Hybrid Solutions

A new hybrid vehicle design shows promise for meeting the fuel efficiency needs of today.

by Ashok Nedungadi, Ph.D., and Mark Walls

Automakers are under constant pressure from environmental activists, as well as government and state agencies, to increase the fuel economy and reduce the emissions of vehicles. This pressure has prompted a careful reexamination of automotive powertrains and encourages a departure from the traditional combustion engine design, which is at best only 35 percent efficient in the case of gasoline engines and 45 percent efficient in the case of diesel engines.

After decades of achieving modest improvements in engine design and operation, it has become increasingly clear that more radical design concepts are needed to meet the fuel economy goals of 80 miles per gallon and ultra-low emissions dictated by groups such as the U.S. government-sponsored Partnership for a New Generation of Vehicles, the Environmental Protection Agency (EPA), and the California Air Resources Board.

One path to meeting these fuel economy goals is through the use of hybrid-electric vehicles. While hybrids typically depend on four components -- an internal combustion engine, an electric motor, a generator, and a battery pack -- the arrangement and integration of these components can be varied to maximize performance and efficiency, and reduce emissions levels.

Dr. Ashok Nedungadi (left) and Mark Walls, both of the Advanced Vehicle Technology Section in SwRI's Engine and Vehicle Research Division, are involved in hybrid vehicle powertrain design, testing, and analysis. The parallel hybrid powertrain in the background is one that the team took from concept to functioning hardware prototype. Contact Nedungadi at (210) 522-3965 and Walls at (210) 522-6880.

The SwRI hybrid choice

In a series hybrid powertrain, propulsion is purely electric. The power is transmitted to the wheels through an electric motor charged by a battery pack and a generator coupled to a combustion engine. The combustion engine is used to charge the battery pack when its state of charge falls below a predetermined threshold.

A parallel hybrid powertrain, however, has a dual propulsion path, comprising both an electric motor and a combustion engine. There are two main advantages of the parallel hybrid over the series hybrid. The parallel hybrid does not require a separate generator because the motor itself acts as a generator. The system also has fewer energy conversions, resulting in less loss of mechanical energy.

Using internal research funds, Southwest Research Institute (SwRI) Engine and Vehicle Research Division staff members have devised a parallel hybrid concept designed to be the most efficient of the parallel hybrid family. It allows the engine speed and load to be decoupled from the vehicle speed and load, thereby enabling the engine to operate in a region of better fuel efficiency and performance. The only commercially available parallel hybrid powertrain that compares in design and performance with the SwRI concept is the recently introduced Toyota Prius.* However, the SwRI concept uses only one electric motor (the Prius has two), making the design mechanically simpler and less expensive.

The SwRI parallel hybrid powertrain consists of a three-cylinder internal combustion engine of 1.0 liter displacement, a power output of 40 kW (peak), an AC induction electric motor of 53 kW (peak), and 24 lead-acid batteries (50Ah rating with a nominal voltage of 12 volts).

A planetary gear arrangement is at the heart of the powertrain. It can either combine or split power produced by the electric motor and the gasoline engine. A sprag (one-way) clutch on the engine and hydraulic brakes on each of the planetary sun, ring, and carrier gears are used for transition between the vehicle's four modes of operation.

The planetary gear box for the hybrid-electric powertrain splits or combines the power from the AC induction motor and combustion engine to form SwRI's patent-pending parallel hybrid powertrain design. Various clutches and hydraulic brakes are housed within the gear box to enable the powertrain to function in four modes of operation.

Powertrain modes

In the "electric" mode, the electric motor supplies the power for the vehicle, exploiting its superior efficiency over the gasoline engine.

In the "charge" mode, the gasoline engine is operated at maximum power, the combustion engine's most efficient state, to supply traction to the wheels. Any excess power from the engine can be transferred by the planetary gear system to the electric motor and used to charge the batteries.

The "assist" mode uses both the gasoline engine and the electric motor for traction. This combination delivers maximum power to the wheels. It is also the least fuel-efficient mode because the electric motor is in traction mode and cannot absorb excess power. Therefore, the engine cannot be guaranteed to provide power at maximum efficiency conditions.

Finally, the "regeneration" mode allows power from the decelerating wheels of the vehicle to be diverted to the electric motor so that the energy can be used to charge the batteries.

