Conference Session B4
Paper 2130
CONTINUOUSLY VARIABLE POWER TRANSMISSION AND
REGENERATIVE BRAKING: INCREASING AUTOMOTIVE MECHANICAL
EFFICIENCY
Christopher Dumm (cmd113@pitt.edu), Joseph Sadecky (jcs119@pitt.edu)
Abstract— This paper describes the mechanics behind and
evaluates the potential impact of the use of Continuously
Variable Transmission (CVT) and “regenerative” braking
technologies in automobiles. The mechanics behind these
systems are detailed and contrasted with currently utilized
technology in order to highlight both the relative merits and
drawbacks of these new systems. Various theoretical and
practical implementations of CVT and regenerative braking
technologies are discussed with a focus on efficiency. The
estimated costs of implementing CVT and regenerative
braking technologies in production automobiles are
considered and discussed in parallel with the estimated
corresponding private and social benefits from efficiency.
While CVT and regenerative braking technologies have not
at present been jointly developed to the point where
widespread implementation is practical, it is believed that
they promise to greatly improve the overall efficiency of
mechanical energy transfer technology.
FIGURE 1
UNITED STATES RETAIL GASOLINE PRICES, AUG. 1990-FEB. 2012 [1]
Two promising new automotive technologies are
Continuously
Variable
Transmission
(CVT)
and
Regenerative Braking systems. Each of these systems has
the potential to significantly improve the energy and fuelefficiency of automobiles. While CVT and regenerative
braking technologies are currently available in a small
selection of automobiles, they are not universally used,
mainly due to limited power transfer capacity and the
difficulties inherent in building robust versions of these
complex systems. However, it is believed that widespread
implementation of these technologies could result in
significant consumer and societal benefits.
One of the most important aspects of these technologies,
from an ethical point of view, is their promise in reducing
vehicular emissions, which logically will result in reduced
air pollution [5, 6]. These technologies also have the
potential to drastically reduce fuel consumption by vehicles,
resulting in increased spending money for consumers [5].
Benefits such as reduced maintenance costs and better
subjective performance might also result from these
technologies’ use [7]. Overall, it is believed that the benefits
of implementing these technologies greatly outweigh their
potential costs of development and implementation, and thus
research into CVT and regenerative braking ought to be
encouraged.
In order to consider how technologies such as CVT and
regenerative braking can improve automotive technology, it
is important to first know how current transmission and
braking technologies work. With this knowledge, one can
understand where inefficiencies in current technology lie and
what improvements might be made. For these reasons, we
will now discuss the mechanics behind current transmission
and braking technologies.
Key
Words—Automotive,
Continuously
Variable
Transmission, Energy Efficiency, Mechanical Energy
Retention, Regenerative Braking
DEMAND AND SUPPLY: THE RISE OF
CONTINUOUSLY VARIABLE TRANSMISSION AND
REGENERATIVE BRAKING
At present, gasoline prices are at an all-time high. The
domestic price of gasoline has increased by approximately
330% percent over the past 24 years (see Fig. 1), and some
sources project the prices to reach between four and five
dollars in mid-2012 [2]. Gasoline is of utmost importance to
global commerce and consumers alike for transportation, so
increased gasoline prices could result in increased prices for
all manner of products and a decrease in worldwide tourism,
to name a few possible consequences [3].
The importance of gasoline in personal transportation and
the desire to reduce costs of transportation has resulted in
great consumer demand for increased fuel-efficiency in
automobiles [3]. Consequently, there is a great deal of
current research into methods of improving automotive fuel
efficiency. As current advertisements show, most research
into new technology has focused on improving engine
efficiency: the internal combustion engine (ICE) is the single
most wasteful system of an automobile in terms of energy
[4]. However, there are several other mechanical systems
where energy-efficiency could be improved.
University of Pittsburgh
Swanson School of Engineering
April 14, 2012
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Christopher Dumm
Joseph Sadecky
manual and automatic transmissions. Both typically have
between four and six gears, but each uses a different method
to switch between them. In automatic transmission vehicles,
the automobile’s computer selects the appropriate gear
reduction for the vehicle and initiates gear changes.
Mechanically, this action is accomplished with a device
called a torque converter, which couples two shafts through
internal movement of a high-pressure fluid while switching
gears (see Fig. 2). Manual transmission systems, however,
switch between gears with an operator-controlled gear lever
and a clutch (a physical coupler for the gear-reduced engine
shaft and output shaft.) To shift, the operator depresses a
pedal which disconnects the automobile’s clutch, and while
disconnected the operator can switch gears [8, 9].
