As the environmental pollution and energy crises are getting more and more remarkable, hybrid electric vehicles (HEVs) have taken on an accelerated pace in the world. A comprehensive overview of HEVs is presented in this paper, with the emphasis on configurations, main issues, and energy management strategies. Conclusions are discussed finally.
In 1834, the first vehicle, actually a tricycle, powered by battery, was developed. But with the improvement in the internal combustion engine (ICE), ICE vehicles (conventional vehicles) have occupied an absolute share in the market; pure electric vehicles (PEVs) have almost disappeared since 1930’s. First, let us investigate the global growth of population and vehicles in the next 50 years [
Growth of population.
Vehicles and population.
It is reported that vehicles emission accounts for 39.2% of the total emission in 2007 [
EV is a road vehicle which involves with electric propulsion [
The major characteristics and features of three types of EV.
Types of EV | PEV | HEV | FCEV |
---|---|---|---|
Energy source | (i) Battery | (i) Battery/ultracapacitor | (i) Fuel cells |
Propulsion technique | (i) Electric motor drives | (i) Electric motor drives | (i) Electric motor drives |
Characteristics and feature | (i) Zero emission | (i) Low emission | (i) Zero emission |
Major techniques | (i) Electric motor control | (i) Electric motor control | (i) Fuel processor |
Regenerative braking | (i) Yes | (i) Yes | (i) Yes |
According to technical Committee 69 (electric road vehicles) of the International Electro technical Commission, an HEV is a vehicle in which propulsion energy is available from two or more kinds or types of energy sources or converters, and at least one of them can deliver electrical energy [
Power flow of HEV.
Series HEV.
Parallel HEV.
Combination HEV.
From Figure
From Figure
From Figure
Typical products of HEV.
Products | Configuration | Automobile companies | Year |
---|---|---|---|
Prius | Combination | Toyota | 1997 |
Insight | Parallel | Honda | 1999 |
Tino | Combination | Nissan | 2000 |
Civic | Parallel | Honda | 2001 |
Lexus LS 600h | Combination | Toyota | 2007 |
Toyota Auris | Combination | Toyota | 2010 |
Lexus CT 200h | Combination | Lexus | 2011 |
Compare to conventional vehicle, HEV can save fuel for the following reasons. HEV can store part of the vehicle’s kinetic energy in battery while braking or downslope in which otherwise is burnt in the brake drums in the form of heat in conventional vehicle. The ICE in an HEV can be designed with a smaller displacement without compromising the performance of vehicle [ HEV can make ICE operate at maximum efficiency point or optimal operating line by regulating the output power of battery to satisfy the required power of vehicle. HEV is a multiple energy system; the main issue of HEV is how to optimize the power flow to obtain best fuel economy or low emission at lower cost, which is often referred to as the energy management (EM) problem. The issue will be discussed in next section in detail.
The most critical aspect in designing HEV is to get most effective results with controlling conversion of energy on the powertrain. Therefore, the controller design of HEV is the key point of the design process. The aim of this section is to explain powertrain control strategies for HEV and review the latest methods of EM strategies in detail. The aim of the control strategies is to satisfy a number of goals for HEV [ Minimization of fuel consumption or maximum of fuel economy. Minimization of emissions. Good drivability.
HEV’s control system is very complex. The multilevel hierarchical control is an important control method for large-scale and complex system [
Block diagram of HEV controller.
The function of driver command interpreter is to compute the driver’s torque demand according to desired speed of vehicle and actual speed of vehicle. Speed of vehicle is controlled by accelerator pedal and brake pedal position. This is a feed control system by adjusting the accelerator pedal and brake pedal position to make the vehicle follow the desired vehicle speed.
Compared to the conventional vehicles, HEV is a multiple energy source, then, how to split the required power among energy sources is called EM. Vehicle system controller performs powertrain control by using EM strategies according to command signals received from driver command interpreter and parameters information feedback from electronic controller. Vehicle system controller can be divided into three function blocks as shown in Figure Required power of vehicle interpreter. Energy management strategies. Torque interpreter.
