ACTA MECHANÌCA ET MOBILITATEM
Vol. 1 – 2017
2017-02-120
THE DEVELOPMENT OF REGENERATIVE BRAKING SYSTEM FOR
MOTOR TANDEM AXLE MODULE IN SERIES HYBRID COMMERCIAL
VEHICLES
Z. Tang,
D. Bhovsar,
Y. Zhang,
L. Oriet,
And E. Lamg,
a
University of Windsor
Canada
Contact Information
[email protected].
ABSTRACT
Since the emission reduction and fuel saving have become the main focus of
nowadays’ automotive societies, the utilizations of new highly energy
efficient hybrid technology are the irresistible general trend to minimize the
load of commercial vehicles engine. This paper will demonstrate the
development of regenerative braking system for single motor powered
tandem axle module in hybrid commercial vehicles such as refuse trucks and
delivery vehicles. The fundamental working principle of this electric motor
tandem axle module is based on a dual-rotor electric motor/generator, which
has two rotors to deliver same torque from rotor shafts’ bevel pinion gears to
differential gears that connected to front and rear tandem axles’ wheels. Due
to the most of commercial vehicles were used to continually start and stop in
designated city routes, the large amount of kinetic energy was lost as heat
during the breaking actions. Thus, the energy efficiency cloud also be
improved with the assist of converting the kinetic energy to electric power by
regenerative braking system. When the brake pedal was pressed, the
vehicle's electric motor would be turned into reverse mode, while working as
a generator to slowing the vehicle and producing electricity which stored in
ultra-capacitor to slowly charge the vehicle's batteries. There are two smaller
brake rotors located on the center of each tandem axle for emergency brake,
so vehicle weight reduction can be achieved as well as lighter unsprang
weight. In this paper, the detailed architecture of this hybrid power system
and brake recovery system will be described. The simulations were executed
to estimate the certain properties in order to prove the efficiency and
suitability of the system including the proportion of energy could be possible
recovered. Moreover, the cost analysis will be indicated along with the
difficulties and future developments.
Keywords:
NOMENCLATURE
INTRODUCTION
ABS
AC
CD
DC
EM
EMF
GHG
ICE
Rr
SOC
Vb
Ve
This Paper describes the structural of an
alternative hybrid driveline which contains a
regenerative braking system for commercial vehicles
like garbage trucks and delivery trucks. With the
growing attention on reducing fuel consumption and
greenhouse gas (GHG) emissions among auto
industries, the passenger hybrid vehicle and electric
vehicle technology updates rapidly within each
passing day. The raise of hybrid vehicle can be giving
credit to the lack of new alternative energy source,
hence the hybrid technology is widely used as
transitional function for current society. Especially,
when it comes to other aspects of automobiles, the
commercial vehicle, which is bearing heavy duties of
Antilock braking system
Alternating current
Aerodynamic drag coefficient
Direct current
Electric motor
Electromotive force
Greenhouse gas
Internal combustion engine
Rolling resistance coefficient
State-of-charge
Back electromotive force voltage
Final acceleration speed, m/s
25
engine load remission. They consistently travel around
40000 kilometers per year and consumes 80-100 liters
per hundred kilometers. [1] Meanwhile, during daily
stop –go routes of operations, heavy commercial
vehicles like refuse truck wastes a tremendous amount
of energy, which result in a highly inefficient driving
cycle.
The hybrid system satisfies the improvement of
these types of vehicles’ fuel economy and brigs less
weight effects caused by batteries and other power
storage devices. Thus, there are two major solutions
from the auto industries, who desired to offer the more
‘green’ trucks to save the world, hybrid electric and
hydraulic hybrid. Both of hybrid electric and hydraulic
hybrid are capable of recovering the wasted kinetic
energy and friction material wear from frequent startstop, but the hybrid electric converts the kinetic energy
to electric power by regenerative braking system,
while hydraulic hybrid uses pump to store the brake
energy into a pressurized fluid storage. For the
alternative drivetrain development, the series hybrid
electric with better evolvability and fuel economy are
chosen in this paper. In order to optimize the truck
performance with fuel efficiency, a regenerative
braking system are introduced along with the modified
electric motor tandem axle module for this application
of series hybrid commercial vehicles.
located as a longitudinally axis from front to rear. As
we know, the torque delivered to the wheels by engine
is applied to the frame at the same time. Meanwhile,
there are also appreciable amount of torque transferred
from driveline to frame by suspension system due to
the rough road. This condition can cause repeated
twisting to all kinks of torsional rigid components that
attached to the frame rails, and half of torque produced
by engine lost. [2] In order to change the repeated
torsion absorption into an exploitable condition and
improve the fuel efficiency of commercial trucks, Dr.