CURRENT APPROACHES: TRANSMISSION
The power transmission systems of modern automobiles are
not a drastic departure from previous technology; in fact,
they are very similar to those of the first automobiles built
over 100 years ago. The transmission system serves to
connect the rotational output of a combustion engine to the
automobile’s wheels. This system can be abstractly broken
up into two subsystems. The larger of these two subsystems
physically is the vehicle’s drivetrain, which includes the
main drive shaft. This shaft is connected to the automobile’s
wheels and causes them to turn. However, the drive shaft
must be turned by the engine. This action is accomplished by
the second subsystem: the gear reduction process [8].
When the automobile’s engine is running, the engine turns
a shaft with energy produced through gasoline combustion.
As the accelerator pedal is depressed, the rate of combustion
is increased, and so the shaft rotates faster. Without a “load,”
another mass for the shaft to turn, the output shaft of the
engine spins at a very high speed, usually several thousand
rotations per minute (RPM). This is similar conceptually to
how a dentist’s drill spins at a very high speed when not in
contact with a tooth. However, when contact is made, the
drill spins much more slowly: when loaded, the energy that
previously created high rotational speeds is expended on the
load. Similarly, a significant load attached suddenly to an
engine would slow the engine down. However, such an
abrupt action would be violent, and could cause damage [8].
The gear reduction process creates a stable, “artificial”
load by connecting the engine output to the wheels through a
set of gears. These gears effectively reduce the angular
velocity, or turning speed, of the shaft significantly. There
are three major effects that make gear reduction valuable. To
begin, the slower output of a “geared-down” engine can be
produced with less power than with a direct engine-shaft
connection. This lower speed is more easily handled by an
operator and thus increases road safety. Finally, the reduced
speed is more stable since the engine has more power
available to respond to load changes: more turning force, or
“torque,” is available with a geared-down engine [8].
However, the amount of torque the engine produces
depends on the gear reduction. Different gear reductions
produce different torques, which have applications in
different situations. In a standard bicycle, several
interchangeable gears are available. Some gears are meant to
help produce small, accurate adjustments when moving at
high speed, while others are meant to help provide power in
situations like hill climbing. Overall, the purpose of gearing
is to reduce the power requirements of the engine (or in this
case, the human). This type of situational adaptability cannot
be replicated by a single gear reduction: with multiple gears,
more efficient power utilization is possible [8].
Just like bicycles, automobiles must have efficient
mechanisms to control torque at the wheels. The
transmission system fulfills this need. Currently, there are
two main approaches to transmission design in vehicles:
FIGURE 2
EXPLODED VIEW OF A TORQUE CONVERTER [9]
Current transmission systems can be improved in the gear
reduction process. Significant losses in energy occur during
the switching action in both types of transmission: the fluid
in a torque converter does not transfer power between shafts
at a 1:1 ratio, and a manual transmission vehicle’s engine
output is wasted when the clutch is disconnected.
Additionally, since each gear reduction is most effective at a
specific speed, energy is not as effectively utilized when the
engine runs at different speeds [7, 8, 9].
CURRENT APPROACHES: BRAKING
Moving vehicles have a great deal of translational kinetic
energy (energy by virtue of displacement). In order to bring
a vehicle to rest, this energy must be eliminated. With the
engine disconnected, friction between a vehicle’s wheels and
a road surface will eventually eliminate this energy, but
efficient highway movement and quick action in
emergencies necessitate a faster-acting method of slowing a
vehicle. Braking systems fulfill this need [8, 10].
Braking systems remove energy with friction, the same
mechanism which slows vehicles on a road. However, in a
braking system, this friction is operator-controlled and of a
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Christopher Dumm
Joseph Sadecky
greater, yet variable, magnitude. The operator controls
motion with braking by reducing the rotational kinetic
energy of the vehicle’s axles: this energy is converted by the
wheels to translational kinetic energy, so a reduction in
rotational kinetic energy results in a reduction of
translational kinetic energy. One braking system variant
(shown in Figure 3), “disc” brakes, use a hydraulic caliper to
clamp several brake pads down onto a rotor, a hardened
metal disc coaxial with the vehicle’s wheels. The rotational
kinetic energy is converted to heat, and is said to be “burned
off” in friction. This process is virtually the same one used
for braking in bicycles, and while other braking system
variants exist, they all have similar mechanics [8, 10].
