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: [e-mail], [e-mail]
Abstract Hybrid electric vehicles are emerging as a practical
TABLE I. Selected hybrid electric vehicles with
solution for meeting increasingly more stringent governmental
integrated transmissions.
standards for fuel economy and emissions. In order to improve
performance, increase efficiency, and reduce costs, there is a
Make Model Integrated Transmission Technology
trend toward more integrated electro-mechanical transmission
Toyota Prius Toyota Hybrid Synergy (THS)
systems for advanced hybrid powertrains. This paper
Ford Escape Ford Hybrid System (FHS)
primarily focuses on the state-of-the-art electro-mechanical
integration of hybrid transmission systems and presents a
GM Tahoe Two-Mode Hybrid Transmission
comprehensive review of various integrated powertrains
BMW X6 Active Hybrid System
including the power-split, two-mode hybrid transmission
systems, and the electric variable transmission. Fundamental
Renault N/A Transmission Infinite VAriable (TIVA)
principles and mechanisms of operation for these integrated
electro-mechanical transmission systems are presented as well.
II. TOYOTA HYBRID SYNERGY DRIVE SYSTEM
Keywords Electric and hybrid electric powertrains, electric Toyota Hybrid Synergy Drive System (THS) is a
variable transmission (EVT), hybrid electric vehicles (HEVs),
leading integrated electro-mechanical hybrid transmission
integrated electro-mechanical transmissions, power split, two-
system mass-produced and commercialized since 1997; it is
mode hybrid.
currently the most popular hybrid system in the market
place. It combines a gasoline engine with two electric
I. INTRODUCTION
motors through a planetary gear set. The same principle is
shared by the Ford Hybrid System (FHS).
Hybrid electric vehicles (HEVs) are no longer new
The planetary gear set is the core component of the
concepts to the public. In fact, they have drawn enormous
THS transmission, shown in Fig. 1. It serves to split the
but still escalating attention from both the public and the
engine power into a mechanical path and an electric path.
researchers since the first modern hybrid car Toyota Prius
By adjusting the portions of these two paths, it achieves the
hit on road in Japan in 1997. Especially recently, with the
variable output speed and torque. The planetary gear set is
soaring prices of the gasoline and the further stringent
comprised of an outer ring gear, an inner sun gear, and a set
governmental emissions control standards, automotive
of planet gears, which are mounted on a movable carrier and
researchers and manufacturers have been urged to come up
mesh with both the ring gear and the sun gear. The carrier
with more efficient solutions. Consequently, more research
the ring gear and the sun gear rotates concentrically.
is being conducted in the areas of vehicle hybridization and
Fig. 2 shows the THS transmission architecture using a
at the same time, the major automotive OEMs are
simplified block diagram. The equation governing the
competing to release new generations of HEVs. It is clear
planetary gear ratios can be derived based on the relative
that the HEV technology is one of the most promising
motions of the gears:
practical solutions for the automotive industry in near-term,
1 , (1)
A substantial portion of the current hybrid electric
vehicle (HEV) research and development is primarily
in which is the angular speed of the sun gear, is the
concentrated on the hybridization of the vehicles
angular speed of the ring gear, is the angular speed of the
propulsion systems in which powertrain integration is of
planet carrier, and k is the ratio of the ring gear radius to the
significant importance [1]-[3]. Different types of advanced
sun gear radius. We could derive the output speed based on
hybrid powertrains have been developed. This paper
the THS configuration:
presents a comprehensive review of various configurations

of HEVs with integrated electro-mechanical hybrid
, (2)

transmission systems. Operating principles are explained
and analyzed for each configuration. Table I lists some of
in which and are the teeth number of the spur gears,
the current commercialized integrated electro-mechanical
is the engine angular speed, and is the generator
transmission systems with typical corresponding vehicles.
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
The static torque relationships between the planetary
high way drive cycles.
gear set can be derived based on Kane s method [4]:

