Lesson Three Text Diesel engine construction (II) 1.Pistons and Pisto
n rodsA piston consists of a lower part—piston skirt(活塞裙) of cast
iron and an upper part or crown(活塞头) made of a special heat-resi
stant steel. They are bolted together and fixed to the piston rod
. Piston crownPiston skirtPiston rodPiston ringThe position of th
e piston parts in relation to each other is secured by means of m
achine-turned recesses(凹槽) and a dowel pin(定位销) in the crown.five
piston rings fitted in chromium plated grooves in the crown. Cas
t iron carrier rings(铸铁镶圈、承磨环), caulked into(嵌入) machined grooves
, are provided for all the piston rings. Cast iron carrier ringsE
ach piston is provided with six or five piston rings. Each piston
is provided with six orThe three topmost rings are narrow(窄的) ri
ngs having diagonal cuts(斜切口), while the next two rings are broad
(宽的) and provided with overlapping end joints(搭接式接口). The lowest
ring is an oil distributor ring(配油环). Diagonal cutsOverlapping en
d jointAll the rings are slightly rounded on their external top a
nd bottom edges to keep the oil film (油膜) on the cylinder liner d
uring the running-in period(跑合期、磨合期) for new piston rings. Extern
al topExternal bottomIn order to control thermal stresses(热应力), t
hin-wall intensively cooled pistons(薄壁强冷式活塞) are used for some ty
pes of modern engines. In this case, the piston has an internal i
nsert in the piston crown, which serves only to direct the coolin
g liquid flow and, thereby, to intensify the conventional ‘cockta
il shaker(振荡冷却)'' effect.The piston rods are bored(钻孔) from the to
p flange to a point opposite the center of the crosshead(十字头). Th
rough this bore, a long pipe is inserted which goes nearly to the
bottom of the bore. PipeThe outside diameter of the pipe is less
than the diameter of the bore, the result being that an annular(
环形的) space is formed between the piston rod and the pipe.Annular
spaceThe lower end of each piston rod is reduced in diameter to f
it 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.Piston rodCrossheadOn each
of the two journals(轴颈) of the crosshead, crosshead shoes(十字头滑板)
are mounted which are guided in the built-in(内装的) crosshead guid
es(十字头导板) of the engine frame. CrossheadCrosshead shoeEngine fram
eCrosshead guideThe position of the crosshead shoes on the crossh
ead is determined by dowel pins. The guide shoes are secured to t
he crosshead by means of tap-bolts(锥体螺钉).The crosshead shoes are
white metal lined(白合金衬), grooves being cut horizontally in the fa
ce of the white metal to ensure an adequate supply of lubricating
oil.The crosshead is short and rigid and the bearings are so con
structed that the bearing pressure between the journal and bearin
g is distributed evenly over the entire length of the bearing. In
order to improve the working conditions of the bearings, the bea
ring pressure is made smaller and the peripheral(圆周的) speed(is ma
de)higher in later designs.The pistons are cooled by oil supplied
from the forced lubrication system(强制润滑系统). The cooling oil is a
dmitted through the pipe, from which the oil is led to the coolin
g spaces of the piston through telescopic pipes(伸缩管) or articulat
ed pipe, and round the internal pipes of the piston rods.The cool
ing oil is conducted from the piston through the internal pipes o
f the piston rods and the ducts(管、道) in the crossheads, as well a
s the slotted pipes from which the oil is led to “control boxes”
with sight glasses(观察镜) on the engine.Sealing between the crankca
se and scavenging air boxes is achieved by means of the piston ro
d stuffing boxes(填料箱、填料函), located in the bottom of the scavengin
g air boxes. Each stuffing is provided with two sealing rings(密封环
) and three scraper rings(刮油环). The sealing rings, which are moun
ted uppermost are in four parts and pressed together around the p
iston rods by means of coil springs(盘簧).Connecting Rod and Main B
earings The crosshead and crankpin bearings (曲柄销轴承) are made of c
ast steel and each consist of an upper and a lower part secured t
o the connecting rod by means of fitted bolts(紧配螺栓), the nut of w
hich are secured.The bearings have white metal bearing surfaces p
rovided with lubrication grooves for the necessary supply of lubr
icating and cooling oil. Shims(垫片) of different thickness are ins
erted between the shells to enable adjustment of the bearing clea
rances (approx. 0.20-0.30mm).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 appropria
te shape. The shells are secured by means of bearing caps and stu
ds, the necessary clearance (approx. 0.30mm) being provided by me
ans of shims of different thickness.All bearings are lubricated f
rom the force-feed lubrication system of the engine, the oil bein
g supplied partly through pipes to each main bearing cap and part
ly to the crosshead, from where the lubricating oil flows through
channels to the crankpin bearings.3. CrankshaftFlywheel飞轮曲柄销Cra
nkpin曲柄臂Crank throw(web)Main bearing journal主轴颈The crankshafts ar
e built up(assembled) types(组/套合式). For semi-built type(半套合式), th
e forged main bearing journals being shrunk(红套) into cast steel c
rank throws(曲柄、曲拐) with a crank journal and two crank throws as o
ne unit. In the fully built type(全套合式), the crank throws are shru
nk on to main journals(主轴颈) and crank journals(曲柄销轴颈). The crank
throws are displaced from (错位) each other to obtain the same numb
er of degrees between the different crank throws. The crankpin jo
urnals (曲柄销轴颈) are provided with large bores for balancing purpos
e.Reading MaterialA. M.A.N. Marine diesel engineThe latest large
bore two stroke M.A.N. engine is the KSZ 90/160B series in cylin
der numbers from 6 to 12 and developing 2,700 K.W. (3,672H.P.)/cy
l.The engine operates at 122 rev/min. with an M.E.P.(mean effecti
ve pressure平均有效压力) of 13 bar and a mean piston speed of 6.5m/sec.
