ZF Has Latest ‘Killer App’ Transmission

DETROIT – Transmission developers and manufacturers swiftly are bringing to market a breed of advanced components that increasingly integrate sophisticated electronic controls.

ZF Friedrichshafen AG highlights at the North American International Auto Show here its latest-generation automatic transmission that markedly improves shift times and shift quality, as well as reduces fuel consumption.

ZF’s new 6 HP 28 is the second-generation of the company’s 6-speed automatic transmission, which changed the powertrain landscape when it was launched in 2001.

The 6 HP 28 continues to offer six forward speeds – some transmission makers have moved on to 7- and even 8-speed automatics – but ZF says shift times have been reduced by 50%, while delivering as much as 3% better fuel economy for gasoline engines and up to 6% for diesels.

“The new automatic 6-speed definitely represents a quantum leap in the field of automatic-transmission development,” Michael Paul-executive vice president, technology, and group executive of the ZF Car Driveline Technology division, says.

The company says the drastically reduced shift times mean the transmission can deliver multiple downshifts virtually without torque interruption.

BMW’s latest X5 cross/utility vehicle benefits from ZF’s second-generation 6-speed automatic transmission.

Sophisticated new control software and a clutch driven by a separate pressure regulator enable optimized torque-converter lockup, which improves fuel economy and quicker “jumps” from one gear to another.

ZF engineers say the transmission can skip as many as four gears thanks to a “target gear” control strategy, which determines the appropriate gear based on numerous powertrain operating parameters and driving conditions.

This new sophistication eliminates many automatic-transmission stumbles.

ZF’s example: A vehicle cruising in sixth gear at highway speed that wants to rapidly accelerate triggers a “decision” to downshift from sixth to second gear in just 100 milliseconds, and the gearshift is completed in 200 milliseconds, “below the limit of physical perception.”

Because of this ability, ZF is billing the new 6 HP 28 as a legitimate competitor to dual-clutch automated manual transmissions, such as the Volkswagen Group’s acclaimed Direct Shift Gearbox.

One of the prime selling points of DSG, which is a manual transmission with electrohydraulic actuation of the clutch (freeing the driver from the need to use a clutch pedal), is that there is no interruption of drive torque when the transmission shifts gears.

The 6 HP 28 already is being used in three new performance-oriented vehicles: the BMW 3-Series Coupe and X3 and X5 cross/utility vehicles, and ZF says several more production vehicles using the 6 HP 28 are on the horizon.

ZF says the new 6 HP 28 can handle engines with a torque output of up to 553 lb.-ft. (750 Nm), which covers a wide range of large-displacement gasoline and diesel engines.

Although other transmission makers are producing engines with more forward speeds, ZF and other transmission experts have said automatics with more than six ratios quickly begin to reach a diminishing return for the added complexity, size and weight of advancing to more than six ratios.

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