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Alpha Diesel (MAN Diesel)
Chapter | eighteen
Alpha Diesel (MAN Diesel) From their introduction in the early 1970s, MAN B&W Alpha Diesel’s kindred 23- and 28-type medium-speed engines were refined and uprated a number of times to address changing requirements from small-ship propulsion and genset drive markets. Upgradings by the Frederikshavn, Denmark-based member of the MAN Diesel group tapped extensive service experience with both designs which respectively featured 225/300 mm and 280/320 mm bore/ stroke dimensions. The last 28/32A engine programme covered a power range from 1470 kW to 3920 kW at 775 rev/min with in-line 6- to V16-cylinder models, optimized for service intervals of 24–30 months based on an annual running time of 6000–7000 h. Outputs from 800 kW to 1920 kW at 825 rev/min or 900 rev/min were developed by 6L, 8L and V12-cylinder variants of the 23/30A series. Early experience with small medium-speed engines burning heavy fuel oil found that exhaust valves could function for around 2000 h (although often less) and had to be scrapped after 3000 h of service. In a few engines, whose fuel was less aggressive and the load moderate, the interval could be extended to 6000 h. Generally, however, the valve’s lifetimes were considered too short in relation to the cost savings made from using heavy fuel and owners opted for marine diesel oil instead. Subsequent 28/32A engine developments raised the maintenance interval to at least 15 000 h, representing a time-between-overhaul of 2 years. Operation on heavy fuel was thus cost-effective for almost all ships, according to the designer, the exceptions being tonnage whose load profiles dictated frequent starting and stopping and low annual running hours. Advances in performance were sought over the years along with improvements in durability and reliability. The specific power rating rose by almost 40 per cent while the brake mean effective pressure remained at around 19 bar and the specific fuel consumption at 190 g/kW h. The general design features of the last 28/32A engine dated from 1990. The engine’s overall appearance was much the same as it was when launched in 1974, with closed-in fuel piping and pumps. But the cross-section (Figure 18.1)
531
532 Alpha Diesel (MAN Diesel)
Figure 18.1 Cross-section of MAN B&W Alpha V28/32A engine
shows that many of the key elements—such as the frame, cylinder liner, piston, cylinder cover and valves—were improved.
Frame The frame was reinforced and cast in nodular cast iron. The support and cooling of the cylinder liner were improved, and the staybolts for the cylinder head given a deeper and enhanced grip in the frame to carry the load from higher combustion pressures.
Cylinder liner 533
Combustion Contributions to reduced fuel consumption and lower exhaust gas emissions were made by (a) accurate start of fuel injection timing, (b) precision-made injection nozzles, (c) fuel pumps with precise fuel metering, (d) modified combustion chambers yielding a compression ratio of 13.9, (e) precisely defined fuel spray geometry and (f) increased injection pressure. Also influential was the MAN B&W turbocharger, introduced in the 1980s and matched to the 28/32A engine.
Fuel valves and pumps Good interaction between the engine and its fuel injection system determined the engine’s torque and power output as well as its emissions and noise generation. Single-plunger injection operated with fuels of diverse viscosities and with peak pressures up to around 1100 bar. The plunger and barrel assembly was provided with a leakage oil return and an extra oil block for leaked fuel. A ring-shaped groove was machined into the pump barrel. Lube oil was forced into the fuel pump housing at the pump foot, preventing the spring guide from sticking in asphaltenes. The very small leakage quantity involved was drawn off through a separate outlet and led into a collecting tank.
Combustion chamber Development of the 28/32A engine during 1988/1990 yielded an improved combustion chamber in which the fuel spray mixed better with the air and avoided spraying at the valves and piston top. In combination with an adjustment of the pressures, the combustion process fostered reduced fuel consumption and lower exhaust gas temperatures, along with minimized exhaust smoke.
Cylinder head, valves and ducts Nodular cast iron was specified for the cylinder head to secure high strength and stiffness. The head featured a bore-cooled flame deck and was fitted with four rotating valves and cooled valve seats. Air and gas ducts were dimensioned to ensure minimum resistance to an optimized air flow and to achieve exhaust valve temperatures of below 400°C. Overhaul intervals exceeding 18 000 h on heavy fuel oil were confirmed in service. The performance of valves and seats was closely monitored. Well-cooled seats, a rotating mechanism and the construction material mean that it is not the valves that dictate the time for maintenance. Regrinding can be carried out twice to give a valve lifetime of up to 40 000 h (but sometimes shorter if the exhaust gases have attacked the valve stem with sulphuric acid).
