If you've looked under the hood of almost any car in the last five years, you might have noticed that there isn't a lot of room in there. The proliferation of electronic controllers and other components in the engine compartment combined with the push for more room in the passenger compartment is placing a premium on underhood real estate.

With automaker engineers acting like high-stakes land brokers, the automotive electronics industry is continually looking for innovative ways to reduce the size of underhood components - as well as increasing the number of features and their reliability.

Linking several electronics systems together via multiplexing, with one or two wires doing the work of several, is one way to solve this puzzle, Making components smaller also helps.

Multiplexing has been around for a quarter of a century, starting with engine-management systems. General Motors Corp. introduced the concept in 1979 to provide diagnostic access to engine-control electronics. In 1981, only 10 GM platforms had limited networks for diagnostic purposes only. Four years later, the Cadillac Eldorado bowed with a multiplex system supporting vehicle operation. By 1993 all GM passenger cars had multiplexed engine management systems. And by 1999 every GM vehicle will feature a multiplexed engine-control system.

Chrysler Corp.'s 1995 product lineup has several examples of multiplex technology in the transmission, body, instrument panel, air conditioning, overhead console, air bag, ABS, trip computer and security systems. Most Chryslers have several multiplex systems. Neon and Jeep Cherokee have one each.

Top-line luxury models like the Mercedes-Benz S-Class, BMW 850 and Infiniti Q-45 implemented multiplex systems several years ago.

The most "visible" use of multiplex wiring systems, however, is the 1995 Lincoln Continental's Memory Profile System, which gives two different drivers their choice of settings for features such as steering assist, suspension firmness and audio presets.

UT Automotive and Motorola Inc. worked together with Ford Motor Co. engineers to design a network capable of communicating more than 200 functional messages. In the process, reliability is increased through fewer connections, says Ford. Multiplexing also saves 100 wires and eight pounds on the Continental.

The goal of multiplexing is to reduce the number of wires and circuits. The total number of circuits in the 1995 Continental is 1,634 or 318 more than in the '94. But the '95 has more features, thanks to the Memory Profile System. Had this year's model included the same features as the '94, it would have required only 1,199 circuits.

"The mission was to prove the benefits of multiplexing for Ford," says Mark Fouts, UTA's principal engineer-wiring systems engineering.

Continental's multiplex system links 10 modules with a two-wire bus. Modules control lighting, vehicle dynamics, driver door and seat, powertrain, air conditioning, instrument panel, ABS and traction control, cellular phone and stereo system.

"Intelligence is resident in each module," says Mr. Fouts. "All modules interpret each electronic message passing through the system, but don't react unless they're programmed to do so."

Robert Jensen, UTA vice president-wiring systems product engineering, says multiplexing is cost-effective because of the features Ford was able to add for very little cost. As applications increase, he says, costs naturally will come down.

"You have to design the whole system from a clean sheet of paper from day one," says Mr. Jensen. "If you add it, you'll get redundancy and miss the full benefits of system integration."

But under the hood isn't the only place where multiplexing offers advantages.

"Driver doors are the most difficult places to wire," says Mr. Jensen. "They have the most content and open and close more often. Multiplexing allowed us to offer more features than we could have without it."

There are 42 circuits in the Continental's driver door. Without multiplexing, there would have to have been 65 circuits, creating a bundle of too many wires to snake to the engine compartment.

In addition to multiplexing, technology is evolving to make electronic components smaller, creating even more room under the hood, behind the dash and other places throughout the vehicle.

The quandary is that while real estate under the hood is at a premium, the demand for functions also is on the rise. Add to that the problem of heat generated by electronic components, and you can imagine the challenges.

"The factors affecting miniaturization include cost, volume and heat dissipation," says Marc Simon, vehicle electronics systems program manager at Siemens Automotive.

The focus on cost drives miniaturization because of the cost of materials. In theory, the less material, the less the system will cost. Obviously smaller components make room for new components and/or allow for a reduction in the size of the engine compartment itself. Miniaturization is limited by heat dissipation, says Mr. Simon. Electronic components generate heat and are in a hot environment under the hood.

As one example the volume of Siemens Automotive's 1986 engine management ECU will have shrunk 72% by the time the 1998 version debuts. In 1986, an 8-bit ECU with 35 input-output pins filled a 90- [in.sup.3] (1,475 [cm.sup.3]) metal box. By 1992, a 16-bit ECU with 55 pins occupied a 60-[in.sup.3] (983-[cm.sup.3]) box and had 30% more computing power.

This slimming down was made possible by an advance in manufacturing technology that allowed robots to more precisely place microprocessors on fiberglass and epoxy printed circuit boards (PCBs).

Siemens goes to a flexible aluminum circuit board for its 1996 model, allowing the board to be folded over and reducing volume from 60 [ins.sup.3](983-[cm.sup.3]) to 42 [ins.sup.3] (688-[cm.sup.3]). Also, the aluminum substrate PCB eliminates the need for a conductive metal casing. Using a plastic cover reduces weight from 25 oz. (709 g) to 11 oz. (312 g).

The next step in the miniaturization process, says Siemens, is to optimize components with higher levels of application-specific integrated circuits (ASICS). Doing that, Siemens has a prototype of an ECU for 1998 that takes up less than 25 [ins.sup.3] (410[cm.sup.3])of space.

Coupling these miniaturization techniques with a ceramic circuit board, which has greater heat dissipation characteristics, Siemens can mount the ECU much closer to the engine than with other substrate materials. Positioning the ECU closer to the engine means there can be fewer and shorter wires as well as fewer connections, increasing reliability. This also allows the ECU to be part of an integrated molded plastic air-fuel delivery system, further maximizing underhood space.

Siemens also is shrinking its air-bag system ECU. The 1987 model, which had to accommodate remote sensors, filled a 48-[in.sup.3] (787[cm.sup.3]) void. In 1993, even with an integrated single-point sensor, the ECU downsizes to 17[ins.sup.3] (279[cm.sup.3]). The 1998 Siemens air bag system ECU will further integrate components and shrink to 7.5[ins.sup.3] (123[cm.sup.3]).

"Sometimes the whole principle of the system increases miniaturization," says Berthold Wolfram, passenger safety systems program manager at Siemens.

Siemens is not alone in reducing the size of its ECUs. Other suppliers of electronics systems, such as Bosch, ITT Automotive and Kelsey Hayes, also are making strides in miniaturization.

The ECUs for Kelsey's new EBC 25 and EBC 30 ABS are up to 40% lighter and half the size of the modules for its EBC 10 and EBC 15 systems introduced three years ago. The new system also can accommodate traction control and serve as the basis for a vehicle stability system.

"A large part of (the size decrease) is a 30% reduction in the number of assembled parts that go into the hydraulics," explains Bob Sullivan, vice president and general manager of Kelsey-Hayes Technologies. On the electronics side, the new ABS ECU has an integrated coil module that combines the solenoid coil and the circuit board, he adds. All of this helps keep the company among the ABS price leaders. Two years ago the company introduced a $200 ABS system.