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55 Cosworth AG2 UAV twin | Dossier either piezo or solenoid. The injection is in two phases on each cycle – a primary ‘pilot’ shot and then the main pulse. He notes that the high fuel pressure calls for high precision in the design and manufacture of the pump to avoid leakage. Indeed, the entire fuel system is a triumph of miniaturisation, and is now proven over thousands of hours of running time. Combustion system Heath notes that the AG2’s dimensions and its operating conditions are such that the combustion strategies developed for contemporary diesel car engines (which are invariably four-stroke, four-valves-per- cylinder turbos) don’t apply. “However, the combustion analysis tools and experience that Cosworth has developed over the years were applicable,” he says. The contemporary four-stroke turbodiesel uses a flat head with a piston crown bowl forming the combustion chamber. In this engine the crown is domed, with the head forming the chamber in the manner of a spark-ignited two-stroke. Taking the place of a central spark plug, the upright injector protrudes into the chamber. There is no additional combustion/pre-combustion chamber. In the case of the four-stroke turbodiesel, to promote mixing, the charge is encouraged to swirl axially into the piston bowl. With this engine the in-cylinder charge motion is tumble-like, using the Schnuerle loop principle to assist the simultaneous entry of the fresh charge and operation of the exhaust in the normal two-stroke manner. The rear-located secondary transfer port is designed to aim charge upwards, encouraging the required looping motion. The chamber in the head is of a smaller bore than the piston, the dome of which extends over its full crown area. The area of the head surrounding the chamber is chamfered to match the corresponding outer area of the dome, creating a tight squish band. This forces almost all the charge into the combustion chamber, which has a slightly convex roof facing the domed surface of the crown. Interestingly, the piston dome has flattened with development. Heath says that, given the loop scavenging, under compression there is not significant bulk motion of the charge to assist mixing with the injected fuel. “The mixing is all in the spray pattern,” he explains. “But you have to avoid wall wetting. The keys here are the fuel pressure, the number and size of holes and how the spray is targeted.” The key to the engine’s operation is therefore in the detail geometry of this chamber and of the design of the injector, which has multiple holes. This combustion system, which has been patented by Cosworth, has coped admirably with the wide – and unpredictable – range of cetane values. Engine structure The crankcase, barrels and heads are billet productions, machined in- house from aluminium alloy 2014. The transfer porting is CNC-machined into the respective crankcase and barrel elements, and each cylinder has a nickel silicon carbide bore coating. Four studs jointly attach the head and barrel on each side of the crankcase. Head-to-barrel sealing is by means of a solid, flat copper ring; a graded selection of rings allows the squish gap to be optimised. The crankcase Unmanned Systems Technology | April/May 2017 Cosworth started its UAV project in 2005 when it became evident that the US military had a policy to use jet fuel – specifically JP5 and JP8, both developed for gas turbine engines – for UAV use. Its first UAV engine was the single-cylinder, naturally aspirated, compression-ignition AE, which was flight tested in 2007 and 2008. The US Navy was sufficiently impressed by that project to fund development of a larger engine, a twin producing 10 bhp, whereas the AE had been a 3 bhp unit. This AG engine was first run in 2010, in climatic and altitude testing at the US Navy’s Pax River facility. The first iteration of the AG was mechanically injected for simplicity and dependability. It could operate at 15,000 ft but the (cam-controlled) injection timing was not at its optimum at that height above sea level, having to be compromised for adequate performance at lower altitudes. That prompted Cosworth to move to common rail injection, exploiting its in-house expertise in engine management system technology. The common rail version of the AG was successfully proven in flight testing to 15,000 ft at Edwards Air Force Base during 2012. The US Navy was not at that stage ready to switch to a new engine; however, in 2014 the AG was revived at the request of the US Air Force Research Laboratory, which was undertaking a study into high- pressure direct injection. The AG was used to benchmark different high-pressure direct injection systems, and proved the worth of its Cosworth-developed system. In 2015, the US Navy re-instigated development funding for the AG, owing to a requirement for higher propulsion performance and more electric power for advanced payloads and a much longer time between overhauls (which was requirement number one by importance). The first AG2 prototype engines were recently completed, and the US Navy has an ongoing Technology Demonstration using the AG2 in the current RQ-21A air vehicle, which will be flying in 2018. History of the project