Transcript ELECTRIC AND HYBRID VEHICLES

ELECTRIC AND HYBRID VEHICLES
Hybridization Ratio
Some new concepts have also emerged in the past
few years, including full hybrid, mild hybrid, and
micro hybrid.
These concepts are usually related to the power
rating of the main electric motor in a HEV. For
example, if the HEV contains a fairly large electric
motor and associated batteries, it can be
considered as a full hybrid. On the other hand, if
the size of the electric motor is relatively small,
then it may be considered as a micro hybrid.
Typically, a full hybrid should be able to operate
the vehicle using the electric motor and battery
up to a certain speed limit and drive the vehicle
for a certain amount of time.
The speed threshold is typically the speed limit
in a residential area. The typical power rating of
an electric motor in a full hybrid passenger car is
approximately 50–75 kW.
The micro hybrid, on the other hand, does not
offer the capability to drive the vehicle with the
electric motor only. The electric motor is merely
for starting and stopping the engine. The typical
rating of electric motors used in micro hybrids is
less than 10 kW. A mild hybrid is in between a
full hybrid and a micro hybrid.
An effective approach for evaluating HEVs is to
use a hybridization ratio to reflect the degree of
hybridization of a HEV.
In a parallel hybrid, the hybridization ratio is
defined as the ratio of electric power to the total
powertrain power.
For example, a HEV with a motor rated at 50 kW
and an engine rated at 75 kW will have a
hybridization ratio of 50/(50+75)kW=40%.
A conventional gasoline-powered vehicle will have a
0% hybridization ratio and a battery EV will have a
hybridization ratio of 100%. A series HEV will also
have a hybridization ratio of 100% due to the fact
that the vehicle is capable of being driven in EV
mode.
Interdisciplinary Nature of HEVs
HEVs involve the use of electric machines, power
electronics converters, and batteries, in addition to
conventional ICEs and mechanical and hydraulic systems.
The HEV field involves engineering subjects beyond
traditional automotive engineering, which was
mechanical engineering oriented.
Power electronics, electric machines, energy storage
systems, and control systems are now integral parts of
the engineering of HEVs and PHEVs.
The general nature and required engineering field by HEVs
In addition, thermal management is also important in
HEVs and PHEVs, where the power electronics, electric
machines, and batteries all require a much lower
temperature to operate properly, compared to a nonhybrid vehicle’s powertrain components.
Modeling and simulation, vehicle dynamics, and vehicle
design and optimization also pose challenges to the
traditional automotive engineering field due to the
increased difficulties in packaging the components and
associated thermal management systems, as well as the
changes in vehicle weight, shape, and weight distribution.
State of the Art of HEVs
In the past 10 years, many HEVs have been
deployed
by
the
major
automotive
manufacturers.
It is clear that HEV sales have grow significantly
over the last 10 years.
Breakdown of HEV sales by model** in the United States in 2009 (in thousands)
The Toyota Prius (2010 model)
Partial list of HEVs available in the United States
The powertrain layout of the Toyota Prius (EM, Electric Machine; PM,
Permanent Magnet)
The powertrain layout of the Honda Civic hybrid
The Ford Escape hybrid SUV
The Chrysler Aspen two-mode hybrid
Challenges and Key Technology of
HEVs
HEVs can overcome some of the disadvantages of battery-powered
pure EVs and gasoline-powered conventional vehicles.
These advantages include:
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optimized fuel economy,
reduced emissions when compared to conventional vehicles,
increased range
reduced charging time,
reduced battery size (hence reduced cost) when compared to pure
EVs.
However, HEVs and PHEVs still face many challenges:
• including higher cost when compared to conventional
vehicles,
• electromagnetic interference caused by high-power
components,
• safety and reliability concerns due to increased
components and complexity,
• packaging of the system,
• vehicle control,
• power management.
CONCEPT OF HYBRIDIZATION
OF THE AUTOMOBILE
Vehicle Basics
Constituents of a Conventional Vehicle
Present-day engine-propelled automobiles have
evolved over many years. Today’s automobiles
initially started with steam propulsion and later
transitioned into ones based on the internal
combustion engine (ICE).
Cutaway view of an ICE
The engine has a chamber where gasoline or diesel
is ignited, which creates a very high pressure to
drive the pistons. A piston is connected through a
reciprocating arm to a crankshaft. The crankshaft is
connected to a flywheel which is then connected to
a transmission system. The purpose of the
transmission system is to match the torque speed
profile of the engine to the torque speed profile of
the load. The shaft from the transmission system is
ultimately connected to the wheels through some
additional mechanical interfaces such as differential
gears.
Transmission system and engine connected together
Vehicle and Propulsion Load
The power generated from the engine is ultimately used to drive a
load. In an automobile this load includes the road resistance due to
friction, uphill or downhill drive related to the road profile, and the
environmental effect of, for example, the wind, rain, snow, and so on.
In addition, some of the energy developed in the vehicle is wasted in
overcoming the internal resistance within the vehicle’s components
and subsystems, none of which are 100% efficient.
