Research paper
Parametric design and regenerative braking control of a parallel hydraulic hybrid vehicle

https://doi.org/10.1016/j.mechmachtheory.2019.103714Get rights and content

Highlights

  • Hydraulic driving unit designed for commercial vehicles with power analysis.

  • Braking control strategy considers braking safety needs and energy recovery.

  • Influence of working pressure and gas volume on energy recovery is analyzed.

  • Different vehicle load and load distributions are investigated.

Abstract

Hydraulic hybrid technologies improve the fuel economy of city-use medium duty vehicles by recovering braking energy and reusing it for driving, most notably vehicle launching. In this paper, a parametric design of hydraulic driving system for a medium duty truck is conducted based on its power demand in typical urban driving cycles. The braking control strategy is designed considering both the safety regulations and braking energy recovery rate. The proportional relationship of hydraulic pump/motor output torque and its working pressure is considered when designing the braking control strategy. The influence of the high pressure accumulator minimum working pressure and the corresponding gas volume at the minimum pressure on the braking energy recovery rate is analyzed. The simulation results show that the proposed hydraulic driving system and braking control strategy helps greatly increase the braking energy recovery rate. Besides, different vehicle mass and load distributions are investigated to analyze the regenerative braking effect on various loading conditions.

Introduction

Improving vehicle fuel economy has become a promising way to solve the environmental degradation and energy shortage problems. Electric driving system and hybrid electric driving system have been widely used in passenger vehicles and light duty vehicles [1], [2], [3], [4]. For these medium duty and heavy duty vehicles, high power is demanded during accelerating and braking [5]. To satisfy the power demand, the size of electric motor and battery should be very big, which dramatically increases the cost and vehicle mass.

With high power density and low cost, hydraulic hybrid driving system has attracted a lot of attention in past decades [6], [7], [8], [9]. Hydraulic driving systems have been applied on delivery vehicles [10], refuse collection vehicles [11], military vehicles [12] and buses [13,14]. Among various driving systems, parallel hydraulic hybrid driving system can be refitted from conventional engine driving vehicle by adding a dual function hydraulic pump/motor (HPM) on driveshaft, which helps to reduce the design cost and simplify the system configuration [15]. In parallel hydraulic hybrid vehicles (PHHV), braking energy is recovered by HPM and stored in the high pressure accumulator. The energy is used for vehicle launching and driving the vehicle at low speed, by which the engine low efficiency operating regions are avoided and vehicle fuel economy is improved.

A balanced regenerative braking control strategy has essential influence on PHHV fuel economy. In braking control strategy design, the braking safety and regenerative braking efficiency should both be considered [16]. The braking control strategy should be designed to maximize the recovery rate of braking energy within safety regulations. The regenerative braking control strategy for electric vehicle (EV) and hybrid electric vehicle (HEV) have been deeply researched [17], [18], [19]. However, for PHHV, there are some specific characteristics should be considered. The PHHV are normally rear wheel driving while most of the HEV and EV are front wheel driving. Consequently more braking force should be allocated to rear wheels in PHHV to maximize recovered braking energy. However, to maintain vehicle stability during braking, the rear wheels should be locked after the front wheels. The braking force distribution between the front wheels and the rear wheels have to be well designed based on the requirements of braking energy recovery and safety restrictions [20].

Some research has made significant contribution to the hydraulic hybrid vehicle (HHV) braking control strategies. In [21], economic commission of Europe regulation is considered when designing the regenerative braking control strategy. Some typical city and urban driving cycles are adopted to validate the control strategy. Reference [22] takes the vehicle load into consideration when designing the regenerative braking control strategy. The braking force distribution lines are different for full load and no load. This control strategy is able to improve the fuel economy and braking performance, but the vehicle load identification during braking is a challenge.

In terms of practical design, the HPM output torque is proportional to its working pressure. A certain minimum high pressure accumulator pressure needs to be maintained to provide hydraulic braking torque required by braking control strategy. Suitable hydraulic driving system parameters should be selected, especially the high pressure accumulator capacity and minimum working pressure. In most research, these parameters are selected intuitively [1,11,23]. A rear driving HHV is researched in [23]. The braking force is more distributed to rear wheel to recover more braking energy. The proportional relationship of HPM output torque and its working pressure is also considered in this research, presented as minimum high pressure accumulator working pressure. Optimization is required to analyze the influence of high pressure accumulator parameters on regenerative braking efficiency and find suitable high pressure accumulator parameters for HHV.

