engine. General experience has been that there are no short
cuts. It is simpler and more satisfactory to measure surfaceignition resistance in an engine
operating under the conditions of interest.

VVTI engines

VVT-i, or Variable Valve Timing with intelligence, is an automobile variable valve timing
technology developed by Toyota, similar in performance to the BMW's VANOS. The Toyota
VVT-i system replaces the Toyota VVT offered starting in 24 December 1991 on the 5-valve per

Unit II: Power System and New Generation Vehicles Page 12 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

cylinder 4A-GE engine. The VVT system is a 2-stage hydraulically controlled cam phasing
system. The Toyota motors CEO has been reported to have said, "VVT is the heart of every
modern Toyota!

VVT-i, introduced in 1996, varies the timing of the intake valves by adjusting the relationship
between the camshaft drive (belt, scissor-gear or chain) and intake camshaft. Engine oil
pressure is applied to an actuator to adjust the camshaft position. Adjustments in the overlap
time between the exhaust valve closing and intake valve opening result in improved engine
efficiency. Variants of the system, including VVTL-i, Dual VVT-i, Triple VVT-iE, and
Valvematic, have followed.

VVTL-i (Variable Valve Timing and Lift intelligent system) is a version that can alter valve lift
(and duration) as well as valve timing. In the case of the 16 valve 2ZZ-GE, the engine has 2
camshafts, one operating intake valves and one operating exhaust valves. Each camshaft has
two lobes per cylinder, one low rpm lobe and one high rpm, high lift, long duration lobe. Each
cylinder has two intake valves and two exhaust valves. Each set of two valves are controlled by
one rocker arm, which is operated by the camshaft. Each rocker arm has a slipper follower
mounted to the rocker arm with a spring, allowing the slipper follower to move up and down with
the high lobe without affecting the rocker arm. When the engine is operating below 6000-7000
rpm (dependent on year, car, and ECU installed), the low lobe is operating the rocker arm and
thus the valves. When the engine is operating above the lift engagement point, the ECU
activates an oil pressure switch which pushes a sliding pin under the slipper follower on each
rocker arm. This in effect, switches to the high lobe causing high lift and longer duration.

In 1998, Dual VVT-i which adjusts timing on both intake and exhaust camshafts was first
introduced on the RS200 Altezza's 3S-GE engine. Dual VVT-i is also found in Toyota's new
generation V6 engine, the 3.5-liter 2GR-FE first appearing on the 2005 Avalon. This engine can
now be found on numerous Toyota and Lexus models. By adjusting the valve timing, engine
start and stop occurs almost unnoticeably at minimum compression. In addition fast heating of
the catalytic converter to its light-off temperature is possible thereby reducing hydrocarbon
emissions considerably.

Toyota's UR engine V8 also uses this technology. Dual VVT-i was later introduced to Toyota's
latest small 4-cylinder ZR engines found in compact vehicles such as the new Toyota Corolla
and Scion xD and in larger 4-cylinder AR engines found in the Camry and RAV4. VVT-iE
(Variable Valve Timing - intelligent by Electric motor) is a version of Dual VVT-i that uses an
electrically operated actuator to adjust and maintain intake camshaft timing. The exhaust
camshaft timing is still controlled using a hydraulic actuator. This form of variable valve timing
technology was developed initially for Lexus vehicles. This system was first introduced on the
2007MY Lexus LS 460 as 1UR engine.

The electric motor in the actuator spins together with the intake camshaft as the engine runs. To
maintain camshaft timing, the actuator motor will operate at the same speed as the camshaft.
To advance the camshaft timing, the actuator motor will rotate slightly faster than the camshaft
speed. To retard camshaft timing, the actuator motor will rotate slightly slower than camshaft
speed. The speed difference between the actuator motor and camshaft timing is used to
operate a mechanism that varies the camshaft timing. The benefit of the electric actuation is
enhanced response and accuracy at low engine speeds and at lower temperatures.
Furthermore, it ensures precise positioning of the camshaft for and immediately after engine
starting, as well as a greater total range of adjustment. The combination of these factors allows

Unit II: Power System and New Generation Vehicles Page 13 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

more precise control, resulting in an improvement of both fuel economy, engine output and
emissions performance.