Oriet came up the idea of structural electric tandem
axle module as Figure 1. This module is propelled by
a dual-rotor electric motor, which has two rotors to
deliver same torque from rotor shafts’ bevel pinion
gears to differential gears that connected to front and
rear tandem axles’ wheels. Instead of wasting the half
of the rotational mechanical torque into bad effects on
frame, the module perfectly exploit the characteristic
of dual rotors motor, which results in better handling
and more efficient driveline. Moreover, as the
reduction of torque applied on the frame side rails, the
weight of chassis is able to be reduced. At same time,
the utilization of electrical drive system induced the
hybrid system into this application, which endows the
brake energy recovery system a more applicable
platform.
TANDEM AXLE MODULE
HYBRID POWERTRAIN SYSTEM
The core of this design is based on Dr. Oriet’s
invention of “Structural Electric Tandem Axle
Module”, which is an electrical powered tandem axle
that provides torque by a dual-rotor motor in the
center. [2]
There are two main hybrid electric systems widely
used recently, parallel hybrid and series hybrid. The
parallel hybrid is the most common hybrid drivetrain
for nowadays’ passenger vehicles. It has an internal
combustion engine (ICE) and an electric motor (EM)
directly joined at an axis by transmission shaft. The
ICE powered by fuel or diesel and the EM powered by
battery are propelling the vehicle together at same
rotational speed. The internal combustion engine also
used as a generator which convert the mechanical
energy to electrical energy that charges the battery. In
the series hybrid system, the ICE only drives the
electricity generator, which recharges the larger
battery packs through convertor. Therefore, the series
hybrid vehicle is propelled by the electrical motor
which powered by batteries. Due to the internal
combustion engine only works as an electric vehicle
range extended device, a higher capacity battery packs
are required for series hybrid vehicles. For this reason,
the power to weight ratio is affected by larger batteries,
but for the most of heavy duty vehicles like garbage
trucks, which weighs about 13 to 28 tones, the series
hybrid system’s weight issue plays a less significant
role. Meanwhile, instead of operating over a wide
variety of engine speed, the optimized engine speed
can be run continuously even during the stop-start
traffic. In this case, the series hybrid system is coupled
to the electric tandem axle module, which has a
turbocharged diesel engine as the primary power
source.
Figure 1. Structural Electric Tandem Axle Module by Dr.
Oriet [2]
Normally, commercial trucks like refuse vehicles have
a chassis, which is combined of pair of frame rails
cross joined with front and rear axles that clamped to
the suspensions. The internal combustion engine of the
trucks provides the torque through the driving shaft
26
REGENERATIVE BRAKING SYSTEM
Figure 2. Series Hybrid Drivetrain Block Diagram
As Figure 2 demonstrated, the series hybrid drivetrain
contains a dual-diesel engine generator, which is also
invented by Dr. Oriet, is consist of two turbo diesel
engines connected by an electric generator with two
hydraulic internal wet-disk clutches between. When
the engines are powered by the diesels from fuel tank,
the inner rotor and the outer rotor of the generator are
rotated in opposite direction by their shafts, which
provides the electric energy. The output of the
generator is convert from alternating current (AC) to
direct current (DC) by electronic convertor rectifier,
which is going through the controller unit to charge the
batteries. The battery pack releases the energy to the
controller unit as the driver press the acceleration
paddle, and the DC from controller directly linked to
the electric motor, which drives vehicle forward or
backward. As Figure 3 showed, the electric motor has
an outer rotor coupled with rear differential by outer
rotor’s pinion gear and an inter rotor coupled with
front differential by inter rotor’s pinion gear. When the
DC current from controller create electromagnetic
field pattern, which starts interacting with the
electromagnetic field pattern from outer rotor’s
permanent magnets, the two rotors begin to rotate in
opposite directions about axis at same speed. [2]
Figure 3. Electric Motor Drive of Structural Electric Tandem
Axle Module by Dr. Oriet [2]
The brake energy recovery system is also
referred to regenerative braking system is of vital
importance to the energy efficiency of the series
hybrid commercial vehicle, especially for the
frequently stop-go vehicles like garbage truck.
