I. Introduction
The application of electrical machines in the drive-line transmission of internal combustion engine (ICE) vehicles has received increased interest in recent years, from both fuel economy/emissions and vehicle performance points of view [1], [2]. This paper reports on the design of a switched reluctance (SR) machine integrated with the flywheel of a down-sized ICE vehicle. The integrated SR machine and flywheel engage the vehicle transmission in the minimal hybrid–electric drive train via an additional clutch mounted within the inner bore of the SR machine rotor, as shown in Fig. 1. The contribution of the flywheel inertial torque significantly reduces the peak torque requirement of the SR machine for engine starting. The torque requirements for engine cold cranking are high, typically 400 for this application, which impose a considerable challenge for the electromagnetic machine design. The contribution from the inertial torque reduces this peak requirement to 70 . Hence, the electrical drive-train cost is lower in comparison with all-electric starter–alternator configurations, the tradeoff being in the cost of the additional clutch and controls. The system, therefore, eliminates the need for a separate starter and alternator, facilitates idle–stop operation, and provides electrical tractive effort for low-speed driving and parking. However, the additional clutch imposes a significant volumetric constraint for the machine design, necessitating a high degree of machine and drive design innovation. A further consideration for system design optimization is the choice of inverter switching strategy and the number of SR machine winding turns per pole such that the machine is capable of realizing power at extended speeds beyond the base-speed limitation imposed by the dc supply voltage, while minimizing the inverter volt–ampere (VA) rating. Previous publications have reported on the optimization of SR machine windings in relation to machine efficiency and inverter VA minimization [3]–[5] but not specifically to realize a constant output power at extended speed or with the additional benefit of continuous phase current excitation. The improvement in extended-speed power capability due to the choice of winding turns per pole and continuous current excitation is reported, and the machine design and simulation procedure are validated via test data from a prototype machine.
Minimal hybrid–electric drive-line concept. (a) Drive-train schematic. (b) Drive-train general assembly.