Using bearing technologies to boost mpg

  • 25-Nov-2009 05:27 EST
Concentric Cam VCT.jpg
The INA Concentric Cam VCT with Integrated Central Valve, check valve, and filter.

A valvetrain technology that debuted on a high-performance sports car engine to improve performance and emissions compliance is being targeted across the vehicle spectrum as a fuel-efficiency enhancement.

The Concentric Cam VCT (variable cam timing) exhaust phasing system developed by Schaeffler Group's INA division was first used on the 2008 Dodge Viper 8.4-L V10 engine. As fitted to the OHV Viper engine, the system's hydraulic solenoid valve is located within the cylinder block.

Schaeffler currently is testing two variants of the Concentric Cam VCT featuring an integrated central hydraulic control valve suitable for both overhead-camshaft and OHV engines, said Craig Dupuis, Product Development Engineer, Engine Components for Schaeffler Group USA.

The integrated unit has only one oil-feed point. "With a remotely located hydraulic solenoid valve, the engine manufacturer must provide a means of supplying the cam-phaser with two oil control passages—addressing advanced and retard directions—in addition to a pressurized oil supply to the valve," explained Dupuis. "With central valve technology, only a pressurized oil supply needs to be provided to the VCT system."

The Concentric Cam VCT enables independent phasing of the intake and exhaust valves. "Depending on the application, applying a Concentric Cam VCT to an SOHC or OHV engine could yield a fuel economy benefit of up to 5%," Dupuis said.

For DOHC engines, a compact version of the Concentric Cam VCT is in development. This system is approximately 25 mm (0.98 in) thick and has an outside diameter of approximately 85 mm (3.3 in). It weighs approximately 800 g (26 oz). Used in the DOHC configuration, the system allows engine designers to adopt advanced combustion concepts that were not previously possible with a conventional cam phaser, Dupuis said.

On an intake camshaft, a late-valve-closing strategy could translate to reduce pumping and throttling losses on a gasoline engine, according to Dupuis. If used on a diesel engine, "the effective compression ratio could be varied to reduce combustion temperatures and thus lower NOx emissions," he noted.

Depending on engine configuration and application, applying a Concentric Cam VCT to the intake side of a DOHC engine equates to potential fuel economy gains of 5 to 10%, claimed Dupuis. It also offers fuel-efficiency synergies when combined with other technologies such as engine downsizing, he said.

New transmission bearing application

Another fuel-focused technology—tandem ball bearings in a transaxle application—is heading toward its first North American application. Development was enabled by Schaeffler's enhanced Bearinx analysis software.

"In the past year, our software programmers added the ability to do friction power energy consumption analysis over the entire vehicle duty cycle," noted Bob Southam, Schaeffler Group's Director of Automotive Engineering for Transmission Applications. 

By replacing four tapered roller bearings with four FAG tandem ball bearings, the analysis showed a 50% reduction in total bearing energy consumption, Southam said.

"We're unsure what that 50% reduction will mean in terms of an mpg increase, but our customers are addressing that and the resulting calculations will be validated through testing," he added.

While bearings and shafts typically are made of steel, the transmission housing that supports the shafts and bearings typically is made of aluminum. The dissimilar metals create unwanted side effects.

"When tapered or tandem bearings are used as shaft supports, axial spacers are generally used between the outer race of one of the bearings and the housing so that when the housing is bolted together, a defined preload is induced on the bearing shaft assembly," explained Southam.

A thermal compensation element (TCE) functions as an axial spacer, expanding at the differential rate between the housing and shaft to maintain a constant preload of the bearing shaft assembly. While a TCE is not a new invention, "the INA product is unique because it is a separate, stand-alone product that can be added to the bearing shaft housing assembly," said Southam.

Prototypes of Schaeffler's proprietary elastomer TCE, which is encapsulated between two axially stamped steel sleeves, are undergoing in-house testing. Retrofitting into a transmission requires axial space of approximately 7-9 mm for most applications. Ideally the TCE would be placed in the housing bore between the bearing outer race and a housing shoulder or shim.

The TCE will enable the axial position of shafts and gears to be accurately maintained at any operating temperature. The product has "the potential of reducing gear noise and improving gear life as well as enabling bearing shaft assemblies to be set with lower preloads for lower bearing friction," claimed Southam.

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