By moses dhilip
kumar
ABSTRACT:
Hybrid vehicles were designed to reduce
emissions and save fuel. With the continuing rise of gas prices and global
warming campaigns, more people are searching for alternatives to decrease
dependence on fuel. This technology will also contribute to the general health
of the public because car emissions pose a threat to one’s health.
These vehicles are able to achieve this
purpose in four ways. It shuts down the diesel engine during stops or idle
periods It has a battery storage that enables it to store and reuse the energy
that it has recaptured It’s able to recapture energy that is usually wasted
while breaking. It relies on two power sources, the diesel engine and the
electric motors to reduce fuel consumption during peak power usage.
This type of vehicle is very convenient for
traffic ridden areas. It also reduces noise emissions when the vehicle is
operating at low speeds. Hybridvehicles are practical and convenient cars for
everyday living in the city.
In this paper we are going to explain about
hybrid vehicle types, their working , emission , and their advantages ,
disadvantages.
INDRODUCTION:
A hybrid
vehicle is a vehicle that uses two or more distinct power sources to
move the vehicle.[1]
The term most commonly refers to hybrid electric vehicles (HEVs), which
combine an internal combustion engine and one or more
electric
motors.
The ultimate goal of the hybrid electric
vehicle is to provide the equivalent power, range, cost and safety of a
conventional vehicle while reducing fuel costs and harmful emissions. At
present a HEV is able to operate nearly twice as efficiently as traditional internal
combustion vehicles. CLASSIFICATION OF HYBRID
VEHICLE:
There are many HEV configurations and design
options that can be grouped in three categories:
Ø series (range-extending HEVs),
Ø parallel (power assist HEVs),
Ø and dual-mode HEVs.
Conventional internal combustion engines
convert the liquid fuel energy into shaft energy. All energy from the
combustion process centers around the crankshaft with the exception of that
lost in the form of heat A typical ICE vehicle only uses approximately 16% of
the liquid fuel energy to move the vehicle. The heat emitted in the combustion
process wastes the majority of the energy while frictional losses from the
hundreds of moving parts in the engine, transmission and the mechanical
connection to the drive wheels consumes the rest. HEV's on the other hand are
designed with energy efficiency in mind.
The main source of energy used in the most
common HEV's today are batteries. A battery contains no moving parts. The only
energy wasted is a very small amount of heat during the course of a discharge
cycle. As previously mentioned, hybrid electric vehicles utilize two different
energy sources. Batteries are usually the main energy supplier for the vehicle
and an auxiliary engine that burns gasoline, diesel fuel, or alternative fuels
such as methanol, ethanol or compressed natural gas provides the auxiliary
power. In some cases, the reverse is true with the batteries providing
auxiliary power during times of high energy demand. The diagrams below(courtesy
of the DOE - HEV Program) compare a conventional vehicle's energy use to that
of a hybrid electric vehicle.
Notice the absence of the idling losses and
the large reduction in engine losses in the hybrid vehicle shown below.
As shown by the chart below, a hybrid
electric vehicle will travel twice the distance of a conventional vehicle on
the same amount of energy. An internal combustion engine is inefficient not
only because of the amount of energy loss incurred in the transfer of energy
from the liquid state to the drive-train, but it also becomes more inefficient
when the vehicle is not moving but the engine is still consuming energy.
Energy
Source/Sink
|
Hybrid Electric Vehicle
|
Internal Combustion
Engine
|
Fuel
|
100
|
100
|
Transmission
Losses
|
-6
|
-6
|
Idling Losses
|
0
|
-11
|
Accessory Loads
|
-2
|
-2
|
Engine Losses
|
-30
|
-65
|
Regenerative
Braking
|
+4
|
0
|
Total Energy
Remaining
|
66
|
16
|
A hybrid electric vehicle is able to utilize
the energy produced by its auxiliary engine, even when the vehicle is not
moving, by storing the energy produced during idling in the battery pack.