Cylinder liner The liner is centrifugally cast from fine-grained perlitic iron. The improved combustion chamber made it possible to introduce a step in the bore diameter at the top of the cylinder with an uncooled flame ring. The inside diameter of
534 Alpha Diesel (MAN Diesel) the ring is somewhat smaller than the liner bore. The piston crown is therefore made correspondingly smaller, allowing carbon coke on the piston top to be kept well clear of the running surface of the cylinder liner. The flame ring— introduced on all new engines from 1992—has proved to be beneficial (Figure 18.2). It avoids liner polishing and micro-seizure, and helps to maintain the piston rings in a good condition; a sound seal against the gas and combustion forces results in a low and stable lube oil consumption.
Piston There were two suppliers of pistons for the 28/32A engine. The designs are different but have both generally performed well. One type is a monobloc piston of nodular cast iron with a splash-cooled piston top and a thin-walled skirt to keep the weight down. The other type is a composite piston with a steel top and a nodular cast iron skirt. Each has an intensively cooled top to keep the rings in good condition and foster a low wear rate. The ring grooves in the pistons are hardened and ground. The top piston ring was a focus of development, the aim being to find the best compromise between low lube oil consumption and good lubrication. All the rings have a chrome layer on the running surface.
Figure 18.2 Section through 28/32A cylinder, highlighting the flame ring
Turbocharger 535
Camshafts The cam shape was optimized and the camshaft material and hardening procedure improved. The rollers were ‘flexible shaped’ to avoid edge loading.
Bearings The application of ‘Rillenlager’ (Mibi’s multi-metal layer bearing) solved the problem of damaged bearings due to catfines in the fuel and extended bearing lifetime to over 30 000 h. This means they can be re-installed at 12 000– 15 000 h service intervals and run for a similar second period.
Turbocharger The MAN B&W turbocharger, introduced in the 1980s, was matched to the 28/32A engine and in conjunction with the compression ratio and valve timing fostered optimized conditions for the valves. Inboard floating bearings, with a reported lifetime of over 24 000 h, are lubricated by the engine lubrication system.
Alpha Diesel (MAN Diesel) From their introduction in the early 1970s, MAN B&W Alpha Diesel’s kindred 23- and 28-type medium-speed engines were refined and uprated a number of times to address changing requirements from small-ship propulsion and genset drive markets. Upgradings by the Frederikshavn, Denmark-based member of the MAN Diesel group tapped extensive service experience with both designs which respectively featured 225/300 mm and 280/320 mm bore/ stroke dimensions. The last 28/32A engine programme covered a power range from 1470 kW to 3920 kW at 775 rev/min with in-line 6- to V16-cylinder models, optimized for service intervals of 24–30 months based on an annual running time of 6000–7000 h. Outputs from 800 kW to 1920 kW at 825 rev/min or 900 rev/min were developed by 6L, 8L and V12-cylinder variants of the 23/30A series. Early experience with small medium-speed engines burning heavy fuel oil found that exhaust valves could function for around 2000 h (although often less) and had to be scrapped after 3000 h of service. In a few engines, whose fuel was less aggressive and the load moderate, the interval could be extended to 6000 h. Generally, however, the valve’s lifetimes were considered too short in relation to the cost savings made from using heavy fuel and owners opted for marine diesel oil instead. Subsequent 28/32A engine developments raised the maintenance interval to at least 15 000 h, representing a time-between-overhaul of 2 years. Operation on heavy fuel was thus cost-effective for almost all ships, according to the designer, the exceptions being tonnage whose load profiles dictated frequent starting and stopping and low annual running hours. Advances in performance were sought over the years along with improvements in durability and reliability. The specific power rating rose by almost 40 per cent while the brake mean effective pressure remained at around 19 bar and the specific fuel consumption at 190 g/kW h. The general design features of the last 28/32A engine dated from 1990. The engine’s overall appearance was much the same as it was when launched in 1974, with closed-in fuel piping and pumps. But the cross-section (Figure 18.1)
531
532 Alpha Diesel (MAN Diesel)
Figure 18.1 Cross-section of MAN B&W Alpha V28/32A engine
shows that many of the key elements—such as the frame, cylinder liner, piston, cylinder cover and valves—were improved.
