Systems in Hybrid Electric Vehicles, Grzegorz Płuciennik, Mechanika Samochodowa, Hybrydowy Napęd
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Integrated Electro-Mechanical Transmission
Systems in Hybrid Electric Vehicles
Yinye Yang,
Student Member, IEEE
, and Ali Emadi,
Senior Member, IEEE
Electrical and Computer Engineering Department
McMaster University, Hamilton, Ontario, Canada
E-mail:
yangy9@mcmaster.ca, emadi@mcmaster.ca
Abstract—
Hybrid electric vehicles are emerging as a practical
solution for meeting increasingly more stringent governmental
standards for fuel economy and emissions. In order to improve
performance, increase efficiency, and reduce costs, there is a
trend toward more integrated electro-mechanical transmission
systems for advanced hybrid powertrains. This paper
primarily focuses on the state-of-the-art electro-mechanical
integration of hybrid transmission systems and presents a
comprehensive review of various integrated powertrains
including the power-split, two-mode hybrid transmission
systems, and the electric variable transmission. Fundamental
principles and mechanisms of operation for these integrated
electro-mechanical transmission systems are presented as well.
TABLE I. Selected hybrid electric vehicles with
integrated transmissions.
Make
Model
Integrated Transmission Technology
Toyota
Prius
Toyota Hybrid Synergy (THS)
Ford
Escape
Ford Hybrid System (FHS)
GM
Tahoe
Two-Mode Hybrid Transmission
BMW
X6
Active Hybrid System
Renault
N/A
Transmission Infinite VAriable (TIVA)
II. TOYOTA HYBRID SYNERGY DRIVE SYSTEM
Toyota Hybrid Synergy Drive System (THS) is a
leading integrated electro-mechanical hybrid transmission
system mass-produced and commercialized since 1997; it is
currently the most popular hybrid system in the market
place. It combines a gasoline engine with two electric
motors through a planetary gear set. The same principle is
shared by the Ford Hybrid System (FHS).
The planetary gear set is the core component of the
THS transmission, shown in Fig. 1. It serves to split the
engine power into a mechanical path and an electric path.
By adjusting the portions of these two paths, it achieves the
variable output speed and torque. The planetary gear set is
comprised of an outer ring gear, an inner sun gear, and a set
of planet gears, which are mounted on a movable carrier and
mesh with both the ring gear and the sun gear. The carrier
the ring gear and the sun gear rotates concentrically.
Fig. 2 shows the THS transmission architecture using a
simplified block diagram. The equation governing the
planetary gear ratios can be derived based on the relative
motions of the gears:
Keywords
—Electric and hybrid electric powertrains, electric
variable transmission (EVT), hybrid electric vehicles (HEVs),
integrated electro-mechanical transmissions, power split, two-
mode hybrid.
I. INTRODUCTION
Hybrid electric vehicles (HEVs) are no longer new
concepts to the public. In fact, they have drawn enormous
but still escalating attention from both the public and the
researchers since the first modern hybrid car Toyota Prius
hit on road in Japan in 1997. Especially recently, with the
soaring prices of the gasoline and the further stringent
governmental emissions control standards, automotive
researchers and manufacturers have been urged to come up
with more efficient solutions. Consequently, more research
is being conducted in the areas of vehicle hybridization and
at the same time, the major automotive OEMs are
competing to release new generations of HEVs. It is clear
that the HEV technology is one of the most promising
practical solutions for the automotive industry in near-term,
A substantial portion of the current hybrid electric
vehicle (HEV) research and development is primarily
concentrated on the hybridization of the vehicles’
propulsion systems in which powertrain integration is of
significant importance [1]-[3]. Different types of advanced
hybrid powertrains have been developed. This paper
presents a comprehensive review of various configurations
of HEVs with integrated electro-mechanical hybrid
transmission systems. Operating principles are explained
and analyzed for each configuration. Table I lists some of
the current commercialized integrated electro-mechanical
transmission systems with typical corresponding vehicles.
