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Common-rail system overview

Sulzer RT-flex. Common-rail system overview

Although common-rail fuel injection is certainly not a new idea, it has only become truly practical in recent years through the use of fully-integrated electronic control based on high-performance computers which allow the best use to be made of the flexibility possible with common-rail injection.
The traditional camshaft has the considerable limitation of fixed timing given mechanically by the cams. Although Sulzer low-speed engines have long had the benefits of double valve-controlled fuel injection pumps with variable injection timing (VIT), and a degree of variable exhaust valve timing being achieved hydraulically in the VEC system, the variation in timing so obtained has been very limited.

Instead electronically-controlled common-rail systems have been adopted in the new Sulzer RT-flex engines to give complete control of the timing, rate and pressure of fuel injection and the exhaust valve operation, allowing patterns of operation which cannot be achieved by purely mechanical systems.
Rather than ‘electronically controlled’, it would be more accurate to describe Sulzer RT-flex engines as being computer controlled. This is because in the RT-flex system, engine functions are fully programmable, perhaps limited only by the designers’ imagination and the laws of nature. The challenge is to use this freedom to create practical benefits for engine users.

Fig 1. Principal elements of the common-rail system on a Sulzer RT-flex engine
The common-rail concept was adopted also because it has the advantage that the functions of pumping and injection control are separated. This allows a straightforward approach to the mechanical and hydraulic aspects of the design, with a steady generation of fuel oil supply at the desired pressure ready for injection. The common-rail concept also has the unique advantage that it allows the fuel injection valves to be individually controlled. Usually there are three fuel injection valves in each cylinder cover, and in the Sulzer RT-flex engines they are operated mostly in unison but under certain circumstances they are operated separately for optimum combustion performance.
The common-rail concept thus provides an ideal basis for the application of a fully-integrated electronic control. The combined flexibilities of common rail and electronic control provide improved low-speed operation, engine acceleration, balance between cylinders, load control, and longer times between overhauls. They also ensure better combustion at all operating speeds and loads, giving benefits in lower fuel consumption, lower exhaust emissions in terms of both smokeless operation at all operating speeds and les NOx emissions, and also a cleaner engine internally with less deposits of combustion residues. Engine diagnostics are built into the system, improving engine monitoring, reliability and availability.
As the common-rail system is built specifically for reliable operation on heavy fuel oil, it detracts nothing from the well-established economy of low-speed marine diesel engines but rather opens up new possibilities for even better economy, ease of operation, reliability, times between overhauls and lower exhaust emissions.
It is more than ten years since development of the Sulzer RT-flex common rail system began and more than 20 years since the first tests were made with electronically-controlled fuel injection in Winterhur, Switzerland.
The early camshaftless systems developed for Sulzer engines relied on integral electronic control but used individual, hydraulically-operated fuel injection pumps. However the change in injection concept from the individual, hydraulically-operated fuel injection pumps to a common-rail system in 1993 was made because the system with individual pumps did not offer potential for further technological development despite it having integral electronic control. Electronic control was found to be insufficient by itself and a new fuel injection concept was recognised as essential. Common rail was seen as the road ahead and it is applied in Sulzer RT-flex engines.
Sulzer RT-flex engines are thus notably different from other electronically-controlled low-speed diesel engines today as Sulzer RT-flex engines are unique in combining the benefits of both common-rail systems and electronic control.

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