72 next cycle to keep peak combustion temperatures low. Each of these five steps in Alpha-Otto’s LTC cycle will be explored and detailed below. “These are all part of the same overall process, and we had the REV and supercharged induction on our last engine in 2020. But, since then, while correlating our engine to Gamma’s GT-Power 1D simulation software and also performing tests using kerosenes, we started gauging heat release and losses against different phasings of the exhaust valve, and discovered we could achieve this combustion mode at low temperatures precisely because of our unique intake and exhaust, with adjustments for the added three stages in between,” Krzeminski explains. Forced, variable induction Air first enters the REV Force’s intake manifold through an initial throttle valve, beyond which is a passage into the cylinders’ intake ports. That passage runs into a belt-driven supercharger, which compresses the air and blows it through a heat exchanger, which can function as an intercooler or a heater. Downstream of the heat exchanger is a secondary bypass valve, which can redirect the supercharged and conditioned air back to the throttle to control or meter (with quick response) how much of the preconditioned air reaches the combustion chamber, and how much gets cycled and fed back into the compressor. “With this arrangement acting as the engine’s forced, variable air pump, rather than relying on the piston, we can precisely control the volume and flow of air into the combustion chamber. “For instance, we can reduce the air’s density by heating it up greatly and putting it under low pressure, so if you’re idling or otherwise need more efficiency, we can really increase the volume of air going into the cylinder, separately from an increase in mass,” Krzeminski says. “This enables us to tailor the properties of the intake air for maximising thermal efficiency, or peak power or continuous power, without needing to change the hardware. It also means we can preheat or pre-cool the air, and increase or decrease the dynamic compression ratio, as is optimal for each of the different fuels or conditions we want to fly. “To run in high efficiency with hydrogen, we heat the air to between 100 C and 200 C, or to fly efficiently with altitude compensation we still might heat the air, but focus more just on increasing pressure to compensate for the ambient air density drop. We have tested REV Force successfully on both kerosenes and hydrogen, and sustained thermal efficiency results of 42-52%.” Achieving such fuel and environmental versatility depends not just on this air preconditioning in and of itself (with successful tests of REV Force running on both kerosene and hydrogen), but also highly optimised control strategies within the ECU to account for the different mechanisms at play. Air can be warmed either by the heater (with EGR, or with liquid coolant running from the exhaust ports and cylinder heads) or by compressing it; throttling upstream of the supercharger inherently puts the compressor at a low flow rate with a high ΔP, creating immense heat. “When we’re looking to go into a high-efficiency mode quickly, relying on the transient response of a coolant system alone to pump heat around the engine can mean you’re not getting heat to the combustion chamber fast enough. Our preconditioning system allows us to create a lot of heated or pressurised air April/May 2025 | Uncrewed Systems Technology A screw-type supercharger from Sprintex is now used to optimise for ΔP across the air preconditioning system in low-speed conditions We have tested REV Force successfully on both kerosenes and hydrogen, and sustained thermal efficiency results of 42-52%
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