Research on powertrain technology of fuel cell electric vehicles

1 Introduction

Fuel cell vehicles are a type of electric vehicle. The electricity generated by the fuel cell is fed to the motor through an inverter, controller and other devices, and then driven by the transmission system, drive axle and other devices to drive the wheels, so that the vehicle can travel on the road. The energy conversion efficiency of the fuel cell is 2-3 times higher than that of the internal combustion engine. The chemical reaction process of the fuel cell does not produce harmful products, so the fuel cell vehicle is a pollution-free vehicle [1-3]. With the requirements for automobile fuel economy and environmental protection, the automobile power system will gradually transition from the current fossil fuel-based system such as gasoline to hybrid power, and will eventually be completely replaced by clean fuel cell vehicles [4].

In recent years, fuel cell systems and fuel cell vehicle technologies have made significant progress [4-5]. World-renowned automobile manufacturers such as Toyota, Honda, General Motors, Daimler-Chrysler, Nissan and Ford Motor Company have developed several generations of fuel cell vehicles [5-12] and announced various strategic

goals for bringing fuel cell vehicles to the market. At present, prototypes of fuel cell cars are being tested, and demonstration projects of fuel cell-powered transport buses are being carried out in several cities in North America. Honda's FCX Clarity has a top speed of 160 km/h[8]; Toyota's fuel cell vehicle FCHV-adv has accumulated 360,000 km of road tests, can start at minus 37 degrees, and can travel from Osaka to Tokyo (560 km) on a single refueling[7]. With the support of China's Ministry of Science and Technology, fuel cell vehicle technology has developed rapidly. In 2007, China successfully developed the fourth generation of fuel cell sedans, which have a top speed of 150 km/h and a maximum range of 319 km. In 2008, 20 fuel cell demonstration vehicles were demonstrated at the Beijing Olympics. In 2010, a total of 196 fuel cell vehicles from domestic automobile companies including SAIC and Chery were demonstrated at the Shanghai World Expo Park[13].

There are still technical challenges in the development of fuel cell vehicles, such as the integration of fuel cell stacks, improving the commercialization of electric vehicle fuel processors and auxiliary parts. Automobile manufacturers are working towards integrating components and reducing component costs, and have made significant progress. However, compared with traditional internal combustion engine cars, fuel cell electric vehicles use "fuel cell + electric motor" to replace the "heart" of traditional cars - the engine and fuel system. The power transmission system of fuel cell cars has undergone major changes, mainly manifested in: the electric motor replaces the internal combustion engine as the driving power source; the clutch and torsional vibration damper are omitted; the multi-speed transmission is usually replaced by a reducer [14,15]. Therefore, the power transmission system of fuel cell vehicles is simplified overall. However, when driving, the fuel cell is the main power source and the battery is the auxiliary energy source. The power required by the car is mainly provided by the fuel cell. It can be said that the selection of automotive fuel cells is crucial to the performance of fuel cell vehicles.

This paper introduces the development of traditional fuel cell vehicle power technology, and discusses in detail the key technologies such as fuel cell electric vehicle power transmission topology architecture, multi-source system management and power system configuration and simulation optimization technology.

2 Power transmission system topology architecture design

The operation of fuel cell vehicles is not a steady-state situation. Frequent starting, acceleration and climbing make the dynamic working conditions of the vehicle very complex. The dynamic response of the fuel cell system is relatively slow. The output characteristics of the fuel cell cannot meet the driving requirements of the vehicle when starting, accelerating sharply or climbing steep slopes. In actual fuel cell vehicles, it is often necessary to use the design method of fuel cell hybrid electric vehicles, that is, to introduce auxiliary energy devices (batteries, supercapacitors or batteries and supercapacitors) and connect them to the fuel cell grid through power electronics to provide peak power to supplement the insufficient output power capacity of the fuel cell when the vehicle is accelerating or climbing. On the other hand, when the power of the fuel cell is greater than the driving power under conditions such as vehicle idling, low speed or deceleration, the excess energy is stored, or during regenerative braking, the braking energy is absorbed and stored, thereby improving the energy efficiency of the entire power system.

2.1 Direct fuel cell hybrid system structure

The power electronics device used in the direct fuel cell hybrid system structure is only the motor controller, and the fuel cell and auxiliary power unit are directly connected to the inlet of the motor controller. For example, Toyota's FCHV-4[16], FIAT-Elettra[17] and Nissan X-TrailFCV[12] all use this similar structural design.