1
,
(1)
in which
is the angular speed of the sun gear,
is the
angular speed of the ring gear,
is the angular speed of the
planet carrier, and k is the ratio of the ring gear radius to the
sun gear radius. We could derive the output speed based on
the THS configuration:
,
(2)
in which
and
are the teeth number of the spur gears,
is the engine angular speed, and
is the generator
angular speed.
978-1-61284-246-9/11/$26.00 ©2011 IEEE
Fig. 2. Block diagram of the THS transmission
Fig. 1. a) Front view b) Simplified block
Architecture of planetary gears
high load conditions. This explains why Toyota Prius has a
higher fuel rating in city driving conditions compared to the
high way drive cycles.
The static torque relationships between the planetary
gear set can be derived based on Kane’s method [4]:
III. TWO-MODE HYBRID TRANSMISSION
Two-mode hybrid transmissions can operate in the
input-split mode and the compound-split mode. The input-
split means that the engine power is transmitted to an input
member and then split through a differential device into two
paths, i.e., the mechanical path and the electric path. The
compound-split means that, except for the input split
differential device, there is another differential device at the
output end, which functions to combine the previously split
power together. The input-split mode is normally used for
low vehicle speeds, while the compound-split mode works
better for high speed or high load conditions. Fig. 3 depicts
the block diagram of the two-mode hybrid transmission,
which GM’s Allison Two-Mode Hybrid Transmission and
BWM’s Active Hybrid System are based on [5]. In addition
to these two operating modes, the transmission also includes
four fixed gear ratios enabling the parallel hybrid operation
as well as performance assistance.
The GM’s Allison Two-Mode Hybrid Transmission
(AHS) is comprised of three planetary gear sets, four
clutches/brakes, two electric generator/motors, and a battery
pack. By controlling the four clutches, the AHS can shift
between the two mode operations and the four fixed gear
ratios operation without any sharp changes in the powertrain
elements.
(3)
(4)
,
, and
are the overall external torques applied on
the sun gear, planet carrier, and ring gear, respectively. The
negative sign “-” indicates that the external torques applied
have opposite rotational directions. Thus, we could derive
the output torque:
,
(5)
is the engine torque, and
is the motor torque.
It is apparent that either the engine speed or the engine
torque is decoupled from the final drive output shaft. This
enables the engine to work at its more optimum operating
points, thus, increasing the fuel efficiency. The integrated
feature of the transmission also improves the vehicle
performance and realizes electric only mode, maximum
power mode, battery charging mode, and regenerative mode
by coordinating the motor and generator with the
environmental conditions. In addition, it reduces the overall
size as well as manufacturing costs by taking off the original
torque converter and the engine starter. However, since the
traction motor always engages with the output shaft, motor
efficiency drops when the vehicle reaches high speeds.
Another issue with the THS systems is the relative low
efficiency of the electric path due to the losses in converters,
battery, and motor windings. We can derive the electric path
power portion as of the total engine power:
i.
Input-split mode, only brake 1 engaged:
Similarly as in the case of THS, we can derive the
output speed and output torque:
1
(6)
(7)
1
in which
is the power that goes through the generator
and
is the total engine power.
Depending on the ratio of the output speed to the engine
input speed, the portion of the engine power that goes into
the electric path could be determined and the corresponding
operating modes could be identified, seen from equation (6).
When there is no engine power transmitted through the
electric path, i.e.,
(8)
It is observed that the G/M 1 serves as the speed
coupler while the GM2 serves as the torque coupler to
regulate the engine speed and torque. Several operating
modes suitable for low speeds can be achieved in the input-
split mode. To have a better understanding of the operating
modes, we will use speed and torque arrow indicators to
analyze the block diagrams. Here we assign the angular
speed and torque to be positive when they have the same
rotational directions as the engine, and we assume, when the
vehicle is moving forward, both the engine and the output
shaft are rotating clockwise, which we will assign an arrow
pointing down in the block diagram shown in Fig. 4. We
would apply the same to the torque, where the down arrow
0
, the output to input speed ratio
is called the “mechanical point”, where there is no electric
path losses in the transmission system. This mechanical
point in THS is typically designed for high drive-cycle fuel
economy, thus compromise the vehicle performance at other
points since there is only one mechanical point in single
mode transmissions. The efficiency drops significantly
especially when the vehicle is operating under high speed or
Fig. 3. Two-mode hybrid transmission architecture.
