Sulzer low speed engines
Active in both four-stroke and two-stroke design sectors, Sulzer’s links with the diesel engine date back to 1879 when Rudolf Diesel, as a young engineer, followed up his studies by working as an unpaid workshop trainee at Sulzer Brothers in Winterthur, Switzerland. The first Sulzer-built diesel engine was started in June 1898. In 1905 the company built the first directly reversible two-stroke marine diesel engine and, five years later, introduced a valveless two-stroke engine with an after-charging system and spray-cooled pistons. Airless fuel injection was applied to production engines in 1932. and turbocharging from 1954.
Low speed crosshead engine designs from Sulzer after 1956 were of the single-acting two-stroke turbocharged valveless type employing loop scavenging and manifested progressively in the RD, RND, RNDM, RLA and RLB series. Details of the RL-type, many of which are still in service, appear at the end of this chapter. A break with that tradition came at the end of 1981 with the launch of the uniflow-scavenged, constant pressure turbocharged RTA series with a single poppet-type exhaust valve (Figures 1 and 2).
The original RTA series embraced six models with bore sizes of 380 mm, 480 mm, 580 mm, 680 mm, 760 mm and 840 mm, and a stroke–bore ratio of 2.86 (compared with the 2.1 ratio of the RL series). These RTA38, RTA48, RTA58, RTA68, RTA76 and RTA84 models—collectively termed the RTA-8 series—were supplemented in 1984 by the longer stroke RTA-2 series comprising 520 mm and 620 mm bore models and the RTA84M model. The RTA-2 series was extended again in 1986 by a 720 mm bore model. An uprated 840 mm bore design, the RTA84C, was introduced in 1988 to offer higher outputs in the appropriate speed range for propelling large cellular containerships (Figure 3).
A higher stroke–bore ratio (3.47) for the RTA-2 series secured lower engine rotational speeds and hence higher propulsion efficiencies. The ratings of the RTA-2 engines were increased in 1987 in line with the power level already offered by the RTA72 model. Various design improvements, together with higher ratings, were introduced for the
Figure 1 Uniflow scavenging system
RTA-2 series in 1988 at the same time that the RTA84C engine was launched. A further upgrading of the RTA-2 series—to RTA-2U status— came in 1992, with a 9 per cent rise in specific power output.
The RTA range was subsequently extended by the introduction in 1991 of the RTA84T ‘Tanker’ engine design, which, with a stroke-bore ratio of 3.75 and a speed range down to 54 rev/min, was tailored for the propulsion of VLCCs and large bulk carriers. The first production engine entered service in 1994. Even longer strokes (4.17 s/b ratio) were adopted in 1995/96 for RTA-T versions of the 480 mm, 580 mm and 680 mm bore models whose key parameters addressed the power and speed demands of bulk carriers up to Cape-size and tankers up to Suezmax size. These RTA48T, RTA58T and RTA68T designs have more compact dimensions than earlier models, giving ship designers more freedom to create short enginerooms, while reduced component sizes and weights facilitate easier inspection and overhaul. Shipyard-friendly features were also introduced to smooth installation.
In 1994 Sulzer anticipated demand for even higher single-engine outputs from very large and fast containerships by announcing the RTA96C engine, a short stroke (2.6 s/b ratio) 960 mm bore design.
RTA engines of all bore sizes have benefited from continual design refinements to increase power ratings and/or enhance durability and
Figure 2 RTA engine combustion chamber components
reliability, based on service experience. In early 1999 the RTA programme was streamlined by phasing out some older types (such as the 380 mm bore model) which had fallen out of demand, a measure also dictated by the need to ensure that all engines could comply with the IMO Annex VI NOx emissions regulations. In mid 1999 the RTA60C model was introduced as the first Sulzer engine designed from the bedplate up to embody RT-flex electronically-controlled fuel injection and exhaust valve actuation systems. Starting with the 580 mm and 600 mm bore designs, RT-flex series versions became available as options throughout the RTA programme (see section below).
Sulzer came under the umbrella of the Wärtsilä Corporation in 1997, the Finland-based four-stroke specialist committing to continued development of low speed two-stroke engines. The current RTA programme, summarized in Figure 4, embraces nine bore sizes
Figure 3 Test results from 9RTA84C engine equipped with an exhaust gas power recovery turbine, according to a propeller characteristic. The engine develops 34 380 kW mcr at 100 rev/min
from 480 mm to 960 mm with various stroke–bore ratios, speed and rating options covering an output band from 5100 kW to 80 080 kW with five-to-14-cylinder in-line models.
Selecting a suitable main engine model to meet optimally the power demands of a given project dictates attention to the anticipated load range and the influence that the operating conditions are likely to
Figure 4 Sulzer RTA engine programme; larger bore RT-flex models are also available
have throughout the entire life of the ship. Every RTA engine has a layout field within which the power/speed ratio (= rating) can be selected. It is limited by envelopes defining the area where 100 per cent firing pressure (that is, nominal maximum pressure) is available for the selection of the contract maximum continuous rating (CMCR). Contrary to the ‘layout field’, the ‘load range’ is the admissible area of operation once the CMCR has been determined. Various parameters have to be considered in order to define the required CMCR: for example, propulsive power, propeller efficiency, operational flexibility, power and speed margins, possibility of a main engine-driven generator, and the ship’s trading patterns.
The layout field, Figure 5, is the area of power and engine speed within which the CMCR of an engine can be positioned individually to give the desired combination of propulsive power and rotational speed. Engines within this layout field will be tuned for maximum firing pressure and best fuel efficiency. The engine speed is given on the horizontal axis and the engine power on the vertical axis of the layout field; both are expressed as a percentage of the respective engine’s nominal R1 parameters. Percentage values are used so that the same diagram can be applied to all engine models. The scales are
Figure 5 Layout field applicable to all Sulzer RTA models. The contracted maximum continuous rating (Rx) may be freely positioned within the layout field
logarithmic so that exponential curves, such as propeller characteristics (cubic power) and mean effective pressure (mep) curves, are straight lines. The layout field serves to determine the specific fuel oil consumption, exhaust gas flow and temperature, fuel injection parameters, turbocharger and scavenge air cooler specifications for a given engine.
The rating points R1, R2, R3 and R4 are the corner points of the engine layout field. R1 represents the nominal maximum continuous rating (MCR). This is the maximum power/speed combination available for a particular engine. A 10 per cent overload of this figure is permissible for one hour during sea trials in the presence of authorized representatives of the enginebuilder. The point R2 defines 100 per cent speed and 55 per cent power. The point R3 is at 72 per cent power and speed. The line R2–R4 is a line of 55 per cent power between 72 and 100 per cent speed. Points such as Rx are power/speed ratios for the selection of CMCRs required for individual applications. Rating points Rx can be selected within the entire layout field.
Sulzer has smoothed the path for selecting the optimum model for a given propulsion project with its PC computer-based EnSel engine selection program which presents a list of all the models that fulfil the power and speed requirements, along with their main data. The program is offered to ship designers, yards, consultants, owners and licensees. The input data calls for the user to specify the power and speed required, and whether or not an Efficiency-Booster power turbine system is wanted. The output data then lists the engines with the appropriate layout fields and details their MCR power, speed and specific fuel consumption, main dimensions and weight, and other relevant information.
RTA DESIGN FEATURES
The RTA design benefited from principles proven in earlier generations of Sulzer R-type engines. The key elements are:
- A sturdy engine structure designed for low stresses and small deflections comprises a bedplate, columns and cylinder block pretensioned by vertical tie rods. The single-wall bedplate has an integrated thrust block and incorporates standardized large surface main bearing shells. The robust A-shaped columns are assembled with stiffening plates or are of monobloc design. The single cast iron cylinder jackets are bolted together to form a rigid cylinder block (multi-cylinder jacket units for smaller bore engines).
- Lamellar cast iron, bore-cooled cylinder liners with back-pressure timed, load-dependent cylinder lubrication.
- Solid, forged bore-cooled cylinder covers with one large central exhaust valve arranged in a bolted-on valve cage; the valve is made from a heat- and corrosion-resistant material and its seat ring is bore-cooled.
- Semi-built crankshaft divided into two parts for larger bore engines with a large number of cylinders.
- Running gear comprising connecting rod, crosshead pin with very large surface crosshead bearing shells (with high pressure lubrication) and double-guided slippers, piston rod and borecooled piston crown using oil cooling. All have short piston skirts.
All combustion chamber components are bore cooled, a traditional feature of Sulzer engines fostering optimum surface temperatures and preventing high temperature corrosion due to high temperatures on one side and sulphuric acid corrosion due to too low temperatures on the other. At the same time, rigidity and mechanical strength are provided by the cooler material behind the cooling bores (Figure 6.).
Comfortable working conditions for the exhaust valve are promoted by: hydraulic operation with controlled valve landing speed; air spring; full rotational symmetry of the valve seat, yielding well-balanced thermal and mechanical stresses and deformations of valve and valve seat, as well as uniform seating; extremely low and even temperatures in valve seat areas due to efficient bore cooling; valve rotation by simple vane impeller; valve actuation free from lateral forces, with axial symmetry; and simple guide bushes sealed by pressurized air.
The low exhaust valve seating face temperature reportedly secures an ample safety margin to avoid corrosive attack from vanadium/ sodium compounds under all conditions. Efficient valve cooling is given by intimate contact with the bore-cooled seat, together with the appropriate excess air ratio in the cylinder. The specific design features of the valve assembly are also said to deter the build-up of seat deposits, seat distortion, misalignment and other factors which may accelerate seat damage.
- Camshaft gear drive housed in a special double column or integrated into a monobloc column, placed at the driving end or in the centre of the engine for larger bore models with a large number of cylinders.
- Balancer gear can be mounted on larger bore engines, when required, to counter second-order couples for four-, five- and sixcylinder models, and combined first- and second-order couples for four-cylinder models.
- A compact integral axial detuner can be incorporated, if required, in the free end of the engine bedplate.
