Unmanned Systems Technology 009 | Ocean Aero Submaran S10 | Simulation and testing | Farnborough report | 3W-110xi b2 TS HFE FI | USVs | Data storage | Eurosatory/UGS 2016 report

52 Opening the pressure relief valve much reduces compression to assist starting. The automatic version can be used in conjunction with the use of the associated generator as a starter motor to allow for a start or a restart controlled entirely by the ECU. The heart of the engine To obtain excellent performance on heavy fuel, 3W didn’t need to alter its regular gas flow system, from air intake through to exhaust. Even the compression ratio, 12.5:1, is the same regardless of the fuel used. Atop the engine is an air filter, positioned directly above the single throttle body, which is centrally mounted to serve both cylinders. Rather than being force fed, the throttle body draws in ambient air from the UAV’s engine bay. The throttle body houses a butterfly controlled by a servo operated by the ECU. It also locates the single port injector (serving both cylinders) downstream of the butterfly. The injector is angled downwards at 45 º relative to the horizontal, entering the airflow where it turns through a right angle to feed down into the crankcase. The fuel system is non-return, and fuel is taken from the tank to the single injector by an electric pump controlled by the ECU that creates a supply pressure of up to 3 bar, according to instantaneous requirement. The incoming fuel-air charge travels through a single reed valve into the middle of the crankcase; 3W uses a conventional reed that has fibreglass- based blades. Wintrich reports that 3W’s first ever twin used a rotary rather than a reed valve but says that route was abandoned on the basis of poor torque. Since then all 3W engines have had a reed valve. Schneurle porting is used to transfer the charge from the crankcase to each cylinder. This means that the combustion chamber is loop scavenged, the 110xi having six transfer ports: four primary and two smaller secondary ones. The ports are dimensioned, located and shaped to direct the incoming charge towards the head. Having reached the domed chamber in the head it is deflected downward, creating a loop that prevents it escaping through the open exhaust port. Avoiding the need for a piston crown deflector, Schneurle porting also creates a beneficial swirling turbulence. The 110xi’s piston crown is flat, and the combustion chamber dome in its head is surrounded by a generous flat squish band. Schudt says detonation is never an issue, so there is no knock sensor. The way the engine is prepared for cold starts avoids any risk of knock upon fire-up. He remarks that a single spark plug is adequate even for ignition of heavy fuel but that twin plugs are used “for safety – that is the main purpose”. He adds that most customers specify the twin spark option for this reason. Should either plug fail then the engine will lose only 150- 200 rpm and thus the mission can be successfully completed. The key inputs to the ECU are a Hall effect crankshaft speed sensor, an ambient pressure sensor (for altitude compensation) and the cylinder head temperature sensors. The ECU sets the throttle position, so there is no need for a position (load) sensor. Communicating with the craft’s control unit, it also controls the ignition coils, the single injector and the fuel pressure. Flow rate control is part of the altitude compensation function of the ECU. Each cylinder has a single exhaust port from which a stainless steel pipe runs down through the underfloor block. Stainless is used in response to the thermal demand of heavy fuel operation. 3W does not add the complication August/September 2016 | Unmanned Systems Technology Dossier | 3W-110xi b2 TS HFE FI The 110xib2 TS HFE FI seen from the underside – note how the exhausts run through a block underneath the crankcase