Burmeister & Wain low speed engines
A pioneering Danish designer and builder of low speed two-stroke crosshead engines, Burmeister and Wain (B&W) focused in the post-War era on high pressure turbocharged single-acting uniflow-scavenged models with one centrally located exhaust valve. The uniflow system fosters effective scavenging of the exhaust gases from the cylinder in an even progressive upwards movement with low flow resistance through to the exhaust manifold via a large diameter poppet-type exhaust valve (Figure 1).
Until 1978 all B&W engines operated on the impulse turbocharging system (Figure 2), but the search for enhanced economy in the wake of contemporary fuel crises dictated a change to the constant pressure system, yielding a saving of around 5 per cent in specific fuel consumption (Figure 3). In the years before its takeover by MAN of Germany in 1980, B&W introduced a number of engine series— successively the K-GF, L-GF, L-GFCA and L-GB programmes—before launching the current MC design portfolio now developed and marketed under the MAN B&W Diesel banner.
All these designs were improved variants of the K-GF series which itself introduced many notable design innovations, such as box frame construction, intensively cooled cylinder components and hydraulically actuated exhaust valves.
A major change to the structure of the company followed its purchase by MAN, resulting in Copenhagen becoming the centre for twostroke engine development of all designs bearing the MAN B&W trade mark.
K-GF TYPE ENGINES
B&W two-stroke engines based on the K-GF series featured many new design features. Notable was the box-type construction of the crankcase and the use of a hydraulically actuated exhaust valve instead of the conventional mechanical rocker arm system used in the earlier K-EF types. A sectional sketch of the K-GF is shown in Figure 4. As this
Figure 1 B&W uniflow scavenging system
model was rapidly superseded by the L-GF and L-GB models, only brief details of the K-GF construction are given here.
Bedplate and frame
The standard K-GF engine bedplate is of the high and fabricated design with cast steel main bearing housings welded to the longitudinal side frames. The standard thrust bearing is of the built-in short type and separated from the crankcase by a partition wall (Figure 5).
Figure 2 B&W uniflow scavenged engines of 1950 and 1970 with impulse turbocharging
The frame or entablature section consists of three units: a frame box of a height corresponding to the length of the crosshead guides, bolted together in the longitudinal direction in the chain drive section only, and two longitudinal girders, also bolted together in this section. This method of construction gives rigid units designed for easy handling and mounting, both on the testbed and when erecting the engine in the ship. The rigid design with few joints ensures oil tightness, and large hinged doors provide easy access to the crankcase.
The crosshead guides, consisting of heavy I sections, are attached at top and bottom in the frame boxes, with the lower attachment flexible in the longitudinal direction.
Cast iron one-piece cylinder sections connected in the vertical plane by fitted bolts are held to the frame section by long tie-bolts which are secured in the main bearing saddles.
Figure 3 Working diagrams for impulse and constant pressure turbocharging
The cylinder liner is provided with a water-cooled flange low down to secure low operating temperatures in the most heavily loaded area. The liners are made of alloyed cast iron with ports for scavenge air and bores for the cylinder lubrication system.
The crankshaft is semi-built for all cylinder numbers with cast steel throws for six- to ten-cylinder engines. Balancing of the engine is undertaken by varying the crankpin bore holes, thus entirely eliminating bolted-on counterweights. Lubrication of the crank bearing is from the crosshead through the connecting rod, thus eliminating stress raising bores in the crankshaft.
The crosshead (Figure 6) is short and rigid and the bearings are so constructed that the bearing pressure between journal and bearing is distributed evenly over the entire length of the bearing. The bearing
Figure 4 Cross-section of K90GF engine
pressure is smaller than previously and the peripheral speed is higher, which improves the working conditions of the bearings.
Interchangeable bearing shells of steel with a 1 mm whitemetal lining are fitted to the crosshead bearings. The shells are identical
Figure 5 K-GF structural arrangement
halves, precision bored to finish size and can be reversed in position during emergencies when a damaged or worn lower half can be temporarily used as an upper shell.
The piston is oil cooled and consists of the crown, cooling insert and the skirt. The cooling insert is fastened to the upper end of the piston rod and transfers the combustion forces from the crown to the rod.
