70 interference (EMI) produced by the engine, while also eradicating its own susceptibility to EMI, which is a growing risk to ECU-driven engines with today’s UAVs integrating more high-voltage and thus high EM-emissions devices. Forced-induction systems At higher altitudes, natural aspiration is insufficient for the airflow requirements of most compression-ignition engines, so forced induction via a turbocharger and a supercharger enables the DHK180 to maintain adequate intake air volumes, mass flow, and pressure for consistent and precise fuel/air mixing. A mechanical supercharger sits between the four cylinder heads, above and feeding into a large, open intake plenum, from which all the cylinders draw air. That supercharger is a twin screw-type, with rotors designed to very close tolerances, chosen by DeltaHawk for its high-efficiency, pressure ratio and volume flow. “The supercharger is critical for starting because the mechanical turning of the crankshaft drives the supercharger to stuff air into the plenum. At that point, the turbo can’t run because there’s no exhaust to drive the turbine,” Webb remarks. “For similar reasons, it also supplies air during idling, lower power settings, and generally at lower altitudes. At higher power and higher altitudes, the turbo does most of the work. And they do run in series: air ideally first comes in through an inlet and filter on the aircraft, the turbocharger draws in air via its compressor and that then goes to an intercooler, and the cooled air then goes into the supercharger. “They’ve been carefully balanced in how much of the boost is coming from either at each level of speed and power. Both also provide some redundancy; a failure of the supercharger triggers a springloaded door underneath to open, so the turbocharger’s air can bypass most of the supercharger straight into the plenum. If the turbo fails, the supercharger keeps running without issue – there’s just some loss of performance at altitude until the problem is fixed.” The turbocharger has been designed in-house (with some undisclosed third-party help) for the higher airthroughputs, exact exhaust flows, pressure ratios and air volumes of the DHK180 versus those of the four-stroke engines that most COTS turbochargers have been designed for. “The FAA’s certification requirements are rigorous for turbos. You have to do burst testing to demonstrate shrapnel containment [and] you have to do overspeed testing, along with other tests, so designing our own from scratch really was non-negotiable,” Webb notes. Considerable CFD analysis contributed not only to the turbo- and supercharger, but also the exhaust channel, which features an ovular cross-section and a bend radius carefully controlled for aerodynamic efficiency, after CFD tests found only the outermost parts of the older, circular channel were being traversed by the exhaust gas. Each cylinder has three exhaust ports feeding into a welded exhaust pipe, each pipe converging in a Y with that of the adjacent cylinder, and the two combined channels then converge in a block that bolts to the turbocharger at the back of the DHK180. “Our exhaust system doesn’t use a wastegate,” Webb says. “They’re failure-prone, and moreover we don’t need one because our engine can’t be over-boosted – it’s a two-stroke. If too much forced air goes into the cylinder, the excess just rushes out of the open exhaust port before it can be compressed and combusted, and that actually helps by cooling the exhaust.” Compression and combustion One may surmise from this that, unlike the aforementioned ATI twostroke diesel, the DHK180 is not uniflow-scavenged (when poppet valves govern the exhaust ports to enable the compression of turbo- or supercharged air); nor does it use a rotary exhaust valve to trap the air, as in the REVolution two-stroke gasoline (issue 33, August/September 2020) from Strange Development, now Alpha-Otto Technologies. The DHK180 is Schnuerle (or ‘loop’) February/March 2024 | Uncrewed Systems Technology The turbocharger has been designed for the higher airthroughputs, exact exhaust flows, pressure ratios and air volumes of the DHK180; COTS turbos were deemed insufficient for certification
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