F-23 aircraft combustion engine

The Hirth F-23 is an air-cooled two-stroke aircraft combustion engine from Hirth Engines, belonging to the 23 Series and rated at 50 HP. It is a two-cylinder boxer engine for light and ultralight aviation, and the maker positions it for aircraft weighing up to 120 kg, which also makes it a candidate for unmanned aircraft and other small experimental platforms.

The key figures from the maker's technical specification are these. Maximum output is 36.7 kW (50 HP) at 6500 rpm, measured according to DIN 70020, and maximum torque is 53 Nm at 6300 rpm. Displacement is 521 cm3, from a bore of 72 mm and a stroke of 64 mm. Mass is 22 kg without the exhaust, which gives a high power-to-weight ratio for the class. The overall dimensions are 286 mm long, 570 mm wide and 338 mm high; the width reflects the opposed-cylinder layout, so installation space is wider than tall. The engine runs counter-clockwise when viewed toward the output shaft.

The engine uses a Mikuni slide carburetor and a CDI ignition system, with dual ignition offered as an option. Fuel is a premix of gasoline and two-stroke oil in a 1:50 ratio, with gasoline of at least 95 octane (RON); a 1:80 to 1:100 ratio is given when BLUEMAX two-stroke oil is used. No separate lubrication system is needed, which simplifies the fuel system for the airframe builder.

For integration, the engine provides a 250 W, 12 V generator output, enough for basic ignition-independent electrical needs such as small avionics, and it starts with a recoil starter. An electric starter is an optional accessory. A Polyflex belt reduction gearbox, the G23, can be fitted in ratios of 1:1.8, 1:2.0, 1:2.5 and 1:3.1 to match propeller speed, and an exhaust system is available according to the maker's technical information. Air cooling means the installation must provide adequate airflow over the cylinders.

The maker states a time between overhauls of 1000 hours. The F-23 is not a certificated aircraft engine: the brochure states it has not undergone the safety and durability testing specified by aircraft standards and is meant only for uncertificated experimental aircraft or vehicles where an engine failure poses no safety risk.

Typical applications are ultralight and light aircraft, experimental airframes and small fixed-wing platforms that need a compact two-stroke powerplant with moderate fuel consumption and simple maintenance.