Unmanned Systems Technology 013 | AutonomouStuff Lincoln MKZ | AI systems | Unmanned Underwater Vehicles | Cosworth AG2 UAV twin | AceCore Neo | Maintenance | IDEX 2017 Show report

60 Dossier | Cosworth AG2 UAV twin Performance Heath admits that in theory the power output could be increased by an exhaust design that is impractical within the confines of a UAV and its silencing requirement. Nevertheless, the AG2 has peak power of 12.3 bhp. The engine is TBO-validated on Cosworth’s dyno running to 10 bhp at 5000 rpm on representative mission profiling. On that basis, the engine requires an overhaul at 250 hours, when the likes of pistons, piston rings and bearings are inspected, then again at 500 hours. Its design life is 750 hours, although some components will continue beyond that timespan. Maximum torque is more than 13 lb-ft (18 Nm) and the package weighs less than 10 kg including exhaust, generator, propeller and ECU (but excluding the generator regulator/rectifier). The AG2 is 468 mm wide, 143 mm tall and 199 mm long. Brake-specific fuel consumption is better than 260 g/kWh (0.43 lb/hph). Heath remarks that this figure is “twice as good” as at least one well-established spark-ignited, jet-fuelled rival two-stroke. This fuel economy advantage can be used to offer increased mission duration or payload capacity. The 750 W electrical generator is not used as the engine starter; instead a handheld electric cranking device is employed. The compression generated by 15 s of cranking raises the temperature of the cylinder sufficiently to initiate combustion; Cosworth has demonstrated that the AG2 can be started in temperatures down to -18 C (0 F). Regardless of whether it is running on diesel, JP5 or JP8, the engine management system automatically optimises operation regardless of temperature or altitude: the engine will operate from -40 C to +50 C and at up to 15,000 ft. Although development has been funded jointly with the US Navy, Cosworth has the rights to commercialise the project, and the AG2 is suitable for UAVs and other small unmanned vehicles in the marine and land spheres. It promises to demonstrate to the world of unmanned systems the formidable potential of a small-displacement engine exploiting compression ignition. April/May 2017 | Unmanned Systems Technology Mindful of the compromise required between weight and strength given compression-ignition loading and the durability target, Cosworth applied its considerable resources in terms of structural FEA and fatigue analysis to the AE/ AG project. It also undertook Simpack multibody dynamic analysis of the cranktrain components. There was the option of putting the generator on the nose or the rear of the crankshaft, in the latter case having its rotating mass next to that of the other main rotating mass, the propeller. Cosworth’s analysis, which embraced crankshaft stiffness, revealed that having both inertias at the rear minimised the required crankshaft mass. For performance simulation, Cosworth used GT Power 1D models, which defined the boundary conditions for the combustion CFD. The innovative combustion system developed for the AE through to the AG2 was optimised at the design stage with the assistance of Converge reactive 3D CFD code. This allowed the design team to screen a wide range of options. “Predictive combustion with detailed chemistry provided insights into the influence of altitude and fuel type on combustion, further helping to identify a robust combination of injector and combustion chamber,” principal engineer John Heath says. “The result is a proprietary combustion chamber design ensuring high efficiency and a wide range of ignitability, leading to a wide range of operability.” CFD was applied to study the cooling flows over the heads and barrels. Thermal FEA used input from the 1D modelling to help define cooling fin geometry in the light of an understanding of its impact on piston and cylinder temperatures. The 1D and 3D analysis was extended to the exhaust system. This allowed the effect of a given intake and exhaust geometry to be evaluated in terms of noise and engine performance. Heath remarks that there was an excellent correlation between the design models and subsequent rig testing of the actual engine. The main Cosworth UAV dyno is a full motoring unit, electrically operated and exploiting control system technology developed for Formula One. It incorporates an intake air chamber in which the pressure can be varied to simulate conditions at altitude (using an electrically driven supercharger to pull pressure down from ambient). A combustion chamber pressure sensor is used to provide full combustion analysis. The very low fuel flow associated with the AG2’s power level has prompted Cosworth to specify a sophisticated Coriolis-type meter to obtain the level of accuracy needed to properly measure brake-specific fuel consumption. The engine can be run on another rig complete with propeller, fitted into a small section of the vehicle in question so as to use the actual engine mounting system together with the relevant fuel and lubricant supply. Control of this rig is again sophisticated enough to replicate a given mission profile. Design and development at Cosworth