Development and Implementation of a Control Strategy for a Hybrid Power Train System in a Classroom Setting

The project, a bench-scale hybrid electric powertrain system, is designed, analyzed and fabricated by students in six modules, starting in their sophomore year and culminating in their final semester as seniors. This complex project has been selected in order to integrate the core mechanical engineering courses: Mechanical Design, Thermodynamics, System Dynamics and Control, and Fluid Mechanics. A bench-scale hybrid-electric vehicle powertrain has sufficient complexity to involve all Mechanical Engineering disciplines and the simplicity to be built by students over the course of five semesters. The work is designed to test two hypotheses: 1. A long-term design project that integrates knowledge from multiple courses strengthens student knowledge retention. 2. A large-scale design project requiring tools from many courses improves student problem-solving and design skills. By integrating five semesters of the mechanical engineering curriculum into a cohesive whole, this project has the potential to transform the way undergraduate education is delivered. Before and after testing is being conducted to assess: a) Change in retention between courses and b) Change in student problem-solving and design skills. Students at Rowan University have built almost all of the “hardware” for the HPT (air engine, planetary gearset, tachometer, etc.) in earlier semesters. The control system is the “capstone” for the five-semester design project, which has been described in an earlier publication [1]. This paper describes the development of the “faculty prototype” of the control system, and gives preliminary results of implementing the control system design project in the classroom. Introduction Toyota has been recognized for developing cutting-edge hybrid systems. Specifically, they have developed and implemented the Toyota Hybrid System (THS) which combines a gasoline engine and an electric motor, with the advantage of not requiring external charging. According to the Toyota [2] the THS II system achieves nearly twice the fuel efficiency of conventional gasoline engines. This system was included in the Toyota Prius. The THS uses a gasoline engine, electric motor, and electric generator to achieve better fuel efficiency which results in reducing exhaust emissions of carbon dioxide (CO2). A simplified diagram of the Toyota Hybrid System (THS) is shown in Figure 1. Figure 1: Diagram of the Toyota Hybrid System (THS) [3] P ge 24409.2 Electric Motor Air Engine Planetary Gears Generator Load Motor/ Generator Load Box Battery Box Power Supply Power Path Electric Path For educational purposes, a simplified version of the THS has been developed at Rowan University. The bench-scale Hybrid Powertrain (HPT) prototype is very similar to the one used in the first generation Toyota Prius, and the differences between the two systems can be seen in Table 1. Figure 2 depicts the HPT diagram and Figure 3 the HPT prototype. The main difference is that instead of using a gasoline engine, it has an air engine powered by shop air at 120psi (8.3bar). The flow of air is controlled by six solenoid valves which are not shown; these will be described below. Also, the prototype has a DC electric motor and DC generator so there is no need for an inverter. The system also includes a planetary gearset. In this case, a differential gearset was used since it was cheaper and more reliable to build this type of geartrain in an educational environment. The result is that the inputs and the output of the planetary gears are linked to different shafts than in the THS. Table 1: Differences between Toyota Hybrid System (THS) and Bench-Scale Hybrid Powertrain (HPT) THS HPT Main Power Source Gasoline Engine Air Engine Electric Motor AC DC Generator AC DC Planetary Gears Sun/ring/planet Differential Sun Linked to generator Linked to motor Ring Linked to motor/wheels Linked to engine Carrier Linked to engine Linked to gen/wheels Inverter Needed Not needed Load Box Not needed Needed Table 2: Planetary gearset connections to inputs and output Input/Output Gear Linked to Shaft Input 1 Sun 1 Electric motor Input 2 Sun 2 Air engine Output Carrier Wheels / Generator The THS system is electronically controlled in order to achieve a variable transmission. It changes to adapt itself to different driving conditions with the aim of working at its most Figure 3: Bench-scale Hybrid Power Train (HPT) prototype developed at Rowan University Output/Wheels