Hirth F-23

The Hirth F-23 is a twin cylinder, horizontally-opposed, two stroke, carburetted or optionally fuel injected aircraft engine designed for use on ultralight aircraft. It is manufactured by Hirth of Germany.^[1][2][3]

The F-23 is intended to compete with the 50 hp (37 kW) Rotax 503 and is differentiated from the Rotax powerplant by offering a horizontally opposed cylinder layout. The F-23 uses free air cooling and piston-ported induction, with dual Bing 34mm slide or optional diaphragm type carburetors. The cylinder walls are electrochemically coated with Nikasil. Standard starting is recoil start. A belt reduction drive system, fuel injection, tuned exhaust and electric start are optional.^[1][2]

The engine runs on a 50:1 pre-mix of unleaded 93 octane auto fuel and oil. Recommended time between overhauls is 1000 hours.^[1][2]

The F-23 produces 50 hp (37 kW) at 6150 rpm and 40 hp (30 kW) at 5500 rpm.^[1][2]

Data from Recreational Power Engineering^[2]

• Type: Twin cylinder, two-stroke, aircraft engine
• Bore: 72 mm (2.8 in)
• Stroke: 64 mm (2.5 in)
• Displacement: 521 cc (31.8 cu in)
• Length: 247 mm (9.7 in)
• Width: 570 mm (22.4 in)
• Height: 338 mm (13.3 in)
• Dry weight: 84 lb (38.1 kg) including reduction unit, full exhaust system and electric start

• Valvetrain: piston-ported
• Fuel system: 2 X Bing 34mm slide or optional diaphragm type carburetor or fuel injection
• Fuel type: unleaded 93 octane auto fuel
• Oil system: 50:1 fuel/oil premix
• Cooling system: free air
• Reduction gear: cog belt with 1.8:1, 2.2:1 and 2.5:1 ratios optional

• Power output: 50 hp (37 kW) at 6150 rpm
• Compression ratio: 9.5:1

• Official website