marine diesel engine gme competence 6

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MARINE DIESEL ENGINES.NOTES FOR BE(Marine Engineering) CADETS.

MARINE DIESEL ENGINE PRINCIPLE AND PRACTICE.

Notes prepared By: Prof. K. Venkataraman. CEng, FIMarE, MIE.

1. Engine Classification:

ENGINE:

Any machine, which produces power, is called an engine.

HEAT ENGINE:

Any engine, which produces power or work from a supply of heat, is called Heat Engine.

The heat can be supplied by burning, i.e. by combustion of fuel.

EXTERNAL COMBUSTION ENGINE:

If the combustion of fuel takes place outside the engine, it is called an external combustion engine, e.g.steam engine, steam turbine, etc.

INTERNAL COMBUSTION ENGINE:

If the combustion of fuel takes place within the engine itself, it is called an Internal Combustion Engine.

Fuel economy, simplicity, and low operational costs make it more popular than external combustion

engines.

CLASSIFICATION OF INTERNAL COMBUSTION ENGINES:

Internal combustion engines can be classified according to different criteria as follows:

1. According to ignition System.

a) Compression Ignition Engine (C. I. Engine)

In this type of engine, the heat of the compressed air itself ignites the fuel. No other means of ignitionare required, e.g. Diesel Engine.

In a Compression Ignition Engine, e.g. Diesel Engine, a piston reciprocates in a cylinder. At downward

stroke of piston, air enters the cylinder. At upward stroke of piston air is compressed. Due tocompression pressure and temperature of air becomes quite high (over 35 bar and 500*C respectively).

Finely atomised fuel oils sprayed into such compressed air ignite spontaneously and produce power.

b) Spark Ignition Engine (S. I. Engine)In this type of engine (Otto engine), the fuel is ignited by the spark produced by a high-tension electrical

circuit. In spark ignition Engine, liquid gasoline is sprayed or drawn through a nozzle or jet into the airstream going to the working cylinder. A combination of mild heating and reduction of pressure partiallyvapourises the gasoline. Proportionate mixing of air and gasoline vapour is done in carburetor. Mixture

enters the cylinder where at a suitable time, an electric spark ignites the mixture, which burns then

quickly and produces power.

Spark Ignition Engine Versus Compression Ignition Engines Similarities.

1. Both are Internal Combustion Engines.

2. Both run on liquid fuels.

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Dissimilarities.

2. According to Operating Cycles.(a) OTTO CYCLE (Constant Volume Combustion Cycle).

It is the ideal air standard cycle for Petrol engine, the gas engine and the high-speed oil engine. Theengines based on this cycle have high thermal efficiency but noisiness results particularly at higher

power due to higher pressures in the cylinders.

(b) DIESEL CYCLE (Constant Pressure Combustion Cycle).

It is the ideal Air standard cycle for Diesel Engine, especially suitable for low speed Diesel Engine but

not for high speed Diesel Engine. (The thermal efficiency is lower than Otto cycle engines but engines

run smoothly due to lower pressures in the cylinder.

(c) DUAL COMBUSTION CYCLE (Constant Pressure and Constant Volume Combustion Cycle).

Modern Diesel Engines do not operate purely on constant pressure combustion cycle but some part ofcombustion process takes place at constant volume while the rest is completed at constant pressure.

In general, this cycle resembles Constant volume combustion Cycle more than constant pressurecombustion cycle. It is suitable for modern Medium and High Speed Diesel Engines. The thermal

efficiency is more than Diesel Cycle but less than Otto cycle. Also noise level is in between the two.

This is a more practical engine.

3. According to Strokes/Cycle.

In an engine, the following events form a cycle:

a) Filling the engine cylinder with fresh air.

S. I. Engine. C. I. Engine.

1. Ignition system required 1. Not required.

2. Draws air and fuel into the system. 2. Draws only air into the cylinder.

3. Compresses air and fuel together. 3. Compresses air only.

4. Fuel is mixed with air at before compressionstarts.

4. Fuel is mixed with air at the end of compression.

5. As too much compression of air and fuelmixture causes pre ignition and detonation

permissible compression ratio is not high(about 7).

5. Only air can be compresses without pre-ignitionand detonation, so compression ratio can be high

(about 16).

6. Efficiency, being proportional to compression,

is limited due to less compression ratios.

6. Higher efficiencies can be obtained due to

possible higher compression ratio.

7. Uses highly volatile liquid fuels so that it can

mix with air at low temperature.

7. Uses less volatile liquid fuels

8. Fuel used is costly. 8. Cheaper fuel can be used

9. More fuel is used for same power. 9. Less fuel consumption.

10. Lighter in weight. 10. Heavier and stronger engines due to higher

pressures involved.

11. Initial cost less. 11. Initial cost high.

12. Smooth operation. 12. Certain roughness in operation encountered,especially in high-speed engines at light loads.

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b) Compressing the air so much that injected fuel ignited readily by coming in contact with hot air

and burns efficiently.c) Combustion of fuel.

d) Expansion of hot gases.

e) Emptying the products of combustion from the cylinder.

Depending on how many strokes of piston are required in completing this cycle, the engines can bedivided into two classes:

1. Four Stroke EngineAn engine, which needs 4 strokes of the piston (2 in and 2 out) to complete one cycle, is called Four-stroke engine.

2. Two Stroke Engine

An engine that needs only 2 strokes of the piston (1 in and 1 out) to complete one cycle is called Two-stroke engine.

4. According to Piston Action:

(a) Single Acting Engine

One end of the cylinder and one face of the piston are used to develop power. The working face is at theend, which is away from crankshaft. Generally, single acting vertical engines develop power on the

down stroke.

(b) Double Acting Engine

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Both ends of the cylinder and both faces of the piston are used to develop power on the upward as well

as on the downward stroke.c) Opposed Piston Engines.

Two pistons travel in opposite directions. The combustion space is in the middle of the cylinder betweenthe pistons. There are two crankshafts. The upper pistons drive one, the lower pistons the other. Each

piston is single acting.

5. According to Piston Connection.

(a) Trunk Piston Type .

The piston is connected directly to the upper end of the connecting rod. A horizontal pin (Gudgeon Pin)

within piston is encircled by the upper end of the connecting rod. This construction is quite common,

especially in small and medium size engines.

(b) Cross Head Type.

The piston fastens to a vertical piston rod whose lower end is attached to a cross head, which slides up

and down in guides. The crosshead carries a crosshead pin, which is encircled by the upper end of the

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connecting rod. This more complicated construction is common in double acting engines and large slow

speed single acting engines.

Crosshead type Engine arrangement.

Comparison between Trunks Piston Versus Cross Head Engine.

Most medium and small size engines use trunk pistons. Resulting side thrust causes the piston to press

against the cylinder wall,first on one side, then on the other. At the top of stroke, when the gas pressureis greatest, side thrust is negligible (due to small connecting rod angle). So most of wear takes place atthe middle of stroke: making piston skirt increases thrust-bearing area, and hence reduces wear. In

medium and small size engines, due to lower gas pressure, units side pressure is so small that neither

piston nor liner wears much.In crosshead engines, crosshead takes the side thrust, which will be high in large engines. So, crosshead

engines have the followingadvantages:

1. Easier lubrication.

2. Reduced liner wear.

3. Uniformly distributed clearance around piston.

4. Simpler piston construction because the Gudgeon pin and its bearing are eliminated.

However these advantages of cross head engines are offset by:

1. Greater complication.2. Added weight.

3. Added height.

4. Careful adjustments.

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6. According to Cylinder Arrangement

a) Cylinder-in-Line Arrangement

This is the simplest and most common arrangement, with all cylinders arranged vertically in line. This

construction is used for engines having up to 12 cylinders. The arrangement is shown in figure below.

(b) V - Arrangement:If an engine has more than eight cylinders, it becomes difficult to make a sufficiently rigid frame andcrankshaft with an inline arrangement. Also engine becomes quite long and takes up considerable space.

So V-arrangement is used for engines with more cylinders, (generally 8, 12, 16) giving about half-length

of engine, more rigid and stiff crankshaft, less manufacturing and installing cost. Angle between twoBanks is kept from 30* to 120* (most commonly 40*, 75*), as shown in the figure.

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(c) Flat Arrangement.

It is a V-engine with angle between the banks increased to 180*. Generally, it is used in trucks, buses,rail cars, etc. where there is little headroom. Arrangement is shown in the following figure.

(d) Radial Arrangement.

In a radial engine, all the cylinders are set in a circle and all point towards the centre of the circle. The

connecting rods of all the pistons work on a single crankpin, which rotates around the centre of thecircle. Such a radial engine occupies little floor space. By attaching the connecting rods to a master disk

surrounding the crankpin, up to 12 cylinders have been made to work on a single crankpin. The

arrangement is shown in the figure below.

