How does the vessel move? RT-Flex! Part.3

In a previous article, I present in main scope, the most popular in marine use, the MAN ME engine. Today I would like to present his biggest opponent: Sulzer RT-FLEX.

History of RT-Flex project

After years of modification, and improvement in the well-known technology of diesel engines we have to wait for the next milestone up to the 20th century. MAN constructs his own, fully programmable version of the engine. In 90’s Wartsila take over Sulzer. Finish giant, Wartsila was specialized in four-stroke engines and compete with two-stroke engines designed by Sulzer. The first real deal for Wartsila, in marine engines comes with the project FOX. At the same time was tested first engine control solutions without the crankshaft. Instead of classic camshaft were used electronic camshaft, known also as common rail, electronic controlled fuel injection, adjusted for heavy fuel and electronic controlled exhaust valve. This solution has a final name known as RT-FLEX, and this is the biggest opponent of MAN ME engines.

In 2002 on the market comes at 7RT-flex60C, which means 7 cylinders, 60 cm diameter of the cylinder. This starts a new family of marine diesel engines. The main idea was to create an engine with high reliability, low emission, low oil consumption, and a minimum of 3 years between overhauls. The new solutions on the market, from an automation point of view, was: Flex system – electronic control system for fuel injection of common rail type and exhaust valve drive.

Of course, there were much more innovations like:

– main bearings co-machined with covers,
– rigid thrust bearing girder cast together with the intermediate piece – welding at the low-stress area,
– crank bearings – two thin bearing shells of white metal,
– conrod big end bearings – bore for rod and cover co-ma-chined,
– guide shoe – single-piece with white metal lining,
– improved stuffing box design – for low system oil consumption,
– Tribo Pack – provides minimal wear of piston, cylinder liner, piston rods – three years’ time between overhauls,
– stiff support of the turbocharger, integrated with air receiver,– air cooler arrangement – hanging tube stacks, fresh water-cooling, simplified maintenance, and cooler washing,
– improved design of water separator in charging air cooling system – essential for engine operating in tropical weather conditions.

But there are not so important from an automation point of view. I am sure You can find more about some mechanical engineering blogs.

Fuel injection development

Fuel injection electronic control was not a new idea applied in the engine. This was in use for high-pressure four-stroke engines. Innovation was to adjust this solution for a two-stroke engine. The biggest issue was the big difference in fuel viscosity and keep the high temperature of injected fuel. The development of these solutions was quite slow. The first modification was done from RTA engines to RTX-2 in 1990. For the next concept of stable pressure rail– Common Rail – and the third generation of RTX-3 system we have to wait up to 1998. Control of exhaust valves and starting valves actuation were also incorporated in that solution. Stages of the development of fuel injection electronic systems are presented below. This solution gives easier maintenance and more flexible operation.

Main Features of RT-Flex control electronics:

– Common fuel rail fed with fuel oil at the pressure of 1000 bar supplied by two independent sections of high-pressure pump,
–Common oil rail fed with oil at the pressure of 200 bar sup-plied by two independent sections of high-pressure pump,
– Servo oil at the pressure of 200 bar used as a working factor in the fuel injection unit, actuating valves control and exhaust valves control,
– Precise control of injected fuel,
– Fuel oil is separated from the control valve – essential for heavy fuel oil,
– High-pressure fuel pump adapted from well-proven four-stroke Sulzer engine ZA40S,
– Variable injection rate shaping and free selection of injection pressure,
– Stable pressure levels in common rail and supply pipes,
– Smokeless operation at all operation speeds,– low, stable running speeds down to 10-12% of nominal speed for maximum continuous engine rating,
– Reduced fuel consumption at part-loads and emission of NOx

Heart of Rt-Flex system

The engine system is controlled by Wartsila Electronic Control System. This is an input/output system, which collects information about engine operation (take input from the sensors) and sends control signals to equipment (make the output signal to equipment). This system is combined together with remote control and alarm system. This is the biggest advantage of the RT-Flex system, by one sensor we can do monitoring, alarming, and control of the engine. Competitive solutions usually have 2 senors, instead of one. The full control system layout looks like at graphic below.

Each cylinder has its own electronic module, which controls the operation, same as MAN-ME engine, but the main difference is, that the cylinder module has also additional tasks like run servo oil pumps. In comparison, MAN has additional electronic units to handle different signals. Additional one FCM units are always running at the online spare position. In case of trouble with any cylinder unit, can be replaced. The RT-Flex solution is easy to do, but after replacing the spare unit from zero position we need to place a new unit in the spare position. The unit has to run a minimum of 2 hours at the engine before can be replaced with one of the cylinders. Unit is constantly monitoring other units and is learning the behavior of the engine. MAN solution is a little bit different because you can replace the existing solution in the system, for example, ACU 2 with broken CCU and engine will run without any problem, but You need to reprogram a unit. Also if You need to replace a new unit, it has to be programmed first. Basically, both solutions are easy and user-friendly.


Same as MAN, RT-Flex can be electronically controlled for improvement of operational flexibility. Optimal combustion process allows reducing minimal engine speed to 12 rpm during maneuvring! Improved engine actuation process by full control over starting air supply, low ignition speed are also fully modified.


  • Mateusz Białas

    Researcher of Autonomous and Remotely Controlled ships. Since 2014 in Maritime Industry. Experience gained at Ro-Ro, Ferry, Container, and Heavy Lifts, design of Navy Ships.

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