Unmanned Systems Technology 028 | ecoSUB Robotics AUVs I ECUs focus I Space vehicles insight I AMZ Driverless gotthard I InterDrone 2019 report I ATI WAM 167-BB I Video systems focus I Aerdron HL4 Herculift

70 also provides the rear crankshaft counterbalancing. A third, 113-tooth gear below it drives the exhaust camshaft. The air intake for the cylinders is achieved by a belt (or ring) of ports running around the circumference of the middle of the cylinder liner. A total of 18 ports run around each cylinder, with actual airflow coming from the turbocharger (via the supercharger) and flowing into and through the crankcase, before being routed down to the inlet ports. The cylinders consist of cast iron liners treated using the Laycarb process for wear resistance. It may seem odd that the cylinders have no reed or rotary intake valves but the WAM-167BB is not a crankcase compression-type two-stroke. Instead, it has long trunk pistons that close off the air ports (except around bottom dead centre) when they are opened to allow scavenge. With no crankcase compression, a blower is needed to provide positive airflow at cranking and low engine speeds, although the turbocharger ‘takes over’ and provides all the differential pressure needed once the engine is producing power. “Many two-stroke diesels have used this same technique, such as those from Detroit Diesel, Electro-Motive Diesel, and MAN Marine & Industrial Engines,” Franklin notes. The fuel pump is installed on the back of the engine, with the MCU attached. The middle gear drives the fuel feed into the pre-combustion chambers via compact injectors, and is responsible for the factory setting of the injection timing. “Those pre-combustion chambers aren’t the easiest thing to pull out and inspect – you have to pull the oil sump off,” Newton notes. “But it’s a good idea to do that anyway, at every 50-hour service, to wash it out, replace the oil and so on.” The third gear is fixed at the end of a camshaft and provides the actuation for the WAM 167-BB’s eight exhaust poppet valves. These have 30° seats arranged in a row down the length of the head, with two at the bottom of each cylinder leading to a bifurcated exhaust port (with one port for each cylinder). An exhaust manifold runs the length of the cylinder head. It carries the exhaust to a centrally mounted turbocharger turbine and separates the exhaust pulses as far as possible for best turbine performance and engine breathing. “A single exhaust valve would need to be large to provide sufficient flow area, and that’s not easy to design in, whereas two valves fit more easily into the arrangement and provide the area required,” Franklin says. As one might expect, the exhaust cam is driven directly by the fuel pump gear, and thus indirectly by the crank-mounted gear. “Any timing adjustment of the fuel or exhaust is done by altering the gears and the relative positions of their teeth,” Franklin explains. “There are three of those identical gears in total running down the back of the engine, each with 113 teeth. Moving a gear by one tooth means one 113th of 360° worth of different timing, and a Vernier adjustment means we can get to within 0.5°.” While many people are familiar with two-stroke engines that use crankcase October/November 2019 | Unmanned Systems Technology Performance chart for the WAM-167BB The engine has a mechanical control unit on the back, which has levers for power output and stopping the engine, both of which are linked to a ‘rack’ control rod in the fuel pump