What you need to know about Sulzer RT-Flex engine

The Sulzer RT-flex engine is essentially a standard Sulzer RTA slow-speed two-stroke marine diesel engine 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, the engine is equipped with a common-rail system for fuel injection and exhaust valve actuation and full electronic (computer) control of engine functions. Two control oil pumps are provided near the engine local control stand, and one of these must always be operational to ensure that the common rail fuel and exhaust valve operation systems can function. The control pump starts automatically once one of the crosshead lubricating oil pumps is started.

The engine is monitored and controlled by a WECS (Wartsila Engine Control System) unit. This is a modular electronic system with separate microprocessor control units for each cylinder. Overall control and supervision is by means of separate, duplicate microprocessor control units.

The cylinder microprocessor control units are mounted on the front of the engine at the common rail in the hot-box, which is located below the top engine platform.

The engine is a single-acting, two-stroke, reversible, diesel engine of crosshead design with exhaust gas turbocharging and uniflow scavenging. Tie rods bind the bedplate, columns and cylinder jacket together. Crankcase and cylinder jackets are separated from each other by a partition, which incorporates the sealing gland boxes for the piston rods. The cylinders and cylinder heads are fresh water cooled.

The exhaust gases flow from the cylinders through the hydraulically operated exhaust valves into a manifold and then on to the exhaust gas turbochargers which work on the constant pressure charging principle.
The charge air delivered by each turbocharger flows through an air cooler and water separator into the common air receiver. Air enters the cylinders through the scavenge ports, via valve groups, when the pistons are nearly at their bottom dead centre (BDC) positions. At low loads two electrically-driven auxiliary blowers supply additional air to the scavenging air space.

The pistons are cooled by bearing lubricating oil supplied to the crossheads by means of articulated lever pipes. The thrust bearing and turning gear are situated at the engine driving end. The fuel and servo oil pumps for the common rail fuel system and exhaust valve actuation are driven by gearwheels from the crankshaft.

The engine is started by compressed air, which is controlled by the electronic starting air system. In case of failure of the engine remote control system (from the bridge or the engine room telegraph) the engine can be controlled from a local (emergency) control stand located on the port side of the engine on the middle platform. There is an ECR back-up control system which is linked with the local (emergency) control system.

The engine lubrication system, with the exception of turbocharger and cylinder lubrication, is supplied by one of two main pumps, which take suction from the main engine lubricating oil sump tank and supply the main bearings. The engine main bearings and thrust block are supplied with lubricating oil by the duty main lubricating oil circulation pump. There are two pumps fitted and these are located at the aft end of the engine with one working and the other switched to automatic standby. The oil is cooled before supply to the engine. Oil from the main bearing system is also supplied, via articulated lever pipes, to cool the working piston crowns. The main bearing and crosshead oil systems are interconnected as the crosshead pumps take their suction from the main bearing LO supply line to the engine.
Two crosshead LO pumps, one working and one on standby, take their suction from the main LO supply to the engine, after the automatic backflush filter and supplies oil at increased pressure to the crosshead bearings and to the servo oil pumps. The bottom end bearings are also supplied with LO from the associated crosshead with the oil flowing down a bore in the connecting rod. The lubrication of crossheads and connecting rod bottom end bearings is made through articulated lever pipes.

The turbochargers are supplied with lubricating oil from the turbocharger LO system. There are, usually two or three turbocharger LO pumps (depending of the engine size and design), one/two operating and one on automatic standby. These pumps supply oil to the turbocharger bearings from the turbocharger LO sump tank via a cooler. The pumps have suction filters and there is also an automatic backflushing filter unit with a back-up bypass filter.

With regard to cylinder lubrication more information can be found on this link.

The fuel oil is supplied to a common rail by the fuel supply pumps which are driven from the crankshaft by a gear system. The fuel pumps are arranged in a V form with four pumps in each bank. The pumps deliver pressurised fuel oil to a collector which then supplies the common fuel rail which is maintained at a pressure of about 1,000 bar at full load (the actual pressure varies with engine load). Recently, for safety and operational reasons the pressure has been reduced to 600 bars. All parts of the high pressure fuel system are sheathed to prevent high pressure fuel leakage from entering the engine room spaces. The fuel supply pumps are driven by a camshaft via three-lobed cams. The lobed cams and the speed of the camshaft means that each pump makes several strokes during a crankshaft revolution. There are six or eight fuel supply pumps (depending on the engine size) and the output of the pumps is such that seven pumps have the capacity to meet the full load requirements of the engine. With only six pumps operational, the engine load must be reduced below maximum. The common fuel rail is divided into two equal sections, one serving the forward six cylinders and the other serving the six aft cylinders. The common rail volume is such that the fuel pressure is constant throughout the operation of the engine.

There are three fuel injectors fitted in each cylinder cover and high pressure fuel oil is supplied to these from the common rail. Each cylinder has its own injection control unit which controls the fuel supply to the injectors from the common fuel rail. Each injection control unit has three rail valves and three injection control valves, one of each for each injector. The rail valve is an electrically operated spool valve which can be moved to each end of its casing by electrical signals from the WECS. The spool valve acts as an open or closed valve and when in the open position it directs control oil to the injection control valve. The injection control valve opens and allows high pressure fuel from the common rail to pass to the fuel injector so beginning fuel injection at that injector. When the WECS signals the spool valve to close, the injection control valve is closed and hence fuel injection stops. Control oil is supplied by the servo and control oil manifold at a pressure of 200 bar. The rail valves are bi-stable solenoid valves with a fast actuation time; the valve is not energised for more than 4ms at any time.

