Developing the high performance derivative

Refining an established engine for high speed vessel propulsion duty must focus on achieving a lighter and more powerful package within a more compact envelope, while also enhancing operating flexibility. The resulting PA6B STC design was released with a nominal maximum continuous rating of 405 kW/cylinder at 1050 rev/min, and a sprint rating of 445 kW/cylinder at 1084 rev/min (Figure 30.37).

Figure 30.36 SEMT-Pielstick PA6B STC cylinder liner
Figure 30.37 Cross-section of SEMT-Pielstick PA6B STC engine

SEMT-Pielstick's development goals aimed to:

• Increase the power rating by 25 per cent.

• Use an operating field allowing operation following a double propeller law.

• Maintain, and if possible reduce, the specific fuel consumption.

• Reduce the weight/power ratio and enhance engine compactness.

• Reduce installation costs.

• Retain the maximum number of existing components.

Power can be raised in two ways: by increasing the brake mean effective pressure and by increasing the piston speed. Increasing piston speed can be effected by increasing the rev/min and by lengthening the stroke. Increasing the rev/min was not attractive because of a need to maintain an acceptable synchronized speed for land power plant applications but also because of these drawbacks: increased specific fuel consumption; higher wear rate; and higher noise levels. Raising piston speed by lengthening the stroke is considered preferable because it reduces specific fuel consumption and fosters optimum performance at starting and low load (the compression ratio is secured without resorting to a combustion chamber that is too flat). This route was therefore selected but with the piston speed limited to 11.5 m/s (representing an increase of 14 per cent with a stroke of 330 mm). To reach the required power rating, the brake mean effective pressure had to be increased by 10 per cent; this was achieved by adopting high performance turbochargers. Attaining the desired specific fuel consumption called for the maximum cylinder pressure to be raised to 160 bar.

Coping with these new parameters dictated redesigning some key components. The existing cylinder head, for example, was incompatible with the targeted specific fuel consumption level. This was partly because its mechanical strength was insufficient in relation to the necessary peak combustion pressure, and partly because the pressure loss through the inlet and exhaust ports would be unacceptable in relation to the intervening gas flow dictated by the increased power. A new cylinder head was therefore designed with a reinforced bore-cooled fire plate and incorporating inlet and exhaust ports with large dimensions. In addition, cooled seats were specified for the inlet valves as well as the exhaust valves to maximize seat reliability and avoid risks of burning.

A redesigned connecting rod was also necessary to address the increased peak compression pressures and the inertia efforts linked to the lengthening of the stroke. The bevel cut design of the original component was abandoned in favour of a straight-cut rod to avoid weak points, such as the bevel cut serrations and the threading anchorage in the shank. Piston cooling by jet replaced the traditional oil supply through the connecting rod from the crankpin, simplifying machining and allowing the bearing shell grooves to be eliminated and yield these benefits: increasing capacity of the bearing shells; stopping cam wear of the crankpin (differential wearing of the pins between the side areas of the plain bearing shell and the central area; that is, the area including the groove); and cutting out the risk of cavitation erosion on the bearing shells at the end of the grooves. The new connecting rod is 10 per cent lighter than its bevel cut forerunner. The weight reduction, along with an improved bearing shell capacity, partly compensates for the increased inertia efforts.

Temperature measurements on the jet-cooled piston head indicated similar levels to those of the original piston, and even lower in some areas. Critical points of the piston were modified to cope with the higher peak combustion pressure: a spherical shape was given to the support spot face used for the piston head/skirt tightening spacers; and the radius under the skirt vault was increased.

Crankshaft dimensions were modified to target the same reliability from the component as before, despite the increased stroke and peak combustion pressure. Finite element analyses of the crankwebs and hydrodynamic calculations led to an increase in the journal diameter from 230 mm to 250 mm, and in the crankpin diameter from 210 mm to 230 mm.

High performance turbochargers were necessary to secure the brake mean effective pressure increase with the required efficiency, a model from MAN B&W's then new NA series—the NA 34S—being selected to meet the performance and compactness parameters. Sequential turbocharging was applied, based on the principle of reducing the number of turbochargers in operation as the engine speed and load fall. The speed of the turbochargers still operating consequently rises and significantly larger quantities of air are thus delivered to the engine.

A simple system was adopted using only two turbochargers, one being switched off at below approximately 50 per cent of the nominal engine power rating. This is effected by closing two flap valves located at the compressor outlet and at the turbine inlet of one of the turbochargers. The designer cites the following benefits from the PA6 engine's sequential turbocharging (STC) system:

• High torque and power ability at reduced engine speed.

• A gain in fuel economy at low and part loads.

• Capability to run the engine at very low loads for extended periods with minimal fouling (the light deposits can be cleaned out by running for half an hour at 50 per cent load).

• Invisible smoke emissions over a wide working range.

• Reduced exhaust temperature.

• Lower thermal stresses in the combustion chamber components at part loads.

A higher output rating naturally reduced the engine's weight/power ratio but other measures were pursued to trim overall weight. The scope for using aluminium alloy was explored for all components where operating stresses (particularly thermal) and class rules allowed, leading to the engine supports, turbocharger support, air manifolds (after the air cooler), lube oil and water cooler support, and lube oil filter support being designed in cast aluminium. Studies assessed other components for which aluminium could not be considered, either to use alternative materials with higher mechanical properties and so reduce thickness, or simply to optimize existing shapes and thicknesses. As an example, specifying high yield point steel sheet for the manifolds connecting the turbocharger to the air cooler allowed a reduction in thickness from 10 mm to 4 mm. The lube oil sump plate was also modified by reducing the material thickness.

A 10 per cent reduction in the original engine weight, along with the increased power output, contributed to a weight/power ratio of 4.8 kg/kW, including all ancillaries. In parallel with the weight trimming studies, SEMT-Pielstick focused on reducing the overall dimensions of both engine and ancillaries.

A key element here is the combi-cooler, integrating one lube oil and one freshwater plate cooler circulated by a common seawater system (Figure 30.38). The combi-cooler's support is used as the rear plate of the cooler and includes as-cast part of the connections to the high temperature freshwater and lube oil systems. Its front plate incorporates as-cast the connections to the low temperature freshwater

Integrated Circuit Wet
Figure 30.38 Fresh and seawater circuits integrated on the PA6B STC engine

loop and the water thermostatic valve. The main self-cleaning lube oil filter is incorporated axially in a cast support located under the combi-cooler, and includes the lube oil thermostatic valve as well as the centrifugal oil filters.

Such solutions fostered compactness, a simple pipeless configuration and good access to the main subjects of maintenance. Integration of the ancillaries on the engine further eases shipboard installation procedures. All pumps (water, oil, fuel make-up), as before, are driven by the engine upon which is also mounted the fuel filter.

A potential for burning an intermediate fuel oil grade such as IF30 was addressed in the development programme, and reflected in the specification of cooled valve seats and exhaust valve rotators. The 75°C temperature necessary to reach an adequate viscosity for its injection can be derived by taking heat from the engine's high temperature freshwater system. In such installations, however, the engine is derated to 360 kW/ cylinder and the time-between-overhauls is reduced.

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