However, a significant reduction in vehicular greenhouse
gas emissions could still be accomplished by increasing
automotive fuel economy. Increased fuel economy reduces
the rate of gasoline combustion in an engine, and therefore
results in reduction of vehicular emissions [1, 6]. Because of
this correlation, it is clear that increased automotive energyefficiency results in a reduced rate of global warming.
It could be argued on the basis of global warming that the
design and implementation of more efficient transmission
and braking technology than currently exists is ethically
mandated. We have determined that current systems are
inefficient in specific areas: transmissions have inefficient
gear reduction, while braking utilizes inefficient mechanics
outright. According to the Second Law of Thermodynamics,
complete retention of the energy lost in these systems is
impossible, but significant improvements can be and are
currently being made in automotive technology [10].
We turn now to the description and analysis of two types
of systems that have the potential to improve automotive
energy-efficiency: Continuously Variable Transmissions and
regenerative braking. In order to analyze these systems, we
intend to first describe the theoretical mechanics of these
systems, and then consider what real-world factors will
reduce these systems’ performances.
Ultimately, in order to be considered worthwhile
additions, CVT and regenerative braking systems must save
enough energy over the vehicle’s lifetime to cover
manufacturing costs. Over the lifetime of a normal vehicle, a
certain amount of energy is expended in materials,
manufacturing, use, and disposal cycles (see Fig. 4). As the
figure shows, approximately 83% of this total energy is
contained in the “use” cycle, which is mainly obtained from
gasoline combustion, while 15% of this energy is used in
manufacture of the entire vehicle [12]. This energy usage
can be considered characteristic of all consumer
automobiles. In all cases, we can safely assume that the
energy costs of implementing CVT and regenerative braking
systems are less than one-third of the total manufacturing
energy cost (5% of total energy). The equivalent of 5% of
total energy is a 6% savings in the use cycle; thus, any
automotive system that provides above a 6% increase in
fuel-efficiency is without question worthwhile. We will
gauge the effectiveness of current CVT and regenerative
braking systems on this basis.
FIGURE 3
A STANDARD AUTOMOTIVE DISC BRAKE SYSTEM [11]
The energy efficiency of modern braking systems of
modern automobiles could also be increased. The energy
“burned off” by friction brakes exits the system into the
atmosphere as heat. While these systems are necessary for
coming to a complete stop and absolutely vital for
emergency stopping, all of the kinetic energy in the system
is entirely wasted during friction braking [10].
THE VALUE OF INNOVATION
Current automotive transmission and braking systems are
energy-inefficient in multiple ways. Using more energyefficient systems in automobiles will logically result in
increased automotive fuel efficiency, which has obvious
benefits in financial savings for consumers [3]. However,
there is an environmental issue of greater importance than
consumer savings impacted by automotive fuel efficiency.
In chemical terms, combustion of gasoline produces
carbon dioxide and other greenhouse gases [1, 6]. The role
of greenhouse gases produced by vehicular emissions in
global warming hardly needs to be stated: the media has
brought this serious environmental threat to worldwide
attention over the past decade [1, 6]. While the rate of
atmospheric pollution would be decreased significantly if
consumer automobile use were eliminated entirely, the
complete elimination of private vehicle usage is absurd.
FIGURE 4
AUTOMOTIVE ENERGY USE OVER PRODUCT LIFE CYCLE [12]
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Joseph Sadecky
EFFICIENT POWER UTILIZATION:
CONTINUOUSLY VARIABLE TRANSMISSION
CVT IN PRACTICE: CURRENT PERFORMANCE
Because a CVT can provide the most efficient gear reduction
for a given situation near-instantaneously, a CVT-equipped
vehicle ought to (and has proven to) be more fuel-efficient
than current vehicles [13]. The consumer benefits of CVT
technology are obvious: increased fuel economy results in
reduced fuel purchases, and therefore less variable cost of
owning and operating an automobile. In societal terms,
emission levels will be reduced as compared to vehicles with
standard transmissions, since gasoline combustion is
minimized [13, 14]. One qualitative benefit of this
technology is smoother performance: in an automobile with
a standard transmission style, a jerk is often felt when
switching between gears, but since CVT gear reduction
occurs without interchanging components, this jerk is
virtually eliminated [14].