7. According to Method Of Fuel Injection.

(a) Air Injection Engine

The fuel is injected into the cylinder by a blast of high compressed air. It was commonly used on early

diesel engines. Being too heavy and complicated, this system is now obsolete.

(b) Airless (or Solid or Mechanical) Injection Engine.

Fuel is injected into the cylinder, through the fuel valve, by high-pressure fuel pump. At present, it is

being used for all types and sizes of diesel engines.

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8. According to method of Charging.

(a) Natural aspirated Engine.

The vacuum is created when the piston moves away from the combustion space draws in the fresh

charge.

(b) Supercharged Engine.

The charge is admitted into the cylinder at a higher than atmospheric pressure. This high pressure isproduced by a pump or blower or exhaust gas turbocharger.

9. According to Fuel Used.

(a) Heavy Oil Engine.It can burn fuels of high viscosities, e.g. 1500 sec. Redwood No. 1 or 350 sec. Redwood No. 1.

(b) Diesel Oil Engine.This uses diesel oil.

(c) Gasoline Engine.

This uses gasoline as fuel. It can also use kerosene. As the 'perfect mixture' of fuel and air is led tocylinder for compression, compression ratio is limited to 7 to avoid self- ignition, power loss, knocking,

etc.

(d) Gas Burning Engine.

It uses gaseous fuels at higher compression. Three ways have been adopted to burn gas at higher

compression. The engines are named accordingly as follows:

i) Gas Diesel Engines

They compress air alone. At the end of compression, they inject the gas at high pressure into the cylinder

just at the moment it is to fire. With gas, a small amount of pilot oil is also admitted to assist the

ignition and to cause smooth and prompt ignition.

ii) Dual Fuel EngineAdmit the gas and air at the same time and compress the gas/air mixture at diesel compression ratio. At

the end of compression, they inject fuel oil, which the high temperature of the gas-air mixture ignites to

fire the mixture. Using lean mixture unlike to perfect mixture of gasoline engine prevents self-ignition.

iii) High Compression, Spark Ignited Gas Engines.Like dual fuel engines, they compress a mixture of gas and air to high pressure, preventing self-ignition

by using a lean mixture but they use spark, instead of oil, for ignition.

10. According to Speed.

1. Slow Speed Engines : 100 to 150 r.p.m.

2. Medium Speed Engines: 300 to 1000 r.p.m.

3. High Speed Engines: More than 1000 r.p.m.

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The following table compares the various aspects of Slow Speed, Medium Speed and High Speed

Engines:

Slow Speed. Medium Speed. High Speed.

1.2.

No Gearing.Four or Two Stroke

acceptable.

Gearing Necessary. FourStroke better.

Gearing Necessary. FourStroke only.

Slow Speed. Medium Speed. High Speed.

3.

4.

5.

6.

7.

8.

9.

10

.

11

.

12

.13

.14.

15

.

16

.

Poor quality fuelacceptable.

Crankcase can be

separated fromcombustion zone.

Less noise and vibration.

Less fatigue failure.

Fewer stresses due toheavier scantling.

Heavy and Large Size.

More head-room

required.Heavy Lifting Gear

required for heavy parts.Engine r.p.m. is

limited by propeller

efficiency.Can have long strokes.

Large bore cylinders.

Heavy & large pistons.

Round sectionconnecting rod.

Failures less and easier tomanage.

Better Fuel required.Diesel oil/Gas oil.

Trunk Piston type.

More noise.

More.

More.

Compact.

Less.Light parts easy to handle.

Engine r.p.m. limited by

piston speed.

Small strokes.

Small bore.

Light and small piston.

I section connecting rod.

More and difficult tomanage.

Distillate Fuel only.

Trunk Piston Type.

Most vibration and noise.

Most.

Most.

Extremely Compact.

Least.Lighter parts can be handled

manually.Engine r.p.m. limited by

piston speed.

Smallest stroke.

Smallest.

Lightest & smallest piston.

I section connecting rod.

Most and very difficult tomanage.

11. According to Bore/Stroke Ratio:

a) Square Engine:If bore/stroke is about one, crankshaft web dimensions become less compared to journal and crankpin.

b) Over Square Engines (Short Stroke)

If bore/stroke > 1, web dimensions (less height, more thickness) are such that webs will be weak. So

generally over square engines are not used.

(c) Long Stroke Engines.

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Generally, engines have stroke/bore >1. This gives crankshafts of good strength. Most common ratio is

stroke/bore = 2. 0: 2.2.

(d) Super-long Stroke Engines.

To have better propeller efficiency and better combustion even with lower grade fuels, lower r.p.m.

engines with longer strokes are gaining popularity. These engines have stroke/bore ratio = 3.

12. According to Use.Engine can be named as Marine, Auto, Tractor, Locomotive, Aero-engines, and Rocket Enginesaccording to their use. On ships they can be called Main Engines if used for propulsion or Auxiliary

Engines, if used for generation of electricity. The Diesel Engines used in Marine power plants are

termed as Marine Diesel Engines.

The Diesel Engines find the following application on board merchant ships.

1. Main Propulsion.2. Electric Power generation.

3. Emergency Pumps (e.g. fire pump).

4. Life Boat.

5. Emergency Generator.6. Emergency Air Compressor

REASONS FOR WIDE USE OF DIESEL ENGINES IN MARINE POWER PLANTS.

1. Small fuel consumption:

Diesel Engine is one of the most efficient heat engines. Hence it gives more power with less fuel. It is anengine of high economy.

2. Cheap fuel:

Diesel engine uses fuel costing very less as compared to other engines.

3. Economy at light loads:

Diesel Engine is not only efficient when it is fully loaded, but also when it is partly loaded.4. Greater Safety:

Diesel fuel is non-explosive and less flammable at normal temperatures and pressures. It requires specialeffort to make it start to burn. This feature makes it very attractive in the marine trade, because it would

be much safer carrying diesel oil on board ships.

Diesel exhaust gases are less poisonous than other engines, because they contain less carbon monoxide.5. Ignition System is not required:

Diesel engines do not require battery or magneto running them.

6. More power can be produced due to more compression allowed.7. Diesel Engine is more robust and stronger.

8. Economy in small sizes:

As great contrast to steam power plant, a small diesel engine has nearly as good an economy as a largeone. This makes it possible to enlarge a diesel engine plant with additional units as the load grows. At allstages of growth, the efficiency is high.

9. Sustained economy in service:

Again in contrast to a steam power plant, diesel efficiency falls off very little during thousands of hoursof use between overhauls.

10. Lightness and compactness:

Diesel engine plants have less weight and space per unit power. It is therefore well suited to portable andmobile installations.

11. Independence of water supply:

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A diesel engine requires very less water in contrast to steam plants.

12. Quick Starting.A cold diesel engine can be started instantly and made to carry its full load in few minutes. It is therefore

ideal for supplying emergency power.

13. Easily in Maneuvering:

A diesel engine can be made to run at full power in either direction.

14.Economy in Labour.Nofire room force is needed.15. Freedom from nuisance:

There are no ashes to be disposed of, no noisy and dusty coal handling and pulverising equipment to

maintain, no smoke, and noise can be easily eliminated. Due to above mentioned reasons, Diesel enginesare quite popular on board ships.

These reasons can very well be regarded as the advantages of Diesel Engines over other prime movers

such as gasoline engines, gas turbines, steam engines, steam turbines and hydraulic turbines.

However, Diesel engines also have certain disadvantages, which can be listed as following:

1. Cost:

Diesel engines, because of the higher pressures at which, they work, require sturdier construction, bettermaterials and closer fits than gasoline engines. Therefore, they cost more to build.

2. Weight:

Because of sturdier construction, weight per power is more than gasoline engines.3. Attendance:

A diesel engine requires more attention than an electric motor running on purchased current. It also

requires more attention per unit of power produced than a large steam turbine.4. Fuel Cost:

Oil used in Diesel engines is costlier than coal. Hence, steam power plants using coal as fuel are cheaper

in operation.

RECENT TRENDS:The Diesel Engines is at present acknowledged to be the best prime mover in a wider range of marine

applications than any other engine. Due to higher efficiency, lower specific fuel consumption andcapability to use cheaper fuel, Diesel engine is preferred to spark ignition Engine, gas turbine and steam

turbine for moderate power applications.

However, small pleasure boats are still powered popularly by spark Ignition engines and very large shipse.g. Tankers and VLCCS are still powered by steam turbines. Gas turbines are popular on naval vessels.

However, Diesel engines are making in rails in heavily these fields too. Even, U.S. merchant ships

dominated heavily by steam propulsion are more and more embracing diesel engines.

In Diesel engines also, there is tough competition between medium and slow speed engines.