The WECS controls the fuel injection system via the Flex Control Module (FCM-20) which not only regulates the start and end of injection but also monitors the quantity of fuel injected. The fuel quantity sensor measures the actual amount of fuel injected and this information is relayed to the control system. The control system then calculates any change in fuel timing required from the engine operating conditions and the actual fuel quantity injected. The functioning of the fuel injection system is monitored at each cycle and changes are made for the next cycle if necessary.
Operation of the rail valves is under the control of the WECS, which can adjust the timing and quantity of fuel injection to suit operating conditions.
Normally all three cylinder fuel injectors, which are of the hydraulically actuated type, operate together but as they are independently controlled it is possible for them to be programmed to operate separately. In the event of one of the fuel injectors or its actuation system failing, the engine may continue to operate with the remaining two injectors. At low engine speeds one or two of the fuel injectors can be cut out for each cylinder to minimise exhaust smoke.
The remaining operational fuel injector(s) operate at longer injection periods with the high fuel pressure maintained by the common rail. With injector(s) cut out the operating injector(s) are changed over every 20 minutes to prevent overheating of the cut out injector(s) and to ensure that all of the injectors have equal running.
The fuel quantity delivered to the engine by the fuel preparation unit is considerably greater than is actually required by the engine with the excess flowing back to the mixing unit of the main fuel preparation unit. The mixing unit is located at the FO circulating pump suction and also takes a FO feed from the low pressure FO supply pump which draws HFO from the duty HFO service tank. From the circulating pumps the HFO flows through the steam heaters and then to the supply manifold for the high pressure common rail supply pumps. A pressure regulating valve, set at 10kg/cm² is fitted between the engine FO inlet and outlet lines and this allows the correct fuel oil supply pressure to be maintained at the engine inlet.
The main engine is designed to operate on HFO during manoeuvring. All pipes are provided with trace heating and are insulated. For reasons of safety, all high-pressure pipes are encased by a metallic hose. Any leakage is contained and led to an alarmed fuel oil leakage tank. The engine may be operated on MDO if necessary.

The starting air system of the RT-flex engine is similar to that of a standard RTA engine except for the control of the cylinder starting air valves which is incorporated in the WECS rather than a starting air distributor. Starting air is supplied to the engine starting air manifold from the starting air receivers via the starting air shut-off valve. The individual cylinders are then supplied with starting air via branch pipes which have flame arresters fitted.
The cylinder starting valve is operated by pilot air and the pilot air valve is controlled electrically by the cylinder control module. The starting pilot air valve is opened and closed directly by the Flex control module (FCM-20) once every revolution at defined crank angles during the starting period. When the engine has started the starting system is shut down.

Each cylinder has a single exhaust valve centrally located in the cylinder cover which is hydraulically opened, but closed by air pressure acting on the piston located below the hydraulic actuating cylinder. When hydraulic pressure is applied to the actuating piston to open the exhaust valve, the air trapped below the air piston is compressed. When the hydraulic opening pressure is removed the air pressure acts on the piston to close the exhaust valve and this is known as the ‘air spring’. The space above the air piston is vented and make-up air is supplied to the space below the piston from the control air system via a non-return valve to replace any leakage that may have occurred.
The exhaust valve is fitted with a series of vanes on the stem known as a spinner. When the exhaust valve is opened, exhaust gas escaping from the cylinder acts on the spinner and causes the valve to rotate. Rotation of the valve evens out the temperature of the valve, and as the valve is still rotating when it reseats it creates a light grinding effect which removes deposits from the valve seat and valve face.
The FCM-20 controls the exhaust valve opening and closing. Hydraulic pressure for opening the valve comes from the servo oil common rail. This is pressurised to 200 bar by the servo oil pumps which are driven by the same gear drive system as the fuel common rail pumps. The FCM-20 controls an exhaust rail valve which then activates the exhaust hydraulic control slide valve and this directs hydraulic oil to and from the exhaust valve actuator unit. The servo oil acts on the lower face of the free-moving exhaust valve actuator piston and as the piston moves upwards, when servo oil pressure is applied, it exerts an hydraulic force on the exhaust valve piston and opens the exhaust valve.
The hydraulic system connecting the upper face of the exhaust valve actuator piston with the exhaust valve piston (the hydraulic pushrod) is filled with engine bearing oil and a connection with the bearing circulation system ensures that the space is always fully charged. This arrangement provides a complete separation of servo hydraulic system and valve actuation/bearing lubricating oil systems, and enables the exhaust valves to be serviced without disturbing the servo oil system.

The RT-flex engine control is shared between the WECS internal engine control and the external propulsion control systems which comprise the remote control system, the safety system, the electronic governor and the alarm monitoring system.

The WECS is the core engine control. It processes all actuation, regulation and control systems directly linked to the engine:

      • Common rail monitoring and pressure regulation
      • Fuel injection, exhaust valve and starting air valve control and monitoring
      • Interfacing with the external systems via the CAN-open or MOD-bus
      • Engine performance tuning, IMO setting and monitoring

The WECS modules are mounted directly on the engine and communicate via an internal CAN-bus. Operator access to the WECS-9520 is integrated in the user interface of the propulsion control system. The manual control panels and the flexView system allow for additional communication with the WECS. The flexView software allows the operator to communicate with the WECS and enables the operator to see operating parameters as required.
Each engine cylinder has its own module for all cylinder related functions; all common functions are shared between the cylinder modules.

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Source and Bibliography:

  • YouTube video credit: Brian Johannesen; Marine Tech Hub; Marine Engineer (PARAMI);

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