Kroeber M4

The Kroeber M4 Köller was a 2-stroke air-cooled horizontally opposed piston engine designed and built by Dr. Kroeber & Sohn G.m.b.H. in Germany in the late 1930s. The M4 proved relatively popular, for powering the ultra-light aircraft and motor-gliders in vogue during the 1920s and 1930s.

M4 Köller
Type
National origin	Germany
Manufacturer	Dr. Kroeber & Sohn G.m.b.H.
First run	1930s
Major applications	Möller Stomo 3, ITS-8

Variants

Kroeber M3 Köller

either a typo or a variant of the M4, (only reference is Cynk's Polish aircraft 1893-1939)^[1]

Kroeber M4 Köller

Main production variant

Applications

Data from AEHS: HOAE Kroeber^[2]

• Grunau Motor-Baby – German motor-glider conversion of a Grunau Baby
• Kocjan Bąk I – Polish motorglider
• Dittmar Condor 'La Falda' (a removable power-egg with M4 and pusher propeller)
• Möller Stomo 3 – German high-speed ultralight aircraft
• Akaflieg München Mü13M Motormerlin
• ITS-8 – Polish motorglider sic Kroeber M3 Köller (only reference to M3)

Engines on display

There are several M4 engines on display, but one of the best preserved and presented is at the Polish Aviation Museum in Kraków.^[3]

Specifications (M4)

Data from Flugzeug-Typenbuch. Handbuch der deutschen Luftfahrt- und Zubehör-Industrie 1944,^[4][5]

General characteristics

• Type: 2-cylinder 2-stroke air-cooled horizontally-opposed
• Bore: 75 mm (2.95 in)
• Stroke: 72 mm (2.83 in)
• Displacement: 0.634 L (38.69 cu in)
• Length: 550 mm (21.7 in)
• Width: 550 mm (21.7 in)
• Height: 646 mm (25.4 in)
• Dry weight: 27.8 kg (61 lb)

Components

• Fuel system: 2x Meco S1/f carburetor
• Fuel type: 1:25 SAE 50 oil / gasoline blend
• Oil system: blended with fuel
• Cooling system: air-cooled

Performance

• Power output:
  

• 18 PS (18 hp; 13 kW) at 2,700 rpm (30 minutes) at sea level
• 16.6 PS (16 hp; 12 kW) at 2,650 rpm (max. continuous / cruise) at sea level

• Specific power: 28.4 PS/L (0.46 hp/cu in; 20.89 kW/L)
• Compression ratio: 6.2:1
• Specific fuel consumption: 0.228 kg/PSh (0.510 lb/(hp⋅h); 0.310 kg/kWh) at max continuous
• Oil consumption: 0.002–0.003 kg/PSh (0.004–0.007 lb/(hp⋅h); 0.003–0.004 kg/kWh) at max continuous
• Power-to-weight ratio: 0.645 PS/kg (0.289 hp/lb; 0.474 kW/kg)
• B.M.E.P.: 2.55 atm (2.58 bar; 37.5 psi)