However, the recent trend is towards having very slow speed super-long stroke engines e.g. SULZER/RTA: M.A.N-B & W/ LMC, due to significant improvement in propulsion efficiency and specific fuelconsumption at low speeds as well as their ability to burn very poor grade fuels which are available

now-a-days.

The worse quality of fuels available and increase in the cost of oil has led to renewed interest in coal-

fired ships. Keeping in view the limited world reserves of oil, coal fired ships seem to provide a good

alternative in 2000s but at present the position of Diesel engines remains unchallenged.

****************************Kv************************************

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End of Engine Classification/ BIT/AMET/BE/Motor/KV/May 2003.

Engine Cycles & Timing Diagrams.

Introduction.We will discuss the full series of the separate steps or events, which follow each other while a diesel

engine is in operation. We will also discuss the timing diagrams of Two Stroke Cycle & Four Stroke

Cycle Engines.

1. Various Steps or Events of a Cycle.

First Step.

Air is introduced into the cylinder because no fuel will burn without air. Burning or combustion is a

process of uniting fuel or combustible with the oxygen in air. The process is chemical reaction which

means that fuel & oxygen, in uniting, change into new substances.

Second Step.

The air must be squeezed or compressed to a high pressure. Two reasons for compressing the air are toget high temperature and high pressure there by higher power. In a diesel engine the air is compressed so

much that it becomes hot and in fact, it will be hot enough to ignite oil that is sprayed into it.

Third Step.

The fuel is injected into the cylinder in the form of fine spray after the air has been compressed and thus

heated to a high temperature. It must be in the form of fine spray so that a cloud of oil droplets willspread through all the air in the cylinder.

Fourth Step.

Combustion takes place after the oil is sprayed in the cylinder. This will generate a large amount of heat.The gaseous mixture gets hotter and grows larger or expands due higher pressure.

It pushes on the piston, which in turn transmits the force through the connecting rod to the crank of the

crankshaft. This will make the crankshaft revolve.

Fifth & Last Step.

When the piston has finished its preceding power stroke and the gases in the cylinder have lost theirpressure, the spent gases must be exhausted.

A cycle is a full series of separate steps or events, which follow each other.For a Four Stroke Cycle Engine, a complete cycle requires four stroke of the piston.

For a Two Stroke Cycle Engine, a complete cycle requires two stroke of the piston.

Four Stroke Cycle & Two Stroke Cycle engines are abbreviations, which do not really make anystatement other than what is stated above.

4-Stroke Cycle Engine.

In a 4-stroke engine, the engine crankshaft makes two revolutions for each working cycle.

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The four corresponding piston strokes are as follows:

1. Suction Stroke, 2.Compression Stroke, 3. Power Stroke & 4. Exhaust Stroke.The engine has air inlet and exhaust valves. By the opening and closing of these valves in proper

sequence, the piston can be made to perform its main function of transmitting power to the crank.

In addition to that the piston also performs subsidiary functions of drawing air into the cylinder,

compressing the air and subsequent expulsion of exhaust gases.

Four Stroke Cycle Engine:

A, B & C: SUCTION STROKE.

(A): Piston at top of stroke, inlet valve open, air intake begins:

(B): Piston descending, air being taken in.

(C): Piston at the bottom of the stroke, all valves closed, air intake full and completed.

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D: COMPRESSION STROKE.

(D): Piston rising. All valves closed. Air being compressed.

E & F POWER STROKE.

(E): Piston at the top of the stroke. Inlet and exhaust valves closed. Injector spraying oil into hot air.

(F): Piston descending. All valves closed. Hot high pressure gas forcing the piston down.

G & H EXHAUST STROKE.

(G): Piston at the bottom of the stroke. Exhaust valve opens.

(H): Piston rising. Exhaust valve opens. Exhaust gas being driven out of the cylinder.

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TIMING DIAGRAM AND POWER CARD OF FOUR STROKE CYCLE:

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The timing diagram shows the closing and opening of the valves. The working cycle is illustrated as

a P - V diagram (pressure-volume). The line l l represents atmospheric line. The piston isconsidered to have just moved over the top dead centre and is on its way down. The air inlet is already

open and because of the partial vacuum created when the piston moves towards its bottom position,

fresh air is sucked into the cylinder. This process is represented in the 'p-v' diagram by the line 1-2

which is termed suction line. This movement of the piston is called 'Suction Stroke".

After the piston has moved over bottom dead centre, the suction valve closes and the volume of air inthe cylinder is compressed during the course of the up stroke of the piston. This is represented by theline 2-3 in the above diagram and termed as compression line. This movement of piston is

compression stroke.

The ignition takes place at point 3 and combustion continues for the duration of fuel injection, ending atpoint 4. After this combustion products expand to point 5 when the exhaust valve opens. Power is

produced between point 4 5.

The pressure drops in the cylinder to the exhaust line from 5 to 6. The exhaust valve remains open tillafter piston passes over the top dead center. The combustible gases are expelled. The line 6 to 1

represents this. The pressure is slightly above atmosphere, because of the resistance in the exhaust pipe.

This stroke is 'exhaust stroke'.

A 4-stroke engine requires two complete revolutions of the crankshaft to finish working cycle.This means inlet, exhaust & fuel valve must only function once for every two revolutions of the

crankshaft.

In order to activate those valves in the correct sequence, it is necessary to operate them from a shaft,which rotates at half the speed of the crankshaft. This is called camshaft.

Two Stroke Cycle Engine:

In this engine, two of the strokes necessary to complete working cycle in a 4-stroke engine are

eliminated.

The remaining strokes are as follows:

--- Compression Stroke.--- Power Stroke

The working cycle is illustrated by 'p-v" diagram in the next page. The compression takes place by 1 to

2. The combustion process and expansion take place as described for a 4-stroke engine. At point 4, the

exhaust valve at top of the cylinder opens (uniflow scavenging).At point 5, the piston exposes the ports in the cylinder wall. The result being that fresh air, known as

scavenge air, flows into the cylinder & flushes out exhaust gases.

Piston covers the port in cylinder wall at 6 and the exhaust valve closes at point 1.The compression beings a new working cycle.

The pressure of scavenge air is little higher than the atmospheric air.

2-stroke engines carry out useful work for each revolution of the crankshaft.

This means fuel and exhaust must function each revolution. The camshaft must rotate at same speed of

the engine crankshaft.

REASONS FOR TIMING.

EXHAUST AND INLET VALVE TIMING.

These deal with the expulsion of the burnt gases and recharging the cylinders with fresh air. The overallefficiency of an engine depends largely upon getting the exhaust gases out or scavenging them.

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The exhaust value is opened after the piston has traveled about 80% of the working stroke. By that time

it has done its useful work, the energy has been spent. The opening of the valve will allow a large part ofthe exhaust gases to be blown out of the cylinder. The cylinder pressure equalises with the pressure in

the exhaust line during this period. This is referred to as the blown down period.

When the piston moves upwards the piston movement expel exhaust gases.

Two Stroke Timing Diagram (Uniflow Type).

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Two Stroke Engine Pressure/Volume Diagram (Uniflow Type).

In a 4-stroke engine, towards the end of exhaust stroke and beginning of section stroke, both exhaust and

inlet values are open. This is called overlap period. This would further help in achieving an efficient

scavenging. Exhaust valve closes after the piston has moved over the top dead centre. The inlet valveremains and the down ward movement of the piston lowers the pressure in the cylinder and thereby

atmospheric air is drawn in. The air in the inlet passages to the inlet valve will gain a high velocity and

in turn kinetic energy. Use is made of this effect to keep the air inlet value open until the piston is past

bottom dead centre. The air then continues to flow into the cylinder until its kinetic energy is lost andairflow ceases. The inlet value is closed now.

In two stroke engines, the events described above as taking place will have to be carried out in about

120* of the crank movement. It will require the assistance of low-pressure air. The speed of opening ofvalve or part has to be rapid so that the pressure of gas falls quickly. It would be easier now for scavenge

air to rush in and get the gases out.

We have to briefly discuss the combustion process to understand fuel timing.Combustion takes place in three distinct stages:

1. Ignition Delay Period, during which some fuel has been admitted but has not yet been ignited.

This is the stage during which fuel is atomised, vapourised, mixed with air and raised in

temperature.2. Rapid or Uncontrolled Combustion, following ignition. The pressure rise is rapid during this

stage.

3. Controlled Combustion. The first stage or the delay period exerts great influence on both enginedesign and performance. The pressure reached during rapid or uncontrolled combustion will

depend upon delay period.

The longer the delay, more rapid and higher is the pressure rise. It is because more fuel will be present in

the cylinder before rate of burning comes under control (in the 3rd stage).

This will cause rough running and diesel knock. But at the same time, there must be certain amount ofdelay period for proper mixing.

