REFINEMENTS TO SULZER ENGINES

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 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 fulllength 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.

A key element of TriboPack is the cylinder liner which is manufactured in cast iron, 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. Piston crowns 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.

MC ENGINE DESIGN FEATURES

All MC series engine models are based on the same design principles and aim for simplicity and reliability, the key elements comprising:

Bedplate: for large bore engines is longitudinal side girders and welded crossgirders with cast steel bearing supports. For the smaller bore engine types the bedplate is of cast iron. It is designed for long, elastic holding-down bolts arranged in a single row and tightened with hydraulic tools. The main bearings are lined with white metal and the thrust bearing is incorporated in the aft end of the bedplate. The aft-most cross girder is therefore designed with ample stiffness to transmit the variable thrust from the thrust collar to the engine seating.

Frame box: this is a single welded unit for the large bore models and a cast iron unit for the smaller types.

Cylinder frame: the cast iron cylinder frames on the top of the frame box make another significant contribution to the rigidity of the overall engine structure. The frames include the scavenge boxes which are dimensioned to ensure that scavenge air is admitted uniformly to the cylinders. Staybolts are tightened hydraulically to connect the bedplate, the frame box and cylinder frames and form a very rigid unit.

Crankshaft: the conventional semi-built, shrink-fitted type crankshaft is provided with a thrust collar. The sprocket rim for the camshaft chain drive is fitted on the outer circumference of the thrust collar in order to reduce the overall length of the engine—except for the high cylinder numbers of the 800 mm bore model and upwards in the programme, for which the chain drive is located between two cylinders. An axial vibration damper is integrated on the free end of the crankshaft.

Connecting rod: in order to limit the height of the engines a relatively short connecting rod, comprising few principal parts, is specified. The large area of the lower half of the crosshead bearing allows the use of whitemetal or tin–aluminium on the small bore engine models. Floating guide shoes mean that most of the alignment work formerly required with crosshead engine pistons can be eliminated. The crankpin bearing for all engine models features thin shells lined with whitemetal.

Cylinder liner: a simple symmetrical design fosters low lubricating oil consumption and low wear rates. The liner is bore cooled on the larger engine models and available in two different configurations— with or without insulation of the cooling water jet pipes—so as to match the cooling intensity closely to the different engine ratings. The joint between liner and cover is located relatively low. This arrangement means that a larger part of the heat-exposed combustion chamber is contained in the steel cylinder cover rather than in the cast iron cylinder liner. Smaller bore engines feature a simple slimtype liner without cooling bores. For both types of liner adequate temperature control of the liner surfaces safeguards against cold corrosion caused by the condensation of sulphuric acid (originating from the sulphur content of heavy fuel) and, at the same time, ensures stable lubrication conditions by preventing excessive temperatures (Figure 10.4).

Cylinder cover: a solid steel component provided with bored passages for cooling water, a central bore for the exhaust valve, and bores for fuel valves, safety valve, starting valve and indicator valve. Piston: an oil-cooled piston crown, made of heat-resistant chrome– molybdenum steel, is rigidly bolted to the piston rod to allow distortionfree transmission of the firing pressure. The piston has four ring grooves which are hard chrome plated on both upper and lower surfaces of the grooves. A cast iron piston skirt (with bronze sliding bands on the large bore engines) is bolted to the underside of the piston crown (Figures 10.5 and 10.6).

Piston rod: the rod is surface treated to minimize friction in the stuffing box and to allow a higher sealing ring contact pressure. The piston rod stuffing box provides effective sealing between the clean crankcase and the combustion area, and has a proven record of very low amounts of drainage oil

Camshaft: the fuel injection pumps and the hydraulic exhaust valve actuators are driven by the camshaft. Cams are shrink-fitted to the shaft and can be individually adjusted by the high pressure oil method. Like its predecessors, the MC engine uses a chain drive to operate the camshaft and thus secures high reliability since a chain is virtually immune to foreign particles. It also enables the camshaft to be positioned higher, shortening the hydraulic connections to the fuel valves and the exhaust valves and, in turn, minimizing timing errors due to elasticity and pressure fluctuations in the pipe system.

