13 IMPORTANT PARTS OF MARINE DIESEL ENGINE

The size ,shape  of particular parts will vary according to engine type and manufacturer.

BASIC PARTS OF MARINE DIESEL ENGINE

1. BEDPLATE

> The Bedplate form the structural base which houses the heavy crankshaft and support the cylinder block.

> It is a platform on which other structural components such as frames or columns and guides may be accurately mounted to support the engine cylinders and ensure the alignment of all working parts.

> The bedplate is seated on stool made above the tank top and secured by foundation bolts.

> Bedplate  withstand heavy, fluctuating stresses from working parts. 

> Bedplate transmit the engine loads, including the propeller thrust, to the 

ship's structure, distributing these over the necessary area, and may complement the ship's strength and propeller shaft alignment.

Construction-:

 > Bedplate generally fabricated from steel plates and casting welding together.

> Longitudinal girder with box section usually used in construction of bedplate which run the length of the Engine.

> Two longitudinal box girders are used,  made of welded steel plates.

> Cross girder connect the two longitudinal girder and each cross girder houses the main bearing bottom shell.

> Cross girder are subjected to heavy cycling loading  due to which adequate stiffeness are provided to raise the natural frequency of vibration.

> A single casting of steel is favoured for making of cross girder.

> The cross girder are attached to longitudinal box girder by fitted bolts or welding.

How the bedplate fastened to the hull structure

> The engine bedplate is supported on a series of chocks fitted around the

underside of the periphery of the base of the bedplate. 

> The chocks sit on the foundation plate which forms part of the inner bottom plating of the hull structure.

> The holding-down bolts pass through holes in the bedplate, chocks

and foundation plate

> When all the holding-down bolts are tightened the bedplate is held fast to the hull structure. 

> The chocks are fitted in place after the engine has been aligned to the intermediate shafting.

Holding-down Bolts or Studs 

> These are screwed into the tank top and a nut fitted and locked underneath.

> Studs will normally have clearance in the bedplate to allow for 

thermal expansion of the engine. 

> These bolts are threaded at each end, and the upper ends have two flats or a square machined on them to take a spanner.

2. ENGINE FRAME

>  Engine frames or columns are used to support the cylinder block from the bedplate. 

> These are known as A frames because of their shape; they are fitted at 

each transverse girder and at both ends of the engine.

 >A frames are fabricated from steel plates welded to form a hollow structure on each side. 

> Transverse stiffening webs are fitted and flanges added for 

necessary connections. 

> Brackets secure the crosshead guides and other internal fittings. 

> Bottom flanges allow the frame to be accurately aligned and secured by fitted bolts and studs to the bedplate and transverse girder. 

> Tie bolts pass vertically within the frame; the pre-stressing of these 

maintains the frame in compression at all times. 

3. THE ENTABLATURE

> The entablature is the name given to the cylinder block which incorporates the scavenge air space and the cooling water spaces.

> It forms the housing to take the cylinder liner and is made 

of cast iron.

> The castings are either for individual cylinders which after machining on the mating surfaces are bolted together to form the cylinder beam.

> The underside of the cylinder beam  is aligned on the A frames and fastened in position using fitted bolts. 

> Fitted bolts used to bolt the entablature, A frames and Bedplate together are for alignment purpose only.

4. TIE BOLTS

> The main gas loads from the cylinder covers are transmitted by long tie bolts. 

> Two bolts are fitted to each transverse girder and they pass up through the casting, through tubes constructed in the engine frames and entablature to the top of the cylinder block

> Locking nuts are hydraulically tightened to pre-stress the structure, maintaining the cylinder block and frames in compression. 

> Tie bolt centres should be as close to the crankshaft axis as possible to reduce bending stress on the girders and to prevent unbalanced loads being transmitted to the welds.

Use Of Tie Bolts 

This rod holds the three major engine components i.e. Cylinder block or entablature, Engine frame, and crankcase in compression and transmits the firing load to the bedplate.

How Firing load Transfered

> The firing load from the cylinder covers is transferred through the cover studs to the cylinder beams.

> The beam transfers the load through the tie bolt nuts and tie bolts to the bedplate cross girders.