Valvematic
It offers continuous adjustment to lift volume and timing. Valvematic made its first appearance
in 2007 in the Noah and later in early-2009 in the ZR High energy and power density batteries
A lithium-ion battery (sometimes Li-ion battery or LIB) is a family of rechargeable battery
types in which lithium ions move from the negative electrode to the positive electrode during
discharge, and back when charging. Chemistry, performance, cost, and safety characteristics
vary across LIB types. Unlike lithium primary batteries (which are disposable), lithium-ion
electrochemical cells use an intercalated lithium compound as the electrode material instead of
metallic lithium. Lithium-ion batteries are common in consumer electronics. They are one of the
most popular types of rechargeable battery for portable electronics, with one of the best energy
densities, no memory effect, and a slow loss of charge when not in use. Beyond consumer
electronics, LIBs are also growing in popularity for military, electric vehicle, and aerospace
applications. Research is yielding a stream of improvements to traditional LIB technology,
focusing on energy density, durability, cost, and intrinsic safety.
Charge and discharge
During discharge, lithium ions Li+ carry the current from the negative to the positive electrode,
through the non-aqueous electrolyte and separator diaphrage.During charging, an external
electrical power source (the charging circuit) applies a higher voltage (but of the same polarity)
than that produced by the battery, forcing the current to pass in the reverse direction. The
lithium ions then migrate from the positive to the negative electrode, where they become
embedded in the porous electrode material in a process known as intercalation.

Construction

Cylindrical 18650 cell before closing

The three primary functional components of a lithium-ion battery are the anode, cathode, and
electrolyte. The anode of a conventional lithium-ion cell is made from carbon, the cathode is a
metal oxide, and the electrolyte is a lithium salt in an organic solvent. The most commercially
popular anode material is graphite. The cathode is generally one of three materials: a layered
oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate), or a spinel
(such as lithium manganese oxide).

Unit II: Power System and New Generation Vehicles Page 14 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

The electrolyte is typically a mixture of organic carbonates such as ethylene carbonate or diethyl
carbonate containing complexes of lithium ions. These non-aqueous electrolytes generally use
non-coordinating anion salts such as lithium hexafluorophosphate (LiPF6), lithium
hexafluoroarsenate monohydrate (LiAsF6), lithium perchlorate (LiClO 4), lithium tetrafluoroborate
(LiBF4), and lithium triflate (LiCF 3SO3). Depending on materials choices, the voltage, capacity,
life, and safety of a lithium-ion battery can change dramatically. Recently, novel architectures
using nanotechnology have been employed to improve performance.

Pure lithium is very reactive. It reacts vigorously with water to form lithium hydroxide and
hydrogen gas is liberated. Thus a non-aqueous electrolyte is typically used, and a sealed
container rigidly excludes water from the battery pack. Lithium ion batteries are more expensive
than NiCd batteries but operate over a wider temperature range with higher energy densities,
while being smaller and lighter. They are fragile and so need a protective circuit to limit peak
voltages.

Electrochemistry
The three participants in the electrochemical reactions in a lithium-ion battery are the anode,
cathode, and electrolyte.

Both the anode and cathode are materials into which, and from which, lithium can migrate.
During insertion (or intercalation) lithium moves into the electrode. During the reverse process,
extraction (or deintercalation), lithium moves back out. When a lithium-based cell is discharging,
the lithium is extracted from the anode and inserted into the cathode. When the cell is charging,
the reverse occurs.

Useful work can only be extracted if electrons flow through a closed external circuit. The
following equations are in units of moles, making it possible to use the coefficient x.

The positive electrode half-reaction (with charging being forwards) is:

The negative electrode half-reaction is:

The overall reaction has its limits. Overdischarge supersaturates lithium cobalt oxide, leading to
the production of lithium oxide, possibly by the following irreversible reaction:

Overcharge up to 5.2 Volts leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray
diffraction

Unit II: Power System and New Generation Vehicles Page 15 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

In a lithium-ion battery the lithium ions are transported to and from the cathode or anode, with
the transition metal, cobalt (Co), in LixCoO2 being oxidized from Co3+ to Co4+ during charging,
and reduced from Co4+ to Co3+ during discharge.

Positive electrodes

Electrode material Average potential difference Specific capacity Specific energy

LiCoO2 3.7 V 140 mA·h/g 0.518 kW·h/kg
LiMn2O4 4.0 V 100 mA·h/g 0.400 kW·h/kg
LiNiO2 3.5 V 180 mA·h/g 0.630 kW·h/kg
LiFePO4 3.3 V 150 mA·h/g 0.495 kW·h/kg
Li2FePO4F 3.6 V 115 mA·h/g 0.414 kW·h/kg
LiCo1/3Ni1/3Mn1/3O2 3.6 V 160 mA·h/g 0.576 kW·h/kg
Li(LiaNixMnyCoz)O2 4.2 V 220 mA·h/g 0.920 kW·h/kg

Negative electrodes

Electrode material Average potential difference Specific capacity Specific energy

Graphite (LiC6) 0.1-0.2 V 372 mA·h/g 0.0372-0.0744 kW·h/kg
Hard Carbon (LiC6) ? V ? mA·h/g ? kW·h/kg
Titanate (Li4Ti5O12) 1-2 V 160 mA·h/g 0.16-0.32 kW·h/kg
Si (Li4.4Si) 0.5-1 V 4212 mA·h/g 2.106-4.212 kW·h/kg
Ge (Li4.4Ge) 0.7-1.2 V 1624 mA·h/g 1.137-1.949 kW·h/kg
Electrolytes
The cell voltages given in the Electrochemistry section are larger than the potential at which
aqueous solutions can electrolyze, in addition lithium is highly reactive to water, therefore,
nonaqueous or aprotic solutions are used.