An emergency "limp" mode allows the small, 1.0-liter gasoline engine to power the vehicle on its own, so that the driver can reach a destination even when the battery pack's charge has dropped below the electric motor's minimum power requirements.

Dr. Bapiraju Surampudi of the SwRI Engine and Vehicle Research Division developed the vehicle control system. He is shown here operating the SwRI-developed Rapid Prototyping Electronic Control System (RPECS) that was adapted for use on the parallel hybrid powertrain. The screen showing the engine speed (upper left), motor speed (upper right) and vehicle speed, as well as the state of the motor, engine, and batteries, is designed to look like the dashboard of a traditional car. The system allows the driver to choose the gear, mode of driving, and speed appropriate to traffic and road conditions. Contact Surampudi at (210) 522-3278.

Control Strategy

The most challenging aspect of the SwRI parallel hybrid powertrain is controlling its two power sources, which compete with one another to provide the appropriate amount of power to the wheels. To achieve optimum power delivery, a control strategy was devised that continuously maintains a power balance between the electric motor and the gasoline engine. The gasoline engine is controlled in speed mode and the electric motor in torque mode.

Thus, the engine speeds and motor torque are precalculated as a function of vehicle speed and driver torque demand. Although these set-point tables are different for each operating mode, the transition from one table to the other is smooth, thereby minimizing drivetrain jerks or other ill effects. The mode switching is transparent to the driver and is a function of vehicle speed and driver torque demand. At all times, the powertrain controller selects the most appropriate mode of operation that meets driver demands while also maintaining the engine in its most efficient operating state.

There can be situations, such as climbing a hill or passing another vehicle, in which the driver may not have the desired torque delivered to the wheels because of a low battery charge. For such events, it is possible to make the small, 1.0-liter engine behave as though it were larger by using a supercharger that can be declutched when not needed. Typically, the supercharger can deliver about 40 percent more power from the engine. In this mode, the engine would operate at a less efficient state. However, driver torque demand would not be compromised. This solution is not possible for series-hybrid configurations, because the engine is not coupled to the wheels in that design.

Data collected from hardware tests were used to validate the computer model of the hybrid powertrain. Engine speeds predicted by the model show a close match to actual hardware test data. Other variables derived from the computer model, such as motor torque, vehicle speed, and fuel consumption, were also validated with the hardware.

Parallel hybrid powertrain simulations

A computer model of the SwRI parallel hybrid powertrain was first developed to assist in selecting the various components of the powertrain, as well as determining the optimum control strategy. The computer model was developed using PATHS™, a comprehensive hybrid vehicle modeling and simulation toolbox developed by engineers from SwRI and the Georgia Institute of Technology. After analyzing numerous computer simulations with different driving schedules and operating conditions, the control strategy and specifications for the components were finalized, and a prototype was fabricated. This powertrain was designed to provide the same driving characteristics as conventional mid-size vehicles, but with twice the fuel economy.

The powertrain was tested for functionality and operation in a test cell, using EPA highway and urban driving schedules in the "assist" mode of operation. Even though assist is the most inefficient mode of operation, fuel consumption measured almost a two-fold improvement in fuel economy (50 miles per gallon in the city driving profile) over conventional mid-size vehicles. Computer simulations indicated that when the regenerative braking and the charge mode are operational, fuel economies are increased to 60 miles per gallon.

Data collected from the test cell was used to validate the computer model of the parallel hybrid powertrain. The same driving profile was executed on the computer model, as well as on the powertrain hardware, and key operating parameters were compared.

Fuel economy measurements were taken of the powertrain using the EPA highway and city driving schedules in the assist mode of operation -- the least fuel efficient of the possible modes. The relatively low battery energy consumption indicates that low-weight batteries can be used on board the vehicle.

Conclusions

A prototype of the combination-of-speeds parallel hybrid powertrain has been developed. Its design, component specifications, and control algorithms were derived from successive computer simulations under varying vehicle operating conditions. The fuel economy measurements are encouraging and have the potential of approaching 60 miles per gallon. A patent is pending on the drivetrain design, and interested parties are invited to license the parallel hybrid concept. Further research is also expected to yield significant reductions in emissions.

* Toyota and Prius are trademarks of the Toyota Motor Corporation.

Published in the Spring 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

Technics Spring 1999 Technology Today
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