- The fuel injection pump and exhaust valve actuator are combined in common units for each two cylinders. The camshaft-driven injection pump with double valve-controlled variable injection timing delivers fuel to multiple uncooled injectors. The camshaftdriven actuators impart hydraulic drive to the single central exhaust valve working against an air spring.
- Constant pressure turbocharging is based on high efficiency uncooled turbochargers; auxiliary blowers support uniflow scavenging during low load operation. In-service cleaning of the charge air coolers is possible. A standard optional three-stage charge air cooler unit can be specified for heat recovery.
Figure 6 The combustion chamber of the RTA series engines is fully bore-cooled. Cooling oil spray nozzles on top of the piston rod direct oil into the bores of the piston crown
- A standard pneumatic engine control system is based on a remote manoeuvring stand in the control room and an emergency manoeuvring stand on the engine itself. The optional SBC-7.1 electro-pneumatic bridge control system is matched to the engine and arranged for engine control and manoeuvring from the wheelhouse or bridge wings.
- Standard optional power take-off drives can be arranged either at the side or at the free end for connecting an alternator to the main engine.
- Sulzer’s Efficiency-Booster System can be specified for the RTA84C, RTA84T and RTA84M engines, as well as for the RTA72U, RTA62U and RTA52U models, to exploit surplus exhaust gas energy in a power recovery turbine.
- RTA engines can satisfy the speed-dependent IMO limits on NOx emissions from exhaust gases. Tuning is facilitated by the electronic variable fuel injection timing (VIT) system.
RTA DESIGN DEVELOPMENTS
The basic RTA engine design has been refined over the years and the range expanded to address changing market requirements. The improvements have yielded wider power/speed fields, increased power outputs, reduced fuel consumption, lower wear rates and longer times between overhauls. Advances in key performance parameters are illustrated in Figure 7. Operating economy has also benefited from the reduced auxiliary power demand of current RTA engines—imposed by electric pumps for cooling water, lubricating oil and fuel supply— which is some 40 per cent less than that of the RL engines.
Sulzer’s introduction of upgraded RTA-2 engine designs called for some modifications to match the higher power outputs and maximum combustion pressures involved (summarized in Figure 8). The shrinkfit of the crankshaft was strengthened to suit the increased torque values; the main bearing was adapted to accommodate the higher loads and ensure optimum oil flow; and the cylinder cover material was changed to take advantage of better fatigue strength at higher loads. Some of the design ideas tested on the 4RTX54 research engine and already applied to the RTA84T engine were also adopted for the RTA-2U series.
Figure 7 Advances in key parameters of RTA series engines since their introduction. (The RTX54 is a Sulzer research engine)
Three fuel injection valves per cylinder were specified for the RTA62U and RTA72U models (though not, because of its smaller bore size, for the RTA52U model which retained two valves per cylinder). The reported benefits of the triple-valve configuration are a more uniform temperature distribution around the principal combustion space components (cylinder cover, liner and piston crown) at the increased maximum combustion pressures, along with even lower temperatures despite the higher loads. Three fuel valves also foster significantly lower exhaust valve and valve seat temperatures.
Figure 8 Main improvements introduced in RTA-2U series engines
Other spin-offs from the research engine included a modified cylinder liner bore-cooling geometry whose tangential outlets of the bores aim for optimum distribution of wall temperatures and thermal strains at higher specific loads. The geometry of the oil cooling arrangements of the piston crown was also modified to maintain an optimum temperature distribution. The good piston running behaviour was maintained by retaining established features of the RTA design: multilevel cylinder lubrication; die-casting technology for cylinder liners; and temperature-optimized cylinder liners. Advances in materials technology in terms of wear resistance have permitted engines to run at higher liner surface temperatures. This, in turn, allows a safe margin to be maintained above the increased dew point temperature and thus avoiding corrosive wear.
Some refinements were introduced, however, to match the new Cylinder cover new running conditions. Four piston rings are specified for the RTA-2U engines instead of the five previously used. The top ring is now thicker than the other rings and has a pre-profiled running face, as on the RTA84C containership engine. The top ring is also plasma coated. The plasma-coated, pre-profiled thicker rings have demonstrated excellent wear results. The radial wear rates were measured at less than 0.04 mm/1000 hours in trials up to and exceeding 13 000 hours of operation.
An important contribution to low wear rates of liners and pistons results from improving the separation and draining of water borne in the cooled scavenge air before it enters the cylinders, particularly at higher engine loads. The RTA-2U engines were specified with a more efficient condensate water separator in the scavenge air flow after the cooler, along with a more effective drain.
A number of design modifications to the RTA series were introduced for the ultra-long stroke RTA48T, RTA58T and RTA68T engines to achieve more compact, lower weight models offering reduced production, installation and maintenance costs (Figures 9 and 10). A time between overhaul for the main components of 15 000 hours was sought. Cutting the manufacturing cost, despite the greater stroke/ bore ratio (4.17) than previous RTA series engines, was addressed by a number of measures: reducing the size and weight of components; simplifying the designs of components and sub-assemblies and making them easier to produce; reducing the number of parts; and designing to save assembly time.
An example of the design changes made to reduce the sizes and weights of components is provided by the cylinder cover, along with its exhaust valve, housing and valve actuator. An overall weight saving of 30 per cent was achieved on each new cover, largely due to the smaller dimensions. Reducing the distance between cylinder centres by around 9 per cent allowed components such as the bedplate, monobloc columns and cylinder block to be reduced in size, resulting in weight savings of 13–14 per cent (Figures 12.11, 12.12 and 12.13). The cylinder block is lower in overall height and thus lighter than in equivalent RTA-2U engines. The freedom for ship designers to create short enginerooms is enhanced by a degree of flexibility in the fore and aft location of the turbocharger and scavenge air cooler module.
The hydraulic jack bolts on the main bearings were eliminated and replaced by simple holding-down bolts that fix the bearing cap directly to the bedplate (Figure 14). In addition, it was possible to simplify
Figure 9 Cross-section of RTA58T engine. Note the high camshaft level allowing the use of shorter high pressure fuel injection pipes for better injection control
the column structure in the region where the jack bolts had needed support and thereby omit machining operations. The scavenge air receiver was simplified by using a ‘half-pipe’ design which is welded to the module incorporating the turbocharger and scavenge air cooler.
Figure 10 Sulzer 4RTA58 engine on test
The result was an easier manufacturing process and significant weight savings compared with equivalent RTA-2U series engines. The singlepiece column structure can be manufactured without the need for machining inclined surfaces: all machined surfaces on the columns are either vertical or horizontal by design. Eliminating the jack bolts on the main bearings also allowed the design of the columns to be made more tolerant to welding quality
Figure 11 Bedplate of RTA58T engine
Figure 12 The monobloc columns of the RTA48T and RTA58T were designed for simpler welding and machining
The exhaust valve actuator was redesigned for the RTA-T engines. The bush of the actuating piston was eliminated and its function integrated directly into the housing, thus reducing the number of components to be produced and the assembly time. Smaller dimensions and a lighter weight (by 27 per cent) were also achieved. A substantial cost saving can be realized if an electrically driven balancer at the forward end of the engine is specified, allowing the previously necessary coupling between balancer and camshaft to be omitted.
Valve-controlled fuel injection pumps are located at the height of the cylinder blocks. The fuel pump blocks can be directly bolted to the cylinder blocks without using any shims. The intermediate gear wheels of the camshaft drive can be aligned more quickly than before
Figure 13 The monobloc dry cylinder blocks of the RTA48T and RTA58T were designed for reduced machining, compactness and lower weight
Figure 14 The main bearings of the RTA48T and RTA58T use holding-down studs instead of the previous hydraulic jack bolts
by means of a built-in device to move the wheels vertically and horizontally into position.
Engines can be built up from modules, starting with the bedplate and crankshaft, and working up with the columns and cylinder blocks. The modules can be pre-assembled at convenient, separate locations in the workshop. All pipe connections are designed to support this type of modular assembly and achieve the shortest possible assembly time in the works or, if necessary, in the ship itself. Piston-running technology was based on experience gained and refined from the RTA-2U and RTA84C engines. The cylinder liner is of cast iron with sufficient hard-phase content and a smooth machined running surface. Bore cooling and three fuel injection valves underwrite favourable temperature levels and distributions. The top piston ring is pre-profiled for easy running-in and plasma coated for low wear rates over extended periods. The multi-level cylinder lubrication system fosters optimum distribution of the lubricating oil and adapts to the longer stroke; and condensate water is taken out of the scavenge air efficiently to avoid contamination of the oil film.
A new type added to the programme in mid-1999, the RTA60C engine was designed to serve faster cargo tonnage such as medium-sized containerships, car carriers, RoRo ships and reefer vessels (Figure 15). A desire for compactness and economical production costs moulded the design, resulting in an engine with a shorter length and lighter weight (5–10 per cent lower) for a given power than others in its class.
A higher rotational speed to suit the intended ship types allowed a reduced piston stroke which, together with the short connecting rod, fostered a significant reduction in engine height: the engine is 8.52 m tall above the shaft centreline and requires a hook height of only 10.4 m for withdrawing pistons (or less by using special tools). The length was minimized by applying measures from the RTA48T and 58T engines, a cylinder distance of 1040 mm being achieved. The sixcylinder RTA60C model has an overall length of 7.62 m, including the flywheel, and weighs 330 tonnes.
The need to keep engine length to a minimum called for particularly good bearing design; thin-walled whitemetal shells are used for crosshead, main and bottom end bearings. The main bearing housing was specifically designed to accommodate and support the thin-walled bearing shell, with four elastic holding-down bolts for each main bearing cap. Two pairs of studs are said to give the most even distribution of
Figure 15 Cross-section of Sulzer RTA60C engine, the first RTA model designed from the start to accept RT-flex systems (camshaft version illustrated)
holding-down load, and also allow the tie rods to be located close to the bearing for efficient transfer of firing pressure loads.
