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Analysis of the Novel Two-Speed Uninterrupted Transmission With Centrifugal
Clutch for Electric Vehicle
Conference Paper · August 2015
DOI: 10.1115/DETC2015-46739
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Proceedings of the ASME 2015 International Design Engineering Technical Conferences &
Computers and Information in Engineering Conference
IDETC/CIE 2015
August 2-5, 2015, Boston, Massachusetts, USA
DETC2015-46739
ANALYSIS OF THE NOVEL TWO-SPEED UNINTERRUPTED TRANSMISSION WITH
CENTRIFUGAL CLUTCH FOR ELECTRIC VEHICLE
Jian Song*
Shengnan Fang
Yuzhuo Tai
Fei Li
Truong Sinh Nguyen
Tsinghua University
State Key Laboratory of Automotive Safety and
Energy, Beijing, 100084, China
Haijun Song
Tsinghua University
State Key Laboratory of Automotive Safety and
Energy, Beijing, 100084, China
Email: hjs127@163.com
ABSTRACT
Current research suggests that the performance of electric
vehicle with a single-speed transmission can still be widely
improved. The novel two-speed uninterrupted transmission
consists of a single-row planetary, a centrifugal clutch, a brake,
and two motor controlling clutch and brake. Changing the
switch of clutch and brake can achieve two-speed uninterrupted
gearshifts. Different from the traditional centrifugal clutch, a
novel adjustable device is specifically designed to control the
switch of clutch. Torque characteristics of clutch prototype are
displayed. Some differences of the gearshift methodology are
specifically designed to compensate for the characteristics of
the electric traction motor and the adjustable centrifugal clutch.
The specific simulation model is established according to the
novel uninterrupted transmission prototype. The dynamic
characteristics of the two-speed uninterrupted transmission
system are analyzed. The higher utilization ratio of the traction
motor power increases the efficiency of the traction motor with
the novel two-speed uninterrupted transmission.
3]. Current research suggests that the performance of electric
vehicle equipped with a single-speed can be improved. Despite
the wide speed range of traction motor, the two-speed
uninterrupted transmission employed instead of a single-speed
transmission can enhance dynamic performance of electric
vehicle. The wheel torque at low velocities of the vehicle can
be increased. In addition, the lower second-gear ratio raises the
vehicle's top speed [4, 5, and 6]. A two-speed transmission can
simultaneously improve efficiency of the electric powertrain,
and increase the mileage of the vehicle.
In order to be compact, easy to control, and less expensive
to manufacture, the two-speed transmission system need be
specifically designed for electric vehicle. Single-speed
powertrain is less expensive, and have an acceptable
performance, which is regard as better ratio of performance to
price. The high efficiency of electric vehicle is well known.
Automatic transmission with hydraulic torque converter (AT) is
lower efficiency, and not suitable for the high efficiency electric
vehicle [7, 8, 9 and 10]. The advantage of Continuously
Variable Transmission (CVT) is reflected in matching to the
internal-combustion engine. Due to the nature of torque/speed
characteristic of traction motor, CVT is not the best option for
electric powertrains. Dual clutch transmission (DCT) with is
considering option for its excellent efficiency. The dual clutch
transmission of pure electric vehicle designed by researchers of
Jilin University of China, has been tested with real vehicle [11].
But the manufacture cost of DCT is higher than other automatic
transmission. In addition, for AT, DCT and CVT, the efficiency
of transmission is Weaken for constantly running electrically-
INTRODUCTION
Different from the internal-combustion engine, traction
motors has a high constant torque from zero to base speed, then
followed by a constant power region for higher angular speed,
which is suitable for the vehicle. In order to minimize the
drivetrain mass, volume losses and cost, fully electric vehicle is
traditionally equipped with a single-speed transmission [1, 2and
*Address all correspondence to this author
1
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driven hydraulic pump to provide the high pressure hydraulic
fluid for the actuation system [12, 13 and 14]. The primary
components of the two-speed uninterrupted gearshifts
transmission designed by the researchers of university of
surrey, are a dual-stage spurgear reduction, a one-way sprag
clutch and a friction clutch. This solution combines the
mechanical simplicity and low cost of a layshaft type gearbox.
But the efficiency of transmission can still be improved for the
friction clutch, which is electro-hydraulically controlled using a
