69 (than in engines burning conventional stoichiometric fuel/air mixtures). That means higher thermal efficiencies, and considerably reduced NOx and soot emissions for such engines. “In expanded terms, LTC is more efficient than conventional combustion because you’re just losing less heat to everything. Look at a typical thermal map of an engine cycle and you see 55-70% or more of your combustion heat going out of your exhaust,” Krzeminski says. Naturally, if a greater proportion of the heat from combustion can be contained within the expansion of the combustion gases, less of that heat is lost through exhaust, transfer into the coolant, the metal engine components and so on. Combustion modes are a defining factor in this: compression-ignition (CI) engines typically achieve about 30-45% in thermal efficiency indexes. Sparkignition (SI) engines conventionally operate at between 20% and 40% thermal efficiency. As Krzeminski explains: “There’s two ways to get thermal efficiencies close to 60%. First, Formula One engines ignite their fuel/air mix in a prechamber before jetting out across their combustion chamber, achieving borderline CI-type combustion in a SI engine, with near spontaneous or auto-ignition of that fuel/air mixture – with inherently no need for a flame front to move across the stroke, like in a typical SI engine. “The Formula One way is an excellent method. I honestly have no criticisms of it, and we take a little bit of inspiration from it for our spark-assisted tech. But the Holy Grail is something like homogeneous charge compression ignition [HCCI]. Our LTC is somewhat akin to HCCI-type operations, and what you’re trying to achieve in both is ‘diffused combustion’ in which combustion happens almost instantaneously across the whole combustion chamber. “That means near-constant volume ignition, with pressure going up exponentially, and without the combustion gases specifically expanding across the space created as the piston thrusts down.” If achieved, diffused combustion adds dramatic thermal and mechanical efficiency to an engine, but LTC has historically posed a number of optimisation challenges, such as optimising for the correct high levels of exhaust gas recirculation (EGR) for variable valve timing and ignition Alpha-Otto REV Force | Dossier Look at a typical thermal map of an engine cycle and you see 55-70% or more of your combustion heat going out of your exhaust Uncrewed Systems Technology | April/May 2025 REV Force’s high thermal efficiency comes from its low-temperature combustion and how it achieves diffused combustion – when combustion occurs almost simultaneously across the chamber
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