Modular Powertrain Structure

The key feature of the MPS is the layout of the powertrain component placeholders, which can be seen in the high-level block diagram in Figure 39. This feature allows permutations of different energy converters and energy storages. This was made possible by having a clear distinction between the energy storage, energy converter, and power transformer. Using the CV and PHEV as examples, the full implementation of the MPS in Simulink can be found in Appendix D.

Figure 38: The structure of the framework within the proposed toolbox

Figure 39: High-level block diagram for layout of the powertrain component placeholders in the MPS

To investigate the workings of the MPS, several powertrain topologies were created based on the available powertrain components that were discussed in Chapter 3. These topologies are shown in Table 12. As seen in the last row of this table, each powertrain

topology is assigned a “Powertrain Variant”, which is an integer used by the optimiser to select the appropriate powertrain during the optimisation runtime.

Table 12: Powertrain architectures to investigate the model framework

Architecture Single Power Source Series Hybrid Parallel Hybrid

Topology CV EV PHEV MHV

Energy Converter #1 ICE EM EM ICE

Energy Storage #1 Fuel Tank Battery Battery Fuel Tank

Energy Converter #2 - - APU -

Energy Storage #2 - - Fuel Tank Flywheel

Powertrain Variant 1 2 3 4

However, it ought to be mentioned that the layout of the MPS will allow the implementation of many more types of powertrain components (and thus creating a larger selection of powertrain topologies), as long as the power “bond” between each powertrain component is observed. The power “bond” is essentially the communication line between the energy storage, energy converter, and power transformer. Each power bond is modelled in terms of effort and flow. This is based on the same distinctions used for Bond Graph modelling, as discussed in Chapter 3.

The following are examples of efforts and flows for their respective energy domains:

 Mechanical: torque and angular velocity

 Electrical: voltage and current

 Hydraulic: pressure and volume flow rate.

The switching mechanism within the framework is facilitated by the “Variant Subsystem” feature of Simulink [142]. Each energy storage and energy converter subsystem acts as a placeholder which contains a library of components. When the optimiser assigns a powertrain variant to the MPS, the respective energy converters and energy storages are populated. Depending on the powertrain configuration selected by the optimiser, the variant subsystem selects the correct energy converter and energy storage, as defined by the respective Powertrain Variant shown in Table 12.

For example, when the optimiser selects “Variant 1” as the desired powertrain topology, the corresponding energy converter and energy storage is the ICE and fuel tank. Similarly, if the optimiser selects “Variant 2” as the desired powertrain topology, the respective energy converter and energy storage is EM and battery. This is shown in Figure 40 and Figure 41 respectively. In both these non-hybrid topologies, only the

primary energy converter and storage are activated; the secondary energy converter and storage are not applicable and subsequently greyed out. The placeholders for the switching components are outlined in red, and the differences in the components between Variants 1 and 2 can be seen here.

Variants 3 and 4 are the series and parallel hybrid architectures, shown in Figure 42 and Figure 43 respectively. In Variant 3, both sets of energy converters and storages are enabled. The switchable component placeholders are once again highlighted in red to aid clarity when comparing the different topologies. For Variant 4, however, the secondary energy converter is once again greyed-out, because there is no conversion of energy domain between the flywheel and the ICE (both in the mechanical domain). Another key enabler for the modular powertrain structure is the power split junction box, which can be seen located between the primary and secondary energy converters and storages. The purpose of this junction box it to regulate the power flow between these two systems, depending on the prevailing hybrid powertrain type. For example, in a series hybrid configuration (Variant 3, Figure 42), the junction box switches into series mode, and the power connection between the EM and the battery is enabled. Therefore, the APU supplements electrical power to the EM, in addition to the battery, in this configuration.

Similarly, in a parallel hybrid configuration (Variant 4, Figure 43), the junction box switches into parallel hybrid mode, and the power connection between the gearbox and ICE is enabled instead. Therefore, the flywheel supplements mechanical power between the gearbox and ICE. By controlling the state of the junction box or by disabling it entirely, the modular powertrain structure is capable of simulating series, parallel, and compound hybrid powertrains, as well as non-hybrid powertrains.

The modular structure described so far ensures that the energy domains between the storage and converters are compatible. For example, the EM will only connect to a battery, and not to a fuel tank. The parameterisation of each powertrain component is stored in individual MATLAB m-files. Although not considered in this thesis, Appendix B shows other possible powertrain topologies that could be created using the MPS for future work.

Structures, which are MATLAB arrays with data fields, are used to store information regarding the inputs and outputs of the simulation. The fields of a structure can contain any kind of data. For example, one field might contain a text string representing a name, another might contain a scalar representing a fuel economy result, and so on. The use of structures allowed the toolbox to be better organised and, consequently, provided convenient access to information. The use of structures in handling data is shown in the m-file in Appendix F, which also details the link between powertrain topology and component parameterisation in MATLAB and the MPS in Simulink.

Figure 40: MPS layout switched to a CV (Variant 1)

Figure 42: MPS layout switched to a series PHEV (Variant 3)