One of the main factors that affect delay period is fuel timing. If it is too early, the delay period is

more because the pressure and temperature are low in the cylinder. If the injection is too late, the fuel

will burn during the expansion stroke. The pressure rise in the cylinder will drop considerably, reducing

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the efficiency. The exhaust temperature will be high and may cause overheating of the engine in severe

cases. So, an optimum angle has to be introduced to get best effect. It depends on delay period. Theinjection is earlier on higher speed engines. Four-stroke or Two-stroke cycle will make no difference on

the point at which injection begins.

Four Stroke Timing Diagrams:

Four Stroke Un-supercharged. Four Stroke Supercharged.

Two Stroke Timing Diagram:

Two Stroke Uniflow Engine. Two Stroke Loop Scavenging Engine.

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Uniflow Scavenge Engine. Loop Scavenge Engine. (Exhaust Valve).

********************************Kv***********************************End of Engine Cycles & Timing Diagrams/ BIT/AMET/BE/Motor/Kv/May 2003.

Engine Indicator and Indicator Diagrams:An engine indicator is used to record pressure/volume or indicator diagrams taken off engines, the areas

of these indicator diagrams represent the work done per cycle of one unit.

There are two types of engine indicators:1. Mechanical type : This records indicator diagrams on paper.

a) Can record pressure within the engine cylinder at any part of the engine cycle.b) Not considered reliable of engine speed more than 150 rpm.

c) Small lightweight models can be used for engines with speeds up to 350 rpm.

d) Mean indicated pressure (m.e.p) from an indicator power diagram.

2. Pressure indicator type - this measures maximum combustion pressure only.

a) Also known as maximum pressure indicator.

b) Compression pressure is recorded with fuel cut off.c) No engine speed limitation.

d) Often used on medium speed engines.e) Does not record indicator diagrams on paper.

Mean Effective Pressure and Indicated Power:

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Power Indicator Diagram.

Referring to the Indicated diagram (Power card), the area of the diagram divided by its length represents

the mean pressure effectively pushing the piston forward and transmitting useful energy to the crank in

one cycle. This, expressed in N/m2, is termed the indicated mean effective pressure (pm).

Power is the rate of doing work (basic unit is the Watt) or:

1 Watt = 1 J/s = Nm/sLet:pm = mean effective pressure (N/m

2).

A = area of piston (m2).

L = length of stroke (m).N = Number of power stroke per second.

Then:

Average force (N) on piston = pm x A newtons.Work done (J) in one power stroke = Pm x A x L newton-metres = joules.

Work per second (J/s = W) = pm x A x L x n watts of power,

Therefore:

Indicated power = pmALn.

This is the power indicated in one cylinder. The total power of a multi-cylinder engine is that multipliedby the number of cylinders, if the mean effective pressure is the same for all cylinders.

Construction and Working Principle of Indicator:

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Engine Indicator.

An engine indicator consists of a small bore cylinder containing a short stroke piston which is subjected

to the same varying pressure that takes place inside the engine cylinder during one cycle of operations.This is done by connecting the indicator cylinder to the top of the engine cylinder in the case of single-

acting engines, or through change over cocks and pipes leading to the top and bottom ends of the engine

cylinder in the case of double-acting engines. The gas pressure pushes the indicator piston up against the

resistance of a spring, a choice of specially scaled springs of different stiffness being available to suit theoperating pressures within the cylinder and a reasonable height of diagram.

A spindle connects the indicator piston to a system of small levers designed to produce a vertical

straight-line motion at the pencil on the end of the pencil lever, parallel (but magnified about six times)to the motion of the indicator piston. The pencil is often a brass point, or stylus, this is brought to

press lightly on specially prepared indicator paper which is scrapped around a cylindrical drum and

clipped to it. The drum, which has a built-in recoil spring, is actuated in a semi-rotary manner by a cordwrapped around a groove in the bottom of it; a hook at its lower end to a reduction lever system from the

engine crosshead attaches the cord, passing over a guide pulley. Instead of the lever system from the

crosshead, many engines are fitted with a special cam and tappet gear to reproduce the stroke of theengine piston to a small scale. The drum therefore turns part of a revolution when the engine piston

moves down, and turns back again when the engine piston moves up, thus the pencil or stylus on the endof the indicator lever draws a diagram which is a record of the pressure in the engine cylinder during onecomplete cycle.

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Line Diagram of Engine Indicator.

Above figures show an engine indicator which is suitable for taking indicator diagrams of steam

reciprocating engines and internal combustion engines up to rotational speeds of about 300 rev/min. Inthis type, the pressure scale spring is anchored at its bottom end to the framework, and the top of the

piston spindle bears upwards on the top coil of the spring, the upward motion of the indicator piston thusstretches the spring.

Types of Indicator Diagrams:Four types of indicator diagrams or cards can be obtained from a slow-running diesel engine:

1. POWER CARD:

This is taken with the indicator drum in phase with piston movement. The area within this diagram

represents the work done during the cycle to scale. This may be used to calculate the power produced

after obtaining the indicated mean effective pressure of the unit.

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2. COMPRESSION DIAGRAM:

This is taken in a similar manner to the power card but with the fuel shut off from the cylinder. The

height of this diagram shows maximum compression pressure. If compression and expansion line

coincide, it shows that the indicator is correctly synchronized with the engine.

3. DRAW CARD or OUT-OF-PHASE DIAGRAM:

Taken in a similar manner to the power card with fuel pump engaged but with the indicator drum 90*

out of phase with piston stroke. This illustrates more clearly the pressure changes during fuel

combustion.

4. LIGHT SPRING DIAGRAM:

Taken similar to power card and in phase with the engine stroke, but this diagram is taken with light

compression spring fitted to the indicator. This shows clearly pressure changes during exhaust and

scavenge in enlarged scale. This can be used to find any defects during those operations.

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TWO-STROKE CYCLE.

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Typical Power Card with Out Of Phase Card taken on the same Diagram.

Trace of a power card taken over a full cycle with the card opened out so that the compression

curve appears to the left of the vertical (tdc) line and the combustion and expansion occurring to

the right of the same line. This is common way for electronic monitors to record events in the

cylinder, again relevant pressures and angles may be well recorded on the print out.

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Card taken by Electronic Device.

Typical print taken from an electronic measuring device. Pressure and their relevant angles are automatically printed on to the card.

Very useful for checking engine performance.

-------------: Early Injection. T.D.C

-------------: Normal Injection.

-------------: Late Injection.

-------------: Late Injection with After Burning. *****************************Kv************************************

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3. Combustion.

This is an exothermic reaction (one in which heat is liberated by the action) between a fuel and oxygen.

Liquid fuels consist of carbon, & hydrogen, in the form of hydrocarbons, with small quantities of

sulphur & traces of other metallic Impurities such as vanadium.A typical fuel analysis, by mass would be:

C = 5%, H2 = 12%, S = 3%, with a C.V. of 44000 KJ/Kg.(19000 BTU/lb.)The oxygen is obtained from the air, which can be considered to contain 77% nitrogen & 23% oxygen

by mass.

The nitrogen plays no active part in the combustion process but it is necessary as it acts as a moderator.

With pure oxygen, the combustion would be violent & difficult to control & it would produce very hightemperatures, creating cooling, metallurgical & lubrication problems.

The reactions, which occur, are:

2H2 + O2 ----------- 2H2O liberating 142 MJ/kg. H2.C + O2 -------------- CO2 liberating 33 MJ/kg. C.

S + O2 --------------- SO2 liberating 9.25 MJ/kg. S.

2C + O2

--------------2CO liberating 10 MJ/kg. C.Combustion will only occur within limits in the air/fuel mixture. If too much air is supplied all the fuel

will be burnt but the excess of oxygen & nitrogen will carry away heat. If too little air is supplied

incomplete combustion will occur, when all the hydrogen will be burnt but only part of the carbon, with

the remainder only burning to carbon monoxide or not burning at all. In diesel engine practice it is usualto supply between 100 & 200% excess air by mass, though 15% is sufficient for a steady flow

combustion process (boiler).

This difference has two reasons:1. As the combustion proceeds in the diesel engine, the fuel finds less & less air to combine with in

a boiler air is constantly being fed in.

2. More air is needed in the diesel engine as it lowers the maximum temperature, allowing Cast ironto be used.

Combustion Process.

Fuel is injected into the clearance volume towards the end of the compression stroke, as a fine mist ofvery small droplets, which have a surface area many times that of the accumulated fuel charge. These

droplets are rapidly heated by the hot compressed air, which has a temperature of between 550* to

650*C, causing vaporisation. The vapour mixes with air and when the mixture exceeds the spontaneous

ignition temperature, (S.I.T.) combustion begins.

