Xtrac springs a new hybridized AMT

  • 10-Jan-2013 02:16 EST
Xtrac 1010 hybrid transmission rear right view.jpg

Xtrac's 1010 hybridized AMT combines with a Magneti-Marelli electric motor for a compact supercar transaxle.

Increasingly stringent fuel economy requirements such as the European Union’s planned 95 g/km rule for 2020 will compel manufacturers of even super sports cars to consider fuel-saving measures such as hybrid-electric drive.

The problem is that electric drive lacks the visceral connection with the driver provided by internal combustion. That is fine for economy cars, but drivers of high-dollar sports cars want that emotional connection, asserts Clive Woolmer, General Manager of the automotive and engineering business unit at Xtrac Ltd.

“I, for one, can’t imagine a silent Lamborghini or a silent Ferrari,” he said.

This compounds the criticisms regarding loss of feedback often aimed at dual-clutch transmissions and their seamless application of power, Woolmer added. Combining the two technologies results in a large, heavy, and expensive transmission that is so smooth as to disconnect the driver from the sensations of piloting an exciting sports car, he argued.

Specialty racing and high-performance sports car transmission supplier Xtrac proposes a solution: a hybrid-electric version of the company’s automated manual transmission (AMT).

Automated manual transmissions are lighter, more compact, and less expensive than DCTs but traditionally suffer from unsatisfactory driving dynamics, as the car lurches through gear changes. This shortcoming limited the popularity of Maserati’s Quattroporte until that car introduced a conventional automatic transmission in place of the AMT.

But a hybridized AMT uses its electric motor to provide “torque infill,” propelling the car forward during gear changes and smoothing out changes in acceleration. In a hybrid AMT, drivers can still feel the gear changes, but they aren’t bothersome, explained Woolmer.

Xtrac’s solution grafts a Magneti-Marelli electric motor to its 1007 seven-speed transmission with transversely oriented gearset (but a longitudinal engine orientation), creating the 1010 hybridized AMT. The resulting transmission weighs 168 kg (370 lb) fully dressed, including the dry clutch. That is about 20% lighter than a comparable DCT, according to Woolmer. About 48 kg (106 lb) of that total are attributable to the electric motor and hybrid components.

Perhaps as importantly, when compared to the hybridized transaxle Ferrari has shown, the 1010 measures 175 mm (6.9 in) shorter from the rear axle centerline to the rear cover, leaving critical space for mufflers and underbody diffuser while permitting shorter rear overhang.

The 7-speed gearbox has a ratio spread of nearly 6:1. Torque infill during shifts will be about 75% of the traction limit, which is less power than from the IC engine but enough to improve shift comfort.

The design lets the electric motor power the rear wheels directly while the engine is disconnected at the clutch during gear-change torque infill at speeds up to 95 mph (152 kph). Because torque from the electric motor doesn’t pass through the clutch, it can be smaller and lighter, with reduced inertia for quicker gear changes, Woolmer observed.

At speeds above 100 mph (161 kph), the electric motor’s torque path through the transmission switches from the output side to the input side, where it can benefit from the multiplication of the gearsets to keep its rpm manageable.

In this mode the system functions similarly to the “push-to-pass” function in Formula One’s Kinetic Energy Recovery System.  In this mode, the 60 kW nominal power motor is able to contribute 120 kW for 60 s.

Xtrac’s goal for the technology is to develop it for high-volume mainstream production models, which are more sensitive to the AMT’s cost advantage over the DCT, said Woolmer.

“At the supercar level you have the ability to develop these technologies that could cascade down into higher-volume vehicles,” he said.  “Some of the feedback we have from OEMs is that they definitely see that as the area to develop transmissions in the next 10 years.”

The company predicts that the first production cars using the 1010 hybridized AMT could appear in three years, with development mules hitting the road later this year.

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