The bedplate is of fabricated design and consists of two I-shaped
longitudinal girders(纵梁) with transverse members(横梁) into which
the cast steel bearing housings are welded. The thrust bearing ca
sing is incorporated(合成一体) into the bedplate. Columns(机架) have be
en replaced by frames constructed of box shaped longitudinal gird
ers. The fabricated top and bottom sections run(贯穿) the full leng
th of the engine and are mounted on the bedplate. Such an engine
frame helps to stiffen(加强) the engine and so protect the bearings
from the deformation forces transmitted via the ship’s double bo
ttom. The cast iron guideways for the crosshead are attached to t
he top section and seal off the crankcase. The cylinder jackets a
re individual castings(铸件) bolted together to form one continuous
member. The one-piece cylinder liners are fitted into the jacket
s from the top and a cast steel strong back is fitted in way of(在
…部位) the combustion space to help absorb gas forces.The scavenge
process is the M.A.N. loop system(回流扫气系统) and the scavenge and ex
haust ports are arranged in two tiers(行) and are carried far arou
nd the liner. The vertical lands between the ports are cooled by
water passing through cast in tubes and thence to passages arran
ged around the liner above the exhaust ports. Each cylinder cover
is in two parts, the lower section is of special thin walled for
ged steel(锻钢) permitting the area in way of the combustion gases
to be intensively cooled whilst absorbing thermal stressesand thi
s is held in position by an upper supporting unit of cast iron, w
hich transmits the gas forces to the cylinder jackets via long st
uds.The joint face between the cylinder cover and the liner is ar
ranged as far away from the combustion process as possible. The g
as loads on the engine structure are constrained(约束) by means of
hydraulically preloaded(预置载荷) tie rods which connect the bedplate
, frame sections and cylinder jackets into a single unit. The cra
nkshaft is of the semi built type(半总成式), with the journal being s
hrunk(红套) into the forged or cast steel crank throws(曲柄臂). With t
he exception of the six-cylinder engine, it(crankshaft) is in two
parts joined by fitted bolts. The coupling flange(联接法兰) for atta
chment to the propeller shaft also accommodates the turning gear(
盘车机) and is integrally forged with the crankshaft.The aft end sec
tion also incorporates a thrust collar(推力环). The drive for the ca
mshaft gear train is taken from a spur gear(正齿轮) which for a six
cylinder engine is at the coupling end and for 7 to 12 cylinder e
ngines the drive is located in the middle of the engine. The pi
ston crown is of forged steel and is designed for intensive cooli
ng, the central supporting body is of cast steel and is mainly su
bjected to(承受) compressive stress. The two components are bolted
together and connected to the piston rod. A single piece piston s
kirt fitted with bronze(青铜) rings guides the piston in the cylind
er liner. The piston ring grooves for a standard engine are flame
hardened(淬火，硬化) against wear. The crosshead pin bearings(十字头销) a
re white metal lined shells lubricated by means of high-pressure
pumps. The piston forces are transmitted over the entire length o
f the crosshead pin then to the connecting rod via the lower half
single bearing shell. The principle of hydrostatic lubrication h
as been applied to the crosshead using high-pressure lubricators.