Real CVTs will exhibit these characteristics, but practical
constraints will have an impact on CVTs’ real-world
performances. Of note is the fact that real CVTs are not
‘infinitely’ variable: upon reconsidering Fig. 4, it is clear
that the precision of the displacement mechanism and
vibrations within the system’s belt will limit the range of
possible CVT gear reductions [13]. However, this range is
certainly greater than that of a standard transmission; thus,
the CVT is superior in this regard.
Of more concern is the belt connecting the pulleys. While
automobile manufacturers produce their own belts, usually
made of high-strength steel, all belts in CVT systems are
under enormous stresses while in operation. These belts are
continually subjected to great tension, and the force
distribution across the belt is not uniform: since the belt
contacts the rollers at its edges, forces keeping the belt in
place act at the belt’s edges [13]. The materials used to
create these belts will deform and break when too much
force is applied; thus, there is a limit to the amount of power
that can be transferred through a pulley-based CVT [13, 14].
At present, the limit of power that a CVT can handle is
approximately 290 horsepower [15]. However, this power
output is characteristic of the small vehicles in which CVTs
are currently being implemented: current sedans have engine
power in the range of 150-300 horsepower [16]. While 290
horsepower is within the sedan power range, it is on the
lower side of current coupe power [17]. However, as CVTs
can transfer power characteristic of sedans, CVT technology
may be considered suitable for current applications.
Quantitatively, CVT systems succeed in energy- and fuelefficiency. As stated previously, a system reducing use-cycle
energy consumption by approximately 6% is sure to make
up for its costs of implementation in energy savings. Current
implementations of CVTs far exceed this 6% increase in
efficiency: The Nissan XTRONIC CVT system increases
fuel economy by 10%, according to in-house research [18],
and the CVT system in a 2012 Subaru Impreza vehicle
provides a similiar boost in fuel economy [19].
Like older systems, Continuously Variable Transmissions
provide a gear reduction mechanism that connects an
automobile’s engine with its drive shaft. However, the
phrase ‘gear reduction’ seems somewhat improper when
referring to CVTs: a continuously variable transmission does
not, in fact, contain any gears at all. In their place, a CVT
uses a form of ‘infinitely variable’ pulley and roller
mechanism to create an effective gear reduction [13, 14].
The most basic CVT system (see Fig. 5) is composed of
two sets of two conical rollers and a belt connecting them.
By changing the distance between the rollers in a set (and
thus the effective radius of the belt’s curvature), it is possible
to create the equivalent of multiple gear reductions, like one
would find in a standard transmission. However, the number
of effective gears that can be created with this technology far
exceeds the four to six gears found in standard
transmissions. Additionally, because the positioning
mechanisms of these rollers are computer-controlled, the
effective gear reduction can be altered on-the-fly in response
to the current speed of the vehicle and desired driver
acceleration. These mechanics mean that the number of
effective reductions that can be created is theoretically
infinite. Additionally, switching between gears can be
accomplished in small time: thus, the name “Continuously
Variable” is apt. Perhaps the most important aspect of CVT
technology, however, is the continued engine-drivetrain
connection while switching gears. The consequences of this
connection are thus: with theoretically infinite gear reduction
capabilities and the continual engine-drivetrain connection,
the energy lost while switching between gears in a standard
transmission is retained. The efficiency of the system is also
improved, as the CVT can provide the most efficient gear
reduction possible for a vehicle at all times [7, 13, 14].
FIGURE 5
AN EXAMPLE OF THE OPERATION OF A PULLEY-BASED CVT SYSTEM [13]
Many different implementations of CVT technology
exist. Each manufacturer builds its own CVTs for use in its
automobiles [14]. However, all versions are, in general,
based on a computer-optimized and adjusted mechanism that
effects a gear reduction between an engine and a drive shaft.
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Joseph Sadecky
This quantitative data supports earlier assertions
regarding efficiency. The reduction in gasoline costs for
consumers resulting from CVT use and the associated
societal benefit of reduction in vehicular emissions clearly
show that CVT implementation is ethically sound. Since
CVT systems can handle the engine power of their target
vehicle market and they provide significant societal benefits,
we may conclude that implementing CVT systems in
modern automobiles is in the best interest of all.
on demand [10]. Therefore, a battery or capacitor would
serve as an excellent energy storage medium. However,
there is a problem present: how does one convert rotational
kinetic energy to electrical energy? This problem has already
been solved, and the solution is currently implemented in
every production automobile.
