Alpha-Otto REV Force | Dossier quickly, and the bypass system means we can quickly dial back and recondition whatever kind we’ve been sending,” Krzeminski says. As well as devising this air preconditioning approach since 2020 (complementary to the pre-existing REV-based scavenging and exhaust control system, and for its planned multi-fuel strategy), Alpha-Otto has engaged in CFD optimisations of its intake manifold’s internal geometry, after finding a slight bias of turbulence favouring the rear cylinder over the front cylinder through testing. Additionally, as we reported five years ago, Strange Development went from a centrifugal supercharger to a Roots-style supercharger to improve intake pressure at low and idle speeds, particularly for starting power. But, more recently, Alpha-Otto has switched to a screw-type device from Sprintex Superchargers. That has helped to greatly optimise for ΔP across the air preconditioning system at low rpm conditions, thanks to the manner in which it compresses air internally, and the fact that it can alter the volume and pressure of air passing through it, thereby also generating heat (none of which a Roots blower does). “People talk about Roots blowers being the most efficient positive displacement devices because you compress after the supercharger and then don’t create heat, especially if you have a bypass valve, but we often actually want that heat, so it’s valuable to us to have a compressor that acts as a secondary heat source,” says Krzeminski. “Although right now the supercharger is oiled from its own sump, we are planning on integrating it with our own oiling system to have everything work more tightly together. We’re also planning to test the overall engine with both this supercharger and a turbocharger as well. “The supercharger is belt-driven, but to offer a high-efficiency mode we really want to be able to dispense with that parasitic loss, so a turbo would enable us to unload the supercharger and maintain a desired compression without shaft efficiency suffering.” Turbulent charge motion Charge motion refers to the quantity of kinetic energy in the air at the desired moment of ignition, and it is a key contributor to both flame propagation in spark-ignited engines, and the chance of successful auto-ignition and diffused combustion, given a homogenous fuel/air mixture across the combustion chamber. “It can be hard to get good charge motion in a four-stroke because they pull in air as the piston goes from TDC to BDC and back again, and the air slows as it flows around the poppet valves and fills up the chamber, unlike in a two-stroke cycle, where the motion induced by the scavenging and the open ports ensures relatively high turbulence and hence charge motion,” Krzeminski says. “Our supercharger pumps air in, and the internal geometry we’ve engineered means we’re directing air into the chamber with the exact angle needed to create as much charge motion as possible before immediately compressing it with the piston and igniting it. While a four-stroke goes through a full, 360° crank rotation to draw in air, we are getting all of our air in just less than 180°. “That first maximises and then preserves the kinetic energy of the air, because our intake port isn’t fully opened until our piston is at BDC. It starts to move back up right after air starts entering the cylinder and then we ignite before TDC.” This quality is critical for achieving a homogeneous mixture of air with almost any fuel, and it is especially crucial to how the REV Force runs smoothly on hydrogen and JP-8. These are two of the most knock-sensitive fuels that AlphaOtto’s target customers wish to use, and knocking occurs primarily as a function of hotspots forming in the cylinder. By keeping the air moving before ignition, the engine achieves not only a uniform distribution of fuel and air across the combustion chamber, but also a uniform temperature spread. Hence, hotspots cannot form and knocking is not initiated. There are a few critical aspects of Alpha-Otto’s engineering that are key to how it achieves a turbulent charge motion. One is the previously 73 Uncrewed Systems Technology | April/May 2025 The domed-piston crown shape helps deflect intake air in a way that aids turbulence and charge motion in the REV Force’s cylinder
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