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

52 Dossier | Cosworth AG2 UAV twin pressure, causing high mechanical loading with an implicit structural requirement and a weight penalty. “That challenge is where our race engine experience came in,” Heath says. “We used our mechanical expertise to make a light engine in the face of that loading. Inherently, a spark- ignition engine will be even lighter, but you need to take into account its higher fuel consumption, because of its lower efficiency. Fuelled for a mission of four-and-a-half hours or more, that means the UAV will typically have a higher take-off weight than one using our engine.” Interestingly, advanced computer modelling revealed that in the case of a piston-ported two-stroke such as this, turbo-supercharging is of limited benefit. “The last thing we close is the exhaust port, so if you try to supercharge you tend to just blow a lot of charge through the engine,” Heath explains. “You would need the complication of an exhaust-trapping valve, and that adds a lot of weight. And running un- throttled, we always have the full airflow at part-load so run very lean, which keeps our exhaust temperature low, so there isn’t much thermal energy to drive a turbine.” The naturally aspirated AG2 has a 5000 rpm routine maximum speed, which is to the benefit of the tip speed of the directly driven propeller; spark- ignition two-strokes tend to run faster. A slower propeller drive speed permits the use of a larger diameter, more efficient propeller for a given tip speed. Moreover, thanks to the lack of an ignition system, the AG2 creates less electromagnetic interference than its spark-ignited counterpart. Plus, there is one less system to go wrong. Heath notes that the key challenges of this engine were the fuel delivery system, the combustion system – and, given that with only air passing through the crankcase, premix was not an option – the lubrication system. General arrangement The general arrangement of the AG2 is that of a horizontally opposed ‘flat’ twin, with the tail of the crankshaft directly driving the propeller and a rotary valve running directly on the nose end. Here we refer to the drive end as the rear, as it would be in the case of a Cosworth race engine (which typically has the timing drive at the front, as is the rotary valve in this case). The charge-air feed into the crankcase is therefore axial with respect to the crankshaft. The two primary transfer ports are at the top and bottom of each cylinder with a secondary transfer to the front, while the exhaust is rearward facing. Heath remarks that the exhaust port is located on the piston pin axis and consequently away from the piston thrust face, which is in the interest of TBO. A high-pressure mechanical fuel pump is driven from the rear of the crankshaft. Its mount is formed integrally with the rear section of the crankcase, and the fuel rail is also integral. Behind that is the generator. Its stator is mounted on the rear face of the crankcase, while its rotor is powered by the tail of the crankshaft and in turn drives the propeller. While a flat-twin can have a April/May 2017 | Unmanned Systems Technology The last thing we close is the exhaust port, so if you try to supercharge you just blow a lot of charge through the engine The AG2 on Cosworth’s sophisticated motoring dyno