refers to the clockwise external torque. The dotted green
arrows in the blocks refer to the speeds, and the solid red
arrows next to the blocks refer to the torques. By using
equation (3) and (4), we can derive the G/M 1 torque:
Fig. 4. Torque and speed analysis of the input-split mode.
(9)
ii.
Compound-split mode, clutch 2 engaged:
In the compound-split mode, the engine power is split
by the first planetary gear set into the mechanical path and
the electric path and they are rejoined together by the third
planetary gear set. Based on equations (1), (3), and (4), we
can derive the output speed and output torque in terms of the
engine and generator/motors input speeds and torques:
Since
and
are defined as the gear ratio between
the ring gears and the sun gears, we have
1
and
1
,
0
, i.e., G/M 1 only has counterclockwise output
torque in the input-split mode. Meanwhile, G/M 2 only
rotates clockwise due to the forward motion of the vehicle.
(a).
0,
0
: All the gear components are
rotating clockwise. G/M 1 serves as a generator and G/M 2
serves as a motor. The transmission is in the low speed
driving mode where the engine drives the vehicle while it
also supplies power to G/M 1.
(b).
0,
0
: Both G/M 1 and G/M 2 serve
as generators. When
(10)
1
(11)
(12)
0
,
0
, the
transmission is in its regenerative braking mode. When
(13)
0
,
still remains positive, thus the
engine supplies all the demanded power to the final drive
while G/M 1 and G/M 2 charge the battery.
(c).
0,
0
: Both G/M 1 and G/M 2 work
as motors. The transmission provides the maximum power
to the final drive when vehicles are accelerating under low
speed with relatively large power demand.
(d).
0,
0
: G/M 1 works as a motor and
G/M2 works as a generator. When
Similarly, we apply speed and torque arrow indicators
to analyze the two-mode transmission operating modes.
Fig. 5 lists all the possible combinations of the speed
directions and torque directions for the two
generator/motors in the compound-split mode. Depending
on the different combinations of torques and speeds, we can
have multiple transmission output and operating modes.
Fig. 5 (a) to (c) show the case when G/M 1 rotates
counterclockwise (arrow up), while G/M 2 rotates clockwise
(arrow down), i.e.
0,
0
. From the speed
equation (10), we see the output speed is the summation of
the engine speed and part of the G/M 1 speed (negative in
this case ). Therefore, the vehicle is operating in the medium
speed range. Fig. 5 (d) to (i) indicate the high speed range
since G/M1 rotates clockwise (arrow down), i.e.
0
.
Furthermore, the cases in the second row, i.e. case (d)
to (f) have
0
, while the cases in the third row, i.e.
case (g) to (i) have
0
. Observed from equation (12),
the first term is negative since
1
,
1
, and
0
for cases (d) to (i). Therefore, with a constant engine
speed, G/M 2 speed can decrease only when G/M 1 speed
increases. This, combining with the analysis of the above
paragraph, indicates the cases in the third row of the
transmission operations modes have the highest speed.
Based on the torque equation (11), the cases in the first
column, i.e., case (a), (d), and (g) indicate large torque
output since the torque of G/M 2 is positive. The remaining
cases have relatively smaller torque output since
0
.
0
,
output torque
remains positive. When
0
,
0
, the transmission is in regenerative
braking mode.
Case (b) and case (d) have the same expression of
output torque and both can work in the battery charging
mode and regenerative braking mode, depending on the
term
. Observed from equation (7), if
G/M 1 changes its rotational direction, the output speed will
be changed accordingly. Therefore, case (b) and (d) work in
different speeds under the input-split mode. This facilitates
the transmission to achieve battery charging or regenerative
braking at different speeds without major changes of the
powertrain elements in the city driving conditions.