Examples of such subsystems or components include the radiator fan,
various pumps, whether electrical or mechanical, motors for the
wipers, window lift, and so on. These items are just a few examples
from a whole list of vehicular loads. The energy lost in these devices is
released eventually as heat and expelled into the atmosphere.
Normally “load” can be related to the amount of opposing force or
torque. But a more scientific definition of load comes from the fact
that it is not defined by a single number or numerical value.
Load is a collection of a set of numbers defined by the speed–torque
or speed–force characteristics in the form of a table or graph, that is,
through a mathematical equation relating speed and torque.
Similarly the engine is also defined by speed–torque characteristics in
the form of a table or graph, that is, through a mathematical equation.
The operating point of the combination of the engine and the load
system together will then be at the intersection of these
characteristics.
Load and engine characteristics of a vehicle
A complete vehicle or automotive system has
various loads. Some of these are electrical devices,
and others are mechanical devices. The electrical
loads are normally run at a low voltage (nominally
12 V). The reason for this, i.e. running the nonpropulsion loads at low voltage, is primarily related
to safety issues. There is an existing manufacturing
base for many of these non-propulsion loads (as
indicated below), where it is easier to take
advantage of the situation and use the existing lowvoltage components, rather than transform the
voltage system.
Examples of these loads are:
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brakes – mechanical (hydraulic or low-voltage electrically assisted);
air-conditioner – generally mechanical;
radiator fan – can be belt driven mechanically (or low-voltage electrical);
various pumps – can be mechanical (or low-voltage electrical);
window lift – electrical;
door locks – electrical;
wipers – electrical;
various lights – non-motor load, low-voltage electrical;
radio, TV, GPS – non-motor, low-voltage electrical;
various controllers – for example, engine controller, transmission
controller, vehicle
• body controller;
• and various computational microprocessors – non-motor, low-voltage
electrical.
Drive Cycles and Drive Terrain
Since a vehicle will be driven through all kinds of
road profiles and environmental conditions, to
exactly know beforehand about which loads the
vehicle will encounter under all circumstances is
difficult.
It is of course possible for one to perform
experiments and place sensors etc. to monitor the
speed and torque of a vehicle, but to do so under
all circumstances for all vehicle platforms is simply
impossible.
Hence, for the engineering studies, a few limited
situations have been developed which more or less
cover typical road profiles and the terrains one can
expect to encounter.
Using a few of these profiles, one can create or
synthesize various arbitrary road profiles. Such
profiles can involve things like driving within a city,
on a highway, across some special uphill or downhill
terrain, to name but a few.
Drive cycles only provide time, an corresponding
speed fluctuations, and labels attached to these
tell us what kind of drive cycle it is, for example,
city, highway, and so on.
So, if a vehicle goes through different driving
situations, partly city, partly highway, and so on,
then one can obtain speed vs. time data by
synthesizing multiple typical drive cycles.
A typical automotive drive cycle
The question then arises about the ways to utilize
the drive cycle information.
Assume that we want to know about the fuel
economy of a particular vehicle X. It is not sufficient
to say that vehicle X does 25 MPG. We also need to
say under what conditions this was obtained. That
is, whether it was under a city drive cycle, or
highway drive cycle, and so on. Then one can
compare another vehicle Y against X, under similar
drive cycle conditions, and make a fair comparison.
As there are different kinds of drive cycles, that
of a passenger car cannot be compared against
the drive cycle of a refuse truck or a postal mail
vehicle, since they have very different kinds of
stop and go driving.
Similarly the drive cycle of a heavy mining
vehicle cannot be compared with the above
either.
Finally, it should be noted that a drive cycle
concerns the road profile through which a vehicle
goes and hence is a situation external to the
vehicle.
However, the response of a vehicle to a given drive
cycle, in terms of fuel economy, will be different
depending on whether the vehicle is a regular ICE
vehicle, fully electric vehicle (EV), hybrid electric
vehicle (HEV), and so on.
Basics of the EV
Why EV?
Although these days people talk more about HEVs which have become
very popular, their underlying system is complex because it has two
propulsion sources. A pure EV is relatively simpler since it has only one
source of energy, that is, a battery or perhaps a fuel cell.
Similarly its propulsion is performed by an electric motor and the need
for an ICE is not there. If the ICE is gone, the vehicle will not need any
fuel injectors, various complicated engine controllers, and all the other
peripherals associated with the engine and transmission. With a
reduced parts count and a simpler system, it will be more reliable as
well.
In addition, an EV is “virtually” a zero-emission vehicle (since
nothing has technically zero emissions in a true global sense).
Of course, if one considers the ultimate source of energy, by
tracing the path backward from the battery to the utility
industry, it will be found that the location of pollution has
been essentially shifted from the vehicle to elsewhere.
Furthermore, an EV is virtually quiet. In fact it can be so quiet
that people have even talked about introducing artificial noise
in the vehicle so that they can hear it, which is something
important to know from a safety point of view.
