Unmanned Systems Technology 036

71 Hirth 3507-01M | Dossier Unmanned Systems Technology | February/March 2021 engineered system for professional end-users. Considerable in-house testing was carried out, including a 150-hour endurance test in line with the Part 33 of the Federal Aviation Regulations on engine airworthiness standards, in early 2019. Having officially entered serial production in mid-2018, more than 250 units of the engine have now been built. Its biggest customer is Martin UAV’s V-BAT (featured in UST 15, August/September 2017). Creating this new version involved in- depth studies of the S1218 to evaluate its potential for further optimisation. The ‘i-power’ direct injection system continues to be analysed in the company’s CFD programs, but Hirth has opted to remove it from the 4201 to provide a simpler product with fewer parts, for improved manufacturability, lower cost, and ease of maintenance and operation. With the competencies gained from these studies, Hirth is now working on the 4202, a heavy-fuel engine based on the 4201, which will be almost identical to its gasoline-burning sibling, albeit with key modifications for Jet A-1 and other kerosenes. The first modification is a heavy-fuel evaporation system, to ensure that the ‘cold’ side of the cylinder (opposite from the exhaust) does not become wet with fuel and interferes with the burn. It consists of a pre-chamber – based in form and operating principle on the resonance tube used in the i-power arrangement,but improved via CFD studies – in which exhaust gases are used to heat the heavy-fuel vapour (further vaporising it and improving its mixing with air) before it is injected into the cylinder. “As with the i-power, there is a resonance effect which means that when the piston is at BDC, the pressure in the pre-chamber is higher than in the cylinder, helping to pull the old exhaust gas and the heavy-fuel spray into the cylinder as the fresh air arrives from the crankcase,” Christian Bitter, head of r&d at Hirth, says. “They mix at about 0.8 lambda, the same as the 3507’s fuel-to-air ratio, and that ensures a controlled but powerful combustion for us to use.” Also, the cylinders have been redesigned by changing the cooling fins from their previously square geometry to a circular, almost spherical shape (with larger outward protrusions as well). This shape, Hirth says, will provide a more even conduction and distribution of heat outwards from the combustion chamber, with a larger surface area for faster cooling. “We are likely to adopt this cooling fin design on our 4201 as well,” Bitter notes. “After prototyping, the next step will be to find suppliers who can provide these cylinders – we’re already in contact with a few two-stroke cylinder makers.” The exhaust system on the 4202 has also been updated to omit the flexible tube on past designs. The newer version of the exhaust has already been prototyped and tested on a 150-hour run, and the engine is expected to be commercially available later this year. Specifications for the 42 Series 4201 Two-stroke flat-twin Naturally aspirated Manifold injection Air-cooled Fuel: gasoline Weight: 5.7 kg Dimensions: 330 x 213 x 160 mm Total displacement: 183 cc Maximum power: 10.67 kW Maximum speed: 6700 rpm Compression ratio: 9.8:1 (static), 6.5:1 (dynamic) TBO: 150 hours Man-hours to strip and rebuild: 16-18 (estimated) 4202 HF Two-stroke flat twin Naturally aspirated Port injection Air-cooled Fuel: Jet A-1 (others planned) Weight: 5.7 kg Dimensions: 330 x 213 x 160 mm Total displacement: 183 cc Maximum power: TBD Maximum speed: 6700 rpm Compression ratio: 9.8:1 (static), 6.5:1 (dynamic) TBO: 150 hours Man-hours to strip and rebuild: 19 (estimated) Some key suppliers Same as for the 3507 (see sidebar: Anatomy of the 3507), unless as stated below or not applicable Block/crankcase: MGH Auxiliary drives: Plettenberg Engine management system: Trijekt Alternator: Plettenberg Main bearings: SKF Seals: Schwarz Exhaust: Zimmermann Schalldaempfer Air filter: Racimex Dynos: K&S Oil filter: Mikuni