Cylinder covers are secured by eight elastic holding-down studs arranged in four pairs, promoting compactness and contributing to shorter engine length, and also simplifying manufacture. A support ring between the cylinder block and the collar of the cylinder liner carries both liner and cylinder cover; it also passes cooling water to the cooling bores and to the cover. A simple water jacket provides the necessary cooling for the portion of the liner length immediately below the bore-cooled top flange of the liner.
The cylinder jacket is a single-piece iron casting, its height determined by the space required for the scavenge air receiver. Access to the piston underside is possible from the receiver side of the engine to allow maintenance of the piston rod gland and piston ring inspection. On the fuel side, one door per cylinder can be opened for inspection and to support in-engine work from outside. The tilting-pad thrust bearing is integrated in the bedplate, the pads arranged to ensure a safe and uniform load distribution. The thrust bearing girder consists of only two steel cast pieces, omitting welding seams in critical corners; the girder is clearly stiffer than in previous designs.
The piston comprises a forged steel crown with a very short skirt; the four piston rings are all 16 mm thick and of the same geometry. Piston running behaviour benefits from an anti-polishing ring incorporated at the top of the liner, preventing deposits on the piston top land from damaging the liner running surface and its lubrication film. Keeping the top land clean also ensures a good spread of lubricant over the liner surface while using a lower cylinder oil feed rate. (More details of the anti-polishing ring are provided in the TriboPack section below.)
An exhaust system featuring tangential gas inlet and outlet in the manifold allows a smooth flow of gases from the exhaust valve to the turbocharger inlet with an energy-saving swirl along the manifold. The combined turbocharger and scavenge air cooler module is designed to accommodate one or two turbochargers, and allows for different sizes of coolers. The flexibility to locate the turbocharger at the aft (driving end) of the engine is particularly suited to modern all-aft ship designs.
The key operating parameters of the RTA60C (see table) are only slightly higher those of the –B versions of the RTA48T, 58T and 68T models, from whose service experience the new engine benefited.
RTA60C engine data
The RTA60C engine was designed from the outset to smooth its acceptance of Sulzer RT-flex fully electronically-controlled fuel injection and exhaust valve systems, which eliminate the camshaft and individual fuel pumps (see below). The debut orders for RTA60C engines—in December 2000—specified RT-flex versions.
RTA50C and RT-flex 50C engines
At end-2002 Wärtsilä Corporation announced a joint venture with Mitsubishi to design and develop a new small bore low speed engine. The Japanese group, which designs and builds its own UEC two-stroke engines , has been a prolific licensed builder of Sulzer engines since 1925.
The new 500 mm bore/2050 mm stroke design was planned for release in two versions: the conventional ‘mechanical’ Sulzer RTA50C and the electronically-controlled Sulzer RT-flex50C. Each type will offer a maximum continuous rating of 1620 kW/cylinder at 124 rev/ min, five-to-eight-cylinder models covering a power range from 5650 kW to 12 960 kW at 99 rev/min to 124 rev/min. Output and speed ratings target Handymax and Panamax bulk carriers, large product tankers, feeder containerships and medium-sized reefer vessels. Both versions will incorporate Sulzer TriboPack measures for enhancing piston-running behaviour, underwriting low cylinder wear rates for three years between overhauls, and minimizing cylinder lube oil consumption (see section below).
High efficiency, compactness and environmental friendliness were set as design goals for the project to be pursued by a joint working group of engineers from both groups. The new engine was assigned for production in Japan by Mitsubishi and its licensees and by other Japanese licensees of Wärtsilä. Separate branding—Sulzer or Mitsubishi—will be applied to the engines, depending on the builder. The first examples were due for completion by end-2004.
RTA50C/RT-flex50C engine data
RTA84T engine refinements
The piston of the RTA84T engine is similar to that of the earlier RTA84M engine but simplified in design. The skirt is bolted directly to the crown, and the increased rigidity of the piston rod can accept higher loads. The crown is oil cooled by the ‘jet/shaker’ cooling principle, with the oil spray nozzles matched to secure optimum piston temperatures. The piston rod gland design (Figure 16) features bronze rings and a modified scraper/gas sealing package on top compared with previous RTA-series engines. This gland principle confirmed the expected appropriate oil leakage rates in the neutral space. The well-proven RTA exhaust valve design was adopted for the RTA84T, embracing an efficiently bore-cooled valve seat, Nimonic valve, hydraulic actuation and a rotation device.
A deeper cylinder cover, with its lower joint between cover and liner, reduces the portion of the liner exposed to gas pressure and high temperature. The bore cooling principle was adapted to control the temperatures and mechanical and thermal stresses in the components adjacent to the combustion space. The cylinder cover is bolted down with eight studs. Compact uncooled fuel injection valves make it possible to place three nozzles symmetrically in the cover despite the very short cylinder pitch. The nozzle tips are sufficiently long for the cap nut to be shielded by the cylinder cover and hence not exposed to the combustion space, Figure 17.
The camshaft was raised to be close to the cylinder covers and is driven via gear wheels. The hydraulic reversing mechanism of the RTA series was retained unchanged. The decision to move the camshaft up towards the engine top was taken after evaluating such factors as engine cost, fuel injection system performance, hydraulic valve actuation and torsional vibration. In particular, with an elongated stroke/bore
Figure 16 Piston rod gland of RTA84T engine; all rings are bronze
ratio of 3.75, the advantage of shorter hydraulic connections for injection and valve actuating pipes was significant in avoiding high pressure losses. The cost of an additional gearwheel (Figure 18) is compensated by the slimmer camshaft, the fuel pump arrangement and the shorter high pressure injection pipes. Higher injection pressures, and hence more efficient and cleaner combustion, are thus facilitated.
The camshaft-driven fuel pump is of the valve-controlled design used in the longer stroke RTA-2 engines and located at a high level, bolted directly to the side of the cylinder block (Figure 19). The double valve-controlled pump principle, traditional in Sulzer low speed engines, offers high flexibility and stability over time in controlling the injection timing. Timing is controlled by separate suction and spill valves regulated through eccentrics on hydraulically-actuated lay shafts. The pump housing incorporates the compact RTA-series arrangement, with fuel injection pump and exhaust valve actuator modules provided to serve one pair of cylinders.
Figure 17 RTA84T fuel injection valve. The nozzle is not exposed to the combustion space, thereby avoiding material burning off
Figure 18 Camshaft drive gear train for RTA84T engine
In comparison with a helix type, the valve-controlled fuel injection pump has a plunger with a significantly greater sealing length. The higher volumetric efficiency reduces the torque in the camshaft. Additionally, says Sulzer, injection from a valve-controlled pump is far more stable at very low loads and rotational shaft speeds down to 15 per cent of the rated speed are achieved. Valve control also offers benefits in reduced deterioration of timing over the years owing to less wear and to freedom from cavitation.
A combination of variable exhaust valve closing (VEC) and variable injection timing (VIT) devices provides an improved degree of setting flexibility. In the upper load range, the specific fuel consumption is optimized through the electronically controlled VIT system, maintaining
Figure 19 RTA84T fuel injection pump with double control valves
the maximum cylinder pressure by injection timing advance. The fuel quality setting (FQS) function is incorporated in the VIT system. Engine efficiency is additionally improved in the lower load range (between 80 and 65 per cent load) via the VEC system (Figure 20). The compression ratio is thereby effectively increased by early closing of the exhaust valve.
The working process of the engine can thus be ‘shaped’ for optimum performance over the whole load range, facilitating its adaptation to a particular ship sailing mode. The resulting benefits promised are
Figure 20 RTA84T engine exhaust valve actuating arrangement with variable closing (VEC) system
the lowest possible fuel consumption in the most common load range, and higher exhaust gas temperatures within that load range compared with other solutions, such as variable turbocharger geometry. (Higher exhaust gas temperatures are valued for waste heat recovery.) The VEC, VIT and FQS devices are electronically regulated from the engine control system. The influence of the VIT/VEC combination on the RTA84T engine’s performance characteristics are shown in Figure 21.
A load-dependent cylinder liner cooling system (Figure 22) helps to avoid cold corrosion of the liner surface over the whole load range and hence to reduce wear rates further. The liner temperature is maintained above the prevailing dew point throughout the load range: the liner is efficiently cooled at full load, while over-cooling at part load is avoided. This is achieved by splitting the cooling water supply to the engine into two lines. Only a restricted amount of water is led through the cylinder liner jacket at part load while the main water flow is led directly to the cylinder cover. The water flow is electronically controlled as a function of the engine load.
Figure 21 Influence of the VIT/VEC combination (solid line) compared with results without VIT/VEC (dashed line) on the performance characteristics of a 7RTA84T engine. The engine has an RI rating of 27 160 kW at 74 rev/min and exploits an exhaust gas power turbine
Cylinder liner lubrication is effected by Sulzer’s multi-level accumulator system. This allows the cylinder lubricating oil to be distributed in very small amounts at each engine stroke, thereby creating an optimum oil film distribution. The main improvement for the RTA84T was the introduction of an electronically controlled flexible oil dosage promoting low wear rates and low lubricating oil
Figure 22 Schematic layout of the RTA84T cylinder jacket cooling water system with a load-dependent controlled bypass past the jacket to the cylinder cover
consumption. The oil feed rate is controlled according to engine load and is adjustable as a function of engine condition. A simplified dualline distributor eliminates the mechanical drive shaft.
Piston-running experience with RTA84T engines in service demonstrated that engines with very long strokes need higher liner wall temperatures than had previously been required by Sulzer’s RTA- 2 series (with stroke/bore ratios of around 3.5) to overcome corrosive wear problems when operating on high sulphur fuels. This experience was applied to benefit later RTA84T production models as well as the design of the smaller RTA-T engines. A number of improvements introduced for the latter series have also been applied to the RTA84T, the refinements including design simplifications to make the engine more manufacturing friendly. A significant saving in weight also resulted, the seven-cylinder model being trimmed by some 6.5 per cent to 960 tonnes.
