An alternate hybrid transmission was proposed to use one electric motor, a planetary gear set, and four fixed gears to realize automated transmission and CVT for a parallel hybrid, as shown in Figure 4.15 [6]. The design is based on the concept of AMTs. It uses a combination of lay shaft gearing and planetary gearing for power transmission. The lay shaft gears on the input shaft and the motor shaft freewheel unless engaged by the shifter–synchronizer assemblies. The carrier of the planetary gear train is connected with the input shaft that picks up the engine torque. The sun is connected with the motor shaft if so engaged. One motor is used either as the driving assisting unit or as the generator in charging and regenerative braking operations. Mode switching and gear shifting are realized by shifters actuated by computer and controlled by step motors as in an AMT. The hybrid system has five operating modes for vehicle driving and one standstill mode for emergency or convenience operations. The six operating modes and the related kinematics are described in the following.
4.5.1 Motor‐Alone Mode
The vehicle is always launched in the motor‐alone mode. In this mode, the motor shaft is engaged by the shifter, and power is transmitted to the final drive by the motor gears.
* © 2006 ASME. Reprinted, with permission, from IASME Journal of Mechanical Design.
Advanced HEV Architectures and Dynamics of HEV Powertrain 93
Vehicle backup is realized by reversing the motor rotation. All other gears freewheel in this mode. The operating parameters for the engine and the motor in this mode are related by
o m
im (4.54)
T i To m m (4.55)
where ωo and ωm are the angular velocities of the output shaft and the motor respec
tively, To and Tm are the output torque and the motor torque respectively, and im is the motor gear ratio.
4.5.2 Combined Power Mode
The combined mode is used when high power is required in situations such as acceler
ating and hill climbing. In this mode, the motor shaft and one of the lay shaft pairs are engaged. One of the four available gears as shown in Figure 4.15 can be selected accord
ing to the vehicle operating condition. The motor and the engine operate at speeds that are mechanically linked by the gear ratios to drive the vehicle jointly, with the operating parameters related as
o e
k m
i im (4.56)
T i T i To k e m m (4.57)
where ωe and Te are the angular velocity and the torque of the engine respectively, and ik (with k = 1–4) is the gear ratio of the lay shaft gear pairs.
Controller Battery
Motor/
generator Motor
shaft
Motor gear
Parking lock
Output to final drive Motor
shifter 4th R
1st 2nd
1–2 Shifter ICE
Engine shaft
Planetary gear train
C S 3rd
Shifter3–4
Figure 4.15 Hybrid transmission proposed by Zhang et al.
Hybrid Electric Vehicles 94
4.5.3 Engine‐Alone Mode
The engine‐alone mode is the most efficient mode for highway cruising. In this mode, a lay shaft gear pair with low gear ratio is engaged to transmit the engine torque to the output shaft with the motor shaft in neutral. The vehicle runs like a conventional vehicle in this mode. The operating parameters are linked by the lay shaft gear ratio:
o e
ik (4.58)
T i To k e (4.59)
4.5.4 Electric CVT Mode
The electric CVT mode provides two degrees of freedom for vehicle operation control that allows optimization of engine operation for best fuel economy. In this mode, the engine drives the vehicle and powers the generator for battery charging at the same time. The sun gear of the planetary gear train is coupled to the motor shaft by the shifter, and the fourth lay shaft gear is coupled to the ring gear. The operating parameters of the system are governed by the characteristics of the planetary gear train as
m s r
s e r
s o
N N
N
N
N i4 (4.60)
T T N
N N T
m s s
s r (4.61)
T T i N
N N i T
o r r
s r e
4 4 (4.62)
where Nr and Ns are the number of teeth of the ring gear and sun gear, respectively; Tr
and Ts are the ring gear torque and sun gear torque, which are distributed from the engine torque at a constant proportion. The output torque To and the angular velocity ωo are determined by the vehicle driving condition. The engine torque and the torque provided to the generator are determined by optimizing the engine efficiency. In the electric CVT mode, the engine speed ωe is optimized at the point for the highest efficiency corresponding to the required torque. The generator speed ωm is controlled such that the engine speed and torque are optimized.
4.5.5 Energy Recovery Mode
In the energy recovery mode, the motor is coupled to the output shaft through the motor gear pair by the shifter, and functions as a generator. The relations for the operating parameters are the same as in the motor‐alone mode, with the power flow reversed.
4.5.6 Standstill Mode
In this mode, the motor is engaged by the shifter to the sun gear and the parking locker is applied to lock the output shaft (ring gear). The lay shaft gears freewheel. This mode can be used to crank‐start the engine or use the engine to charge a low battery at
Advanced HEV Architectures and Dynamics of HEV Powertrain 95
standstill. It can also be used as a generator for household electricity or other conveni
ences if a bidirectional power converter is provided. The parameters are
e s
rN s m
N N (4.63)
T N
N N T
m s
r s e (4.64)