Programme Expansion

New models have entered the MC programme since its launch in response to propulsion market trends at both ends of the power spectrum. A notable addition in 1986—the S26MC series—took the low speed engine deep into small-ship propulsion territory, offering an output per cylinder of 365 kW at 250 rev/min. The nominal power of the 260 mm bore/980 mm stroke design has since been raised to 400 kW at 250 rev/min and the four- to twelve-cylinder programme covers an output range from 1100 kW to 4800 kW. With a stroke-bore ratio of 3.77:1, this rating corresponds to a mean piston speed of 8.2 m/sec and a mean effective pressure of 18.5 bar. A firing pressure of around 170 bar yields a specific fuel consumption of 179 g/kWh at the maximum continuous rating. Reliable operation on poor quality heavy fuel oil with a viscosity of up to 700 cSt/50°C is promised.

The key components of the S26MC engine (Figure 10.7) are based on those well proven in the established larger bore models but with modifications to suit the intended small-ship market. Design amendments since its introduction include an improved axial vibration damper and refinements to the crankshaft. The cylinder cover and fuel injection system have also been upgraded, the auxiliary blower arrangement simplified and the exhaust valve provided with a new closing system with built-in damping.

Similar refinements have benefited the L35MC engine, which pioneered the MC programme. An upgrading in 1992 sought improved performance and reduced maintenance costs, the new maximum power rating of 650 kW/cylinder at 210 rev/min, mean effective pressure of

18.4 bar and mean piston speed of 7.35 m/sec calling for increased diameters of the main and crankpin journals, and reinforcement of the main bearing housing and bedplate. The crosshead bearing, thrust bearing and piston and piston rod were also reinforced, and the cylinder frame, camshaft arrangement and auxiliary blower arrangement simplified.

A longer-stroke S-version of the 350 mm bore design was introduced in 1993, this S35MC model delivering 700 kW/cylinder at 170 rev/ min (since increased to 740 kW/cylinder at 173 rev/min.) A number of components were modified in line with the longer stroke (1400 mm compared with 1050 mm) but the main design features were unchanged.

The strong and rigid main structure of the S35MC is essentially the same as the L35MC but the bedplate and framebox are wider and higher due to the longer stroke of the semi-built crankshaft. The connecting rod and piston rod are necessarily longer but the cylinder frame is identical. The main bearings are wider in order to keep the bearing load inside the area ensuring good service results. Most of the components in the S35MC combustion chamber are also similar to those of the L-version. The exhaust valve design was unchanged but an improved sealing system was introduced to reduce further the wear of the valve stem. The cylinder cover is slightly higher to allow for the increased combustion space. Multi-level cylinder lubrication was adopted to ensure sufficient lubrication of the liner. A modified piston pack comprises two high rings in grooves 1 and 2, and two rings of normal height in grooves 3 and 4. All the rings are made from an improved material. The fuel pump plunger diameter was increased to satisfy the larger injection volume per stroke; and improved sealing was introduced to prevent any fuel from leaking into the main lube oil which is common to the crankcase and camshaft. The camshaft diameter was increased to accept the larger torque from the fuel pumps.

The S35MC and the 1994-launched S42MC series were conceived to improve the propulsive efficiency of plants for small-to-medium sized ships by lowering the engine speed, these 350 mm bore and 420 mm bore models, respectively, distinguished by stroke-bore ratios of 4:1 and 4.2:1. The later introduction of the S46MC-C model (also exploiting a 4.2:1 stroke-bore ratio, the highest of any production engine in the world) addressed the needs of smaller tankers and bulk carriers which can also be served by the S42MC and L or S50MC engines. The 460 mm bore model extended the number of ideal combinations of power, speed and number of cylinders for a given project (Table 10.2 and Figure 10.8).

MAN B&W LOW SPEED ENGINES Table 10.2 S46MC-C engine

Bore 460 mm

Stroke 1932 mm

Stroke-bore ratio 4.2:1

Speed 129 rev/min

Mean effective pressure 19 bar

Output/cylinder 1310 kW

Mean piston speed 8.3 m/sec

Cylinders 4-8

Specific fuel consumption (mcr) 174 g/kWh

Cross Section View Man 6s50mc

Choice in the mid-bore range (500 mm, 600 mm and 700 mm models) was widened from 1996 by uprated versions supplementing the existing S50MC, S60MC and S70MC engines. These new S50MC-C, S60MC-C and S70MC-C models, which share the same design features as the S46MC-C, are more compact (hence the C-designation) and offer higher outputs than their established equivalents (an L70MC engine is shown in Figure 10.9). Stroke-bore ratios were raised from 3.82 to 4:1 and the increased power ratings correspond to a rise in the mean effective pressure to 19 bar. Supporting the higher ratings are modified turbocharging and scavenge air systems as well as modifications of the combustion chamber configuration and bearings. Installation space savings were achieved by reducing the overall length of the C-engines (by around 1000 mm in the case of the six-cylinder S50MC-C engine) and the overhauling height requirement compared with the original designs. The masses are also lower (by 25 tonnes or 13 per cent for the 6S50MC-C) which yields benefits in reduced vibration excitations. The MC-C engines can be 100 per cent balanced.

