A trucker's dream rig gets 6 m.p.g.

It may not seem like much to someone with a fuel-stingy economy car, but to the people who build heavy-duty trucks, six miles per gallon will be a dream come true.

And it willbe the culmination of a hectic research effort aimed at improving truck technology. As with the auto industry, the effort began in earnest following the oil embargo days of 1973-74, as truck manufacturers fought for more fuel economy and to maintain sales through two recessions.

Considering that most heavy-duty trucks are getting 3 1/2 to 4 1/2 miles to a gallon of diesel fuel, reaching six miles a gallon would mean a fuel saving of more than 70 percent.

A report on a recent test of an engine built by the Cummins Engine Company stated that the 72,000-pound truck "turned in an impressive 5.25 miles per gallon average fuel economy" on a trip from Florida to California.

To reach this lofty level, US truck manufacturers are scrambling to modify engines, cut weight, make tires roll more efficiently, and even streamline the bodies of their trailers.

At the same time, they are being forced to comply with tougher air pollution and noise standards. But these standards comes from government, not the marketplace. Cus tomers who spend $50,000 to $80,000 on a vehicle provide all the pressure truck builders need to make the 18-wheelers more fuel efficient.

"Trucks are tools," said Sheridan Brinley, spokesman for the Motor Vehicle Manufacturers Association. "People buy them to perform a job, and looks are secondary." Many trucks today travel 100,000 miles a year. Mr. Brinley explained. A 25 percent fuel savings on such a vehicle can mean savings of over

Also, truck experts point out, while six miles per gallon migh not seem like much, it is far more fuel efficient than a car, considering the weight involved. An automobile with an mpg of 24 may be six times as fuel efficient as the best truck. But it might only weigh 4,000 pounds, including pasengers. A truck and its cargo, meanwhile, often weights 80,000 pounds, or 20 times as much.

While truck and engine manufacturers work on the engines, the truck builders and companies that use trucks are taking some of the easier steps to save fuel. Many trucks have been carrying wind deflectors on top of their cabs for several years.

"There can cut wind drag up to 35 percent at 55 miles per hour," Mr. Brinley said. "And they can mean a fuel savings of 2-to-6 percent said. "Any they can mean a fuel savings of 2-to-6 percent, depending on the type."

And while steel-belted radial tires are becoming increasingly popular on automobiles, their reduced resistance to rolling almost makes them a necessity on trucks.

A truck that is ligher is also less resistant to roling. So manufacturers are taking weight off faster than a new member of Weight Watchers.

"Anything we can take out of the truck enables the operator to haul more payload and stay within legal weight limits," said Walter May, senior vice-president for production and engineering for Mack Trucks Inc. If payload is not a problem, he added, less weight means the engine doesn't have to work as hard -- more fuel savings.

To cut weight, Mr. May explained, Mack and other companies are trying to find as many places as possible to use something lighter than steel. "We're using aluminum and fiber glass in as many noncritical areas as possible," he said. Fiber glass is being tried on many parts of the cab body, including the hood, fenders, doors, instrument panels, and battery boxes. "We're considering making the whole cab out of fiber glass," he added.

Noting that the General Motors Chevrolet Division has announced fiber glass springs for its 1981 corvettes, James Cote, an engineer with the company's GMC Truck Division, expects the material will one day be used in truck springs.

But it is in the engines that manufacturers have the most room for innovation , and where the greatest fuel saving lie. Here, terms like turbocharging, turbocompounding, torsional isolation, and even ceramics are being bandied about by an energy-conscious industry.

Perhaps the most promising innovation is turbocompounding, a technique that improves on the benefits of turbocharging to provide more power while taking much of what was formerly "waste" heat generated by the engine and using it to produce even more power. "The exhaust from the turbocharger is ducted to a low-pressure power turbine which is coupled to the engine crankshaft.

One of the effects of such measures is to reduce the amount of horsepower needed to move a truck.

"It takes less horsepower to drive a truck today than its used to," said Carl Ahlers, assistant vice-president for research and engineering at Cummins Engine. "We think all the things going on in the marketplace are going to work to drive horsepower down."

The never engines, explained Jerry Holmes, spokesman for the Detroit Diesel Allison division of General Motors, are designed to operate at a higher torque, which means they can reach their maximum power at a lower RPM [revolutions per munite] level. "This means you can pull heavier loads at slower [engine] speeds and your engine isn't doing as much work," Mr. Holmes said.

"The new engines can operated at a greater RPM range, which means they don't need as many gears," said Dario Sabatini, president of Dario's Diesel Service in Worcester, Mass. "Without so many gears the driver doesn't have to shift as often." As a result, trucks with 10.13, or even 15 gears are expected to be replaced by those with eight or nine speeds.

One of the more interesting concepts in engine technology is still in the experimental stage -- ceramic parts. So far, Cummins Engine is the only firm looking into this idea seriously, but it has its origins in an idea that is thousands of years old: a brick oven retains heat longer than a steel one.

Noting that 30 percent of an engine's heat is sent out the exhaust pipe and 35 percent goes to the cooling system, Mr. Ahlers points out that this "leaves only 35 percent that comes out as shaft work . . . so over the two-thirds of the heat is wasted." By saving some of this heat in ceramic partS, the engine does not have to produce as much original heat.

The primary drawback to full development of ceramic parts, or even a totally ceramic egine, Mr. Ahlers says, is stress.

"We have ceramics that can stand up to the heat," he says." But not up to the stresses of an engine. They just break up from the force of all the work they have to do."

The truck industry has given itself some incentives to save fuel through the voluntary truck and bus fuel conservation program. Set up in 1975, the program involves the Departments of Transportation and Energy, the Environmental Protection Agency, and the truck-building industry. It is an effort to coordinate resources and technology to promote fuel-saving advancements and driving techniques. The program has been saving about 100,000 gallons of fuel a day, Mr. Brinley of the Motor Vehicle Manufacturers Association said.