DETROIT, MICH.--Indications that the Diesel type airplane engine, recently developed by Capt. L. M. Woolson, chief aeronautical engineer of the Packard Motor Car Co., will become a commercial reality and possibly a revolutionary factor in airplane engine design, is seen here in the announcement of the concern that it will begin construction immediately of a $650,000 plant to produce the engines in large quantity for the commercial market. The new plant, according to the announcement by Hugh J. Ferry, treasurer of the Packard firm, will be completed and in operation within five weeks. Between 600 and 700 men will be employed and, according to expectations, production will be carried on at the rate of about 500 Diesel engines per month by July. The Packard Diesel was announced first in October, following experiments covering several years. The original engine was placed in a Stinson-Detroiter, which was flown successfully by Captain Woolson and Walter Lees, Packard pilot. Since that time Captain Woolson has built four of the engines, all of 200 hp. capacity, developing 1 hp. for every 2 lb. of weight. The Diesel, installed on the Stinson-Detroiter, it was said, now has had 200 hr. flying time, and gives not the slightest indication that it will need an overhauling for some time. The other three engines have been tested on the block in the company's research plant. It is claimed by the builders that the Packard Diesel will produce a saving of about 20 per cent. in fuel consumption as compared with engines using gasoline. It is claimed further that the Diesel will prove far more reliable in construction than any airplane engine yet developed. Evidence of this, it was pointed out, is seen in the performance of the initial Diesel. DETAILS NOT ANNOUNCED Although neither Mr. Ferry, nor Captain Woolson, would disclose any technical details as to the engine's construction in making it applicable to airplane use, the secret of its success was reported to be an especially designed pumping device creating high compression necessary for Diesel firing. Since announcement of the engine, the Packard factory has been literally a Mecca for engineers from many parts of the world wishing to see the engine. The Crown Prince of Spain, in Detroit last fall, was given a flight in the Diesel powered Stinson. None of the construction secrets, however, have been divulged, it was said.