Liquid electrolytes in lithium-ion batteries consist of lithium salts, such as LiPF6, LiBF4 or LiClO4 in an
organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate. A liquid
electrolyte conducts lithium ions, acting as a carrier between the cathode and the anode when a
battery passes an electric current through an external circuit. Typical conductivities of liquid
electrolyte at room temperature (20 °C ) are in the range of 10 mS/cm (1 S/m), increasing by
approximately 30–40% at 40 °C and decreasing by a slightly smaller amount at 0 °C.

Unfortunately, organic solvents easily decompose on anodes during charging. However, when
appropriate organic solvents are used as the electrolyte, the solvent decomposes on initial
charging and forms a solid layer called the solid electrolyte interphase (SEI)., which is
electrically insulating yet provides sufficient ionic conductivity. The interphase prevents
decomposition of the electrolyte after the second charge. For example, ethylene carbonate is
decomposed at a relatively high voltage, 0.7 V vs. lithium, and forms a dense and stable
interface.

A good solution for the interface instability is the application of a new class of composite
electrolytes based on POE (poly(oxyethylene)) developed by Syzdek et al. It can be either solid
(high molecular weight) and be applied in dry Li-polymer cells, or liquid (low molecular weight)
and be applied in regular Li-ion cells. Another issue that Li-ion technology is facing is safety.
Large scale application of Li cells in Electric Vehicles needs a dramatic decrease in the failure

Unit II: Power System and New Generation Vehicles Page 16 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

rate. One of the solutions is the novel technology based on reversed-phase composite
electrolytes, employing porous ceramic material filled with electrolyte.

Advantages
 Wide variety of shapes and sizes efficiently fitting the devices they power.
 Much lighter than other energy-equivalent secondary batteries.
 High open circuit voltage in comparison to aqueous batteries (such as lead acid, nickel-
metal hydride and nickel-cadmium). This is beneficial because it increases the amount of
power that can be transferred at a lower current.
 No memory effect.
 Self-discharge rate of approximately 5-10% per month, compared to over 30% per
month in common nickel metal hydride batteries, approximately 1.25% per month for
Low Self-Discharge NiMH batteries and 10% per month in nickel-cadmium batteries.
According to one manufacturer, lithium-ion cells (and, accordingly, "dumb" lithium-ion
batteries) do not have any self-discharge in the usual meaning of this word. What looks
like a self-discharge in these batteries is a permanent loss of capacity. On the other
hand, "smart" lithium-ion batteries do self-discharge, due to the drain of the built-in
voltage monitoring circuit.
 Components are environmentally safe as there is no free lithium metal.

Disadvantages
Cell life
 Charging forms deposits inside the electrolyte that inhibit ion transport. Over time, the
cell's capacity diminishes. The increase in internal resistance reduces the cell's ability to
deliver current. This problem is more pronounced in high-current applications. The
decrease means that older batteries do not charge as much as new ones (charging time
required decreases proportionally).
 High charge levels and elevated temperatures (whether from charging or ambient air)
hasten capacity loss. Charging heat is caused by the carbon anode (typically replaced
with lithium titanate which drastically reduces damage from charging, including
expansion and other factors).
 A Standard (Cobalt) Li-ion cell that is full most of the time at 25 °C irreversibly loses
approximately 20% capacity per year. Poor ventilation may increase temperatures,
further shortening battery life. Loss rates vary by temperature: 6% loss at 0 °C, 20% at
25 °C, and 35% at 40 °C. When stored at 40%–60% charge level, the capacity loss is
reduced to 2%, 4%, and 15%, respectively. In contrast, the calendar life of LiFePO4 cells is
not affected by being kept at a high state of charge.

Internal resistance
 The internal resistance of standard (Cobalt) lithium-ion batteries is high compared to
both other rechargeable chemistries such as nickel-metal hydride and nickel-cadmium,
and LiFePO4 and lithium-polymer cells. Internal resistance increases with both cycling
and age. Rising internal resistance causes the voltage at the terminals to drop under
load, which reduces the maximum current draw. Eventually increasing resistance means
that the battery can no longer operate for an adequate period.

