Mechanical Configurations of Hybrid Electric Vehicles

Jonathan Shoop

Engineering 5

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Due to increased pressures on auto manufacturers, there has been an upswing in the production of cars designed to reduce emissions and fuel consumption. This trend has resulted in several offshoots, the most notable of which is the hybrid vehicle, also known as the HV. Within the hybrid family, there is a range of designs. Farthest from the internal combustion driven automobile is the electric vehicle (technically not an HV), in which no combustion engine is used. Near the middle is the “soft hybrid,” consisting of a powerful integrated starter generator (ISG) to allow engine shutdown during idle periods as brief as stopping at a light and provide rapid restart. The design also allows for the internal combustion engine for a given car to be smaller, as much of the torque can be shared by the electric motor. However, even once this design is generally accepted, there are numerous variations in control strategy that can alter the vehicles performance.

The first part of my paper will describe the various control strategies. These are differentiated by various configurations of drive architecture and state of charge control. My paper with then focus on the ramifications of renewable braking on safety and energy consumption. Finally, in conclusion I will give a brief summary of the problems associated with the commercial manufacture of HEVs.

Drive Architecture

Figure 1: Series Structure (NREL.gov)

There are two basic structures in the construction of a hybrid. The first is a series, in which the internal combustion engine (ICE) is used to power an electric motor and to charge the batteries. The batteries power the electric motor. In this set-up, only the electric motor supplies the torque to drive the vehicle. This means that the electric motor would have the same torque and power requirements as a full electric vehicle. The first Swarthmore HEV used this design

Figure 2: Parallel System (NREL.gov)

The second basic structure is the parallel series, of which there are two primary variants. Both consist of separate electric and combustion engines which can simultaneously power the vehicle by means of a torque combination device. Their primary difference stems from the location of the transmission: the first variant placing it just after the ICE, but before the combination with the electric motor; the second placing it after the two are combined (Emdahi, Ehsani, Miller 198). A parallel series can be designed so that the ICE can power the batteries. The second Swarthmore HEV, SwiftR, is designed so that this can be done by means of a “through the road” system, wherein the ICE drives the front wheels. The kinetic energy is then transferred by the road to the rear wheels, which are driven by the electric motor, and can be switched from aiding the ICE to recharging the batteries (Knouf, E-90 2002 Integrated Report; Miyasaka, E-90 2002 Powerpoint ).

Figure 3: Through the Road Parallel (SwiftR) (Miyasaka, E-90 2002 Powerpoint)

State of Charge Control

State of charge control is divided into two categories, either charge depleting or sustaining. Charge sustaining strategies tend to be preferable, as they supply a larger amount of available energy at any given time and are not optimized for only a given drive cycle as the battery is kept at a higher state of charge under this set up. However, in a charge depleting strategy, the efficiency of the vehicle is higher, at least while the battery is charged. These two options for state-of-charge control can combined with either a series or parallel system structure to create four possible control strategies (Emdahi, Ehsani, Miller 199).

Control Strategies

To combine a series system with a charge sustaining strategy would result in the use of the ICE to attempt to mirror the battery usage by the electric motor. Closely related to this is the charge depleting series hybrid used by the first Swarthmore HEV, in which the ICE would only run once the battery has been depleted past a certain threshold. The effect of running the motor only at a low state-of-charge is that the ICE will only be run at its most efficient operating point until the battery is recharged. This serves as a genuine improvement over the charge sustaining series in this case. However, one result of this strategy is a tremendous wear on the battery, resulting in a shorter life span.

A charge sustaining parallel hybrid is designed so that the electric motor supports the ICE, in order to keep it in its highest efficiency ranges. Once the batteries have dropped below a certain threshold, the control strategy shifts, so that the ICE will exert extra toque in order to recharge the battery above the minimum. In this case, the electric motor would still be used, overriding the recharging, to exert extra torque if necessary. In this design, the batteries are never depleted, so the car has theoretically unlimited range in relation to its electric power and will always operate at a high level of performance (in relation to handling, rather than efficiency and emissions). One example of a charge sustaining parallel hybrid is the Honda Insight. The car is designed so that the electric motor will only start the engine and assist, but never independently power the car (Honda).

The final control strategy is the charge depleting parallel hybrid, in which the electric motor is used for all propulsion below a threshold speed, which decreases as the battery decreases. Above that speed, the ICE provides the propulsion with the electric motor in support. This ensures that the ICE will not operate in low efficiency regions, but the trade off is that the battery can easily be depleted and vehicular performance degraded (Emadi, Ehsani, Miller 198-204). A commercial example of this strategy is the Toyota Prius. However, the parallel only goes so far, as below 50% battery power (or way above), the car switches to a strategy similar to charge sustaining parallel, bringing up the ICE to standardize power. This design is optimized for a “stop and go” situation (Emdahi, Ehsani, Miller 204; Toyota).