> The gas pressure on the piston acts downwards. The force created is transmitted through the piston, piston rod, connecting rod, crankpin, crank webs and journals on to the lower halves of the main bearings supported in the bedplate cross girders. 

> The upward acting force on the cylinder cover is transmitted through the cover studs into the cylinder beam.

> It is then transferred into the tie bolts and the bed plate cross girder where the downward acting force is balanced at the main bearings.

How Tie Bolts Tightened 

> Large tie-bolts are tightened with an adaptation of a hydraulic jack which loads the tie-bolt in tension. 

> The tie-bolt nut is usually drilled to take a toggle bar or slotted to take a hook spanner. 

> When the correct pull is on the tie-bolt the tie-nut is pulled up hand-tight

> The pressure in the jack is then released leaving the tie-bolt tight. 

> The load placed on the tie-bolt by the hydraulic jack is controlled by the hand-pump pressure, which is indicated on the pump pressure

gauge. 

> The pump pressure to as given in the engine instruction manual.

Consequence Of slack Tie Bolts 

> Engine with slack tie-bolts, the cylinder beam flexes and lifts at the

location of the slack tie-bolt. 

> The landing faces of the tie-bolt upper and lower nuts, and the landing faces of the cylinder beam on the A-frame fret and faces are eventually destroyed.

> Fretting  in an uneven pattern where the cylinder beam lands,

and the tie-bolts are tightened the alignment of the cylinders to the line of the piston stroke is destroyed.

> Vibration increases and  loosening of foundation bolts and chocks.

> Misalignment  of crankshaft will be occur.

5. CRANKSHAFT

> A crankshaft consists of a number of cranks  which are rotated by piston forces transmitted through the connecting rods and bottom end bearings.

> Every crank is made up of two crank webs joined by a common crankpin to which the bottom end bearing is fitted.

> A main bearing will support the shaft at each journal.

> The crankshaft receives and transmits the full working power of the engine and is subjected to fluctuating bending, torsion and shear stresses. 

How To Take Crankshaft Deflections 

> Misalignment of an engine crankshaft occurs due to wear of main bearings or from distortion of the engine bedplate.

> It can be detected by measuring deflections of crankshaft webs for each unit of the engine. 

> If misalignment exists the crank webs will open and close slightly.

> It is measured by means of a clock or dial gauge fitted between adjacent webs at a point in line with the outside of the journals furthest from the crank pin. 

> A spring extension rod will hold this in position. 

> The first measurement is taken with the engine just beyond bottom dead centre position with the gauge close to the side of the connecting rod. 

> The engine is now rotated by the turning gear and stopped at each 

quarter turn where gauge readings are taken as plus or minus values. 

> The final reading is taken near bottom centre, with the connecting rod on the opposite side of the gauge to the first reading. 

> The first and last readings are averaged to use as an approximation for bottom centre position. This procedure is repeated for each unit in 

turn. 

> All readings are recorded and these should be compared with previous values preferably with the ship in a similar load condition and at similar temperatures. 

> Total deflection vertically and horizontally is calculated for each crank.

> The vertical total will be proportional to misalignment between the bearings due to weardown. The horizontal total indicates side wear in the bearings.

6. CAMSHAFT 

> A camshaft is necessary to operate the valves and fuel pumps which control the engine cycle. 

> Each valve or pump is actuated by a cam follower which rises or falls as the cam rotatate .

> profile or shape of each cam is designed to give the correct timing, speed and height of lift to its corresponding follower.

>Reversible engines may have servomotors fitted to their camshaft to readjust the timing of the cams when the engine is to run astern.

Purpose Of Cams

> Cams are used on diesel engines to operate the mechanisms connected with opening and closing exhaust valves and air inlet valves, driving fuel pumps, airstarting valve pilot valves, etc.

7. CONNECTING ROD 

> It connects the top and bottom end bearings, and convert the reciprocating forces on the piston into rotating power in the 

crankshaft. 

> Connectimg Rods are machined from a steel forging shaped at each end to accommodate the  bearings. 

> Oil hole is bored through the centre of the rod to allow passage of lubricating oil between the bearings .

8. CROSSHEAD BEARING

> The crosshead pin connects the piston rod to the connecting rod.  

