80 Dossier | INNengine Rex-B converts combustion power into shaft motion via the cam track (especially amid reaction forces at the cam-roller interface, which may be large and impact efficiency given the shallow angles at some points), and therefore the amount of fuel it consumes relative to power output at peak or cruise rpm. INNengine has yet to publish SFC figures. On efficiency, Ruben comments, “Rex-B’s kinematic curve shows similar conversion of combustion energy into rotational force as crank engines. And net output favours the cam’s due to lower friction losses: the contact areas for our conversion are further from combustions, in cooler and more oiled areas.” Objective advantages of the engine include an inherently compact cylindrical shape (owing to the arrangement of the cylinders in parallel around the output shaft, rather than at right angles to the shaft as in radial, flat and inline engines). That, combined with the number of combustions per shaft revolution, give it high ratios of power to weight and to size. The two-stroke spark-ignited engines we have previously featured rarely exceeded 3 kW/kg. As some examples, we have covered a 2.5 kg engine displacing 100 cc and producing 7.3 kW, a 3 kg engine displacing 116 cc and outputting 8.4 kW, a 28 kg engine displacing 548 cc and producing 36 kW, and a 50 kg engine displacing 578 cc and outputting 164 kW. As a reminder, the 125 cc Rex-B weighs 4.6 kg and produces 16 kW, while the 288 cc version weighs 9 kg, so if INNengine’s claims are accurate, the 288 cc Rex-B is the most gravimetrically power-dense UAV engine we have encountered. At the time of writing though, insufficient data is available from UAV engine companies for a comparison of volumetric power densities. Juan notes though,“It’s not only about power. We have higher torque relative to power than most; our pilot test clients are actually finding that desirable for applications like hydraulic pumps and power generators. “Our piston linear speeds can be higher than in conventional engines, as lateral load is moved from the piston head to the piston body, where lubrication is abundant and heat is lower. And even if linear speed stays the same, our other moving parts move more slowly, reducing friction and improving efficiency, durability and reliability.” As a last benefit to note, the RexB’s vibration profile benefits from the elimination of second-order vibrations, given the absence of traditional big-end journals swinging the piston from side to side. The guide plate also holds the piston rods in place to prevent the limited sideways motion that could otherwise occur as the rollers run in the cams. Future plans Going forward, INNengine plans to continue producing and optimising trial units of the Rex-B and E-Rex in-house to court customer and partner interest. INNengine also plans to run its engines on green fuels including hydrogen to see how its engines could enable sustainable aviation and road transport without the power and energy losses of going fully electric. December/January 2024 | Uncrewed Systems Technology Rex-B Two-stroke (with two combustion events per shaft rotation) Cam-driven engine Gasoline (with 50:1 pre-mix, existing version, or 400:1, new lubrication system) Spark-ignited ECU EFI or carburetted Naturally aspirated Air-cooled Schnuerle scavenged 125 cc version Gasoline (with 50:1 pre-mix) Bore x stroke: 40 x 25 mm Weight: 4.6 kg Dimensions: 214 x 146 x 146 mm Maximum continuous power output: 21.5 bhp (16 kW) at 6000 rpm Maximum continuous torque: 27 Nm TBO: in progress (threshold is 500 hours, goal is 1500) 288 cc version Gasoline (with 400:1 pre-mix) Bore x stroke: TBD Weight: 9 kg Dimensions: 269 x 188 x 188 mm Maximum power output: 55 bhp (41 kW) at 6000 rpm Electric power output: 20 kW Maximum continuous torque: 68.2 Nm TBO: In progress (threshold is 500 hours, goal is 1500) Specifications The cylinder heads are cooled by a combination of direct external airflow and internal charge flow
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