The crown and skirt are held together by screws while a heavy Belleville spring is used to press the crown and the cooling insert against the piston rod. The crown has chromium plated grooves for five piston
Figure 6 Crosshead arrangement
rod and transfers the combustion forces from the crown to the rod. The crown and skirt are held together by screws while a heavy Belleville spring is used to press the crown and the cooling insert against the piston rod. The crown has chromium plated grooves for five piston rings while the rod has a longitudinal bore in which is mounted the cooling oil outlet pipe. The oil inlet is through a telescopic pipe fastened to the crosshead, and the oil passes through a bore in the foot of the piston rod to the cooling insert (Figure 7).
The cylinder cover is in two parts, a solid steel plate with radial cooling water bores, to which a forged steel ring with bevelled cooling water bores is bolted. The insert has a central bore for the exhaust valve cage and mountings for fuel valves, a safety valve and an indicator valve.
The cover is held against the cylinder liner top collar by studs screwed into the cylinder block and the nuts are tightened by a special
Figure 7 Combustion chamber and piston showing bore-cooling arrangement
hydraulic tool to allow simultaneous tightening and correct tension of the studs.
The valve is similar to earlier types but with hydraulic actuation, a feature originally unique to the K-GF series (Figure 8). The valve consists of a cast iron cage and a spindle, with steel seats and the valve mushroom faced with Stellite for hard wearing. Studs secure the valve housing to the cylinder cover.
The hydraulic actuation system is based on a piston pump driven by a cam on the camshaft, and the pressure oil is led to a working cylinder placed on top of the exhaust valve housing. The oil pressure is used to open the valve and closing is accomplished by a ring of helical springs.
The camshaft is divided into sections, one for each cylinder, enclosed and suspended in roller guide housings with replaceable ready-bored
Figure 8 Components of hydraulically actuated exhaust valves
bearing shells. The cams and couplings are fitted to the shaft by the SKF oil-injection pressure method and the complete camshaft unit is driven by chain from the crankshaft. For reversing of the engine, the chain wheel floats in relation to the camshaft which is driven through a self-locking crank gear; reversing crankpins are turned by built-on hydraulic motors.
Until 1978 all B&W engines were equipped with turbochargers operating on the impulse system, as was the case for the K-GF when first introduced. All B&W engines (including the K-GF) were subsequently specified
Figure 9 Fuel pump
Figure 10 Fuel injector
with a constant pressure charging system in which the exhaust pulses from the individual cylinders are smoothed out in a large volume gas receiver before entering the turbine at a constant pressure. This system gives a fuel consumption which is some 5 per cent lower. For part load operation and starting electrically driven auxiliary blowers are necessary. The arrangement of the constant pressure turbocharged K-GF engine is shown in Figure 4.
L-GF AND L-GB ENGINES
The oil crisis in 1973 and the resulting massive increase in fuel prices stimulated enginebuilders to develop newer engines with reduced specific fuel consumption. B&W’s answer was the L-GF series combining constant pressure turbocharging with an increase in piston stroke: an increase in stroke of about 22 per cent results in a lowering of the shaft speed by around 18 per cent, leading to greater propulsive efficiency when using larger diameter propellers and around 7 per cent increase in thermal efficiency.
The design of the L-GF series engine was heavily based on that of the K-GF and the necessary changes were mainly those relating to the increase in piston stroke and modifications to components to embody thermodynamic improvements (Figure 11). A major component design change was the cylinder liner, made longer for the increased stroke, and featuring cooling bore drillings forming generatrices on a hyperboloid to ensure efficient cooling of the high liner collar without cross borings with high stress concentration factors.
The cylinder covers are of the solid plate type used for the K-GF, while the pistons also are of the original oil cooled type but with
Figure 11 Short and long stroke 67GF engines
somewhat improved cooling caused by the stronger ‘cocktail shaker’ effect resulting from the greater quantity of oil in the elongated piston rod. The design of the crankshaft, crosshead, bearings, and exhaust valve actuating gear are, in the main, similar to the components introduced with the K-GF series.
Figure 12 12L90GFCA engine of 34 800 kW at 97 rev/min
Figure 13 Load diagram for L90GFCA engine
Constant pressure turbocharging
The increased stroke and thus reduced rev/min of L-GF engines compared with K-GF engines was not aimed at developing more power but an improvement in ship propulsive efficiency. The uniflow scavenging system has the advantage of a good separation between air and gas during the scavenging process, and the rotating flow of air along the cylinder contributes to the high scavenging efficiency and clean air charge.