III. TWO-MODE HYBRID TRANSMISSION
(3)

Two-mode hybrid transmissions can operate in the

(4)
input-split mode and the compound-split mode. The input-

, , and are the overall external torques applied on split means that the engine power is transmitted to an input
the sun gear, planet carrier, and ring gear, respectively. The member and then split through a differential device into two
negative sign  - indicates that the external torques applied paths, i.e., the mechanical path and the electric path. The
have opposite rotational directions. Thus, we could derive compound-split means that, except for the input split
the output torque: differential device, there is another differential device at the
output end, which functions to combine the previously split

, (5)
power together. The input-split mode is normally used for

low vehicle speeds, while the compound-split mode works
is the engine torque, and is the motor torque.
better for high speed or high load conditions. Fig. 3 depicts
It is apparent that either the engine speed or the engine
the block diagram of the two-mode hybrid transmission,
torque is decoupled from the final drive output shaft. This
which GM s Allison Two-Mode Hybrid Transmission and
enables the engine to work at its more optimum operating
BWM s Active Hybrid System are based on [5]. In addition
points, thus, increasing the fuel efficiency. The integrated
to these two operating modes, the transmission also includes
feature of the transmission also improves the vehicle
four fixed gear ratios enabling the parallel hybrid operation
performance and realizes electric only mode, maximum
as well as performance assistance.
power mode, battery charging mode, and regenerative mode
The GM s Allison Two-Mode Hybrid Transmission
by coordinating the motor and generator with the
(AHS) is comprised of three planetary gear sets, four
environmental conditions. In addition, it reduces the overall
clutches/brakes, two electric generator/motors, and a battery
size as well as manufacturing costs by taking off the original
pack. By controlling the four clutches, the AHS can shift
torque converter and the engine starter. However, since the
between the two mode operations and the four fixed gear
traction motor always engages with the output shaft, motor
ratios operation without any sharp changes in the powertrain
efficiency drops when the vehicle reaches high speeds.
elements.
Another issue with the THS systems is the relative low
efficiency of the electric path due to the losses in converters,
i. Input-split mode, only brake 1 engaged:
battery, and motor windings. We can derive the electric path
Similarly as in the case of THS, we can derive the
power portion as of the total engine power:
output speed and output torque:


1 (6)


(7)


in which is the power that goes through the generator

and is the total engine power.

1 (8)

Depending on the ratio of the output speed to the engine
input speed, the portion of the engine power that goes into It is observed that the G/M 1 serves as the speed
the electric path could be determined and the corresponding coupler while the GM2 serves as the torque coupler to
operating modes could be identified, seen from equation (6). regulate the engine speed and torque. Several operating
When there is no engine power transmitted through the modes suitable for low speeds can be achieved in the input-


split mode. To have a better understanding of the operating
electric path, i.e., 0, the output to input speed ratio

modes, we will use speed and torque arrow indicators to
is called the  mechanical point , where there is no electric
analyze the block diagrams. Here we assign the angular
path losses in the transmission system. This mechanical
speed and torque to be positive when they have the same
point in THS is typically designed for high drive-cycle fuel
rotational directions as the engine, and we assume, when the
economy, thus compromise the vehicle performance at other
vehicle is moving forward, both the engine and the output
points since there is only one mechanical point in single
shaft are rotating clockwise, which we will assign an arrow
mode transmissions. The efficiency drops significantly
pointing down in the block diagram shown in Fig. 4. We
especially when the vehicle is operating under high speed or
would apply the same to the torque, where the down arrow
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:
Since and are defined as the gear ratio between
In the compound-split mode, the engine power is split
the ring gears and the sun gears, we have 1and 1,
by the first planetary gear set into the mechanical path and
0, i.e., G/M 1 only has counterclockwise output
the electric path and they are rejoined together by the third
torque in the input-split mode. Meanwhile, G/M 2 only
planetary gear set. Based on equations (1), (3), and (4), we
rotates clockwise due to the forward motion of the vehicle.
can derive the output speed and output torque in terms of the
(a). 0, 0: All the gear components are engine and generator/motors input speeds and torques:
rotating clockwise. G/M 1 serves as a generator and G/M 2

(10)
serves as a motor. The transmission is in the low speed

driving mode where the engine drives the vehicle while it


1 (11)
also supplies power to G/M 1.