RTA84T-B and -D versions
Following the introduction of the RTA48T and RTA58T models in 1995, the same design concepts were applied to the RTA84T ‘Tanker’ engine—originally introduced in May 1991—resulting in a Version B in 1996. Easier manufacturing and enhanced service behaviour were realised, with no change in power output. In July 1998 a lower specific fuel consumption (down by 2 g/kWh) was gained by applying ‘low port’ cylinder liners (scavenging air inlet ports with a reduced height) in combination with higher efficiency turbochargers. There is no penalty in either higher component temperatures or too low exhaust gas temperatures, and the low ports give a longer effective expansion stroke in the cycle (Figure 23).
The RTA84T-B engine was uprated at end-1998 from 3800 kW/ cylinder at 74 rev/min to 4100 kW/cylinder at 76 rev/min to create the Version D through tuning and turbocharging matching measures. The power available from a seven-cylinder engine was thereby increased from 27 160 kW to 28 700 kW, better addressing the propulsive demands of VLCCs with higher service speeds. Nine-cylinder RTA84T-D models, each developing 36 900 kW, were specified to power a series of 442 500 dwt tankers with a service speed of 16.5 knots.
Structural, running gear, combustion chamber, fuel injection, turbocharging and scavenge air system design details are similar to those of the RTA84C and RTA96C engines described below. The large stroke-to-bore ratio of the RTA84T, however, allows a relatively deeper combustion chamber with more freedom in the layout of the fuel spray pattern. The semi-built crankshaft has to cater for the longest stroke ever applied in a Sulzer engine; to limit the crankshaft weight for production, assembly and transport, the main journals and crankpins are bored. The design of the crank has a good transverse width at the upper part of the web, allowing the latter to be slim longitudinally.
Figure 23 Surface temperatures measured on the combustion chamber components of the RTA84T-B engine at full load R1 rating. The thickness of the lines represents the circumferential variation in temperature
The favourable torsional vibration characteristics allow six-cylinder engines to use a viscous damper in many cases instead of a Geislinger damper.
RTA84T-B and -D engine data
A further development of the RTA-T programme resulted in the 680 mm bore RTA68T-B engine, the first example of which entered service in November 2000. The design benefited from all the reliability improvements incorporated in the earlier RTA-T engines, with parameters similar to those of the smaller bore RTA48T-B and 58T-B models. TriboPack design measures (see below) contributed to pistonrunning behaviour, and a new design of piston rod gland featured oil scraper rings with grey cast iron lips: the system oil is fully recirculated and a dry neutral space effected.
RTA68T-B engine data
RTA84C and RTA96C engines
Introduced in September 1988 for propelling the coming generation of larger and faster containerships, the RTA84C was developed from the RTA84, already established in that market. In 1993 its power output was increased by six per cent, the cylinder cover modified and the number of fuel nozzles increased from two to three; these measures contributed to a reduction in the thermal load of the combustion chamber. The current performance of the series, shown in the table, equates to a maximum output of 48 600 kW at 102 rev/min from the 12-cylinder model.
RTA84C engine data
Increasing sizes of post-Panamax containerships dictated more powerful engines and stimulated the introduction of the RTA96C series in December 1994. The 960 mm bore design was fully based on the RTA84C (Figures 24 and 25), which it supplemented in the programme. The selection of a stroke (2500 mm) some 100 mm longer than that of the RTA84C engine for the RTA96C design (Figure 26) was influenced by demands for the highest reliability. By adopting a longer stroke, the absolute depth of the combustion chamber could be proportionally increased to give more room for securing the best combustion and fuel injection parameters, and better control of temperatures in the combustion chamber components. Additionally, the slightly longer stroke fosters a simplified crankshaft design with enhanced reliability, since the shrunk-in main journals do not cut the journal fillets at the inner sides of the crankwebs.
Figure 24 A 12-cylinder Sulzer RTA84C engine on test
Figure 25 Crosshead and connecting rod assembly for RTA84C engine
A time-between-overhaul (TBO) of three years from the RTA96C engine’s key components was sought, a goal underwritten by: a cast iron (preferably die cast) cylinder liner with the necessary amount of wear-resistant hard-phase particles and a smoothly machined and honed surface for quick and trouble-free running-in; bore cooling of all the main combustion chamber components; three fuel injection valves symmetrically distributed in the cylinder cover contribute to evenness of temperature distribution; cylinder oil lubrication of the liner surface via two levels of quills to achieve effective and economical distribution; and a top piston ring pre-profiled and plasma coated to secure the lowest wear rate to reach the three-year TBO goal with sufficient margin. Diametral cylinder liner wear rates of around 0.03 mm/1000 hours
Figure 26 Cross-section of RTA96C engine, designed for propelling ultra-large containerships
are reported for RTA96C design, in service. All new engines further benefit from TriboPack measures to enhance piston-running behavior (see below).
The semi-built crankshaft comprises combined crankpin/web elements forged from a solid ingot, with the journal pins then shrunk into the crankweb. The main bearings have whitemetal shells, and the main bearing caps are held down by a pair of jack bolts in the RTA84C and by a pair of elastic holding-down studs in the RTA96C. The crosshead bearing is designed to the same principles as for all other RTA engines; it also features a full-width lower half bearing. The crosshead bearings have thin-walled shells of whitemetal to yield a high load-bearing capacity. Sulzer low speed engines retain a separate elevated-pressure lube oil supply to the crosshead; this provides hydrostatic lubrication which lifts the crosshead pin off the shell during every revolution to ensure that sufficient oil film thickness is maintained under the gas load: crucial to long term bearing security.
The combustion chamber was recognized as the most important design area because of its key influence on engine reliability and the high power concentration. Component design was based on established practice and benefited from work carried out for the medium bore RTA48T and RTA58T engines. Bore cooling technology, Wärtsilä suggests, provides an escape from the rule that larger components (resulting from a larger bore) when subjected to thermal loading will also have higher thermal strains. With bore cooling, the thermal strains in the cylinder cover, liner and piston crown of the RTA96C can be kept fully within the values of earlier generations of RTA engines, as can the mechanical stresses in those components.
The solid forged steel, bore-cooled cylinder cover is secured by eight elastic studs, and the central exhaust valve of Nimonic 80A material is housed in a bolted-on valve cage. Anti-corrosion cladding is applied to the cylinder covers downstream of the injection nozzles to protect the covers from hot corrosive or erosive attack. The pistons feature a forged steel crown with a short skirt; the crown is cooled by combined jet-shaker oil cooling achieving moderate temperatures on the crown and a fairly even temperature distribution across the crown surface. No coatings are necessary.
A high structural rigidity with low stresses and high stiffness is important for low speed engines. The RTA84C and RTA96C designs exploit a well proven structure with a ‘gondola’ type bedplate surmounted by very rigid A-shaped double-walled columns and cylinder blocks, all secured by pre-tensioned vertical tie rods. Both bedplate and columns are welded fabrications designed for minimum machining. The cylinder jacket is assembled from individual cast iron cylinder blocks bolted together to form a rigid whole. The fuel pumps are carried on supports on one side of the column and the scavenge air receiver on the other side of the cylinder jacket.
Access to the piston under-side is normally from the fuel pump side but is also possible from the receiver side of the engine, to facilitate maintenance of the piston rod gland and also for inspecting piston rings. The tilting-pad thrust bearing is integrated in the bedplate. The use of gear wheels for the camshaft drive allows the thrust bearing to be very short and stiff, and to be carried in a closed rigid housing.
The three uncooled fuel injection valves in each cylinder cover have nozzle tips sufficiently long for the cap nut to be shielded by the cylinder cover and not exposed to the combustion space. The camshaftdriven fuel injection pumps are of the double-valve controlled type, traditional in Sulzer low speed engines. Injection timing is controlled by separate suction and spill valves regulated through eccentrics on hydraulically-actuated lay shafts. Flexibility in timing is possible through the variable fuel injection timing (VIT) system for improved part-load consumption, while the fuel quality setting (FQS) lever can adjust timing according to the fuel oil quality. The valve-controlled fuel injection pump, in comparison with a helix type, has a plunger with a significantly greater sealing length. The higher volumetric efficiency reduces the torque in the camshaft; additionally, injection from a valve-controlled pump is far more stable at very low loads, and rotational shaft speeds down to 15 per cent of the rated speed are achieved. Valve control also has the benefits of less deterioration of timing over the years owing to reduced wear and freedom from cavitation.
The camshaft is assembled from a number of segments, one for each fuel pump housing. The segments are connected through SKF sleeve couplings, each segment having an integral hydraulic reversing servomotor located within the pump housing. The camshaft drive is a traditional Sulzer arrangement, effected in this case by three gearwheels housed in a double column located at the driving end or in the centre of the engine. The main gearwheel on the crankshaft is in one piece and flange mounted.
Scavenge air is delivered by a constant pressure turbocharging system based on one or more turbochargers, depending on the number of engine cylinders. For starting and during slow running, scavenge air delivery is augmented by electrically-driven auxiliary blowers. The scavenge air receiver incorporates non-return flaps, an air cooler and the auxiliary blowers; the turbochargers are mounted on the receiver, which also carries the fixed foot for the exhaust manifold. Immediately after the cooler, the scavenge air passes through a water separator comprising a row of vanes that divert the air flow and collect the water. Ample drainage is provided to completely remove the condensed water collected at the bottom of the air cooler and separator. Effective separation of condensed water from the stream of scavenge air is thus accomplished, a necessity for satisfactory piston-running behaviour.
RTA engines have simple seating arrangements with a modest number of holding-down bolts and side stoppers (14 side stoppers are needed for a 12-cylinder RTA96C engine). No end stoppers or thrust brackets are needed as thrust transmission is provided by fitted bolts or thrust sleeves applied to a number of the holding-down bolts (Figure 27). The holes in the tanktop for the thrust sleeves can be made by drilling or even flame cutting. After alignment of the bedplate, epoxy resin chocking material is poured around the sleeves. The engine is equipped with an integrated axial detuner at the free end of the crankshaft, and a detuner monitoring system developed by Wärtsilä is standard equipment.
