Common Rail: The Way Ahead for Ship Propulsion

Transcription

1 Common Rail: The Way Ahead for Ship Propulsion Kaspar Aeberli Director, Marketing & Sales Support, Marine Wärtsilä Switzerland Ltd, Winterthur Summary Common-rail fuel injection, made possible by fully-integrated electronic control, is a major milestone in the development of large marine diesel engines. The combination of common rail and electronic control offers unprecedented flexibility of operation which opens up further avenues in engine development to meet market requirements for ship propulsion now and in the future. The first series-built low-speed engine with electronically-controlled common-rail fuel injection has been in service for two years with excellent results. Further engines are being ordered by shipowners who recognise that common-rail fuel injection is a desirable development. Today common-rail technology provides benefits in smokeless operation at all operating speeds, lower fuel consumption, reduced maintenance costs and lower steady running speeds for better manoeuvring. Further progress can be expected along these lines over the coming years. * This paper is based on papers and presentations at the International Symposium on Combustion Engines and Marine Engineering (ISCEM 2003), Busan, Korea, October 2003, and the 4th International Ship Propulsion Systems Conference, Manchester, November Statistics are updated to January Fig. 1: The first nine-cylinder Sulzer RT-fl ex60c engine during its official shop test at HSD Engine Co Ltd, Korea. [04#007] 1 Wärtsilä Corporation, January 2004

2 Introduction Common-rail fuel injection is now a practical proposition for large, low-speed marine diesel engines. The first such Sulzer RT-flex engine has been at sea for than two years with excellent service experience and the system is to be employed in the most powerful marine engines built today. Although common-rail fuel injection is certainly not a new idea, it has only become truly practical now through the use of fully-integrated electronic control which allows full 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. The change to electronically-controlled common-rail systems has been made to ensure that the timing, rate and pressure of fuel injection and the exhaust valve operation are fully controllable, allowing patterns of operation which cannot be achieved by purely mechanical systems. The common-rail concept was adopted 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 commonrail 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 Fig. 2: Section of rail unit for a Sulzer RT-fl ex96c engine. The fuel rail is on the left and the servo oil rail is on the right, with control units superimposed on both. [03#016] Fig. 3: Supply unit on a 7RT-fl ex60c engine with the fuel pumps in a Vee-form arrangement on the right and servo oil pumps to the left of the gear drive. [03#039] 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 and lower exhaust emissions in terms of both smokeless operation at all operating speeds and less NO X emissions. Engine diagnostics are built into the system, improving engine monitoring and reliability. 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 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 Winterthur, Switzerland. The early camshaftless systems developed for Sulzer engines 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, 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. To summarise, common rail is seen to qualify as the 2 Wärtsilä Corporation, January 2004

3 Fig. 4: Side elevation of the 12-cylinder Sulzer RT-fl ex96c engine showing the supply unit in the middle and the rail units near the cylinder tops. The engine is some 22.6m long.. [03#085] way ahead for the further development of low-speed marine engines because: The technical concept is well founded Reliability is becoming well proven first engine exceeding 12,000 running hours More engines are already in service Sulzer RT-flex engines are available for your projects today with powers of ,080 kw Market demand is clear already confirmed orders for 61 engines Benefits available today are worthwhile with the current execution Future potential exists for continuing development Fig. 5: Test rig for the common-rail system of the RT-fl ex96c engine. An injection control unit (ICU) is mounted on a common rail which is of the length for six cylinders. To the left of the ICU are the three fuel injection valves for one engine cylinder. [03#120] Well-founded technical concept Compared to the conventional, mechanically-controlled low-speed two-stroke marine engine, the Sulzer RT-flex engine is provided with unique features through the fullyintegrated electronic control of fuel injection and exhaust valve actuation [1]. This makes it possible to replace various mechanical parts: the camshaft and its gear drive, the complete fuel injection pump units including the exhaust valve actuator pumps and reversing servomotors, and all their related mechanical control gear. These parts are replaced by four principal elements in the Sulzer RT-flex system: the rail unit along the side of the cylinders (Fig. 2), the supply unit on the side of the engine (Fig. 3), a filter unit for the servo oil, and the integrated electronic control system. The common rail is a pipe running the length of the engine just below the cylinder cover level. Together with related pipework, it is neatly enclosed with ready access 3 Wärtsilä Corporation, January 2004

