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67 designed primarily to run on Jet A-1, JP-4, JP-5 and JP-8 fuels. Built mostly from aluminium alloy, the 3507 weighs 38 kg, measures 543 x 442 x 380 mm, and displaces 625 cc. Its maximum continuous power output is 41.5 kW (55.7 hp) which it achieves at 6100 rpm (with a redline of 6600 rpm) and produces up to 65 Nm of torque. Christian Bitter, head of r&d at Hirth, says, “One of the biggest challenges was finding the best suppliers for all the different parts we would need, because in 2017 a lot of companies from 2013 and previous years just weren’t around anymore, or they weren’t making the parts we’d used before. Also, the low- volume nature of the UAV world often makes it hard to establish effective supply chains. “In time though, we found ideal suppliers, and we redesigned and requalified the S1208, keeping fairly close to the original design to avoid introducing new things that could disqualify the thousands of hours of testing before 2013.” Knock management Knocking is a critical concern for both gasoline and heavy-fuel engines – particularly the latter, as they are more sensitive to it – as it can severely damage the cylinder walls, pistons and piston rings. Compression-ignited engines get around this problem by being made much stronger and heavier than their spark- ignited counterparts. However, the extra weight would have run counter to Hirth’s customers’ requests for higher power- to-weight ratios at the power outputs required, which is why the company went down the spark-ignition route. Pre-mixed heavy fuel and air is especially prone to knocking, owing to high cylinder temperatures earlier in the compression stroke than gasoline. To compensate for that, the 3507’s cylinder and combustion chamber have been designed for a lower compression ratio than that of its gasoline predecessors. “The cylinder and combustion chamber have to be designed precisely to accommodate heavy fuels properly without allowing intermittent damage to parts, reductions in the TBO or excessive weight,” Bitter says. “Thermal management is also a huge factor. You have to counter any excessive heat but you also can’t cool too much, because then you might have uneven or insufficient vaporisation of fuel droplets, and wind up with a wet cylinder. You have to strike a careful balance between the two.” The cylinder head temperature sensor is key to detecting when temperatures are becoming excessive, with 200 C programmed as the maximum safe temperature for the head. Bitter says though that temperature is not always the best predictor of knocking. High-resolution pressure sensors in the cylinder heads have been the preferred tool for detecting when knocking is happening. Hirth installed them in the 3507 prototype’s cylinder heads (as it does in every prototype) to map out when and how the ECU should compensate for the risk of knocking, but they are not used in the production engines owing to their high cost. Direct injection Historically, achieving high control (and thus high quality) of heavy-fuel combustion across the speed and power bands of spark-ignited engines has been challenging, owing to the sub-optimal properties of the fuels. However, modern direct injection technology achieves these levels of control. In our earlier Hirth dossier, the company discussed in detail its ‘i-power’ direct injection system installed on the S1218. This is designed for effective pre-mixing and vaporisation of both Hirth 3507-01M | Dossier Unmanned Systems Technology | February/March 2021 UMS Skeldar has been the primary customer for the 3507, as well as being a major contributor to its development (Courtesy of UMS Skeldar)