All automobiles contain a battery, which is used to start
the engine, power the headlights and onboard computer, and
so on. This battery discharges continually to power the
vehicle’s systems: a way to recharge it in operation was
needed. The solution was to use the engine itself to charge
the battery. The device that accomplishes this is the
alternator, which is composed of two main components: a
set of magnets attached to a rotating shaft and a coil of wire
that encloses the travel area of the magnets (see Fig. 4).
When the magnets spin within the coil, a rotating magnetic
field is created and an electric current is induced in the coil,
which can then be used to power a circuit, or for the purpose
of regenerative braking, to charge a battery or capacitor [21].
RETAINING ENERGY: REGENERATIVE BRAKING
A regenerative braking system’s purpose is to remove
translational kinetic energy from a vehicle, store that energy
to be accessed later (as opposed to friction brakes, which
remove the energy from the system outright), and to
reconvert the stored energy to translational energy when
required [20]. Therefore, a regenerative braking system has
three essential components: a way to remove translational
kinetic energy efficiently from a vehicle; some way to store
that energy efficiently; and a way to convert the stored
energy back into translational energy. We now consider
some possible methods of regenerative braking.
To begin, recovery of heat energy from the friction
brakes is not an option: it is extremely difficult to recover
and use heat energy [10]. Therefore, a regenerative braking
system needs to act in place of the friction brakes in
removing energy. The standard braking system removes
rotational kinetic energy from the vehicle’s axles, so it
seems most logical to remove energy from the axles in a
similar way. As always, a complete, controlled stop requires
friction brakes, but a regenerative braking system can retain
some energy that is normally lost. In addition, since
regenerative braking involves decreased use of the friction
brakes, a vehicle’s brake pads will not need replacement as
frequently, saving the consumer money.
The two most promising methods of regenerative braking
currently under research (and limited implementation) are
regenerative braking based on kinetic energy storage and
electrical energy storage. Each of these systems has its own
strengths and weaknesses, but overall, each provides a
feasible way to implement regenerative braking, which
means that when used efficiently, the range and therefore
fuel economy of the automobile will be extended.
FIGURE 4
EXPLODED VIEW OF A STANDARD AUTOMOTIVE ALTERNATOR [21]
This phenomenon is explained by Faraday’s Law of
Induction. When applied to this situation, this law states that
a magnetic field can induce a current in a wire, and vice
versa [10]. This principle is the same one that explains how
an electric motor works: the motor operates by passing a
current through a coil of wire, which then creates a rotating
magnetic field and induces rotational motion in a series of
magnets attached to an axis [10]. In fact, the motor and the
alternator are the same physical device: the specific
motor/alternator behaviors are created by either passing a
current through the coil or moving a magnetic field inside
the coil [10]. The dual uses of this device are the key to
electrical regenerative braking technology.
Electric vehicles such as the Chevrolet Volt use a motor
and a supply of batteries in place of the conventional ICE
and gasoline combination [22]. When running, the motor is
powered by current from the batteries. This supply can be
switched on and off at will, but the connection to the
vehicle’s drive shaft remains; consequently, if the drive shaft
turns while the current to the motor is off, current will be
induced in the motor. This induced current can be used to
charge a capacitor or battery, which can be used as an
Faraday’s Law of Induction and Electric Regenerative
Braking
We have established that the best way to remove kinetic
energy is by removing rotational kinetic energy from the
vehicle’s axes in a similar way to friction brakes, but we still
need the actual components that will perform the
regenerative braking task. Let us consider the storage
medium. The first energy storage devices one tends to think
of are energy storage units are batteries and capacitors: they
can store a large amount of electrical energy and release it
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Christopher Dumm
Joseph Sadecky
electrical energy source for the vehicle [22]. The production
of current allows regenerative braking action.
For an example of how this system might be useful
practically, imagine a conventional vehicle coasting down a
hill. The vehicle would naturally pick up speed as it goes
further down the hill, necessitating the use of brakes to
maintain a constant speed. But in this configuration, kinetic
energy is converted to electric current in the motor (which
acts as an alternator). This current can be used to recharge
the batteries of the car, which results in an increase in stored
energy that can be used to drive the vehicle further without
recharging the vehicle.