> Side of the crosshead pin are mounted the crosshead slippers.

> The slippers run up and down in the crosshead guides as the piston and rod are reciprocating and prevent the top of the connecting rod from moving sideway.

> This bearing transmits the full gas load from the piston to the connecting rod and crankshaft.

 Why Crosshead Bearing are difficult to lubricate

> The top of the connecting rod swings about the pin and changes direction each time the piston reaches mid stroke.

> The relative speed between bearing and pin at mid stroke is zero and accelerates to a maximum as the piston approaches top or bottom dead center.

> Then decelerates back to zero again as the piston approaches mid stroke and the con rod changes direction. 

> This means that hydrodynamic lubrication, where the pin is separated from the bearing by a wedge of oil only occurs over part of the swing.

Effective Lubrication Of Crosshead Bearing

> For  effective lubrication the pin has a large diameter. This increases the relative speed between pin and bearing.

> The bottom halves of the bearing shells have oil gutters cut in them to assist the distribution of oil. 

> Oil is supplied to the crosshead using a telescopic pipe for efficient lubrication.

> The pin is highly polished to a mirror finish.

 Cause Of Crosshead Bearing Failure

1 Misalignment of engine running gear.

2 Uneven suface  finish of crosshead pins.

3 Quality of white-metal.

4 Insufficient supply of lubricant.

S Impure lubricant, or water contamination.

6 Excessive firing pressure.

9. CYLINDER LINER

> Cylinder liner forms the cylindrical space in which the piston reciprocates.

> Materials for liners must provide adequate strength and fatigue life, readily transfer heat, resist abrasion and corrosion, able to retain a film of lubricating oil on working surfaces.

> The liner can be manufactured using a superior material to the cylinder block.

> The liner thickness must give adequate strength to resist the internal gas load but is limited by the necessity to transfer heat rapidly to reduce the thermal stress.

Cylinder Liner Cooling 

> This is often referred to as jacket cooling and is carried out by circulating fresh water between the outer surface of the liner.

> The water space extends over the upper part length of the liner.

> Water enters at the lower end of the jacket, flowing upwards and leaving at the top to pass on to cool the cylinder cover.

Bore Cooling

> In this number of individually small holes are bored within the thickness of a part so that water can be passed close to the heated surface.

> The holes are bored upwards and at an angle so that they approach the internal surface of the liner at a tangent. 

> Holes are then bored radially around the top of the liner so that they join 

with the tangentially bored holes.

> Cooling takes place near the hot surface, thermal stress is greatly reduced.

Gauging a Cylinder Liner

> After the cylinder cover or upper piston has been removed and the liner has been cleaned, the locating gauge is placed on the port or starboard side of  liner.

> A series of horizontal chalk marks is made in the liner corresponding to

the position of the micrometer locating holes.

>  The locating gauge is then placed on the opposite side of the liner.

> The extension of the internal micrometer is placed in the top locating hole and the micrometer head is swung in a horizontalarc on the level of the chalk mark until the right 'feel' is obtained on the micrometer head. >The micrometer is then read and the measurement recorded.

> The process is repeated down the liner and the various diameters of the

cylinder.

Cylinder Liner Wear 

It is mainly due to friction, abrasion and corrosion.

Frictional Wear - It takes place between the sliding surface of cylinder liner and piston rings. It will depend upon the materials involved, surface conditions, efficiency of cylinder lubrication.

Corrosion - It occurs mainly in engines burning heavy fuels, particularly with high sulphur content. It is caused by acids formed during combustion. Sulphuric acid corrosion may be caused in the lower part of the liner if the jacket cooling water temperature is too low. This may 

allow vapour present after combustion to condense. The moisture formed will absorb any sulphur present to form sulphuric acid and corrode the liner.

Abrasion - It may take place from the products of mechanical wear, corrosion  and combustion—all of which form hard particles which may act as abrasives.

Adhesion or Scuffing -It is a form of local welding between particles of the piston ring and the liner rubbing surface, resulting in very rapid wear. It may occur if the lubricating oil film between ring and liner is removed due to excessive temperature, insufficient supply.

10. PISTON & PISTON ROD

> The Piston comprises of two pieces the crown and the skirt. 