motor driven actuator [15].
THE NOVEL TRANSMISSION
The novel two-speed transmission system is specifically
designed for electric vehicle in this article. The novel twospeed transmission consists of a single-row planetary, a
centrifugal clutch, a brake, and two motor used to control the
clutch and brake, which is displayed in the Figure 1. The singlerow planetary is used to transmit the power. The sun of
planetary is connected to electric traction motor of the vehicle,
and the carrier connected with the main reduction gear is used
to be output of planetary set. The centrifugal clutch is located
between the ring and the carrier, and the brake is used to brake
the ring. The centrifugal clutch and the brake is act as the
operation of the gearshift. Controlling the switch of clutch and
brake can achieve two-speed uninterrupted gearshifts. Control
motor is used to control the centrifugal clutch and the brake.
So, there is not hydraulic pump to provide the high pressure
hydraulic fluid for the actuation system. Therefore, the
efficiency of novel two-speed uninterrupted transmission
system is higher. Certainly the final drive and the differential is
display in the Figure 1.
Figure 1 the novel two‐speed uninterrupted transmission
The centrifugal clutch is disengaged and the brake is
engaged in the first gear. The planetary is decelerator. The ratio
of the first gear is decide by:
i1  1  k
(1)
Where k is ratio of teeth of ring gear and the teeth of sun
gear. The centrifugal clutch is engaged and the brake is
disengaged in the second gear. The planetary is one body
transmission. The ratio of the second gear is decide by
i2  1 .when the centrifugal clutch is gradually engaged from
disengage and the brake is simultaneously disengaged from
engage, the transmission is up to the second gear from the first
gear. The gear shift process is display in the Figure 2. The
powertrain of the gear shift is uninterrupted during the traction
motor power on when the action of centrifugal clutch and brake
is controlled synchronously.
First gear
Second gear
Figure 2 gear shift between the first gear and the second gear
need be controlled in the automatic transmission. Different
from the traditional centrifugal clutch, the novel adjustable
device driven through motor is specifically designed to control
the switch of clutch.
ADJUSTABLE CENTRIFUGAL CLUTCH
The traditional centrifugal clutch is automatically engaged
in a determined rotation speed. But the switch of the clutch
2
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The novel centrifugal clutch consists of driving disc, shoe,
adjustable pin, carrier, and drum, which are displayed in the
Figure 3. Power is input from driving disc, and is output from
the drum. Shoe1 and shoe2 rotating with the driving disc is
tightened up by the spring through adjustable pin.
shoe, M shoe is changing. When M shoe is bigger than
M spring , the clutch is engaged. M shoe is zero when carrier is
collinear with the line connecting the shaft of shoe1 and that of
shoe2.
The pressure between shoe and drum of centrifugal clutch
is smaller than other clutch driven by high pressure hydraulic
fluid. The wet centrifugal clutch designed is better in the heat
dissipation capability than a dry centrifugal clutch. But the
coefficient of friction of wet clutch is approximate 0.1, which is
smaller than that of the dry clutch, which is approximate 0.3.
The output torque of the dry centrifugal clutch is bigger than
that of the wet centrifugal clutch. The dry centrifugal clutch is
final selection in order to get a bigger output torque.
The output torque of the centrifugal clutch is affected by
the direction of the centrifugal shoes rotation. In the Figure 3,
the output torque of clockwise rotation is much more that of
anti-clockwise rotation. The output torque of the centrifugal
clutch is less than that of other clutch. In order to amplify the
torque of the centrifugal clutch, the direction of the centrifugal
shoes rotation is clockwise in the Figure 3.
The prototype of the novel centrifugal clutch is displayed
in the Figure 5. A stepper motor is used as the control motor.
When the clutch is turning the shaft and the shell of the control
motor are both in rotation. Adjusting the relative rotation angle
between the shaft and the shell of the motor can control the
switch of clutch. The control motor is connected with the
control circuit through a conductive ring.
Figure 3 the novel centrifugal clutch
Adjusting the arm of spring force can control the switch of
centrifugal clutch. Switch of clutch depends on the contrast of
M shoe (moment of centrifugal force) and M spring (moment
of spring force), which are displayed in the Figure 4. M shoe
and M spring are given by:
M shoe  Fshoelshoe

