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74 our redesign of the cylinder heads to keep their distance from any potential knocking from heavy fuels,” Bitter adds. “That ensures that the increased power from combustion goes towards moving the crankshaft rather than overheating the exhaust. “The compression ratio is 9.5:1 when you consider the whole stroke, and 6.9:1 when you calculate it for the stroke starting once the exhaust port is closed.” Liquid cooling While many of Hirth’s engines are designed to use air cooling – including its other, upcoming heavy-fuel engine the 4202 – the 3507 has a liquid- cooling circuit. The reason for this comes down to power differences. For engines running from 15 to 28 hp, an air-cooling system provides sufficient thermal management while also keeping weight and mechanical complexity low. “Air cooling has worked well for our customers, even for Martin UAV’s V-BAT, which has to stay stationary for key portions of its flight and VTOL,” Bitter says. “The V-BAT is capable of maintaining an ongoing rush of airflow with its ducted integration of our 4201, enough to cool the engine even during the high power draw when hovering. “On the 55.7 hp 3507 though, the cooling demands are much higher, especially when it’s in a helicopter like the UMS Skeldar UAV. Its hover time severely reduces airflow, and maintaining a cooling airflow in a helicopter during a hover is challenging.” In Hirth’s liquid-cooling circuit, a heat exchanger – typically a radiator – cools a water-glycol mix, which is piped from the radiator to the engine system by a water pump. The pipe splits in two shortly after this, with one part going to the compressor and the other to the engine. The latter has two inlet ports for the water, each beneath its cylinder’s exhaust port. The distribution of coolant through the cylinders is achieved through an open-deck channel, based on similar in- house designs from the 35 Series and February/March 2021 | Unmanned Systems Technology The 3507 engine’s crankcase is cast from ALSi10MgCu as two halves that split horizontally as one lower section and one upper, the latter featuring the mounting points for the cylinders. The two halves are screwed together with a total of 18 fasteners: eight M8 studs and 10 M6 screws. All mating surfaces across the block halves are sealed using Omnivisc 1050 as a liquid gasket. The design of the crankcase has been optimised to avoid having any especially large amounts of aluminium in any section, as an excess of it in any one place could cause significant and uneven temperature gradients in how the block cools and solidifies. Formation of air bubbles or blowholes in the aluminium is reduced by ensuring that the metal cools smoothly and evenly throughout either part, otherwise it could weaken its structural integrity, particularly against vibration. After each casting, Grabert Aluformguss X-rays the block halves to ascertain that there is no porosity in the aluminium. After that, the key joining surfaces of the crankcase – such as the mounting seats for the crankshaft’s bearings – are CNC- machined for fitting and holding other parts such as the bearings in place. This is essential for ensuring the crankshaft runs exactly parallel with the length of the crankcase. Each of the two cylinders is cast from AlSi10Mg, an aluminium alloy well- known for high corrosion resistance, strength and thermal conductivity, and given T6 heat treatment to further enhance its thermal properties. The cylinder design features a 76 mm bore and a 69 mm stroke, for 312.5 cc displacement per cylinder. Each cylinder is bolted to the upper engine block section using four studs, with the interior coated using nickel silicon carbide (at 55 µm thickness). Two O-rings seal between each cylinder and its head, for gases as well as liquid coolant flowing upwards from the circumferential water jackets. The cylinder heads are also cast from T6-treated AlSi10Mg. Six M8 screws fasten each head to its cylinder via inserts that run through the water jackets. The design of each cylinder head also features mounting inserts for the spark plug, the air injector (from the direct fuel-injection rail) and the water temperature sensor, as well as an outlet for the coolant on one side. The crankshaft consists of six parts: three shaft-and-counterweight pieces and an additional central counterweight cut from 16MnCr5 case-hardened steel, and two crank pins milled from 42CrMo4 steel (set at 180 º to each other for alternating firing order). Anatomy of the 3507 Exploded view of the 3507’s crankshaft