Tech Briefs
Bosch EHB for new SL Class
 Among the most important performance features of the SL's electrohydraulic braking system are the dynamic building up of brake pressure and the precise monitoring of driver and vehicle behavior using sensors. Click to enlarge  |
The new Mercedes SL Class has an impressive package of cutting-edge dynamic handling control systems that includes a new electrohydraulic brake system Mercedes-Benz calls Sensotronic Brake Control (SBC) along with Active Body Control (ABC) and the Electronic Stability Program (ESP). Among the most important performance features of the SBC electrohydraulic braking system are the dynamic building up of brake pressure and the precise monitoring of driver and vehicle behavior using sensors. In emergency situations, SBC instantly increases the pressure in the brake lines and applies the pads to the brake discs so that they can grip instantly with full force when the brake pedal is pressed. Furthermore, thanks to variable brake proportioning, SBC offers enhanced safety when braking on bends.
SBC was developed with the help of Bosch, which labels is electrohydraulic brake (EHB) system. The system provides the brakes with a fluid supply from a hydraulic high-pressure reservoir sufficient for several braking events. A piston pump driven by an electric motor enables a controlled brake fluid pressure of 14 to 16 MPa (2030 to 2320 psi) from a gas diaphragm reservoir. When the brakes are activated, the EHB control unit calculates the desired target brake pressures at the individual wheels. Braking pressure for each of the four wheels is regulated individually via a wheel pressure modulator, which consists of one inlet and one outlet valve controlled electronically. Normally, the brake master cylinder is detached from the brake circuit, with a pedal travel simulator creating normal pedal feedback. If ESP intervenes, the high-pressure reservoir supplies the required brake pressure quickly and precisely to the wheel brakes.
"The crucial performance feature of the EHB is that it raises braking comfort," according to Gunther Plapp, Executive Vice President, Development, ABS and Brakes Division of Robert Bosch GmbH. The vehicle controller intervenes early and stabilizes the vehicle without the typical ABS feedback and is almost undetected by the driver. This allows vehicle guidance functions such as ESP and Adaptive Cruise Control (ACC) to be designed in a less intrusive way. Additional advantages include a dry brake function, which carries out regular short and weak brake impulses on wet roads to dislodge the brake-disc water film and ensure full and immediate braking. A "traffic assistant" can brake the vehicle with predefined deceleration when the driver removes his/her foot from the accelerator pedal; a "drive away assistant" can prevent rolling backward on a hill and simplifies the drive away process.
- Kevin Jost
ZF six-speed debuts on BMW's 7 Series
 ZF brought its 6HP26 six-speed automatic transmission from concept to production in less than four years. |
After appearing as a concept nearly two years ago at the SAE 2000 World Congress, the ZF Group began producing in July the "world's first" production six-speed automatic transmission, the 6HP26. The passenger-vehicle transmission went from development to production in less than four years, and will appear first on the redesigned 2002 BMW 7 Series.
In a July article in AEI ("Transmission options," page 64), ZF's Group Vice President for Product Development Gerhard Wagner said that many "industry experts predict that (its) six-speed automatic transmissions will become the preferred solution for vehicles with longitudinally installed engines because of their compact designs."
The 6HP26 is not only more compact, but in test simulation it was shown to offer a 13% reduction in mass, 5% better acceleration, and a 5-7% fuel-consumption savings while offering "fundamentally better performance" than the standard five-speed automatic transmission it replaces, the 5HP24. Fuel consumption was achieved not only through the addition of the sixth gear, but via a new low viscosity oil developed by ZF as well as Stand-by-Control (SBC). SBC is an electronic control system that essentially controls the input clutch in the transmission so that the engine is disconnected from the driveline when the vehicle is stationary, even when in gear.
The new transmission for high-performance cars with eight- to twelve-cylinder engines has a maximum input torque of 600 N•m (440 lb•ft) at 4200 rpm. The overall gear-ratio range of 6.04 was selected to achieve comparatively low speeds in the "long" sixth gear ratio and at high speeds.
ZF engineers based the six-speed automatic transmission on the Lepelletier geartrain. Using this concept, the 6HP26 was designed with significantly fewer components. For the current five-speed automatic transmissions, three geartrains and six clutch elements were needed; the new transmission requires only one simple and one Ravigneaux geartrain in combination with five clutch elements. The result is a transmission with a mass of only 89 kg (196 lb) with oil. It is also 50 mm (2 in) shorter.
The use of plastic also reduced mass, with the oil pan and base body of the Mechatronik module no longer made of metal. The Mechatronik module is a combination of hydraulics and electronics that fine-tune all shifting processes in the transmission for enhanced shift quality and increased driver comfort. The shifting electronics not only give signals to the driver, but also receive information about the current driving situation; thus, it is able to recognize and react to (with optimal shift timing) the condition of the transmission, drag resistance (e.g., weight and slope), and driver behavior. In this way, it can range from very sporty to very economical driving styles.
According to ZF, it will be adding to both the top and bottom ends of the six-speed transmission range, covering a torque range from about 300 to 750 N•m (220 to 550 lb•ft).
- Jean L. Broge
Improved Goodyear run-flats for Lexus
 Inside the Goodyear Eagle F1 GS-D EMT run-flat tire. Click to enlarge |
New, lighter-weight run-flat tires from Goodyear are optional on the 2002 Lexus SC 430 sedan. Goodyear's new Eagle F1 GS-D EMT run-flat tire in size 245/40ZR18 culminated in 18 months of tire development and "optimization of tire shape and insert and tread compounds," said Jan Racke, Lead Engineer for Goodyear's Asian original-equipment passenger-tire programs.
Lexus specified that the ride, handling, and rolling resistance of the run-flat tire equal conventional tire performance. According to Racke, the sidewall inserts used for reinforcement employ a special rubber compound for improved run-flat mileage performance without increasing tire weight or sacrificing ride performance.
Goodyear's run-flat technology, introduced in 1993, features a sidewall that is built to maintain its shape and driveability after losing all air pressure. The tires often perform so well when "flat" that cars must come equipped with sensors to alert the driver to lost pressure; otherwise, a tire may be driven on until it is damaged beyond repair. The outer computer-designed mold shape is said to improve ride and handling while interior compounded deflation supports sustain the vehicle when the tire is deflated.
In addition to improved ride and handling, the tires provide low rolling resistance for improved fuel efficiency and wet handling, said Byron Meyer, Marketing Manager for Goodyear original-equipment consumer tires, as well as offer the comfort of never having to change a flat.
Designed to be driven with no air pressure for up to 160 km (100 mi) at 89 km/h (55 mph) and still be fully repairable, the F1 GS-D EMT represents an 80-km (50-mi) improvement over the Eagle Aquasteel, Goodyear's previous generation of run-flats.
Run-flat tires are a $400 option on the 224-kW (300-hp) SC 430. Models that come equipped with the tires will not carry a spare, Racke added.
Goodyear has several run-flat tire projects under way at its technical centers in Akron, OH, and Luxembourg in Europe, and is in discussions with inflation-pressure sensor firms to develop more standardized run-flat tire systems.
"We're finding that specifically tuning run-flat tire performance on new cars such as the Lexus SC 430 is the best way to increase run-flat capabilities. Look for more applications down the road," Meyer added.
- Jenny R. Hessler