VLCC project planners were given a wider choice with the introduction of a longer stroke 900 mm bore model, the S90MC-T design, tailored to the propulsion of large tankers. The layout flexibility enables operators to select maximum continuous speeds between 64 and 75 rev/min in seeking optimum propeller efficiency. The design parameters (Table 10.3) addressed the key factors influencing the selection of VLCC propulsion plant, defined by MAN B&W as: the projected ship speed, the propeller diameter that can be accommodated and engine compactness. A specific fuel consumption of 159 g/kWh is quoted for a derated S90MC-T engine served by high efficiency turbocharger(s).

The dimensions of the S90MC-T engine were required not to exceed those of the 800 mm bore S80MC series which had established numerous references in the 'new generation' VLCC market. MAN B&W suggests that VLCCs with speed requirements of more than 15 knots can take advantage of the S90MC-T engine, either as a full-powered six-cylinder model or as an economy-rated seven-cylinder model. The S80MC engine, in seven-cylinder form, is considered an attractive solution for VLCCs required to operate with speeds up to 15 knots.

Refinements also sought to maintain the competitiveness of the S80MC design. Engine length, and hence engineroom length, represents dead volume and is normally to be minimized. The S80MC therefore benefited from a remodelling of its bedplate and chain drive/thrust bearing to shorten the length by 700 mm. The engine can now be pushed deeper aft into the hull, promoting increased cargo capacity from a given overall ship length or reduced ship length for the same

Figure 10.9 L70MC Mk 5 engine cross-section

capacity. The introduction of an optimized fuel system in conjunction with improved timing (already implemented on smaller bore MC engines) raised thermal efficiency equivalent to a reduction in specific fuel consumption (sfc) approaching 3 g/kWh. The sfc can be further improved at part load by specifying a high efficiency turbocharger

Table 10.3 S90MC-T engine

Bore Stroke

Stroke-bore ratio Nominal speed Mean piston speed Mean effective pressure Output/cylinder Cylinders

Specific fuel consumption (mcr)

9GG mm 3188 mm 3.B4:1

166 g/kWh

7B rev/min 7.97 m/sec 18 bar and an adjustable exhaust gas bypass, an option which can be applied in conjunction with a TCS power turbine system.

The S-MC-C series was expanded by a 900 mm bore design, the first example of which entered service in mid-2000; the first units of another new type, the S80MC-C, were ordered in the same year. The 7S80MC-C and 6S90MC-C models have proved popular for VLCCs and ULCCs requiring higher service speeds than before, their outputs respectively 7 per cent and 15 per cent higher than the 7S80MC Mark 6 engine (Figure 10.10).

Demands for slightly higher outputs from the L60MC and L70MC engines were addressed by adopting S-MC-C design principles for these two types, with a mean effective pressure of 19 bar and a mean piston speed of approximately 8.5 m/s. The output of the L60MC-C engine is 2230 kW/cylinder at 123 rev/min, while the L70MC-C engine delivers 3110 kW at 108 rev/min. Among the targeted applications of the L-MC-C series are feeder containerships.

980 mm bore MC/MC-C engines

The propulsion demands of the largest and fastest longhaul containerships were comfortably met until the mid-1990s by short stroke K90MC and K90MC-C engines (Figure 10.11) with an upper output limit of 54 840 kW. Market interest in a new generation of 6000 TEU-plus ships with service speeds of 25 knots or more stimulated the introduction in 1994 of the K98MC-C series which, in 12-cylinder form, could deliver 68 520 kW (Figure 10.12). This 980 mm bore/ 2400 mm stroke model shared key operating parameters with the 900 mm bore MC/MC-C engines, notably a mean effective pressure of 18.2 bar and a mean piston speed of 8.32 m/sec. A propeller speed requirement of 104 rev/min was selected as the design basis, resulting in a stroke-bore ratio of 2.45 and an output per cylinder of 5710 kW. (A cylinder bore of 980 mm was not new to the marine engine industry:

Power estimation 300,000 dwt VLCC

Shaft power BHP -,

35,000-

M: Specified engine MCR inclusive 10% 40,000-engine margin S: Service rating inclusive 15% sea margin, loaded ship in service PD: Propeller design, loaded ship at design draught with clean hull and at calm weather 30,000-

conditions

25,000-

8S80MC-C 42,240 BHP

6S90MC-C 39,900 BHP

7S80MC-C 36,960 BHP

7S80MC Mk 6 34,650 BHP

Engine margin

15% Sea margin

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