Favoured for many years for cruise ship, ropax ferry and icebreaker propulsion, the Sulzer ZA40S rotating-piston medium speed engine recently logged its last installation with the delivery of Cunard’s Queen Victoria
Sulzer four-stroke Z-engines matured over several decades from a 400mm-bore family founded by the ZH40 two-stroke medium speed engine, which was designed in the early 1960s and installed as propulsion plant in icebreakers, ferries and cruise ships. The uniflow-scavenged two-stroke design was succeeded in 1972 by a four-stroke version (the Z40) which was in turn completely redesigned and replaced in 1982 by the ZA40 engine.
A longer stroke (560mm instead of 480mm) ZA40S engine was introduced in 1986 to succeed the Z40 and ZA40 designs. The performance development steps over the years are illustrated (see page 91).
In autumn 1988, three years after its announcement with a cylinder output of 660kW, the ZA40S engine was uprated to 720 kW/cylinder, following successful service experience and heavy fuel endurance tests. The new rating was associated with an increased maximum cylinder pressure of 165 bar and mean effective pressure of 24.1 bar, underwritten by thermodynamic and mechanical optimisations.
Some design modifications were also introduced to improve reliability and durability. These included an exhaust valve with a 45-degree seat angle achieving better cooling without reduction of mechanical and thermal safety at the higher engine load and maximum cylinder pressure, as well as more efficient seating due to a wider seat area.
Sulzer’s combined ‘jet-shaker’ piston cooling principle was also adopted, the conventional shaker effect in the piston crown being supplemented by oil jets sprayed through nozzles; this offered advantages in lowering crown temperatures, thereby inhibiting carbon formation and gaining an improved washing effect.
Additionally, a new version of the waste gate in the exhaust gas manifold before the turbine was introduced to secure a better match between engine and turbocharger at the increased power rating.
A classic feature retained throughout successive generations of Sulzer Z-type engines was the rotating piston. Sulzer patented the concept in 1937 after tests on a number of research engines with cylinder bores from 90mm to 420mm confirmed the special benefits offered for highly loaded trunk piston engines.
A ratchet mechanism transformed the swinging motion of the connecting rod into a smooth rotation of the piston. The connecting rod had a spherical small end allowing some 40 per cent more bearing area than a gudgeon pin bearing.
The rotating piston was eventually adopted for the Z-type design, first introduced in two-stroke form in 1964. The concept remained unique to Z-engines until its adoption in 1995 for GMT’s upgraded 550mm-bore VA55 medium speed engine. (The Italian designer benefited from a technology transfer arrangement with Sulzer; both later became members of the Wärtsilä Corporation.)
For rotating the piston, the connecting rod is provided with two pawls positioned slightly out of the centre of the spherical small end bearing. When the connecting rod performs its swinging movement relative to the piston the pawls impart an intermittent rotating motion to a toothed rim, from which it is transmitted to the piston by an annular spring. The flexible connection maintains the forces necessary for rotating the piston at a constant low level.
These merits were cited by Sulzer for the rotating piston:
• even temperature distribution around the piston crown as there were no particular inlet and outlet zones
• small and symmetrical deformation from a top end bearing of spherical design and having a relatively large area
• optimum sealing and working conditions for the piston rings because the small, symmetrical deformations of the piston allowed the smallest running clearance between piston and cylinder liner
• low and stable lubricating oil consumption because the small piston running clearances minimised piston slap and obviated the need for the traditional oil cushion, thereby allowing the oil scraper ring to be located at the lower end of the piston skirt
• good margin for unfavourable running conditions with the smallest risk of seizure because the grey-iron piston skirt was always turning to a fresh part of the cylinder liner surface.
The ZA40S engine programme was later uprated to 750 kW/cylinder at 510 rpm on a mean effective pressure of 25.1 bar and with a mean piston speed of 9.5 m/sec. Six, eight and nine-cylinder in-line and V12, 14, 16 and 18-cylinder models covered a power band from 4,500kW to 13.5MW.
Crankcases were fabricated for in-line cylinder engines while V-form versions featured a cast iron monobloc structure. The originally fabricated two-part design of the Z-engine frame was superseded by a monobloc casting for the ZA40 and ZA40S in-line models, as had always been standard for the V-engines.
The cylinder heads of ZA40 and ZA40S engines were bore cooled. The measured bottom deformations under gas load were reportedly only one-third those of the conventional double-bottom head, and the mechanical stresses were also found to be low in the same proportion. The drilled cooling passages permitted a uniform temperature adjustment within a few degrees at the desired low level.
Good valve seat sealing with optimum heat transfer to the exhaust valve seats was promoted by the resulting low thermal distortion as well as the high mechanical stiffness.
The rotating piston for the ZA40 and ZA40S engines benefited from simplification and bore cooling of the crown was also introduced. An optimised arrangement of the cooling bores reduced stresses by around 10 per cent, with practically no change in crown temperatures. The substantially stiffened periphery secured by bore cooling also resulted in reduced thermal deformation of the crown and hence improved working conditions for both piston and rings.
The top ring of the ZA40S piston featured a Sulzer plasma-coated running face while the running faces of the other two rings were chromium plated. All three rings were barrel shaped to foster initial running-in and to avoid a need to hone smoothened cylinder liners at routine overhauls.
A combination of bore-cooled rotating pistons and plasma-coated rings – the flanks of the rings and their grooves were also chromium plated – promoted low wear rates and long piston life. Rotation and negligible distortion due to the spherical top end, as well as no transmission of gas forces through the cast iron piston skirt, underwrote a small skirt clearance.
A connecting rod with marine-type bottom end featured large crankpin diameters and hydraulically-tightened studs for easy maintenance; no interference with the bottom end bearing was necessary at piston overhauls. Shims could be applied to achieve the different compression ratios for three engine tuning options.