The process can be divided into four phases :

1. Injection delay.2. Ignition delay.

3. Constant volume combustion.

4. Direct burning.

Injection delay:

A time lag of about 0.005 seconds occurs between trapping the fuel charge in the pump barrel and

starting injection into the engine cylinder. This is due to:a) Elasticity of high-pressure fuel lines & system.

b) Slight compressibility of the fuel charge.

c) Leakage past the pump plunger & injector needle.d) Opening delay of the pump discharge valve & injector needle.

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In a slow speed engine the lag period accounts for up to 5* of crank movement. In a high speed engine it

may account for 20* or more and because of point (a) it is necessary to use fuel lines of similar lengthfor all cylinders, when the fuel pumps are grouped together.

Ignition Delay.

Ignition delay is another short period of time delay, which is sufficient to account for several degrees ofcrank angle. Several factors are involved:

a) Spreading and penetrating of the fuel in to the clearance volume space.b) Heating of the fuel to cause vaporization & then exceeding the fuels spontaneous

ignition temperature.

c) Mixing of the fuel & air in the clearance volume space before detonation.

Constant Volume Combustion.

Ignition occurs at T.D.C. when the fuel charge, which has entered during the ignition delay period, burns

rapidly causing a sharp rise in cylinder pressure with little movement of the piston occurring. Modernfour stroke engines may attain 100 bar; at this point where as a two stroke engines are likely to operate

with pressures of 75 to 98 bar.

Direct Burning.The remainder of the fuel burns as it enters the cylinder and mixes with air. The excess air and

combustion gases prevent high temperatures and rapid combustion so the pressure remains about

constant. Injection and combustion should cease simultaneously at the end of this period.

Factors Affecting Combustion.

In order to attain good combustion it is essential that:a) Sufficient air is supplied.

b) Compression is high enough to give a temperature above the spontaneous ignition temperature.

c) Good mixing of the air and fuel is obtained.

All of these give problems. The factors affecting combustion are:

1. Atomisation.2. Penetration.

3. Turbulence.

1. Atomisation.

The rate of heat absorption and burning depends upon the surface area of the fuel particles. As this mustbe rapid it follows that the surface area needs to be big & this is achieved by breaking up the fuel into

small droplets. The amount of the fuel pressure, diameter of injector nozzle holes and the viscosity of

the fuel, affect the process.

2. Penetration.

To use all the air in the combustion space it is necessary to give the fuel particles sufficient energy to

enable them to penetrate to the extremes of the space. This is controlled by the fuel pressure, the size ofthe particle & the length to diameter ratio of the nozzle hole (From 2:1 to 5:1). The latter also controlsthe angle of spray.

3. Turbulence.

To aid mixing of fuel with air and atomisation, friction between the fuel & air is needed. Friction is afunction of the relative velocity between the fuel particle and the air, and may be obtained by either of

two methods.

a) Fuel seeks air.

b) Air seeks fuel.

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a) The air is static or slow moving and the mixing energy is obtained from the fuel particles.

Injection pressures of 200 to around 1000 bars are needed from multi-holed nozzle injectors.Advantages are, simplicity, economy and easier for cold starting the engine. The latter because

little air movement means reduced heat loss to the cold liner and piston crown (also assists in the

burning of heavy fuel). Disadvantages are in producing and sealing high fuel pressures.

b) The air is made to swirl rapidly at the end of the compression stroke by using a pre-designedcombustion chamber. Single holed nozzles and lower fuel pressures are used, 70-100 bars.

Advantages are simplicity of injection, equipment and rapid combustion (useful in high speedengines). Disadvantages are complicated combustion chambers and high rate of heat loss tosurroundings. Causes difficulties in cold starting, sometimes needing cylinder combustion space

heating system.

In practice, a combination is often used minimum fuel pressures being used with a small degree of swillproduced by vaned inlet valves or tangentially cut scavenge ports. Quantity of swirl causes half the

liner circumference to be traversed during combustion.

Combustion Faults.

Detonation.

The combustion process is regarded as a controlled explosion with a flame front speed of about 25 m/s.

However if combustion conditions are not correct double ignition may occur and a detonation mayresult. The latter occurs when the mixture is rapidly compressed by an initial ignition and the remaining

mixture is overheated and burns almost instantaneously (Flame speed 2000 m/s). The detonation can set

up very high pressures, temperatures and causes vibration of the cylinder and piston. It also reduces theefficiency of the engine as energy is absorbed producing the vibration.

After burning.

This occurs when combustion extends into the expansion period after the injector has closed. It is causedby poor ignition qualities or very poor atomization and produces high exhaust pressures and

temperatures.

Injection timing.

Early injection produces high firing pressures; late injection produces low firing pressures and high

exhaust pressures. In both cases the engine power is reduced.All these faults could be seen very clearly in indicator cards of each unit.

Ideal Combustion.

To obtain maximum thermal efficiency, the combustion process should be carried out as close to the

Otto cycle as practically possible. This means, the rate of rise of pressure should be as rapid as possible,without exceeding the designed mechanical and thermal loading. To achieve maximum mean effective

pressure the fuel remaining after the initial period of rapid rise, should be burned at a rate which will

hold the cylinder pressure constant, at the maximum design value until the fuel is burned.Some of those factors affecting the ideal combustion can be considered as follows.

Injection timing.Using jerk injection system, it has been found that the shortest delay period occurs when it includesT.D.C.

1. Early injection results in increased delay since the pressure and temperature are still

rising, so auto injection energy has not been reached.2. Late injection causes increased delay since the piston is accelerating away from the

cylinder head and temperature and pressure fall rapidly.

In each case, the rate of pressure rise is increased due to the large quantity of the fuel in the combustionspace before the chemical reaction is initiated. The reaction, which follows involves a massive amount

of fuel and approximates to detonation.

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This results in Diesel knock, the effects of which are determined objectionable. Many engines are

timed later than that which gives maximum mean effective pressure to reduce the rate of pressure riseand the maximum pressure. This however involves some sacrifice in efficiency and power output.

Engine R.P.M.

Since the delay period is determined mainly by the fuel characteristics, it follows that delay tends to be

independent of engine speed. The delay angle however will vary with engine speed and haveconsiderable influence on the pressure / crank angle diagram.

In each case 10 deg. BTDC & 20deg. BTDC the delay angle is increased with increase in speed.

- - - - - - -: High Speed. -----------: Slow Speed.

Other factors influenced by engine speed may include.

1. Fuel spray characteristics (since fuel pumps are engine driven and pressure and temperature in

cylinder affect secondary atomisation).

2. Volumetric Efficiency (since the piston speed & valve opening characteristics influence the gasexchange process).

3. Combustion chamber wall temperature (since rate of heat input & rate of heat conductiondetermines the wall temp).

Fuel / Air Ratio.

As fuel is being injected there will be local fuel-air ratios varying from infinity near the injector to zero

where fuel vapour has not yet reached. Provided the vapourisation is not complete before injectioncommences the amount of fuel injected would have no direct effect on the delay period. However, with

reduced Fuel /Air ratio, combustion temperatures are lowered, which reduces the cylinder wall

temperature. With some engines, this may have the effect of increasing the delay period.

Varying Fuel/Air Ratio Diagram.

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From the above diagram it may be seen that:

1. The delay period is not effected.2. There is little reduction in rate of pressure rise.

3. Provided that only a small proportion of the fuel is injected during the delay, it will have limited

effect, on the maximum pressure.

Combustion can take place with extremely low Fuel/Air ratios, probably due to burning taking placeclose to the injector where the local F/A is high enough for stable reactions to occur.

Turbulence.The turbulence effect is probably associated more with the mixing process rather than with propagationof chemical reactions.

Turbulence takes place possibly in two ways:

1. Primary turbulence : Due to the way in which the air enters the cylinder. In large diesel enginesthis is produced by the angularity of the inlet ports, near the end of compression, when the air

density is high, the effective swirl will be greatly reduced.

2. Secondary turbulence : Squish, is produced, by the shape of the piston crown and cylinder head.The air is made to move readily inward and across the path of the automised fuel. This may help

to secure short second and third combustion stages.

Turbulence after complete combustion, say due to detonation, can break down the cool insulating layer

of gas near the cylinder head wall, which will:1. Reaches cylinder wall temperature locally (Hot spot).

2. Increases heat loss to cooling water.

3. Breaks down the oil film on the cylinder walls. Promotes micro seizure and service wear.

Compression ratio.

The compression ratio determines the air pressure and the temperature at the moment of fuel injection

and will have a considerable influence on the degree of secondary automisation, the delay period, andthe rate of rise of maximum pressure. Increasing the compression ratio alone, in the range used for diesel

engines, has only a marginal effect on the power developed and cycle efficiency.