4 Fig. 6: Test rig for a fuel supply pump of the RT-fl ex96c engine. To the left of the vertical pump is the fuel collector. The pump delivers fuel to the common-rail test rig shown in figure 5. [03#121] from above and the side. The common rail provides storage volume for the fuel oil, and has provision for damping pressure waves. There is no need for energy storage under gas pressure. The volume of the commonrail system and the supply rate from the fuel supply pumps are such that the rail pressure is very stable with negligible pressure drop after each injection. The common rail is fed with heated fuel oil at the usual high pressure, up to 1000 bar, ready for injection. The supply unit has a number of high-pressure pumps running on multi-lobe cams with a speed-increasing gear. The pumps have suction control to regulate the fuel delivery volume according to engine requirements. Although the pumps run on a camshaft, it cannot be compared with the traditional engine camshaft. It is very short and of much smaller diameter, and is quite differently loaded as there is no sudden, jerk action but rather the pump plungers have a steady reciprocating motion. With multi-lobe cams and the speed-increasing gear drive, each fuel supply pump makes several strokes during each crankshaft revolution. The result is a compact supply unit. Servo oil for injection control and exhaust valve actuation is provided at a pressure of 200 bar by a number of hydraulic pumps also on the supply unit. They deliver to a second common rail alongside the fuel rail. For the first engines, the supply unit is arranged low on the manifold side but subsequent engines have it higher on the opposite side. This keeps the engine footprint small so that the engines can be located far aft in ships with fine afterbodies. Fuel is delivered from the common rail through a separate injection control unit for each engine cylinder to standard fuel injection valves (Fig. 5). 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, although they normally act in unison, they can also be programmed to operate separately 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 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 the servo oil rail. The electronically-controlled actuating unit for each cylinder gives full flexibility for valve opening and closing patterns. This unit utilises exactly the same Sulzer rail valves as are used for controlling fuel injection. Being proven in service Although engine testing of the Sulzer RT-flex system began in June 1998, the real proof of the capability of the electronically-controlled common-rail system has come with the first series-built RT-flex engine [2]. Entering service in September 2001, this engine was built to operate using only the electronically-controlled commonrail system with no alternative. It went to sea as a fully industrialised product, ready for continuous heavy-duty commercial operation. It achieved this performance with very good success. The engine is a Sulzer 6RT-flex58T-B, of 11,275 kw at 93 rev/min, and is installed in the bulk carrier Gypsum Centennial. By the end of 2003, it had accumulated more than 12,000 running hours. The service experience of this engine has already been reported [3, 4, 5]. One key point from the operation of the RT-flex engine has been that the ships engineers quickly became comfortable operating the engine, and were only accompanied by a Wärtsilä engineer up to the end of May Throughout its operation, the service experience has been very good. Although there were a number of teething problems during the first few months, they have mostly been remedied or new components are under development. Even when there were problems, in most cases they 4 Wärtsilä Corporation, January 2004

5 Fig. 7: The bulk carrier Gypsum Centennial, Sulzer 6RT-fl ex58t-b. [01#148] Fig. 8: Aframax tanker Sea Lady, Sulzer 6RT-fl ex58t-b. [03#105] Fig. 9: Multi-purpose carrier Wladyslaw Orkan, Sulzer 7RT-fl ex60c. [03#115] Fig. 10: Reefer Carmel Ecofresh, Sulzer 7RT-fl ex60c. [04#002] did not interfere with normal ship operation as they caused either just an alarm signal or the engine to slow down. Some faults were rectified during normal scheduled engine halts while the majority, concerning commonrail and electronic components, could be rectified by briefly slowing the engine and replacing components. Six unplanned shutdowns occurred in the first couple of months operation, but since then there has only been a single stoppage. The fact that the whole design of the common-rail system was made in-house proved invaluable when troubleshooting problems. In-house knowledge allows quick diagnosis of problems and prompt identification of suitable remedies. By the end of 2003, a total of four ships had been delivered with Sulzer RT-flex engines (Figs. 7 to 10). The other three are: The Aframax tanker Sea Lady in Japan entered service in August 2003 with a 6RT-flex58T-B engine The multi-purpose carrier Wladyslaw Orkan built in China was delivered in November 2003 with a 7RT-flex60C engine The reefer Carmel Ecofresh, also with a 7RT-flex60C, was delivered from Portugal also in November Overall the service experience with these RT-flex engines has been very satisfactory. In demand for newbuildings Sulzer RT-flex engines have been very well received by shipowners. The research engine and the first series-built engine attracted their interest right from the outset. By January 2004, a total of 61 Sulzer RT-flex engines had been built or were on order, aggregating 2.26 million kw (3.08 million bhp), see Table 1. The concept has therefore been extended to other Sulzer low-speed engine types (Fig. 11). The Sulzer RT-flex58T-B and RT-flex60C are now in service. The next type to be built will be the largest bore size, the Sulzer RT-flex96C which will also be developed in a 14-cylinder version to give 80,080 kw (108,920 bhp) for jumbo container liners. The RT-flex84T-D will be specifically for ULCCs and VLCCs, and the RT-flex68T-B will also be included. The smallest RT-flex engine is the new RT-flex50 currently under development which extends the range down to 5650 kw (7700 bhp). With the increasing numbers of RT-flex engines being ordered, their manufacture is also being extended to more 5 Wärtsilä Corporation, January 2004

