轮机英语课文

LESSON 3
DIESEL ENGINE CONSTRUCTION I
Pistons and Piston Rods
A piston consists of a lower part--piston skirt of cast iron--and a upper part or crown made of a special heat-resistant steel. They are bolted together and fixed to the piston rod. The position of the piston parts in relation to each other is secured by means of machine-turned recesses and a dowel pin in the crown. Each piston is provided with five or six piston rings fitted in chromium plated grooves in the crown. The two or three topmost rings are narrow rings having diagonal cuts, while the next two are broad and are provided with overlapping end joints. The lowest ring is an oil distributor ring. All the rings are slightly rounded on the external top and bottom edges to keep the oil film on the cylinder liner during the running-in period for new piston rings.
In order to control thermal stresses, thin-wall intensively cooled pistons are used for some types of modern engines. In this case, the piston has an internal insert in the piston crown, which serves only to direct the cooling liquid flow and, thereby, to intensify the conventional 'cocktail shaker' effect.
The piston rods are bored from the top flange to a point opposite the centre of the crosshead. Through this bore, a long pipe is inserted which goes nearly to the bottom of the bore. The outside diameter of the pipe is less than the diameter of the bore, the result being that an annular space is formed between the piston rod and the pipe.
The lower end of the piston rod is reduced in diameter to fit a bore in the crosshead, this being secured to the piston rod by a nut. The piston rods are provided with dowel pins to ensure the correct assembly of the components.
On each of the two journals of the crosshead, crosshead shoes are mounted which are guided in the built-in crosshead guides of the engine frame. The position of the crosshead shoes on the crosshead is determined by dowel pins. The guide shoes are secured to the crosshead by means of tap-bolts.
The crosshead shoes are white metal lined, grooves being cut horizontally in the face of the white metal to ensure an adequate supply of lubricating oil.
The crosshead is short and rigid and the bearings are so constructed that the bearing pressure between the journal and bearing is distributed evenly over the entire length of the bearing. In order to improve the working conditions of the bearings, the bearing pressure is made smaller and the peripheral speed higher in later designs.
The pistons are cooled by oil supplied from the forced lubricating system. The cooling oil is admitted through the pipe, from which the oil is led to the cooling spaces of the piston through telescopic pipes or articulated pipes and round the internal pipes of the piston rods. The cooling oil is conducted from the pistons through the internal pipes of the piston rods and ducts in the crossheads, as well as the slotted pipes from which the oil is led to ‘control boxes’with sight glasses on the engine.
Sealing between the crankcase and scavenging air boxes is achieved by means of the piston rod stuffing boxes, located in the bottom of the scavenging air boxes. Each stuffing box is provided with two sealing rings and three scraper rings. The sealing rings, which are mounted
uppermost, are in four parts and pressed together around the piston rods by means of coil springs.
Connecting Rod and Main Bearings
The crosshead and crankpin bearings are made of cast steel and each consist of an upper and lower part secured to the connecting rod by means of fitting bolts, the nuts of which are secured. Compression shims are provided between the bottom end bearing and the palm of the connecting rod.
The bearings have white metal bearing surfaces provided with lubrication grooves for the necessary supply of lubricating and cooling oil. Shims of different thicknesses are inserted between the shells to enable adjustment of the bearing clearances (approx. 0.20-0.30 mm).
The main bearings, which support the crankshaft, each consist of two shells of cast steel with white metal bearing surfaces having lubricating grooves of an appropriate shape. The shells are secured by means of bearing caps and studs, the necessary clearance (approx. 0.30 mm) being provided by means of shims of different thicknesses.
All bearings are lubricated from the force-feed lubrication system of the engine, the oil bearing supplied partly through pipes to each main bearing cap and partly to the crosshead, from where the lubricating oil flows through channels to the crankpin bearings.
Crankshaft
The crankshafts are built up types. For the semi-built type, the forged main bearing journals are shrunk into cast steel crank throws with a crank journal and two crank throws as one unit. In the fully built type, the crank throws are shrunk on to main journals and crank journals. The crank throws are displaced from each other to obtain the same number of degrees between the different crank throws. The crankpin journals are provided with large bores for balancing purpose.
