Sulzer RT-flex. Conclusion

Common rail is now an industrial standard for diesel engines. It has been proven to be an tremendous step forward for all sizes of diesel engines from automotive engines up to the largest low-speed two-stroke engines.
In this environment, Sulzer RT-flex engines have become well accepted by shipowners. Shipowners’ confidence is being encouraged by the good operating experience with the growing number of RT-flex engines in service.
The combination of common-rail concepts and fully-integrated electronic control applied in Sulzer RT-flex engines clearly has excellent potential for future development. It gives the large degree of flexibility in engine setting and operation, together with reliability and safety, which are required to meet the challenges in future marine engine applications in terms of emissions control, optimised fuel consumption, insensitivity of fuel quality, ease of use, operational flexibility, etc.

Fuel consumption flexibility

Sulzer RT-flex. Fuel consumption flexibility

Sulzer RTA engines have always been highly competitive in fuel consumption right across the load range owing to the use of variable injection timing (VIT). Variable exhaust valve closing (VEC) was also added in RTA84 T engines in 1991 to reduce further the part-load BSFC. These benefits already been carried over to the electronically-controlled common-rail systems of the RT-flex engines. 

Very slow running

Sulzer RT-flex. Very slow running

Sulzer RT-flex engines have also demonstrated their ability to run stably at very low speeds, lower than engines with mechanically-controlled injection.
They can run without smoking at about ten percent nominal speed. This makes for easy ship handling when manoeuvring or in river and canal passages.
Such slow running was well confirmed in service in the Gypsum Centennial. Slow running was taken to a new “low” during the testing in May/June 2004 of the first 12-cylinder RT-flex96C engine. Owing to its number of cylinders, it could run steadily at just seven revolutions per minute. 

Low exhaust emissions

Sulzer RT-flex. Low exhaust emissions

A clearly visible benefit of Sulzer RT-flex engines is their smokeless operation at all ship speeds. It helps give a “green” image.
This was well demonstrated in the testing of the first RT-flex engine and during the sea trials of the Gypsum Centennial.

Benefits from the Sulzer RT-flex system

Sulzer RT-flex. Benefits from the Sulzer RT-flex system

At its heart, the Sulzer RT-flex engine is the same reliable, basic engine as the existing Sulzer RTA engine series. The power ranges, speeds, layout fields and full-power fuel consumptions are the same for both engine versions.
For shipowners, the principal benefits of Sulzer RT-flex engines with their electronically-controlled common rail systems are:

  1. reduced part-load fuel consumption
  2. smokeless operation at all running speeds
  3. very low, stable running speeds at about ten per cent nominal speed
  4. easy engine setting for less maintenance
  5. longer times between overhauls expected, primarily through better load balance between cylinders and cleaner combustion at all loads.

Operation and maintenance

Sulzer RT-flex. Operation and maintenance

Sulzer RT-flex engines are designed to be user friendly, without requiring ships’ engineers to have any special additional skills. Indeed the knowledge for operating and maintenance of RT-flex engines can be given in the same form as Wartsila usual one-week courses for Sulzer RTA-series engines given to ships’ engineers and owners’ and operators’ shore staff. The training time usually given to the camshaft system, fuel pumps, valve actuating pumps and reversing servomotors is simply given instead to the RT-flex common-rail system.

Reliability and redundancy

Sulzer RT-flex. Reliability and redundancy

Reliability and safety has the utmost priority in the RT-flex system. Although particular attention is given to the reliability of individual items of equipment in the RT-flex system, the common-rail concept allows for increased reliability and safety through its inherent redundancy.
High-pressure fuel and servo-oil delivery pipes, the electrically-driven control oil pumps, and essential parts of the electronic systems are duplicated for redundancy. The duplicated high-pressure delivery pipes have spot cocks at both ends to isolate any failed pipe. Each single pipe is adequate for the full delivery. All high pressure pipes are double-walled for safety.