 To power larger devices, such as electric cars, connecting many small batteries in a
parallel circuit is more effective and efficient than connecting a single large battery.

Safety requirements

Unit II: Power System and New Generation Vehicles Page 17 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

If overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture. In
extreme cases this can lead to combustion. Deep discharge may short-circuit the cell, in which
case recharging would be unsafe. To reduce these risks, Lithium-ion battery packs contain fail-
safe circuity that shuts down the battery when its voltage is outside the safe range of 3–4.2  V
per cell. When stored for long periods the small current draw of the protection circuitry itself may
drain the battery below its shut down voltage; normal chargers are then ineffective. Many types
of lithium-ion cell cannot be charged safely below 0°C.

Other safety features are required in each cell:

 shut-down separator (for overtemperature)

 tear-away tab (for internal pressure)
 vent (pressure relief)
 thermal interrupt (overcurrent/overcharging)

These devices occupy useful space inside the cells, add additional points of failure and
irreversibly disable the cell when activated. They are required because the anode produces heat
during use, while the cathode may produce oxygen. These devices and improved electrode
designs reduce/eliminate the risk of fire or explosion. These safety features increase costs
compared to nickel metal hydride batteries, which require only a hydrogen/oxygen
recombination device (preventing damage due to mild overcharging) and a back-up pressure
valve.

Specifications and design

 Specific energy density: 150 to 250 W·h/kg (540 to 900 kJ/kg)
 Volumetric energy density: 250 to 620 W·h/l (900 to 1900 J/cm³)
 Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285 W·h/l)

Because lithium-ion batteries can have a variety of cathode and anode materials, the energy
density and voltage vary accordingly.

Lithium-ion batteries with a lithium iron phosphate cathode and graphite anode have a nominal
open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese
cobalt (NMC) oxide cathode with graphite anodes have a 3.7 V nominal voltage with a 4.2 V
max charge. The charging procedure is performed at constant voltage with current-limiting
circuitry (i.e., charging with constant current until a voltage of 4.2 V is reached in the cell and
continuing with a constant voltage applied until the current drops close to zero). Typically, the
charge is terminated at 3% of the initial charge current. In the past, lithium-ion batteries could
not be fast-charged and needed at least two hours to fully charge. Current-generation cells can
be fully charged in 45 minutes or less. Some lithium-ion varieties can reach 90% in as little as
10 minutes.

Charging procedure

Stage 1: Apply charging current until the voltage limit per cell is reached.

Stage 2: Apply maximum voltage per cell limit until the current declines below 3% of rated
charge current.

Unit II: Power System and New Generation Vehicles Page 18 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Stage 3: Periodically apply a top-off charge about once per 500 hours.The charge time is about
three to five hours, depending on the charger used. Generally, cell phone batteries can be
charged at 1C and laptop-types at 0.8C, where C is the current that would discharge the battery
in one hour. Charging is usually stopped when the current goes below 0.03C but it can be left
indefinitely depending on desired charging time. Some fast chargers skip stage 2 and claim the
battery is ready at 70% charge.

Top-off charging is recommended when voltage goes below 4.05 V/cell. Typically, lithium-ion
cells are charged with 4.2 ± 0.05 V/cell, except for military long-life cells where 3.92 V is used
for extending battery life. Most protection circuits cut off if either 4.3 V or 90 °C is reached. If the
voltage drops below 2.50 V per cell, the battery protection circuit may also render it
unchargeable with regular charging equipment. Most battery protection circuits stop at 2.7–3.0 V
per cell. For safety reasons it is recommended the battery be kept at the manufacturer's stated
voltage and current ratings during both charge and discharge cycles.

Variations in materials and construction

It has been suggested that Nanoball batteries be merged into this article or section.
(Discuss) Proposed since September 2010.

The increasing demand for batteries has led vendors and academics to focus on improving the
power density, operating temperature, safety, durability, charging time, output power, and cost
of LIB solutions.

