Fundamental of Sulzer Common Rail Fuel Injection
The common rail is a manifold running the length of the engine at just below the
cylinder cover level illustrated in Fig. 2.5a and 2.5b as an example of the common-rail fuel
systems in Sulzer RT-flex engine. It provides a certain storage volume for the fuel oil, and
has provision for damping pressure waves. The common rail and other related pipework
are neatly arranged beneath the top engine platform and readily accessible from above (Fig.
2.7). The common rail is fed with heated fuel oil at the usual high pressure (nominally
1000 bar) ready for injection. The supply unit has a number of high-pressure pumps
running on multi-lobe cams. The pump design is based on the proven injection pumps used
in Sulzer four-stroke engines.
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Fig. 2.5a: Schematic of the common-rail systems for fuel injection and exhaust valve
actuation in the Sulzer RT-flex engine [2.4]
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each engine cylinder to the standard fuel injection valves (Fig.2.6) that are hydraulically
operated in the usual way by the high-pressure fuel oil. The control units, using
quick-acting Sulzer rail valves, regulate the timing of fuel injection, control the volume of
fuel injected, and set the shape of the injection pattern. The three fuel injection valves in
each cylinder cover are separately controlled so that they may be programmed to operate
separately or in unison as necessary. The common-rail system is purpose-built for
operation on just the same grades of heavy fuel oil as are already standard for Sulzer
RTA-series engines. For this reason, the RT-flex system incorporates certain design
features not seen in other common-rail engines using middle-distillate diesel oils. The key
point is that, in the RT-flex system, the heated heavy fuel oil is kept away from the
precision quick-acting rail valves. The key features of the Sulzer common-rail system are
thus:
• Precise volumetric control of fuel injection, with integrated flow-out security
• Variable injection rate shaping and variable injection pressure
• Possibility for independent action and shutting off of individual fuel injection valves
• Ideally suited for heavy fuel oil
• Well-proven standard fuel injection valves
• Proven, high-efficiency common-rail pumps
• Lower levels of vibration and internal forces and moments
• Steady operation at very low running speeds with precise speed regulation
• Smokeless operation at all speeds (Figs. 2.8 and 2.9)
As indicated in Fig. 2.5b the RT-flex system encompasses more than the fuel injection
process. It includes exhaust valve actuation and starting air control [2.4]. The exhaust
valves are operated in much the same way as in existing Sulzer RTA engines by a
hydraulic pushrod but with the actuating energy now coming from a servo oil rail at 200
bar pressure. The servo oil is supplied by high-pressure hydraulic pumps incorporated in
the supply unit with the fuel supply pumps. The electronically-controlled actuating unit for
each cylinder gives full flexibility for valve opening and closing patterns (Fig. 2.5b). This
unit utilizes exactly the same Sulzer rail valves as are used for controlling fuel injection.
common rail (CR) technology will be applied to cut emissions. Fig. 2.8 shows some results of
tests performed on a test engine 7L 16/24 CR (common rail) in HFO operation for 50 % load
at constant speed (1200 rpm): there is a positive influence of the rail pressure on smoke
emission indeed, but the increased fuel consumption rate (sfoc) at very high rail pressures has
to be taken into account too (Fig. 2.9) [2.4].
All functions in the RT-flex system are controlled and monitored through the integrated
Wärtsilä WECS-9500 electronic control system (Fig. 2.5a). This is a modular system with
separate microprocessor control units for each cylinder, and overall control and supervision
by duplicated microprocessor control units. The latter provide the usual interface for the
electronic governor and the shipboard remote control and alarm systems. Sulzer RT-flex
engines are designed to be user friendly, without requiring ships’ engineers to have any
special additional skills. Indeed, the knowledge for operation and maintenance of RT-flex
engines can be included in Wärtsilä’s usual one-week courses for Sulzer RTA-series engines
given to ships’ engineers. Training time usually given to the camshaft system, fuel pumps,
valve actuating pumps, and reversing servomotors is simply given instead to the RT-flex
system.
At its heart, the Sulzer RT-flex engine is the same reliable, basic engine as the existing
RTA engine series. The power ranges, speeds, layout fields and full-power fuel consumptions
are the same for both engine versions. The Sulzer RT-flex engine offers a number of
interesting benefits to ship owners and operators:
• Reduced running costs through lower part-load fuel consumption and eventually longer
times between overhauls
• Reduced maintenance requirements, with simpler setting of the engine, for example
adjustment of mechanical injection pumps is no longer necessary, and ‘as-new’
running settings are automatically maintained
• More balanced engine operation. The common-rail system with volumetric control
gives excellent balance in engine power developed between cylinders and between
cycles, with precise injection timing and equalized thermal loads
• More predictable maintenance costs owing to better balanced engine operation and
better retaining of engine settings over many running hours
• Extendable times between overhauls through better engine running conditions, and
better prediction of maintenance timing
• Better fuel economy in actual in-service load range
• Flexibility to optimize fuel consumption at selected service loads within compliance
with the NOX emission limit in Annex VI of the MARPOL 73/78 convention
• Smokeless operation at all operating speeds.
• The RT-flex system is based on well-proven hardware
• Full electronic common-rail control with integrated monitoring functions
• Lower steady running speeds, in range of 10–12 rev/ min obtained smokelessly
through sequential shut-off of injectors in all cylinders. Selective shut off of injectors
gives more balanced engine operation than cutting out cylinders
• Built-in overload protection
• Built-in redundancy, as 100 percent power can be developed using three of the four
fuel pumps, and with one servo oil pump out of action.
