Bai Giang Chi Tiet 1.2

8/10/2019 Bai Giang Chi Tiet 1.2

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CHAPTER 2

STATIC PARTS CONSTRUCTION OF DIESEL ENGINES

2.1 BEDPLATE

2.1.1. General construction

- Bedplate is subjected to the entire weight of the engine, receives the combustion

gases pressure and action forces of the moving parts of the engine. Therefore, the bedplate

must be constructed with the sufficient strength and rigidity to resist the bending in the

longitudinal direction.

- The lower part (bottom) of the bedplate forms a basin to contain or to collect

circulating lubricating oil of the engine. It is also called crankcase. The crankcase of the

engine is made either of a single forging, or of separate construction and then is welded to the

lower side of the bedplate.

The bottom of the crankcase is usually slopewise arranged to the flywheel side or the

middle of the engine (it is convenient for flowing of the lubricating oil). When the engine is

working the big end of the connecting rod must not be reached to the lubricating oil surface.

- The bedplate and framework are fastened to each other by long bolts.

- The bedplate is secured to the ships structure by bolts.

2.1.2. The bedplate of low-speed large engines

- The bedplate of the large engines is normally made of welding steel plates. It allows

to reduce the weight and to increase rigidity of the bedplate.

Figure 2.1 is an illustration of the bedplate construction of large engines.

Figure 2.1 Bedplate construction of a low-speed large engine

- Basically, the bedplate consists of longitudinal girders, transverse girders (cross-

girders) and strengthening bars.

The transverse girders strengthen the rigidity of the bedplate in transversal direction.

The center of the girders is formed the saddle to support the main bearings of the engine.


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These transverse girders, together with partitions divide the bedplate into spaces

corresponding to the number of the cylinder. The lower part of the partitions is made a hole to

circulate the lubricating oil in the crankcase.

Figure 2.2 shows the transverse partition of the large engines.

Figure 2.2 Construction of the transverse partition

- In multi-cylinder diesel engines with long axial dimension, the bedplate is normally

built up of separate sections, which are secured to each other by bolts.

2.1.3. The bedplate of high and medium-speed engines

For high and medium-speed, small engines the bedplate is generally made of cast iron

or cast steel (figure 2.3)

Figure 2.3 Bedplate of high and medium-speed, small engines


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2.2 MAIN BEARINGS

The main bearings are placed on the transverse girders of the bedplate. Each main

bearing consist of a bearing cap and two bearing shells (bearing liners).

Figure 2.4 Main bearing

1Crankshaft; 2Transversegirdersof the bedplate;3Bearing shell; 4Bearing cap; 5 - Bolt

2.2.1. Main bearing shells

- The bearing shell has a form of a cylindrical half. Inside of the bearing shells iscovered by a thin anti-corrosion layer (white metal or copper-lead alloy, figure 2.5).

Figure 2.5 Main bearing shell

1Upper shell; 2Lower shell

- The upper shell is drilled one or some holes, these holes connect to the annulargroove inside the shell for purpose of distribution the lubricating oil on the shell surface.


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- Edges of the bearing shells are made with a shallow cavity to form oil wedge when

the crankshaft rotating.

- To prevent the bearing shells from rotating and longitudinally shifting, the lower

shell is provided with a pin, which fits in the hole on the bedplate. In some cases, edge of the

lower or upper shell is made with a convex claw on the back that fits in the hollow of the

bedplate or the bearing cap.

- There are two kinds of bearing shell: thin wall and thick wall type

+ Thin wall bearing shells has a thickness of 3 ~ 5 mm. This type of bearing

shells are made of steel, interior is anti-friction metal, next layers are nickel and zinc,

outside surface is covered by a thin anti-corrosion layer (figure 2.6).

Thin wall bearing shells are normally used for high-speed engines.

Figure 2.6 Construction of thin wall bearing shell

+ Thick wall bearing shells has a thickness of 7 ~ 15 mm. They are made ofcast iron, the surface is cast white metal or other soft metal. On the assembling

surfaces between the two bearing shells are placed thin shims to adjust the oil

clearance when necessary.

Thick wall bearing shells are generally used for low-speed engines. However,

in recent years the type of thin wall bearing shell is commonly used for all of the

engines.

2.2.2. Main bearing caps

- Bearing caps are used for pressing bearing shells on the bedplate and to ensure the

contact without clearance between the two bearing shell in all working condition of theengine.

- To secure the main bearing caps on the bedplate, they can use bolts (figure 2.7a) or

jacks, in this case, two ends of the jack press on the bearing cap and engine frame (figure

2.7b).


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a) b)

Figure 2.7 Fastening between bearing caps and bedplate by bolts (a) and jack (b)

- The main bearing caps are normally made of cast iron, their cross section has a form

of rectangle or letter I. The main bearing caps are drilled the centripetal groove to fed the

lubricating oil to the bearing shells and journals (figure 2.8).