6 MW RTA48T-B Speed rpm RT-flex50 RTA50 RTA52U-B RT-flex58T-B RTA58T-B RT-flex60C RTA62U-B RT-flex68T-B RTA68T-B RTA72U-B RT-flex84T-D RTA84T-D RTA84C Sulzer RT-flex Fig. 11: Sulzer low-speed marine engine programme with the RT-fl ex engines highlighted. The six Sulzer RT-fl ex engine types cover a power range of 5650 to 80,080 kw ( ,920 bhp). [03#017] RT-flex96C RTA96C factories. So far, they have been, or will be manufactured in six factories in four countries; namely Wärtsilä s own factory at Trieste, Italy, and five licensees in Korea, Japan and China PRC. Benefits for shipowners The benefits of the electronically-controlled common-rail systems of Sulzer RT-flex engines are outlined above in the Introduction. Attention here is focused on the particular aspects of fuel consumption, slow-running behaviour and exhaust emissions. Although Sulzer RT-flex engines can be applied to advantage in any ship types their benefits will carry different weight in different ship types according to their types of operation and their owners priorities. For example, the very slow running ability and reduced part-load fuel consumption are expected to be particularly attractive for very large and ultra large crude oil carriers (VLCC and ULCC), while their smokeless operation might be highlighted more by liner operators. Fuel consumption flexibility A key feature of the electronically-controlled common-rail system is the complete flexibility allowed in the timing, rate and pressure of fuel injection and in exhaust valve operation. This is employed to advantage in the newly introduced feature of fuel consumption flexibility. At the first stage of development of RT-flex engines, the main objective has been to achieve the same performance standards as are achieved in the mechanical-camshaft engines, particularly with respect to power, speed, fuel consumption, exhaust emissions, cylinder pressures, etc. Thus the curve of brake specific fuel consumption (BSFC) of the first RT-flex engines has been the same as with corresponding RTA engines, or perhaps slightly lower in the part-load region. As the fuel injection pressure at part- Table 1: Numbers of Sulzer RT-fl ex engines delivered or on order at the end of December Type Cylinder No. No. Engines RT-flex96C RT-flex96C 31 RT-flex84T-D 7 3 RT-flex84T-D 3 RT-flex60C RT-flex60C 15 RT-flex58T-B RT-flex58T-B 12 Total 61 load is kept higher with the common-rail injection system, combustion is sufficiently better to have a beneficial effect on fuel consumption. The new step is to introduce flexibility into the choice of fuel consumption curve. Two fuel consumption curves are available as standard for RT-flex engines, the new alternative given by Delta Tuning being a lower curve with reduced BSFC below 90 per cent load (Fig. 12). In both cases, the RT-flex engines comply with the NO X regulation of Annex VI of the MARPOL 73/78 convention, often referred to as the IMO NO X regulation. 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). 6 Wärtsilä Corporation, January 2004

