Southwest Research Institute^® (SwRI^®) News

Southwest Research Institute's innovative hybrid drivetrain now available for licensing

San Antonio, Texas -- March 29, 1998 -- Engineers in Southwest Research Institute's^® (SwRI^®) Engine and Vehicle Research Division have designed and built an innovative parallel hybrid drivetrain, capable of an estimated fuel economy of close to 60 miles per gallon for a mid-size vehicle operating the Federal Transport Protocol (FTP) urban driving cycle. The design, for which a patent is currently pending, was funded under SwRI's internal research and development program.

Besides achieving excellent fuel economy, the drivetrain can periodically recharge the battery while the vehicle is running. This saves the driver from having to recharge the battery pack every 60 to 100 miles, a problem that affects electric vehicles.

"The most innovative feature of the design, however, is that it is possible to decouple the engine speed from the vehicle road speed," says SwRI project manager Dr. Ashok Nedungadi. "This allows the engine to operate in a region of better fuel efficiency and lower emissions while the vehicle travels at the rate commanded by the driver. In contrast to other parallel hybrid designs, the SwRI arrangement avoids the inevitable inefficiencies that occur when there is a fixed ratio between the engine and the vehicle speed. It also enables the drivetrain to optimize efficiency and performance, much as a continuously variable transmission."

The drivetrain test-stand consists of a one-liter, 40 kilowatt (54-horsepower) gasoline engine and a 53-kw (67 hp) AC induction motor powered by 24 deep-cycle, lead-acid batteries. These batteries can tolerate larger current draws than conventional batteries. An SwRI-designed planetary gear system receives power from the electric motor and the engine, then diverts it to the drivetrain using one of four modes of operation. These modes are assist, combining engine and motor power to provide propulsion; electric, using only battery power for propulsion; charge, where part of the engine power drives the vehicle and the rest is absorbed by the electric motor operating as a generator to charge the battery pack; and regenerative braking, which uses the kinetic energy from the decelerating wheels to recharge the batteries.

Prior to development of the hardware, a detailed computer model of the drivetrain was created using the Performance Assessment Tool for Hybrid Systems (PATHS), a modeling and simulation software program developed at SwRI.

The modeling was followed by extensive computer simulations to observe the performance of various selected drivetrain components as they operated under different driving cycles. The results of these simulations were used to select hardware components for the drivetrain as well as develop and test the drivetrain control strategy. All parts used in the prototype were "off-the-shelf" except for the transmission casing, which was custom-designed and fabricated.

Finally, the prototype drivetrain was successfully tested in a dedicated test cell using a variety of FTP urban driving profiles. Key measurements made in the prototype tests were used for validation of the developed computer model.

"In addition to making some important practical innovations in parallel hybrid drivetrain design, we achieved a very satisfactory agreement between predictions made using the computer model and the subsequent hardware," says Nedungadi. "This proved the usefulness of PATHS as a design and analysis tool and was another rewarding feature of the research program."

The most challenging aspect of controlling the drivetrain was the use of two power sources that could potentially 'fight' each other if not controlled in a coordinated manner. This problem was solved through use of a control strategy that maintains a balance between the two power sources. Because it was important for the engine to operate in a narrow speed range, the engine was controlled in speed mode while the motor was controlled in a torque mode. The speed and torque set points were calculated as a function of vehicle speed and driver torque demand.

This control strategy allows the drivetrain to operate in its most efficient mode, such as assist or charge, depending on the driver torque demand. The transition from one mode of operation to another is totally transparent to the driver and is completely automated.

The described drivetrain technology is available for licensing from SwRI. For additional information contact Ashok Nedungadi at (210) 522-3965.

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