Design and Optimisation of the Propulsion Control Strategy for a Pneumatic Hybrid City Bus

A control strategy has been designed for a city bus equipped with a pneumatic hybrid propulsion system. The control system design is based on the precise management of energy flows during both energy storage and regeneration. Energy recovered from the braking process is stored in the form of compressed air that is redeployed for engine start and to supplement the engine air supply during vehicle acceleration. Operation modes are changed dynamically and the energy distribution is controlled to realize three principal functions: Stop-Start, Boost and Regenerative Braking. A forward facing simulation model facilitates an analysis of the vehicle dynamic performance, engine transient response, fuel economy and energy usage. To identify respectively (1) the maximum overall fuel economy, (2) the maximum amount of air and energy recovered during the braking and (3) the minimum loss of available energy during acceleration, a number of variables in the control strategy are selected in an optimisation process. Three optimisation algorithms are compared in different aspects of the control strategy: (1) using the Pattern Search to optimise the initial air tank pressure for every stop-start event in order to maximize the pressure increment in the air tanks; (2) conducting the Genetic Algorithm optimisation to find out the best gear change strategy during braking in order to maximize the energy recovery to the air tanks; and (3) implementing the multiobjective optimisation to simultaneously minimize the fuel consumption and the loss of available energy in the air flow during acceleration. The rationale for the choice of optimisation methods is explained and recommendations made for the development of energy management strategies in which a variety of different vehicle functions contribute to an overall fuel economy benefit.