Mack Trucks last week unveiled a new line of diesel engines that company officials said will improve trucks' gas mileage and durability.

The new Econodyne series includes six-cylinder and V-8 engines, said Mack executive vice president Walter M. May.

"The new engine will have a significant impact on the [trucking] industry," said H. Kenneth Tooman, Mack vice president. "The increased cost of diesel fuel, rising almost on a daily basis, makes the Econodyne engines even more important."

[In the District last Friday, Mack and Garrett Corp., both subsidiaries of the Signal Companies and part of the Industrial Turbines International consortium, demonstrated the GT 601 turbine engine. In testing, the engine offers higher performance and efficiency than even advanced diesel engines.]

The diesel engine's fuel efficiency comes from a newly disigned cooling system mounted on the chassis.

A heat exchanger mounted in front of the radiator reduces the air coming into the engine to 100 degrees Fahrenheit, down from the 220-degree level achieved in 1973 when Mack introduced its first cooling system, May said.

The cooler air permits more complete fuel combustion, giving better gas mileage.

The new engine will not cost more than current engines because price is based on horsepower, officials said.

The Hagertown plant of Mack Trucks, with headquarters in Allentown, Pa., will design and manufacture the new engines.

May said that with predictions of diesel fuel prices rising to $2 a gallon, gas savings on a truck with one of the new engines could be more than $1,000 a year.

Last year Mack Trucks lost money for the first time in more than three decades, but its top official believes sales may pick up enough to put some laid-off employes back to work.

Alfred Pelletier, board chairman and chief executive officer, said Mack plans in the next few months to increase its production rate slightly at assembly plants in Allentown and Macungie, Pa., because of small increases in sales orders.

That, he said, would mean additional work at the plant here, which produces engines, transmissions and rear axles. About 1,000 of the 4,500 employes at the Hagerstown plant are currently off the job.

Pelletier termed the slight sales increase "the first sign of improvement.

"That means there'll be some people coming back," he added. "It will do something for Hagerstown. . . . It won't be a lot but it'll do something."

Mack is trying to regain its strength in several areas, including expansion of its foreign sales efforts and development of news-model transmissions and trucks, said Pelletier.

Mack posted an $18.2 million loss last year on sales of $1.5 billion. Mack last recorded a loss in 1949.

But the figures showed that sales began to recover from October through December. Losses continued during that period, but Mack narrowed the gap by posting a less-than-expected loss of $4.9 million on sales that just $22 million less than the $434.5 million mark of the same period in 1979.