Additionally HEV's recover 10% or more of the energy consumed in propelling the
vehicle during deceleration by reversing the direction of current flow from the
drive motors. The motors become generators and energy is placed back into the
battery by a process known as regenerative braking. This also aids in
prolonging the life of the braking system.
Only the electric motor is connected to the
wheels in a series hybrid. In a “classical” series HEV, an electric generator,
coupled with an engine, supplies electricity for the battery which, in turn,
feeds the electric motor. Generally, the engine/generator set keeps the battery
charged between 60-80%. When the battery reaches the lower limit, the engine starts.
Similarly, when the battery reaches the upper limit, the engine will shut off.
However, in some series HEVs, electric power to the motor can come from both
the batteries and the engine/generator set. Since only the electric motors are
connected to the wheels, the engine can run at optimum performance greatly
reducing emissions.
 |
Entire drive power transmitted electrically
May require larger batteries Requires on-board charging Requires some off-board charging Optimization by separating engine speed from vehicle speed Engine never idles, thus reduces overall emissions May not require a transmission Requires heavy-duty motor |
S
E R I E S |
Parallel hybrids have mechanical connections
to the wheels from both the electric motor(s) and the engine allowing the
vehicle to accelerate faster than a series HEVs. These vehicles do not need a
dedicated generator and are connected to the electric grid for recharging the
batteries (Although the electric motor could be used as a generator to recharge
the batteries via a clutch). In a parallel HEV, the electric motor assists the
engine during start-up and acceleration.
An example of a parallel hybrid vehicle is
the HIMR bus (HINO Motors) in Japan.
Dual-mode hybrids are basically parallel
hybrids with a separate generator that also allows recharging of the batteries.
In normal driving conditions, the engine moves both the wheels and the
generator, which in turn supplies power to the electric motor and the
batteries. During full-throttle acceleration or under heavy load, the motor
gets a power boost from the battery.
 |
Engine can fuel batteries as well as drive wheels
Requires medium - large batteries Requires on and off-board charging Accelerates faster due to dual power sources Engine idles Packaging of components less flexible May not require a transmission Requires heavy-duty motor |
D
U A L - M O D E |
Perhaps the most successful dual- mode
hybrid is the Toyota Prius. In the Prius, when the vehicle starts moving, the
engine shuts down and only the electric motor drives the wheels, drawing its
power from the batteries.
Additionally, hybrids can be defined as
"charge sustaining" or "charge non-sustaining". In a
"charge sustaining" HEV system, the hybrid power source is capable of
providing sufficient energy, independent of the storage device(usually a
battery), to drive the vehicle just like it was a conventional vehicle. As long
as the hybrid has fuel for the engine, the vehicle will operate.
The hybrid power source in a "charge
non-sustaining" HEV is only able to provide recharging energy and cannot
supply the necessary energy to drive the vehicle by itself. If an HEV requires
an instantaneous 120 kW to accelerate, and the hybrid power source is only
capable of supplying 60 kW, the HEV is considered a "charge
non-sustaining" system because the engine/generator cannot produce the
required energy necessary to accelerate the vehicle. This system must have
additional energy from the storage device (battery) to meet the energy needs of
the vehicle. A "charge non-sustaining" system is often referred to as
a "range extender" because its intended to extend the range of the
vehicle.
Why put a combustion
engine in an electric vehicle?
. An average commute to work is around 40 miles. Currently, EV's are
forecasted to have a range
in the 80-100 mile range
using advanced battery technology such as NiMH. Surveys indicate
this
range to be borderline for what commuters desire.
Transportation needs go beyond the typical family car. There are
trucks, buses and other
commercial vehicles that can also benefit from EV technology, but
the size these types of
vehicles further reduces the range that current battery technology
can supply. This is where the
hybrid concept becomes very
valuable. Because there are two energy sources in a HEV, the
auxiliary power source which
is typically an internal combustion engine, can be greatly reduced
in size. A good example of
this is the hybrid electric HMMWV, a military applications vehicle
designed and built by PEI Electronics as part of a DARPA grant. The
original engine in the
vehicle was approximately 9 Liters, and has been replaced by a 288 volt
lead acid battery and a 2
liter auxiliary engine.