High compression ratio, do however increase cylinder friction loss, ring leakage, and starting torque

requirements. With highly pressure charged engines, the cylinder air charge is increased which allows

more fuel to be burnt, but if working close to the Otto cycle the maximum pressure can be high. To limitthe maximum pressure and therefore maximum stress the engine is designed to operate with lowest

compression ratio consistent with satisfactory running and starting.

Turbocharging.

This has the tendency of raising both the pressure and temperature at the point of fuel injection. This is

beneficial in reducing the delay period and the rate of pressure rise. The degree of supercharging islimited not so much by combustion considerations but by durability and reliability of the components

concerned in stressing the high maximum cylinder pressure and high heat flow rates.

Air inlet and Jacket water Temperatures: Increasing both of above:1. Reduce the delay period.

2. Reduce rate of pressure rise.

3. Reduce heat flow to the cylinder coolant.4. Reduce power developed due to reduced air mass.5. Increases cylinder wall temperature.

6. Increases cycle efficiency due to reduced heat loss.

Increasing the air inlet temperature has the effect of down rating the engine and lowering the smokethresh-hold. In each case this is due to the reduced air mass. This effect can be pronounced when

operating in the tropics, where both air and sea temperatures are high. One should keep in mind that,

while operating in cold climates, where sea and air temperatures are low, the inlet air temperature shouldnot be brought down too low, as humidity in the air may cause corrosive damage to cylinder liners.

***********************End of Combustion/BIT/AMET/BE/Motor/Kv/May 2003********************************

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4. Marine Diesel Structural Parts:

MAIN STRUCTURE OF MODERN LARGE POWER SLOW SPEED DIESEL ENGINE.

Type: MAN/B&W: MC Type Engine.

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Bed Plate:The bedplate is a substantial, rigid structure which forms the base on which the engine is built. It issupported by the ship structure through the double bottom arrangement, but this support does not reduce

the rigidity needed & in fact with some modern vessels, the hull is left flexible and the bedplate stiffened

so that a simple four-point attachment to the hull can be used. This reduces the distortions developed inthe bedplate when hull deflection occurs.

Forces applied to the bedplates:1. Firing load from cylinders.2. Side thrust from guide faces.

3. Unbalanced inertia forces in the running gear.

4. Weight of engine structure & running gear.5. Torque reaction from propeller.

6. Hull deflections due to hogging, sagging, racking.

7. Vibration due to torque variations, shock loading.8. Thermal stresses due to atmospheric and lubricating oil temperature changes.

9. Inertia & gyroscopic forces due to ship's movement in heavy seas.

In addition to withstanding forces due to the above causes,, the bedplate should provide.

1. An oil tight chamber to contain the oil splash & spray of the forced lubricating oil system.2. A drainage grid to filter out large particles before they enter the oil sump or drain tank.

3. A housing for the thrust bearing.

Having provided for all the above the bedplate should also be small & light to keep the overall size andmass of the engine to a minimum.

Basic Structure:

The bedplate consists of longitudinal and transverse girders as shown below:

Longitudinal Girders may be single or double plate construction.

Single plate type. Double plate type.

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Their purpose is to maintain longitudinal alignment by providing sufficient rigidity to withstand the

hogging & sagging of the hull structure & provide a stiff support for attachment of the transversegirders.

The double plate form is stiffer but more complex than the single plate and makes access for holding

down arrangements more difficult. The single plate form is becoming more popular with the use of box

bedplates and similar construction of columns.Transverse Girders are deep plates lying between the longitudinals and fitted with pockets to carry the

main bearings. A deep plate is needed to give sufficient stiffness (I-value) to resist the firing loadwithout bending. Inadequate stiffness will cause distortions of the bearing pocket, which will nip themain bearings, gripping the crankshaft journal and causing wiping of the bearing.

The girders may be of single or double plate construction with a flat plate on the top to give a landingfor the A frames or equivalent.

The double plate arrangement provides the greatest strength & stiffness but holes must be cut in theplate to allow access for welding and inspection. These holes must be large enough to allow easy entry

by a welder and can, seriously weaken a double plate arrangement for a small engine. To restorestrength & stiffness a tube may be welded through the girder holes.

Depending upon the material used, the attachment of the transverse girders to the longitudinal girders

may differ most are welded but some may be bolted if the girder is cast as this reduces repair difficulties

allows stress relieving of the girder only and lessens risk of distortions.

Types of Bedplates.

The two most common types are:1. Box type.

2. Trestle type.

1. Sulzer, B & W, MAN and Doxford, all use the box or flat bottom type as it can be mounted

directly to the tank top plating (via chocks) and is suitable for fabricated construction.

2. G.M.T. & Mitsubishi are examples of engines still using the trestle type. This type provides a

deep and therefore stiff transverse section. To accommodate this deep suction however thebedplate must be seated on special built up stools in the double bottom structure or a special well

must be left in the double bottom structure. Both complicate the double bottom structure. If the

well is used an added attraction is a reduction in engine height.

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Flat or Box type Bedplate Construction.

Tie-bolt.

Trestle type Bedplate Construction.

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Bedplate Materials.

1. Fabricated mild steel . Slow speed engines Sulzer, Doxford.

2. Cast iron . Medium speed engine (small).

3. Composite type. Fabricated mild steel longitudinal girders and cast steel transverse girders.

Engines that use above are B & W, G.M.T. Mitsubishi, MAN. Sulzer.

1. The all welded form of construction gives the lightest bedplate (about 25% less than C.I.) withthe greatest strength against shock loads & the highest guarantee of manufacture. It is also theeasiest to repair. However it possesses poor vibration damping characteristics & due to the

multitude of welds is liable to cracking. To ensure freedom from distortion the welding sequence

must be correct and after welding the bedplate requires stress relieving by heating to 600*C andholding for 1 hour/inch (25 mm) of plate thickness. Normal plate thickness is 1 - 2 (35-50

mm). The size of the bedplate is controlled by lifting equipment available and the size of the

stress-relieving furnace. Because of these factors plates are normally made in at least two parts.Transverse girders are normally cut from a single plate and supporting ribs welded on below the

bearing pockets. Pockets are usually of cast steel. Examples: M.A.N. & SUIZER Engines.

2. Cast Iron : It is never used for large bedplates any more as the quality of generator of a defect free

casting is not good enough. Frequently used for small engines however. The main advantage is

the materials ability to absorb vibration (not shook), which limits vibration transmission throughthe engine & reduces the frequency of cracking in the bedplate. Any cracks are difficult to repair

& require a Metalock type repair, which cannot be effected by ship's staff. The material has a

low tensile strength and is usually supported by tie-bolts. Examples: Only small medium or high-speed engines use this type of bedplate.

3. Composite construction involves fabricated mild steel for the longitudinal girders and cast steel

for the transverse girders. This system has the advantage of a continuous transverse girder withthe bearing pocket integral. Strengthening ribs can be cast in and the complete unit stress

relieved before bolting or welding to the longitudinal girders. The cast steel must be of wieldablequality, up to 0.23% C. The steel has a higher resistance to cracking compared to fabricated mild

steel due to the irregular grain flow and lack of welds. Examples: B & W, Doxford, G.M.T.,

Mitsubishi engines.

The following surfaces of the bedplate must be machined:

1. Top face: For attachment of A frames.

2. Bottom seating face: For chocks, tie-bolt heads and oil sump pan.

3. End face: For thrust block housing, turning gear & end chocks.4. Side face: For side chocks and Entablature cover plates.

Faults found in Bedplates:1. Cracks.2. Oil leaks.

3. Loose chocks.

4. Loose A frames.

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1. Cracks usually occur:

i) Under bearing pockets on fabricated mild steel bedplates.

ii) Radially around tie bolt & frame boltholes.

iii) Between longitudinal and transverse girders.iv) Around lightening holes.

v) At the base of serrated seating for main bearing keeps.

Causes may be:

a) Bearing wear & therefore overloading.

b) Slack tie bolts.

c) Vibration.d) Poor welding or stress relieving.

e) Stress risers on welds -(Coarse welds should be ground).

Repair :

For mild steel & cast steel crack chipped out and welded, but care should be taken to ensure a minimum

distortion by determining the optimum welding sequence.

For Cast Iron the crack should be arrested by drilling a small hole, sketch or photograph the crack for

future assessment. The crack could be Metallocked or supported by a mild steel doubling plate, boltedon, if serious.

2. Oil leaks:

i) Sump pan.

ii) Doors and casings.iii) Crank case relief valves.

iv) Bedplate cracks.

3. Chocks may fret if the holding down bolts get lack and due to the movement of bed plate chocksbed into the tank top. As a temporary measure the chock should be shimmed up and the bolt

hardened down and as soon as possible the chock should be removed, the tank top faced up bygrinding and a new, thicker chock prepared and re-bedded.

Bedplate inspection.