Typically, a disc brake is most used braking system for
current automobiles to slow the rotation of the wheels
by the friction between brake pad and brake rotor. In
this case, the friction pads are pressed into brake disc
by the piston of the fixed caliper, which contains
hydraulic force from brake fluid cylinders. Another
type of braking, known as drum brakes, slows the
rotation of the wheels by pressing the friction pad to
the inner surface of drum of the wheel. Both of the
traditional braking systems turn the most of kinetic
energy of the moving vehicle into a tremendous
amount of heat energy by friction. [3] Therefore, the
recovery of vehicle kinetic and penitential energy in
conventional braking systems becomes the one of
major development in hybrid vehicles.
As mentioned before, the characteristic of
electric motor in hybrid electric vehicles is enable to
allow the bi-directional power flow, which helps the
braking system to seize the kinetic energy as electricity
by using the motor as a generator. The electricity from
generator can be saved in an energy storage device like
battery or ultra-capacitor. According to Lester J.
Erlston’s illustrations, “a division of a vehicle’s total
kinetic energy, based on a realistic assumption that
50% of the available kinetic energy from a braking or
slowing event is able to pass through the recovery
system, and shows that a) 40% of this energy will be
stored in the batteries for future use, and b) that the
foundation brakes will experience a load reduction
from 95% to 45% of the total kinetic energy for a
comparable non-kinetic energy recovery system
braking event, the other 5% being for losses due to
aerodynamic drag and road-to-tire friction.” [4]
In the configuration of the single motor tandem
axle module powered hybrid commercial vehicles, the
electric motor has an outer rotor connected with rear
differential and an inter rotor connected with front
differential. Both front and rear differentials are
rotationally coupled with the drive shafts of front and
rear tandem axle. When the braking action is started,
the controller unit which connect directly to the
electric motor drops the applied voltage to be less then
back electromotive force (back-EMF) voltage (Vb), so
armature torque is reversed with armature current.
Therefore, the speed of the rotor of electric motor is
falling with the drive shaft connected to the wheels, as
along as the power is generated by greater EMF. As
we known, the battery has limited capacity, which is
more likely to be overloaded by the suddenly large
amount of recovered energy. In order to extend the
battery life and improve the efficacy, the DC current,
generated by electromagnetic field, is stored into an
ultra-capacitor first. The controller units will dispatch
27
the energy follow from the ultra-capacitor to slowly
charge the battery and power the electric motor later.
The braking performance is always a priority
consideration when design a braking system. The
capability of braking system is defined as the response
speed of stopping vehicles and the stability of the
vehicles’ braking action at any road conditions. In
order to stop the vehicle as fast as possible, the braking
system has to provide enough torque to slow the
rotation of each wheel. Meanwhile, this amount of
torque is supposed to be controlled by antilock braking
system (ABS), which avoids the wheels to be locked
to cause uncontrolled skidding. [4] Due to the function
of the single motor tandem axle module powered
hybrid commercial vehicles, the braking torque by
electric motor/generator is easier to be controlled for
each tandem axle. However, the torque control for
each wheel is still required, hence two brake rotors are
introduced to site aside of differential mechanisms on
each tandem axle. These two rotors are smaller than
big disc brakes’ for heavy duty vehicles, since the
tremendous amount of loads on discs are extracted by
motor/generator. For this reason, vehicle weight
reduction can be achieved as well as lighter unsprang
weight. Furthermore, conventional brake system is
applied to the front axle of the trucks.
SIMULATION AND ANALYSIS
This application is simulated in Matlab and AVL
Cruise to estimate the certain properties in order to
prove the efficiency and suitability of the system.
Since no prototype system has been built and tested,
certain calculations and assumptions are made to
generate the required data by simulation model.
First of all, in order to run the vehicle in different
operation patterns, a control strategies have been
developed in controller, which gives commands to
different components. The maximum state-of-charge
of peaking power source (Max. SOC-of-PPS) control
strategy are used in this application, since it is enable
to satisfy the power demands of the battery with the
most optimized and sustainable engine operations for
efficiency and emission. According to Gao, Y. and
Ehsani, M, “This control strategy is considered to be
the proper design for vehicles for which the
performance relies heavily on the peak power source.
