New ZF 8-speed automatic gives 2012 Chrysler large cars 30 mpg economy

  • 07-Jul-2011 02:58 EDT
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Cutaway of the ZF 8HP 8-speed rear-drive automatic, a version rated for 450 N·m throughput. It is projected to help the 2012 Chrysler 300 deliver 30 mpg highway. (Photo by Paul Weissler)

A 2012 Chrysler 300 is cruising down the road at 70 mph (112 kph) and the tachometer shows just 1400 rpm. Earlier, a 0-60 mph (0-98 kph) acceleration in the same car was achieved in just 7.2 s. The engine isn’t a 5.7-L Hemi V8, nor is it a diesel. It’s Chrysler's Pentastar 3.6-L V6, and it's not yet equipped with direct fuel injection.

Outwardly the 300 is the same large, rear-drive sedan that was U.S. EPA certified at 27 mpg highway for 2011. But for 2012 Chrysler expects it to be certified at 30 mpg highway, an 11% increase. The 300's city fuel economy number also is expected to rise 5%, from 19 to 20 mpg.

The 300's impressive new highway fuel economy number comes from use of the ZF 8HP 8-speed automatic transmission. The impact of this unit, and some of the future technologies it enables, goes beyond the effect of adding three gears to Chrysler’s 5-speed automatic, for example, or adding a couple extra ratios to ZF's own 6-speed automatic.

The 8HP was chosen for its overall efficiency. So the potential future highway fuel economy of the Chrysler 300 could approach the “magic” 35 mpg number with the Pentastar V6 + 8HP powertrain.

The Chrysler applications —initially the 300 and Dodge Charger—are for a version of the transmission with a torque rating of 450 N·m (331 lb-ft). The Pentastar peak output is 260 lb- ft (353 N·m) and 292 hp (218 kW).

Doing it HIS way

ZF introduced the 8HP in 2009. Higher peak-torque-throughput versions (rated for 700 N·m/515 lb-ft and 900 N·m/662 lb-ft) are used by Audi, Range Rover, BMW, Bentley, and Rolls-Royce.

One physically small but technically important item is the “HIS” (hydraulic impulse storage), a transmission oil accumulator that provides a lower cost, quieter, built-in way to pressurize the transmission hydraulic system on an idle stop than by using an electric oil pump. The HIS accumulator permits vehicle driveaway just 350 ms after the engine restarts.

Powertrain stop-start and direct fuel injection, for which the 3.6 V6 was design-protected, are two technologies that provide potentially large fuel economy benefits as Chrysler looks to meet forthcoming regulations in the most cost-effective way.

The 8HP itself is a model of efficiency, claimed Dr. Mircea Gradu, Chrysler's Director of Transmission and Driveline Engineering. He pointed out that while there are other 8-speed automatics in use, this one has the following fuel-economy-enhancing features:

• There are only five friction locking elements (three clutches, two brakes), and in any gear just two are open. Further, a low-viscosity lubricant is used (5.6 cSt vs. 7.5 cSt in the Chrysler 5-speed), which reduces spin losses. So total energy losses from drag through the transmission are under 2%.

• Although there are four planetary gearsets, the total bill of material is less (467 parts vs. 471) than the ZF 6-speed with its three planetary sets. At about 91 kg (201 lb), weight of the 8HP is about 2-3 kg (4.4-6.6 lb) less than the ZF 6-speed, and the package size is the same. The 8-speed is just 3 kg (6.6 lb) heavier than Chrysler's 5-speed.

• The oil pump is offset and chain-driven, so it is sized according to engineering requirements, rather than by dictates of a conventional location on a transmission main shaft. The pump itself is responsible for at least one percentage point of the fuel economy boost.

• The torque converter includes an advanced damper adapted from the ZF 6-speed, which permits the converter clutch to lock up at lower speeds, even in top gear. The clutch can, however, partially unlock if necessary to reduce NVH in top gear.

• The relatively tall first gear (4.71) reduces the amount of torque converter multiplication required. With the early lockup of its clutch, the converter is used almost exclusively for vehicle launch.

• The transmission gearing (4.71, 3.14, 2.10, 1.67, 1.29, 1.00, 0.84, 0.67) means small ratio steps (1.50 or below) and a large overall ratio spread of 7.03 (4.71/0.67). The large spread contributes to fuel economy.

The rear axle ratios selected —2.65 for the 300 RWD, 3.06 for the AWD model—locate shift points according to brake specific fuel consumption (BSFC) maps. And further, Chrysler has 40 control maps that modify the shift patterns for different operating conditions.

Large skip-downshifts possible

Fuel economy is important, but the transmission still has to enable good acceleration and smooth shifting. The small ratio steps help improve shift quality. In addition, each shift requires just one application of a friction locking element and one release, and it takes just 200 ms, another contributor.

Some of those 40 control maps also improve performance and shift quality, and they cover both the rear-wheel-drive and all-wheel-drive systems. Adaptive strategies could permit large skip downshifts, such as 8th to 2nd.

The first Chrysler applications of the 8HP will be sourced from a ZF plant in Germany, Dr. Gradu said. However, Chrysler has plans for extensive use of the transmission and, as a result, will soon start to build it under license from ZF, on a special line at its Kokomo, IN, transmission plant, he said.

Although not announced, likely next applications are on the Jeep rear-drive lineup and the Dodge Durango.

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