Fig. 5. Torque and speed analysis of the compound-split mode.
The operating difference between the cases of the
second column and the third column in the same row relies
on the torque difference between the engine and G/M 2. In
all these cases,
0
, the summation of the engine
torque and the G/M 2 torque determines the G/M 1 torque
and the operation mode for G/M 1 based on equation (13).
The second column represents the cases when
0
,
and the third column represents the cases when
0
.
In addition, for those cases in the second and third columns,
if the output torque is negative, the transmission is
absorbing the regenerative power. It can achieve this by
having both the generator/motors working as generators like
case (b) in the medium speed range and case (f) in the high
speed range, or only one of the generator/motors serves as
generator to absorb the energy like in case (c), (e), and (i).
For case (d) and case (h), both of the generator/motors
serve as motors to assist the engine to propel the final drive.
Since the case (h) has a higher speed output, if the power
applied is the same, case (d) would have a higher torque
output due to the power conservation law.
In summary, a wide range of torques over a large span
of speeds can be realized since the generator/motors have
full ability to switch between generating and motoring
modes of operation. The transmission can also change its
function without any sudden change in the powertrain
elements. Besides, the engine could operate in the fuel
optimum regions by controlling the speeds and torques of
the two generator/motors. The peak power and speed
demands for the electric machines are reduced due to the
engine-motor integration in the compound-split mode, thus,
reducing the packing size and manufacturing costs. By
introducing the compound-split mode, the system also
brings in another two mechanical points in addition to the
mechanical point in the input-split mode, see Fig. 6. This
realizes relatively low energy flow through the electric path
over a large span of transmission ratio, reduces the energy
losses, and thus improves the fuel efficiency.
iii.
Fixed gear ratios:
GM’s Allison two-mode hybrid transmission is
equipped with four fixed gear ratios apart from the two
continuous variable modes. By applying two clutches at the
same time, it can switch from the continuous variable modes
to fixed gear ratio modes. Fig. 7 shows the four fixed gear
ratio configurations of the GM Allison two-mode hybrid
transmission.
The first fixed gear ratio takes place when brake 1 and
clutch 4 are engaged. Clutch 4 locks the sun gear of the
second planetary gear set to its ring gear so that all the gears
in the first gear set and the second gear set have the same
angular speed and, thus, the engine, G/M 1, and G/M 2 have
the same rotational speed. The output speed is proportional
to the engine speed, and by using the energy conservation
law, we could derive the torque output:
(14)
1
(15)
The first fixed Gear is used to increase the output
torque when high torque is required during vehicle
acceleration operation, especially during the low speed
range, where it could be switched from the input-split mode
by simply engaging clutch 4. Both of the generator/motors
can work as motors to boost the vehicle acceleration.
The second fixed gear ratio is on when brake 1 and
clutch 2 are engaged. It can also be directly switched from
the input-split mode by engaging the clutch 2. The second
fixed gear ratio is typically used during the transition from
input-split mode to compound-split mode. It engages clutch
2 at the beginning of the second fixed gear ratio, and
releases brake 1 at the end of this mode. Thus, a smooth
transmission is achieved between the two continuous
variable modes. Equations (16) and (17) give its speed
output and torque output:
(a) The first fixed gear ratio (b) The
e second fixed gear ratio
(c) The third fixed gear ratio (d) Th
Fig. 7. AHS fixed gear
e fourth fixed gear ratio
r ratio configurations.
ission operation
Fig. 6. GM Allison two-mode transmi
split mode, clutch 2 engages to real
ratio, and brake 1 is released to sw
split mode at the end of this
generator/motors can either freewh
they can assist the engine to provi
beginning of the compound-split m
in the reverse direction as a genera
up to zero again, the transmiss
mechanical point. Thereafter, G/M
G/M 2 switches to a generator to a
speed.
