3. Components and vehicle modelling
3.3 Single electric motor hybrid configuration – modelling in Simulink
3.3.3 Power management
77 To determine the amount of ideal torque that the electric axle has to provide in order to follow this strategy, two quantities are required at any given time:
- External torque demand: this is the value of the overall torque needed to follow the target speed profile.
- Optimal torque: this is the value of torque which guarantees the optimal efficiency for a given angular speed.
The external torque demand can be estimated using different approaches. It is possible to calculate it in analytical ways, as described in paragraph 1.6.1, but in this case another approach has been used. Since different models have been realized for all the different driving situations and architectures, it is possible to run a simulation using the only-ICE architecture model for a certain procedure, and log the output engine torque.
The torque profile acquired corresponds to the external torque demand to follow that exact procedure test.
Obviously in a power management strategy that will be implemented in the vehicle’s on board electronics this is not the correct strategy. In this case, in fact, an analytical calculation or an estimate based on look-up table is necessary. For the aim of this work, the approach previously described was implemented.
Regarding the optimal torque, this quantity is defined by a simple 1-D look-up table, giving as input the engine’s crankshaft rotating speed.
Figure 55 - Optimum engine torque curve
However, in hybrid mode, the optimal torque curve could be inappropriate for some types of driving situations. Since the highest engine efficiencies are achieved at higher loads, it can
78 happen that the optimal torque is always too high, while the engine load is relatively low, for example in a city speed profile. This will result in an excess of energy stored in the batteries, and therefore an excessive increase of the battery’s SOC.
So, a multiplying factor has been introduced (a), which multiplies the optimal torque curve and scales it in order to stabilize the SOC to a goal value. Another method to reduce the SOC when the optimal torque is too high is to switch frequently to full electric mode when the vehicle is travelling at idling or low speed conditions, taking advantage of the energy previously collected at higher efficiency and reducing the SOC.
The ideal electric torque is then calculated as:
𝐶𝐸𝑉𝑖𝑑 = 𝐶𝑒𝑥𝑡− 𝑎 ∙ 𝐶𝑜𝑝𝑡
The obtained value is then processed according to different operating strategies that will be described below.
18.104.22.168 SOC control strategies
According to the battery’s specifications, it is necessary that its SOC remains controlled in a specific range. Therefore, if during the cycle the SOC goes below 25% or above 90%, the ideal torque calculated is processed in order to force the SOC to remain in the selected range.
To do so, three parameters have been introduced: a gain, and two boundaries (upper and lower). The following table describes the choice of these parameters corresponding to the different situations:
Table 6 - SOC control strategy parameters
Condition Gain Upper boundary Lower boundary
SOC<SOCmin 1.2 0 N.D.
SOCmin<SOC<SOCnorm 1 100 N.D.
SOCnorm<SOC<SOCmax 1 N.D. 100
SOC>SOCmax 1.2 N.D. 0
If the SOC is too low (< SOCmin), the torque is limited to negative values (it is only possible to apply an electric braking torque that will charge the battery, and it is not possible to drain energy from the battery). Furthermore, the amount of braking torque is raised by 20%
If the SOC is between SOCmin and SOCnorm, the system sets an upper boundary for the motoric torque to 100 Nm (related to the engine’s rotating speed), and restores the gain to 1. If the
79 SOC is included in the regular operating range (SOCnorm<SOC<SOCmax) there are no boundaries to the torque and the gain is unitary. If the SOC overcomes the SOCmax it will be only possible to exert a motoric torque. Therefore the lower boundary is set to 0 and the motoric torque is boosted by 20% in order to favor the battery discharge.
22.214.171.124 Powertrain mode selection
Another task for the power management subsystem is to define the vehicle’s driving mode between electric only and hybrid mode.
In the first mode the engine is shut off and the power is delivered only by the electric motor, exploiting the charge stored in the batteries. The charge is restored when the vehicle switches again to hybrid mode, by shifting up the engine’s load.
This allows to eliminate the engine’s operating points in the lower loads, increasing the engine’s efficiency. Moreover, this strategy allows to remove the idling phase.
So, the subsystem will give as an output a parameter called “mode”, which is equal to zero during the full electric mode, and equal to one in hybrid mode.
This parameter will multiply the output signals of the TruckSim model, in order to simulate the engine turning off, such as the engine output torque and power, and the fuel rate.
Furthermore, if this parameter switches to zero, the required power to follow the speed profile defined in the test procedure (which is the engine’s output power coming from TruckSim) will come from the electric machine.
So, the motoric power that goes into the battery block will switch to this value, corrected by the motor’s efficiency, as described in section 3.3.1.
The input parameter that define the powertrain mode are:
- Vehicle’s speed;
- Required power;
- Current mode.
Table 7 - Powertrain mode selection strategy
Current mode Vehicle’s speed Required power Next mode
1 >20 km/h - 1
1 <20 km/h > 90% of motor’s maximum power 1 1 <20 km/h < 90% of motor’s maximum power 0
0 >30 km/h - 1
0 <30 km/h > motor’s maximum power 1
0 <30 km/h < motor’s maximum power 0
80 As it is explained in table 7, if the vehicle’s speed drops below 20 km/h and the required power (which can be related to the throttle pedal position) is less than 90% of the electric motor’s power, the powertrain switches to electric mode.
Once that the mode is 0, in order to switch back to hybrid mode, the vehicle’s speed has to raise to at least 30 km/h, or the required power has to overcome the maximum power that the electric motor can provide. This prevents the mode to flicker from 0 to 1 an excessive number of time when the threshold values of speed and torque are reached.
126.96.36.199 Simulink model
Once that the power management strategies have been defined, it was possible to implement the subsystem in the Simulink model.
Figure 56 - Power management subsystem
The required torque value is given by a 1-D look-up table that contains the required torque profile, obtained by launching a simulation of the system where have been previously disconnected the input torque signals, and logging the output engine torque.
81 The optimum torque curve is implemented in another 1-D look-up table, which takes as an input the current engine rpm. The torque scale parameter is needed to scale the optimum torque curve, in order to balance the battery SOC and avoid abnormal behavior of the system.
The ideal torque referred to the engine’s angular speed is obtained as described in section 188.8.131.52, and is later multiplied by the gain and saturated by two external boundaries, calculated in a further subsystem.
This subsystem is organized in two sections: the first is dedicated to limit the ideal torque in order to implement SOC control strategies, as explained in paragraph 184.108.40.206, and the second section’s task is to define the powertrain’s operating mode.
The Simulink modelling of this subsystem is reported below:
Figure 57 - SOC control strategy subsystem
Figure 58 - Powertrain mode selection subsystem