【正文】 then operate like full hybrids once the batteries have discharged to a predefined level. Fundamentals of Regenerative Braking 7 One of the most important features of HEV’s is their ability to recover significant amounts of braking energy. The electric motors can be controlled to operate as generators during braking to convert the kiic energy of the vehicle into electrical energy that can be stored in the energy storage system and reused. However, the braking performance of a vehicle also greatly affects vehicle safety. In an emergency braking situation the vehicle must be stopped in the shortest possible distance and must be able to maintain control over the vehicle’s direction. The latter requires control of brake force distribution to the wheels [12]. Generally, the braking torque required is much larger than the torque that an electric motor can produce [12]. Therefore, a mechanical friction braking system must coexist with the electrical regenerative braking. This coexistence demands proper design and control of both mechanical and electrical braking systems to ensure smooth, stable braking operations that will not adversely affect vehicle safety. Energy Consumption in Braking Braking a 1500 kg vehicle from 100 km/h to 0 km/h consumes about kWh of energy based on Equation . 221mvE? If 25 percent of this energy could be recovered through regenerative braking techniques, then Equation can be used to estimate that this energy could be used to accelerate the vehicle from 0 km/h to about 50 km/h, neglecting aerodynamic drag, mechanical friction and rolling resistance during both braking and accelerating. This also assumes that the generating and driving modes of the electric motor are 100% efficient. This suggests that the fuel economy of HEV’s can be greatly increased when driving in urban centres where the driver is constantly braking and accelerating. Note that the amount of energy recovered is limited by the size of the electric motor and the rate of which energy can be transferred to the ESS. Methods of Regenerative Braking There are two basic regenerative braking methods used today. These methods are often referred to as parallel regenerative braking and series regenerative braking. Eac h of these braking strategies have advantages and disadvantages that will be discussed in this section. 8 Parallel Regenerative Braking During parallel regenerative braking, both the electric motor and mechanical braking system always work in parallel (together) to slow the vehicle down [48]. Since mechanical braking cannot be controlled independently of the brake pedal force it is converting some of the vehicle’s kiic energy into heat instead of electrical energy. This is not the most efficient regenerative braking method. However, parallel regenerative braking does have the advantages of being simple and cost effective. For this method to be used, the mechanical braking system needs little modification and the control algorithms for the electric motor can be easily implemented into the vehicle. This method also has the added advantage of always having the mechanical braking system as a backup in case of a failure of the regenerative braking system. Series Regenerative Braking During series regenerative braking the electric motor is solely used for braking. It is only when the motor or energy storage system can no longer accept more energy that the mechanical brakes are used [48]. This method requires that the mechanical braking torque be controlled independently of the brake pedal force and has the advantage of being the most efficient by converting as much of the vehicle’s kiic energy into electrical energy . The downfall of this method is that it brings many costs and plexities into the system. For this method to function properly a brakebywire system has to be developed which either uses an electrohydraulic brake (EHB) or an electromechanical brake (EMB). Both of these types of brakes require brake pedal simulators and redesigned brake systems which can bee costly. Since these systems are brakebywire there are also many redundancies required with sensors, processors and wiring for safety which add to the plexity of the system. Current Regenerative Braking Systems The current regenerative braking system in most HEV’s (. Toyota Prius) is the more costly electrohydraulic braking (EHB) system. This system uses a brake pedal simulator, which is separate from the hydraulic braking circuit, to establish driver braking demand. The braking demand is then propo