The relevance of this law is that the wheels of an
automobile are themselves nothing more than flywheels that
contact the ground. If the wheels were connected (through
the drive axle) to a flywheel, this flywheel could slow the
vehicle by charging and could move the vehicle from rest,
which is a regenerative braking action.
REGENERATIVE BRAKING IN PRACTICE: ITS
IMPLEMENTATION AND IMPACT
The fuel savings that regenerative braking systems promise
to provide stand on their own merits: it is clear that
regenerative braking would increase automotive energy- and
fuel-efficiency and therefore decrease vehicular emissions,
in addition to reducing maintenance costs. However, we
must evaluate whether these systems are truly practical and
valuable in the real world. What factors might reduce
regenerative braking systems’ efficiencies?
Inertia and Kinetic Regenerative Braking
The regenerative braking systems described would certainly
be useful for smaller vehicles capable of running on an
electric motor, but larger vehicles such as vans or trucks
require power that current electric motors are incapable of
providing, and so would use an ICE. However, an ICE does
not have the motor-alternator relationship to harness for
regenerative braking, and the battery network required for
alternator-based regenerative braking adds significant
weight, costs and complexity, as evidenced by the $40,000+
price of the Chevrolet Volt [22]. Therefore, a different
method must be used to implement regenerative braking in
these vehicles.
Capacitors and batteries are electrical energy storage
units appropriate for regenerative braking systems, but their
use requires conversion of rotational kinetic energy to
electrical energy. This conversion is not strictly necessary:
only a need to remove and store energy from the drive axles
exists. One device capable of storing significant mechanical
energy is a flywheel, which is essentially a massive disc. A
flywheel is installed on an axis, and according to (1), when
spun, or “charged”, has rotational kinetic energy
proportional to its mass and angular velocity [10]. As a
flywheel can act as an energy storage unit, the remaining
question is of how to transfer energy between the main axles
of a vehicle and a flywheel.
𝐸𝑘 =
1
2
1
𝐼𝜔2 ; 𝐼𝑓𝑙𝑦𝑤ℎ𝑒𝑒𝑙 = 𝑚𝑟 2
2
Electric Regenerative Braking in Practice
In an electrical regenerative braking system, there is
really only one component to discuss: the alternator/motor’s
efficiency. As we know from the Second Law of
Thermodynamics, the alternator can never retain all
rotational energy in the system, but does it make the 5%
benchmark necessary to justify its cost? To begin, it should
be noted that the costs of implementing regenerative braking
in electric cars will naturally be minimized. Because the
motor can be used as an alternator, implementing a
regenerative braking system would simply involve
controlling a charging connection to the vehicle’s battery
packs by computer.
The merits of electrical regenerative braking are clear
when considering electric hybrid vehicles such as the
Chevrolet Volt. In EPA testing, the Volt had an alternator
efficiency of over 90%, which, over the vehicle’s life cycle,
will result in far greater energy savings than the costs of
installing the regenerative systems [22]. A 2010 study on the
impact of regenerative braking on vehicular emissions
concurs with this savings: test bed vehicles and simulations
suggested that regenerative braking could increase energy
efficiency by at least 30%, which is clearly above our 6%
guideline [6].
This real-world data supports our analysis. Electric
regenerative braking systems are extremely effective at
improving automotive efficiency; thus, these systems have
the potential to significantly reduce vehicular emissions.
(1)
According to the Law of Conservation of Angular
Momentum, the total angular momentum in a closed system
is constant [10]. The consequence of this law is best
explained by example. Imagine a flywheel being spun at a
fast speed. If another identical but stationary flywheel were
connected to the shaft of the first flywheel, the spinning
flywheel would decelerate and the stationary flywheel would
accelerate: they would come to an equivalent speed less than
that of the original flywheel. The rotational kinetic energy of
the first flywheel would be distributed between both
flywheels when connected.
Kinetic Regenerative Braking in Practice
The kinetic-based regenerative braking system can be
evaluated in the same way as the electrical braking system.
The main component that needs to be discussed, in this case,
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Christopher Dumm
Joseph Sadecky
is the efficiency of the flywheel as an energy storage
mechanism.