Exhaust valve: hydraulic oil supplied from the actuator opens the exhaust valve and the closing force is delivered by a pneumatic springwhich leaves the valve spindle free to rotate. The closing of the valve is damped by an oil cushion on top of the spindle. The rotation force is provided by exhaust gas acting on vanes fitted to the valve stem. Extended service life from the valve is underwritten by Nimonic valvespindles and hardened steel bottom pieces, specified as standard on the large bore engines. The bottom piece features patented chamberin-seatgeometry.

Fuel pump: larger engine models incorporate pumps with variable injection timing for optimizing fuel economy at part load; the start of fuel injection is controlled by altering the pump barrel position via a toothed rack and a servo unit. Individual adjustment can be made on each cylinder. Additionally, collective adjustment of the maximum pressure level of the engine can be carried out to compensate for varying fuel qualities, wear and other factors. Both adjustments can be effected while the engine is running. The pump is provided with a puncture valve which prevents fuel injection during normal stopping and shutdown. Fuel oil system: the engine is served by a closed pressurized fuel oil system, with the fuel preheated to a maximum of 150∞C to ensure a suitable injection viscosity. The fuel injection valves are uncooled. The fuel system is kept warm by the circulation of heated fuel oil, thus allowing pier-to-pier operation on heavy fuel oil.

Reversing mechanism: the engine is reversed by a simple and reliable mechanism which incorporates an angularly displaceable roller in the fuel pump drive of each cylinder. The link connecting the roller guide and the roller is self locking in the Ahead and Astern positions. The link is activated by compressed air which has proved to be a very reliable method since each cylinder is reversed individually. The engine remains manoeuvrable even if one cylinder fails: in such a case the relevant fuel pump is set to the zero index position.

PROGRAMME EXPANSION

A longer stroke S-version in1993. Component modified but design features we're unchanged.

BEDPLATE n framebox wer wider n higher

Longer con rod n piston rod

Cyl frame s identical

Wider main brgs

Comb chamber components unchanged

Cyl cover s little higher to increase combustion space

Exh vv same but improved sealing arrangement to reduce wear of vv stem.

Multi level cyl lubrication

2 high rings on top

Fuel pp dia increased and improved sealing

Camshaft dia increased

The new S50/60/70MCC are more compact and offer higher outputs than their established equivalents L70. Stroke bore ratio wer raised from 3.8 to 4:1 and mep to 19bar and mean piston speed of 8.5m/s. Supporting the higher rating are modified turbocharging and scavenge air systems, modification of combustion chamber and brgs.(by around 1000mm in 6S50MCC). Masses are also lower by 13% which yuelds in reduced vibr excitation.

S80MC remodelled BEDPLATE and chain drive/thrust bearing to shorten the length by 700mm.

LARGE BORE ENGINES

The structural design of the K98MC, K98MC-C, S90MC-C and S80MCC engines is primarily based on that of the compact S-MC-C medium bore models. The main differences between the established K90MC/ MC-C engines and the new design are as follows:

Bedplate 

All the new large bore engines are designed with thin-shell main bearings of whitemetal. The rigidity of the bearing housing was increased substantially to reduce stress levels and deformations. Furthermore, the change from the traditional stay bolts to the twin stay bolt design, with bolts screwed into the top of the cast bearing part, means that the geometry of the main bearing structure is simplified and an improved casting quality is achieved. Finally, to improve the fatigue strength, a material with upgraded mechanical properties was specified. The use of twin stay bolts, fitted in threads in the top of the bedplate, has almost eliminated deformation of the main bearing housing during bolt tightening. The match between main bearing cap and saddle, secured in the final machining, is thus maintained in the operational condition, yielding a highly beneficial effect on main bearing performance. A triangular plate design engine frame box was adopted for later K98MC/MC-C engines when this was introduced on the S90MC-C and S80MC-C engines. The new design provides continuous support of the guide bar, thus ensuring uniform deformation of the bar and a more even contact pattern between guide shoe and guide bar that enhances guide shoe performance. In addition, the continuous support contributes to a significant reduction of the stress level in the areas in question. The holes in the supporting plate make it possible to inspect all longitudinal welding seams from the back, and thus ensure that the specified quality is secured.