LIB types
Target
Area Technology Researchers Date Benefit
application
Lucky Goldstar
[57] Hybrid electric
Manganese Chemical, NEC,
Cathode vehicle, cell 1996 durability, cost
spinel (LMO) Samsung,[58] Hitachi,[59]
phone, laptop
Nissan/AESC[60]
Segway Personal
University of moderate density
Transporter,
Texas/Hydro-Québec, (2 A·h outputs 70
power tools,
Lithium iron [61]/Phostech Lithium amperes)
aviation products, 1996
phosphate Inc., Valence operating
automotive hybrid
Technology, temperature >60
systems, PHEV
A123Systems/MIT[62][63] °C (140 °F)
conversions
Lithium nickel
Imara Corporation, density, output,
manganese 2008
Nissan Motor[64][65] safety
cobalt (NMC)
power, safety
LMO/NMC Sony, Sanyo (although limited
durability)
durability, cost
Lithium iron
University of Waterloo[66] 2007 (replace Li with Na
fluorophosphate
or Na/Li)
University of Dayton
Lithium air automotive 2009 density, safety[67]
Research Institute[67]

Unit II: Power System and New Generation Vehicles Page 19 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

5% Vanadium-
doped Lithium
Binghamton University[68] 2008 output
iron phosphate
olivine
automotive
(Phoenix
Motorcars),
electrical grid output, charging
(PJM time, durability (20
Interconnection years, 9,000
Lithium-titanate Regional cycles), safety,
Anode Altairnano 2008
battery (LT) Transmission operating
Organization temperature (-50–
control area,[69] 70 °C (-58–
United States 158 °F)[72][dead link]
Department of
Defense[70]), bus
(Proterra[71])
Lithium vanadium density (745Wh/l)
Samsung/Subaru.[73] automotive 2007 [74]
oxide
Cobalt-oxide
nano wires from density,
MIT 2006
genetically thickness[75]
modified virus
Three-
Dimensional (3D)
specific capacity >
Porous Particles high energy
2000 mA·h/g, high
Composed of Georgia Institute of batteries for
2011 efficiency, rapid
Curved Two- Technology [76] electronics and
low-cost synthesis
Dimensional (2D) electrical vehicles [77]
Nano-Sized
Layers
Iron-phosphate
nano wires from density,
MIT 2009
genetically thickness[78][79][80]
modified virus
explosive
Silicon/titanium detection
dioxide sensors,
composite nano biomimetic
density, low
wires from University of Maryland structures, water- 2010 [81]
charge time
genetically repellent
modified tobacco surfaces,
virus micro/nano scale
heat pipes
Porous portable
high stability, high
silicon/carbon Georgia Institute of electronics,
2010 capacity, low
nanocomposite Technology electrical vehicles,
charge time[82]
spheres electrical grid

Unit II: Power System and New Generation Vehicles Page 20 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

density[83][84] (shift
from anode- to
nano-sized wires wireless sensors cathode-limited),
Stanford University 2007
on stainless steel networks, durability issue
remains (wire
cracking)
Laboratoire de
Réactivité et de Chimie density (1480
Metal hydrides 2008
des Solides, General mA·h/g)[85]
Motors
Silicon stable high specific capacity
Nanotubes (or energy batteries 2400 mA·h/g,
Georgia Institute of
Silicon for cell phones, ultra-high
Technology, MSE,
Nanospheres) laptops, 2010 Coulombic
NanoTech Yushin's
Confined within netbooks, radios, Efficiency and
group [86]
Rigid Carbon sensors and outstanding SEI
[87]
Outer Shells electrical vehicles stability
durability, safety
Electrode LT/LMO Ener1/Delphi,[88][89] 2006
(limited density)
Université Paul
Nanostructure Sabatier/Université 2006 density
Picardie Jules Verne[90]

Fuel cells
A fuel cell is an electrochemical cell that converts chemical energy from a fuel into electric
energy. Electricity is generated from the reaction between a fuel supply and an oxidizing agent.
The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte
remains within it. Fuel cells can operate continuously as long as the necessary reactant and
oxidant flows are maintained.

Fuel cells are different from conventional electrochemical cell batteries in that they consume
reactant from an external source, which must be replenished [1] – a thermodynamically open
system. By contrast, batteries store electric energy chemically and hence represent a
thermodynamically closed system. Many combinations of fuels and oxidants are possible. A
hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other
fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide.[2

Design
Fuel cells come in many varieties; however, they all work in the same general manner. They are
made up of three segments which are sandwiched together: the anode, the electrolyte, and the
cathode. Two chemical reactions occur at the interfaces of the three different segments. The net
result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an
electric current is created, which can be used to power electrical devices, normally referred to
as the load.

At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively
charged ion and a negatively charged electron. The electrolyte is a substance specifically
designed so ions can pass through it, but the electrons cannot. The freed electrons travel

Unit II: Power System and New Generation Vehicles Page 21 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

through a wire creating the electric current. The ions travel through the electrolyte to the
cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react
with a third chemical, usually oxygen, to create water or carbon dioxide.

A block diagram of a fuel cell

The most important design features in a fuel cell are:

 The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.
 The fuel that is used. The most common fuel is hydrogen.
 The anode catalyst, which breaks down the fuel into electrons and ions. The anode
catalyst is usually made up of very fine platinum powder.
 The cathode catalyst, which turns the ions into the waste chemicals like water or carbon
dioxide. The cathode catalyst is often made up of nickel.