From a technical viewpoint, the EV has another benefit. In the ICE, which is a
reciprocating engine, the torque produced is pulsating in nature. The flywheel
helps smooth the torque which would otherwise cause vibration. In the EV
the motor can create a very smooth torque and, in fact, it is possible to do
away with the flywheel, thus saving material and manufacturing cost, in
addition to reducing weight.
And finally, the efficiency of an ICE (gasoline to shaft torque) is very low. The
engine itself has about 30–37% efficiency for a gasoline and about 40% for a
diesel engine, but by the time the power arrives at the wheel, the efficiency is
just 5–10%. On the other hand, the efficiency of the electric motor is very
high, on the order of 90%. The battery and power electronics to drive the
motor also have high efficiency. If each of these components has an efficiency
on the order of 90%, by the time the battery energy leaves the motor shaft,
the overall efficiency will be something like 70%. This is still substantially
higher than that in the ICE.
Constituents of an EV
The complete EV consists of not only the electric
drive and power electronics for propulsion, but
also other subsystems to make the whole
system work. One needs a battery (or a fuel cell)
to provide the electrical energy.
System-level diagram of an EV.
Vehicle and Propulsion Loads
There is a significant amount of commonality between
the loads in an EV and a regular automobile. Hence, just
like a regular vehicle, some of these loads are electrical
devices and others are mechanical devices. As noted
earlier, those loads which are electrical normally run at a
low voltage (nominally 12 V), with the exception of the
propulsion load, that is, the propulsion motor, which runs
at a high voltage (several hundred volts). The reason for
this has to do with safety primarily. And, of course, the
existing manufacturing base for many of these nonpropulsion loads can be used to advantage by using the
existing low-voltage components, rather than
transforming the voltage system.
Examples of these loads are same as
those noted previously:
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propulsion or traction motor – high-voltage electrical load;
brake motor (if a fully or partially electrical brake system is used) – low voltage;
air-conditioner motor (if electrical) – low voltage;
radiator fan (if electrical) – low voltage;
various pumps (if electrical) – low voltage;
window lift – low voltage;
door locks – electrical;
wipers – electrical;
various lights – non-motor load, low-voltage electrical;
radio, TV, GPS – non-motor, low-voltage electrical;
various controllers, for example, engine controller, transmission controller, vehicle
body
controller;
various computational microprocessors, digital signal processors (DSPs) – nonmotor,
low-voltage electrical.
The above list more or less covers the various loads
in the vehicle, including propulsion loads. The
propulsion load can be several kilowatts for a mild
hybrid vehicle regenerative braking system, up to
say 50kW or a few hundred kilowatts for propulsion
in a hybrid vehicle. The various pumps and fans can
be only a few hundred or less watts, whereas some
small motors like door lock motors could be just a
few tens of watts. Similarly the lights can range
from a few tens to about a hundred watts.
Basics of the HEV
Why HEV?
Previously we discussed the architecture of a purely EV. As we saw, the
EV propulsion uses an electric motor for propulsion. The energy comes
from the battery (or perhaps a fuel cell). The battery bank in a pure EV
can be quite large if the vehicle is to go a few hundred miles on one
full charge to begin with. The reason for this is that battery technology,
as it stands today, does not have a very high energy density for a given
weight and size, compared to a liquid fuel like gasoline. Although new
batteries like lithium-ion batteries have a much higher energy density
than the existing lead acid or nickel metal hydride batteries, it is still
much lower compared to liquid fuel.
As noted earlier, the HEV is a complex system
since it has two propulsion sources.
Comparatively a pure EV is simpler since it has
only one source of energy, namely, a battery or
perhaps a fuel cell. In the EV the propulsion is
produced by only the electric motor and there is
no ICE. This removes the need for fuel injectors,
complicated engine controllers, and all other
peripherals. Hence, with a reduced parts count,
the system is simpler and more reliable.
Of course, there is an efficiency improvement in
the HEV compared to the ICE, but it will still be
lower than in the EV. The overall efficiency will
depend on the relative size of the ICE and the
electric propulsion motor power.
A variant of the HEV is found in locomotives and in very high powered
off-road vehicles. In a number of variants of such systems there is no
battery. The ICE is used to drive a generator which creates AC power.
This power is translated to DC and then to another AC power required
to drive an electric motor.
The problem with this system is that the engine has to be run
continuously to produce the electricity. The advantage is that it does
not need a battery. Furthermore, the ICE can be run at an optimal
speed to achieve the best possible efficiency. One problem with this
system is that it does not lend itself to regenerative energy recovery
during braking. The battery helps regenerative energy recovery by
allowing storage and it can also be coordinated more optimally in
terms of when the ICE or the electric motor should be run.
Constituents of a HEV
As noted earlier, an EV is simpler than a HEV.
As we can see, the only difference between this
diagram and the one for the EV is that this one
has an additional subsystem called IC engine,
along with the necessary interface and the
controller. Otherwise the two diagrams are
identical.
System-level diagram of a HEV.
Basics of Plug-In Hybrid Electric Vehicle (PHEV)
Why PHEV?