76 there’s COTS systems we’ve bought and integrated for that,” Krzeminski says. “However, in addition to having the REV open and shut the exhaust port, we’ve added a back pressure valve – a normal, servo-controlled butterfly – just downstream of the REV. That allows us to trap and create a ‘slug’ of exhaust air that stops fresh intake air from leaving the combustion chamber. Because of this, we’ve found no hydrogen in our exhaust when using the upstream port injection.” Alpha-Otto has also looked into creating its own DI system, pressurising fuel in the pump before driving it into an injector for it to mechanically spray the atomised fuel via hydraulic pressure, and the company anticipates optimising and manufacturing such systems for defence applications relying on JP-8, owing to that fuel’s corrosiveness and the durability limitations of electronic injectors when handling it. “But there are so many optimisation problems with DI, including droplet size, dispersion, wall washing and, of course, mixture homogeneity. As it stands, our port injector is positioned on the floor of the intake port and angled up into the cylinder, such that it sprays up into the combustion chamber at the cylinder head pocket,” Krzeminski says. The company has found this strategy resembles DI enough, particularly in terms of the fuel efficiency gains versus an earlier, more conventional injector location and angle, that its tests show little gain when spraying from the direct injector in the cylinder head. It is meanwhile considering trialling a sleeve-mounted, sideways injection system (akin to Suter Industries’ semi-direct injection; see Issue 60), but notes its present port-injection approach is already optimised for diffused combustion, so any new approach will incur fresh, painstaking months of optimisation to suit its LTC mode. “All that said, the two-stroke architecture lends itself well to DI integration. We have a sizeable combustion pocket inside the cylinder head, so we have a lot of room for packaging an injector through that,” Krzeminski notes. “The one we’ve used there is a bent stream injector, from which fuel flows down into the cylinder, but we have done a lot of work to determine the best time to inject, and we’ve actually found the best time is actually while the intake port is still open, because we get a tremendous amount of charge motion at that instant, so that’s how we get the most mixing. Traditional spark-ignition engines, by contrast, tend to inject later in the combustion cycle. “We’ve looked into strategies built around that, and there are benefits to mixture homogeneity, especially if you can do multiple quick injections throughout the compression stroke, but other than enabling kerosene operations, especially JP-8, we are still leaning more towards our port injection plus diffused combustion approach because of its fuel-agnosticism benefits, and because it doesn’t bring the drawbacks of having to inject fuel at very high pressures. “But, if a defence customer specifically wanted JP-8 – and by and large, we’ve found the boots on the ground do – then we’d likely optimise for the last 1% of power with a DI plus CI version of REV Force.” Spark-assisted ignition One area where LTC falls short of Alpha-Otto’s wishes is when aiming for peak power. At such speeds diffused combustion becomes difficult to manage, and a spark plug in the cylinder head pocket kicks in to continue power output until operations return from peak power to peak efficiency mode. “Auto-ignition is a spontaneous reaction by nature, and hence unpredictable, so we’re continuing to compare pure auto-ignition versus our spark-assisted auto-ignition. “The latter encompasses moments where we have almost ideal conditions for auto-ignition, but still need the spark plug to push it over the edge into combustion, as triggered by the ECU when auto-ignition isn’t happening,” Krzeminski says. Those moments are likely to come more often than expected and not only when running at redline speeds. While Alpha-Otto is undaunted when asked to achieve auto-ignition in lab April/May 2025 | Uncrewed Systems Technology The back pressure valve enables added control over exhaust gas movements above the rotary exhaust valve
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