1. Cracks.2. Corrosion. This may be due to moisture or acidic compounds in the oil. If the bedplate has been

painted, remove flaking paint and cheek for pitting. After that do not repaint.

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3. Cleanliness. Check for sludge and carbon building up in corners, under bearings, behind bearing-

cover studs, etc.4. Loose connections - bolted transverse girders, A-frames, oil pipes, sump grids, chocks and

holding down bolts.

5. Oil leaks - through cracked welds, loose sumps, leaking seals.

6. Faulty welding - on new engines - under cutting, blowholes, slgg; etc.7. Faulty castings - porosity, blowholes, inclusions etc.

MAN Engine Bedplate.

Engine Bedplate sketch.

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Bedplate

The bedplate acts as the main strength member, maintains correct alignment and supportsthe weight of the components. it must be capable of withstanding the fluctuating forces

created during operation and transmit them to the ships structure. In addition it may alsocollect lubricating oil. In slow speed engine design, it consists of a deep longitudinal box

section with stiffening in the form of members and webs.

Transverse members are fitted between each throw of the crankshaft. These

support the main bearing saddles and Tie -rod connection. They are attached to the structure

by substantial butt welds.

To reduce the engine height the sump of the bedplate may be sunken allowing itto fitted into a recess in the ships structure.

Plate and weld preparation is required with welds of the double butt type ifpossible. Regular internal inspection of the parts especially the transverse girder is required

for fatigue cracking. Tie bolts should be checked for tighteness.

Box girders-A box girder is stronger and more rigid then I or H section girder of the samec.s.a.

From the simple beam bending equation we have;

M /I = s /y = E/R

M=Bending moment

I=2nd moment of area of the cross sections =Stress

y=distance from the axis of bending to the outer faceE= modulus of elasticity

R-radius of curvature of the bending.

This can be arranged into

s = (M/I) . y

It can be seen that for the same bending moment on a symmetrical shape ofsame size, the stress is reduced on the increasing 2nd moment of area. The second moment

of area increase with moving of material away from the axis of bending towards the extremes

of the section.

Because of this the commonest way of construction a fabricated bedplate is bycreating two box section girders and tie them using transverse girders.

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The advent of the small bore slow speed has seen the use of single side

bedplates. A box section is then created by using a box section crankcase structure rather

than the more traditional A-frame.This has the advantages of reducing width as well asweight and increasing the amount of fabrication so reducing assembly times.

Due to the weight penalty, the use of cast iron is generally limited to smaller

units where fabrication becomes impractical. However, cast iron has internal resilienceallowing it to dampen down vibrations, this has led to its usage on some medium speed

installations, especially passenger carriers, where noise and vibration suppression isimportant. .

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The most highly loaded pat of a bedplate is the transverse girder. Classificationsocieties require that residual stress is removed after construction.

The transverse girder acts as a simple beam with the forces of combustion acting

on the piston passing down through the bearing. The forces acting on the head are passedthrough the Tie rods.

It can be seen that to reduce the bending moment the tie rods have to bebrought closer to the crankshaft. The limit to this is the securing arrangement required for

the main bearing keep. One method is to use two instead of one bolts which can be made of

smaller diameter. Sulzer use an alternative and very successful method in the form of jackingbolts. These jack against the bottom of the A-frame.

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.

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ENGINE CHOCKS.

These are needed between the bedplate and tank top to ensure that any variations in the surface of the

tank top does not cause misalignment. Up to 200 chocks per engine may be fitted. They also permit any

chaffing or fretting to be repaired by adjustment of individual chocks and any subsequent distortionsafter fitting (due to settlement) to be corrected.

End chocks are fitted at each end of the long girder to position the engine, absorb collision loads and in

the case ofthe integral thrust block, absorb propeller thrust & propeller excited vibrations.Side chocks are needed to absorb side loads due to components of unbalanced reciprocating forces andthermal expansion. They also prevent chaffing of the supporting chocks and tank top and also help the

holding down bolts resist the lateral forces when the vessel is rolling.

Chocks are usually made of cast iron or steel. Cast Iron chocks are popular because:

1. Easy to form.

2. High compressive strength & low malleability.The chock retains its shape under load reducing the chance of bolt slackening & therefore bolt fracture.

Unfortunately this also means that the chock is hard and liable to bed into the tank top or bedplate. It is

also brittle and therefore liable to fracture under excessive impact loads, hence minimum chock

thickness should not be less than 30 mm. Steel is used to reduce those problems and allow easier fitting.Steel chocks should be used for clearances less than 30 mm.

Epoxy resin is increasing in popularity and now widely used for small, medium & large engines. The

compound has the following advantages:i) Elimination of fitting & machining.

ii) Increased support as large areas of the bedplate can be used.

iii) Elimination of breakage, fretting and slackness.iv) Improved resilience, which absorbs vibrations, reduces noise and gives greater

ductility.

The compound is suitable for any bedplate, which can be fitted with a sealing dam to contain thecompound while it is setting (may take up to 24 hrs with some heating, around 16*C necessary). It can

be used on new engines or as a replacement on old engines. Where the chock is deep, steel rollers areadded to the resin chock to increase strength & Durability.

Fitting of engine chocks:

Process is more or less similar for cast iron, steel or resin chocks.1. Bedplate is aligned on the tank top using temporary chocks, jackscrews or wedges using the

sagging wires pilgrim or optical alignment method.

2. Crankshaft is budded & deflections taken after the engine is fully built up and the vessel isfloating in even keel with all transmission shafts in place.

3. Metal chocks are machined slightly oversize and then hand filed and scrapped. It is bedded in its

place and fitted. Minimum 70 to 80% bedding is required. For bedding purpose the chock couldbe tapered up to 1/100 from outside to inside.

4. For Resin Chocks the surfaces are cleaned, a dam prepared around the chock area, holding

down bolts placed in position and greased and all surfaces sprayed with a releasing agent. Resin

is mixed and poured into position. When solid, temporary support can be removed and after 24hours, holding down bolts tensioned. A 1mper mm of chock thickness is allowed for shrinkage

5. Crankshaft deflections are retaken to confirm alignment. The deflection reading should be the

same at the end of fitting the chocks as it was when taken before fitting as per step 2.

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A third material, rubber is used for some installations; usually high speed diesel engines in small vessels

These are resilient mountings and fitted to reduce vibration transmission from engine to hull or viceversa. Very careful selection of the right size and stiffness must be made in order to obtain the optimum

operation and sufficient flexibility must be arranged in all connections to the engine to prevent any

restrictions. This includes the output shaft coupling. Generally flexible hoses connect all pipelines and

for engine exhaust pipe line a good metal exhaust bellow is fitted. 4 to 8 mountings are normal.

Some points regarding Epoxy Resin:Resin chocking is a recent development in engine chocking arrangement. It has been widely used nowfor large, medium and small engines. It has following advantages:

1. Reliable permanent alignment without machining foundation, bedplates or chocks.

2. Provides uniform precise mounting for superior retention of critical alignment.3. Resists degradation by fuels, lubricants, eliminates corrosion in chock area.

4. Non-fretting.

5. Reduces noise levels, maintaining the alignment and hold down bolt tension.6. The modulus of resin helps to maintain crankshaft deflection and machinery alignment during

hull flexure or distortion.

MOST IMPORTANTThis is a liquid; it conforms to all irregularities in the fitting surface, providing a precise contact fit

between machinery bases and foundations (after solidification).

Properties of the chock after it has cured:i) Compressive strength: 1330 kg/cm2.

ii) Tensile strength: 350 kg/cm2.

iii) Shear strength: 380 kg/cm2.iv) Heat distortion temperature: 93*C.

Bedplate holding down bolts.Holding down bolts may be fitted or clear. If collision & side chooks are used the bolts are usually clear.

If not the bolts at the flywheel end are fitted, remainder clear, to ensure the coupling to output shaft isnot strained. Bearing faces of bolt heads & nuts must be normal to the bolt shank & parallel to each

other to prevent any bending stresses. If necessary the bedplate & tank top may be machined.

Traditional holding down bolt arrangement.

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Procedure for fitting the above holding down bolt.

a) Harden the stud into screwed tank top to achieve watertight seal on the conical face.b) Tighten lower nut, tack weld or caulk over thread for locking.

c) Tighten upper nut.

Modern method of holding down bolt arrangement.The traditional method suffers the problem of fretting cast iron chocks and bolt failure particularly under

slow speed diesel machinery. In modern days to eliminate the above problem long elastic bolts withextended collars are used.These bolts possessed high resilience and are highly stressed when tightened. As such when strained

while in service, there will be less reversal of stress which results in reduced possibility of fatigue

failure.When these bolts are tightened and slightly stretched, the bedplate, chock and tank top seating are under

compression. When holding down bolts come under strain while in service, the parts under compression

expand and the mating surfaces of the chocks remain in contact with the bedplate and tank top seating.Fretting is hence avoided.