Figure 27 Arrangements for transmitting propeller thrust to the RTA84T, RTA84C and RTA96C engine seatings; the inset shows the thrust sleeve for the thrust bolts
A standard all-electric interface is employed for engine management systems—the DENIS (Diesel Engine Interface Specification)—to meet all needs for control, monitoring, safety and alarm warning functions. It matches remote control systems and ship control systems from a number of approved suppliers.
RTA96C uprating and programme expansion
The first RTA96C engine, an 11-cylinder model, started tests in March 1997 and was followed into service by eight-, nine- and 10-cylinder models and numerous 12-cylinder versions. The design was originally offered with an output of 5490 kW/cylinder and up to 12 cylinders but the rating was increased by four per cent in 2000 to 5720 kW/ cylinder at 102 rev/min (see table). At the same time, the programme was extended to embrace an in-line 14-cylinder model delivering 80 080 kW (the first time such a low speed engine configuration had featured in a production programme) and suitable for powering singlescrew post-Panamax containerships with capacities up to 10 000 TEU and service speeds up to 25 knots. (Figure 28).
A 14RTA96C engine would measure 27.31 m long overall to the flywheel flange ¥ 10.93 m high over the shaft centreline (or 13.54 m overall) and weigh 2300 tonnes dry (compared with 2050 tonnes for a 12RTA96C). The individual elements, such as the two crankshaft sections, are close to the maximum lifting capacities of contemporary enginebuilders which impose a limit on engines with higher than 14 cylinders. The camshaft for engines with more than 14 cylinders would need to be split into three parts, and at least two camshaft drives would become necessary. Such engines would therefore have to adopt the Sulzer RT-flex common rail systems for fuel injection and exhaust valve actuation, which eliminate the camshaft (see below).
With a length less than 15 per cent longer than the established 12- cylinder engine, the longitudinal and torsional rigidity of the 14RTA96C model would be adequate for the expected ship structures. The torsional vibration characteristics are also considered acceptable for the envisaged firing order. The crankshaft material and the shrink fit of the journals in the webs, however, would be redefined to suit the increased torque transmitted to the propeller shaft.
RTA96C engine uprating data
RTA96C service experience
Some cases of main bearing damage were restricted to a specific series of containerships, and involved the local breaking-out of whitemetal in the lower shells and some local fretting. The fretting problem was
Figure 28 Side elevation of a 14-cylinder RTA96C engine
solved by applying a so-called key-slot solution with the aim of increasing the radial forces pressing the bearing shell into the girder, and by some other small design modifications. With the exception of one particular main bearing, no connection could be found with the bearing load and shaft orbit that could explain the breaking-out problem. As a countermeasure, the lower shell of the main bearing next to the thrust bearing is now machined together with its respective bearing cap to ensure better bearing geometry. The more precise alignment of the bearing cap and shell, along with a key between the cap and shell to secure tight fitting of the shell in the bearing girder when tightening down, gradually yielded results.
Some reports of cracks in A-frame columns were first received in mid-1999 after some 5000 to 12 000 running hours. A project team was established to investigate the matter and all other engines in service were inspected. Although the cracks occurred in seven of the eight engines, they were in only some of the double-walled A-frame columns manufactured to the original design, and were of various degrees of severity. The cracks were initiated at the inboard ends of two pairs of horizontal ribs on the columns and propagated vertically and in the direction of the crosshead guide rails. The root of the problem was a combination of lack of welding quality and somewhat higher than anticipated local stress level.
All cracked A-frame columns were repaired by welding. At the same time, the inboard ends of all horizontal ribs in the engines concerned were cut back to limit the stress concentration and thereby eliminate the possibility of cracking. Subsequent measurements showed that the modified shape of the ribs reduces the stress concentrations in the columns down to around half, and thus is not as sensitive to imperfect welding. The same shape of horizontal rib is now applied on all RTA96C engines, whether repaired or built as new; the shape was adapted from the modified rib design introduced in December 1997 but slightly revised for ease of manufacture.
Some leaks were experienced in the cylinder pressure relief valve located in the cylinder cover. These were caused by corrosion which arose because the shortened valve was too cold through being too far from the combustion space. An improved valve design replaced those in existing engines, while later engines were fitted with valves of a traditional design which operate at higher temperatures and hence avoid the original problem.
A number of scavenge air receivers suffered cracks in the form of rupturing of the upper and lower longitudinal welded seams; these were found to have been caused by resonant vibration in the dividing wall. As a remedy, additional stiffening was applied to the dividing walls of the affected receivers (and adopted as standard for new engines) to change the resonant frequency and thereby reduce vibration amplitudes.
A stiffer turbocharger support was developed for new engines after vibration had caused problems in the supports of one of the turbochargers of the first 10RTA96C engine. Excessive transverse movement of the exhaust manifolds on the first such models was also recorded. This was remedied by stiffening the manifold support in the transverse direction, thus halving the manifold movements. Excessive movements of the manifolds also led to the breakage of cooling water discharge pipes; the adoption of flexible pipes of a proprietary make as standard solved the problem.
A number of broken cylinder cover studs have been noted, mostly broken at the first thread at the top but some fractured at the bottom. The lower fractures were initiated by corrosion because of improper sealing; the upper fractures resulted from materials and thread qualities beyond the specification. The design was changed to a wasted stud and a special nut to reduce stress levels and ensure a good safety margin.
TriboPack for extended TBO
The time-between-overhaul (TBO) of low speed marine diesel engines is largely determined by the piston running behaviour and its effect on the wear of piston rings and cylinder liners. Addressing this, Sulzer introduced a package of design measures in 1999, which are now standard on all new RTA engines and retrofittable to existing engines. The TriboPack technology enables the TBO of cylinder components, including piston ring renewal, to be extended to at least three years and also allows a further reduction in the cylinder lubricating oil feed rate. The design measures (Figure 29) are:
- Multi-level cylinder lubrication.
- Fully and deep-honed cylinder liner with sufficient hard phase.
- Careful turning of the liner running surface and deep honing of the liner over the full length of the running surface.
- Mid-stroke liner insulation and, where necessary, insulating tubes in the cooling bores in the upper part of the liner.
- Pre-profiled piston rings in all piston grooves
- Chromium-ceramic coating on the top piston ring.
- RC (Running-in Coating) piston rings in all lower piston grooves.
- Anti-polishing ring at the top of the cylinder liner.
- Increased thickness of chromium layer in the piston ring grooves.
Figure 29 The Sulzer TriboPack design measures for improving piston-running behaviour
A key element of TriboPack is the cylinder liner manufactured in cast iron, which needs a controlled hard-phase content and the best grain structure in the running surface for both good strength and running behaviour. Careful machining followed by full deep honing to remove all damaged hard phase from the liner surface reportedly delivers an ideal running surface for the piston rings, together with an optimum surface microstructure. Deep honing of the full liner running surface is a prerequisite for maximizing the benefits of TriboPack, says Wärtsilä: its experience has shown that plateau honing of a wave-cut liner is not adequate because, once the plateau is worn down, the rings run on liner metal whose hard phase structure was damaged during machining. This damaged hard phase must be removed by deep honing.
Pistons have four rings, all of the same thickness. The chrome-ceramic top ring, proven in Wärtsilä four-stroke engine practice, has a cast iron base material. The running face is profiled and coated with a layer of chromium as a matrix into which a ceramic material is trapped. High operational safety and low liner and ring wear have been demonstrated, with a much better resistance to scuffing than any other ring material, Wärtsilä asserts. Good performance is conditional, however, on using the chrome-ceramic rings in conjunction with a deep-honed liner. The other piston rings have a running-in and anti-scuffing coating which fosters a safe and swift running-in of the engine when the liners are deep honed.
The anti-polishing ring (APR) prevents the build-up of deposits on the top land of the piston which can damage the lube oil film on the liner and cause bore polishing. Deposit build-up can be heavy in some engines, especially those running on very low sulphur content fuel oil (less than one per cent sulphur) combined with an excessive cylinder lube oil feed rate. If such deposits are allowed to accumulate, they inevitably touch the liner running surface over a large part of the piston stroke. The lube oil film can then be wiped off, allowing metalto- metal contact between the piston rings and liner; in the worst case there can be scuffing.
Applied as standard for some years on Wärtsilä four-stroke engines, the thin alloy steel APR is located in a recess at the top of the liner and has an internal diameter less than the cylinder bore to reduce the clearance to the piston top land. It does not need to be specifically fixed, as the thermal expansion of the hot ring keeps it tightly in place. The steel material was selected to ensure and maintain a high safety margin against thermal yielding. Excessive deposits are scraped off the piston top land at every stroke while they are still soft, thus preventing hard contact between the deposit and the liner wall surface. The oil film on the liner wall remains undisturbed and can fulfil its function. The APR also stops the upward transportation of new lube oil by the layer of deposits to the top of the liner where it is burned instead of being used for lubrication. The ring is thus effective in allowing the lube oil feed rate to be kept down to recommended values.
Load-dependent cylinder lubrication is provided by Sulzer’s multilevel accumulator system, the lubricating pumps driven by frequencycontrolled electric motors. On the cylinder liner, oil distributors bring oil to the different oil accumulators. For ease of access, the quills are positioned in dry spaces instead of in way of cooling water spaces.
It is also important that the liner wall temperature is adapted to keep the liner surface above the dew point temperature over the whole of the piston stroke to avoid cold corrosion and maintain good piston-running conditions. The upper part of the liner is bore cooled with cooling water passing through tangential drillings in the liner collar. The mid-stroke region of the liner is cooled by a water jacket, and only the lower part is uncooled. There is often a tendency for liner temperatures to be too low, thus leading to corrosive wear from the sulphuric acid formed during combustion. Wärtsilä applies two insulating techniques to secure better temperature distributions. For some years, PTFE insulating tubes have been fitted in the cooling bores of the liner. As part of TriboPack, the liner is now also insulated in the mid-stroke region by a Teflon band on the water side. The insulating tubes are adapted according to the engine rating to ensure that the temperature of the liner running surface is kept above the dew point temperature of water over the full length of the stroke and over a wide load range.