M spring  Fspring lspring
(2)
Where Fshoe is the centrifugal force of the shoe, and
Fspring is the spring force, and l shoe is the arm of centrifugal
force of the shoe, and l spring is the arm of spring force. l shoe
is changed when the carrier is relatively rotated to the shoe and
the adjustable pin is simultaneously rotated along the track of
the shoe.
Figure 5 prototype of the novel centrifugal clutch
After the clutch is engaged, adjusting the arm of spring
force can control change the pressure between the shoes and
drum, and the torque of clutch is adjusted. The torque of clutch
is not only adjusted along with the arm of spring force, which is
decided by the angle of spring force, but also changed along
with the angular speed of the shoe, which is decided by vehicle
speed. The torque of the clutch prototype changed with the
angle of spring force and angular speed of shoe, is displayed in
the Figure 6.
Figure 4 moment of centrifugal force and spring force
The clutch is disengaged when M shoe is smaller than
M spring . When the adjustable pin rotated along the track of the
3
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..

T
T
=J
θ
carrier
sun_i
carrier_o
planetary

 J planetary  J carrier  J ring  J sun

..

 T carrier_i -T carrier_o =J carrier θ carrier

 T clutch ≥ k
T carrier_i

1 +k
T
 brake = 0
Where
(4)
J ring is the inertia of the ring.
The dynamic process of the upshift and downshift for such
an uninterrupted transmission system during power-on is
similar to the dynamic process of DCT (dual clutch
transmission). The first difference is between planetary
transmission and fixed shaft transmission. Secondly, the
adjustable centrifugal clutch is different from the clutch
installed in traditional DCT.
In the torque phase of upshift, the brake is engaged at
lowest torque in order to the sun braked. Then the centrifugal
clutch is gradually engaged and the brake is simultaneously
disengaged. The dynamic characteristics is decided by:
..
1

 T sun_i - 1+k T carrier_i =J sun ( 1+k) θ carrier

..
(5)

 T carrier_i -T carrier_o  T clutch =J carrier θ carrier

 T brake  T clutch ≥ k T carrier_i

1+k

Figure 6 clutch torque changed with the angle of spring force and
angular speed of shoe
DYNAMIC MODEL
The principle prototype of the novel two-speed
uninterrupted transmission is currently in the process of
building, which is about to be experimental tested at Tsinghua
University of China. In order to analysis the dynamic
performance and design the control system, the dynamic
processes of the overall system are analyzed in the following.
In the first gear the centrifugal clutch is disengaged and the
brake is engaged. The dynamic characteristics is decided by:
..
1

 T sun_i - 1+k T carrier_i =J sun ( 1+k) θ carrier

..
(3)
 T
-T carrier_o =J carrier θ carrier
 carrier_i
k

 T brake ≥ 1+k T carrier_i

 T clutch = 0
the input torque of transmission from the traction motor, and
Tcarrier_i is the input torque of carrier, and Tcarrier_o is the
In the end of the torque phase of upshift, the braked torque
of the sun is almost offered by the toque of the centrifugal
clutch and the torque of brake is almost zero. Then the next
inertia phase is coming. In the inertia phase of upshift, the
brake is quickly disengaged, and the centrifugal clutch is
increase the pressure between shoe and drum, then completely
engaged. The dynamic characteristics is decided by:
..
1

 T sun_i - 1+k T carrier_i =J sun θ sun

..
 T
(6)
 T clutch =J carrier θ carrier
carrier_i -T carrier_o

..
k

 T clutch  1+k T carrier_i  J ring θ ring

 T brake  0
output torque of ring to the final drive, and Tbrake is the
Where θ ring is the angular acceleration of the ring, and
braked torque, and Tclutch is the toque of the centrifugal
θ sun is the angular acceleration of the sun. The inertia phase
Where Tsun_i is the input torque of the sun, which is also
clutch, and
..
..
J sun is the inertia of the sun, and J carrier is
of upshift is end when the following equation is established:
 .
θ
 .
 θ
..
the inertia of the carrier, and θ carrier is the angular
acceleration of the carrier.
In the second gear the centrifugal clutch is engaged and the
brake is disengaged. The dynamic characteristics is decided by:
.
sun
ring
.
=
=
.
θ
carrier
.
θ
(7)
carrier
.
Where θ ring , θ carrier , and θ sun are respectively the
angular speed of the ring, the carrier, and the sun.
4
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The dynamic process of the downshift is similar to the
dynamic process of the upshift.
and
Where  is rotating mass increase coefficient of vehicle,
 is road incline. Where u1 and u2 are respectively 50
km/h and 80 km/h. Where a1 and a2 are respectively the
accelerations of 0-50 km/h and 50-80 km/h. Finally, traction
motor map selected in the simulation is displayed in the Figure
7.
The selection of the gear ratio depends on parameter of the
whole vehicle and the electric traction motor. k of the
planetary gear set is 1.745, and the ratio of the final drive is
3.996. According to the above analysis the ratio of the planetary
gear set in the first gear is 2.745.
SIMULATION
The parameter of electric vehicle selected in the simulation
is displayed in Table 1. According to the national standard of
China, the maximum speed is no less than 80 km/h, which is
determined by the ratio of planetary and the final drive. The
accelerating time of 0-50 km/h is no more than 10 seconds, and
the accelerating time of 50-80 km/h is no more than 15 seconds
[16].
Table 1 parameter of the vehicle
parameter
value
Mass(m)
Drag coefficient( CD )
1570kg
0.355
Frontal area( A )
2.1m2
wheel rolling radius(r)
0.265m
According to the required maximum speed of electric
vehicle, the required power of traction motor is decided by:
Pmax1  (mgf 
CD A 2
umax )umax
21.12
(8)
Where f is rolling resistance coefficient, which is 0.014.
Where umax is the required maximum speed of electric vehicle.
According to the required acceleration of 0-50 km/h and 50-80
km/h, the required power of traction motor is decided by:
CD A 2

