74 Dossier | Alpha-Otto REV Force discussed air preconditioning, by which air pressure, volume, temperature and velocities are optimised to maximise turbulence, so controlling the interplay between the air and fuel at hand. The next aspect concerns the geometry of how the intake ports are oriented into the combustion chamber, and Krzeminski describes this as being the most important contributor. While traditional transfer ports are forgone due to the absence of crankcase scavenging, each cylinder’s air inlet nonetheless starts on one side of the block and then splits into two pathways that wrap around the cylinder, running in opposite directions, each one entering the cylinder at the opposite side to the air inlet. “As well as that wraparound motion, the ramp and the angle created inside the intake port are really critical, not just for creating charge motion but also for mixing fuel and air by forcing the air up into the top of the combustion chamber, so it then splashes out and down,” he explains. “We have also redesigned all our port openings with something of a bellmouth shape, which naturally helps the gas ‘grab’ the sides of the walls, resulting in a perfect laminar flow with no pressure drop along the way, unlike a straight pipe, which tends to result in a low pressure drop and struggles to preserve kinetic energy of the gases.” Another vital geometric factor comes from Alpha-Otto’s piston crown, which features a dome shape carefully optimised to deflect intake air upwards in the manner suggested, and aid turbulence and charge motion, but without accidentally causing air to spray towards the exhaust port. “The air goes above the exhaust port and then pushes up into the combustion chamber. Since we have a cross-flow scavenging design, with the intake ports across from the exhaust port, albeit lower down, getting the piston crown shape perfect was absolutely critical, both for incoming air and for enabling burnt exhaust gases to rush out efficiently too,” Krzeminski adds. “Some of your readers might remember the really old two-stroke piston crowns that had a weird sort of ‘wing’ shape on top of them, for a similar kind of deflection effect; an asymmetric piston wouldn’t work in our application, as our piston crown shape has to let air in as well as it lets exhaust out, and work together really intricately with the port locations and alignments.” The timing and metering by which the REV allows gases out of the chamber has also been critical, and precisely tuned, to controlling the turbulence, charge motion, and mixing (this is explained further on). Optimising the port and piston geometries, along with the air preconditioning and exhaust management, took extensive CFD work. Beyond simulations, Alpha-Otto sought novel means of recording airflow experiment results inside the cylinder, including the use of a seethrough cylinder head, placements of ribbons inside of the cylinder and close examination of oil deposit patterns inside, to understand where the air was flowing into and throughout the cylinder. “Also, given that we have persistent airflow, with some air blowing into the piston skirt even when the intake port is closed, we’ve adjusted our piston-ring design such that there’s a compression ring and an oil-control ring at the bottom of the skirt, contrasting with the two compression rings around the top of the skirt,” Krzeminski adds. “You’ll remember that we keep the crankcase as sealed from the cylinder and as close to vacuum-like conditions as possible to prevent drag losses on the crankcase as we go from 6000 rpm to 8000 rpm, so it’s absolutely crucial we avoid even a tiny amount of blow-by.” Injection question The REV Force engine has been equipped with two direct injectors and four port injectors across its two cylinders, as part of Alpha-Otto’s strategy of researching and enabling a wide range of fuels within one engine. “Being able to inject fuel directly into the engine after the ports shut is advantageous for liquid-based fuels where hydrocarbon contamination of the exhaust stream is a concern, and for DI of gasoline or ethanol-based biofuels. DI really isn’t novel anymore, and April/May 2025 | Uncrewed Systems Technology Alpha-Otto anticipates that it may stick with port injection due to the advantages of its strategy there, although direct injection will be most useful in JP-8 applications
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