There are a few variants of automatic transmissions such as automated manual trans
mission (AMT), CVT, and dual‐clutch transmission (DCT). Each of these technologies has its own penetration levels in different regions of the world (North America, Europe, or Asia). The advantages of DCT include high efficiency, low cost, and driving comfort.
Conservative estimates peg DCT technology at around 10% of the global market by 2015. Table 4.1 c ompares the advantages and disadvantages of CVT, AMT, and DCT.
DCT technology is well suited for both high‐torque diesel engines and high‐revving gas engines. Some of the major drivers for DCT include
● flexible and software tunability
● gear ratio flexibility the same as that of manual lay shaft transmissions allowing greater compatibility with any engine characteristics.
4.4.1 Conventional DCT Technology
A typical DCT architecture has a lay shaft with synchronizers used for maximum efficiency. It also has launch clutches (either wet or dry) used with electronics, along with mechanical or hydraulic actuation systems to achieve the automatic shifting. Lay shaft transmissions yield an efficiency of 96% or better as compared to 85–87%
e fficiency of automatic transmissions [3, 4].
Figure 4.11 shows the diagram of a DCT‐based transmission. It is a typical setup found in many of the latest vehicle models with a DCT. It consists of two coaxial shafts, each having the odd and even gears. It is tantamount to having two transmissions, hence the name.
In a DCT system, the two clutches are connected to two separate sets of gears. The odd gear set is connected to one of the clutches and the even gear set to the other clutch. It is necessary to preselect the gears to realize the benefits of the DCT system. Accordingly, the off‐going clutch is released at the same time as the on‐coming clutch is engaged. This gives an uninterrupted torque supply to the driveline during the shifting process. This preselec
tion of gears can be implemented using either complicated controllers such as fuzzy logic or simple ones such as selections based on the next anticipated vehicle speed.
4.4.2 Gear Shift Schedule
Initially, when the vehicle starts, gear N1i is synchronized. Therefore, engine torque is transmitted to the final drive through gears N1i and N1m. Vehicle speed increases as
Table 4.1 Qualitative comparison of automatic and manual transmissions.
Aspects Automatic
transmission Manual
transmission Desired transmission
Cost Expensive Lower Low
Efficiency Moderate High High
Ease of use Easy Hard Easy
Comfort Good Poor Good
* © 2009 IEEE. Reprinted, with permission, from 2009 IEEE VPPC Conference.
Hybrid Electric Vehicles 88
the odd clutch engages. When vehicle speed reaches a certain threshold, gear N2i is synchronized. As the even clutch engages (the odd clutch disengages), engine torque is shifted from gear N1i to N2i. Hence engine torque is transmitted through gear N2i and N2m. As vehicle speed increases, N3i is synchronized. Then the odd clutch would engage and the even clutch would disengage. This process will continue until the vehicle speed becomes stable (from N3i to N4i, from N4i to N5i, and from N5i to N6i).
During downshift, the process is reversed. For example, assume initially that N4i is synchronized and the even clutch is engaged. During downshift, N3i is synchronized before the even clutch opens. When the even clutch disengages and the odd clutch engages, engine torque is transferred from N4i to N3i. Similarly, N2i would be synchro
nized before the even clutch engages.
Since all transitions in a DCT are managed by gear synchronizers and two clutches, there is no need for a torque converter in a DCT. The transitions (gear shifting and torque shifting) are very smooth. Control of the synchronizers and clutches, or shift controller, is computerized in the vehicle. The shift controller decides the upshifts or the downshifts of the transmission as per the gear shift schedule as shown from left to right in Figure 4.12.
This controller intelligently preselects the higher or the lower gear depending on the current and desired vehicle velocity.
4.4.3 DCT‐Based Hybrid Powertrain
The diagram for a DCT‐based hybrid powertrain is shown in Figure 4.13 [5]. The trans
mission is a six‐speed AMT. The hybrid powertrain consists of two motors with each coupled mechanically onto the two shafts using a standard gear reduction. Due to the presence of the motor/generator, the vehicle can be reversed without the reverse gear.
The odd shaft houses gears 1, 3, and 5, and the even shaft houses gears 2, 4, and 6. The two motors can also be operated as generators as needed by the hybrid control strategy.
Engine
N1m N2m
N6m
N5m N3m
Odd gear N4m clutch
Even gear clutch
Nf o Nf i
N2i
N4i N6i
Differential N1i
N3i N5i
Figure 4.11 Dual‐clutch transmission. Note that the reverse gear is omitted in the diagram.
Advanced HEV Architectures and Dynamics of HEV Powertrain 89 Gear shift schedule
Vehicle speed (kmph) 120
100 80 60 40 20 0
0
Throttle (%)
20 40 60 80 100 120
1 2
1 2 2 3
2 3 3 4
3 4 4 5
4 5 5 6
5 6
Figure 4.12 Gear shift schedule.
