Ever since Henry Ford started building cars, engineers have been trying to improve the efficiency of the internal combustion engine. Believe it or not, there are engineers in Annapolis who are still trying -- and they have just raised $2.5 million to finance the work of their fledgling company.

The company is Sonex Research Inc., a 5-year-old firm whose founders believe they have found a commercially acceptable way to improve combustion efficiency -- whether the engine is burning gasoline, diesel fuel or some other type of fuel.

"It is not an engine technology," explained Sonex Chairman Andrew A. Pouring, "it's a technology that improves combustion efficiency."

The efforts of the young company are chronicled in the prospectus that accompanied the firm's first public offering. The front cover carries the familiar words: "These securities . . . involve a high degree of risk . . . and should only be purchased by those who can afford to lose their entire investment."

Despite warnings aplenty, the new stock issue sold out and reportedly found a number of buyers among friends and acquaintances of the people involved in the company. Investors also may have been attracted by the impressive credentials listed for a dozen officers and directors drawn from the scientific, business and educational communities. The president of the company is R. William McNealy Jr., 56, who spent 16 years as an executive at American Motors Corp.

Sonex, whose stock began trading over the counter April 18, sold 500,000 units at $5 each. A unit consists of one share of common stock and one warrant. For every two warrants a stockholder owns, he will be entitled to purchase one share of common stock at $6 a share until April 17, 1988. The underwriters were Rosenkrantz, Ehrenkrantz, Lyon & Ross Inc. of New York.

The new investors bought 17 percent of the company's stock (their money represented 56 percent of the company's capital) while the old shareholders retained 83 percent of the stock, (representing 44 percent of the capital). Company insiders thus kept control of most of the stock.

With virtually no revenue, the company has so far chalked up losses of about $2.4 million. Sonex is seeking a $1.5 million revenue bond from the Maryland Energy Finance Administration.

Sonex technology experiments were begun in the mid-1970s by Richard F. Blaser, an aeronautical engineeer, who worked with Pouring, then a professor of aeronautical engineering at the U.S. Naval Academy. They invented and patented what they called the "heat-balanced engine."

At the moment, Sonex officials are trying to move their theories and experiments from the laboratory to the commercial market. They are building a motorcycle engine for a European manufacturer and modifying a Ford Escort to run on Sonex technology.

And what sort of reaction have they had from the auto industry?

"Very guarded," replied Pouring. "Certain segments would like to see us succeed . . . and others are skeptical."

Shares of Radiation Systems took a solid hit late last week when the Sterling, Va., antenna manufacturer announced it had ended its March quarter with earnings of 10 cents a share, far below the 20 to 25 cents analysts had expected. The reason given was that orders in the military pipeline had been delayed. The stock quickly fell from $12.75 Wednesday to $10 on Thursday and closed Friday at $9.75.

The upside for Radiation Systems was that the analysts, while surprised with the earnings drop, were not dismayed. Eliot H. Benson, research director at Ferris & Co. in the District, said he saw the price drop as a "buying opportunity," and Mae Graves O'Leary, analyst at Scott & Stringfellow in Richmond, said her firm still liked the stock, especially at the lower price.

At Radiation Systems, President Richard E. Thomas was optimistic. "Orders not booked are not lost," he said, noting that military orders are slow to move these days because of the intense scrutiny -- especially price scrutiny -- being given to military procurement by Defense Department officials.

Looking ahead, Thomas predicted that business will improve, and he forecast earnings of $1.20 on sales of $45 million for fiscal 1986, ending June 1986. That would be way up over the $1 earned for fiscal 1984 and the 70 cents now projected for fiscal 1985.

Luskin's, the Baltimore appliance chain that advertises itself as the "Cheapest Guy in Town," is selling one-third of its stock (about 1.35 million shares) to the public. Founder Jack Luskin and his family will retain the other two-thirds. The shares are expected to sell for between $15 and $18, with the offering being handled by Prudential Bache. Luskin's, which has 25 stores in the Baltimore-Washington area, rang up almost $90 million in sales in 1984.

It's called a Beneficial Assignee Certificate (BAC), and it recently began trading on the New York Stock Exchange as "shares" in the CRI Insured Mortgage Investment (CRIIMI) Limited Partnership of Rockville. Put the two names together and you've got CRIIMI-BACs -- which sounds like a new kind of candy bar.

Not only is the CRIIMI-BAC an unusual name for a NYSE listing but, says the company, its BACs are the first to find their way onto the Big Board.

Working with Merrill Lynch, CRI Inc. sold the BACs for $20 each (minimum 100 BACs) in a public offering last June. It sold 9. 1 million BACs for a total of $182 million. The BACs closed Friday at $20.375.

The money was used to buy a package of 45 Ginnie Mae mortgages on private multifamily housing properties in several states, including Maryland, Virginia and the District. The government-insured mortgages, said President William B. Dockser, were bought at deep discounts, averaging 69.2 percent of face value. Investors, he said, can expect a yield of 10 percent with the hope of an increased yield to 12 or 13 percent if the mortgages are prepaid -- which most often happens when a multifamily property is sold for cash or goes condo.

CRI Inc., which Dockser said has been in business since 1974, is the nation's largest syndicator of rental housing. The firm's investment portfolio includes 53,000 apartment units, 3,178 hotel rooms and more than 1 million square feet.

Atlantic Research Corp. of Alexandria will split its stock 3-for-2 to shareholders of record on May 17. The new shares will be distributed June 21 . . . Smithfield Foods of Arlington has repurchased 83,200 shares of its comon stock from O'Sullivan Corp. of Winchester, Va., in a private transaction. No price was disclosed. Smithfield stock closed Friday at $8.

Madison National Bank of D.C. is offering a discount brokerage service for customers with home computers. Investors can request stock quotes and can buy and sell securities. The bank is organizing the service with the help of Security Pacific Brokers Inc. . . . Standard & Poor's April stock guide boosted the common stock rating of Washington Gas from A-minus to A. The top rating is A-plus. Meanwhile, E. I. L. Instruments of Sparks, Md., was rated B after not having been rated.