This includes vehicles with frequent stop–go driving
patterns.” [7] When the demanded traction power is
greater, the engines is going to operates in its optimal
region as showed in Figure 4 along with the peaking
power source (battery) providing power to electric
motor. If the SOC of the battery is less than 100%, the
power from generator is charging the battery, and if
SOC of the battery is reaching its maximum capacity,
the generator powers the motor directly. When the
driver steps on the brake pedal, a negative power is
requested to be produced by motor. If the requested
braking power is higher than the maximum
regenerative braking power, the mechanical brake on
each axle must be commended to participate.
Figure 4. Example of engine characteristics and optimal
operating region [7]
During the process of building the model in AVL
Cruise, the characteristics of each components needs
to be estimated by following calculations in Matlab.
According to the Newton’s second law, the tractive
force acting between wheels and ground is the force
that accelerating and decelerating the vehicles. Thus,
in order to find the required power rating of the
traction motor, the tractive force 𝐹𝑡𝑟𝑎𝑐 must be
estimated first by Equation 1.
F𝑡𝑟𝑎𝑐 = 𝑚
𝑑𝑣
𝑑𝑡
+ 𝑚𝑔𝑠𝑖𝑛𝜃 + 𝐹𝑎𝑒𝑟𝑜 + 𝐹𝑅𝑅
(1)
Where m is the total vehicle mass with contents
in 23000 Kg, dv is the speed difference of the vehicle,
g is the gravity acceleration in 9.80 m/s 2 , θ is the
road grading, 𝐹𝑎𝑒𝑟𝑜 is the aerodynamic drag force,
and 𝐹𝑅𝑅 is the rolling resistance force. After multiply
Equation 1 by vehicle final speed 𝑉𝑓 , the power rating
of the motor P𝑡𝑟𝑎𝑐 can be estimated as Equation 2.
𝑑𝑣
2
+ 𝑚𝑔𝑉𝑒 𝑠𝑖𝑛𝜃 + 𝑚𝑔𝑅𝑟 𝑉𝑒 +
𝑑𝑡
3
1
𝜌𝑎𝑖𝑟 𝑔𝐶𝐷 𝐴𝑓 𝑉𝑒3
P𝑡𝑟𝑎𝑐 = 𝑚𝑉𝑒
5
(2)
Where 𝑉𝑒 is the final acceleration speed in 105
km/h as the speed limits for urban commercial vehicles
in Toronto, Canada, 𝜌𝑎𝑖𝑟 is the air density in 1.202
kg/m3, 𝑅𝑟 is the tire rolling resistance coefficient
assumed as 0.01, 𝐴𝑓 is the front area of the normal
rear loading collection vehicle in 10.75m2 , and 𝐶𝐷 is
the aerodynamic drag coefficient assumed as 0.5. [7]
With consideration of less than 5 seconds 0 to
105km/h acceleration on flat ground ( θ =0), the
average power needed with no regenerative braking is
483kw.
Meanwhile, the power supply from the most
optimized and sustainable engine/generator operations
for efficiency and emission is also occurred at a
constant speed with zero-degree road, which is
obviously less then power demand for acceleration.
Since the energy lost in generator, motor drive, and
28
battery is all assumed as ten percent each, the
efficiency of electric drive at constant speed can be
indicated as 72.9%. In this case, the power output from
generator is equal to tractive power divided by 72.9
percent. Meanwhile, based on Mehrdad Ehsani and
Yimin Gao’s theories, at a stop-go urban driving
conditions, the satisfaction of engine/generator power
is also able to be checked by compare to the average
load power from Equation 3. [7]
𝑃𝑢𝑟𝑏𝑎𝑛𝑎𝑣𝑒 =
1 𝑇
1
∫ (𝑚𝑔 𝑅𝑟 + 𝜌𝑎𝑖𝑟 𝐶𝐷 𝐴𝑓 )𝑉𝑑𝑡 +
𝑇 0
2
1 𝑇
𝑑𝑉
(3)
∫0 𝑚𝑔 dt
𝑇
𝑑𝑡
As result of tandem axle drive system, there are
three axles in the truck which obtains the braking force
during deceleration. Due to the regenerative braking is
only equipped with tandem axle in this application, the
efficiency of the regenerative braking is based on the
brake energy from tandem axle. As we known, the
overall power required to stop the vehicle is same
value and opposite direction of tractive power of
acceleration, hence the brake power is equal to the
negative result of Equation 1. The Equation 4 is used
to generate the tandem axle braking force, where the h
is the height of the center of gravity, l is the distance
between front axle and the center of tandem axle, j is
the deceleration rates of the vehicle, and b is the
distance from center of gravity to the center of tandem
axel. [8]
𝐹𝑥1 = 𝑚𝑔(
ℎ𝑗 2
+
𝑏𝑗
)
heavier weight problems also inevitably occur in this
truck because of large battery. Meanwhile, with dual
diesel engine/generator, ultra-capacitor, and large
battery on board, the estimated cost of the hybrid truck
will not be any cheaper than the conventional hybrid
trucks. Fortunately, due to the convenient maintenance
of the self-contained tandem axle module and fuel
efficiency of regenerative braking, the future cost can
be reduced in a significant amount.