(b). 0, 0: Both G/M 1 and G/M 2 serve


(12)
as generators. When 0, 0, the



transmission is in its regenerative braking mode. When
(13)


0, still remains positive, thus the

Similarly, we apply speed and torque arrow indicators
engine supplies all the demanded power to the final drive
to analyze the two-mode transmission operating modes.
while G/M 1 and G/M 2 charge the battery.
Fig. 5 lists all the possible combinations of the speed
directions and torque directions for the two
(c). 0, 0: Both G/M 1 and G/M 2 work
generator/motors in the compound-split mode. Depending
as motors. The transmission provides the maximum power
on the different combinations of torques and speeds, we can
to the final drive when vehicles are accelerating under low
have multiple transmission output and operating modes.
speed with relatively large power demand.
Fig. 5 (a) to (c) show the case when G/M 1 rotates
(d). 0, 0: G/M 1 works as a motor and
counterclockwise (arrow up), while G/M 2 rotates clockwise

(arrow down), i.e. 0, 0 . From the speed
G/M2 works as a generator. When 0,

equation (10), we see the output speed is the summation of
output torque remains positive. When
the engine speed and part of the G/M 1 speed (negative in

0, 0, the transmission is in regenerative this case ). Therefore, the vehicle is operating in the medium

speed range. Fig. 5 (d) to (i) indicate the high speed range
braking mode.
since G/M1 rotates clockwise (arrow down), i.e. 0.
Furthermore, the cases in the second row, i.e. case (d)
Case (b) and case (d) have the same expression of
to (f) have 0, while the cases in the third row, i.e.
output torque and both can work in the battery charging
case (g) to (i) have 0. Observed from equation (12),
mode and regenerative braking mode, depending on the
the first term is negative since 1, 1, and

term . Observed from equation (7), if
0 for cases (d) to (i). Therefore, with a constant engine

speed, G/M 2 speed can decrease only when G/M 1 speed
G/M 1 changes its rotational direction, the output speed will
increases. This, combining with the analysis of the above
be changed accordingly. Therefore, case (b) and (d) work in
paragraph, indicates the cases in the third row of the
different speeds under the input-split mode. This facilitates
transmission operations modes have the highest speed.
the transmission to achieve battery charging or regenerative
Based on the torque equation (11), the cases in the first
braking at different speeds without major changes of the
column, i.e., case (a), (d), and (g) indicate large torque
output since the torque of G/M 2 is positive. The remaining
powertrain elements in the city driving conditions.
cases have relatively smaller torque output since 0.
Fig. 5. Torque and speed analysis of the compound-split mode.
The operating difference between the cases of the
iii. Fixed gear ratios:
second column and the third column in the same row relies
on the torque difference between the engine and G/M 2. In
GM s Allison two-mode hybrid transmission is
all these cases, 0, the summation of the engine equipped with four fixed gear ratios apart from the two
torque and the G/M 2 torque determines the G/M 1 torque continuous variable modes. By applying two clutches at the
and the operation mode for G/M 1 based on equation (13). same time, it can switch from the continuous variable modes
The second column represents the cases when 0, to fixed gear ratio modes. Fig. 7 shows the four fixed gear
and the third column represents the cases when 0. ratio configurations of the GM Allison two-mode hybrid
In addition, for those cases in the second and third columns, transmission.
if the output torque is negative, the transmission is The first fixed gear ratio takes place when brake 1 and
absorbing the regenerative power. It can achieve this by clutch 4 are engaged. Clutch 4 locks the sun gear of the
having both the generator/motors working as generators like second planetary gear set to its ring gear so that all the gears
case (b) in the medium speed range and case (f) in the high in the first gear set and the second gear set have the same
speed range, or only one of the generator/motors serves as angular speed and, thus, the engine, G/M 1, and G/M 2 have
generator to absorb the energy like in case (c), (e), and (i). the same rotational speed. The output speed is proportional
For case (d) and case (h), both of the generator/motors to the engine speed, and by using the energy conservation
serve as motors to assist the engine to propel the final drive. law, we could derive the torque output:
Since the case (h) has a higher speed output, if the power