Figure 2.8 Oil groove for lubricating journal and bearing shell

For high-speed, small engines without bedplate, the underslung main bearings

construction is used. In this case, the main bearing caps are fastened to the engine frame(framework) by bolts, below the engine is arranged a light oil tray to collect the lubricating oil

(figure 2.9).


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Figure 2.9 Underslung main bearing of the diesel D100

- In case of engines without thrust bearing, the main bearing toward the flywheel

(flywheel side main bearing) is used as a thrust bearing to prevent the crankshaft from axial

(longitudinal) shifting. In this case, the bearing shells are provided with shoulders, which rest

on the edge of the journal (figure 2.10).

Figure 2.10 Construction of a stop bearing


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2.3 FRAMEWORK

2.3.1. Framework of large engines with the long piston stroke

- The framework consists of separate frames with a form of letter A. The A frames are

placed on the transverse girders (transverse partition) of the bedplate and secured by studbolts. Outside of the frames is welded by steel plates to make the framework. In figure 2.11 is

shown construction of A frames.

Figure 2.11 Construction of A frames

- The framework must be arranged openable covers to inspect inside of the crankcase

(these covers are also called crankcase door).

- The framework must be provided with vent pipe, oil mist detector, and safety deviceto prevent the crankcase from exploding.

2.3.2. Framework of small engines with the short piston stroke

- For the engines with the cylinder diameter of about 100 ~ 300 mm, the framework

and the cylinder block are cast in one piece, the bedplate are separately made and secured to

the framework by the stud bolts (figure 2.12a).

a) b)

Figure 2.12 One-piece construction of framework and cylinder block


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- For the smaller size engines they also use one-piece construction of the framework

and the cylinder block. In this case the engine without the bedplate, the main bearings are

hung on the lower part of the framework (figure 2.13b). It permits to reduce the weight of the

engine.

- For the V-shape engines, the stud bolts are used to fasten the framework, cylinder

block and cylinder cover. The main bearings are hung on the framework (figure 2.13).

Figure 2.13 Framework of V-shape engines

2.4 CYLINDER BLOCK AND CYLINDER LINERS

2.4.1 Cylinder block

- The cylinder block is placed on the framework and secured by stud bolts.

- The cylinder block has a form of rectangular parallelepiped, inside of the block is

bored (the holes) to fit the cylinder liners (figure 2.14).

Figure 2.14 Cylinder block of 4-stroke engines


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- The cylinder liners are separated by vertical partitions. Cooling water for the cylinder

liners can circulate either in interior spaces of the cylinder block and the partitions or between

the cylinder block and the cylinder liners.

- The cylinder block construction of the 2-stroke engines is more complicated in

comparison with the 4-stroke engines. Besides the cooling spaces, the cylinder block of the 2-

stroke engines must have spaces and passages for the scavenging air. In addition, for the

return flow scavenging 2-stroke engines the cylinder block is arranged the spaces andpassages for exhaust gases (figure 2.15).

Figure 2.15 Cylinder block construction of 2-stroke engines

- For the low-speed large engines, the cylinder block is normally made of individual

units and then they are secured to each other by bolts (figure2.16).

Figure 2.16 Separate cylinder blocks of low-speed large engines

2.4.2. Cylinder liners

- Cylinder liners work in hard condition:

+ To withstand high pressure and high temperature;

+ To withstand wear due to friction;

+ To be subjected to mechanical and thermal stresses;


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+ To be subjected to corrosion due to the acid component contained in the

combustion gases.

- Cylinder liners are usually made of cast iron, the upper part of the cylinder liners has

a thicker wall than the lower part (figure 2.17).

Diameter of the upper part of the cylinder liner is a little larger than its normal

diameter to prevent the piston and rings from seizing.

Figure 2.17 Construction of a cylinder liner

- The upper part of the cylinder liners is made with a shoulder, when assembling this

shoulder is rest on the supporting flange of the cylinder block. The face surface of the

shoulder is made with an annular groove to fit a gasket for sealing between the cylinder liner

and the cylinder cover.- Outside surface of the cylinder wall is provided with annular grooves to fit rubber

rings (or O-rings) for sealing the cooling water chamber.

- To increase the ability of anti-corrosion, the working surface of the cylinder liners is

either plated with chrome (for the cylinder liners made of cast iron) or soaked with nitrogen

for the cylinder liners made of cast steel. Out side of the cylinder wall is generally cover with

thin bakelite layer.

- To reduce friction in working process, the working surface of the cylinder liners

must be lubricated.

+ For small engines with short piston stroke, the cylinder liners are lubricatedby splash method.

