Hirth 3701

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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]

Development

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 34mm slide carburetors or optionally fuel injection. The cylinder walls are electrochemically coated with Nikasil. Standard starting is electric start and recoil start is not available as an option. The reduction drive system available is the G-50 gearbox, with reduction ratios of 2.16:1, 2.29:1, 2.59:1, 3.16:1, or 3.65:1.^[1]

The engine runs on a 50:1 pre-mix of unleaded 93 octane auto fuel and oil, or optionally 100:1 oil injection.^[1]

Variants

3701 high torque/low rpm engine: Three-cylinder in-line, two stroke, aircraft engine with three Bing 34mm slide carburetors or fuel injection. Low piston port timing produces 84 hp (63 kW) at 4950 rpm with a very flat torque curve.^[1]
3701 high performance engine: Three-cylinder in-line, two stroke, aircraft engine with three Bing 34mm slide carburetors or fuel injection. High piston port timing produces 100 hp (75 kW) at 6000 rpm with a steep torque curve.^[1]

Applications

Specifications (3701 high performance)

Data from Recreational Power Engineering^[1]

General characteristics

Type: Three cylinder, two-stroke, in-line, aircraft engine
Bore: 76 mm (3.0 in)
Stroke: 69 mm (2.7 in)
Displacement: 939 cc (57.3 cu in)
Length: 490 mm (19.3 in)
Width: 280 mm (11.0 in)
Height: 420 mm (16.5 in)
Dry weight: 108 lb (49.0 kg), including electric starter and exhaust system. With optional gearbox total weight is 127 lb (57.6 kg), plus radiator.

Components

Valvetrain: piston ported
Fuel system: 3 X Bing 34mm slide type carburetors or fuel injection
Fuel type: unleaded 93 octane auto fuel
Oil system: 50:1 fuel/oil premix or oil injection
Cooling system: liquid
Reduction gear: G-50 gearbox with reduction ratios of 2.16:1, 2.29:1, 2.59:1, 3.16:1, or 3.65:1

Performance

Power output: 100 hp (75 kW) at 6000 rpm
Compression ratio: 9.5:1

References

↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Recreational Power Engineering (undated). "3701S - 2 cycle 100hp". Archived from the original on 15 September 2008. Retrieved 2009-12-18. Check date values in: |date= (help)
↑ 2.0 2.1 Ultraligero.Net (undated). "3701 - 2 cycle - 100hp". Retrieved 2009-12-18. Check date values in: |date= (help)
↑ Von Koschier, Angelo (November 2003). "Helicycle". Retrieved 2009-12-18.

External links

Official website on Archive.org

v t e Hirth and Heinkel-Hirth aircraft engines

Piston engines	HM 8 HM 60 HM 150 HM 500 HM 501 HM 504 HM 506 HM 508 HM 512 HM 515

Two-strokes	F-23 F-30 F-33 F-36 F-263 2702 2703 2704 2706 3002 3003 3202 3203 3502 3503 3701 3702 3703

Gas turbines	HeS 1 HeS 3 HeS 8 HeS 30 HeS 40 HeS 50 HeS 60 HeS 011

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