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

54 Dossier | 3W-110xi b2 TS HFE FI of an exhaust valve, which in the context of a two-stroke is a device that alters the effective area of the exhaust port to suit differing operational conditions. Schudt notes that while this can offer certain theoretical performance advantages, in practice, owing to its exposure to the exhaust stream, it represents a maintenance issue, and many customers would not accept the frequency with which it would need to be cleaned. Each cylinder’s exhaust discharges into its own silencing canister. “Having separate exhaust exits helps with exhaust pulse tuning,” explains Schudt. “Otherwise, using one bigger joint canister, the exhaust gas flows can interfere with each other. So for performance and safety, having separate canisters is better.” Engine structure The 110xi can be supplied for use in a pusher or a tractor configuration, and can be located with its induction system mounted above or below the main engine structure. The crankshaft is longitudinally aligned with the propeller at one end, the generator at the other. Schudt remarks that this arrangement is “safer” than the common arrangement of having the generator immediately behind the propeller, and notes that it means there is no interference between it and the crankshaft speed sensor. The crankcase is formed in three sections – the main power section, a snout between that and the propeller, and on the other end the generator housing. All three house crankshaft bearings. The fact that the crankcase is formed in three sections can be useful for adaptation to differing requirements: in particular, the section incorporating the generator housing can be tailored to a specific application. The main, box-like centre crankcase section is split vertically into left- and right-hand sides, each in turn carrying its respective cylinder. Four bolts tie the two halves of the main section together, two running through from each side. Each cylinder is then attached to its crankcase face by four short (25 mm) bolts passing through a lower flange into the adjacent case structure. Wintrich notes that this structural arrangement ensures that, in operation “the cylinders sit properly and seal properly”. 3W’s approach is always to have a combined barrel and cylinder head, which minimises weight and avoids any combustion chamber sealing problem. Between each cylinder and the crankcase is a single-layer asbestos- free gasket. “Multi-layer gaskets are not necessary in these small engines,” notes Schudt. The crankcase and the cylinders are aluminium castings, the latter using a different alloy from the former in view of the temperature of the combustion chamber. Accessing the transfer ports when the head is integral is always a challenge, but this cylinder is precision cast using a sand core, and Schudt reports that the porting is fully finished as cast. Reciprocating and rotating assembly Since this is a two-stroke engine, both cylinders fire concurrently. The crankshaft has separate crankpins for each cylinder in a 180 º configuration, such that the cylinders ‘box’ towards and away from each other. Thus the pistons move in phase with each other, and the primary force generated by one piston counteracts that generated by the other at all times, resulting in primary and secondary balance. However, since the pistons are not in line with each other – there is a 50 mm bore offset between the horizontally opposed cylinders, each of which has its own crankpin – there is a rocking couple along the crankshaft. The two outer crankshaft throws are each formed as a counterbalancing web. This reduces vibration to the extent that no other damping is required in the engine. No flywheel is required. The forged steel crankshaft runs in four all-metal ball bearings, sealed for life using a special grease that expands to account for wear of the balls. Patented by NGK, no supply of lubricant is required for this type of bearing. All engine lubrication requirements are taken care of by a fuel-oil premix, so there is no need for a separate August/September 2016 | Unmanned Systems Technology The 110xib2 TS HFE FI running on the test bed at 3W