Their cost is considerable and an additional 40,000 for the bolts of a 6-cylinder engine is typical.

Epoxy Resin chock.

Chockfast system needs only simple bolts and nuts to give permanent engine security. It is claimedthat the use of pour able resin chocks overcomes bolt stretching, slack nuts and bolt failure, while also

offering considerable economies when erecting the engine since perfect matching takes place between

engine bed-plate and the unmachined tank-top seating.

Line diagram for holding down bolt and chock.

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Modern slow speed main engine bed plate arrangement over ships structure.

Bedplate holding down arrangement for the above engine with long bolt.

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Earlier engines bedplate and holding down arrangement showing main chock, side chock, end chock

and hydraulic stretching tool.

Chocking arrangement with tall bolts and washer system of holding down bolts.

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Inspection requirement pertaining to holding down bolts and engine chocks.

Holding down bolts are strained while in service and thus required to be tightened up occasionally if

troubles with bedplates is to be prevented. Even the most imperceptible movement of the bed plate will

cause fretting to occur on the bedded mating surfaces of the bed plate, chock and foundation plates. If

fretting occurs in areas covering a number of adjacent chocks, the crankshaft may be seriously damagedthrough misalignment.

New installations should have the bolts checked after a few running hours and at least every six monthsafter that. A record should be kept. These holding down bolts should be checked fully if the vessel hadmet with an accident, such as grounding near engine room, fire in engine room or near the engine room

and collision.

Bedplate Inspection

1. Cracks (split around the parts mentioned earlier).2. Faulty welding - on new engines (under cutting, blowholes, slag etc.).

3. Faulty castings - porosity, blowholes, inclusions etc.

4. Corrosion.

5. Cleanliness - sludge and carbon build up in corners, under bearings, cover studs etc.6. Loose connections - bolted transverse girders, A-frames, oil pipes, chocks and holding down

bolts.

7. Oil leakage.

***************************************Kv******************************************End of Bedplate/chocks/BIT/AMET/BE/KV/May 2003.

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Engine Frames and Cylinder Blocks:

Engine Frames.

These fit between the bedplate and cylinder block beam. They are sometimes referred to as the

entablature. They serve the following functions.a) Support the cylinder blocks, turbo-chargers, camshaft and driving gear, scavenge belt etc.

b) Provide a facing for the girders & absorb the guide forces.

c) Develop an oil tight easing, for forced lubricating oil system, & support pipes &

walkways.

A Frames.

In old engines the frames were of cast iron and made hollow to reduce weight without reducing rigidity.

The frames or columns were held in compression by tie-bolts. These frames were later fabricated frommild steel tube and plate with guides of cast iron bolted onto the frames. This type of arrangement uses

individual frames at each transverse girder position of the bedplate with the longitudinal spaces between

frames filled by plates bolted to the frames. The structure is strong and rigid in the transverse plane butrelatively flexible longitudinally. This makes oil tight fixing of the side covers difficult unless very

heavy covers or longitudinal stiffness are used. It also produces a weak structure if exposed to internal

pressure from a crankcase explosion and will allow alignment of the cylinder blocks to the bedplate tovary in relation to ship movement.

The A-frame construction is now being abandoned in favor of longitudinal girder construction.

Improved methods of prefabrication which can be relied upon to produce large, distortion free units has

allowed longitudinal girders to be manufactured so that the longitudinal stiffness of the structure can beincreased without altering the transverse stiffness. This also contributes to the bedplate stiffness and

reduces effects of hull hogging and sagging. MAN engine manufactures claim that the bedplate only

contributes 17% to the overall stiffness compared to 60% for the traditional A-frame construction.

In the Sulzer engine the fabricated longitudinals form a sandwich by enclosing a cast iron centerpiece

at each transverse girder spaces. The cast iron centerpiece forms the crosshead guides. The structure isbolted together.

In the B&W engine the entablature retains the A transverse section but both longitudinals and

transverse components are fabricated into a box form. The guide faces are bolted to the transversecomponents. The entablature is formed in two pieces connected at the camshaft drive position at the

middle of the engine.

In the MAN engine, regular box shaped fabrications are used, again with longitudinal and transverse

sections welded together to form a single unit. The layer sizes (more than 700 mm bore) have the box

divided into 2 on the horizontal plane. The upper box has openings on the back into which the cast ironguide faces are bolted. In the Doxford-J engine a continuous girder is fabricated for the guide side of

the framework with the columns at each main bearing position welded to the longitudinal. The front of

the engine is left more open to allow easy access to the running gear.Apart from increased stiffness which reduces:

i) Misalignment,

ii) Bearing distortion,

iii) Vibration,The structure is more oil tight, as fewer joints are required & the structure works less. It is also easier

to build the engine & ensure equivalent alignment when the engine is reassembled in the ship.

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M.A.N. Engine Bedplate, Lower frame, Upper frame and Cylinder jacket.

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Transverse section of Sulzer Engine, showing all internal bolts and fittings.

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Doxford Engine Structural Arrangement. B&W Engine Structural arrangement.

Tie Bolts.These are fitted to relieve the frames of tensile stress.

The bolts are mounted between the transverse girder of the bedplate and the upper face of the cylinder

jacket. As this in variably makes the bolt very long it is sometimes fitted in two lengths joined at thebase of the cylinder jacket. Hydraulic tightening tensions the bolt and this pre-tensioning should be

sufficient to keep the frames in compression throughout the engine cycle. This produces a substantial

tensile stress in the bolts requiring them to be checked frequently.Transmission of Firing Load.

In most single acting engines, apart from Opposed Piston Engines, the long tie bolts transmit the main

gas loads from the cylinders. Two bolts are fitted to each transverse girder and they pass through thecasting through tubes constructed in the engine frames and through the entablature or cylinder jackets

where locking nuts are fitted. Tie-bolts are prestressed during assembly and carry the firing forces from

the cylinder cover to the transverse, beam and thence the ship's hull. Tie-bolts should be as close to the

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crankshaft axis as possible to minimise bending stress on the transverse girders of the bedplate and to

prevent unbalanced loads being transmitted to the welds.The further the tie-bolts are the greater will be the bending stress. Hence any method of bringing the tie-

bolts close together will decrease the stress. Therefore Sulzer and Fiat engines have used jacking

bolts between A-frames and main bearing upper half keeps. This ensures that the tie-bolts are as close as

together as possible. Great care must be taken that the tie-bolts are correctly tensioned before tensioningjacking bolts otherwise if the tie-bolts were tensioned after jacking bolts, the latter and main bearing

keeps could be over-stressed.

Sulzer Engine Main Bearing Jack bolt arrangement (See page 40 drawing for full details).

CONSEQUENCES OF RUNNING AN ENGINE WITH SLACK TIE BOLTS:Cylinder beam would flex and lift at the location of the slack bolt landing faces of the tie bolt upper and

lower nuts, landing faces of the cylinder beam on the frame would fret and machined faces would

eventually get destroyed. The fitted bracing bolts between the cylinder jackets will also slacken and thefit of the bolts would be lost.

If fretting has occurred in an uneven pattern where the cylinder beam lands, and the tie bolts are

tightened, the alignment of cylinder to the piston stroke will be destroyed. The fitted bracing bolts

between the cylinder jackets will also slacken and fit of the bolts will be lost.Fretting may make the nut landing face out of square and if tie bolts are tightened on the damaged face,

a bending moment will be induced in the tie bolt, this may cause an uneven stress pattern in the tie bolt

which could lead to early fatigue failure. Damage may take place in the bedplate in way of cross girder.

Rigidity of the whole structure will be destroyed. Guide force will have to be absorbed by the framebolts and dowels, which may stretch and slacken allowing the structure to work. This may destroy the

piston alignment. Guide faces and bars may get slackened (these are bolted to the supporting structure)

TENSIONING OF TIE RODS AND CHECKING THE PRETENSION (SULZER RLA. ENGINE).

Bedplate, columns, cylinder jackets are greatly relieved of the gas forces set up and freed from tensile

stresses when tie rods are properly tensioned. In order to avoid vibration all tie rods are held in positionby special guide bushes located on the lower end of the cylinder jackets. These bushes are of two parts

and clamped on to the rods. Clamping bolts jam the tie rods in the bores. Tie rods are pretensioned by

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hydraulic tensioning device. Tightening is carried out in two steps to avoid to reduce additional stresses

on the jackets.Note: For new engines it is recommended that all tie rods be checked for correct pretension after the first

year of service and if necessary pretensioned to the valve specified. After that it is sufficient to make

random cheeks during major overhaul. The bolts shou

marine diesel engine gme competence 6

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