As an engine’s mean indicated pressure increases the amount of exhaust gas energy supplied during the scavenging period, relative to the impulse energy during the blow-down period, constant pressure turbocharging is advantageous; also for uniflow scavenged engines with unsymmetrical exhaust valve timing. Theoretical calculations for improved fuel economy showed a possible gain of 5–7 per cent in specific fuel consumption by using constant pressure turbocharging.
The most obvious change with constant pressure turbocharging is that the exhaust pipes from each valve body led to a common large exhaust gas receiver instead of to the turbochargers as in the impulse system. When the cylinders exhaust into a large receiver the outflow of gas is quicker because the large gas impulse at the commencement of the exhaust period is levelled out in the gas receiver and the outflow of gas will not be retarded, as in the case of impulse turbocharging where a pressure peak is built up in the narrow exhaust pipe before the turbocharger. The opening of the exhaust valve can be delayed about 15, thereby lengthening the expansion stroke and improving the efficiency and reducing the fuel consumption. The energy before the turbine is less than for impulse turbocharging but as the pressure and temperature before the turbine is nearly constant the turbine can be adapted to run at peak efficiency and the blower can supply sufficient air above 50 per cent of engine load. The scavenging air pressure is increased and the compression pressure somewhat decreased compared with impulse turbocharging.
A small auxiliary blower is necessary for satisfactory combustion conditions at loads up to about 50 per cent. Two blowers, of half capacity, are used for safety, and even with one of these out of action the other with half the overall capacity is satisfactory for starting and load increase. The engine will still run at down to 25 per cent load but with a smoky exhaust.
The change from K-GF to L-GF resulted in a 2 per cent improvement in the specific fuel consumption and the lower speed accounts for a 5 per cent improvement in propeller efficiency. Constant pressure turbocharging adds a further 5 per cent to the improvement, resulting in a reduction by 12 per cent in specific fuel consumption between the L-GF and K-GF types. The next engine model of the constant pressure turbocharged type was the L-GFCA, shortly succeeded by the L-GB type.
L-GB TYPE ENGINES
A further improvement in specific fuel consumption was yielded by the L-GB and L-GBE series engines. By using the optimum combination of longer stroke, higher output and higher maximum pressure, and the newest high efficiency turbochargers, much lower fuel consumption rates were achieved. The L-GB series had an mep of 15 bar at the same speed as the L-GFCA to give an increase in power of 15 per cent and an increase in the firing pressure from 89–105 bar. Accordingly, the important Pmax/mep ratio is almost the same but the specific fuel consumption is some 4 g/kWh lower than the L-GFCA series. A further economy rating is obtained with engines of the L-GBE type by holding the Pmax at reduced engine output (so-called ‘derating’, as outlined
Figure 14 L-GB/GBE engine cross-section
in Chapter 5: the practice of offering propulsion engines in both normal and derated versions was adopted by many engine manufacturers, even for four-stroke medium speed engines).
Bedplate and main bearing
The bedplate consists of high, welded longitudinal girders and welded crossgirders with cast steel bearing supports. For the four- and five- cylinder engines the chain drive is placed between the aftermost cylinder and the built-in thrust bearing. For the six- to twelve-cylinder engines the chain drive is placed at the assembling between the fore and aft part. For production reasons, the bedplate can be made in convenient sections. The aft part contains the thrust bearing. The bedplate is made for long, elastic holding-down bolts tightened by hydraulic tool.
The oil pan is made of steel plate and welded to the bedplate parts. The oil pan collects the return oil from the forced lubricating and cooling oil system. For about every third cylinder it is provided with a drain with grid.
The main bearings consist of steel shells lined with whitemetal. The bottom shell can, by means of hydraulic tools for lifting the crankshaft and a hook-spanner, be turned out and in. The shells are fixed with a keep and the long elastic studs tightened by hydraulic tool.
The thrust bearing is of the B&W–Michell type. Primarily, it consists of a steel forged thrust shaft, a bearing support, and segments of cast iron with whitemetal. The thrust shaft is connected to the crankshaft and the intermediate shaft with fitted bolts.
