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Test Drive: VW Turbo Li-ion Jetta Hybrid Offers Economy, Performance

22 August 2013

Volkswagen literally translates as people's car, and the recently introduced Jetta Hybrid is certainly that, at least in terms of a starting price the average working stiff can afford (just under $25k), which is about half the cost of other hybrids coming out of Europe.

VW engineers have refined the hybrid gasoline-electric architecture as opposed to revolutionizing it. The result is a vehicle that is responsive in acceleration and handling, while posting EPA mileage ratings of 42 mpg (city) and 48 mpg (highway). On a test drive, the latter was achieved—according to the trip computer—in roughly 75 miles with the cruise control set at 65 mph.

According to VW's Mark Gillies, manager of product technology communications, the company is looking to bring more electrified powertrains into the U.S. market in order to reach the looming tightening of CAFE mileage standards of 54.5 mpg mandated by 2025, with a 2017 interim review.

The Jetta Hybrid powertrain consists of a 150-horsepower (184 ft-lbs torque) 4-cylinder, 1.4 l, turbocharged, direct-injection gasoline engine and a 27-horsepower (20.1 kW), 114 ft-lbs torque electric motor. Driveline components and software limit the combined output of the engine and motor to a maximum of 170-horsepower and 184 ft-lbs of torque, notes Gillies. Volkswagen’s advertising, highlighting the Jetta Hybrid drivetrain's performance, claims the car achieved 187.147 mph at the Bonneville Salt Flats in Utah.

Coasting (or "sailing" as VW terms it), regenerative braking and the gas engine are used to charge the 1.1kWh Li-ion battery. The high energy density of Li-ion chemistry, combined with the modest battery capacity, results in a small enough package under the floor of the trunk to allow for both rear seats to fold down, (unlike some other hybrids) for a convenient boost in cargo capacity.

Running on only the electric motor, the Jetta Hybrid can travel up to 37 mph, which can be kicked up to 44 mph if the E-Mode (extended electro-mode drive) button is pushed. Electric-only range is limited to 1.2 miles because of the small battery in this non-plug-in hybrid.

Technology enablers

A key component in achieving high mileage is the 7-speed, dual-clutch direct-shift gearbox (DSG) automatic transmission, which the company claims is 20 percent more efficient than conventional automatics. The transmission’s dual-clutch units "anticipate" the next gear for more efficient shifting, while seven speeds allow the engine to run longer near optimum-efficiency rpm.

Combining the DSG with a turbo engine in a mid-size car is a first for the Jetta Hybrid, says VW's Gillies. Integration and packaging of the hybrid architecture also required attention to design detail because of the various driving modes, including the need to disconnect the engine from the transmission and then reconnecting it after cruise or a stop.

On the electronics side, the Bosch power electronics are key to smooth hybrid functioning. Housed in a compact aluminum "shoebox," they function as an interface between the battery and electric motor. The pulse inverter takes direct current from the battery and converts it into alternating current for the motor.

During regenerative braking, the motor acts as an AC generator and the pulse inverter changes the current to DC to recharge the battery. In addition, a DC-DC converter takes the high Li-ion battery voltage and converts it into 12 V power for the conventional electrical system running the radio, headlights, electric power steering, etc. If headlight and other systems are nearly all in use, upwards of 150 A can be coursing through the power electronics unit.

"The more efficiently the power electronics work, the greater the vehicle's range," notes Joachim Fetzer, an executive manager with Bosch Gasoline Systems Division—with the current system having 92 percent efficiency. He adds the latest generation of hybrid power electronics (2.3 model) is designed to work within a voltage range of 150-430 V, and can power an electric motor up to 134-horsepower (100 kW).

Design refinements

On the road, in addition to high mileage, the Jetta Hybrid has crisp handling plus a very quiet cabin compared to a gasoline-powered Jetta. While the electric motor contributes to lower noise, other design features provide further noise reduction, according to Gillies. These include thicker glass, quieter powertrain mounts, low rolling-resistance tires, exhaust system acoustical treatment and an independent rear suspension that is designed without a noise-conducting torsion beam.

Aerodynamic drag is about 10 percent lower, at CD = 0.275, than previous Jettas, also contributing to better mileage. A body undertray, the low rolling-resistance tires, rear spoiler and diffuser, front air dam and flush air intake all contribute to the drag reduction.

Overall, the Jetta hybrid provides a sporty driving package with economy, good handling and quiet operation.



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