DieselShip

MAN B&W low speed engines         

MAN B&W Diesel’s roots are closely entwined with the early days of the diesel engine through both its German and Danish branches, respectively MAN and Burmeister & Wain. Both groups evolved distinctive low speed two-stroke crosshead engine designs before MAN acquired control of B&W in 1980. MAN subsequently discontinued the development of its own loop-scavenged engines at Augsburg and centred all low speed design and R&D operations in Copenhagen, pursuing the refinement of the MAN B&W uniflow-scavenged programme in the shape of the MC series.

The prototype, an L35MC model, entered service in 1982 and the first large bore example, a six-cylinder L60MC engine, was started on the Christianshavn testbed in September 1983. The full L-MC programme was introduced in 1982 with bore sizes of 350 mm, 500 mm, 600 mm, 700 mm, 800 mm and 900 mm. Refinement of the MC design and the introduction of new bore sizes (260 mm, 420 mm, 460 mm and 980 mm) and stroke options continued through the 1980s/1990s in line with service experience and market trends. The current portfolio embraces 11 bore sizes from 260 mm to 1080 mm and K (short stroke), L (long stroke) and S (super-long stroke) variations, with stroke–bore ratios ranging from 2.44 to 4.2:1 and rated speeds from 56 rev/min to 250 rev/min.

Output demands from around 1100 kW to 97 300 kW are now covered by four- to 14-cylinder in-line models in the MC programme, whose individual rating envelopes are illustrated in Figure 1. Additions to the portfolio and key parameter refinements over the years—progressing to Mark 7 versions with mean effective pressures of 19 bar or higher—are shown in Table 10.1. Performance curves are typified by those for an S60MC engine (Figure 2).

Electronically-controlled ME versions of the 500 mm to 1080 mm bore models (detailed below) were progressively introduced from 2001 to offer the same outputs as their MC engine equivalents.

Choosing the most appropriate model for a given ship and propulsion duty is eased by MAN B&W’s Computerized Engine Application System, which includes an integral speed and power prediction programme

Figure 1 MAN B&W Diesel’s MC engine programme. A K108MC model has taken the output threshold to 97 300 kW.

for ships and the corresponding main engine selection programme. Optimized solutions (bore size, number of cylinders, speed/power ratings) can be determined on the basis of both technical and economical data input. The system—comprising a number of integrated sub-programmes which are hierarchically arranged—facilitates calculations of such parameters as fuel consumption, utilization of exhaust gas heat and maintenance costs, and allows economic comparisons of the various options.

The layout diagram in Figure 3 (in this case for an S60MC engine) shows the area within which there is full freedom to select the combination of engine power (kW) and speed (rev/min) that is the optimum for the projected ship and the expected operating profile. The engine speed (horizontal axis) and engine power (vertical axis)

are shown in percentage scales. The scales are logarithmic which means that, in this diagram, exponential curves like propeller curves (3^rd power), constant mean effective pressure curves (1st power) and constant ship speed curves (0.15 to 0.30 power) are straight lines. The constant ship speed lines, shown at the uppermost part of the diagram, indicate the power required at various propeller speeds in order to keep the same ship speed, provided that, for each ship speed, the optimum propeller diameter is used, taking into consideration the total propulsion efficiency.

An engine’s layout diagram is limited by two constant mean effective pressure lines L1–L3 and L2–L4, and by two constant engine speed lines L1–L2 and L3–L4. The L1 point refers to the engine’s nominal maximum continuous rating (mcr). Based on the propulsion and

Figure 2 Performance curves for S60MC engine

engine running points, the layout diagram of a relevant main engine can be drawn in. The specified mcr point (M) must be inside or on the limitation lines of the layout diagram; if not, the propeller speed has to be changed or another main engine model must be chosen. It is only in special cases that point M may be located to the right of line L1–L2.

The specified mcr is the maximum rating required by the yard or owner for continuous operation of the chosen engine. Point M can be any point within the layout diagram. Once the specified mcr point has  been selected, and provided that the shaftline and auxiliary equipment are dimensioned accordingly, that point is now the maximum rating

Figure 3 Ship propulsion running points and engine layout for S60MC model

at which an overload of 10 per cent is permissible for one hour in 12 hours. The continuous service rating (S) is the power at which the engine is normally assumed to operate; point S is identical to the service propulsion point (SP) unless a main engine-driven shaft generator is installed.

All MC engines can be delivered to comply with the IMO speeddependent NOx emission limits for the exhaust gas, measured according to the ISO 8178 test cycles E2/E3 for heavy duty diesel engines. NOx emissions from a given engine will vary according to the engine load and the optimizing power. Specific fuel consumption (sfc) and NOx emission levels are interrelated parameters and an engine offered with both a guaranteed sfc and the IMO NOx limitation will be subject to a tolerance of 5 per cent on the fuel consumption.