Optimal Operation of a Hybrid Hydraulic Electric Architecture (HHEA) for Off-Road Vehicles over Discrete Operating Decisions

Most off-highway construction and agriculture equipment use hydraulics, which has unmatched power density, for power transmission and throttling as a means for control. Trends towards better efficiency and electrification motivated a novel Hybrid Hydraulic-Electric Architecture (HHEA) which could significantly reduce energy consumption even in high power machines that would be too costly to electrify directly. This is achieved by using a set of common pressure rails to transmit the majority of power hydraulically and modulating the power with small electric motor-drives to achieve precise control. This paper proposes a computationally efficient, Lagrange multiplier method for computing the optimal sequence of pressure rail selections to minimize energy use. This is needed to evaluate HHEA's energy saving potential and for iterative architecture design and sizing. An interesting complication is that the cost function is not fully defined until the candidate control sequence is fully specified. This issue is dealt with by decomposing the original problem into a set of subproblems with inequality constraints that can be solved efficiently. A case study of an off-road construction machine demonstrates that the HHEA reduces energy consumption by 2/3 compared to the baseline load sensing architecture.