The auxiliary power unit expands the total energy capacity of the power system and increases the vehicle's driving range after one hydrogen refueling; it expands the power range of the system and reduces the power load borne by the fuel cell. Many plug-in hybrid fuel cell vehicles also often adopt this architecture, such as Ford's Edge Plug-in fuel cell car and GM's Volt Plug-in fuel cell car [18]. This plug-in hybrid vehicle will effectively reduce the consumption of hydrogen fuel. In addition, the presence of the auxiliary power unit enables the system to have the ability to recover braking energy and increase the reliability of system operation. The reasonable distribution of load power between the fuel cell and the auxiliary power unit can also improve the overall operating efficiency of the fuel cell [4].

In the system design, a bidirectional DC/DC converter can be added between the auxiliary power unit and the power system DC bus. This makes the control of the charging and discharging of the auxiliary power unit more flexible and easy to implement. Since the bidirectional DC/DC converter can better control the voltage or current of the auxiliary power unit, it is also the execution component of the system control strategy.

2.2 Parallel power system structure

Another architecture is the parallel fuel cell hybrid system structure. This construction usually installs a DC/DC converter between the fuel cell and the motor controller, and the terminal voltage of the fuel cell is matched with the voltage level of the system DC bus through the boost or buck of the DC/DC converter. This system is different from the above architecture in that the design of this power system does not consider energy feedback recovery. Therefore, although the system is simple, its efficiency is relatively low.
Although there is no longer a coupling relationship between the voltage of the system DC bus and the power output capacity of the fuel cell, the DC/DC converter must maintain the voltage of the system DC bus at the voltage point (or range) that is most suitable for the operation of the motor system. For the AC motor drive system, a DC/AC converter is usually required. At present, this type of architecture system is only used in some small or experimental vehicles. For example, the Autonomy and Hy-wire developed by General Motors in 2002 are both based on this architecture [10]. In 2008, the Tongji University-ThyssenKrupp Joint Laboratory used this architecture to develop a small fuel cell vehicle [19] and studied the impact of the fuel cell stack system on the performance of the entire vehicle.


3 Management and optimization of multi-energy systems for fuel cell vehicles

Fuel cells are not suitable as the sole driving energy source of the power system. Auxiliary energy systems must be selected to reasonably supplement the energy required to drive electric vehicles, cover power fluctuations, increase peak power, absorb feedback energy, and improve the transient characteristics of fuel cell output power. Currently, major automobile developers have adopted auxiliary power to improve the performance of fuel cell vehicles (as shown in Table 1).

3.1 Power battery auxiliary energy system

At present, lead-acid batteries [20] have been eliminated due to their low specific energy and specific power. The power batteries commonly used in automobiles are mainly nickel-metal hydride batteries and lithium-ion batteries.

Table 1 Typical fuel
cell electric vehicles

Nickel-metal hydride batteries are alkaline batteries that are not prone to aging, do not require pre-charging, and have good low-temperature discharge characteristics. Their energy density can exceed 80 Wh/kg, they can travel a long distance on a single charge, and they can discharge smoothly when working at high currents. The power systems of FCHV-4[6], High-lander FCHV-adv[7] and General Motors Chevrolet Equinox[9] are all integrated with fuel cells and nickel-metal hydride batteries. However, the battery charge of nickel-metal hydride batteries will drop sharply in high temperature environments, and they have problems such as memory effect and heating during charging. In the fuel cell hybrid system, the SOC of the nickel-metal hydride battery should be maintained between 40% and 60%, the charge and discharge current should be in the range of 160-240 A, and the temperature should be maintained near room temperature to ensure system safety and economy[21,22].

Lithium-ion batteries have the advantages of small size, high energy density (>120Wh/kg), high safety and pollution-free. Honda FCX Clarity[8], GM Chevrolet Sequel[10] Lithium and Nissan X-Trail FCV[12] all use lithium-ion batteries as auxiliary energy systems for fuel cell vehicles. The energy density of lithium-ion batteries is 1.5-3 times that of nickel-metal hydride batteries. The average voltage of its single cell is 3.2V, which is equivalent to the voltage value of three nickel-zinc or nickel-metal hydride batteries connected in series. Therefore, it can reduce the number of battery assemblies and reduce the probability of battery failure caused by the voltage difference of single cells, thereby increasing the service life of the battery pack.

Lithium-ion batteries have the advantages of low self-discharge (only 5%-10%). When stored in a non-use state, the internal structure is quite stable and almost no chemical reaction occurs[4,5]. Since lithium-ion batteries do not contain heavy metals such as cadmium, mercury and lead, they will not cause pollution to the environment during use. For electric vehicles, lithium-ion batteries are easy to arrange and install on the vehicle and are a more ideal energy storage medium. Tools such as Simulink and Dymola are often used to simulate and analyze battery systems[23] to improve the efficiency and life of batteries.

