The Hirth 3701 is an in-line three-cylinder, two-stroke, carburetted aircraft engine, with optional fuel injection, designed for use on ultralight aircraft and small homebuilts.
[1][2] The 3701 was developed as a liquid-cooled and narrower installation alternative to the four-cylinder Hirth F-30 air-cooled engine of 85 to 110 hp (63 to 82 kW).
[1][2] The 3701 features triple Bing 34 mm slide carburetors or optionally fuel injection.
The cylinder walls are electrochemically coated with Nikasil.
[1] The engine runs on a 50:1 pre-mix of unleaded 93 octane auto fuel and oil, or optionally 100:1 oil injection.