One disadvantage of kinetic-based regenerative braking is
that the stored power is not universally distributable within
the automobile. In an electric system, stored power can be
used to power any of the vehicle’s components, but the
kinetic energy stored by a flywheel can only be used to turn
the vehicle’s axles unless an alternator is installed to convert
the flywheel’s rotational energy to electrical energy.
However, this disadvantage is offset by a significant
increase in energy storage capabilities. Electrical systems’
energy storage capabilities are limited by the maximum
storage potential of batteries or capacitors [10], whereas
flywheel-based systems are only limited by the maximum
speed of the flywheel before centripetal force tears the
flywheel apart [10, 22, 23].
One other limiting factor in energy storage is friction
between the flywheel shaft, bearings, and drivetrain, as well
as air resistance. Just like an ordinary braking system, these
sources of friction will reduce the flywheel’s rotational
kinetic energy over time. Friction between the bearings,
shaft, and drivetrain can never be fully eliminated when a
physical connection between them exists, but the impact of
air resistance can be minimized [10, 20]. One method of
minimizing this impact is the placement of the flywheel in a
sealed chamber under vacuum [20, 23]. However, creating
such a system requires a significant investment in materials
and manufacture [20]. The method of reducing air resistance
used in Formula racing’s KERS (Kinetic Energy Recovery
System, pictured in Fig. 5) is a partial vacuum: the flywheel
is located within a chamber that can be placed under partial
vacuum by a built-in vacuum pump [20, 23]. This method
results in reduced air friction, reduced costs as compared to a
sealed vacuum chamber, and ultimately increased energy
retention within the vehicle [20, 23].
Formula racing’s KERS systems have experimentally
shown a recovery of 70% of braking energy [20].
Theoretically, when scaled to the size of a standard vehicle,
this energy savings could recoup the system’s cost over a
year of driving [20]. While there are currently no production
automobiles with flywheel-based regenerative braking
systems, based on the KERS systems’ efficiency, it seems
highly probable that such a system’s energy savings would
justify the system’s cost. When considering the
environmental benefit of reduced emissions due to reduced
fuel consumption, this justification becomes even stronger.
right, so it seems that a combination of these technologies in
the same vehicle would be even more efficient.
FIGURE 5
A FORMULA 1 RACING KERS SYSTEM [23]
Currently, there are no production systems that
concurrently implement true CVT and regenerative braking
technologies. This may seem surprising given the increased
efficiency of these technologies. However, it is important to
remember that while the theory behind these systems may be
relatively simple, building robust physical systems from
basic materials for production use requires significant testing
and design cycles to ensure quality and safety.
Nevertheless, information is available on one system that
currently implements regenerative braking and a system
similar to CVT technology: the Toyota Hybrid Synergy
Drive used in their Prius vehicle. This system’s fuelefficiency is due in a large part to their regenerative braking
systems and their planetary gear drive [24], which has
similar characteristics to the CVTs discussed. The 50 miles
per gallon fuel-efficiency statistic of this vehicle attests to
the efficient performance of this system, and to the likely
performance of a vehicle with true CVT and regenerative
braking technologies implemented concurrently [5].
CONSUMERS, SOCIETY AND THE ENVIRONMENT:
JUSTIFICATION FOR CONTINUED DEVELOPMENT
Based on our evaluation, it is clear that Continuously
Variable
Transmission
and
regenerative
braking
technologies could significantly affect the world when fully
implemented. Both CVT and regenerative braking offer
significant increases in automobile fuel economy when
installed in vehicles. This increase in fuel economy results in
a decrease in overall vehicular emission of greenhouse gases
from gasoline combustion: by increasing the range an
automobile can travel with a constant supply of fuel, these
technologies will directly assist in reducing current growth
of global warming.
However, these technologies have additional benefits.
While CVT and regenerative braking lead to reduced
emissions and minimized air pollution, they also provide
significant private benefits that would interest consumers. A
consumer would desire an automobile with both of these
CVT WITH REGENERATIVE BRAKING: MAJOR
INCREASE IN MECHANICAL EFFICIENCY
The CVT and regenerative braking systems that have been
described are unquestionably valuable additions to modern
automobiles. These systems individually provide significant
consumer savings in fuel costs and maintenance costs as
well as a major environmental benefit in reduced emissions.
CVT and regenerative braking are each efficient in their own
7
Christopher Dumm
Joseph Sadecky
[14] Markus, Frank. (2000, January). “Torqueing Up the Gearless Tranny.”
