Unmanned Systems Technology 005 | Selex ES Falco UAV | Sense and avoid systems | RCV Engines DF70 | DSEI show report | Fuel cells | CUAV Expo, InterDrone and CUAV Show reports | SLAM

52 Dossier | RCV Engines DF70 UAV boxer twin rubbing friction or even partial seizure. Mason refers to this as “valve grip”, which the use of a DLC-coated VRV running within a nickel silicon carbide coated bore helps to counter. However, there remains a thin line between excessive clearance and insufficient clearance. Mason explains that the necessary sealing is accomplished by careful tolerancing between the VCV and its bore, with materials carefully selected considering thermal expansion and distortion. Moreover, the VRV is allowed to float radially so that cylinder pressure pushes it against the intake and exhaust ports, minimising the leakage path. The width of the VRV aperture and the intake and exhaust port geometry together determine the valve timing. The intake valve opens at 24º BTDC and closes at 60º ABDC; the exhaust valve opens at 60º BBDC and closes at 24º ATDC. Each cylinder has its own charge air intake with a throttle body that contains a barrel throttle. When the barrel is wide open, the charge has a straight tube to run through to the intake port exit controlled by the VRV. The intake and the exhaust geometry is tuned for performance in the normal manner of a four-stroke engine, exploiting pressure wave reflection to enhance volumetric efficiency. In general, lengthening the exhaust primary pipes enhances performance, but the gain is marginal when the implicit increase in weight is taken into account. The DF70 is run by an electronic management system. It uses distributorless ignition, and there is the option of dual ignition for redundancy – if one spark plug fails then the other will get the craft home (and in the case of this boxer, one of the two cylinders is enough to limp home). Interestingly, the dual ignition has been found to enhance combustion, as seen in improved fuel consumption figures. Mason notes that the 9 mm plugs stay very clean, regardless of the fuel used, which is a function of using the VRV. Fuel is injected into the inlet manifold just upstream of the VRV using a single injector per cylinder. This, notes Mason, avoids the carbonisation and other problems that can occur with a direct injection system running heavy fuel. The fuelling also includes altitude compensation. A fuel return system allows the electric fuel pump supplying both cylinders to run at a constant speed. Fuel pressure (nominally 2.5-3.0 bar) is measured by a sensor in the ECU, and fuel injection times are then adjusted to allow for changes in this pressure, making the system robust to changes in pressure that occur during normal use. The use of a 50:1 fuel-oil mixture saves the weight and complication associated with a dedicated lubrication system, as normally found on a four- stroke engine. Lubrication of the VRV gear drive is provided by the fuel-oil mixture blowing past the VRV; likewise, lubrication of the bottom end is provided by the fuel-oil mixture blowing by the single piston ring gap. The ‘boxing’ motion of the pistons creates alternating compression and expansion of the internal air-oil mist within the crankcase. While this Dec 2015/Jan 2016 | Unmanned Systems Technology The use of a 50:1 fuel-oil mixture saves the weight and complication of a dedicated lubrication system, as on a four-stroke engine The DF70 has an innovative vertical rotary valve drive