Time was when four-wheel drive was simple. You turned your ignition key, popped the car in gear, and you were in full-time-four-wheel drive.

Then came the oil embargo. Anything that guzzled an extra drop of gas was targeted for redesign. So part-time-four-wheel drive became the rage. And with it came an evolution of new features: manual hub lock; automatic hub lock; shift-on-the-fly; push-button access. Unfortunately, none of the part-time versions offered the convenience and performance of full-time four-wheel drive.

Now, a new system is taking a giant step back toward convenience. The product of a team effort by Ford Motor Company and Borg-Warner Automotive, it combines full-time performance with part-time fuel economy.

Called Control Trac by Ford, it acts as an automatic form of four-wheel drive. Using sensors, microprocessors, and a specially designed electromagnetic clutch, the unit quickly switches from two-to four-wheel drive when it senses wheel slip.

Gone are the inconveniences associated with part-time-four-wheel drive. Drivers don't have to ruminate over their choices; they don't have to wonder whether they're in four-wheel drive; they don't need to possess a shred of knowledge about the technology. "This system is totally automatic," says Stephen G. Lyons, Ford Division general marketing manager. "You just set it and then you forget it."

Oddly, the market force behind the new automatic four-wheel drive system is just as quirky as the oil embargo that launched the trend toward inconvenience. Utility vehicles are automotive's fastest growing segment, and a healthy portion of new utility owners aren't yet familiar with four-wheel drive. Ford estimates that two-thirds of its Explorer buyers are new to the market. Hence, the need for a no-fuss system.

What's more, few utility drivers ever leave the highway. Though many envision themselves driving up mountainsides, estimates reveal that only 7% will ever go off-road. That means that four-wheel drive might play its most important role when the vehicle hits an icy patch on the way to the grocery store.

In the past, most part-time systems were less useful on the way to the grocery store. Because part-time transfer cases typically have no differential, manufacturers warned users to keep such vehicles in two-wheel drive on dry pavement. Otherwise, they could generate "wind-up torques" that would affect the vehicle's handling.

Control Trac suffers from no such operating limitations. Though it contains no differential, its use of microprocessor logic and the electromagnetic clutch enable it to address highway slippages without generating wind-up torques.

What's more, Control Trac is smaller, lighter, and, reportedly, more reliable than competing versions. "We've been reviewing this system for three years," notes Ford Explorer Chief Engineer Keith Takasawa. "It's gone through all kinds of testing: sand pits up to the wheel hubs; snow conditions; mountainsides; highway miles. It's a very robust design."

User friendly. The idea for electronically controlled four-wheel drive emerged at Borg-Warner Automotive's Sterling Heights, MI, facility in 1985. At the time, all automakers were endowing their engines with electronic control, and many were working on electronic transmission control. "We saw a need for a user-friendly, fuel-efficient, four-wheel-drive system," recalls Ronald A. Schoenbach, vice president of product engineering for Borg-Warner Automotive. "And we envisioned electronics providing it."

That year, Borg-Warner engineers thought out the rough concept, drew it up, and prototyped it on an existing vehicle. The prototype called for an electronically controlled clutch with a differential. The system was crude, though: Its original clutch controller was a rheostat.

Within a year, the controller had grown larger and more complex. A breadboard electronic version-complete with sensor amplifiers and bulky control modules-consumed the entire back end of a station wagon. Input data came from variable reluctance sensors installed at the front propeller shaft, rear propeller shaft, throttle, brakes, and steering. The goal: Control clutch actuation by controlling current.

Up to that time, Borg-Warner engineers had intended to use the clutch in conjunction with a mechanical differential. But, in 1988, they realized that electronic control had opened a new avenue. "We asked ourselves: "Why do we need a differential?'" Schoenbach recalls. "There was all of that iron, all that weight, all that cost. And here, we saw that we could control the clutch pack very precisely without it." Thus was born the Torque-On-Demand transfer case.

Such a design, however, was considered unconventional for a so-called part-time/full-time system. With a part-time/full-time system, users expect to employ four-wheel drive on the highway. But accepted practice held that a planetary or bevel gear differential was needed to prevent generation of wind-up torques on a dry road. Designs without mechanical differentials have long been known to cause particular problems during tight turns and parallel parking, when wheels on one side of the vehicle travel a different distance than wheels on the other side. Forcing all four wheels to operate in synchronization during such manuevers causes drive-line wind-up.