In this paper, based on the power analysis of a medium duty PHHV and the braking control strategy, hydraulic driving system parameters are designed. The effectiveness and the energy benefits of the regenerative braking control strategy is validated by simulation. The main contribution of this paper is that the research results provide a practical parametric design methodology of the hydraulic driving system for PHHV, which is normally done empirically at the moment. The safety requirements and the energy benefit are compromised during the regenerative braking control strategy design. More importance is put to the braking safety here. The vehicle parameter sensitivity analysis verifies the effectiveness of the hydraulic driving system under different vehicle loading condition, which validates the practicality of the hydraulic driving system and promotes its commercial use.

The rest of this paper is organized as follows. In Section 2, the structure and working principle of the PHHV are proposed. The modeling of hydraulic driving system is conducted in Section 3 followed by the power analysis in Section 4. Energy management strategy is designed in Section 5, mainly focusing on the regenerative braking control strategy. Based on the braking torque requirement, high pressure accumulator parameters are optimized in Section 6. Parameter Sensitivity of vehicle load and load distribution are analyzed in Section 7. Section 8 gives a conclusion of the paper.

Section snippets

PHHV structure and working principle

The structure of the PHHV is shown in Fig. 1 [11,24]. Based on a conventional engine driving vehicle, the hydraulic driving system is installed at the driveshaft. In the engine driveline, engine power is transferred to the rear wheels via the engine clutch, AMT, driveshaft, differential and half shafts. In the hydraulic driving system, the high pressure accumulator is used to store high pressure oil and the low pressure accumulator is used to store low pressure oil. HPM could provide driving

Modeling of PHHV

The powertrain system is modelled in the Simulink environment of Matlab as rigid body, linear system. The system is solved using ODE4 with at fixed sample time of 0.1 s. As the focus of this paper is on system control and regenerative braking for energy recovery purposed, a number of assumptions have been made to reduce modeling complexity. By assuming the system is a rigid body, the degrees of freedom of the system are reduced, eliminating vibration response of the system and increasing

Power analysis of a medium duty truck

With high power density, the hydraulic driving system gets more energy benefits in city used medium and heavy duty vehicles with frequent starting and braking, such as delivery vehicles and refuse collection vehicles [27,28]. This paper selects a widely used medium duty truck as target vehicle. This truck is frequently refitted into refuse collection vehicle. The vehicle parameters are shown in Table 1.

In this section, half load is used as the vehicle mass to analyze the vehicle power demand.

Energy management control strategy

Due to low energy density of the accumulator, the hydraulic driving system is mainly used to recover the braking energy and launch the vehicle. The engine is still the primary power source for driving. During the vehicle launching, the hydraulic driving system is first used if hydraulic torque is available. The engine takes over the HPM to drive the vehicle when the hydraulic energy is used up or a minimum desired driving speed is achieved.

During braking, the braking force is composed of

High pressure accumulator parameters design

The HPM efficiency is influenced by working pressure, displacement and speed. As shown in Fig. 5, the HPM generally has relatively higher efficiency under higher pressure and higher displacement. The HPM has better working efficiency with the working pressure from 15 MPa to 30 MPa. The high pressure accumulator pressure during working is determined by its minimum working pressure and the initial gas volume at the minimum pressure. Increasing the accumulator minimum working pressure is favorable

Parameter sensitivity analysis

The hydraulic system parameters are selected based on the vehicle mass with half load which is 7000 kg. The vehicle mass is variable between 3300 kg and 10,700 kg with different load conditions. It is necessary to analyze the regenerative braking benefit under different vehicle loads, including both load distribution and total load. Fig. 13 shows the braking energy recovery rate across the range of vehicle mass.

From Fig. 13, the hydraulic system gets good energy benefit within the no load to

Conclusions

In this paper, HHV hydraulic driving system parameters are designed based on vehicle power analysis and energy management control strategy, especially the regenerative braking control strategy. Braking safety is considered as the most important factor when designing the braking control strategy. The distinctive characteristics of the HPM such as its output torque is proportional to its working pressure are taken into consideration in the parametric design process. By analyzing different

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This project was supported by the Australian Research Council under Discovery Early Career Researcher Award (DE0170100134).

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