The thrust shaft has a collar for transfer of the ‘thrust’ through the segments to the bedplate. The thrust bearing is closed against the crankcase, and it is provided with a relief valve.
Lubrication of the thrust bearing derives from the system oil of the engine. At the bottom of the bearing there is an oil sump with outlet to the oil pan.
The frame section for the four- and five-cylinder engine consists of one part with the chain drive located aft. The chain drive is closed by the end-frame aft. For six- to twelve-cylinder engines the frame section consists of a fore and an aft part assembled at the chain drive. Each part consists of an upper and a lower frame box, mutually assembled with bolts.
The frame boxes are welded. The upper frame box is on the back of the engine provided with an inspection cover for each cylinder. The lower frame box is on the front of the engine provided with a large hinged door for each cylinder.
The guides are bolted onto the upper frame box and offer possibility for adjustment. The upper frame box is provided on the back side with a relief valve for each cylinder and on the front side with a hinged door per cylinder. A slotted pipe for cooling oil outlet from the piston is suspended in the upper frame box.
The frame section is attached to the bedplate with bolts. The stay bolts consist of two parts assembled with a nut. To prevent transversal oscillations the assembly nut is supported. The stay bolts are tightened hydraulically.
Cylinder frame, cylinder liner and stuffing box
The cylinder frame unit is of cast iron. Together with the cylinder liner (Figure 15) it forms the scavenging air space and the cooling
Figure 15 Components of L-GB cylinder liner
water space. At the chain drive there is an intermediate piece. The stay bolt pipes and the double bottom in the scavenging air space are water cooled. On the front the cylinder frame units are provided with a cleaning cover and inspection cover for scavenging ports. The cylinder frame units are mutually assembled with bolts.
Housings for roller guides, lubricators and gallery brackets are suspended on the cylinder frame unit. Further, the outside part of a telescopic pipe is fixed for supply of piston cooling oil and lubricating oil. At the bottom of the cylinder frame unit there is a piston rod stuffing box. The stuffing box is provided with sealing rings for scavenging air and oil scraper rings preventing oil from coming up into the scavenging air space.
The cylinder liner is made of alloyed cast iron and is suspended in the frame section with a low located flange. The uppermost part of the liner has drillings for cooling water and is surrounded by a cast iron cooling jacket. The cylinder liner has scavenging ports and drillings for cylinder lubrication.
The cylinder cover is made in one piece of forged steel and has drillings for cooling water. It has a central bore for the exhaust valve and bores for fuel valves, safety valve, starting valve and indicator valve (Figure 16).
The cylinder cover is attached to the cylinder frame with studs tightened by a hydraulic ring covering all studs.
Exhaust valve and valve gear
The exhaust valve consists of a valve housing and a valve spindle. The valve housing is of cast iron and arranged for water cooling. The housing is provided with a bottom piece of steel with Stellite welded onto the seat. The spindle is made of heat resistant steel with Stellite welded onto the seat. The housing is provided with a spindle guide. The exhaust valve housing is connected to the cylinder cover with studs and nuts tightened by hydraulic jacks. The exhaust valve is opened hydraulically and closed by a set of helical springs. The hydraulic system consists of a piston pump mounted on the roller guide housing, a high pressure pipe, and a working cylinder on the exhaust valve. The piston pump is activated by a cam on the camshaft.
Cover mounted valves
In the cylinder cover there are three fuel valves, one starting valve, one safety valve, and one indicator valve.
Figure 16 Components of L-GB cylinder cover
The fuel valve opening is controlled by the fuel oil pressure and it is closed by a spring. An automatic vent slide allows circulation of fuel oil through the valve and high pressure pipes and prevents the compression chamber from being filled up with fuel oil in the event of a sticking spindle in a stopped engine. Oil from venting and other drains is led away in a closed system.
The starting valve is opened by control air from the starting air distributor and closed by a spring. The safety valve is spring loaded. The indicator valve is placed near the indicator gear.
The crankshaft for four- and five-cylinder engines is made in one part, and for six- to twelve-cylinder engines it is made in two parts assembled at the chain drive with fitted bolts. The crankshaft is semi-built with forged steel throws.
The crankshaft has in the aft end a flange for assembling with the thrust shaft. The crankshafts are balanced exclusively by borings in the crankpins, though in some cases supplemented by balance weight in the turning wheel.