> The crown is subject to the high temperatures in the combustion space.

> The crown also carries the piston ring grooves .

> The cast iron skirt acts as a guide within the cylinder liner.

> Piston rod is bolted to the underside of the piston. The other end of piston  rod is attached to the crosshead pin.

> Pistons are cooled either using water or the crankcase oil. Water has a better cooling effect than oil, but there is a risk of leakage of water into the crankcase.

> Modern engines have oil cooled pistons.

Oil Piston Cooling

> Cooling oil is supplied under pressure to the crosshead via either telescopic pipes or swinging links with glands. 

> From the crosshead it passes through holes bored in the piston rod to the cooling spaces in the piston. It returns via the crosshead to a collector.

> Cooling oil enters from the central tube within the connecting rod .

> Then flows under the centre of the crown before passing through bore cooling holes in a radial direction to the toroidal space at the piston edge. > Then it swirl down behind the piston ring grooves before returning to the crosshead by the concentric space outside the tube in the connecting rod.

Piston Ring

1. Compression Ring - Rings for sealing the gases above the piston and preventing gas leakage called compression rings or pressure rings.

2. Scraper Rings - Rings for controlling the amount of lubricating oil passing up or down the cylinder wall, or spreading the oil evenly around the cylinder, called oil control rings or scraper rings.

3.  Oil Spreader Ring- Rings used for spreading oil evenly around the circumference of a cylinder called oil-spreader rings.

Purpose of Piston Ring 

 a) sealing of combustion gases 

 b) Spreading the oil on the liner surface. 

 c) Transfer of heat from piston to the liner.

11 CYLINDER HEAD

> The cylinder head is cover for the liner and block, which also seals the combustion chamber at the top.

> It sustains dynamic thermal and mechanical loads caused by combustion pressure and temperature. 

> It houses the exhaust valve, fuel injectors, starting air valve, indicator cock, & cooling water passage.

12 EXHAUST VALVE 

> Exhaust valve is a valve that releases burned gaeses from a cylinder.

> Exhaust valves open inwards into the cylinder, so that the gas pressure in the cylinder will maintain positive closing under pressure in the 

cylinder and ensure their non-return action.

> Gas pressure will act upon the area of the valve lid to hold it against the seat and supplement the closing action of the springs.

>During opening, the valve springs are compressed and these force the valve to close as the cam peak leaves the follower.

> Rotation of valves during operation will assist in removal of deposits between seat and landing and ensure an even temperature and wear around the valve. Rotation may be carried out mechanically by Rotocap.

> Modern two stroke crosshead engines have a hydraulically operated air spring exhaust valve. The cam operates a hydraulic pump instead of a push rod.

> Two-stroke crosshead marine engine, has a single exhaust valve per unit, mounted in the centre of the cylinder head. 

Hydraulic Operated Exhaust Valve

> The cam operates a hydraulic pump instead of a push rod. 

> Oil displaced by the pump operates a piston in the exhaust valve which pushes the valve open. 

> Instead of mechanical springs, the valve has an "air spring". Air at 7 bar is led via a non-return valve to the underside of a piston attached to the valve spindle.

> When the valve opens, the air underneath the piston is compressed. 

> The expansion of this compressed air, when the hydraulic pressure is relieved assists in the closing of the valve. 

13. FUEL INJECTOR

> The fuel is delivered by the fuel pumps to the fuel injectors or fuel valves.

> For the fuel to burn completely at proper  atomisation and penetration needed into combustion space so that they mix with the oxygen. 

> Fuel injectors achieve this by making use of a spring loaded needle valve. 

> The fuel under pressure from the fuel pump is fed down the injector body to a chamber in the nozzle just above where the needle valve is held hard against its seat by a strong spring. 

> As the fuel pump plunger rises in the barrel, pressure builds up in the chamber, acting on the underside of the needle .

> When this force overcomes the downward force exerted by the spring, 

the needle valve starts to open. The fuel now acts on the seating area of the valve, and increases the lift. 

> As this happens fuel flows into the space under the needle and is forced through the small holes in the nozzle where it emerges as an "atomised spray".

> At the end of delivery, the pressure drops sharply and the spring closes the needle valve.