Mid-stroke insulation and, where necessary, insulating tubes are therefore used to optimize liner temperatures over the piston stroke. An insulation bandage in the form of Teflon bands with an outer stainless steel shell is arranged around the outside of the liner to raise liner wall temperatures in the mid-stroke region. Mid-stroke insulation is known to be particularly useful for sustained engine operation at low power outputs, while the TriboPack gives an additional safety margin in abnormal operating conditions (for example, against excessive carbon deposits built up on the piston crown).
While trying to avoid corrosive wear by optimizing liner wall temperatures, it is necessary to keep as much water as possible out of the cylinders. Highly efficient vane-type water separators fitted after the scavenge air cooler and effective water drain arrangements are thus vital for good piston running behaviour. Load-dependent cylinder lubrication is provided by the Sulzer multi-level accumulator system, which ensures the timely quantity of lube oil for good piston running. The lube oil feed rate is controlled according to the engine load and can also be adjusted according to the engine condition.
Piston rod glands
Time-between-overhauls of crosshead engines are partly defined by the piston rod glands, in the sense that their removal for exchange of elements is often connected to a withdrawal of the piston and piston rod assembly. The gland elements and piston rods therefore need to have a long life expectation (TBO of three years or more). At the same time, they have to assure sealing of the crankcase from the piston underside, limit contamination of crankcase system oil by combustion residues, and keep the oil consumption at a reasonable level for maintaining oil quality.
Recent improvements have introduced additional gas-tight top scraper rings, stronger springs for the other scraper rings, enlarged drain channels for the scraped-off oil, and the exclusive application of bronze scraper rings on fully hardened rods. The drain quantities from the neutral space were reduced by a factor of three. Additionally, the scraped-off oil is reusable without any treatment and therefore can be directly fed back internally in the gland box to the crankcase. System oil consumption figures were significantly reduced. The design of the gland box housing was modified, allowing it to be dismantled either upwards during piston overhaul or downwards without pulling the piston.
Complete retrofit packages available for all RTA engines in service comprise the newly-designed upper scraper, new middle sealing and new lower scraper groups, and some modifications on the gland box housing. The upper scraper group consists of a two-piece housing with newly-designed oil scraper rings made of bronze, newly-designed gas-tight sealing rings and modified tension springs. The oil scraper rings consist of four segments conforming better to the piston rod. Two new seal rings in three parts and adapted tension springs were introduced for the middle seal group. For the lower scraper group, all rings are of bronze, since Teflon has an inferior performance when there is an increased amount of hard particles in the oil residues coming from the piston underside. Here, the new scraper rings comprise three slotted segments for adaptability to the piston rod; they are provided with grooves at the top to promote draining of the scraped oil.
The actual surface condition and shape of the piston rod is of paramount importance, Wärtsilä advises. Ideally, the new glands should be used with hardened piston rods. Existing rods can be retained, however, providing their surface condition and geometry are acceptable, before introducing any new stuffing box elements. If the rod is worn down, roughened or otherwise surface damaged it can be ground to standard diameters of 2 mm or 4 mm undersize and then surface hardened.
Exhaust valve behaviour
The exhaust valve is subjected to hot gases and the temperature resistance of its seat and body is therefore crucial. Nimonic valves combined with proper seat cooling have yielded excellent service behaviour and long life times. When the RTA96C engine was introduced, and its shallow combustion space created difficult conditions for combustion chamber components, some exhaust valves were additionally coated with Inconel alloy. After limited running times, however, there was some cracking of the coating originating from the centre hole, with loosened material, making removal by grinding necessary. Noncoated valves, on the other hand, showed excellent performance, remaining free of cracks after over 14 000 running hours, without any loss of material. Today’s standard therefore is the non-coated valve.
Burning off of material on piston crowns is very dangerous as hole formation leads to direct contact of the combustion flame with the piston cooling oil system and dire consequences. The use of the combined shaker and jet cooling system in RTA engines assures piston crown temperatures below 400 ∞C and thus eliminates such burning.
In seeking longer times-between-overhauls it is desirable to keep the engine ‘closed’ for as long as possible; this calls for additional systems to monitor the condition inside the cylinder without having to dismantle the engine for access. A growing family of products has been developed by Sulzer over many years to support the engine operator, these MAPEX tools complementing and expanding the functions of standard remote control systems. They include features for monitoring, trend analysis and planning, as well as for management support for spare parts ordering and stock control and maintenance, and are applicable to new engines and retrofit to existing engines.
A basic tool, MAPEX-CR (Combustion Reliability), continuously monitors the pressure in the cylinder. It can therefore detect and eliminate various malfunctions that may occur during engine operation while allowing the engine to run at optimum conditions. Previously, cylinder pressure could only be measured periodically by either a traditional indicator or a peak pressure indicator, or more recently by temporarily attached electronic sensors. Newly developed pressure sensors are sufficiently robust to be installed permanently for continuous on-line measurement.
MAPEX-CR monitors several key parameters (peak pressure, pressure gradient, compression pressure, pressure ratio and mean indicated pressure) along with their deviations. Evaluations of these parameters enables defects—such as permanently overloaded running gear, unbalanced cylinder load share, injection problems or broken piston rings—to be detected at an early stage. The system also generates an automatic alarm signal when permissible limits are exceeded.
MAPEX-PR (Piston Running) is a useful tool for checking pistonrunning behaviour without opening the engine. It continuously monitors the temperatures of the cylinder liners and the cooling water, and analyses them according to several alarm criteria which also take into account the engine load. Alarm signals are generated in the case of abnormal events. The past history of temperatures can also be reconstructed to enable relevant conclusions to be drawn. A unique ability is claimed to detect high friction on the cylinder liner at an early stage, this ensuring the reduction or even prevention of major consequential damage through piston scuffing. Other conditions indicated by the system include increased scavenge air temperature, excessive fluctuations in cooling water temperature, too high or too low average temperature of the whole engine, and excessive deviation of individual cylinder temperatures. Irregularities in the engine can be detected before serious damage occurs.
A new tool for managing condition-based maintenance of Sulzer twostroke engines combines the expert knowledge and wide experience of Wärtsilä Corporation engineers. The CBM Management tool is offered in three different solutions, depending on the extent that customers want to outsource this activity.
A wide range of products for diesel engine monitoring, trend analysis, diagnosis and management already support condition-based maintenance. CBM Management takes these a step further, with the aim of optimizing the balance between long TBOs, low spare part costs, less time off-hire and high reliability from the engine. Wärtsilä appreciated that, although customers use the same engine types in their fleet, they experience quite different maintenance costs and varying engine reliability. This arises from various factors that influence the condition of the engine, such as the quality of fuel, lube oil and spare parts, and the quality of routine inspections and overhauls.
Analysis of cases of extensive damage to engine components had shown that long before the damage occurred it would have been possible to foresee that something was going wrong. With the appropriate expert knowledge and a systematic analysis of available information, it is often possible to predict problems and to prevent expensive maintenance costs. Wartsila’s solution was to collect the expert knowledge of engineers from its technical service organization worldwide and store it in a common database.
Some data are specific to an individual engine: shop trial results, sea trial results, electronically stored information from general service reports made by Wärtsilä, and performance data collected onboard. All these were combined with the database of expert knowledge, enabling the resulting expert system to automatically generate advice on the condition of the specific engine. The CBM Management tool comprises a Data Collector (hardware and software) and an Expert System (hardware and software).
The Data Collector is a hand-held computer whose main purpose is to enable the ship’s engineers to collect data in a standardized way. It is automatically loaded with engine performance data from the control system through a standard interface but manually fed with maintenance and inspection data. For systematic and efficient manual input, the Data Collector contains a structured inspection guide for the engine, supported by interactive templates for measurement records and performance sheets. This guide exists for the following twelve function groups: piston performance; piston rod gland condition; scavenge air flow; combustion performance; fuel injection pumps; camshaft; engine structure; gears and wheels; bearings; crankshaft; engine control system; and vibration dampers.
The Data Collector takes into account different conditions of important components, such as piston rings, cylinder liners and bearings. A picture gallery helps the ship’s engineers to understand the terminology used. The Data Collector reduces inspection time and partly replaces the necessary paperwork through the automatically created templates.
The Expert System is the heart of the tool and runs on a PC using data from the Data Collector. There are three steps from collected data to real expert advice: calculate trends; assess engine condition by comparing the data with a weighted set of reference cases based on the expert knowledge of Wärtsilä engineers; and assess the condition of the engine components and parts, and offer advice to the ship’s engineers on achieving optimized TBOs. The system supports the operator with graphs, pictures, trends and text recommendations. Communication is possible between ships, the operator’s headquarters and Wärtsilä’s CBM centre in all directions.
Three different solutions are offered to suit the individual requirements of owners and operators:
- CBM Virtual Expert: the operator performs data collection and analysis, and Wärtsilä’s expert knowledge is given as far as possible without further involvement. The onboard engineers carry out data collection, the expert system being used on the ship as well as at the operator’s HQ. Such a solution typically suits the needs of owners and operators who have their own technical experts.
- CBM Report Agreement: the operator collects the data and Wärtsilä undertakes the analysis. The owner or operator uses only the Data Collector, while the data is analysed at Wärtsilä’s Two-Stroke CBM centre in Switzerland, which sends the customer a regular report. This type of agreement typically suits owners and operators without their own technical office.
- CBM Inspection Agreement: Wärtsilä carries out both data collection and analysis. Wärtsilä service engineers visit the ship in port, collect the data and send it to the CBM centre, from where the customer receives a report. Such a solution suits owners and operators without a technical office and who want to outsource inspections.