Electronic control

Sulzer RT-flex. Electronic control

All functions in the Sulzer RT-flex system are controlled and monitored through the Wartsila Engine Control System (WECS). This is a modular electronic 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. The microprocessor control units, or electronic control units, are mounted directly on the engine, either on the front of the rail unit or adjacent to it.

Operating pressures and system energy

The normal operating pressure for the fuel rail ranges up to 1000 bar. It is lowered for the best compromise  between BSFC (brake specific fuel consumption) and NOx emissions according to the respective engine load and to keep the parasitic energy demand low.

Exhaust valve control

Sulzer RT-flex. Exhaust valve control

The exhaust valves are operated by a hydraulic “push rod”, being opened by hydraulic oil pressure and closed by an air spring, as in the Sulzer RTA engines with mechanical camshafts. But for RT-flex engines the actuating energy now comes from the servo oil rail. There is one exhaust valve actuator (also known as the partition device) for each cylinder.

Injection control unit

Sulzer RT-flex. Injection control unit

Fuel is delivered from the common rail to the injection valves through a separate ICU for each engine cylinder. The ICU regulates precisely the timing of fuel injection, accurately controls the volume of fuel injected, and sets the shape of the injection pattern. The ICU has an injection control valve and a Sulzer electro-hydraulic rail valve for each fuel injection valve. The rail valves receive control signals for the beginning and end of injection from the respective electronic unit of the WECS (Wartsila Engine Control System). 

Rail unit

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.

Control oil

Sulzer RT-flex. Control oil

Control oil is supplied at a constant 200 bar pressure at all engine speeds by two electrically-driven oil pumps, one active and the other on standby. Each pump has its own pressure-regulating valve and safety valve attached.
The control oil system involves only a small flow quantity of the fine filtered oil. The control oil serves as the working medium for all rail valves of the injection control units (ICU). The working pressure of the control oil is maintained constant to ensure precise timing in the ICU. It is also used to prime the servo oil rail at standstill thereby enabling a rapid starting of the engine.

Servo oil

Sulzer RT-flex. Servo oil

Servo oil is used for exhaust valve actuation and control. It is supplied by a number of swashplate-type  axial-piston hydraulic pumps mounted on the supply unit. The pumps are of standard proprietary design and are driven at a suitable speed through a step-up gear. The working pressure is controllable to allow the pump power consumption to be reduced. The nominal operating pressure is up to 200 bar. The number and size of servo oil pumps on the supply unit depend on the engine output or number of engine cylinders. There are between three and six servo oil pumps.

Supply unit

Sulzer RT-flex. Supply unit

Fuel and servo oil are supplied to the common-rail system from the supply unit which is driven through gearing from the engine crankshaft.
In the first few RT-flex engines, the supply unit is on the exhaust side of the engine so that it could be lower down without interfering with access to the crankcase. However, for all subsequent engines, the location of the supply unit has been standardised on the front of the engine (on the same side as the rail unit) and at about mid height. This keeps the engine “footprint” small so that the engines can be located far aft in ships with fine afterbodies.

RT-flex sizes

Sulzer RT-flex. RT-flex sizes

The hardware in the RT-flex system is being developed in four principal sizes for the six engine types currently in the programme. The six RT-flex engine types cover a power range of 8100 to 80080 kW (11000 to 108920 BHP).
This illustrates one of the advantages of the common-rail system in that hardware is standardised for groups of engine types, not just for the various cylinder numbers.

Sulzer RT-flex system

Sulzer RT-flex. Sulzer RT-flex system

Sulzer RT-flex engines are essentially standard Sulzer RTA low-speed two-stroke marine diesel engines except that, instead of the usual camshaft and its gear drive, fuel injection pumps, exhaust valve actuator pumps, reversing servomotors, and all their related mechanical control gear, they are equipped with a common-rail system for fuel injection and exhaust valve actuation, and full electronic control of engine functions.
There are four elements in the Sulzer RT-flex common-rail system: the rail unit along the side of the cylinders, the supply unit on the side of the engine, a filter unit for the servo oil, and integrated electronic control system, including the crank angle sensor.

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.

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