A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as
current increases, due to several factors:

 Activation loss
 Ohmic loss (voltage drop due to resistance of the cell components and interconnects)
 Mass transport loss (depletion of reactants at catalyst sites under high loads, causing
rapid loss of voltage).

To deliver the desired amount of energy, the fuel cells can be combined in series and parallel
circuits, where series yields higher voltage, and parallel allows a higher current to be supplied.
Such a design is called a fuel cell stack. The cell surface area can be increased, to allow
stronger current from each cell.

Proton exchange membrane fuel cells

In the archetypical hydrogen–oxygen proton exchange membrane fuel cell[4] (PEMFC) design, a
proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode

Unit II: Power System and New Generation Vehicles Page 22 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

sides. This was called a "solid polymer electrolyte fuel cell" (SPEFC) in the early 1970s, before
the proton exchange mechanism was well-understood. (Notice that "polymer electrolyte
membrane" and "proton exchange mechanism" result in the same acronym.)

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into
protons and electrons. These protons often react with oxidants causing them to become what is
commonly referred to as multi-facilitated proton membranes. The protons are conducted
through the membrane to the cathode, but the electrons are forced to travel in an external circuit
(supplying power) because the membrane is electrically insulating. On the cathode catalyst,
oxygen molecules react with the electrons (which have traveled through the external circuit) and
protons to form water — in this example, the only waste product, either liquid or vapor.

In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including diesel,
methanol (see: direct-methanol fuel cells and indirect methanol fuel cells) and chemical
hydrides. The waste products with these types of fuel are carbon dioxide and water.

Construction of a high temperature PEMFC: Bipolar plate as electrode with in-milled gas
channel structure, fabricated from conductive composites (enhanced with graphite, carbon
black, carbon fiber, and/or carbon nanotubes for more conductivity);[5] Porous carbon papers;
reactive layer, usually on the polymer membrane applied; polymer membrane.

Unit II: Power System and New Generation Vehicles Page 23 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the
cell ensures the collection of electric current.[6]

The different components of a PEMFC are (i) bipolar plates, (ii) electrodes, (iii) catalyst, (iv)
membrane, and (v) the necessary hardwares. [7] The materials used for different parts of the fuel
cells differ by type. The bipolar plates may be made of different types of materials, such as,
metal, coated metal, graphite, flexible graphite, C–C composite, carbon–polymer composites
etc.[8] The membrane electrode assembly (MEA), is referred as the heart of the PEMFC and
usually made of a proton exchange membrane sandwiched between two catalyst coated carbon
papers. Platinum and/or similar type of noble metals are usually used as the catalyst for
PEMFC. The electrolyte could be a polymer membrane.

Proton exchange membrane fuel cell design issues

 Costs. In 2002, typical fuel cell systems cost US$1000 per kilowatt of electric power
output. In 2009, the Department of Energy reported that 80-kW automotive fuel cell
system costs in volume production (projected to 500,000 units per year) are $61 per
kilowatt.[9] The goal is $35 per kilowatt. In 2008 UTC Power has 400 kW stationary fuel
cells for $1,000,000 per 400 kW installed costs. The goal is to reduce the cost in order to
compete with current market technologies including gasoline internal combustion
engines. Many companies are working on techniques to reduce cost in a variety of ways
including reducing the amount of platinum needed in each individual cell. Ballard Power
Systems have experiments with a catalyst enhanced with carbon silk which allows a
30% reduction (1 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in
performance.[10] Monash University, Melbourne uses PEDOT as a cathode.[11] A 2011
published study[12] documented the first ever metal free electrocatalyst using relatively
inexpensive doped carbon nanotubes that are less than 1% the cost of platinum and are
of equal or superior performance.
 The production costs of the PEM (proton exchange membrane). The Nafion membrane
currently costs $566/m². In 2005 Ballard Power Systems announced that its fuel cells will
use Solupor, a porous polyethylene film patented by DSM.[13][14]
 Water and air management[15] (in PEMFCs). In this type of fuel cell, the membrane must
be hydrated, requiring water to be evaporated at precisely the same rate that it is
produced. If water is evaporated too quickly, the membrane dries, resistance across it
increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and
oxygen combine directly, generating heat that will damage the fuel cell. If the water is
evaporated too slowly, the electrodes will flood, preventing the reactants from reaching
the catalyst and stopping the reaction. Methods to manage water in cells are being
developed like electroosmotic pumps focusing on flow control. Just as in a combustion
engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel
cell operating efficiently.
 Temperature management. The same temperature must be maintained throughout the
cell in order to prevent destruction of the cell through thermal loading. This is particularly
challenging as the 2H2 + O2 -> 2H2O reaction is highly exothermic, so a large quantity of
heat is generated within the fuel cell.
 Durability, service life, and special requirements for some type of cells. Stationary fuel
cell applications typically require more than 40,000 hours of reliable operation at a
temperature of -35 °C to 40 °C, while automotive fuel cells require a 5,000 hour lifespan
(the equivalent of 150,000 miles) under extreme temperatures. Current service life is

