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

The AG2’s air-cooled barrel 51 team’s brief wasn’t to convert an existing engine to run on jet fuel, which inevitably implies the use of spark ignition. The two-stroke option was taken to minimise weight compared with a reciprocating four-stroke. The focus was to run on the kerosene-based JP5 and JP8 jet fuels used by the US military; JP5 has the higher flashpoint of the two, and is consequently favoured by the US Navy since it minimises the fire risk on aircraft carriers. Both fuels are used by the US military for ground vehicles in place of diesel, but they have no standard in terms of cetane rating (indicating propensity to ignite). Whereas regular roadside diesel tends to have a cetane rating of about 50 (on a scale of 0-100), JP5 and JP8 typically come in at 42-45, although on occasion fuel in the mid-30s has been experienced by Cosworth. At the same time, these fuels have a very low – and equally unpredictable – octane rating. That necessitates a spark-ignition engine developer specifying a low compression ratio at the cost of engine efficiency. By contrast, the low cetane value calls for a very high compression ratio for successful compression ignition, which gives good efficiency. Heath does not wish to reveal the actual compression ratio, which is significantly higher than that of a comparable spark-ignited two-stroke. Both this and the lack of throttling inherent in compression-ignition operation are the basis of superior efficiency compared with a spark-ignition counterpart. Heath notes that, as can be seen in the specific fuel consumption map on page 54, this high efficiency is maintained over the whole engine operating envelope. On the other hand, the downside is high cylinder Cosworth AG2 UAV twin | Dossier Unmanned Systems Technology | April/May 2017 The AG2 undergoing dyno testing Low cetane value means a high compression ratio for successful compression ignition, giving good efficiency