A power management strategy for hybrid output coupled power-split transmission to minimize fuel consumption
Abstract
The objective of this work is to prove the hypothesis that there can be found at least one architecture of hydraulic hybrid that, when combined with a sophisticated control strategy, can yield better fuel economy than its electric hybrid counterpart. Investigation into various possible layouts of a power-split based hydraulic hybrid transmission leads to the selection of output-coupled power-split drive. Different power management strategies are proposed and analyzed from the criterion of optimality. The objectives of a power management can vary; reducing emissions, minimizing fuel consumption, maintaining satisfactory drivability, reducing drivetrain noise or a combination of these goals. In this study, the focus is achieving the minimum fuel consumption over a general drive cycle without compromising the driving performance. An instantaneous optimization based power-management has been proposed first that achieves engine management and maximizes the transmission efficiency at every instant during the drive cycle. For the purpose of optimal control based power management, hydraulic hybrid powertrain is modeled in the state-space framework. A cost function has been formulated that puts a direct penalty on the overall fuel consumption and also penalizes any drop in driving performance. Constraints arising from the physical limitations of the components are also considered. A Dynamic Programming (DP) based approach has been utilized that computes the optimal control trajectory. Although the solution given by DP is cycle-specific and noncausal in nature, it serves as the benchmark for any sub-optimal control. A practical and implementable control strategy based on Stochastic Dynamic Programming (SDP) has been proposed next. Driving power demand at the wheels has been modeled as stationary Markov chain. Power management based on SDP has the form of nonlinear, time-invariant state feedback. All three power management strategies result in significant improvement in fuel economy with hydraulic hybrids compared to the conventional or electric hybrid counterparts. These strategies are also analyzed with regard to their practicality of implementation, computational effort and the optimality of the results.
Degree
Ph.D.
Advisors
Ivantysynova, Purdue University.
Subject Area
Automotive engineering|Mechanical engineering|Energy
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