Hybrid electric auxiliary power sources have several advantages over
traditional internal
combustion engines. To begin with, alternative fuels (such as
natural gas, propane, liquefied
natural gas and compressed
natural gas)are neither corrosive nor toxic, they possess a high
ignition temperature, are lighter than air, and have a narrow
flammability range, making
alternative fuels inherently safe fuels as compared to other fuel
sources. Alternative fuels do not
contaminate soil or water.
Alternative fuels will always rise to the atmosphere out of doors,
unlike other fuels, which are
heavier than air and can pool, either as a liquid or a vapor, upon the
ground. Alternative fuels
contain a distinctive odorant, which allows for detection at 0.5%
concentration in air, well
below levels, which can cause drowsiness due to inhalation, and well
below the weakest
concentration that can support combustion.
These fuels further reduce harmful
emissions. Because the engine is sized to some average load, with the battery
providing energy for peak loads, the engine can be tweaked or computer
controlled to operate at its maximum efficiency. The engine does not need to
speed up or slow down as the load varies.
Natural gas is the cleanest burning
alternative fuel. Exhaust emissions from natural gas powered engines are much
lower than those from equivalent gasoline-powered vehicles. For instance,
natural gas emissions of carbon monoxide are approximately 70 percent lower,
non-methane organic gas emissions are 89 percent lower, and oxides of nitrogen
emissions are 87 percent lower. In addition to these reductions in pollutants,
natural gases also emit significantly lower amounts of greenhouse gases and
toxins than do gasoline vehicles.
Natural gas produces little or no
evaporative emissions during fueling and use. For gasoline vehicles,
evaporative and fueling emissions account for at least 50 percent of a
vehicle's total hydrocarbon emissions. Using natural gas as an alternative fuel
in a HEV can reduce carbon dioxide exhaust emissions by almost 20 percent.
Exposure to the levels of suspended fine
particulate matter found in many U.S. cities has been shown to increase the
risk of respiratory illness. Diesel exhaust is under review as a hazardous air
pollutant. Engines operating on natural gases produce only tiny amounts of this
matter.
Per unit of energy, natural gases contain
less carbon than any other fossil fuel, and thus produce lower CO2 emissions
per vehicle mile traveled. While natural gas powered engines do emit methane,
another principle greenhouse gas, any slight increase in methane emissions
would be more than offset by a substantial reduction in CO2 emissions compared
to other fuels.
Utilizing natural gas in the auxiliary
engine also reduces the emission levels of carbon monoxide (approximately 70
percent lower than a comparable gasoline vehicle) and volatile organic
compounds. Although these two pollutants are not themselves greenhouse gases,
they play an important role in helping to break down methane and some other
greenhouse gases in the
atmosphere, and thus increase the global rate of methane decomposition.
atmosphere, and thus increase the global rate of methane decomposition.
While regular gasoline contains more energy
per unit volume, as compared to alternative fuels (shown in table below), the
difference in cost between gasoline and alternative fuels makes up for the lost
energy on a per mile basis.
Fuel and Primary or Typical Composition
|
Energy Available for Power in One Gallon
|
Factor : Gallons required for same mileage as gasoline
|
Unleaded Regular
Gasoline
(C8H15-18) |
114,000 BTU
|
1.00 Gallon liquid
|
Natural Gas
(CH4)
|
114,000 BTU
|
1.00 Gallon liquid
|
Liquefied
Natural Gas (LNG) (CH4)
|
76,000 BTU
|
1.50 Gallon liquid
|
Diesel (C16H34)
|
128,000 BTU
|
0.89 Gallon liquid
|
Propane
(HD5)(C3H8)
|
82,450 BTU
|
1.38 Gallon liquid
|
Methanol (CH3OH)
|
57,000 BTU
|
2.00 Gallon liquid
|
M85 (85%
methanol,15% gasoline)
|
65,500 BTU
|
1.74 Gallon liquid
|
Research is still taking place in an effort to make hybrid electric
vehicles even more efficient as well as more environmentally friendly. The main
emphasis at this point is in energy storage methods. Primary vehicle energy
storage devices such as the fuel cell have the potential to increase energy
efficiency. A fuel cell is an electrochemical device in which a fuel reacts
with oxygen to release electrons, producing electricity. The fuel cell's
greatest benefit is that it produces zero emissions.