(14)
applied is the same, case (d) would have a higher torque

output due to the power conservation law.
1 (15)
In summary, a wide range of torques over a large span
of speeds can be realized since the generator/motors have
The first fixed Gear is used to increase the output
full ability to switch between generating and motoring
torque when high torque is required during vehicle
modes of operation. The transmission can also change its
acceleration operation, especially during the low speed
function without any sudden change in the powertrain
range, where it could be switched from the input-split mode
elements. Besides, the engine could operate in the fuel
by simply engaging clutch 4. Both of the generator/motors
optimum regions by controlling the speeds and torques of
can work as motors to boost the vehicle acceleration.
the two generator/motors. The peak power and speed
The second fixed gear ratio is on when brake 1 and
demands for the electric machines are reduced due to the
clutch 2 are engaged. It can also be directly switched from
engine-motor integration in the compound-split mode, thus,
the input-split mode by engaging the clutch 2. The second
reducing the packing size and manufacturing costs. By
fixed gear ratio is typically used during the transition from
introducing the compound-split mode, the system also
input-split mode to compound-split mode. It engages clutch
brings in another two mechanical points in addition to the
2 at the beginning of the second fixed gear ratio, and
mechanical point in the input-split mode, see Fig. 6. This
releases brake 1 at the end of this mode. Thus, a smooth
realizes relatively low energy flow through the electric path
transmission is achieved between the two continuous
over a large span of transmission ratio, reduces the energy
variable modes. Equations (16) and (17) give its speed
losses, and thus improves the fuel efficiency.
output and torque output:
(a) The first fixed gear ratio (b) The
e second fixed gear ratio
(c) The third fixed gear ratio (d) The fourth fixed gear ratio
Fig. 7. AHS fixed gear
r ratio configurations.
Fig. 6. GM Allison two-mode transmi
ission operation
split mode, clutch 2 engages to real
lize the second fixed gear
ratio, and brake 1 is released to sw
witch into the compound-
split mode at the end of this transition. Both of the