Unit II: Power System and New Generation Vehicles Page 24 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

7,300 hours under cycling conditions. Automotive engines must also be able to start
reliably at -30 °C and have a high power to volume ratio (typically 2.5 kW per liter).
 Limited carbon monoxide tolerance of some (non-PEDOT) cathodes.

High temperature fuel cells

SOFC
A solid oxide fuel cell (SOFC) is extremely advantageous “because of a possibility of using a
wide variety of fuel”. Unlike most other fuel cells which only use hydrogen, SOFCs can run on
hydrogen, butane, methanol, other petroleum products and producer gases from biomass
gasification [18]. The different fuels each have their own chemistry.

For SOFC methanol fuel cells, on the anode side, a catalyst breaks methanol and water down to
form carbon dioxide, hydrogen ions, and free electrons. The hydrogen ions meet oxide ions that
have been created on the cathode side and passed across the electrolyte to the anode side,
where they react to create water. A load connected externally between the anode and cathode
completes the electrical circuit. Below are the chemical equations for the reaction:

Anode Reaction: CH3OH + H2O + 3O= → CO2 + 3H2O + 6e-

Cathode Reaction: 3/2 O2 + 6e- → 3O=

Overall Reaction: CH3OH + 3/2 O2 → CO2 + 2H2O + electrical energy

At the anode SOFCs can use nickel or other catalysts to break apart the methanol and create
hydrogen ions and carbon monoxide. A solid called yttria stabilized zirconia (YSZ) is used as
the electrolyte. Like all fuel cell electrolytes YSZ is conductive to certain ions, in this case the
oxide ion (O=) allowing passage from the cathode to anode, but is non-conductive to electrons. It
is a durable solid, advantageous in large industrial systems, and a good ion conductor.
However, YSZ only works at very high temperatures, typically about 950 oC. Running the fuel
cell at such a high temperature easily breaks down the methane and oxygen into ions. A major
disadvantage of the SOFC, as a result of the high heat, is that it “places considerable
constraints on the materials which can be used for interconnections”.[19] Another disadvantage of
running the cell at such a high temperature is that other unwanted reactions may occur inside
the fuel cell. It is common for carbon dust (graphite) to build up on the anode, preventing the
fuel from reaching the catalyst. Much research is currently being done to find alternatives to
YSZ that will carry ions at a lower temperature.

MCFC
Molten carbonate fuel cells (MCFCs) operate in a similar manner, except the electrolyte consists
of liquid (molten) carbonate, which is a negative ion and an oxidizing agent. Because the
electrolyte loses carbonate in the oxidation reaction, the carbonate must be replenished through
some means. This is often performed by recirculating the carbon dioxide from the oxidation
products into the cathode where it reacts with the incoming air and reforms carbonate.

Unlike proton exchange fuel cells, the catalysts in SOFCs and MCFCs are not poisoned by
carbon monoxide, due to much higher operating temperatures. Because the oxidation reaction
occurs in the anode, direct utilization of the carbon monoxide is possible. Also, steam produced

Unit II: Power System and New Generation Vehicles Page 25 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

by the oxidation reaction can shift carbon monoxide and steam reform hydrocarbon fuels inside
the anode. These reactions can use the same catalysts used for the electrochemical reaction,
eliminating the need for an external fuel reformer.

MCFC can be used for reducing the CO 2 emission from coal fired power plants [20] as well as gas
turbine power plants.[21]

Fuel cell efficiency

The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more
power means drawing more current, which increases the losses in the fuel cell. As a general
rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves
as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For
this reason, it is common to show graphs of voltage versus current (so-called polarization
curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning
that 50% of the energy content of the hydrogen is converted into electrical energy; the
remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel
might leave the system unreacted, constituting an additional loss.)

For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is
equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the
reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V.
(This voltage varies with fuel used, and quality and temperature of the cell.) The difference
between these numbers represents the difference between the reaction's enthalpy and Gibbs
free energy. This difference always appears as heat, along with any losses in electrical
conversion efficiency.