The connecting rod is of forged steel. It has a Tee-shaped base on which the crank bearing is attached with hydraulic tightened bolts and nuts with Penn-securing. The L90GBE engine has shims placed between the base and the crank bearing, as this engine type needs a smaller compression chamber because of a higher compression ratio. The top is square shaped on which the crosshead bearings are attached with hydraulic tightened studs and nuts with Penn-securing. The bearing parts are mutually assembled with bolts and nuts tightened by hydraulic jacks.
The lubrication of the crank bearing takes place through a central drilling in the connecting rod.
The crank bearing is of steel cast in two parts and lined with whitemetal. The bearing clearance is adjusted with shims. The crosshead bearings are of cast steel in two parts and provided with bearing shells.
The piston consists of piston crown, piston skirt and cooling insert for oil cooling (Figure 17). The piston crown is made of heat-resisting steel and is provided with five ring grooves which are hard-chrome plated on both lands. The piston skirt is of cast iron. The piston rings are right- and left-angle cut and of the same height.
The piston rod is of forged steel. It is fixed to the crosshead with a hydraulic tightened stud. The piston rod has a central bore which, in connection with a cooling oil pipe and the cooling insert, forms inlet and outlet for cooling oil.
The crosshead is of forged steel and is provided with steel cast guide shoes with whitemetal on the running surfaces. A bracket for oil inlet from the telescopic pipe and a bracket for oil outlet to slit pipe are mounted on the crosshead.
Figure 17 Components of L-GB piston
Fuel pump and fuel oil high pressure pipes
The fuel pump consists of a pump housing of nodular cast iron and a centrally placed pump cylinder of steel with sleeve and plunger of nitrated steel. The plunger has an oblique injection edge which will automatically give an optimum fuel injection timing. There is one pump for each cylinder. In order to prevent fuel oil from being mixed into the separate lubricating system on the camshaft the pump is provided with a sealing device.
Figure 18 Components of L-GB connecting rod and crosshead
The pump gear is activated by the fuel cam, and the injected volume is controlled by turning the plunger by a toothed bar connected to the regulation mechanism. Adjustment of the pump lead is made with shims between top cover and pump housing.
The fuel pump is provided with a pneumatic lifting device: this can, during normal operation and during turning, lift the roller guide roller free of the cam.
The fuel oil high pressure pipes have protecting hoses. The fuel oil system is provided with a device which, through the pneumatic lifting tool, disconnects the pump in case of leakage from the high pressure pipes.
Camshaft and cams
The camshaft is divided into sections for each cylinder. The individual sections consist of a shaft piece with one exhaust cam, one fuel cam, one indicator cam, and coupling parts. The exhaust and fuel cams are of steel with a hardened roller race, and are shrunk on the shaft. They can be adjusted and dismounted hydraulically.
The indicator cams, which are of cast iron, are bolted onto the shaft. The coupling parts are shrunk on the shaft and can be adjusted and dismounted hydraulically.
The camshaft is located in the housing for the roller guide. The camshaft bearings consist of two mutually interchangeable bearing shells, which are mounted in hydraulically tightened casings.
Chain drive and reversing
The camshaft is driven from the crankshaft by two 41/2-inch chains. The chain drive is provided with a chain tightener and guidebars support the long chain strands. The camshaft is provided with a hydraulically actuated reversing gear turning the camshaft to the position corresponding to the direction of rotation of the crankshaft.
Starting air distributor, governor and cylinder lubricators are driven by separate chain from the intermediate wheel.
Four-, five- and six-cylinder engines are prepared for moment compensators, which can be fixed to the fore and aft ends of the frame section and are driven by the camshaft through flexible couplings. The moment compensator will reduce the second order external moments to a level between a quarter of the original figure and zero.
The engine rev/min is controlled by a hydraulic governor. For amplification of the governor’s signal to the fuel pump there is a hydraulic amplifier. The hydraulic pressure for the amplifier is delivered by the camshaft lubricating oil system.
The cylinder lubricators are mounted on the cylinder frame, one per cylinder, and interconnected with shaft pieces. The lubricators have built-in adjustment of the oil quantity. They are of the ‘Sight Feed Lubricator’ type and each lubricating point has a glass. The oil is led to the lubricator through a pipe system from an elevated tank. A heating element rated at 75 watt is built into the lubricator.
