Robert D. Mitchell
Product Information Manager
ALL SEASON TRACTION AVAILABLE ON MOST 1994 BMW MODELS TO ENHANCE DRIVER CONTROL WHEN THE GOING GETS SLIPPERY Woodcliff Lake, New Jersey...Much of BMW's engineering effort is devoted to enhancing the driver's ability to control his or her vehicle, especially under challenging driving conditions. For 1994, the Bavarian automaker has taken another decisive step in this direction by making its All Season Traction system available in all 6-, 8- and 12-cylinder models as either optional or standard equipment. Many types of traction control systems are available on the market today. At the bottom end of the technological spectrum, some respond to incipient wheelspin by applying the brakes to the driving wheels only; others merely reduce engine power. BMW has taken the "high road" by engineering a system that not only does both -- and does so by relatively sophisticated means -- but adds a further mode of operation to help improve driver control under conditions of reduced traction. Appropriately, we call our system All Season Traction. STANDARD OR OPTIONAL ON ALL 6-, 8- AND 12-CYLINDER MODELS FOR 1994 In 1993, the All Season Traction system was available only on our V-8 5-Series models and the 7- and 8-Series. For 1994, availability has been extended to all four BMW Series as follows: 3-Series - optional on all 6-cylinder models (325i Sedan, 325is Coupe, 325i Convertible) 5-Series - standard on 530i Touring, optional on all other models 7-Series - standard on 750iL, optional on 740i and 740iL 8-Series - standard on all models. According to the technology employed in the various models, the All Season Traction is either fully integrated with the anti-lock braking system (ABS) or configured as a separate control unit that utilizes the ABS wheel-speed sensors. In either case, the operation is the same and the basic system components are as follows: Electronic control system, with inputs from wheel-speed sensors and outputs to engine controls and brake system Engine controls (electronic throttle, Digital Motor Electronics engine-management system) Brake hydraulic control unit (shared with ABS). SOPHISTICATED COMBINED CONTROL OF ENGINE AND BRAKES TO LIMIT WHEELSPIN All Season Traction influences both engine power and the rear brakes (all BMW models sold in the U.S. have rear-wheel drive) to limit wheelspin. The system continually processes data from the ABS wheel-speed sensors. Anytime a drive wheel begins to lose traction -- this is indicated by one or both drive wheels significantly exceeding the rotational speed of the other wheels -- the All Season Traction system goes into action. It does so in the following steps: Reduction of engine torque. BMW has engineered this function to achieve not only smooth, driver- friendly action but also minimum effect on exhaust emissions. Four types of action take place to reduce engine torque once the system has sensed incipient wheelspin: Progressive closing of the electronic throttle. This servo-controlled throttle, which has no mechanical connection to the accelerator pedal, functions by electric/electronic means either in tandem with the normal mechanical throttle (6- and 8-cylinder engines) or as the normal throttle (V-12 engines, in which there are two electronic throttles). Retarding of ignition timing. Via the DME system. Smooth phase-out of ignition and fuel injection. Also via the DME system. Prevention of any shifting by automatic transmission (not applicable with manual transmission) so as not to add a possibly disturbing influence. Selective application of rear brakes. If the reduction of engine torque is not sufficient to bring wheelspin under control, All Season Traction then begins to apply the rear brakes -- individually as one or the other wheel tends to spin until the spin is brought under control. Extensive tests have shown that there is no perceptible increase in brake wear even if All Season Traction is used often. ADDITIONAL CONTROL ACTION UNDER DECELERATION An additional refinement of All Season Traction, not found in many competing systems, is the so-called Motor Speed Regulation (MSR) under deceleration. On very slippery surfaces, merely letting off the accelerator pedal could cause enough engine-braking effect (i.e., the engine slowing down the car) to induce wheel slip. If wheel slip begins to occur under deceleration in this manner, MSR utilizes the system's sensing and control capabilities to increase engine speed slightly, achieving a further improvement in driving stability under these difficult conditions. BUILT-IN SAFEGUARDS Many equally sophisticated safeguards have been built into the All Season Traction system. If, for example, the driver applies the brakes while the system is acting on the brakes to control wheelspin, the system's control function is immediately canceled. And because it is inherently smoother to control engine torque than the brakes, the system releases rear-wheel brake action as soon as engine torque has been reduced to a level that will control wheelspin. The system is also self-diagnosing. It includes two redundant main microprocessors, which continuously compare data; if they aren't identical, the system switches off automatically. Anytime a system fault is recognized by the diagnosis circuitry, an instrument-panel warning light comes on; every relevant function (individual wheel-speed sensors, hydraulic valves, low brake fluid, idle speed, etc.) is monitored. Under certain rare conditions, such as starting off in deep snow, traction may be better served by switching off the system. This is done with a console switch; however, so that the system is always ready to perform, each time the ignition is switched on, All Season Traction is automatically switched on too. In the BMW models available with manual transmission (325 models, 525i and 530i Sedan, 850CSi, All Season Traction offers a rare combination of capabilities: Few competitive models are available with traction control and a manual transmission. BMW believes that All Season Traction makes a significant contribution to Better Driving by helping the driver cope with difficult road conditions, and is pricing the system so that it is highly competitive with other makes' traction control systems.