In the process of driving mode switching, it is difficult to control the engine because of its long response time and non-linearity compared with the motor. When the Ev-mode is switched to Eng-mode, Hev-mode, or Char-mode, the motor must start the engine. The combustion is unstable at the initial stage of the engine ignition, resulting in a large fluctuation of engine torque. Thus, the torque of the hybrid system changes dramatically, and then transfers this to the whole vehicle, affecting the vehicle’s ride comfort [
27,
28]. Therefore, it is necessary to coordinate the torque of the engine, the motor, and the wet clutch. The whole process of mode switching is illustrated in
Figure 8. The driver’s driving intention is recognized according to the signals from the accelerator pedal and the braking pedal, after which the required torque of the vehicle is obtained. The engine’s and motor’s target torques will be obtained by the torque distribution strategy. The target working mode will be confirmed according to the required torque, SOC of battery, and vehicle’s velocity. The mode switch is judged by comparing the current working mode with the target one. If the mode switch occurs, the coordinated torque control strategy is adopted to control the engine, the motor, and the wet clutch. After the coordinated control process is finished, the working mode will be updated and the mode switch process is finished.
4.1. Control of the Wet Clutch
The plug-in hybrid power system must start the engine quickly by engaging the clutch during the process of the motor starting the engine. In this process, part of the motor torque is used to drive the normal running of the vehicle, and part of the motor torque is provided to drag the engine by the sliding torque of the wet multi-plate clutch. During this process, the motor torque and clutch torque should be coordinated to ensure that the engine starting process will not cause excessive torque fluctuation.
In this process, the clutch torque can be calculated by [
11]:
where
is the clutch torque,
is the clutch pressure,
and
are the speed of the motor and engine, respectively,
is the friction factor,
is the friction pair’s number,
is the area of friction plate, and
is the clutch plate’s effective radius.
The clutch’s oil pressure is controlled by the EPV. The average current of the EPV’s proportional solenoid can be calculated by [
29]:
where
is the PWM’s duty cycle,
is the PWM’s period,
is the steady voltage,
is the equivalent resistance of the proportional solenoid, and
is the time constant of the EPV.
According to Equation (14), the clutch torque is a function of the engaging oil pressure, which is actually the outlet pressure of the EPV. When the spool of the EPV is in equilibrium state, the outlet pressure of the EPV is proportional to the average current of the proportional solenoid [
30]. According to Equation (15), after keeping the PWM’s period (
) unchanged, the average current of the proportional solenoid is proportional to the PWM’s duty cycle (
), and when the PWM’s duty cycle (
) varies from 0 to 100%, the average current of the proportional solenoid is changed from 0 to the steady current (
). Therefore, the wet clutch’s torque control is converted into the control of the PWM’s duty cycle (
).
In the process of controlling the clutch engagement, the control of the initial PWM’s duty cycle () and the changing rate of the PWM’s duty cycle () in sliding stage are the most important, which has a great influence on the engagement performance of the wet clutch. The PWM’s duty cycle of the EPV is controlled through a fuzzy logic control.
A. The fuzzy control of the initial PWM’s duty cycle.
When the clutch controller receives the engagement instruction, the controller outputs the initial PWM’s duty cycle
to the proportional solenoid, the spool moves, the outlet valve opens, and the clutch’s oil pressure rises rapidly to build the initial oil pressure. At the same time, the increment of the initial PWM’s duty cycle
is calculated according to the accelerator pedal’s opening
and the change rate of the accelerator pedal’s opening
. Then, the initial PWM’s duty cycle can be obtained by the following:
Fuzzy control is applied to calculate the increment of the PWM’s duty cycle
.
and
are selected as the inputs of the fuzzy controller,
is selected as the output of the fuzzy controller. All of the parameters of
,
, and
are divided into seven fuzzy language sets: VS (very small), S (small), MS (minor small), M (medium), MB (middle big), B (big), and VB (very big). The fuzzy logic control rule of the increment of the initial PWM’s duty cycle
is shown in
Table 3. When the driver steps on the accelerator pedal deeply and quickly, the increment of the initial PWM’s duty cycle
must be increased to meet the requirement for strong power. When the driver steps on the accelerator pedal slowly, the increment of the initial PWM’s duty cycle
must be declined to meet the ride comfort.