CBM solutions were expected to be generally available by mid-2003. A rapid growth in expert knowledge was anticipated from service engineers using the data collecting tool as a standard instrument.
RT-FLEX ELECTRONIC ENGINES
The Sulzer RT-flex system, which will be progressively offered as an option for all models in the RTA programme, resulted from a project originated in the 1980s to develop an electronically-controlled low speed engine without the constraints imposed by mechanical drive of the fuel injection pumps and exhaust valve actuation pumps . Traditional jerk-type fuel injection systems combine pressure generation, timing and metering in the injection pump with only limited flexibility to influence the variables. In contrast, Sulzer’s common rail system separates the functions and gives far more flexibility for optimizing the combustion process with injection and valve timing.
RT-flex engines are essentially the same as their standard RTA equivalents but dispense with the camshaft and its gear drive, jerktype fuel injection pumps, exhaust valve actuator pumps and reversing servomotors. Instead, they are equipped with common rail systems for fuel injection and exhaust valve actuation, and full electronic control of these functions (Figure 30).
The following benefits for operators are cited for the RT-flex system:
- Smokeless operation at all running speeds.
- Lower steady running speeds (in the range of 10–12 per cent nominal engine speed) obtained smokelessly through sequential shut-off of injectors while continuing to run on all cylinders. Very steady running at 12 rev/min has been demonstrated.
- Reduced running costs through lower part-load fuel consumption and longer times-between-overhauls.
- Reduced maintenance requirements, with simpler setting of the engine; the ‘as new’ running settings are automatically maintained.
- Reduced maintenance costs through precise volumetric fuel injection control, leading to extendable TBOs. The common rail fuel system and its volumetric control yields excellent balance
Figure 30 The fuel pumps, valve actuator pumps, camshafts and drive train of the standard Sulzer RTA engine are replaced by a compact set of supply pumps and common rail fuel system on the RT-flex engine
Figure 31 Sulzer RT-flex electronically-controlled common rail systems
in engine power developed between cylinders and between cycles, with precise injection timing and equalized thermal loads.
- Reliability underwritten by long term testing of common rail system hardware and the use of fuel supply pumps based on proven Sulzer four-stroke engine fuel injection pumps.
- Higher availability resulting from integrated monitoring functions and from built-in redundancy: full power can be developed with one fuel pump and one servo oil pump out of action. High pressure fuel and servo oil delivery pipes, and electronic systems, are also duplicated.
- A reduced overall engine weight: approximately two tons per cylinder lower in the case of a 580 mm bore RT-flex engine compared with its conventional RTA counterpart.
The common rail for fuel injection is a manifold running the length of the engine at just below the cylinder cover level; the rail and other related pipework are arranged on the top engine platform with ready accessibility from above (Figure 32). The common rail is fed with heated fuel oil at a high pressure (nominally 1000 bar) ready for injection into the engine cylinders. The fuel supply unit embraces a number of high pressure pumps mechanically driven from the crankshaft and running on multi-lobe cams, which increase their supply capacity and hence reduce the number needed. A four-pump set is sufficient for a six-cylinder RT-flex 58T-B engine. The pump design, based on fuel injection pumps used in Sulzer four-stroke engines, has suction control to regulate the fuel delivery volume according to engine requirements (Figure 33).
Figure 32 Schematic layout of Sulzer RT-flex common rail fuel system
Heated fuel oil is delivered from the common rail through a separate injection control unit for each engine cylinder to the standard fuel
Figure 33 Three of the six fuel oil pumps for a Sulzer 7RT-flex60C engine
injection valves which are hydraulically operated in the usual way by the high pressure fuel. The control units, exploiting quick-acting Sulzer solenoid 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 key features of the common rail system are defined as:
- Precise volumetric control of fuel injection, with integrated flowout security.
- Variable injection rate shaping and free selection of injection pressure.
- Stable pressure levels in common rail and supply pipes.
- Possibility for independent control and shutting-off of individual fuel injection valves.
- Ideally suited for heavy fuel (up to 730 cSt at 50∞C) through clear separation of the fuel oil from the hydraulic pilot valves.
- Proven standard fuel injection valves.
- Proven high efficiency common rail fuel pumps.
The fuel injection pressure can be freely selected up to more than 1000 bar over the whole load range. In combination with different injection patterns, this provides the opportunity to optimize the engine in several ways: for example, for low emission levels or improved fuel efficiency at non-optimum loads. (See Environmental Performance section below.)
The injection system can be adapted to different patterns, such as: pre-injection, with a small part of the fuel charge injected before the main charge; triple injection, with the fuel charge injected in three separate short sprays in succession; and sequential injection, with individual actuation of the fuel injection nozzles so that injection timing is different for each of the three nozzles in a cylinder. Different shapes of cylinder pressure profile during the engine cycle can thus be created which, with free selection of the rail pressure, allows the optimum pattern to be selected in each case for the loads and performance optimization target of the engine.
Selective shut-off of single injectors is valuable for low manoeuvring speeds or ‘slow steaming’ as this facility fosters better injection and atomization of the small quantities of fuel needed. In such modes, the common rail system is controlled to use the three injection valves in sequence. Regulated by an electronic governor, the RT-flex engine demonstrated very steady running at a lowest speed of 12 rev/min.
Exhaust valves are operated in much the same way as in conventional Sulzer RTA engines by a hydraulic ‘pushrod’, but with the actuating energy coming from a servo oil rail at 200 bar pressure. The servo oil is supplied by hydraulic pumps mechanically driven from the same gear train as the fuel supply pumps. An electronically-controlled actuator unit for each engine cylinder—operated by hydraulic pressure from the servo oil rail—gives full flexibility for valve opening and closing timing. Two redundant sensors inform the WECS-9500 control system (see below) of the current position of the exhaust valve. Lube oil from the engine is used as servo oil to keep the system simple and compatible. Before entering the servo oil circuit the oil is directed through an additional six-micron filter with an automatic self-cleaning device to ensure reliability and a long lifetime of the actuator units and solenoid valves.
All functions of the RT-flex system are controlled and monitored through the integrated Wärtsilä WECS-9500 electronic control system. This modular system has separate microprocessor control units for each cylinder, with overall control and supervision by duplicated microprocessor control units which provide the usual interface for the electronic governor and remote control and alarm systems.
The full load efficiency of RT-flex engines is the same as their conventional RTA engine equivalents but improvements in part-load fuel economy are gained. This results from the freedom allowed in selecting the optimum fuel injection pressure and timing, and exhaust valve timing, at all engine loads or speeds, while maintaining efficient combustion at all times, even during dead slow running. A similar freedom in exhaust valve timing allows the RT-flex system to keep the combustion air excess high by earlier closing as the load/speed is reduced. Such a facility is not only beneficial for fuel consumption; it also limits component temperatures, which normally increase at low load. Lower turbocharger efficiencies at part load normally result in low excess combustion air with fixed valve timing.
Another contribution of the RT-flex system to fuel economy cited by Wärtsilä is the capability to easily adapt the injection timing to various fuel properties influencing poor combustion behaviour. Variable injection timing (VIT) over load has been a traditional feature of Sulzer low speed engines for many years, using a mechanical arrangement primarily to keep the cylinder pressure high for the upper load range. This is much easier to arrange in an electronicallycontrolled engine.
Environmental performance: A very wide flexibility in optimizing fuel injection and exhaust valve processes enables RT-flex engines to comfortably meet IMO limits on NOx emissions. The most visible benefit cited is smokeless operation at all ship speeds, underwritten by superior combustion. The common rail system allows the fuel injection pressure to be maintained at an optimum level irrespective of engine speed. In addition, at very low speeds, individual fuel injectors are selectively shut off and the exhaust valve timing adapted to help keep smoke emissions below the visible limit. In contrast, engines with traditional jerk-type injection pumps have increasing smoke emissions as engine speed is reduced because the fuel injection pressure and volume decrease with speed and power, and they have no means of cutting off individual inj ection valves and changing exhaust valve timing. (Figure 34).
Figure 34 Smoke emissions with conventional fuel injection and with Sulzer RT-flex common rail technology for engines burning heavy fuel and marine diesel oil
As all settings and adjustments within the combustion and scavenging processes are made electronically, future adaptations are possible simply through changes in software, which could be easily retrofitted to existing RT-flex engines. A possibility is to offer different modes for different emissions regimes. In one mode, the engine would be optimized for minimum fuel consumption while complying with the global NOx limit; then, to satisfy local emissions regulations, the engine could be switched to an alternative mode for even lower NOx emissions while the fuel consumption is allowed to rise.
RT-flex engines in service
The RT-flex system was first applied to a full-size research engine in June 1998 at Wärtsilä’s facilities in Switzerland and represented the third generation of electronically-controlled Sulzer diesel engines. The first RT-flex engine, a six-cylinder RT-flex58T-B model, was tested in January 2001 and later installed as the propulsion plant of a 47 950 dwt bulk carrier, which was handed over in September that year. The engine’s slow-running capability was demonstrated by steady operation at speeds down to 12 rev/min. Subsequent orders called for sevencylinder RT-flex60C models, the first Sulzer low speed engine designed from the bedplate up with electronically-controlled common rail systems (Figure 35). The RT-flex option was extended to all bore sizes in
Figure 35 Sulzer 7RT-flex60C engine on test at Wärtsilä’s Trieste factory
the RTA series, the first contract for a 12-cylinder RT-flex96C engine developing 68 640 kW being booked for containership propulsion in early 2003.
RL TYPE ENGINES
The RTA engine’s immediate predecessor in the Sulzer low speed programme was the loop-scavenged RL series, of which examples remain in service and merit attention here.
The RLA56, a small bore two-stroke engine introduced in 1977, incorporated the basic design concept of the successful RND and RNDM series but extended the power range at the lower end. This engine, of comparatively long stroke design, was the first model in the RLA series. It retained many of the design features of the then most recent economical loop-scavenged RLB type (Figures 36 and 37), both engines using many features of the earlier RND-M series.
