Uncrewed Systems Technology 049 - April/May 2023

88 Dossier | Rotron Power RT600-HC of our rotor’s combustion pockets over the years to have a more leading shallow shape, with greater compression towards the rear flank of the rotor to achieve the best squish characteristics.” Exhaust ejector The improvements in the design of the combustion chamber have made for a more effective squish and a more efficient burn in the RT600, while also producing greater concentrations of heat on the leading edges of the rotor. To counter that, a leading-fin design has been adopted that more effectively conducts heat away from the combustion pocket that sits in each of the rotor’s three sides. This in turn makes for a faster and more efficient heat exchange between the rotor’s internal fins and the cooling air that passes through the rotor. In 2015, the RT600’s rotor was either charge-cooled as per the LCR version, or cooled using an exhaust ejector system as per the XE (although in our previous article, Rotron referred to it as an ‘exhaust extractor). These days, both the HC and XE versions use a system that is an evolution of the original exhaust ejector, in this case having been optimised for aerospace rather than racing. “We can still do charge cooling, but that inherently has a power limitation, because it heats the charge and reduces its density before it can enter the combustion chamber,” Head comments. “That could have benefits for heavy fuel or multi-fuel operations, but if you really want to ensure cool, clean air in your gasoline combustion chamber, we have an exhaust ejector.” The system works by using exhaust gas velocity to generate a vacuum, which in turn draws cooling air through the engine via the Venturi effect – as used by Teledyne Energy Systems ( UST 35, December 2020/January 2021). Teledyne’s fuel cell uses a Venturi ejector to passively manage its reactant gases, without the need for pumps, actuators or other moving parts that could fail while a vehicle was out in space or underwater. In the Rotron ejector, exhaust gases exiting the combustion chamber flow into a manifold, connected to which is a channel running from an oblong aperture on top of the centre plate, which serves as an outlet for cooling air. The wide shape of the manifold relative to the prior exhaust outlet causes a drop in local static pressure, and the ensuing pressure differential between the manifold and the air channel induces a vacuum in the latter. Inlets for cooling air sit on the side plates, and the vacuumdraws air into themthrough the stationary gear on the side plates, picking up oil and helping to lubricate the shaft and rotor bearings in the process before being sucked out of the centre plate aperture, into the ejector manifold and finally expelled fromthe engine. Inside the sealed elements of the rotor housing is a lemon-shaped aperture. In addition to holding the stationary gear and eccentric shaft, this aperture is where the cooling air passes and is briefly concentrated to conduct heat away from the rotor. As the aperture is completely sealed, the cooling air is prevented from entering and contaminating the combustion chamber. Given the oiling assistance it provides to the bearings, what enters the ejector typically ends up being a mist of air and two-stroke oil. “Since 2015, we’ve matured our exhaust ejector into a piece of technology that we can integrate into lots of different aircraft and engine configurations,” Head says. “Of course, it has to be tuned for fluid dynamics and specialist materials depending on the use case, but as with any Venturi pump system it has no moving parts, which keeps it 100% reliable. “And since the energy of the exhaust gases would just shoot out of the engine unused without this system, essentially we’re recuperating otherwise wasted energy without any detriment to engine power.” The absence of pumps, belts and gears also keeps the system lightweight; even the ejector manifold weighs no more than a typical set of mufflers and exhaust pipes. The expansion of exhaust gas in the manifold does create some noise, but Rotron says its customers are satisfied with the present configuration and its performance. April/May 2023 | Uncrewed Systems Technology Ignition is performed using four spark plugs per combustion chamber, which fire in a phased order that increases fuel efficiency and makes for a cleaner burn