The two titans of the aircraft engine business, General Electric and Pratt & Whitney, are locked in a lobbying struggle which provides a rare glimpse of the pressures behind many national defense decisions.

At stake for the companies is not only up to $8 billion in Pentagon business but, for the long run, the dominant position in the figher engine market.

"I've never seen snything like it," said one congressional source who has been at the receiving end of the gloves-off lobbying. "Lobbyists from GE and P&W have been swarming all over the place."

If General Electric wins this battle, its engines could end up replacing Pratt & Whitney engines now powering the Air Force's F16 fighter-bomber and the Navy's F14 fighter.

If Pratt & Whitney prevails, its engines would stay in those planes while both companies compete for improved engines under Pentagon development contracts.

In Congress, Rep. Joseph P. Addabbo (D-N.Y.), the new chairman of the House Appropirations defense subcommittee, is General Electric's big gun. General Electric is a major employer in New York state.

Rep. Robert N. Giaimo (D-Conn.), chairman of the House Budget Committee and a member of the House Appropriations subcommittee, is taking a leading role in defending Pratt & Whitney's position. Pratt & Whitney, with headquarters in East Hartford, is a giant employer in Connecticut.

Both companies have mobilized their lobbying teams for the battle over who will build the military engines of the future. George C. Troutman is GE's point man on Capitol Hill, while his counterpart at Pratt & Whitney is Clark MacGregor, former chairman of President Nixon's 1972 re-election committee.

The Pentagon's new budget fails to portray the dimensions of the battle. It reveals only that $16 million would be obligated in fiscal 1980 to pursue something called "alternative fighter engine 101X."

However, the 101X would be a derivative of the engine GE built for the B1 bomber which President Carter canceled. So the company lobbied heavily to get the $16 million included in the Pentagon's fiscal 1980 budget now before Congress.

The Air Force and Navy under 101X would jointly search for a replacement for the Pratt & Whitney TF30 engine in the Navy's F14 fighter and the Pratt & Whitney F100 engine powering both the Air Force's F15 and F16 fighters.

Congressional studies now underway have come up with a rough estimate of $6 billion to $8 billion if the engines on the F14 and F16 were replaced by the 101X. (The F15 has two engines, in contrast to the single engine powering the F16, so their replacement is not as likely.)

Since General Electric is already building the engines for the Navy F18 fighter and the Air Force A10 attack plane, getting the money to replace Pratt & Whitney engines on the F14 and F16 would make General Electric dominant in the military engine field.

Executives of both companies are well aware of this and are thus depolying lobbyists all around Capitol Hill and the Pentagon to make their case. Telegrams, briefings, memos and face-to-face lobbying are all part of this campaign.

Last last year, when the Pentagon budget was being put together, General Electric in lobbying for the $16 million for the 101X wrote a memo declaring that the Partt & Whitney engine in the F14 "has been so troublesome and caused so many crashes."

Navy improvements to the F14 engine, continued General Electric, "may well not be adequate." Besides, argued GE in the memo sent to Washington influentials, the national defense would be well served by having a second source for these fighter engines.

Pratt & Whitney, in one of its memos sent to Congress a few days ago, complained that it was being frozen out of the competition for the new engine. The Air Force, charged Pratt & Whitney, is gearing up for a "sole source" selection of the General Electric 101X.

"We strongly urge you to take action to preclude release of funding" for the 101X program "until Congress has had opportunity to review program," said MacGregor in a telegram sent out to lawmakers on Feb. 7.

The review demanded by Partt & Whitney is now underway. The main forum to date has been the House Appropriations defense subcommittee now going over the Pentagon's fiscal 1980 budget. Air Force witnesses, in defending the 101X program, have said that they want to look at an alternative to the Pratt & Whitney engines.

Rep. Jack Edwards of Alabama, ranking Republican on the defense subcommittee, has called for a separate set of hearings on this engine competition and sent a long list of questions to Defense Secretary Harold Brown, including the following:

Why is a new competition getting underway now when the Air Force chose the Pratt & Whitney engine over the competing GE one back in 1975?

How can the Pratt & Whitney engine in the F15 and F16 fighter planes be called troublesome when "only one of the F15 accidents" has been attributed to engine failure?

Would it be worth putting the GE 101X in the F16 when the plane's performance would be improved by "a marginal 10 to 15 percent?"

Edwards wrote Brown that of the total $94.4 million earnarked to pursue an alternative fighter engine in the fiscal years 1971 to 1981, $81.1 million would go to General Electric and $13.3 million to Pratt & Whitney.

"Is this to be a competitive program as directed by Congress." Edwards asked in a question pushed to the forefront of the congressional consideration of the Pentagon budget, "or will it go sole source to General Electric?"