(16)
generator/motors can either freewh during this stage, or
heel

they can assist the engine to provi output torque. At the
ide
1 2 3 2 3
1 3 2 (17)
2 beginning of the compound-split mode, G/M 1 still rotates
m
1 2 2
in the reverse direction as a generator. When its speed goes
The third fixed gear ratio is realized b
by engaging both
up to zero again, the transmission reaches its second
clutch 2 and clutch 4. This achieves a 1:1 i
input-output gear
mechanical point. Thereafter, G/M 1 turns to a motor while
ratio since all the gears in the three planet
tary gear sets are
G/M 2 switches to a generator to a
achieve a constant engine
all locked together. The output torque is the
e combination of
speed.
the three input sources torques:
We can observe from Fig. 6 that the engine speed is
kept in a bonded region where it locates in its optimum
t
(18)
operation regions and, thus, achiev
ving high fuel efficiency.
Fixed gear 4 provides an overdrive ge
ear ratio. Brake 3
Meanwhile, continuous variable sp output is smoothly
peed
locks the G/M 2 while clutch 2 engages to output the speed
realized by coordinating the generator/motors and switching
and torque. It can also be used in the high speed range so
h
the clutches and brakes. Besides, three mechanical points,
that only the engine drives the vehicle in its
s efficient regions
points 2, 3, and 4, are allocated thro
oughout the transmission
with G/M 1 powering the accessories o generating if
or
operation range achieving relativel low portion of engine
ly
battery recharging is demanded.
power flowing through the electric p
path.
1 2
(19)
1 2 1
IV. ELECTRIC VARIABLE TRANSMISSION
1 2 1 2 1
(20)
Electric Variable Transmission (EVT) consists of two
n
1 2 2
concentrically arranged electric machines that are
c
mechanically connected and electri
ically linked [6]. It takes
iv. GM Allison two-mode transmission op
perations:
in the engine power from one of its
s mechanical ports, splits
Two-mode hybrid transmission con numerous
ntains
the engine power into both mech path and electric
hanical
operating modes by controlling the generato
or/motors as well
path by using dual rotors mech and eventually
hanism,
as switching the clutches and brakes. By pr
roperly choosing
combines the power again to the ou
utput final drive shaft. Fig.
the operating modes according to the road conditions, the
d
8 shows a typical configuration of th
he EVT system.
two-mode hybrid transmission can achieve significant fuel
e
The dual rotor mechanism is the core of EVT. It is
savings while providing uncompromised p
performance. Fig.
composed of one stator and two rot
tors which are labeled as
6 shows a typical two-mode hybrid transm operating
mission
the inner rotor and the outer rotor. T
The inner rotor is directly
with the numbers made up for illustration.
connected to the engine output crankshaft and the outer
c
In the reverse drive range, G/M 2 rotat
tes in the reverse
rotor is sandwiched in between the i
inner rotor and the stator,
direction and provides all the output torque
e. In the forward
connecting to the output shaft. The inner rotor and the outer
drive range, G/M 1 serves as a generator a
at low speed. The
rotor comprise the first electric ma defined as GM1,
achine,
transmission is operating in the input-split mode. When the
while the stator and the outer rot comprise the second
tor
speeds of the engine, G/M 1, and G/M 2 are
e synchronized as
electric machine, defined as GM2 Rather than having a
2.
shown at point 1, clutch 4 engages to switch the
fixed stator, GM1 generates power by the relative rotational
transmission into the first fixed gear ratio supplying the
o,
speeds of the two rotors. The th
hree-phase windings are
maximum power to the final drive by hav both of the
ving
typically wound on the stator and the inner rotor via slip
d
generator/motors working as motors. Clut 4 disengages
tch
rings and carbon brushes and the connected to AC/DC
en
when the speed increases to certain level, a
and G/M 1 turns
converters through the DC link to th
he battery.
back to a generator. The first mechanical point is reached
From Fig. 8, we derive the input-output relationship
i
when the speed of G/M 1 reduces to zero, transmitting all
assuming steady state situations and ignoring all the losses.
the engine power through the mechanica path. With its
al
The engine input power is:
speed drops below zero, G/M 1 turns into a motor again.
o
(21)
During the transition from input-split mode to compound-
 Fig. 9. EVT control strategy. Fig. 10. Axial-field EVT.
Fig. 8. Electric Variable Transmission (EVT).
Apart from the above discussed radial-field EVT, an
axial-field type of EVT is also possible [7]. It still utilizes a
is the engine crankshaft output speed and is the
dual rotor mechanism, where an inner rotor with double-side
transmission input torque. Part of this engine power is
imbedded magnets is sandwiched between a stator and outer
transmitted through GM1 to the output shaft while the rest
rotor. However, the difference lies in the magnetic flux
of the power goes through the AC/DC converter into the
direction between these three components, in which the flux
DC link:
is aligned parallel to the rotational axis rather than in the
(22)
radial direction. Fig. 10 shows this topology.
(23)
V. CONCLUSION
The electric power could either supply the power to
Integrated electro-mechanical hybrid transmissions can
GM2, or charge the battery. The total output power to the
achieve higher fuel efficiency, lower emissions, and better
final drive is:
performance. Both mechanical and electrical systems are
(24)
needed to be integrated in order to achieve an advanced
electro-mechanical hybrid powertrain. Such integrated
is the power provided by the battery. Output torque
systems are highly promising for advanced electric drive
is:
vehicles. As these technologies mature and costs are

reduced, we could expect higher mass production volumes
(25)

of commercialized vehicles with integrated electro-
A relatively simple control strategy can be applied on mechanical transmissions gradually replacing more and
EVT: GM1 regulates the transmission output speed and more of the conventional internal combustion engine only
GM2 regulates the output torque. Fig. 9 shows the speed vehicles.
and torque control for EVT. Point 1 stands for the demanded
engine speed and torque from the final drive shaft. Since we
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