Sulzer RT-flex. Rail unit
The rail unit is located at the engine’s top platform level, just below cylinder cover level. It extends over the length of the engine. It is fully enclosed but has good maintenance access from above and form the front. The rail unit contains the rail pipes and associated equipment for the fuel, servo oil and control oil systems. The starting air system is not included in the rail unit.
For engines with up to eight cylinders, the rail unit is assembled as a single unit. With greater numbers of cylinders, the engines have a mid gear drive and the rail unit is in two sections according to the position of the mid gear drive in the engine.
The fuel common rail provides storage volume for the fuel oil, and has provision for damping pressure waves. There is no need for energy storage under gas pressure. The volume of the common-rail system and the supply rate from the fuel supply pumps are such that the rail pressure is very stable with negligible pressure drop after each injection.
In the RT-flex Size I, the high-pressure pipe for the fuel rail is modular with sections for each cylinder and flanged to the individual injection control units for each cylinder.
With the Size IV, the high-pressure fuel rail was changed to a single-piece rail pipe to shorten assembly time and to simplify manufacture. A single length of rail pipe is installed in each section of the rail unit. The only high-pressure pipe flanges on the Size IV pipe are the end covers.
Fig 6. The two sections of rail unit for a 12-cylinder RT-flex96C engine during the course of assembly
The common rail system is designed with very high safety margins against material fatigue. The fuel rail pipe for instance has a very special inner shape to keep the stress amplitude in cross-bored drillings remarkably low. The fact that, by definition, common rail have almost constant pressure levels further increases the safety against high cycle fatigue cracking compared to conventional injection and actuator systems with high pressure cycles.
The high-pressure rail is trace heated from the ship’s heating system, using either steam or thermal oil. The simplification of the fuel rail for Size IV, without intermediate flanges, compared with that for Size I allowed the trace heating piping also to be simplified. The trace heating piping and the insulation are both slimmer, allowing easier service access inside the rail unit.