Unmanned Systems Technology 017 | AAC HAMR UAV | Autopilots | Airborne surveillance | Primoco 500 two-stroke | Faro ScanBot UGV | Transponders | Intergeo, CUAV Expo and CUAV Show reports

65 Primoco 500 four-cylinder, 500 cc two-stroke | Dossier deformation in the face of the loadings to which they are subject.” He also notes that, compared with the reference engine used in the One 100, the 500 has longer con rods for increased mechanical efficiency, and a larger diameter piston pin for greater strength. The piston, CNC-machined from stamped semi-fabricated (specially selected) aluminium alloy, is uncoated. It carries two identical 1.0 mm axial height piston rings, this twin approach being for durability, says Smat. He declines to disclose ring material or coating (if any) other than to say that the ring specification is “applicable for a nickel silicon carbide-coated bore”. The individual cylinder barrels are investment cast from aluminium alloy, while the heads are CNC-machined from 7075 T651 aluminium alloy billet. A metallic head gasket comprising a single solid ring of a copper-based alloy is fitted at the head-barrel interface. “This [one-use-only] item is to regulate the heat and ensure a good seal in the face of combustion pressure,” notes Smat. With its all-aluminium structure, the dry weight of the 500 is the aforementioned 25.5 kg as a complete, ready-to-run powertrain package including propeller hub, carburettor, air filter, exhaust, generator, engine control unit and wiring. Belt driven from the rear of the engine, the generator is a rugged automotive unit that delivers 28 V and 55 A, and in itself accounts for 6.5 kg of the total. Engine operation All engine cooling is via a flow of air over the structure with its finned barrels and cylinder heads. No intake flap to adjust this ram-supplied flow is considered necessary, Smat reports. In normal two-stroke style, lubrication is via a pre-mix of fuel and oil, in this instance in a ratio of 30:1. The engine runs satisfactorily on 95 octane (RON) petrol, Smat reports, adding that Primoco is happy to develop an Avgas-fuelled version should a customer want one. The piston crown is domed, in typical two-stroke fashion. The cylinder head has an inner dome surrounded by a sloping outer section that forms a tapering squish area (decreasing in volume towards the periphery) within the combustion chamber. That chamber is served by twin spark plugs, set at an included angle of 70 º and arranged to fire together. Smat does not consider twin spark plugs to be advantageous; on the contrary they add weight, complexity and cost compared with a single plug set-up. He even thinks that, given the 500’s 54 mm bore dimension, combustion might be improved through having a single central point of ignition for the mixture. VM Motor race engines use a single plug, for example. However, twin plugs are traditionally specified in aero applications for reason of redundancy, and Primoco considers twin plugs will be advantageous when it comes to future certification of the engine. “Certification officials like twin plugs,” as Smat puts it. The ignition system is distributorless, Unmanned Systems Technology | December/January 2018 CAD illustration of the 500, showing the underslung intake and exhaust systems Smat does not consider twin spark plugs to be advantageous; they are more complex and costly than a single plug set-up