Fuel cells are not heat engines and so the Carnot cycle efficiency is not relevant to the
thermodynamic efficiency of fuel cells. [28] At times this is misrepresented by saying that fuel cells
are exempt from the laws of thermodynamics, because most people think of thermodynamics in
terms of combustion processes (enthalpy of formation). The laws of thermodynamics also hold
for chemical processes (Gibbs free energy) like fuel cells, but the maximum theoretical
efficiency is higher (83% efficient at 298K [29] in the case of hydrogen/oxygen reaction) than the
Otto cycle thermal efficiency (60% for compression ratio of 10 and specific heat ratio of 1.4).
Comparing limits imposed by thermodynamics is not a good predictor of practically achievable
efficiencies. Also, if propulsion is the goal, electrical output of the fuel cell has to still be
converted into mechanical power with another efficiency drop. In reference to the exemption
claim, the correct claim is that "limitations imposed by the second law of thermodynamics on the
operation of fuel cells are much less severe than the limitations imposed on conventional energy
conversion systems". Consequently, they can have very high efficiencies in converting chemical
energy to electrical energy, especially when they are operated at low power density, and using
pure hydrogen and oxygen as reactants.

It should be underlined that fuel cell (especially high temperature) can be used as a heat source
in conventional heat engine (gas turbine system). In this case the ultra high efficiency is
predicted (above 70%).

In practice
For a fuel cell operating on air, losses due to the air supply system must also be taken into
account. This refers to the pressurization of the air and dehumidifying it. This reduces the

Unit II: Power System and New Generation Vehicles Page 26 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

efficiency significantly and brings it near to that of a compression ignition engine. Furthermore,
fuel cell efficiency decreases as load increases.

The tank-to-wheel efficiency of a fuel cell vehicle is greater than 45% at low loads and shows
average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle)
is used as test procedure. The comparable NEDC value for a Diesel vehicle is 22%. In 2008
Honda released a fuel cell electric vehicle (the Honda FCX Clarity) with fuel stack claiming a
60% tank-to-wheel efficiency.

It is also important to take losses due to fuel production, transportation, and storage into
account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel
efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid
hydrogen.[36] In addition to the production losses, over 70% of US' electricity used for hydrogen
production comes from thermal power, which only has an efficiency of 33% to 48%, resulting in
a net increase in carbon dioxide production by using hydrogen in vehicles . However, more than
90% of all hydrogen is produced by steam methane reforming.

Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone
power plants based on discontinuous sources such as solar or wind power, they are combined
with electrolyzers and storage systems to form an energy storage system. The overall efficiency
(electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is
between 30 and 50%, depending on conditions. While a much cheaper lead-acid battery might
return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen,
and is therefore better suited for long-term storage.

Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and
hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and
used to heat water in a micro combined heat and power (m-CHP) application. When the heat is
captured, total efficiency can reach 80-90% at the unit, but does not consider production and
distribution losses. CHP units are being developed today for the European home market.

Fuel cell applications ; Power

Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote
weather stations, large parks, rural locations, and in certain military applications. A fuel cell
system running on hydrogen can be compact and lightweight, and have no major moving parts.
Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they
can achieve up to 99.9999% reliability. This equates to around one minute of down time in a two
year period.

Since electrolyzer systems do not store fuel in themselves, but rather rely on external storage
units, they can be successfully applied in large-scale energy storage, rural areas being one
example. In this application, batteries would have to be largely oversized to meet the storage
demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical
device).

One such pilot program is operating on Stuart Island in Washington State. There the Stuart
Island Energy Initiative has built a complete, closed-loop system: Solar panels power an
electrolyzer which makes hydrogen. The hydrogen is stored in a 1,900 L at 1,400 kPa, and runs
a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence.

Unit II: Power System and New Generation Vehicles Page 27 of 28

 AU 2032: NEW GENERATION AND HYBRID VEHICLES, UNIT - 2

Cogeneration

Configuration of components in a fuel cell car.

Micro combined heat and power (MicroCHP) systems such as home fuel cells and cogeneration
for office buildings and factories are in the mass production phase. The system generates
constant electric power (selling excess power back to the grid when it is not consumed), and at
the same time produces hot air and water from the waste heat. MicroCHP is usually less than 5
kWe for a home fuel cell or small business. A lower fuel-to-electricity conversion efficiency is
tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized
as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined
heat and power efficiency is still lower than 100%, typically around 80%. In terms of exergy
however, the process is inefficient, and one could do better by maximizing the electricity
generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC)
comprise the largest segment of existing CHP products worldwide and can provide combined
efficiencies close to 90% (35-50% electric + remainder as thermal) Molten-carbonate fuel cells
have also been installed in these applications, and solid-oxide fuel cell prototypes exist.

Unit II: Power System and New Generation Vehicles Page 28 of 28