Main bearing

Tin-aluminium (Sn40Al) has proved very reliable as the bearing material for the smaller engines, the alloy having a much higher fatigue strength at elevated temperatures. The material may be applied to large bore engines to maintain the reliability of the main bearings. Dry-barring on the testbed in some cases caused light seizure of Sn40Al main bearing shells on S-MC-C engines, but the problem was eliminated either by pre-lubrication with grease, high viscosity oil or by PTFEcoating the running surface of the shells.

Engine alignment Traditionally, bedplate alignment, especially on large tankers, has been performed on the basis of a pre-calculated vertical position of the bearings, as well as of the engine as such, and possibly also involving an inclination of the entire main engine. On completing the precalculated alignment procedure, it has been normal practice to check the alignment by measuring the crankshaft deflections. Such checks are normally carried out either in drydock or with the ship afloat alongside at the yard in a very light ballast condition. Owing to repeated cases of bearing damage, presumably caused by the lack of static loads in the normal operating conditions (ballast and design draught), MAN B&W introduced modified alignment procedures for bedplate and shafting (crankshaft and propulsion shafting) as well as modifying the vertical offset of the main bearing saddles. In the modified bedplate alignment procedure, the importance of the so-called sag of the bedplate is emphasised in order to counteract the hog caused by hull deflections as a result of the loading down of the ship, and partly by deformations due to the heating up of the engine and certain tanks.

Combustion chamber 

A reconfigured combustion chamber (Figure 10.14) was developed for the new large bore engines (800 mm bore and above), the key features being: 

1. Piston crown with high topland. 

In order to protect the piston rings from the thermal load from combustion the height of the piston topland was increased (Figure 10.15). The resulting increased buffer volume between the piston crown and the cylinder wall improves conditions for the rings and allows longer times-between overhauls. The high topland was first introduced in the mid1990s, the positive service experience leading to its use for all new engine types.

2. Piston crown with Oros shape. With increasing engine ratings, the major development challenge with respect to the combustion chamber components is to control the heat load on them. The short-stroke large bore engines have a rather flat combustion chamber because of the relatively smaller compression volume; this makes it more difficult to distribute the injected fuel oil without getting closer to the combustion chamber components. Furthermore, the higher rating means an increased amount of fuel injected per stroke. All this makes it more difficult to control the heat load on the components in short-stroke engines compared with long-stroke engines of the same bore size. 

The heat load on the cylinder liner has been reduced by lowering the mating surface between cylinder liner and cylinder cover as much as possible (to just above the uppermost piston ring at top dead centre). This means that the greater part of the heat load is absorbed by the cylinder cover, which is made of steel and thus more resistant to high heat loads. In addition, the cylinder cover is water cooled, making it relatively easy to control the temperature level. The piston is cooled by system oil, which means a lower cooling efficiency compared with the cooling of the cylinder cover. Oil cooling of the piston, however, offers a number of advantages. The optimum way of reducing the temperature level on the piston is to reduce the heat load on it, this being secured by redesigning the shape of the combustion chamber, including the piston, to provide more space around the fuel valve nozzles. The new piston shape was termed Oros (Greek for small mountain). The result of the increased distance from the fuel valve nozzles to the piston surface was simulated by CFD analyses, and the optimum shapes of piston crown and cylinder cover determined from these simulations. Tests on several engine types verified the simulations. A significant reduction in temperature was obtained after development tests on K90MC engines with various layouts of fuel oil spray pattern. Temperature measurements on the piston crown and exhaust valve are shown in Figure 10.16. The reduction in maximum piston temperature was approximately 90∞C, this result being attained without impairing the temperature level on the oil side of the piston or the temperature on the exhaust valve.

Piston ring pack

 The Controlled Pressure Relief (CPR) top ring with relief grooves is now standard on all MC engines and has proved very effective in protecting the cylinder liner surface as well as the lower piston rings against excessive heat load. The CPR ring has a double lap joint, and an optimum pressure drop across the top piston ring is ensured by relief grooves (Figure 10.17). With increasing mean indicated pressures, the traditional angle-cut ring gap may result in higher thermal load on the cylinder liner; this load is significantly reduced by the CPR ring as no gas will pass through its double lap joint. The relief grooves ensure an almost even distribution of the thermal load from the combustion gases over the circumference of the liner, resulting in a reduced load on the liner as well as on the second piston ring. 