READING MATERIAL
A.MAN MARINE DIESEL ENGINE
The latest large bore two stroke MAN engine is the KSZ 90/160B series in cylinder number from 6 to 12 and developing 2,700 kW (3,672 HP) /cyl. The engine operates at 122 r/min, with an M.E.P. of 13 bar and a mean piston speed of 6. 5 m/s.
The bedplate is of fabricated design and consists of two I-shaped longitudinal girders with transverse members into which the cast steel bearing housings are welded. The thrust bearing casting is incorporated into the bedplate.
Columns have been replaced by frames constructed of box shaped longitudinal girders. The fabricated top and bottom sections run the full length of the engine and are mounted on the bedplate. Such an engine frame helps to stiffen the engine, and so protect the bearings from the deformation forces transmitted via the ship's double bottom. The cast iron guide ways for the crosshead are attached to the top section and seal off the crankcase.
The cylinder jackets are individual castings bolted together to form one continuous member. The one piece cylinder liners are fitted into the jackets from the top and cast steel strong back is fitted in way of the combustion space to help absorb gas forces. The scavenge process is
the MAN loop system and the scavenge and exhaust ports are arranged in two tiers and are carried far around the liner. The vertical lands between the ports are cooled by water passing through cast in tubes and hence to passages arranged around the liner above the exhaust ports.
Each cylinder cover is in two parts, the lower section is of special thin walled forged steel permitting the area in way of the combustion gases to be intensively cooled whilst absorbing thermal stresses and this is held in position by an upper supporting unit of cast iron, which transmits the gas forces to the cylinder jackets via long studs. The joint face between the cylinder cover and the liner is arranged as far away from the combustion process as possible. The gas leads on the engine structure are constrained by means of hydraulic preloaded tie rods which connect the bedplate, frame sections and cylinder jackets into a single unit.
The crankshaft is of the semi built type, with the journal being shrunk into the forged or cast steel crank throws. With the exception of the six-cylinder engine, it is in two parts joined by fitted bolts. The coupling flange for attachment to the propeller shaft also accommodates the turning gear and is integrally forged with the crankshaft. The aft end section also incorporates a thrust collar. The drive for the camshaft gear train is taken from a spur gear which for a six cylinder engine is at the coupling end and for 7 to 12 cylinder engines the drive is located in the middle of the engine.
The piston crown is of forged steel and is designed for intensive cooling; the central supporting body is of cast steel and is mainly subjected to compressive stress. The two components are bolted together and connected to the piston rod. A single piece piston skirt fitted with bronze rings guides the piston in the cylinder liner. The piston ring grooves for a standard engine are flame hardened against wear.
The crosshead pin bearings are white metal lined shells lubricated by means of high-pressure pumps. The piston forces are transmitted over the entire length of the crosshead pin then to the connecting rod via the lower half single bearing shell. The principle of hydrostatic lubrication has been applied to the crosshead using high-pressure lubricators. This is achieved by means of high pressure pumps with two plungers and rods operated by the oscillating movement of the connoting rod, high pressure oil is delivered to the lower part of the crosshead bearing, the pumps press oil into the bearing gap when the load on the shell is at its lowest. This ensures that the oil film is maintained at all times and on all loads between the journal and the shell.
Scavenging of the cylinder liner is on the loop scavenge system and constant pressure of the exhaust gas is used to drive the turbochargers. During running up and in the lower partial load range the turbochargers are assisted by electrically driven auxiliary blowers connected before the compressors of the main blowers. In the upper power range the auxiliary blowers are disconnected. Diffusers are fitted in the exhaust system and facilitate charge removal and scavenging in each cylinder. The diffuser also prevents pressure pulses from one cylinder interfering with another during operation.
B.MAN-B&W MC ENGINE
MAN-B & W K 90 MC-C engine is a large crosshead type two-stroke engine with a bore of 900 mm, a 2,300 mm stroke and an operating speed of 104 r/min. It is constructed with
between six and twelve cylinders.
Developed as one of the extensive range of the manufacturer's MC engines, it is of the power and speed best suited to large, fast container ships. The increase in running speed is obtained by a slight decrease in engine stroke. High thermal efficiency is maintained by an increase in mean effective pressure (M.E.P.).