Fuel cell vehicles are an attractive advance
from battery powered vehicles. They offer the advantages of battery power, but
can be re-energized quickly and could go longer between refueling. There are
many different types of fuel cells currently under development and advances are
still being made on units that have not made it to the production line. The
most common type of fuel cell is known as a PEM fuel cell. These fuel cells
have a Proton Exchange Membrane through which the atomically smaller protons
migrate through while the free electrons which are larger are conducted through
the external circuit as electricity.
Shown at right, is a Ballard PEM fuel cell. The fuel cell consists
of two electrodes, the anode and cathode, separated by a polymer membrane
electrolyte. Each side of the electrodes is coated on one side with a thin
platinum catalyst layer. The electrodes, catalyst and membrane together form
the membrane electrode assembly. Hydrogen fuel dissociates into free electrons
and protons(positive hydrogen ions) in the presence of the platinum catalyst
and the anode. The free electrons are conducted in the form of usable electric
current through the external circuit. The protons migrate through the membrane
electrolyte to the cathode. At the cathode, oxygen from the air, electrons from
the external circuit and protons combine to form pure water and heat.
Individual fuel cells produce about 0.6 volt and are combined into a fuel cell
stack to provide the amount of electrical power required.
Within the fuel cell stack(shown at left), gases(hydrogen and air)
are supplied to the electrodes on either side of the PEM through channels
formed in the flow field plates. Hydrogen flows through the channels to the
anode where the platinum catalyst promotes its separation into protons and
electrons. On the opposite side of the PEM, air flows through the channels to
the cathode where oxygen in the air attracts the hydrogen protons through the
PEM. The electrons are captured as useful electricity through an external
circuit and combine with the protons and oxygen to produce water vapor on the
cathode side.
One of the major challenges with fuel cells
will be infrastructure. There are few fuel stations that sell hydrogen. Using a
reformer to convert other fuels to hydrogen adds to the cost of fuel cells and
the packaging of the fuel system. Thus, although the development of fuel cells
for motor vehicles may be close, normal use may take
many years.
Another promising device which can store energy is the flywheel.
Flywheels store energy mechanically. To absorb energy, the flywheel converts
electrical energy into kinetic energy using a built in motor, making the
flywheel's rotor spin faster. To deliver energy, the kinetic energy stored in
the rotor is converted to electrical energy using the same motor with the
polarity of the field coils reversed, which causes the energy to flow into the
battery.
The flywheel operates on the same principal
as the regenerative braking feature, but uses the speed of the rotor rather
than the momentum of the vehicle to generate energy.
CONCLUSION
Hybrid cars consume less fuel, they also emit less
fumes. Hybrid vehicle emissions today are getting close to or even lower than
the recommended level. The recommended levels they suggest for a typical
passenger vehicle should be equated to 5.5 metric tons of carbon dioxide. Hybrid
and electrical vehicles in an attempt to gauge how quickly the vehicle stock
could be hybridized and or electrified in the United States. The
analogs are 1) the electric motors in
U.S. factories in the early 20th century, 2) diesel electric locomotives on
U.S. railways in the 1920-1945 period, 3) a range of new automotive
features/technologies introduced in the U.S. over the past fifty years, and 4)
e-bike purchases in China over the past few years. These analogs collectively
suggest it would take at least 30 years for hybrid and electric vehicles to
capture 80% of the U.S. passenger vehicle stock