But Borg-Warner engineers were convinced that proper application of the electromagnetic clutch would alleviate drive-line wind-up, even without the presence of a differential. "It all depends on the control algorithms," explains Dan Showalter, chief engineer of advanced design. "You can get wind-up torque if you don't program it right."

The design team gained confidence in their concept during 1989, when they made a breakthrough in the clutch's control system. Development of closed-loop control smoothed its operation. Using it, the clutch made smaller adjustments, but did it more frequently. Ultimately, the microprocessor would review input from sensors every 20 msec and decide if the front axle needed more torque.

Using this technique, the unit's computer watched for wheel slip. If it sensed as little as half an rpm difference between the front and rear axles, it sent a power signal to the clutch. The clutch engaged, diverting torque to the front axle in 10% increments, until it alleviated the wheel slip. As a result, the system could control runaway wheel speed in as little as a third of a wheel revolution.

Drive-off. In 1991, members of Borg-Warner's engineering team travelled to Ford's light truck engineering facility in Dearborn. There, they presented the basics for the Torque-On-Demand transfer case. They told Ford engineers about the electromagnetic clutch, the closed-loop control system, the sensors, and the elimination of the planetary gear differential. "We slid a set of keys across the table and said: "Here, this is more than a nice, fancy paper study," Schoenbach recalls. "Here's an operating prototype. Go drive it. Tell us what you think.'"

Though Ford engineers reserved judgment, Borg-Warner executives believe that the prototype was a key to success. Although the transfer case was an early mechanical prototype, it was fully functional, with years of research and close to a million dollars in development behind it. "It improves your credibility when you can hand over the keys to their vehicle," Schoenbach says.

Months later, Ford engineers called Borg-Warner with a proposal: Build an all-wheel-drive system for the new Explorer, to be introduced in '95. Borg-Warner's all-wheel-drive model would go head-to-head in a drive-off against two other manufacturers' versions.

At the drive-off, staged in October, 1991, Borg-Warner showed up in Dearborn with three identical, dark red, Eddie Bauer Explorers. One of the three contained the Torque-On-Demand system, another incorporated a viscous coupled four-wheel drive, and the third used the electromagnetic clutch with a mechanical differential. Borg-Warner engineers never saw their competitors' systems.

In the drive-off vehicles, the Borg-Warner design team also incorporated another new concept: a rotary selector knob on the dashboard. The selector knob enabled users to choose two-wheel drive, four-wheel drive, or low-range-four-wheel drive. The decision to add the knob was based on feedback from customers who didn't like pushbuttons or couldn't see LED displays. The engineers borrowed a heater knob from a Taurus, backlit it, and installed it on the instrument panel. Result: The Borg-Warner four-wheel systems took on the appearance of finished products. "Our hardware looked like it was showroom," notes Robert L. Seaman, vice president of marketing and sales for Borg-Warner Automotive. "It reflected that we hadn't "kludged' it up in a hurry."

A blind jury evaluation followed, with a cast of engineers driving the vehicles up a slippery hill at the Ford proving grounds. By November, Borg-Warner received word that the Torque-On-Demand system-soon to be known as Control Trac-had been selected for the Explorer.

Desert testing. Still, the development work was far from finished. Led by Showalter, Borg-Warner engineers set out to bullet-proof the new transfer case. A key area of concentration: clutch plate material.

Early in the development, Borg-Warner had employed a sintered bronze clutch material that exhibited an operating condition commonly known as "stick-slip." The slippage had inspired engineers to replace it with a paper-based material mounted atop metal. The paper offered a better coefficient of friction and solved the slippage problem. Seaman credits Borg-Warner's automatic transmission components division for the solution.

Now, however, another dilemma surfaced: heat. Heat was burning out the new clutch material. To solve the problem, engineers studied the clutch's torque. "Heat was a problem, not because of the material, but because we had inadequate clutch torque," Showalter recalls.

