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

55 RCV Engines DF70 UAV boxer twin | Dossier Mason notes that the rotation of the VRV “creates a paddle-wheel effect that enhances mixture preparation.” He adds, “The side of the valve aperture hits the charge, causing it to change direction, so the 3D geometry of that aspect of the valve is very important for performance.” Mason further explains, “The in-cylinder air motion is quite different from a poppet valve engine, as the inlet flow is close to the centre of the cylinder rather than to the side. If the port is fully open then the air motion will be tumble as the scallop in the valve directs the air into the cylinder. “The rotary valve creates a high level of in-cylinder air motion. Approaching TDC, this bulk air motion will break down and, combined with the large squish area, this results in a high level of turbulence in the compact chamber. “While the spark plug is offset on one side of the combustion chamber formed by the valve, as the valve exposes it the homogeneous mixture generated by the turbulence achieves consistent combustion,” continues Mason. “As the cavity formed by the valve is the combustion chamber in the axis of the cylinder, flame propagation is similar to having a central spark plug.    “As the valve sees both inlet and exhaust flow, its temperature distribution is similar to that of a piston. The combination of a high level of turbulence and absence of a hot exhaust valve means the engine is insensitive to detonation and can operate over a wide range of air-to-fuel ratios.” Customers run the DF70 successfully on a broad range of fuels, including gasoline and a range of kerosene-based heavy fuels – even paraffin. Heavy fuel (Jet A-1, JP5 and JP8) is inconsistent in terms of octane rating, and is in any case very low at about 15-25 RON. Nevertheless, the VRV combustion system can cope. In his work, Internal Combustion Engine in Theory and Practice , the late Charles Fayette Taylor, Professor Emeritus of Automotive Engineering at MIT, who directed the Sloan Automotive Laboratories at MIT from 1926 to 1960, identified five ways to avoid detonation – a small bore, a high velocity through the intake porting, a short ratio of flame path to bore, the absence of hot surfaces in the end gas region and the use of squish area, in particular in the end-gas region. “Our technology ticks all those boxes,” notes Mason. He reports that detonation is not in RCV’s vocabulary. “We once ran an engine on n-heptane, the fuel that is used to provide a zero rating on the RON and MON octane scales. Even then we could only find detonation on extreme occasions.” Mason adds that the DF70’s combustion system is particularly robust. It can operate successfully across a broad air-to-fuel ratio band as well as over a wide range of engine load and speed combinations, from part- to wide-open throttle. VRV versus the poppet valve Compared with an equivalent poppet valve engine, the main drawback of the VRV is the way in which it compromises compression ratio. Moreover, increased valve area relative to a poppet valve engine is no real advantage when valve opening rate is taken into account. A poppet valve can be made to open quicker than a VRV. On the other hand, the VRV presents no restriction to flow once aligned with the port, which enables high volumetric efficiency to be obtained at high rpm. In addition, the VRV lowers the component count, and there are no valve springs or associated dynamic issues. There is also no valve-to-piston clearance issue to limit crankshaft speed potential, plus there is no hot exhaust valve or hot residuals to promote detonation. Mason notes that, compared with an equivalent poppet valve engine, there is less friction attributable to the valvetrain. This is particularly significant in the context of a small-displacement engine, since the smaller the displacement, the proportionally higher the valvetrain friction. More important than that though, he stresses, is inherently superior volumetric efficiency. He says, “Compared with Unmanned Systems Technology | Dec 2015/Jan 2016 Key people at RCV Engines (from left): Keith Lawes, Eric Hill and Brian Mason