Tech Briefs - Tokyo Motor Show supplier technology
February 2002
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Bosch twin differential
 
The radial-type, torque-sensing, limited-slip differential unit from Bosch acts as the center differential, splitting torque between the front and rear axles. Inside it is the axial-type torque sensing limited-slip differential that divides the power for the front axle between the two wheels.Click either image to enlarge |
Packaging is a key challenge for vehicle designers during this era of increasing interest in all-wheel-drive (AWD) vehicles. The conundrum is especially keen when manufacturers want to add AWD to an existing front-wheel-drive (FWD) vehicle. The problem is where to locate the center differential in an already crowded FWD transaxle case. A common approach, such as that employed in the Ford Escape and the Acura MDX SUVs, is to forgo a center differential, using a computer-controlled coupling to send power to the rear wheels when it detects the front wheels beginning to slip.
The problem with that approach, according to Bosch Automotive Systems Corp., is that "pain is needed for the painkiller to work." Pain, in this case, is slippage in the traction of the front tires. The resulting understeer is undesirable and is frequently corrected by activation of a vehicles' stability control system, as well as the transmission of power to the rear wheels by the computer-controlled coupling.
The result of such an arrangement is a vehicle that handles very much like a normal FWD car, but that can respond to a loss of traction at the front by then sending power to the rear wheels. For an SUV that may occasionally encounter slippery conditions, the compromise is satisfactory.
But for sedans, whose owners may not enjoy the understeer of a FWD car, a more balanced power transfer is needed so that the rear wheels do as much work as the front wheels. That is the job of Bosch's new compact twin differential, which incorporates a center differential into its design. Power is divided between the front and rear axles before the front-wheel differential, then splits it between the two wheels. The rear differential also splits the power it receives from the center differential that is integrated into the transaxle via a power takeoff and driveshaft to the rear.
The Bosch twin differential lets manufacturers tailor the front-to-rear torque bias to best suit the application, considering such factors as the vehicle's weight distribution and its target market. Sport sedans, for instance, suggest a rear torque bias to give the driver better ability to "steer with the throttle." An SUV might use a 50/50 split and a family car might have a forward torque bias to send more power to the wheels at the heavier end of the car.
The twin differential combines an axial torque-sensing differential for the two front wheels and a radial torque-sensing center differential to split power between the front and rear axles. Because it acts to reduce instances of traction loss, rather than responding to such instances, the twin differential reduces the frequency of traction and stability control system activation. It also works with such systems when slippage occurs, reducing the wear on brakes from their use by the traction and stability control systems.
Sachs DynaStart
 Sachs' entry in the burgeoning race to 42-V integrated starter/generators and hybrid-electric powertrains looks similar to competing designs, but targets durability and low-cost manufacturing as its calling cards. |
Suppliers are moving quickly to support manufacturers' migration from 14- to 42-V electrical systems, each seeking to expand its traditional niche to include new components. While it seemed natural for long-time alternator and starter suppliers to develop flywheel-mounted integrated starter/generators (ISG), clutch expert Sachs may be a less obvious candidate.
However, the company believes that its experience inside the harsh environment of the transmission bellhousing positions it to offer ISG flywheel and clutch modules to manufacturers. Combining that experience with recent technological improvements in electric motors used for automotive power, its DynaStart ISG has benefits over similar systems.
A specific advantage that Sachs cites is a large air gap of 1-1.5 mm (0.04-0.07 in) between the rotor and stator that lets the ISG mount directly to the crankshaft. Crankshaft wobble could cause destructive contact between the parts if the gap was insufficiently large. Use of NdFeB permanent magnets permits torque as high as 400 N•m (295 lb•ft) and power of 12 kW from the DynaStart when used as an assist motor in a hybrid-electric vehicle.
As a starter, the DynaStart draws a battery current of between 100 and 300 A, and as a generator it develops a high level of power at idle and is efficient through 85% of the internal-combustion engine's operating range.
A simple single-tooth winding makes the DynaStart very suitable for large-scale production, according to the company, at prices described as "attractive." Sachs has already inaugurated pilot production, with a production line capable of building 1000 units per year. Once manufacturers move to 42-V systems, the company plans to expand to large-scale production of 100,000 units per year.
