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This paper deals with design optimisation of hydraulic hybrid drivelines during early concept design phases. To set the design parameters of a hybrid driveline such as gear ratios, pump/motor displacements and size of energy storage, the energy management of the hybrid machine needs to be considered as well. This is problematic since a nested design and control optimisation normally requires substantial computer power and is time-consuming. Few previous studies have treated combined design and control optimisation of hydraulic hybrid vehicles using detailed, non-linear component driveline models. Furthermore, previously proposed design optimisation methods for on-road vehicles are not suitable for heavy off-road machines operating in short repetitive cycles with high transient power output. The paper demonstrates and compares different optimisation approaches for design and control optimisation combining deterministic dynamic programming and non-gradient based numerical optimisation. The results show that a simple rule-based energy management strategy can be sufficient to find the optimal hardware design even though non-optimal control laws are used.

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This article presents simulation and experimental results of an alternative scheme for reducing the consumption of compressed air in pneumatic positioning systems with external loads. The circuit adds a fast-switching on/off valve interconnecting the cylinder chambers on a typical system configuration comprising a symmetrical cylinder and a 5/3 directional proportional valve. The proportional valve is operated using a proportional-integral controller with dead zone compensation, and the auxiliary fast-switching on/off valve is controlled using a pulse-width modulation technique associated with another proportional-integral controller. An algorithm based on the pressure difference and the control signal for decision-making regarding compressed air saving is proposed. The results presented in this article show that the proposed scheme reduced the compressed air consumption by 47%-54%. The simulation results show that the presented approach can also be applied to circuits using asymmetrical cylinders with or without external loads.