7 BSFC 25% 50% 75% Load 100% RTA engines RT-flex engines: Basic tuning Delta Tuning Fig. 12: The new, alternative BSFC curve for RT-fl ex engines given by Delta Tuning compared with the original BSFC curves of Sulzer RTA and RT-fl ex engines. All curves shown are for engines complying with the IMO NOx regulation. [03#117] This has been particularly true for the RTA84T engines which since their introduction in 1991 have also had the advantage of variable exhaust valve closing (VEC) which further reduces the part-load BSFC. These benefits have already been carried over to the electronically-controlled common-rail systems of the RT-flex engines. The question, of course, arises as to why the BSFC could not be lowered at all engine loads/speeds. It is technically possible to do so. With RT-flex engines all the relevant parameters can be continuously varied so that the engine can follow any specified BSFC curve as engine load and speed are varied. The key limitation today, however, is in the need to comply with the IMO NO X regulation. Owing to the natural laws of physics and combustion chemistry, there is always a trade off between lower fuel consumption and greater NO X emissions. At the present state of technology, if the BSFC curve for an engine is lowered then there is an increase in NO X emissions. However, the IMO NO X limit is specified as a weighted average of NO X emissions measured at certain engine loads and speeds. Thus it is possible to accept increases in NO X emissions in some ranges of engine load if there is a compensating reduction in NO X emissions at other engine loads. This explains the shape of the new BSFC curve given by Delta Tuning. The BSFC is lowered in the mid- and low-load range, thereby increasing the NO X emission levels at those load points, but then has to be increased at high engine loads (90 100% load) for a compensating reduction in NO X levels. The overall result is that an Fig. 13: Sulzer RT-fl ex engines have the unique ability to cut off individual fuel injectors, here shown schematically. This feature is used to assure clean combustion for smokeless, stable running at very low speeds. [03#118] RT-flex engine with Delta Tuning also complies with the IMO NO X regulation. The fuel saving is attractive but it would not benefit all ship types to the same magnitude. It is of greater advantage to those ships which spend a high proportion of their time sailing in ballast or at reduced speeds. Very slow running Another key advantage of Sulzer RT-flex engines is their ability to run stably at very low speeds, lower than engines with mechanically-controlled injection. They can run without smoke at per cent nominal speed. This ability will be very useful during transits of the Suez Canal, or during long port approaches and river passages. Very low, stable running speeds are also advantageous for tankers during lightering operations or when loading at offshore oil fields. Such slow running has been well confirmed in service in the Gypsum Centennial. It is made possible by the precise control of injection, together with the higher injection pressures achieved at low speed, and the cutting out of injectors at low speeds. Cutting out injectors is enabled by the separate control of individual fuel injection valves (Fig. 13). This feature is unique to the Sulzer RT-flex system. Usually the injection valves operate in unison but, as the engine speed is reduced one injection valve can be cut out and at a lower speed a second injection valve can be cut out. Thus at minimum speed, the engine runs on all cylinders but with just one injection valve in each cylinder. Fig. 14: [02#123] Usual operation: all nozzles in unison Selective cut-out of injection valves Smokeless operation at low speed Two nozzles Single nozzle Time 7 Wärtsilä Corporation, January 2004

8 Filter Smoke Number [FSN] ON OFF Aux. Blower HFO 380 cst 3% sulphur 0.1% ash Smoke visibility limit Conventional low-speed engine 6RT-flex58T-B with Common Rail Engine Load [%] Fig. 15: Smokeless operation on heavy fuel oil at all speeds was confirmed by measurements on the first Sulzer 6RT-fl ex58t-b engine, shown here in comparison with those typical for Sulzer RTA engines. [02#010] Reducing the number of injection valves in service makes injection of the reduced fuel quantities more efficient, especially as the injection pressure is kept up to a higher value than in a mechanically-injected engine at low speeds. If the RT-flex engine then runs for a period in single-injector operation, the electronic control system switches between the three injection valves in a cylinder so that the thermal load is equalised around the combustion chamber. This arrangement provides more stable operation with better distribution of engine load and thermal loads than if very slow running was to be achieved by cutting out whole cylinders. Low exhaust emissions A clearly visible benefit of RT-flex engines is their smokeless operation at all ship speeds. This was well demonstrated in the Gypsum Centennial (Figs. 15 and 16). The superior combustion performance with the commonrail system is achieved by maintaining the fuel injection pressure at the optimum level right across the engine speed range. In addition, the selective shut-off of single injectors and an optimised exhaust valve timing help to keep smoke emissions below the visible limit at very low speeds. The flexibility of the RT-flex engines will also allow considerably lower NO X emissions. With common-rail injection, a wide variety of injection patterns can be generated (Fig. 17). The injected quantity of fuel can be divided, for pre-injection, triple injection, etc. The Sulzer RT-flex engine, with its individual fuel valve control, also has the unique ability to individually vary the injection timing and sequence between the three fuel injectors in each cylinder and thus to generate a tailor-made heat release. In engine tests, this degree of flexibility has proved useful to reach NO X emissions of 20% below the IMO NO X limit with a moderate BSFC increase of 2.3%. High reliability and redundancy Reliability is crucial for all ship propulsion engines whatever the size or type of ship. Though it is especially true for the engines of, for example, large oil tankers with their pollution potential, and large container ships with their high-value cargoes and tight sailing schedules. Not only has reliability been an important requirement in the development of Sulzer RT-flex engines but the inherent redundancy of the common-rail concept also contributes to ship safety. This is an attribute unique among fuel injection systems to the common-rail concept. The multiple fuel and servo oil supply pumps have Fig. 16: Smokeless operation in practice the Gypsum Centennial [02#041] 8 Wärtsilä Corporation, January 2004

