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Hybrid Electric Vehicles
INTRODUCTION
1.1 Overview of Hybrid Electric Vehicles
A hybrid electric vehicle (HEV) is a vehicle that uses an internal combustion engine (ICE) and an electric motor (EM) as propulsion systems to increase the system efficiency. These vehicles are the short term solution to the reduction in the demand for fossil fuels. As shown in Figure, typical fuel consumption of an ICE is high at low speed and decreases as the speed increases beyond 45 km/h . For the same distance, if you drive the vehicle at 50 km/h rather than 15km/h, you would increase the engine efficiency by 45 percent. If the propulsion power can be provided by an EM at low speed, the combined efficiency will be better than when an ICE alone is used. However, HEVs pose a challenging energy management problem of effectively splitting the required torque between the EM and the ICE. It’s obvious that this decision will greatly impact the gas mileage of the vehicle, and state of charge of the battery as an energy source.
Much research has been done in recent years in HEV control, and several strategies have been proposed. These strategies include optimal, intelligent, as well as rule-based approaches. Each method has its degree of complexity, strength, and limitation. Some of these control strategies are; 1- Parallel electric assist control strategy, 2- Fuzzy logic control for optimum fuel use, 3- Fuzzy logic control for vehicle efficiency, 4- Adaptive control strategy, 5- Torque split control strategy (Honda Insight Model). Figure 1.2 illustrates a basic scheme for HEV control structure. Basically, a control strategy of a HEV determines the torque/power split between ICE and EM for a certainspeed, torque, and acceleration.
The difficulty of implementing these control strategies lies either in obtaining the information required to implement the strategy, the lack of availability of expert knowledge for the intelligent control strategy, or tuning the rules for simple strategies
Hence, hybrid vehicle control focused on the torque/power split between the combustion engine and the electrical motor to operate them at the most efficient torque speed characteristics. They require a built-in controller, torque/power splitter and other transmission integrated devices which make the system very complex and expensive. In addition, choosing proper traction motor and its supply system will affect the system efficiency immensely because of the increase of electrical energy consumption by using high power EM and high capacity battery.
1.2 Research Motivation:
This research proposes the design of a novel control strategy that improves gas mileage of a traditional single engine vehicle by using an add-on package which includes an electric motor with battery and motor controller system. It is titled “add-on” because of its compact structure and simplicity to implement on existing non hybrid vehicles. The idea suggests that if an external EM, battery, and motor control system are added to the vehicle as a standalone package, the control strategy will only determine the EM torque contribution according to the vehicle speed, battery state of charge (SOC), and actual load of the vehicle. So, the motor torque controller is independent from the driver pedal; unlike a typical HEV (compare Figures). The conceptual design of the proposed control strategy is shown in Figure. Using actual speed, SOC, and actual vehicle load information, see Figure 1.3, the controller determines the torque contribution of EM using EM-ICE efficiency maps. Efficiency maps are obtained using experimental data, and illustrate the change of motor efficiency.
For instance, if the ICE operates at 10% efficiency, 90% of the energy is wasted during that drive. As seen on Figure, the efficiency maps for the ICE and EM of first generation Toyota Prius illustrates the maximum efficiency regions. ICE has 45 percent maximum efficiency at high torque regions while EM has 90 percent. Before applying the energy management strategy, one has to know the torque needed by the vehicle at a given speed.
HYBRID ELECTRIC VEHICLES
2.1 HEV Configurations
The decrease in fuel consumption could be enhanced with proper design of power train components, such as downsizing the ICEs, and adding higher power EMs with well-designed
power management strategies for the vehicle such as recapturing the kinetic energy lost during braking or driving the vehicle with EM at low speeds where ICE are known to be inefficient.
The transportation busses, military vehicles, and automobiles may require different speed-torque drive characteristics. For this reason, different configurations of HEVs are developed for various vehicular applications. But generally, HEVs evolved out of two basic configurations: series and parallel. However, the second generation Toyota Prius has series-parallel configuration. In this chapter, the different configurations of HEVs are introduced, compared with each other, and power flow diagrams are explained.
2.1.1 Series HEVs
The series HEV configuration, see Figure 1, has the simplest control structure because there is no direct mechanical connection between the ICE and the wheels. All the propulsion power comes from the EM while the ICE is only used to charge the battery used to power the EM or its battery The biggest advantage of a series HEV is the simplicity of its drive train which is due to the decoupling between the ICE and the wheels permit the ICE to be operated on its most efficient operating region while maximizing fuel efficiency for generating power needed by the EM. Another advantage is the near zero emission when compared with the other HEV configurations; the ICE is not primary power source and works at regions of minimum emission.
On the other hand, the presence of a generator is a big disadvantage for this configuration. Furthermore, the electric motor should provide enough power for maximum gradeability, acceleration and highway cruising. So the rapid depletion due to high speed cruising and frequent acceleration demands should be taken into consideration during the battery design. Based upon these advantages and disadvantages, the series HEVs are best suited to low speed, and interrupted type driving. As explained, during deceleration the electric motor acts as generator and converts kinetic energy to electrical energy which is referred to as regenerative braking. Part of the kinetic energy of the vehicle is transformed into battery power through the EM and charges the battery.
2.1.2 Parallel HEVs
A parallel hybrid vehicle is one in which more than one energy source can provide propulsion power. The ICE and the EM are configured in parallel with a mechanical coupling which blends the torque/power coming from the two different sources. Unlike series HEVs, both power sources (ICE and electric motor) can be utilized to drive the vehicle with the help of an integrated gear/clutch system that give opportunity of providing the desired power either solely from the prime mover or from both motors. As illustrated in Figure, energy from the fuel tank and the battery is transferred to the wheels via two separate mechanisms. During high speed cruising, only the selected prime mover is utilized for propulsion and this is typically the ICE for parallel HEVs. During interrupted cruising, ICE charges the battery through the transmission and EM, which eliminates the necessity for an extra component like a generator. At the regenerative
braking, the battery is charged via EM that acts as generator. For instance, Honda Insight uses this kind of HEV configuration with a 995 cc single overhead VTEC engine which has maximum power of 76 HP at 5,700 rpm and a maximum torque of 113 Nm at 1,500 rpm with the assistance of an electric motor which produces 10 kW at 3000 rpm. In parallel HEVs, the need for a generator is eliminated, which decreases the weight and the cost of the configuration. If EM is not the only power source, then EM could be used for power regeneration during deceleration and converts kinetic energy to electrical energy.