Manoeuvring system (without bridge control)
The engine is provided with a pneumatic manoeuvring and fuel oil regulating system which transmits orders from the separate manoeuvring console to the engine.
By means of the regulating system it is possible to start, stop, reverse and control the engine. The speed control handle in the manoeuvring console activates a control valve which gives a pneumatic speed-setting signal to the governor dependent on the desired number of revolutions. The start and stop functions are controlled pneumatically. At a shutdown function the fuel pumps are moved to zero position independent of the speed control handle.
Reversing of the engine is controlled pneumatically through the engine telegraph and is effected via the telegraph handle.
Reversing takes place by moving the telegraph handle from ‘Ahead’ to ‘Astern’ and by moving the speed control handle from ‘stop’ to start’ position. Control air then moves the starting air distributor and, through the pressurizer, the reversing mechanism to the ‘Astern’ position.
Turning gear and turning wheel
The turning wheel has cylindrical teeth and is fitted to the thrust shaft; it is driven by a pinion on the terminal shaft of the turning gear which is mounted on the bedplate. The turning gear is driven by an electric motor with built-in gear and brake. Further, the gear is provided with a blocking device that prevents the main engine from starting when the turning gear is engaged. Engagement and disengagement of the turning gear is executed by axial transfer of the pinion.
The engine is provided with gallery brackets placed at such a height that the best possible overhaul and inspection conditions are obtained. The main pipes of the engine are suspended in the gallery brackets. A crane beam is placed on the brackets below centre gallery manoeuvring side.
Scavenging air system
The air intake to the turbocharger takes place directly from the engineroom through the intake silencer of the turbocharger. From the turbocharger the air is led via charging air pipe, air cooler and scavenging air pipe to the scavenging ports of the cylinder liner. The charging air pipe between turbocharger and air cooler is provided with a compensator and insulated.
The engine is as standard arranged with MAN or BBC turbochargers. The turbochargers are provided with a connection for Disatac electronic tachometers, and prepared for signal equipment, to indicate excessive vibration of the turbochargers. For water cleaning of the turbine blades and the nozzle ring during operation, the engine is provided with connecting branches on the exhaust receiver in front of the protection grid.
Exhaust gas system
From the exhaust valves the gas is led to the exhaust gas receiver where the fluctuating pressure will be equalized and the gas led further on to the turbochargers with a constant pressure. After the turbochargers, the gas is led through an outlet pipe and out in the exhaust pipe system.
The exhaust gas receiver is made in one piece for every cylinder and connected to compensators. Between the receiver and the exhaust valves and between the receiver and the turbocharger there are also inserted compensators.
For quick assembling and dismantling of the joints between the exhaust gas receiver and the exhaust valves, a clamping band is fitted. The exhaust gas receiver and exhaust pipe are provided with insulation covered by a galvanized steel plate.
Between the exhaust gas receiver and each turbocharger there is a protection grid.
The engine is provided with two electrically driven blowers which are mounted in each end of the scavenging air receiver as standard. The suction sides of the blowers are connected to the pipes from the air coolers, and the non-return valves on the top of the outlet pipes from the air coolers are closed as long as the auxiliary blowers can give a supplement to the scavenging air pressure.
The auxiliary blowers will start operating before the engine is started and will ensure complete scavenging of the cylinders in the starting phase, which gives the best conditions for a safe start.
During operation of the engine the auxiliary blowers will start automatically every time the engine load is reduced to about 30–40 per cent, and they will continue operating until the load is again increased to over approximately 40–50 per cent.
In cases when one of the auxiliary blowers is out of service, the other auxiliary blower will automatically function correctly in the system, without any manual readjustment of the valves being necessary. This is obtained by automatically working non-return valves in the suction pipe of the blowers.
Starting air system
The starting air system contains a main starting valve (two ball valves with actuators), a non-return valve, a starting air distributor and starting valves. The main starting valve is combined with the manoeuvring system which controls start and ‘slow turning’ of the engine. The ‘slow turning’ function is actuated manually from the manoeuvring stand.
The starting air distributor regulates the control air to the starting valves so that these supply the engine with starting air in the firing order.
The starting air distributor has one set of starting cams for ‘Ahead’ and ‘Astern’ respectively, and one control valve for each cylinder.