Frame The frame was reinforced and cast in nodular cast iron. The support and cooling of the cylinder liner were improved, and the staybolts for the cylinder head given a deeper and enhanced grip in the frame to carry the load from higher combustion pressures.
Cylinder liner 533
Combustion Contributions to reduced fuel consumption and lower exhaust gas emissions were made by (a) accurate start of fuel injection timing, (b) precision-made injection nozzles, (c) fuel pumps with precise fuel metering, (d) modified combustion chambers yielding a compression ratio of 13.9, (e) precisely defined fuel spray geometry and (f) increased injection pressure. Also influential was the MAN B&W turbocharger, introduced in the 1980s and matched to the 28/32A engine.
Fuel valves and pumps Good interaction between the engine and its fuel injection system determined the engine’s torque and power output as well as its emissions and noise generation. Single-plunger injection operated with fuels of diverse viscosities and with peak pressures up to around 1100 bar. The plunger and barrel assembly was provided with a leakage oil return and an extra oil block for leaked fuel. A ring-shaped groove was machined into the pump barrel. Lube oil was forced into the fuel pump housing at the pump foot, preventing the spring guide from sticking in asphaltenes. The very small leakage quantity involved was drawn off through a separate outlet and led into a collecting tank.
Combustion chamber Development of the 28/32A engine during 1988/1990 yielded an improved combustion chamber in which the fuel spray mixed better with the air and avoided spraying at the valves and piston top. In combination with an adjustment of the pressures, the combustion process fostered reduced fuel consumption and lower exhaust gas temperatures, along with minimized exhaust smoke.
Cylinder head, valves and ducts Nodular cast iron was specified for the cylinder head to secure high strength and stiffness. The head featured a bore-cooled flame deck and was fitted with four rotating valves and cooled valve seats. Air and gas ducts were dimensioned to ensure minimum resistance to an optimized air flow and to achieve exhaust valve temperatures of below 400°C. Overhaul intervals exceeding 18 000 h on heavy fuel oil were confirmed in service. The performance of valves and seats was closely monitored. Well-cooled seats, a rotating mechanism and the construction material mean that it is not the valves that dictate the time for maintenance. Regrinding can be carried out twice to give a valve lifetime of up to 40 000 h (but sometimes shorter if the exhaust gases have attacked the valve stem with sulphuric acid).
Cylinder liner The liner is centrifugally cast from fine-grained perlitic iron. The improved combustion chamber made it possible to introduce a step in the bore diameter at the top of the cylinder with an uncooled flame ring. The inside diameter of
534 Alpha Diesel (MAN Diesel) the ring is somewhat smaller than the liner bore. The piston crown is therefore made correspondingly smaller, allowing carbon coke on the piston top to be kept well clear of the running surface of the cylinder liner. The flame ring— introduced on all new engines from 1992—has proved to be beneficial (Figure 18.2). It avoids liner polishing and micro-seizure, and helps to maintain the piston rings in a good condition; a sound seal against the gas and combustion forces results in a low and stable lube oil consumption.
Piston There were two suppliers of pistons for the 28/32A engine. The designs are different but have both generally performed well. One type is a monobloc piston of nodular cast iron with a splash-cooled piston top and a thin-walled skirt to keep the weight down. The other type is a composite piston with a steel top and a nodular cast iron skirt. Each has an intensively cooled top to keep the rings in good condition and foster a low wear rate. The ring grooves in the pistons are hardened and ground. The top piston ring was a focus of development, the aim being to find the best compromise between low lube oil consumption and good lubrication. All the rings have a chrome layer on the running surface.
Figure 18.2 Section through 28/32A cylinder, highlighting the flame ring
Turbocharger 535
Camshafts The cam shape was optimized and the camshaft material and hardening procedure improved. The rollers were ‘flexible shaped’ to avoid edge loading.
Bearings The application of ‘Rillenlager’ (Mibi’s multi-metal layer bearing) solved the problem of damaged bearings due to catfines in the fuel and extended bearing lifetime to over 30 000 h. This means they can be re-installed at 12 000– 15 000 h service intervals and run for a similar second period.
Turbocharger The MAN B&W turbocharger, introduced in the 1980s, was matched to the 28/32A engine and in conjunction with the compression ratio and valve timing fostered optimized conditions for the valves. Inboard floating bearings, with a reported lifetime of over 24 000 h, are lubricated by the engine lubrication system.