Suter Industries | Dossier rings, and gradually into the cylinders, where it burns up as conventional in most two-stroke UAV engines. “But it bears mentioning that we’ve closely studied both the performance and the insides of our engines, which are lubricated using premix, and our oilsprayed engines, and right now the latter approach, as designed into the 288SDI, exhibits the least carbon build-up among any of the engines we’ve built,” Giussani notes. Kehe adds: “That was surprising to us, as I’ve worked with oil-burning kerosene engines, and experienced firsthand how bad their carbon build-ups can be. “In the past, I’d seen other such engines exhibit much faster drop-offs in performance due to the exhaust port getting clogged up with carbon deposits, or their compression ratios would degrade because there was carbon collecting in the piston crown or under the cylinder head. So we’re very satisfied with how we’ve engineered the lubrication in the 288-SDI.” Knocking countermeasures While some manufacturers will go through exhaustive programming to embed an anti-knocking strategy into the ECU of a new heavy fuel engine, Suter’s approach to prevent knocking has focused on optimising mechanical design and stability to ensure consistently good conditions for combustion. Part of this comes from the determination that not every single combustion process can be controlled, because supplied engines cannot be instrumented so heavily as to be monitored comprehensively for perfect adjustments of key factors, such as fuel-injection timing, injection quantities, ignition timing and gas exchange. It also comes from the consensus within Suter that the most important thing for combustion stability by far is ensuring controlled, consistent conditions around the spark plug (matching what is needed by the fuel) at the point of ignition. “In a spark-ignited gasoline engine, you basically try to fight knocking by speeding up the combustion process. In a spark-ignited kerosene engine, you get a naturally high speed of combustion, but that induces combustion to start in an unpredictable, sometimes erratic manner,” Giussani says. “The appropriate response should therefore be to optimise engine design around the perfect speed, with clear definitions of when we want the firing to start, when we want combustion to start, and how we want the combustion modulated.” Mechanically optimising for combustion stability required extensive work in fluid dynamics and the design of the cylinderhead interior. This ensured precise microturbulences inside the cylinder to achieve closely-repeated tumbles and (to a lesser extent) swirls in every combustion cycle across cruising speeds and all the way up to wide-open throttle conditions. To evaluate how well such stability was being achieved, Suter focused on measuring the indicated mean effective pressure (IMEP) inside the cylinder, analysing hundreds of engine cycles’ worth of data to examine the variance of IMEP from one cycle to the next. “Having large variations would mean clusters of both good and bad cycle stability, hence indicating some problems akin to a lot of misfiring happening. What we wanted was very minor variations, meaning very stable combustion about an average model,” Giussani explains. “As well as being important to prevent knocking, stabilising and reducing IMEP variations means you have a basis to start working on improving 77 Uncrewed Systems Technology | February/March 2025 Copious fluid dynamics simulation and testing were key to optimising combustion stability in the HF TOA 288-SDI Mechanically optimising for combustion stability required extensive work in fluid dynamics and the design of the cylinder-head interior
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