Alpha-Otto REV Force | Dossier conditions, and will gladly demonstrate it for any well-intentioned visitors, in the field, fuel inconsistency is a widespread problem. “It’s very hard to get ideal conditions in terms of fuel purity. A tank of supposed 87 Octane just doesn’t look the same in one location versus another,” Krzeminski explains. “Even automotive hydrogen comes in purity levels from 95% upwards, and price increases exponentially with purity. Hence, there’s a lot of push in the hydrogen community for engineers to power mobility on low-purity hydrogen, because that can cut the cost by 50% compared with max-purity hydrogen. So it’s critical to always have the spark as backup in case the ignition event isn’t happening when we expect it to, so that combustion happens no matter what.” Lastly, the spark plug acts as a useful backup amid high transients, with Alpha-Otto noting that, in general, diffused combustion works best when the engine runs at a consistent, stable rpm that is suited to efficiency maximisation. So, while some end-users would doubtlessly want to use at least some of the REV Force’s 127 kW for shaft horsepower, particularly when driving a main propeller, the company concedes that its ultimate value in the uncrewed space would likely be as a constant speed range extender in hybrid powertrains (or as a ground-based stationary power system for fieldrecharging battery packs). Variable REV timing As indicated, a back pressure valve is now installed downstream of the cylindrical REV, bringing an extra degree of control over exhaust gas movement in the REV Force engine. A few other qualities about this core aspect of the engine’s technology have also changed since our investigation five years ago. For one, the shape of the valve has been optimised to achieve better control fidelity over airflow. “The last valve was almost like a switch: air either flowed through the exhaust port or it didn’t. We wanted to change both the port and valve shape to allow more finite variations in flow, and more finely tune how air moves into and through the exhaust system,” Krzeminski explains. “So, while the REV from five years ago was very straight, now the orifices and port are more ovular, which allows a more sinusoidal curve in terms of airflow rates as it rotates, and opens and closes.” Furthermore, while some may recall that we previously reported on REVolution’s rotary valve being beltdriven by the crankshaft, Alpha-Otto has disclosed to us that the design (in both REVolution and REV Force) actually features an additional planetary gear component between the belt and the rotary valve. That gearbox is servoactuated during operation to phase the relative angle between the rotary valve and crankshaft, hence serving as a variable valve-timing (VVT) mechanism. “Much like when one is developing a cam phaser for VVT, we’ve spent a lot of time evaluating crank position and exhaust valve position, and changing the latter depending on either the combustion mode we want to run or the speed load point,” Krzeminski says. The planetary gear works continuously, phasing the valve anywhere over a 360° range. That means, for instance, that the valve could be made completely wide open, or bordering on shut, as the piston fires downwards and opens the inner exhaust outlet to the combustion chamber, effectively enabling 77 Uncrewed Systems Technology | April/May 2025 The rotary exhaust valve now features a curving shape, which helps tune the air movements into and through the exhaust port It’s very hard to get ideal conditions in terms of fuel purity. A tank of supposed 87 Octane just doesn’t look the same in one location versus another
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