We are currently extending the research to add a
front axle regenerative braking system, which would
allow the whole system to capture higher proportion of
brake kinetic energy.
Also, the more accurate
simulation method of evaluating series hybrid truck’s
fuel efficiency and regenerative braking system’s
energy efficiency is worth to be developed in the
future.
ACKNOWLEDGEMENTS
This paper utilized principles that is based on Dr.
Oriet, Leo’s invention of Structural Electric Tandem
Axle Module, and was academically supported by Dr.
Lang, Edward.
REFERENCES
1.
(4)[8]
2.
The power, which is available for regeneration,
should be the overall braking power required to stop
the vehicle 483 kW. The supplied power from
regenerative braking is estimated by multiplying the
tandem axle braking force with the average velocity of
50 km/ h, which results in 377 kW.
3.
2𝑙𝑔2
2𝑙𝑔
DIFFICULTIES AND FUTURE
DEVELOPMENTS
The architecture and characteristic of
regenerative braking system for single motor powered
tandem axle module in hybrid commercial vehicles
has been preliminarily presented above. In general,
this hybrid application turned the repeated torque from
propelling devices into an exploitable source and
improved the fuel efficiency of conventional
commercial trucks. However, there are still certain
difficulties exist in this concept, and also prevails in
current heavy duty hybrid trucks. First of all, even
though this hybrid drivetrain system is able to perform
very well under typical urban road with substantial
regenerative energy, it still does not possess the fast
acceleration abilities and grade ability. [6] Secondly,
as all the other types of hybrid vehicles suffered, the
4.
5.
6.
7.
8.
Gurdas Sandhu, H. Christopher Frey, Shannon
Bartelt-Hunt, Elizabeth Jones, “Real-World
Activity and Fuel Use of Diesel and CNG Refuse
Trucks,” Presentation at PEMS International
Conference & Workshop, April. 2014.
Leo P. Oriet, “Structural Electric Tandem Axle
Module,” U.S. Patent No. 0090505. Apr. 2, 2015
Nathan C. Nantais. “Active Brake Proportioning
and Its Effects on Safety and Performance”.
University of Windsor: Faculty of Mechanical
Engineering Graduate Studies and Research,
2006.
Erlston, L. and Miles, M., "Retrofittable
Regenerative Braking in Heavy Vehicle
Applications," SAE Technical Paper 2008-012558, 2008, doi:10.4271/2008-01-2558.
Gao, Y. and Ehsani, M., "Electronic Braking
System of EV and HEV---Integration of
Regenerative Braking, Automatic Braking Force
Control and ABS," SAE Technical Paper 200101-2478, 2001, doi: 10.4271/2001-01-2478.
AIR RESOURCES BOARD, “Technology
Assessment: Heavy-Duty Hybrid Vehicles,”
http://www.arb.ca.gov/homepage.htm, accessed
Jun. 2016.
Mehrdad Ehsani, Yimin Gao, Sebastien E. Gay,
Ali Emadi, “Modern Electric, Hybrid Electric,
and Fuel Cell Vehicles: Fundamentals, Theory,
and Design,” (CRC Press, 2004), 239-257, ISBN:
978-1-4200539-8-2.
Reza N. Jazari, “Vehicle Dynamics: Theory and
Application,” (Springer, 2008), 68-71, ISBN:
978-1-4614-8544-5.
29