Long term service experience has confirmed the effectiveness of the inherent design measures embodied in MAN B&W Diesel MC/MC-C and ME/ME-C two-stroke engines for direct coupling to controllable pitch propellers (CPP) or de-clutchable propellers.

CPP installations featuring MAN B&W two-stroke engines with bore sizes up to 70 cm are particularly customary for tankers deployed in offshore loading or where ice class is demanded. Such a propulsion plant often embraces a shaft generator, usually, a simple synchronous generator requiring a constant engine speed to deliver electrical power or a generator capable of delivering constant frequency power at varying engine speeds.

All sizes of MAN B&W two-stroke engines may also be specified for plants in which the propeller is de-clutchable. In these installations, the engine can be operated for power generation at full speed with the propeller de-clutched at loads from 0% to 100% for prolonged periods.

Load profile flexibility
The generator load profiles outlined below are normal for MAN B&W MC/MC-C and ME/ME-C engines, which are designed to run at all load conditions at high engine speed, including very low loads:
 

• For simple synchronous generators, engine operation at (or close to) full speed is necessary to exploit the shaft generator in seagoing conditions. Thus, when using the generators in harbour, or whenever the ship is operated with zero-pitch on the propeller, it is necessary to operate the engine at full speed and down to 10-20% load (normal consumption of a propeller in zero pitch).
   
• With constant frequency generators, a relatively wide engine speed is available for electrical power production. When there is no need for propulsive power, therefore, the engine speed is reduced and electrical power can be delivered with reduced propeller speed and, thereby, reduced loss of power. A full engine speed and low load combination is not called for in this scenario.
   
• Full speed and loads down to 0% can be expected for plants with engines connected to a de-clutchable propeller and a large generator, for example, those serving North Sea shuttle tankers.

MC/MC-C and ME/ME-C engines applied in these plant configurations are designed for continuous operation in a flexible load range. Special care is paid to the design of the following components/structures to allow the 0% engine load and 100% engine speed case (the full speed-idle load mode):

Main bearing assembly in the bedplate, bearing shells and main bearing cap.
 

• Crankpin bearing assembly.
• Crosshead bearing assembly.
• Piston/piston rod assembly.
• Piston rod/crosshead assembly.
• Main bearing, crankpin bearing and crosshead bearing.

It is important to emphasise that the full speed-idle load mode is fully applicable for continuous operation; the mode is relevant for many applications of engines connected to various types and sizes of shaft generator.

The design practice described above also increases safety margins against fatigue damage in structural parts for fixed pitch propeller installations.

Structural integrity of main assemblies
The most complex assembly to analyse is that of the main bearing cap in the bedplate. An important step is evaluating the contact pressure between the bearing shell and the bearing saddle, this pressure distribution being calculated during assembly when the main bearing caps are tightened. Controlling the contact pressure distribution is crucial in avoiding fretting of the back of the bearing shell.

In other steps of the analysis, contact pressure distribution is superimposed with the variation in contact pressure distribution originating from the various running conditions. The full speed-idle load mode is naturally of special importance as this running condition requires the most attention in avoiding bearing shell back fretting.

Elasto-hydrodynamic bearing analysis
Crosshead bearings show a significantly different load pattern when operating in the full speed-idle load mode. At the full load condition (100% engine load and 100% engine speed) the bearing load mainly points downwards, thus creating high oil film pressures in the lower shell of the crosshead bearing.

Much effort is being directed towards achieving optimal conditions with respect to load-carrying capacity and oil film buildup in this highly loaded bearing. In the full speed-idle load mode, however, the upper shell of the crosshead bearing participates in the load-carrying process.

Elasto-hydrodynamic bearing analyses are always conducted at the design stage of new or re-designed (uprated) engine types, and the full speed-idle load mode is always taken into consideration. For this mode, the maximum oil film pressure peaks at two crank angles (57 degrees and 339 degrees), in both cases impacting on the upper crosshead bearing shell. The calculated oil film pressures are nevertheless well within the design limits for continuous operation.

Similar elasto-hydrodynamic calculations are performed for the other principal bearings in the engine, for verification over the complete load range including the full speed-idle load mode.