Required power of vehicle interpreter is a function block to convert the driver’s torque demand to power demand. HEV is a multiple energy system, different from conventional vehicles which can only output power, battery not only can output power but also absorb power. For a required power, how to split the power required between two energy sources and mechanical brake in order to minimum the fuel consumption or emission is the hot topic among technology developers. In the next section, EM will be discussed. Then, torque interpreter converts the power demand of ICE and mechanical brake to torque demands of ICE, generator, motor, and mechanical brake.
Block diagram of vehicle system controller.
Electronic controller is the embedded system which carries out the commands from the vehicle system controller to make corresponding parts work. Electronic controller in Figure
Engine control unit is an electronic control unit (ECU) for controlling ICE; it makes the ICE output desired torque coming from vehicle system controller command signal by injecting fuel into the combustion chambers of the ICE. ICE’s operating point can be described by torque and speed. In series HEV, there is no mechanical connection between ICE and transmission, then how to control the speed of ICE? There is a mechanical connection between ICE and generator, so ICE’s speed is controlled by generator’s torque demand. Motor is the final drive device and coupled by mechanical connection to transmission, and so the motor’s speed depends on the driving cycle, similarly, motor’s operating point can be described by torque and speed usually; the torque demand of motor is computed by driver’s torque demand due to mechanical connection to transmission. MCU makes the motor operate at desired torque using FOC technology usually. Motors, used for traction usually, are also able to become generator during braking or down slope. Hence, the vehicle kinetic energy, which otherwise is burnt in the brake drums in the form of heat, can be converted into electrical energy and sent back to the battery. If the battery is unreceptive, then electronic braking system control unit can be work.
There have been a number of surveys on EM strategies for HEV. Early EM is designed based on rules for its effectiveness in real time supervisory control of power flow in a hybrid powertrain, which is set up on the basis of heuristics, intuition, and human expertise. For example, the maximum torque ICE can produce is low at low speed, while the motor is high at low speeds. So, heuristics EM strategies are to make the vehicle run in purely electrical mode from standstill to a low vehicle speed. When the vehicle speeds up from this speed, the ICE will turn on and operate based on optimal operating line. Global optimal control methods are adapted subsequently. In a word, EM for HEV can be divided into two main methods as shown in Figure
Energy management control strategies for HEV.
Some global optimal control methods based on optimal control theory have been developed recently. These methods aim at minimizing the energy loss or fuel consumption over a given period of time [
Dynamic programming was founded to solve the shortest distance early. EM problem is similar with shortest distance problem, and so it is commonly solved by DP. DP requires gridding of SOC over a given driving cycle with a time discretization step
Although global optimal control methods can obtain global optimum solution, it requires substantial amount of computational time and cannot be implemented real time. One method that often permits a reduction of computational time is the instantaneous control methods, and it only optimizes at instantaneous time. The greatest advantage of the methods is possible applied for real time. Instantaneous control methods can be divided into deterministic instantaneous methods, fuzzy control methods and equivalent fuel consumption method.
Deterministic instantaneous methods are designed via lookup tables to split the requested power between two energy sources. Deterministic instantaneous methods include power follower control, thermostat [
In equivalent fuel consumption method [
Environment protection and energy crisis have urged the development of EV. However, PEV is not widely used currently. The main reason is that they could not satisfy the consumers’ need due to high initial cost and short driving range. Thus, BEV will be designed mainly for short range, such as community transportation. Although FCEV has long-term potential for future main stream vehicles due to zero emission and comparable driving range with conventional vehicles, the major challenge for developing FCEV is how to investigate low-cost FC and refueling system. Consequently, HEV can meet consumers’ need currently and will grow in faster rate. The main issue of HEV is how to optimize the multiple energy sources to obtain best fuel economy or low emission at lower cost.
This paper has presented an overview of HEV with focus on the configurations, main issues, especially the control of HEV, and it elaborates the EM approaches. The EM problem is the hot research area until now, and it is also the critical technology for HEV. Global optimization approach can obtain global optimum, but it requires substantial amount of computational time and does not implement real time, so the global optimization approach, such as DP, usually serves as a benchmark for evaluating other EM strategies. Instantaneous control methods only optimize at instantaneous time. So, the real-time application becomes possible.