Refined over many years in the Z40 and ZA40 engines, a well proven exhaust valve arrangement was retained for the ZA40S models: Nimonic exhaust valves with rotators and water-cooled valve seat inserts but no valve cages.
The higher rigidity of the bore-cooled cylinder head and the generally lower temperature levels fostered by the enhanced turbocharging system further improved the sealing conditions of the exhaust valves in the ZA40S engine. Longer times-between-overhauls were thus secured, the intervals coinciding with those for the piston.
A new generation of aluminium/tin alloy bimetal-type bearings was introduced for both main and bottom end bearings of ZA40 and ZA40S engines. These ‘Rillenlager’ and bimetal types dispensed with the traditional overlay of the previous trimetal type to give the following reported benefits: sustained fatigue resistance and embeddability; better wear resistance for heavy fuel operation; improved local self-healing and emergency running behaviour; and better adaptability after inspections.
Loading of the spherical top end bearing remained some 15-20 per cent lower than that of a conventional gudgeon pin design. The big end of the ZA40 and ZA40S connecting rod was also enlarged from the Z40 design, resulting in a reduction of the dynamic stresses of around 20 per cent. The bolts were hydraulically tightened for easier and faster maintenance procedures.
Helix-controlled fuel injection pumps were provided for each cylinder, with fuel leakage into the lubricating oil prevented by an oil barrier at the lower end of the plunger sleeve. Depending on the engine application, helix control edges could be chosen for constant or variable injection timing (CIT or VIT).
A simple arrangement of short high pressure fuel pipes to the injection valves, together with circulation of preheated fuel oil up to the level of the pumps, facilitated pier-to-pier operation on heavy fuel (with appropriate circulation and preheating).
The injection valves were equipped with sleeve-type nozzles cooled by fresh water and provided with rounded-off inner edges of the spray holes. Rounded-off spray holes were traditional for Sulzer nozzles, yielding a more stable spray pattern over longer service periods with a narrower scatter of injection rates between individual cylinders.
A more corrosion-resistant sleeve material was introduced to replace the original material, whose punishment threshold was not large enough to cope with corrosion attacks by insufficiently treated cooling water (resulting in some tip breakages).
A variable fuel injection timing system was standard for ZA40S engines not equipped with a charge air waste gate. The helix of the pump plunger was shaped so as to raise Pmax throughout the part-load range and thereby reduce part-load specific fuel consumption.
The single-pipe exhaust system serving the ZA40S engine delivered a reasonable combination of the advantages of the pulse system (partially retaining the kinetic energy of the exhaust gases) with the simplicity of a constant pressure system. This had been recognised in the 1950s but could only be realised when modern high efficiency turbocharger designs became available.
Turbocharger matching could be demanding, however, with the short valve overlap necessary for low fuel consumption at full load; it could lead to surging at part loads and, furthermore, a high efficiency turbocharger showed a pronounced decrease in charge air pressure at part load.
Such difficulties were overcome in the ZA40S engine by using charge air bypasses and waste gates to create load-adaptable turbocharging systems that often enabled the engines to surpass performance with conventional systems.
The ZA40S system, Sulzer asserted, could be tailored to give low fuel consumption through the normal engine load but could also be adapted to yield the high torque required at part load in the propulsion of icebreakers and shortsea shuttle ferries.
Progressively opened up above 85 per cent load, the optional waste gate valve allowed higher boost pressures to be used at part load for a substantial improvement of part-load specific fuel consumption while avoiding excessive cylinder pressures at higher loads.
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 500mm-bore/660mm-stroke ZA50S design extended the power range of the programme to 21.6MW at 450 rpm from six, eight and nine-cylinder in-line and V12, 14, 16 and 18-cylinder models.
A shorter and lighter engine than rivals in the same power class was sought by the designers, and attention was paid to reducing the headroom requirement and minimising the 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 its smaller bore precursor, 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.
An 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. Hydraulic actuation – in conjunction with pneumatically-controlled, load-dependent timing to provide variable inlet closing – gave flexibility in valve timing, fostering lower exhaust gas emissions and improved fuel economy.
Variable inlet closing, together with optimised supercharging, delivered a very flat fuel consumption characteristic across the whole load range of the engine. A considerable reduction in smoke levels in part-load operation was also secured.
A robust spheroidal cast iron engine block with underslung crankshaft – targeting maximum rigidity with low stresses – was designed for direct installation on resilient mountings.
The symmetrical design of the rotating piston (steel crown and spheroidal graphite cast iron skirt) achieved even distributions of thermal load and wear around the piston. The crown was bore cooled with oil jet-shaker cooling, and the skirt provided with inner lubrication from lube oil fed to the cylinder liner outwards from within the piston.
• Both ZA40S and ZA50S series were inherited by Wärtsilä when the Finnish group acquired Sulzer in the 1990s, although the latter design was eventually withdrawn from the programme.
Built by Wärtsilä Italia in Trieste, the ZA40S engine remained in demand for large cruise ships until the mid-2000s, its prestigious references including Carnival Cruise Lines’ 109,500gt Destiny class and P&O’s 109,000/116,000gt Grand Princess class tonnage from Fincantieri. Each diesel-electric power station for these ships is based on four V16-cylinder and two V12-cylinder models with a combined rating of 63.4MW.
Cunard’s new 90,000gt/2,014-passenger Queen Victoria has provided a home for the last four ZA40S engines to be built. The two V16-cylinder and two V12-cylinder models develop an aggregate output of 63.4MW, each engine driving an ABB alternator for propulsive and auxiliary power.
A ‘dry package’ of measures securing low emissions and reduced smoke includes optimised pistons and fuel injection nozzles and variable inlet valve closing to facilitate Miller timing. MP
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