Pmax_ a1  (mgf cos  mg sin  21.12 u1  ma1)u1

CA
P
 (mgf cos  mg sin  D u22  ma2 )u2
 max_ a2
21.12
Figure 7 traction motor map selected in the simulation
According to the prototype of the novel two-speed
uninterrupted transmission system, the specific simulation
model is established in matlab/Simulink, which is displayed in
the Figure 8.
(9)
Figure 8 electric vehicle model with two‐speed uninterrupted transmission
5
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In order to contrast the character of the novel two-speed
uninterrupted transmission, a simulation model of electric
vehicle with single-speed transmission is established in
matlab/Simulink. Certainly, the traction motor, the planetary set
and the final drive used in the two-speed transmission system
model are the same as that used in the single-speed
transmission system model.
For the vehicle with single-speed transmission, the traction
motor power, vehicle speed and motor torque are displayed in
the Figure 9. The vehicle speed only reach 30 km/h in the first
ten seconds. It is obvious that the required acceleration of 0-50
km/h is not realized. If the ratio of final drive is increased, the
required acceleration of 0-50 km/h can be realized, but the
maximum vehicle speed will be reduced. When traction motor
works in constant torque from zero to base speed, the speed is
relatively low, and the efficiency is often poor, which can be
revealed in the traction motor map. The traction motor power
reach constant power when the vehicle speed rises to 70 km/h
at the 25th second. The low utilization ratio of the traction
motor power result in efficiency loss.
Figure 10 traction motor power, vehicle speed and motor torque with
the novel two‐speed uninterrupted transmission
The vehicle speed, clutch torque and brake torque of
vehicle with the novel two-speed uninterrupted transmission are
displayed in the Figure 11. Controlling the switch of clutch and
brake achieves two-speed uninterrupted gearshift when the
vehicle speed rises to 51 km/h at the 8.4th second.
Figure 9 traction motor power, vehicle speed and motor torque with
single‐speed transmission
For the vehicle with the novel two-speed uninterrupted
transmission, the traction motor power, vehicle speed and
motor torque are displayed in the Figure 10. It is obvious that
the required acceleration of 0-50 km/h and 50-80 km/h is
realized. The vehicle speed reach to 56.76 km/h during 10
seconds, and reach to 90.7 km/h during 25 seconds. When the
vehicle speed reach to 51 km/h, the upshift is going on. The
traction motor power reach constant power when the vehicle
speed rises to 24 km/h at the 3th second. The higher utilization
ratio of the traction motor power increases the efficiency of the
traction motor.
Figure 11 vehicle speed, clutch torque and brake torque of vehicle with
the novel two‐speed uninterrupted transmission
CONCLUSION
The novel two-speed uninterrupted transmission with
centrifugal clutch is specifically designed for electric vehicle in
6
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[7] Serrarens A. Roadmap of DTI’s Efficient Power shift
Transmissions[C]//Proceedings, CTI Symposium on Innovative
Automotive Transmissions, Berlin. 2009.
this article. Controlling the switch of clutch and brake can
achieve two-speed uninterrupted gearshifts. Different from the
traditional centrifugal clutch, a novel adjust device driven
through motor is specifically designed to control the switch of
clutch. The clutch torque is not only adjusted along with the
angular speed of the shoe, which is decided by vehicle speed,
but also changed along with the arm of spring force, which is
decided by the angle of spring force. Torque characteristics of
clutch prototype are displayed. In order to analysis the dynamic
performance and design the control system, the dynamic
processes of the novel two-speed uninterrupted transmission are
analyzed, and the specific simulation model is established. The
higher utilization ratio of the traction motor power increases the
efficiency of the traction motor with the novel two-speed
uninterrupted transmission.
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