The latest developments made on the PC2 and PA6 engine families are presented in this paper. As far the PC4 engine, it should be referred to the presentation made at last ISME Yokohama Congress [1].
2. LATEST PC2 ENGINE DEVELOPMENT
There are two ways to achieve competitive engine prices per kilowatt:
- to increase the specific engine power,
- to reduce manufacturing costs.
Both have been used by S.E.M.T. Pielstick for the latest developments of the PC2 engine family.
As the specific power of an engine is only depending of its brake mean effective pressure and mean piston speed levels, it is evident that it should be sought for the highest values of these parameters, without jeopardising engine reliability and maintenance costs.
State of the art values are today for this medium-speed engine category (fig. 4):
- brake mean effective pressure: about 24 bar,
- mean piston speed: about 10 m/s. Using these values allows to set a target of 750 kW/cyl, which represents a 36% increase from the PC2.6 engine rating.
A 10m/s piston speed could be achieved by choosing:
- an engine speed of 600 rpm
- an increased stroke to 500 mm which results in a strokebore ratio of 1.25; a good compromise to keep low manufacturing cost while getting satisfactory performances.
The denomination of this enlarged stroke PC2 engine is: PC2.6 B.
As this development implied deep modification on most of the main engine components, the step by step strategy of development has been used once again, i.e.:
- a first step at a running speed kept at 500 rpm,
- a second step at the target speed of 600 rpm.
First step
A comprehensive description of the first step development has been given in the last ISME Congress [1].
The major change in the engine design from the previous PC2 Vee-type engines is the use of a nodular cast-iron crankcase instead of a steel welded solution.
This allowed to get rapidly service results with the stroke of 500 mm and to fully assess the reliability of the new crankcase, while progressing in the second step of development.
For this first step, the engine rating was set at 615 kW/cyl at 500/514 rpm for land use and 520/530 rpm for ship propulsion engines.
Second step
The main problems arising with the 20% engine speed increase (from 500 to 600 rpm) are:
- higher pumping losses through the cylinder head during exhaust and intake strokes,
- increased mechanical loads resulting from inertia forces and from torsional vibrations of the crankshaft and camshafts,
- higher excitation frequencies of the engine structure vibration modes.
This required deeper modification of following engine components (fig. 5)
・cylinder head,
・connecting rod,
・camshaft,
・crankshaft
Further modifications had to be made on:
・piston,
・cylinder liner.