Engineers beefed up the capacity and equipped a fleet of test vehicles, which they took to the Anza-Borrego Desert in southern California. They made 11 trips in all, subjecting the prototypes to heat, mountain driving, and deep sand. It was the most rigorous test possible for a four-wheel drive, with the clutches being called upon for almost continuous delivery of torque.

At first, the environment quickly burned out the clutches. But as engineers gained knowledge, the clutches improved. "It got to the point where we would go out there and thoroughly demolish the test vehicles, but the clutches would still look good," Showalter recalls.

Back at the lab, engineers tortured the four-wheel-drive system in every imaginable way. What would happen if the rear wheels lost traction while backing down a mossy boat ramp? William R. Kelley, manager of advanced engineering development, built a test stand to find out. "We asked one fundamental question: "Do we have enough torque capacity for every imaginable Explorer use?'" Kelley says.

When Ford engineers later questioned the use of powder metal in the clutch's ball-ramp cams, Kelley devised another technique for testing the cams at maximum load for millions of cycles. To simulate service conditions for a separate concern, he also worked with MTS Systems Corp., Minneapolis, MN, to build a special simulator. Says Seaman: "In my 21 years of transfer case and manual transmission development, I have never seen a product more thoroughly tested."

Light, quiet, simple. By the time the product was introduced late in 1994, it had been so thoroughly tested that it had served in two national off-road championship vehicles. At only 70 lbs, it also laid claim to being the lightest and smallest transfer case on the market. The next closest, according to Borg-Warner engineers, weighed in at 83 lbs and 100 lbs.

Control Trac's transfer case might also be the auto industry's quietest, says Kelley. Borg-Warner engineers employed magnesium to minimize weight, then used finite element analysis to improve its acoustic characteristics. The electronics, which once filled the back end of a station wagon, are no larger than a cigarette pack.

Since its introduction to the press late in 1994, Control Trac's evaluations have been unswervingly positive. "The transition from two- to four-wheel drive is nearly imperceptible, except for the additional traction," noted Automobile magazine in a typical review. Adds the Detroit Free Press: "The simplicity of the system is sure to make a hit with folks who want the security of four-wheel drive when the going gets slippery."

The system, engineers believe, fills a growing need among a new class of car buyers. "Customers are demanding that we make it simple and make it work," concludes Seaman. "And that's precisely what we've done here."

Four-Wheel Drive Primer

Few automotive components are more misunderstood than four-wheel drive systems. Following are explanations of basic four-wheel-drive categories.

Part time. These systems operate in four-wheel drive when they're off-road and two-wheel drive on the highway. Since gaining popularity on Army vehicles in World War II, they've appeared in many configurations. Part-time systems are lighter and offer better overall mileage, but generate undesirable torque effects on dry pavement if left in the four-wheel-drive mode.

Full time. Known also as all-wheel drive, these systems operate off-road and on. They use planetary or bevel gear differentials to allow wheel slip on tight turns on dry pavement. Large and heavy, they lost favor during the oil embargo of the 1970s.

Part time/full time. The best of both worlds. These systems incorporate a planetary or bevel gear differential, but typically must be manually shifted from two-wheel to four-wheel drive. Control Trac is the first automatic part-time/full-time system to use electronic control and no differential.

Control Trac's Impressive Debut

The fastest way for an automotive component to earn a reputation is on the racing circuit. Unfortunately, the reputation it earns may not always be a good one.

"Sometimes firms are a little reluctant to race because they don't want to be embarrassed by a big failure," notes Robert L. Seaman, vice president of marketing and sales for Borg-Warner Automotive. Never is such caution more warranted than in off-road races.

Yet, the Torque-On-Demand transfer case already has served as the four-wheel-drive system on the Borg-Warner-sponsored Rampage Racing Team. The transfer case has served on world champion off-road trucks in successive years: a Ford Bronco in the Class 3 World Championships in 1993; and the F-150 in the Class 4 in 1994. The Bronco never lost a race after it used the Control Trac transfer case. In all trucks, the four-wheel-drive systems never experienced a failure.

Off-road racing experience, Seaman says, serves as the ultimate test bed for four-wheel-drive systems. "We have not just relied on laboratory and proving ground experience," he says. "With the Control Trac, we've gone a step beyond."