B. The fuzzy control of the PWM’s duty cycle changing rate in sliding stage.
After the initial PWM’s duty cycle reaches the target value, the wet clutch establishes its initial oil pressure. The main and driven plates of the clutch start contacting, sliding, and transmitting torque. During the sliding stage, the driver’s intention and the degree of the clutch’s engagement should be determined, and the fuzzy control is used to set the appropriate changing rate of the PWM’s duty cycle.
The change rate of the accelerator pedal’s opening
and the speed difference of the clutch plate
are selected as this fuzzy controller’s inputs, whereas the output of the fuzzy controller is the changing rate of the PWM’s duty cycle.
,
and
are also divided into seven fuzzy language sets: VS, S, MS, M, MB, B, and VB. The fuzzy rules of the changing rate of the PWM’s duty cycle
are shown in
Table 4. In the sliding stage, when the accelerator pedal is stepped on quickly, the driver wants to complete the mode switching process as fast as possible, so the changing rate of the PWM’s duty cycle should be increased quickly. If the driver steps on the accelerator pedal slowly, the comfort should be fully considered; thus, the PWM’s duty cycle should be increased slowly. If
is large, which indicates that the clutch engagement degree is low, then
should be declined to meet the ride comfort; if
is small, which means that the clutch engagement degree is high and the clutch oil pressure is high, then
should be increased to reduce the sliding friction work as much as possible.
The integrated fuzzy control system of the PWM’s duty cycle control of the EPV is shown in
Figure 9. As shown, the wet clutch’s oil pressure is controlled by regulating the EPV’s duty cycle through the dual fuzzy control.
4.2. Driving Mode Switch’s Control Strategy
The driving mode switch can be divided into two types: the mode switch with clutch engagement and the mode switch without clutch engagement. The first type takes place mainly while the Ev-mode is switched to Eng-mode, Hev-mode, or Char-mode. This type of mode switch requires motor-assisted engine starting. During the motor-assisted engine starting process, to achieve continuous control of the clutch oil pressure, to ensure that the clutch is engaged rapidly and smoothly, and to take advantage of the fast response characteristic of the motor, the dual fuzzy control strategy for a wet clutch and the coordinated torque control strategy are combined to control the clutch’s oil pressure and the motor’s torque. Taking the switching from Ev-mode to Eng-mode as an example, the combined control strategy for motor-assisted engine starting is shown in
Figure 10. The combined control strategy for the switch from Ev-mode to Eng-mode- can be divided into four stages.
The first stage: the whole vehicle runs in Ev-mode, the engine is closed, the clutch is separated, and only the motor provides the normal driving torque of the vehicle:
The second stage: the vehicle control unit (VCU) sends out the mode switch command, and the clutch begins to engage. The engagement oil pressure is controlled by regulating the PWM’s duty cycle through the above fuzzy control system. At this time, the motor must not only provide the torque to run the vehicle, but also must provide the clutch torque to start the engine:
The third stage: when the engine speed is equal to the motor speed, the engine is ignited. Since the output torque of the ignition engine is not stable, it is necessary to use the motor to obtain torque compensation:
The fourth stage: when the absolute value
of the difference between the engine torque and the engine target torque is within the allowable range
, the clutch is locked, the motor is closed, the vehicle is driven by the engine alone, and the mode switching ends:
The type of the mode switch without clutch engagement takes place mainly while the Eng-mode, Hev-mode or Char-mode is switched to Ev-mode, or the mode switches between Eng-mode, Hev-mode, and Char-mode. The torque coordinated control strategy for this type of mode switch is that the torque changing rate of the engine is controlled and the motor provides torque compensation. Taking the switching from Eng-mode to Ev-mode as an example, the torque coordination control strategy for the clutch separation process is shown in
Figure 11.
The torque coordinated control strategy for this switch can be divided into three stages.
The first stage: the whole vehicle runs in Eng-mode, the motor is closed, the clutch is locked, and only the engine provides the driving torque of the vehicle:
The second stage: at the beginning of mode switching, the motor control unit (MCU) receives the instructions from the vehicle controller, and the motor starts. The motor gradually increases the torque while the engine gradually decreases the torque, and the torque decreasing rate of the engine is controlled:
where
is the engine torque before change,
is the target engine torque after changing rate control,
is the changing rate of the engine torque, and
is the engine’s actual output torque.
The third stage: when the absolute value of the difference between the motor torque and the motor target torque is within the allowable range , the engine shuts down and the clutch separates quickly. The motor alone drives the vehicle. The mode switching ends.