Design of electric vehicle propulsion system incorporating flywheel energy storage

Battery electric vehicles are crucial for moving towards a zero emission transport system. Though battery electric vehicle technology has been rapidly improving, it is still not competitive to the conventional vehicles in terms of both cost and performance. The limited driving range and high cost are significant impediments to the popularity of battery electric vehicles. The battery is the main element which affects the range and cost of the vehicle. The battery has to meet the requirements of sufficient power and energy, quick recharge, safety, low cost and sufficient life. However the battery can either provide high power or high energy but not both. Hybridisation of the energy source is one of the methods to improve the energy efficiency of the vehicle, which would involve combining a high energy battery with a high power source. High power batteries, ultracapacitors and high speed flywheels are the potential high power sources that could be used. Out of these, the high speed flywheel in combination with a mechanical transmission is an attractive high power source for the battery electric vehicle due to its favourable characteristics of high specific power, sufficient high specific energy, high energy efficiency, long cycle life, quick recharge and low cost . This thesis presents and critically assesses a concept of a mechanically connected flywheel assisted battery electric vehicle propulsion system for a modern passenger car application. The main contribution of this thesis is the analysis of the effect of utilizing a mechanically connected flywheel in a hybrid energy storage with Li-ion batteries on the energy efficiency of the electric vehicle. The starting point of the research was to create a base electric vehicle model based on current technology. An analysis of the battery electric vehicle, its various components and control strategy and various approaches to model it was discussed which led to the creation of the baseline model. Simulations using the baseline model on three real world driving cycles representing urban, extra urban and motorway conditions, showed the potential for improving the energy efficiency of the vehicle by utilizing a power handling device that could transmit power directly to the driveline such as a mechanically connected flywheel. Hybridisation of the energy storage with the incorporation of the mechanically connected flywheel was presented. The flywheel was sized and a road data analysis was performed to support the sizing analysis. To accomplish the integration of the flywheel with the driveline, a fundamental analysis of the mechanical power split continuously variable transmission was conducted which showed various ways of obtaining the desired ratio range for the flywheel operation according to vehicle requirements. The speed ratio, power flow and efficiency were derived for three different types of transmissions. This analysis produced a simple methodology that can be applied to design a transmission for flywheel energy storage to provide any required speed ratio coverage and predict its efficiency in both directions of power flow, which is an important contribution of the thesis. The hybrid vehicle layout was presented and all its components were discussed. Further to obtain the maximum potential for improvement in energy consumption with the hybrid vehicle, optimisation of the energy management strategy was conducted. The optimisation problem was complex because of factors such as the small storage capacity of the flywheel, the kinematic constraints and the slipping of clutches. Dynamic programming was used to find optimal energy management strategy on the three real world driving cycles, which was the first instance of its implementation for such a powertrain; another important contribution of the thesis. The results were compared with baseline using a quasi static backward model. There was significant reduction in energy consumption for the more aggressive motorway cycle, less for the extra urban cycle, while there was a small increase in energy consumption for the relatively less aggressive urban cycle. However significant reduction in battery stress was observed for all the cycles which is expected to lead to improvements in battery life and lower operating costs. To provide a further step in implementation, a predictive energy management strategy was applied in the backward model for the hybrid vehicle based on dynamic programming with short computation time and utilizing limited future journey information which showed good performance in comparison to the benchmark simulation results. Finally the control was tested in a forward dynamic simulation to verify its suitability for real life implementation, and showed small deviation in performance compared to the backward simulation.