This is "General Motors Research Laboratory", a part of the conglomerate of sections and companies of the General Motors Group.

-For aviation engines its best standard was Allison.

-A group of GMRL engineers such as Goldthwaite, Hallet, Wittner, etc ... developed an idea of ​​higher performance engines based on the use of two-stroke cycle (double effective strokes of the four-stroke) but with twin cylinders and a common combustion chamber.

“The12 cylinders (24) boxer from GMRL”

-The first demonstration was a project of 12 twin cylinders (total 24 cylinders). There was a 10-cylinder project to save weight and size.

“Radial version with the same idea: the U-173”

-They needed supercharging to force the mixture into the cylinders. The intake was done through the base of one and the exhaust through the other one.

-The U-173 gave only 90 hp. With later models the power and the number of cylinders increased. The U-744 was a radial with six double cylinders.

“U-744 radial engine”

-They made an engine with ten radially arranged cylinders, the U-1240, also in the early 1930's.

“U-1240 side view”

“U-1240 rear view drawing”

-The superchargers were generally Roots as on the U-250, but centrifugal ones were also used as on the X-250.

“U-250, with the Roots compressor on the foreground”

“X-250 front view cross-section”

-This engine has a large-diameter, centrifugal compressor at the rear and taking in through the carburetor.

-They were used on unmanned aircraft around 1941.

"X-250, with centrifugal compressor"

"Drone designed by Cessna with X-250"