This is achieved by means of high pressure pumps with two plunge
rs and rods operated by the oscillating(振荡) movement of the conne
cting rod, high pressure oil is delivered to the lower part of th
e crosshead bearing, the pumps press oil into the bearing gap whe
n the load on the shell is at its lowest. This ensures that the o
il film is maintained at all times and on all loads between the j
ournal 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(起动) and in the
lower partial load range the turbochargers are assisted by elect
rically driven auxiliary blowers connected before the compressors
of the main blowers.In the upper power range the auxiliary blowe
rs are disconnected. Diffusers(扩压器) are fitted in the exhaust sys
tem and facilitate(有利于) charge removal and scavenging in each cyl
inder. The diffuser also prevents pressure pulses from one cylind
er interfering with another during operation. MAN-B&W MC Eng
ine MAN-B & W K 90 MC-C engine is a large crosshead typ
e two-stroke engine with a bore of 900 mm, a 2,300 mm st
roke and an operating speed of 104 r/min. It is constructe
d 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 su
ited to large, fast container ships. The incr
ease in running speed is obtained by a slight decrease in engine
stroke. High thermal efficiency is maintained by an increase in m
ean effective pressure (M.E.P.). Construction can be consider
ed generally as typical for the whole range. The engine bedplat
e is of rigid box form, fabricated from steel plates with m
ain bearing supports of cast steel. Welded ‘A’ fr
ames are assembled into a frame box which contains the c
rankcase, the crosshead guides and also supports the wheels
for the chain drive of the camshaft. A cast ir
on cylinder frame accommodates the scavenge space between the cyl
inder jacket and the diaphragm（隔板）, both of which are water-coole
d. Long pre-stressed tie bolts are fitted between the top of the
frame and the underside of the bedplate girders. The cylin
der liner is of alloy cast iron, its upper flange（凸缘） 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 combust
ion space. Cylinder lubricating oil is injected
at one level in the liner. Pistons have a chrome-molybde
num（铬钼） alloy steel crown with hard chrome-surfaced ring gr
ooves in which four compression rings are fitted.
In this particular model a protective layer of Inco
nel（铬镍铁） is welded to part of the crown surface to preven
t 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 fro
m 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（淬火） reduces wear on the pis
ton rod at the diaphragm gland. The rod is bolted at the top of a
cylindrical crosshead which is of large diameter and incorporate
s a full length bottom half bearing shell. Floating guide shoes a
re attached at each end. The crankshaft may he either semi-buil
t 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 e
nd) bearing have thin-wall（薄壁） shells. White metal is
used for the guide surfaces. The exhaust valves are oper
ated hydraulically under oil pressure from cam-timed actu
ated pistons. They have air compressed springs which
allows them to be rotated by vanes. The valve
spindles are usually manufactured by the hot isostatic pre
ssure (HIP) method（热均压法）, a compound Nimonic（镍铬钛合金） and austeni
tic（奥式体） steel part construction. Valve housing is cooled
at its seat and spindle guide bush but its upper duct is u
ncooled to avoid low temperature corrosion. Fuel pum
ps are cam driven and timed by the plunger helix（螺旋面）.
An adjustable barrel allows variable ignition timing
to maintain combustion efficiency at low speed and can be adjuste
d to match the ignition quality of fuels. Pump timing i
s changed for astern operation by a link connected at t
he cam roller guide which is activated by compressed air. Each
pump supplies three identical fuel injectors for the corres
ponding 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 consta
nt pressure system, with uncooled turbochargers. Two a
uxiliary blowers are fitted to operate at low charge air pressure
or at low engine revolutions. A number of waste heat recovery（废热
回收） and power take-off systems（动力引出装置） can be operated under runn
ing conditions. C. VIBRATION DAMPER 减 振 器
The vibration damper serves to reduce the torsional vibratio
n（扭振） generated by the crankshaft. The damper consists of a hous
ing and a free rotating inertia ring （惯性环）supported by
an axial bearing. It forms a totally enclosed unit. T
he free space between inertia ring and damper housing is filled w
ith 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 vibratio
n in the crankshaft, and each variation generated by such a v
ibration will affect the movement of the inertia ring.
The resulting shift of the ring to the damper hous
ing is opposed by the viscous fluid and the vibration w
ill be reduced. The energy, absorbed during the process, is converted into heat and then radiated to the surrounding air. Hence, proper ventilation（通风） of the damper is essential. Maintenance The viscous damper fluid is during engine operation subjected to ageing （老化，变质）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 （粘度）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（取样） 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. |
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