TheKroeber M4 Köller was a 2-stroke air-cooled horizontally opposed piston engine designed and built byDr. Kroeber & Sohn G.m.b.H. inGermany 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.

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

Data from AEHS: HOAE Kroeber^[2]

• Grunau Motor-Baby – German motor-glider conversion of aGrunau 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 sicKroeber M3 Köller (only reference to M3)

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

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

• 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)

• 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

• 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)

• Boxer engines
• Kroeber aircraft engines
• 1930s aircraft piston engines

• CS1 errors: ISBN date
• CS1 German-language sources (de)
• Articles with short description
• Short description matches Wikidata