Figure 36 Cross-section of Sulze RLB90 engine
Figure 37 Longitudinal section of RLB90 engine
A number of RND-M engine features were retained for the RLA and RLB type, namely:
- Constant pressure turbocharging and loop scavenging.
- A bore-cooled cylinder liner and one-piece bore-cooled cylinder cover and water-cooled piston crowns.
- Double guided crossheads.
- New cylinder liner lubrication system with accumulators for the upper liner part and thin-walled aluminium–tin crosshead shell bearings.
Major new design features peculiar to the RL types were:
- A new bedplate design with an integrated thrust block; a new box type column design.
- A semi-built or monobloc type crankshaft without a separate thrust shaft.
- Location of the camshaft gear drive at one end for engines of four to eight cylinders.
- Multiple cylinder jackets.
- A bore-cooled piston crown.
- Modified crosshead.
- A new design of air receiver.
Bedplate with thrust block
For the RL type engine bedplate a new concept of great simplicity was applied. Both the crossgirders and the longitudinal structure are of single-wall fabricated design giving very good accessibility for the welded joints. The central bearing saddles are made of cast steel and only one row of mounting bolts on each side is used to secure the bedplate to the ship’s structure.
The completely new design feature was the method of integrating the thrust block into the bedplate (Figure 38) allowing an extremely compact design and saving engine length. The first crankshaft bearing on the driving end is a combined radial-axial bearing. The bedplate is a one-piece structure for all engines from four to eight cylinders but, if required, it can be bolted together from two halves.
Thick-walled whitemetal lined bearing shells are used to support the crankshaft and guarantee an optimum safety for the running of the crankshaft.
In addition to the traditional semi-built type crankshaft, a monoblock continuous grain flow forged type can be used. From four to eight
Figure 38 Integrated thrust bearing and camshaft drive for RL engine
cylinders, the crankshaft is a one-piece component with an integrated thrust collar section. Only one journal and pin diameter was used for all engines up to eight cylinders. For RLA engines larger than the RLA56, the crankshaft was semi-built, and the thrust shaft separately bolted to the crankshaft.
For the small RLA56 engine a new method of frame construction was used. Cast iron central pieces, on which the crosshead guides are bolted, are sandwiched between two one-piece fabricated side-frame girders. This replaced the traditional construction of ‘A’ frame bolted to the bedplate with side plates attached with access doors forming the enclosure. Larger RL type engines use ‘A’ shaped columns of fabricated double wall design, assembled with longitudinal stiffening plates to constitute a rigid structure between the bedplate and cylinder blocks (Figures 39 and 40).
The cylinder jackets are made of fine lamellar cast iron produced as single block units bolted together in the longitudinal plane to form a
Figure 39 Structural arrangement of bedplate, columns and cylinder jackets for 12- cylinder Sulzer RL engine
Figure 40 Arrangement of columns for RLA56 engine
single rigid unit, and held on top of the frame section by long tie bolts secured in the bedplate. For the small RLA56 engine, multi-cylinder blocks consisting of two or three cylinders were standard and provide great rigidity of the structure. The arrangement of the cooling water passages in the jackets ensures forced water circulation and optimal water distribution around the exhaust canal.
Because of the steadily rising charge air pressures, the design of the air receiver was modified to allow the use of automatic welding techniques. Instead of a rib-stiffened plain side plate, a semi-circular pressure containment was fitted with an integral air inlet casing. The auxiliary blower is mounted on the front end of the receiver, thus eliminating inclined ducts.
Combustion chamber components
The combustion chamber of RL type engines is principally of the same shape as that of the RND-M. The cylinder cover is basically a one-piece steel block with cooling bores, while an identical arrangement of bore cooling is applied to the upper collar of the cylinder liner which is made from lamellar cast iron, a material of good heat conductivity and wear resistance. Figure 41 shows the arrangement with cover, cylinder liner and piston.
A new type of piston crown was introduced for the RL series engines. This combustion chamber component similarly uses a bore cooling arrangement (Figures 42 and 43) as previously only applied to the liner and cover. Water cooling was retained but the piston bore cooling uses a somewhat different mechanism to the force flow system of the liner and cover.
The cooling space of the piston crown is approximately half filled with cooling water and, as a result of piston acceleration and deceleration, a ‘cocktail shaker’ effect is produced to provide excellent heat removal. This effect is capable of ensuring efficient heat transfer under all prevailing load conditions in order to keep the vital temperatures on the piston crown and around the piston rings within suitable limits.
As a result of this new design the crown temperatures were lowered compared with the previous construction. Forged steel blocks were specified for piston crown manufacture but cast steel versions were also used.
Lubrication of the cylinder is through six quills mounted in the upper area of the liner just above the cylinder jacket, as shown in Figure
Figure 41 Combustion chamber components for RLA90 engine
44. The oil distribution grooves have a very small angle of inclination to avoid blow-by over the piston rings and small but regular quantities of cylinder oil supplied by an accumulator system. Two further lubrication points are provided below the scavenge ports on the exhaust side with the necessary oil pumps positioned on the front side of the engine above the camshaft drive.
The crosshead is similar to that used for RND engines and has double guided slippers. The pin size was increased for safety and thin-walled half shells of the aluminium–tin type were used. The pin itself is of
Figure 42 Detail view of RL engine piston with bore cooling
Figure 43 Arrangement of telescopic pipes for RL engine piston cooling
forged homogeneous steel of symmetrical design, and can therefore be turned around in case of damage. The piston rod is connected to the pin by a single hydraulically tightened nut, while the slippers, made of cast steel and lined with whitemetal, are bolted to the ends of the pin. The cast iron double guide faces are fitted to the engine columns.
The connecting rods of traditional marine type have a forged normalized steel bottom end bearing, lined with white metal, held in place by four hydraulically tensioned bolts. Compression shims are provided between the bottom end bearing and the palm of the connecting rod.
The piston (Figure 42) consists of a water-cooled cast steel crown, a cast iron skirt with copper bandages and a forged steel piston rod, with the piston rings fitted in chromium plated grooves.
Figure 44 Cylinder lubricator for RL engine
The water-cooled piston has proved very reliable when running on heavy fuels and the use of water cooling has resulted in practically negligible system oil consumption. (With oil cooling, oil is consumed usually as a result of thermal ageing on hot piston walls. Oil leaks from oil-cooled pistons may also occur on other engine types.) The two-part gland seals for the piston rod and telescopic piston cooling pipes can be inspected while the engine is running and can be dismantled without removing the piston. The double-gland diaphragm around the piston rod completely separates the crankcase from the piston undersides, preventing contamination of the crankcase oil by combustion residues or possible cooling water leakages. In addition, the fresh water piston cooling water system is served by an automatic water drain-off when the circulating pumps are stopped. This avoids leakages when the ship is in port.
Sulzer RL engines all employ the constant pressure turbocharging system and with the high efficiency Brown Boveri series 4 turbochargers include provision for automatic cleaning. The layout of the turbocharging arrangement is shown in Figure 45.
The use of the piston undersides to provide a scavenge air impulse
Figure 45 Constant pressure turbocharging and operation of under-piston supercharging (RL engine)
eliminates the need for large electrically driven auxiliary blowers. At higher loads a simple flap valve opens to cut out the piston underside pumping effect with a consequent improvement in fuel consumption, whilst a small auxiliary fan incorporated in the scavenging system improves the smoke values at the lowest loads. The piston underside scavenge pump facility allows the engine to start and reverse even with total failure of all turbochargers. The auxiliary fan and engine will even operate at up to 60 per cent load, thus giving a ‘take home’ facility.
The turbochargers are mounted on top of the large exhaust gas receiver with the scavenge air receiver which forms part of the engine structure beneath. The charge air is passed down through seawater cooled intercoolers which are mounted accessibly alongside the scavenge air receiver. Flap-type valves which operate according to the scavenge air pressure direct the air inside the three-compartment air receiver to the piston underside (at low scavenge air pressure) or direct to the scavenge ports when the higher air pressure from the turbochargers keeps the underside delivery flap valves shut.
For RL type engines equipped with only one turbocharger a separate turbocharger/air intercooler module is available as a standard option. This unit can be located adjacent to the engine at either the forward or aft end, thus considerably reducing the mounting height and width of the main engine. The turbocharger module consists of a base frame onto which the turbocharger, charge air cooler, air ducts and cooling water pipes are solidly mounted with flexible connections to the engine.
Upgrading RLB engines
Based on field tests carried out between 1991 and 1995, a retrofit package of modifications was introduced for Sulzer RLB90 engines in 1993 and for RLB66 engines in 1995. The main element is the change to loop-cooling in the cylinder liners, an improved cooling technique that reduces the thermal and mechanical stresses in the top collar of the liner (Figure 46). By optimizing the running surface temperature, the system also reduces corrosive wear and extends the time-betweenoverhauls up to two years.
Figure 46 Loop cooling of the cylinder liner benefits Sulzer RLB90 and RLB66 engines
The retrofit package comprises the loop-cooled cylinder liners as well as:
- A new fully gas-tight, vermicular top piston ring of increased thickness.
- A modified top piston ring groove with increased height and increased thickness of chromium layer.
- Multi-level lubrication.
- A new water guide jacket to suit the new liner.
- A modified condensed water drain from the scavenge air receiver.
As RLB engines exploit loop scavenging with both scavenge and exhaust ports in the lower part of the liner, the fully gas-tight top ring significantly reduces the lateral forces at the piston skirt in addition to reducing the blow-by of hot combustion gases—both of which increase the piston skirt life. The package can be applied to the engine either as a full package or step-by-step (intermediate package) depending on service experience with the liners and whether the general pistonrunning behaviour meets today’s expectations. It is also possible to replace individual original liners with loop-cooled units whenever they need to be renewed; there is no problem in running loop-cooled and original design liners together in the same engine.