Low-Cost Pathway to Ultra Efficient City Car: Series Hydraulic Hybrid System with Optimized Supervisory Control

A series hydraulic hybrid concept (SHHV) has been explored as a potential pathway to an ultra-efficient city vehicle. Intended markets would be congested metropolitan areas, particularly in developing countries. The target fuel economy was ~100 mpg or 2.4 l/100km in city driving. Such an ambitious target requires multiple measures, i.e. low mass, favorable aerodynamics and ultra-efficient powertrain. The series hydraulic hybrid powertrain has been designed and analyzed for the selected light and aerodynamic platform with the expectation that (i) series configuration will maximize opportunities for regeneration and optimization of engine operation, (ii) inherent high power density of hydraulic propulsion and storage components will yield small, low-cost components, and (iii) high efficiency and high power limits for accumulator charging/discharging will enable very effective regeneration. The simulation study focused on the SHHV supervisory control development, to address the challenge of the low storage capacity of the accumulator. Two approaches were pursued, i.e. the thermostatic SOC control, and Stochastic Dynamic Programming for horizon optimization. The stochastic dynamic programming was setup using a set of naturalistic driving schedules, recorded in normal traffic. The analysis included additional degree of freedom, as the engine power demand was split into two variables, namely engine torque and speed. The results represent a significant departure from the conventional wisdom of operating the engine near its “sweet spot” and indicate what is preferred from the system stand-point. Predicted fuel economy over the EPA city schedule is ~93 mpg with engine idling, and ~110 mpg with engine shut-downs. INTRODUCTION The energy security and climate change challenges provide strong impetus for pursuing ultra-efficient vehicle concepts. The fuel economy of passenger vehicles shows very strong dependency on vehicle mass and aerodynamic drag, but hybridization enables departures from the trend and significant leaps in fuel economy improvements. The mechanisms for improvements include possibility for engine downsizing, optimization of engine operation, regeneration and engine shut-downs when the vehicle is stopped. The relative contribution of each depends on the architecture, e.g. parallel, powersplit or series, selection of component and the type of application. In the context of a small passenger vehicle, the series configuration holds a promise of maximizing benefits through flexibility in controlling the engine and effective regeneration, since by default the traction motor will be sized generously. However, larger sizes of components in the hybrid driveline create a cost challenge. Hence, this study explores the potential of the hydraulic hybrid propulsion system, with the expectation that inherently high power density of hydraulic propulsion and storage components will yield compact, low-cost components [1]. While state-of-the art hydraulic pump/motor technology for mobile applications includes very advanced designs, manufacturing is mature and can easily be setup in any region of the world. The series hydraulic hybrid driveline comprises propulsion pump/motors coupled to front and rear differentials and a hydro-pneumatic accumulator for energy storage. Coupling another hydraulic pump to the engine creates a power generation sub-system. The SAE Int. J. Engines | Volume 2 | Issue 2 505 particular advantages of hydraulics relevant for series architectures are the high-efficiency of the hydraulic pump/motor and high power limits for accumulator charging/discharging. They both contribute to very effective regeneration, and to some extent mitigate the effect of multiplying efficiencies in the propulsion chain. The comparatively low energy density of the hydraulic accumulator creates a special challenge and requires novel approaches to development of supervisory control. The features of the parallel and series hydraulic hybrid vehicle architectures have been investigated before, particularly in the context of heavier vehicles [2, 3, 4] and the optimization of the design and control strategies led to impressive fuel economy improvements. However, the application of a series hydraulic hybrid concept to a very small car has only recently caught attention [5, 6, 7]. It is our goal to utilize some of the advanced methodologies previously demonstrated in studies of heavy vehicles to maximize the fuel economy of the SHHV system for a small car. The particular focus is on the supervisory control development. This is of critical importance for SHHV analysis since the low storage capacity of the hydraulic accumulator creates a special challenge for controller development, very different than in the case of electric system. The lowcost objective requires keeping the component sizes as small as possible, therefore emphasizing the impact of power management. The aim of this paper is to demonstrate a hydraulic hybrid system for an ultra small city car. The target fuel economy is ~100 mpg or 2.4 l/100km in city driving. Hence, the baseline for the study is a first-generation Honda Insight IMA Hybrid Electric Vehicle, as its aluminum structure and aerodynamic body represent the state-of-the-art for a given category. The series hydraulic hybrid powertrain is designed and analyzed for the selected platform using a simulation developed in SIMULINK. Two approaches for supervisory control development are pursued, i.e. the Thermostatic State-Of-Charge (SOC) control and Stochastic Dynamic Programming (SDP) for horizon optimization of the supervisory policy. In the case of Thermostatic control, we challenge the conventional wisdom of operating the engine at the “sweet spot” and search for optimal SOCthreshold and power threshold during charging. In the SDP investigation, we take the notion that system effects dominate over the component-centric approach a step further. Namely, the technique includes selection of engine torque and speed, rather than just the power level, thus allowing departures from the best BSFC line if it benefits the overall powertrain efficiency. The paper is organized in three major sections. First, we explain the modeling of the baseline Honda Insight HEV. This includes in-depth review of its control strategies and electrical system operation based on data available in literature [8]. The second section proposes the new series hydraulic hybrid vehicle configuration. Modeling of hydraulic components and integration of the complete SHHV system is included. Detailed description of the two control approaches follows, i.e. the Thermostatic SOC management and SDP horizon-optimization. The stochastic dynamic programming is setup using a set of naturalistic driving schedules, recorded in normal traffic. Results of both supervisory control methodologies are discussed to uncover the mechanism of fuel economy improvements. Finally, we discuss the findings and the compare the fuel economy results of the SHHV vehicle to the baseline IMA HEV. The paper ends with conclusions. BASELINE ISA HYBRID CONFIGURATION A low cost city car with very high fuel economy will require a very light body with superb aerodynamics. Honda Insight (first generation) is used as a starting point for a light vehicle platform because it represents the state-of-the art in addressing both the light weight and aerodynamic drag. Honda Insight is a pretransmission parallel electric hybrid, as shown on the schematic in Figure 1. The motor augments the engine torque under high load and hence is also called Integrated Motor Assist (IMA). Honda Insight does not have a conventional alternator and uses the IMA for other electrical needs. Honda Insight in North America has a 5 speed manual transmission. Manual transmission offers higher efficiencies at steady state speeds but forces the driver to choose the optimal gearing to keep engine operation near optimal. High fuel economy of Honda Insight is attributed to its small engine and attention to every source of vehicle losses. The impact of hybridization, including the ability to regenerate will be highlighted later in the paper through comparisons with the SHHV. Honda Insight model is used as a baseline vehicle for performance and fuel consumption comparison. The model will be validated using the published results on Honda Insight, [8]. Table 1 gives the specifications of the baseline Honda Insight vehicle. Figure 1 : Hybrid electric vehicle architecture with Integrated Motor Alternator SAE Int. J. Engines | Volume 2 | Issue 2 506