Engine
Motor 1/
generator
Motor 2/
generator
Nm1o Nm1i
Nm2i N1m Nm2e
N2m
N6m
N5m N3m
N4m
Nf o Nf i N1i
N2i
N4i N3i N5i
N6i Differential
Odd gear clutch
Even gear clutch
Figure 4.13 Hybrid powertrain based on dual‐clutch transmissions. Reverse gear is not needed due to the fact that the motors can be used to back up the vehicle.
Hybrid Electric Vehicles 90
4.4.4 Operation of DCT‐Based Hybrid Powertrain
The DCT‐based hybrid powertrain shown in Figure 4.13 has seven operating modes when the vehicle is in motion and one additional operating mode for standstill charging.
4.4.4.1 Motor‐Alone Mode
The vehicle is always launched in the motor‐only mode unless the battery’s state of charge (SOC) is below the minimum level. In this mode, the gears are selected accord
ing to the shift logic controller. The vehicle operates in this mode up to a maximum speed defined by the controller, provided the SOC is greater than the minimum SOC for the battery as per the system design. Since the engine does not operate in this mode, the dual clutches are disengaged to prevent any backlash to the engine. Either motor can be used for the launch and backup of the vehicle. The equations for this mode are
o m
g a m
i i i (4.39)
T i i i To a g m m (4.40)
4.4.4.2 Combined Mode
This mode is selected when a high torque is required for situations such as sudden acceleration or climbing a grade. This mode is also selected if the vehicle speed becomes more than the maximum speed defined by the controller in the motor‐alone mode.
Both the engine and the motor provide the propulsion power to the drive shaft.
Depending on the vehicle speed, the transmission shift controller selects the appropri
ate clutch and the gears. The power flow is shown in Figure 4.14. The equations for this mode are
o m
g a m e
i i i i ig a (4.41)
T i i i To g a m m i i Tg a e (4.42) 4.4.4.3 Engine‐Alone Mode
This mode involves the engine as the only source of propulsion. The engine controller ensures that the engine transmits power to the lowest possible gear ratio such that the engine remains in the best efficiency window. The equations for this mode are
o e
i ig a (4.43)
T i i To a g e (4.44)
4.4.4.4 Regenerative Braking Mode
The motor is coupled to the output shaft through gears, and it can function as a genera
tor as well. It is used to recover the energy during braking to charge the battery.
Depending on the current clutch that is used, the controller decides which motor is to
Advanced HEV Architectures and Dynamics of HEV Powertrain 91
be operated in this mode. In case the motor torque is not sufficient to brake the vehicle, a conventional braking system is used to supplement the braking demand.
The equations for this mode are
in m
m g a
i i i (4.45)
Tin i i i Tm a g m (4.46)
4.4.4.5 Power Split Mode
This mode is used to charge the battery when the vehicle is in motion. The vehicle controller decides on this mode if the engine supplies more power than that required to drive the vehicle. The excess power is then used to charge the battery. The motor on the same lay shaft that drives the output shaft is selected to act as the generator to charge the battery. The motor controller selects the correct motor depending on the shaft that is transmitting the power to the final drive. The equations for this mode are
o m
a m g e
i i i i ia g (4.47)
T i i T To a g e m (4.48)
4.4.4.6 Standstill Charge Mode
This mode can be used to crank‐start the engine or charge the battery when the vehicle is in standstill position. The controller opts for this mode when the battery SOC is lower
Engine
Nm1o Nm1i
Nm2i Nm2e
N1m N2m
N6m
N5m N3m
Odd gear N4m clutch
Even gear clutch
Nf o Nf i N1i
N2i
N4i N5i N3i
N6i Differential
MG1
MG2
Figure 4.14 Power flow in the combined mode.
Hybrid Electric Vehicles 92
than the minimum SOC level permitted by the design. This is the only operating mode when the engine is cranked and the vehicle is in standstill position. Since the vehicle is not moving and no power is transmitted to the drive train, all the gears are disengaged for safety. The kinematic equations for this mode are
o 0 (4.49)
T T ie m m (4.50)
e im (4.51)
4.4.4.7 Series Hybrid Mode
This mode offers a very interesting option for the DCT‐based hybrid powertrain. The engine is operated in a region near its sweet spot (by adaptively changing the gear ratios) so that the torque generated from the engine is used by one of the motors to generate electricity. This electricity is then used by another motor on the other shaft to drive the vehicle. This therefore gives the option of having the DCT‐based hybrid powertrain operating as a series hybrid. The kinematic equations for this mode are
o m
a m g
i i i (4.52)
T T i i io m m g a (4.53)