+ For large engines with long piston stroke (especially crosshead engines), the

cylinder liners are lubricated by Bosch type pumps through oil holes that drilled on the

upper part wall of the cylinder liners.

To prevent the lubricating oil from returning, at the inlet of each lubricating oil

nozzle on the cylinder wall is provided with non-return valves. The oil holes inside the

liners are connected by waved grooves (or straight grooves) to distribute the

lubricating oil over the working surface (figure 2.18).


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Figure 2.18 Cylinder liner with oil holes for lubricating

Each cylinder normally has 6 ~ 12 oil holes depending on its diameter. The

cylinder lubricating oil is supplied to the cylinder liners when the first piston ring fully

covers the oil holes in upward stroke of the piston and it is finished when the last

piston ring passes these holes.

- Assembly of the cylinder liners:

The cylinder liners are fitted in the cylinder block, under surface of the cylinder

shoulder rest on the supporting flange of the cylinder block. The lower end of the liner is left

free so that it can easily expand in longitudinal direction when be heated during operation.

Radial clearance between the cylinder shoulder and the supporting flange also must be

large enough for expansion.

- Classification of cylinder liners: Depending on features of the cooling water

chamber, the cylinder liners are classified as follow:

+ Wet type liners: In this case, cooling water chamber exists in space between

the outside of the liner wall and inside of the cylinder block. The cooling water

chamber is sealed by rubber rings (O-rings), which are provided on upper and lower

parts of the liners. This type of liner is commonly used for marine diesels (figure

2.19a).

a) b)

Figure 2.19 Types of cylinder liners


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+ Dry type liners: The cylinder liner is installed in the bore of the cylinder

block. In this case the liner wall did not directly contact with the cooling water, the

cooling water is circulated in the space of the cylinder block. The dry type liner is only

used for small diesel engines.

+ Integral cylinder liners: The integral liner also has the cooling water

chamber, but in this case the cooling chamber is included in the liner (figure 2.19b).

This type of the liner can be found in opposed piston engines.

+ Cylinder liners of the engines with high supercharging level:

This type of liner is commonly applied for medium and low-speed engines

with high supercharging level. To reduce thermal stress of the cylinder liner, the upper

part of the liner has many holes drilled in tangential direction to provide water

passages (figure 2.20).

Figure 2.20 Cylinder liners of marine diesels with high supercharging level

2.5 CYLINDER COVERS

- The cylinder cover forms a part of the combustion chamber. The cylinder covers

work in condition of high temperature and high pressure due to burning of the fuel in thecombustion chamber. Beside of this, due to design features the cylinder covers have many

cavities, so it is easy to arise thermal stress and crack.

- The cylinder covers are normally made of cast iron or cast steel, for small engines

the covers are sometimes made of aluminum alloy.

- The cylinder cover may have a form of a cylindrical, hexagonal, octagonal block or a

parallelepiped. For the small engines with the cylinder diameter smaller than 200mm, the

cylinder cover is cast in one piece for all cylinders or a group of cylinders.

- On the cylinder covers there are cavities for installing the fuel injection valves,

starting valve, indicated and safety valve, intake and exhaust valves (for 4-stroke engines).

Each cylinder may have 2 or 4 intake and exhaust valves (figure 2.21).


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Figure 2.21 Cylinder cover sections of 4-stroke engines

In figure 2.21 is illustrated construction of the cylinder cover of a 4-stroke diesel

engine. For high-speed small engines the intake and exhaust valves are directly installed on

the cylinder cover. In low-speed large engines the valve casing constructions with cooling

space are used (figure 2.22)

Figure 2.22 Constructions of valve casing

- The cylinder covers of return-flow scavenging 2-stroke engines are simpler than that

of 4-stroke engines because they have not intake and exhaust valve.

- The cylinder covers are cooled by water, the cooling water is circulated from the

cooling chamber of the cylinder liners through the joint pipes that sealed by rubber rings.

To prevent the cooling water from generating vapour pocket in the cooling space, the

outlet of the cooling water is arranged on the highest position of the covers.

To seal the combustion chamber, when assembling, between two surfaces of the

cylinder cover and liner shoulder must be placed a copper or steel gasket. Fastening the

covers with cylinder liners must obey regulations of manufacturer.


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Review questions

1.

State duty of the bedplate and describe its construction.

2. State duty of the main bearing and describe its construction.

3. Describe the construction of the cylinder block.

4.

State working conditions and methods for lubricating cylinder liners. Representclassification of cylinder liners.

5. How many types of cylinder liner are used for marine diesel?

6. Describe the construction of the cylinder liner.

7. Describe the construction of the cylinder cover

8. State attentions that must be paid for cooling and assembling cylinder covers.

Documents

Bai Giang Chi Tiet 1.2