A dry permanently engaged starter, a latching piston assembly and mid-lock Variable Cam Timing (VCT) highlight next-generation stop-start solutions that enable automatic shut down and restart of an internal combustion engine to reduce engine idle time.
Combining the technologies, currently in development at Schaeffler Group, on a passenger vehicle equipped with an automatic transmission will net a 4 to 8% city cycle fuel economy gain, according to Schaeffler Group engineers who provided AEI with details during an exclusive interview at the firm's Troy, MI, R&D center.
With multiple patent applications for the stop-start components' packaging and design-features, Schaeffler's permanently engaged starter uses a dry, sprag-type one-way clutch with a much larger working radius of the inner ring and the outer ring, compared with a conventional, oil lubricated one-way clutch.
"The larger radius reduces the required forces, which in turn reduces the stress on the clamping sprags and allows the dry design to succeed," said Jeff Hemphill, Region Director of Technical Product Development for Schaeffler Group USA.
This system's unique design also means that a path for oil to reach the one-way clutch from the engine is not needed, simplifying the implementation, according to Hemphill. In addition, there is no drag on the engine because additional oil seals are not required on the axially-compact dry one-way clutch.
Another feature of Schaeffler's stop-start solution is a patents-pending latching piston assembly that keeps the transmission in first gear for engine restart. Key components of the assembly are an over-stroke piston (which nests on the main piston) to operate a snap-spring that holds a pop-off valve in a closed position. This prevents oil drainage from the transmission piston.
"The latching piston helps provide torque as soon as the engine is rotating, which occurs in approximately 400 ms—nearly the same speed as an electrically driven pump," said Hemphill.
Automatic transmissions need the engine running to maintain hydraulic pressure, which is why existing production stop-start solutions use electric pumps or large accumulators. In contrast, the latching piston assembly requires no electric power when the engine is off to maintain the automatic transmission's hydraulic load.
Schaeffler's mid-lock VCT enhances the performance of a stop-start system. Most conventional VCTs lock the rotor and stator together at an end stop-meaning a full advanced or a full retarded position. But an end-stop default lock position limits the camshaft timing design strategy, according to Craig Dupuis, the company's Product Development Engineer for Engine Components.
"Because the rotor can be locked in an intermediate position between the end stops on a mid-lock VCT, this intermediate locking position permits an optimum camshaft timing setting for reduced cold-start emissions as well as reduced NVH during engine start and at idle speed," explained Dupuis.
A mid-VCT's locking mechanism uses hydraulic pressure to unlock.
"To lock at engine shut down, the mid-lock VCT uses a combination of hydraulic pressure and camshaft torque to move the camshaft into its default locked position," said Dupuis.
In a typical engine start event, the oil pressure required to operate a VCT does not arrive until after engine ignition occurs, but Schaeffler engineers have developed a solution that allows the VCT to unlock and operate the cam-phaser prior to ignition.
Schaeffler has tested multiple engines with a prototype active pressure accumulator installed within the camshaft.
"It is possible that the patents-pending active pressure accumulator can be combined with a mid-lock VCT to enhance the locking and unlocking function and allow for cam phasing in operating conditions where there is little to no oil pressure," said Dupuis.
According to Hemphill, Schaeffler's dry permanently-engaged starter, the latching piston assembly, mid-lock VCT, and active pressure accumulator could be production-ready as early as MY2013.
"These low-cost, stop-start solutions will be more beneficial to vehicles equipped with gasoline turbocharged direct-injection engines versus conventional fuel-injected engines," he noted.