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

54 represents a power-sapping pumping loss, in this instance it is seen as beneficial as a means of creating motion of the air-oil mist in the interest of lubricating the internal components. Lawes explains that, although only a small amount of lubricant is introduced per engine stroke, oil will build up in the crankcase until the amount entering the crankcase each stroke reaches equilibrium with the amount leaving each stroke via the crankcase breather. That breather is positioned on the centreline of the crankshaft at the rear of the engine in a position where the oil is centrifuged away from the exit point. This minimises the rate at which oil leaves via the breather, and the oil level builds to a fairly high level in the crankcase before the equilibrium point between oil in and oil out is reached. “This is a conventional method of lubricating the crankcase on nearly all four-stroke model aero engines,” Lawes says. “No problems with bottom end or valvetrain durability have been experienced during extensive testing.” An issue associated with the use of heavy fuel is carbonisation – parts of the engine that get hot when running are prone to carbon build-up. The original rotating cylinder concept made piston cooling a challenge, so it was not well suited to the use of heavy fuel. In contrast, the current stationary-cylinder- plus-VRV solution lends itself to a simple and hence light air-cooling system, even when using heavy fuel. Lawes notes that a heavy-fuel engine needs to be kept within a 40 C temperature window: if it runs too hot then there will be carbon build-up, whereas if it runs too cold it will misfire. Air cooling can obtain the correct running temperature, avoiding the additional weight and complexity of an oil or water cooling system. The secret is in the design of the engine cooling fins and in working with the developer of the craft it will power to obtain sufficient flow from the engine cowling. Normally, adjustable louvres can provide the optimum level of cooling for any given operational situation. However, in the case of a rotor craft, the lack of the wash that would be created by the propeller of a fixed-wing craft means that a cooling fan needs to be used. The turbulence in the combustion chamber generated by the rotation of the VRV assists starting; however, when using heavy fuel a major issue is always starting from cold. In view of this there is a small electrical heater in each intake port and one next to each VRV. The fuel itself is not heated but these four cartridge heaters are switched on for a short period to obtain the induction and combustion chamber temperatures necessary for the ignition of heavy fuel. Lawes says, “Starting from cold on heavy fuel requires more complex cold- start algorithms than with gasoline. These, along with the rest of the calibration factors, are set at the factory and require no operator intervention. The one extra operation required for cold start on heavy fuel is pre-heating the engine via the conventional diesel glow plugs, and typically takes one or two minutes.” Engine operation To keep the compression ratio down to 8.5:1 the piston crown is slightly dished. A higher compression ratio is avoided since it promotes excessive blow-by through the VRV; this compression ratio compromise is the key drawback with using the VRV. While the bore is 36 mm, the external diameter of the VRV is 20 mm, and its wall thickness is 4 mm. It follows that the base of the VRV contributes 4 mm to a total effective squish band width of no less than 12 mm. Dec 2015/Jan 2016 | Unmanned Systems Technology Selection of DF70 crankshafts A heavy-fuel engine needs to be kept within a 40 C window: if it runs too hot then there will be carbon build-up, if it runs too cold it will misfire