PA6B STC design

High rigidity without compromising overall engine weight is achieved by a stiff one-piece crankcase (Figure 30.34) of nodular cast iron specially treated for shock resistance, with transverse bolt connections between both crankcase sides through the underslung-type bearing caps. Integrated longitudinal steel piping supplies lubricating oil to the main bearings.

Figure 30.34 SEMT-Pielstick PA6B STC crankcase

Large dimension main journals yield a particularly large bearing surface and conservative pressures which underwrite prolonged bearing life, as do the large surface area connecting rod bearing shells. The alloy steel one-piece forged crankshaft (Figure 30.35) has high frequency hardened crankpins and journals, and is bored to feed lubricating oil to the connecting rods.

Figure 30.35 SEMT-Pielstick PA6B STC crankshaft

A nodular cast iron cylinder head was configured to achieve improved air and gas flow rates for the PA6B STC version. The composite piston (steel crown and aluminium skirt) is fitted with five rings. The forged steel connecting rod is fitted with large surface bearing shells to yield high durability under high firing pressures. Special low wear characteristics were sought from the centrifuged cast iron material specified for the cylinder liner (Figure 30.36). The camshaft is formed from several sections for ease of dismantling.

Operational flexibility and power output are fostered by the singlestage sequential turbocharging system which is based on two turbochargers and delivers a large combustion air excess at partial loads. Supercharging is effected by one turbocharger for engine loads up to 50 per cent of the nominal power, its effort boosted by the second identical turbocharger at higher loads. Switching from single to twin turbocharger mode is performed automatically by opening two flap valves. Engine performance at prolonged low load is improved with respect to fuel consumption, smoke emission, fouling resistance and transient performance. Additionally, engine utilization is expanded towards the high torque/low rev/min area.

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