The dynamic process of its charge and discharge can be described by the Thevenin model as follows [23,24]:


3.2 Supercapacitor system

Supercapacitor is a new type of energy storage element. It has high discharge power like electrostatic capacitor and large charge storage capacity like battery [23,25]. Because its discharge characteristics are closer to electrostatic capacitor, it is still called "capacitor".

If supercapacitor is used as the only auxiliary energy source, there are still many shortcomings. For example, when electric vehicles are restarted after a long shutdown, due to the self-discharge effect of supercapacitor, the power supply of the on-board auxiliary system will not be guaranteed when the energy output of the fuel cell is not stable [5]. Moreover, the energy density of supercapacitor is very low. If a certain energy storage capacity is to be achieved, the size of the equipment must be increased. At present, supercapacitors are purchased together with other power batteries as auxiliary power systems and used in fuel cell vehicles [4,25,26]. In order to overcome the characteristics of supercapacitors, the impedance method can be used to model instead of the simple RC loop model [23]. The current SOC of supercapacitors is mainly based on the output voltage of supercapacitors:


3.3 Combination and control of multi-source energy

Fuel cell electric vehicles are equipped with the above two topological configurations, combined with power batteries and supercapacitors, to achieve better results. At present, the three main energy combinations are: 1) fuel cell + power battery, General Motors Chevrolet Equinox and others use this combination [9,10,12]; 2) fuel cell + supercapacitor, such as Honda's FCV-3 and Mazda FC-EV [4]; 3) fuel cell + power battery + supercapacitor, such as Honda FCHV-4 [8]. Tadaichi [6] studied the flow of energy under different conditions. By comparing the three types of energy used in the vehicle, a multi-energy energy management system was designed based on the fuel cell engine output power prediction control strategy, realizing the optimal management and control of the three types of energy [26].

4 Power system configuration and simulation optimization technology

4.1 Fuel cell system simulation technology

There are two methods for modeling the fuel cell system in fuel cell vehicles. One is to establish a relatively complex one-dimensional or multi-dimensional physical model based on the theories of electrochemistry, engineering thermodynamics, fluid mechanics, etc. [27]. This model can establish a corresponding model according to the structural parameters of different fuel cells to analyze the influence of various factors such as pressure, temperature, humidity, flow, catalyst, pipeline structure, etc. on the operation of the fuel cell. However, this model is complex and not intuitive, and the calculation speed is slow. The other method uses a simpler mathematical empirical model combined with corresponding commercial software [24,26]. This method is intuitive and fast, but the model can only be used for specific fuel cell systems, and its establishment depends on experimental data.

4.2 Vehicle power transmission system simulation optimization technology

The ultimate goal of fuel cell vehicle simulation is to simulate and analyze the working conditions of the fuel cell system and even the entire vehicle power system based on the fuel cell model, combined with the relevant models of the subsystem and power transmission system. This system optimization method is mainly carried out in combination with actual use, and is generally divided into two types [24,27].

When the actual road conditions are unknown, T. Gabriel Choi et al. [28] from Ohio State University studied the control measurements of the FIAT Panda model for the power requirements of fuel cell plug-in electric vehicles: the setting method of control measurements under offline global optimization and dynamic optimization. Energy optimization control methods for home charging and fuel cell hybrid applications. Guezennec et al. [29,30] studied the use of energy by driving habits and optimized the power system and size capacity.

When the actual use conditions are known, Xie Changjun et al. [26] studied the optimization methods under conditions such as cruising acceleration, Francisco et al. [31] studied the design method of the power system capacity of fuel cell electric vehicles under rural routes, urban routes and a combination of the two, and studied the efficiency and energy consumption of the power system under different auxiliary energy systems, providing a reference for the design of fuel cell power systems. Keshav S et al. [32] used the power system simulation analysis tool (PSAT) to analyze the performance of the fuel cell vehicle system including the fuel cell stack and other components, and found that when using a single auxiliary energy, the lithium battery has the best effect (Table 2). When lithium batteries and supercapacitors are used together, the efficiency can reach 9%. In addition, the mechanical structure of the fuel cell and its dynamic response also need further consideration [14].

5 Summary

The fuel cell stack in a fuel cell electric vehicle can only maintain the average power requirement of the vehicle. The use of an auxiliary energy system improves the efficiency of the fuel cell vehicle. This paper focuses on the key technologies of fuel cell vehicle power conventional technology, and discusses in detail the key technologies such as fuel cell electric vehicle power transmission topology architecture, multi-source system management, and power system configuration and simulation optimization technology. The research in this paper has important reference value for the design and manufacturing of fuel cell electric vehicle power conventional technology.