Za50s Engine

Rising demand for compact, high output medium speed diesel propulsion installations—particularly from cruise and ferry sectors— stimulated Sulzer to introduce in 1995 a larger bore derivative to complement the successful ZA40S engine. This 500 mm bore/ 660 mm stroke ZA50S design extended the power range of the programme to 21 600 kW at 450 rev/min from six, eight and nine inline and V12-, 14-, 16- and 18-cylinder models (Figures 26.16 and 26.17). The ZA50S engine is no longer in the Wartsila programme, its output range covered by the Wartsila 46 design.

A shorter and lighter engine than rivals in the same power class was sought by the ZA50S designers, and attention was paid to reducing the engineroom headroom requirement and the minimum distance necessary between engines in multi-engine plants. The connecting rod design contributed to a particularly low dismantling height.

The ZA50S engine inherited the main features of the ZA40S, notably the rotating piston, fully bore-cooled combustion space, high stroke-bore ratio, cylinder head without valve cages, and load-adaptable turbocharging with single-pipe exhaust system. The opportunity to innovate was taken, however, by adopting hydraulic actuation for the gas exchange valves, standard for many years in low speed two-stroke engine practice but introduced here for the first time to a medium speed four-stroke design (Figure 26.18).

Hydraulic actuation-in conjunction with pneumatically controlled

load-dependent timing to provide variable inlet closing-yields flexibility in valve timing, fostering lower exhaust gas emissions and improved fuel economy. Variable inlet closing, together with optimized super-

Figure 26.17 Eight-cylinder ZA50S engine charging, delivers a very flat fuel consumption characteristic across the whole load range of the engine. A considerable reduction in smoke levels in part-load operation is also secured.

A high stroke-bore ratio, similar to that of the ZA40S engine, underwrites a compression ratio promoting clean and efficient combustion over the whole engine load range. The combustion chamber is nevertheless sufficiently deep for the clean combustion of heavy fuel, with low smoke emissions and low thermal loadings of the surrounding components. The standard ZA50S engine meets the nitrogen oxide emission limits set by the IMO; further reductions are possible by engine tuning refinements.

A robust engine block with underslung crankshaft combines maximum rigidity with low stresses, the designer reports. The spheroidal cast iron block is designed for direct installation on resilient mountings. The main and bottom end bearings follow experience with the ZA40S engine, featuring bimetal aluminium/tin-alloy running surfaces on thin-walled shell bearings. The bimetal-type bearings, which have demonstrated fatigue resistance and a long life expectancy, also exhibit

Figure 26.18 Hydraulic exhaust and inlet valve drive for Sulzer ZA50S engine. The merits cited include: constant valve timing for the exhaust valves; variable valve timing for the inlet valves (improving fuel economy, emissions and part-load operation); no valve clearance adjustment; and low noise operation

Figure 26.18 Hydraulic exhaust and inlet valve drive for Sulzer ZA50S engine. The merits cited include: constant valve timing for the exhaust valves; variable valve timing for the inlet valves (improving fuel economy, emissions and part-load operation); no valve clearance adjustment; and low noise operation high resistance against wear and corrosion, together with good embeddability and emergency running behaviour.

The symmetrical design of the rotating piston achieves even distributions of thermal load and wear around the piston. The piston comprises a steel crown and spheroidal graphite cast iron skirt. The crown is bore cooled with oil jet-shaker cooling; and the skirt is provided with inner lubrication, whereby lube oil is fed to the cylinder liner outwards from within the piston. Effective lubrication of the liner, rings and pistons, with low lube oil consumption, is reported from ZA40S installations. Full bore cooling of the combustion chamber— the cylinder head, liner and piston crown—fosters optimum temperatures and temperature distribution, low mechanical stresses and low thermal strains.

The ZA50S engine inherited the proven exhaust valve concept of earlier Z-type engines, based on water-cooled seat inserts, Nimonic valves and mechanical rotators but no valve cages. The inlet valves also feature mechanical rotators. The times between valve overhauls are arranged to coincide with those for the pistons.

A key contribution to economy and performance derives from another feature shared with the ZA40S engine: optimized supercharging incorporating a simplified single-pipe exhaust system. Appropriate use is made of waste gates and charge air bypasses to suit the individual application. The reported benefits are realized in low fuel consumption at both full and part loads, improved load acceleration behaviour and reduced thermal loading of engine components.

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