Car and Driver Magazine. [Online]. Available:
http://www.caranddriver.com/features/torqueing-up-the-gearless-tranny
[15] (2011.) “Nissan Maxima Specifications.” Nissan USA. [Online.]
Available: http://www.nissanusa.com/maxima/specifications.html
[16] (2011.) “2012 Sedan Buying Guide.” Edmunds.com. [Online.]
Available: http://www.edmunds.com/sedan/2012/buying-guide.html
[17] (2011.) “2012 Coupe Buying Guide.” Edmunds.com. [Online.]
Available: http://www.edmunds.com/coupe/2012/buying-guide.html
[18] (2010.) “XTRONIC CVT.” Nissan USA. [Online.] Available:
http://www.nissanglobal.com/EN/TECHNOLOGY/OVERVIEW/xtronic_cvt.html
[19] (2011, April.) “Subaru Debuts All-new 36-MPG 2012 Impreza at New
York International Auto Show.” Subaru of America. [Online.] Available:
http://media.subaru.com/index.php?s=43&item=241
[20] Abrams, Michael. (2012, January). “Stopping Power.” American
Society of Mechanical Engineers. [Online]. Available:
http://www.asme.org/kb/news---articles/articles/automotivedesign/stopping-power
[21] Whaley, D.M.; Soong, W.L.; Ertugrul, N. (2004, September.)
“Extracting More Power From The Lundell Car Alternator.” Australian
Universities Power Engineering Conference. [Online.] Available:
http://itee.uq.edu.au/~aupec/aupec04/papers/PaperID82.pdf
[22] Donovan, John. (2011, November 7). “Regenerative Braking.”
Electronic Engineering Times. [Online]. Available: Academic OneFile:
Gale Document Number GALE|A271822363
[23] (2011.) “Kinetic Energy Recovery Systems (KERS).” Formula 1
World Championship. [Online.] Available:
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[24] (2012.) “Hybrid Synergy Drive: Power Split Device.” Toyota.
[Online.] Available:
http://www.hybridsynergydrive.co.za/hybrid/view/hybrid/en/page477
systems because they reduce the variable costs of owning a
vehicle in terms of gasoline and in replacement of parts such
as brake pads. In addition, there are benefits such as the
smoother driving experience these systems provide.
When CVT and regenerative braking systems are
implemented individually, they have a dramatic impact on
automotive fuel efficiency. Currently no production
automobile uses both CVT and regenerative braking
technology, but preliminary results from similar systems
suggest that, on a large scale, these systems will significantly
reduce current rates of air pollution through increased
energy-efficiency. Furthermore, since CVT and regenerative
braking systems have significant consumer benefits,
consumers themselves are likely to lead a push toward the
use of these technologies. These systems are clearly
important in the ongoing fight to reduce global warming,
and they have a great deal of merit in terms of benefits to all
of society. Therefore, we conclude that Continuously
Variable
Transmission
and
regenerative
braking
technologies are of vital importance to the world, and that
further research, development, and implementation of these
technologies ought to continue.
REFERENCES
[1] United States Energy Information Administration. (2012, February.)
“Weekly US Regular All Formulations Retail Gasoline Prices.” [Online.]
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http://tonto.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=EMM_EPM
R_PTE_NUS_DPG&f=W
[2] Segall, Laurie. (2010, December.) “Ex-Shell president sees $5 gas in
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[7] Simanaitis, Dennis. (2003, December.) “CVTs Are Coming of Age –
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[12] Ashby, Michael; Shercliff, Hugh; Cebon, David. (2007.) Materials:
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[13] Yao, Chao-Hsu. (2008, January.) “Automotive Transmissions:
Efficiently Transferring Power from Engine to Wheels.”
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ADDITIONAL RESOURCES
Grable, Ron. (1994, December). “The continuously variable transmission:
minimum effort, maximum efficiency.” Motor Trend. [Online]. Available:
Academic OneFile: Gale Document Number GALE|A15971206
ACKNOWLEDGMENTS
We would like to thank the University of Pittsburgh Writing
Center and the Bevier Engineering Library for their support
and assistance throughout our research and writing
processes. We would like to thank Mr. James Hayes, Mr.
Matthew Goodwill, and Mr. Jonathan Dumm for their
constructive advice and comments. In addition, we would
especially like to thank Mrs. Diane Kerr for her continual
support and guidance throughout our writing and revision
processes.
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