Measurements confirmed that the peak temperature on No.2 piston ring was reduced from 300∞C in association with an oblique cut top ring to 150∞C with the CPR top ring. No.2 ring retains its spring force and times-between-overhauls are considerably extended. Furthermore, the pressure drop across the top piston ring has been optimized with respect to wear on the liner, piston rings and ring grooves. Thanks to the double lap joint, the pressure drop will be almost constant irrespective of the wear on the liner and rings. This contrasts with the traditional angle-cut ring, with which the cylinder condition slowly deteriorates as the liner wears. With the CPR rings, MAN B&W asserts, a continually good cylinder condition and low wear rate can be expected over the whole lifetime of the liner. Alu-coating of the sliding surface of piston rings was introduced to ensure safe running-in. The aluminium-bronze alloy coating, a type of bearing material, has proved effective in protecting ring and liner surfaces during the running-in period. Alu-coated rings make it possible to load up a completely new engine on the testbed within only five hours, fostering time and fuel savings; running-in can also be performed with reduced cylinder lubrication. The lifetime of the coating is 1000– 2000 hours, depending on such factors as the cylinder oil feed rate and surface roughness of the liner. The technical explanation is that no scuffing occurs with the combination of aluminium bronze/grey cast iron as the wear components in the cylinder. Alu-coated rings allow the normal increase in cylinder oil feed rate after changing rings and/or liners to be dispensed with.

Piston ring groove

 In order to extend the interval before reconditioning of the piston ring grooves is required the chromium layer has been increased to 0.5 mm and induction hardening of the grooves before chrome plating introduced. The chromium layer is thus better supported by the base material and the risk of cracking of the brittle chrome reduced, fostering a longer service life. 

Piston cleaning (PC) ring

 Incorporated in the top of the cylinder liner, the PC-ring has a slightly smaller inner diameter than the liner and hence scrapes off ash and carbon deposits built up on the piston topland (Figure 10.18). Without such a ring, contact between the topland and the liner wall could wipe off the injected cylinder oil, preventing the lubricant from performing its optimized role. In some cases, deposit formation on the topland could cause bore polishing of the liner wall, contributing to deterioration of the cylinder condition. Introducing the PC-ring eliminates contact between deposits on the topland and the liner, promoting an enhanced cylinder condition and lube oil performance.

Cylinder liner

 The liners of the large bore engines are bore cooled; the cooling intensity is adjusted to maintain an optimum temperature level and to ensure optimum tribological conditions for the cylinder lube oil. For years MAN B&W Diesel used a wave-cut liner surface, which was modified a few years ago to a semi-honed surface to facilitate running-in of the highly-loaded engines. The original wave-cut had a depth of approximately 0.02 mm. Experience showed that a deeper cut is advantageous in increasing the lifetime of the oil pockets, which, together with the Alu-coated piston rings, leads to very safe runningin. An improved semi-honed wave-cut liner surface was therefore introduced.

Fuel valve The fuel valve design was changed a few years ago from a conventional type to the mini-sac type. The aim was to reduce the sac volume in the fuel nozzle and curb dripping, thus improving combustion; introducing the mini-sac valves reduced the sac volume to approximately one third of the original (see the chapter on Fuel Injection). To improve combustion even further, however, a new slide-type valve was introduced for all large bore engines, completely eliminating the sac volume (see the chapter on Exhaust Emissions and Control). A significant improvement in combustion is accompanied by reduced NOx, smoke and particulate emissions. The reduced particulates also improve the cylinder condition, and the wear rates of cylinder liner, piston rings and ring grooves are also generally lower with slide-type fuel valves. Slide-type valves were introduced on the K98MC engine from the beginning, their positive effect confirmed on the testbed. With fuel nozzles optimized with respect to heat distribution and specific fuel consumption, the NOx emission values associated with this engine are described as very satisfactory.