# Development of a Hybrid Electric Vehicle Simulation Tool with a Rule-Based Topology
## 2 Materials and Methods
### Control Strategies of the Simulation
#### 2.2.2 Rule-Based Control Strategies  
Rule-based control topologies are introduced in this section for serial, parallel and serial/parallel hybrids. Serial hybrids allow a complete decoupling of the ICE from the powertrain, which makes it easier to implement efficiency-increasing ICE measures that react sensitively to transient behavior. Such hybrid concepts show a high degree of freedom regarding the charging process of the battery. It can easily be charged at vehicle stop and at low vehicle speeds, for instance. An additional positive effect of the serial hybrid powertrain is its electrical driving experience. In contrast, there is the long efficiency chain from the ICE to the wheel. The overall losses of this mechanical/electrical path cannot be neglected, although the ICE can be operated at optimum efficiency. This becomes particularly clear at high vehicle speeds, as the high speeds of the EMs cause the efficiency of the machines to drop sharply. In comparison, a parallel concept has a significantly lower degree of freedom in the charging process of the battery. Likewise, the customer's desired electric driving experience cannot be achieved in every driving situation. Although the ICE cannot be operated with optimum efficiency at every point due to the mechanical coupling, this can be approximated by a large number of gear stages combined with an operating point shift by charging/discharging the battery while driving. The efficiency chain from the ICE to the wheel is significantly shorter compared to the serial concept. Part of the mechanical power generated by the ICE is always fed directly to the output, bypassing the electrical path. A fuel consumption advantage over the serial approach can be expected. In case of serial/parallel hybrids, it is able to combine the advantages of both approaches. Due to the dynamical coupling and structural complexity, the serial/parallel hybrid driving system possesses multiple working modes to adapt to different driving conditions. As an example, in this paper, the author introduces a simple logic control for rule-base strategies in the following sections. This is to show the flexibility of the tool in adapting any logic algorithm by the user. 1. Parallel hybrid topology Among 9 different modes (the different modes described in Section 2.2.1), 7 different operating modes are selected for parallel hybrids in the HEV simulation tool, so it is necessary to divide the different states into operating ranges, because the torque demand from the ICE will be a major control parameter, since the ICE is directly connected to the wheels in parallel HEVs. To clarify the system of mode switching, a rule-based control topology for the parallel hybrids is proposed for optimal system efficiency. The operation regions are divided according to the test data of the engine [7], as shown in Figure 3. As shown in this figure, the line \"Te_min\" is the very low torque curve, with a relatively poor efficiency of ICE, \"Te_low\" is the minimum torque, and \"Te_ch\" is the limit of torque of charging mode. Region IV (in green) is the area with minimum fuel consumption, based on the example of test data of an engine. The dash line \"Te_eff\" represents the optimum area for minimum fuel consumption at a given engine speed and \"Te_max\" is the expected limit of engine torque in this hybrid simulation, which is close to the solid line for full loads of the engine. Above the line, a combination of the power sources, ICE and EMs, is required to carry out very high power requests. Based on the different regions, the operating system is divided into five parts, which are: e-driving mode (Region I), e-driving or engine driving mode (Region II), charging mode (Region III), engine driving mode (Region IV), and engine and EM hybrid mode (Region V). To formulate the control strategy of the parallel hybrids, two other parameters are considered, which are battery SOC and vehicle speed. The vehicle speed is just used to realize the IDEL mode, and the SOC is the other main parameter for the control logic.