70 Dossier | Alpha-Otto REV Force delay, or for the formation of a dilute cylinder charge with homogeneity across bore and stroke. Several modes of LTC have been proposed and researched over the last decade, including the aforementioned HCCI, as well as premixed charge CI (PCCI), partially premixed LTC (PPLTC) and reactivity controlled CI (RCCI). Alpha-Otto’s approach has come about as a result of r&d into areas not strictly related to UAV engines, although its technology comes to us with the company having identified that its newest offerings – in LTC, efficient multi-fuel operations, and hybridisation – bear watching for people seeking high-power output and high-efficiency UAV operations amid insecurity over which fuels ICEs may need to run on in the future, be it in defence, commercial or civil applications. Stages of Alpha-Otto’s LTC While the two-cylinder engine block looks unchanged from its form five years ago (and the core principle of using a rotating valve at the exhaust port end to trap supercharged air and boost engine power is still there), its ancillaries have been revamped considerably, with the new LTC operating principles broadly stemming from a close interaction between them. “The pumping work of this engine is separate from the power cell. Upstream, our air preconditioning and induction system controls the air going into the engine. Downstream, our scavenging and exhaust control system controls the air leaving the engine – with the REV component being a critical part of the latter,” Krzeminski tells us. “Depending on how exactly we want to fill or evacuate the combustion chamber, we can control for the ΔP [delta P] across that chamber, based on how we control the ways those upstream and downstream systems work together. We can increase the gas pressures upstream or downstream, and control their flow rates as they pass through the combustion chamber. “But it is important to note that those two systems – and hence our solution and LTC mode – could never work without the two of them, and how we’ve very carefully tuned them to interact perfectly over the last five years. Having exhaust control without pressure from upstream, or having upstream pressure without the ability to trap the added intake air in the combustion chamber, just doesn’t work. We have both, and it’s taken a lot of CFD and ECU work to get it right.” An understanding of the latest technologies defining this engine is best achieved through the lens of its LTC process. This consists of five stages (chronologically and in terms of the related engine parts), with the aforementioned air preconditioning and induction system naturally coming first, so as to precisely regulate intake air temperature and pressure in a way that achieves Alpha-Otto’s desired dynamic compression ratio and conditions ideal for homogenous, diffused combustion. Then, a turbulent but sustained charge motion in the combustion chamber further hones those conditions and enables a thorough fuel/air mixing, as well as stabilising the subsequent combustion with a lean mixture and uniform diffusion. The injection of fuel that follows is precisely timed, angled and quantified to prevent premature ignition (this objective being aided through deliberate cooling of the chamber, enabled by the intake and exhaust systems mentioned earlier) and optimise the subsequent combustion. Here, a DI system enables knocksensitive fuels such as kerosene and hydrogen to enter the cooled chamber after the ports have closed, again reducing pre-ignition risks, and this aids efficient, homogenous fuel/air mixing (although Alpha-Otto admits its port injection yields such positive and consistent LTC results that DI may not be necessary). Diffused combustion may then occur via auto-ignition, or through what the company calls “spark-assisted ignition” in the event of poor fuel quality, which interferes with the process. Then, as mentioned, a scavenging process managed using the REV – responsible for trapping the intake air to increase dynamic compression earlier in the operating cycle – enables precise exhaust flow and internal EGR to optimise the exchange of gases inside the cylinders, controlling residuals and conditions in the chambers for the April/May 2025 | Uncrewed Systems Technology Alpha-Otto’s direct injection lets knock-sensitive fuels enter the cooled chamber after the ports close, reducing pre-ignition risks and helping the homogenous fuel/air mix
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