9 Fig. 17: Some of the fuel injection patterns possible with common-rail injection: preinjection, triple injection and sequential injection. Note the sharp beginning and end of the needle lift in all three types of pattern. Sequential injection is unique to RT-fl ex engines as the individual fuel injection valves are separately controlled. [03#123] Pre-injection Triple injection Sequential injection Needle lift Injection pressure Cylinder pressure Rail pressure adequate redundancy for the engine to deliver full power with one fuel pump and one servo oil pump out of action, and a strictly proportional reduction in power should further pumps be out of action. High-pressure fuel and servo-oil delivery pipes, and the electronic systems are also duplicated for redundancy which extends to the separate control of individual fuel injection valves. Future potential The common-rail systems incorporated in Sulzer RT-flex engines give the best potential for the future development of low-speed marine engines. The reasons can be summarised as: Simplest solutions are given by common rail for fuel injection and exhaust valves Free choice for all parameters for fuel injection and exhaust valves Precise volumetric fuel injection control Separate control of individual fuel injection valves unique to RT-fl ex Separation of functions delivery is handled at the supply unit and control is locally at each cylinder unique to RT-fl ex No external pressure storage or damping is needed unique to RT-fl ex Stable pressure level in common rail Separation of heated fuel oil from precision parts ideal for heavy fuel oil In-house design, with standardisation for different bore sizes Pumping power minimised through suction pressure control and rail pressure regulation Minimised pumping volume Redundancy for reliability and safety Built-in monitoring Quick and easy maintenance. To put it simply, our experience with various fuel injection and valve operating systems camshaft-operated, hydraulically-operated and common rail has confirmed that electronic control alone is not sufficient. It needs to be combined with common rail for progress as, for example, in Sulzer RT-flex engines. It is clear, however, that the direction for future engine developments will need to match shipowners requirements. Some of the possible avenues to be investigated might be: Flexibility for further improving fuel consumption and lowering exhaust emissions Extendibility, for example to include direct water injection for reducing NO X emissions Making engines self-adapting, with built-in feedback control for longer times between overhauls, varying fuel quality, varying waste heat recovery demand, varying ambient conditions, complying with local emissions regulations, etc. Not all developments will be restricted to newbuildings or require additional hardware. Sulzer RT-flex engines have the benefit that new operational features can be added by simply updating the system software. Conclusion From all the experience so far, Sulzer RT-flex engines are clearly a major step forward in the technology of large marine diesel engines. The combination of common-rail concepts and fully-integrated electronic control is the only solution giving the degree of flexibility, together with reliability and safety, required to meet the challenges in future marine engine applications in terms of emissions control, optimised fuel consumption, insensitivity to fuel quality, ease of use, operational flexibility, etc. References 1. Stefan Fankhauser and Klaus Heim, The Sulzer RT-flex: Launching the era of common rail on lowspeed engines, CIMAC 2001, Hamburg. 2. Stefan Fankhauser, World s first common-rail lowspeed engine goes to sea, Wärtsilä, Marine News, No , pp Kaspar Aeberli and John McMillan, Common Rail at Sea: The Sulzer RT-flex engine, The Motor Ship Marine Propulsion Conference 2002, Copenhagen. 4. Huber, Konrad and Beat Güttinger, First year of service successful for first Sulzer RT-flex, Wärtsilä, Marine News, No , pp Kaspar Aeberli, Experience with Sulzer Common- Rail Engines, The Motor Ship Marine Propulsion Conference 2003, Hamburg. 9 Wärtsilä Corporation, January 2004