In reflecting upon these strategies, it is apparent that there is often a trade-off between performance, fuel economy, and emissions control in a vehicle. Additionally, no commercial HEV available utilizes a charge depleting strategy (The Prius only uses such to a certain extent, but then switches to a charge sustaining) because performance would never be compromised, even for better fuel economy and lower emissions. However, the Prius does show an attempt to combine the beneficial aspects of the various strategies, in a bid to lower gas consumption and emissions without decreasing performance. This shows a general trend towards a mixture of these pure theoretical designs in application, so that the most common type of hybrid is truly a series-parallel charge sustaining-depleting car.

Braking

These diverse models for control strategies all have common underlying systems, notably those for braking. In a regular car, simple friction brakes work perfectly well for braking. However, in a hybrid, where energy conservation is the goal and there is sufficient storage capacity for excess energy, it behooves the designers to install a regenerative braking system. At low decelerations, this system will allow the car to recuperate the majority of the energy involved in braking and recycle it back into the propulsive system.

The HV motor/generator (M/G) plays the pivotal role in the power transfer resulting from the braking. The actual braking is accomplished by an electro-hydraulic brake (EHB) system in place of the anti-lock brakes. The EHB system consists of a hydraulic electronic control unit, which replaces the actual anti-lock brake system, and an actuator control unit which serves as the sensors for brake pedal pressure and speed.

To actually brake, a computer reads the information from the actuator control unit, analyzes the pressure on the brake pedal and its application speed, and determines how much braking force is needed. Initially the computer utilizes only the HV M/G regenerative braking. This capacity can only be used to a certain extent in a given braking situation. Therefore, any additional braking required as interpreted by the computer is done by the electro-hydraulic brakes. The valves of this unit release brake fluid into the calipers to fulfill the braking requirements much as any common disc or drum brake. Because of the large reliance on computers, in case of an electrical power systems failure, the electronic valves open, and the regenerative braking system is bypassed, thus ensuring safety.

Several requirements of a regenerative braking system are that they maintain balance between the rear and forward brakes, as well as keeping the vehicle dynamics stable. Additionally, regenerative braking maintains the functionality of conventional hydraulic assist anti-lock brakes, as well as providing several advantages over them. The first and most obvious is the increased fuel economy. The second is the reduced wear on the standard brakes through the use of regenerative braking. The EHB system requires a shorter range of motion for the pedal with a lower force required for stopping, thus resulting in greater driver comfort with braking. And, finally, the RBS isolates the driver from the pedal pulsations during breaking associated with anti-lock brakes. This pulsation is believed to cause drivers to release their pressure during braking in emergency situations (Emdahi, Ehsani, Miller 206-208).

Problems with Commercial Manufacturing of HEVs

In designing a hybrid electric vehicle, space is at a premium, because of the necessity for both an ICE and an electric engine, as well as the additional batteries. This challenge can be seen in the Swarthmore HEV, in which it was necessary to completely remove the rear seats to emplace the additional four car batteries, electronics, and electric motors and controllers. In a standard commercial car, however, this remedy is unacceptable, as the car is first and foremost a mode of transportation. Therefore, limitations in maximum battery capacity, as well as a more manageable reduction in ICE size are required.

Conclusion

In the future it seems assured that Hybrid Vehicles will be assuming a larger place in commercial and private vehicle sales, as gas prices continue to rise and consciousness of the depletion of our natural resources and the pollution cars produce increases. This will only be spurred on by the increased efficiency of these vehicles, especially in the field of mechanical/electrical energy conversion. This process has been given a jump start by the National Renewable Energy Laboratory. As part of the Department of Energy, they are tasked with the development and analysis of new technologies, especially in the field of hybrid and fuel cell vehicles. Their program, the ADvanced VehIcle SimulatOR (ADVISOR) is useful in the creation of more fuel efficient and lower emission vehicles, as it allows quick analysis of previously tested parts in relation to their affect on the aggregate whole (NREL). Over time, these new advances in technology and available tools will lead toward the ideal of a zero emissions vehicle, epitomized by the Electric Car.

Figure 4: Toyota Prius (Toyota.com)

Link to the Old Site

Link to Bibliography

Email me at jshoop1@swarthmore.edu