We can observe from Fig. 6
kept in a bonded region where it
operation regions and, thus, achiev
Meanwhile, continuous variable sp
realized by coordinating the genera
the clutches and brakes. Besides,
points 2, 3, and 4, are allocated thro
operation range achieving relativel
power flowing through the electric p
lize the second fixed gear
witch into the compound-
transition. Both of the
heel during this stage, or
ide output torque. At the
mode, G/M 1 still rotates
tor. When its speed goes
ion reaches its second
1 turns to a motor while
achieve a constant engine
(16)
1
2
3
1
2
2
2
3
2
1
3
2
(17)
The third fixed gear ratio is realized b
clutch 2 and clutch 4. This achieves a 1:1 i
ratio since all the gears in the three planet
all locked together. The output torque is the
the three input sources torques:
Fixed gear 4 provides an overdrive ge
locks the G/M 2 while clutch 2 engages to
and torque. It can also be used in the high
that only the engine drives the vehicle in its
with G/M 1 powering the accessories o
battery recharging is demanded.
1
2
by engaging both
input-output gear
tary gear sets are
e combination of
that the engine speed is
t locates in its optimum
ving high fuel efficiency.
peed output is smoothly
tor/motors and switching
three mechanical points,
oughout the transmission
ly low portion of engine
path.
(18)
ear ratio. Brake 3
output the speed
h speed range so
s efficient regions
or generating if
1
2
1
(19)
IV. ELECTRIC VARIABLE
TRANSMISSION
1
2
1
1
2
2
1
2
(20)
Electric Variable Transmission
concentrically arranged electric
mechanically connected and electri
in the engine power from one of its
the engine power into both mech
path by using dual rotors mech
combines the power again to the ou
8 shows a typical configuration of th
The dual rotor mechanism is
composed of one stator and two rot
the inner rotor and the outer rotor. T
connected to the engine output c
rotor is sandwiched in between the i
connecting to the output shaft. The
rotor comprise the first electric ma
while the stator and the outer rot
electric machine, defined as GM2
fixed stator, GM1 generates power
speeds of the two rotors. The th
typically wound on the stator and
rings and carbon brushes and the
converters through the DC link to th
From Fig. 8, we derive the i
assuming steady state situations an
The engine input power is:
n (EVT) consists of two
c machines that are
ically linked [6]. It takes
s mechanical ports, splits
hanical path and electric
hanism, and eventually
utput final drive shaft. Fig.
he EVT system.
the core of EVT. It is
tors which are labeled as
The inner rotor is directly
crankshaft and the outer
inner rotor and the stator,
inner rotor and the outer
achine, defined as GM1,
tor comprise the second
2. Rather than having a
by the relative rotational
hree-phase windings are
d the inner rotor via slip
en connected to AC/DC
he battery.
input-output relationship
d ignoring all the losses.
iv.
GM Allison two-mode transmission op
perations:
Two-mode hybrid transmission con
operating modes by controlling the generato
as switching the clutches and brakes. By pr
the operating modes according to the road
two-mode hybrid transmission can achieve
savings while providing uncompromised p
6 shows a typical two-mode hybrid transm
with the numbers made up for illustration.
In the reverse drive range, G/M 2 rotat
direction and provides all the output torque
drive range, G/M 1 serves as a generator a
transmission is operating in the input-split
speeds of the engine, G/M 1, and G/M 2 are
shown at point 1, clutch 4 engages
transmission into the first fixed gear ratio
maximum power to the final drive by hav
generator/motors working as motors. Clut
when the speed increases to certain level, a
back to a generator. The first mechanical
when the speed of G/M 1 reduces to zero
the engine power through the mechanica
speed drops below zero, G/M 1 turns into
During the transition from input-split mode
ntains numerous
or/motors as well
roperly choosing
d conditions, the
e significant fuel
performance. Fig.
mission operating
tes in the reverse
e. In the forward
at low speed. The
mode. When the
e synchronized as
to switch the
o, supplying the
ving both of the
tch 4 disengages
and G/M 1 turns
point is reached
, transmitting all
al path. With its
o a motor again.
to compound-
(21)
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