Construction can be considered generally as typical for the whole range. The engine bedplate is of rigid box form, fabricated from steel plates with main bearing supports of cast steel. Welded ’A’ frames are assembled into a frame box which contains the crankcase, the crosshead guides and also supports the wheels for the chain drive of the camshaft. A cast iron cylinder frame accommodates the scavenge space between the cylinder jacket and the diaphragm, both of which are water-cooled. Long pre-stressed tie bolts are fitted between the top of the frame and the underside of the bedplate girders.
The cylinder liner is of alloy cast iron, its upper flange lands on top of the frame and has bore cooling. It is secured by a forged steel cylinder cover which is also bore cooled and is shaped internally to accommodate most of the combustion space. Cylinder lubricating oil is injected at one level in the liner. Pistons have a chrome-molybdenum alloy steel crown with hard chrome-surfaced ring grooves in which four compression rings are fitted. In this particular model a protective layer of Inconel is welded to part of the crown surface to prevent high temperature corrosion. The piston is oil cooled, oil being supplied by a telescopic gland to the crosshead and then through the piston rod. It is returned from the crosshead to a slotted pipe in the crankcase. A short cast iron skirt is added. The crown is bolted to the piston rod at an inner support ring.
Surface hardening reduces wear on the piston rod at the diaphragm gland. The rod is bolted at the top of a cylindrical crosshead which is of large diameter and incorporates a full length bottom half bearing shell. Floating guide shoes are attached at each end.
The crankshaft may he either semi-built up or of welded construction, with large journals and pins. All crankcase bearings are of white metal. Main bearings have thick shells, crankpin (bottom end) and crosshead (top end) bearing have thin-wall shells. White metal is used for the guide surfaces. The exhaust valves are operated hydraulically under oil pressure from cam-timed actuated pistons. They have air compressed springs which allows them to be rotated by vanes. The valve spindles are usually manufactured by the hot isostatic pressure (HIP) method, a compound Nimonic and austenitic steel part construction. Valve housing is cooled at its seat and spindle guide bush but its upper duct is uncooled to avoid low temperature corrosion.
Fuel pumps are cam driven and timed by the plunger helix. An adjustable barrel allows variable ignition timing to maintain combustion efficiency at low speed and can be adjusted to match the ignition quality of fuels. Pump timing is changed for astern operation by a link connected at the cam roller guide which is activated by compressed air. Each pump supplies three identical fuel injectors for the corresponding unit. Injectors are uncooled but they circulate hot fuel oil directly while their needle valves are in the closed position.
The engine operates with a constant pressure system, with uncooled turbochargers. Two auxiliary blowers are fitted to operate at low charge air pressure or at low engine revolutions. A number of waste heat recovery and power take-off systems can be operated under running conditions.
C. VIBRATION DAMPER
The vibration damper serves to reduce the torsional vibration generated by the crankshaft. The damper consists of a housing and a free rotating inertia ring supported by an axial bearing. It forms a totally enclosed unit. The free space between inertia ring and damper housing is filled with a high viscous fluid. When the crankshaft rotates at constant speed, the inertia ring will follow this movement. The combustion pressure exerted on each piston generates vibration in the crankshaft, and each variation generated by such a vibration will affect the movement of the inertia ring. The resulting shift of the ring to the damper housing is opposed by the viscous fluid and the vibration will be reduced.
The energy, absorbed during the process, is converted into heat and then radiated to the surrounding air. Hence, proper ventilation of the damper is essential.
Maintenance
The viscous damper fluid is during engine operation subjected to ageing and the fluid becomes more viscous. In sending fluid samples on a regular base to the damper manufacturer the rate in increase of the viscosity can be determined. In the analysis the quality of the fluid is mentioned and a forecast of the number of running hours still to go is given. A sampling interval time is 6,000 hours advised. Dampers with a too high fluid viscosity may cause crankshaft breakdown.
The damper cover has two filling holes located 180o opposite each other and closed by screw-in filler plugs. If one of these plugs is accessible, a fluid sample container can be screwed in and a sample taken with the damper in place. On a spare damper, sampling must be effected with the damper in the vertical position. For liquid sampling, a special kit can be ordered from Stork-Wartsila Diesel.。