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Learning Objectives

Understand the various terms related to diesel engines

Differentiate between a crosshead engines and trunk piston

engine

Explain working principle of 2-stroke and 4-stroke engines

State advantages and disadvantages of slow speed diesel

engine

Explain meaning of scavenging and supercharging

Explain Diesel Engine ancillary system

Diesel Engines

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Internal Combustion (I.C.) Engines are

Diesel Engines

Petrol Engines

Jet Engines

Gas turbines

In I.C. Engine combustion takes place inside the engine ascompared to steam engines and steam turbines where

combustion takes place out side the engine i.e. in boilers

(known as External Combustion Engine)

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Are one type of I.C. Engines which use D.O. or F.O.

Classified as Compression Ignition (C.I.)engines where fuel oil is ignited

inside the engine cylinder by heat of compression of airUnlike petrol engine where fuel is ignited by electric spark

Application of Diesel Engines on Ship:

Main Engine

Aux EngineLifeboat Engine

Emergency Generator

Emergency Compressor

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Use of cheaper grade of fuel

Higher plant efficiency (beyond 50% ) with

improved waste heat recovery unit

40% less fuel consumption than steam plant

Less complex than steam plantHigher reliability & better manoeuvre ability

Less maintenance cost

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Piston actionSingle acting

Double acting

Engine speed

Slow speed engine (55 to 150 rpm)Medium speed engine (300 to 850 rpm)

High speed engine (850 to 3000 rpm)

Piston connectionTrunk piston type engine

Crosshead type engine

Cylinder arrangementInline arrangement

Vee arrangement

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TDC

BDC

Bore

Stroke

Swept volume

Clearance volumeCompression ratio

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Stroke of a Piston

This is the distance traveled by the piston from the TDC

(Top Dead Centre) to BDC (Bottom Dead Centre) or viceversa. Stroke is determined by the length of the crank.

Swept Volume

This is the volume swept by the piston during one stroke.Swept Volume is the product of 'Piston Area' and 'Stroke'.

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Clearance Volume

This is the volume on top of the cylinder when the piston is at TDC.

The clearance volume space forms the Combustion Chamber.

Compression Ratio

This is the ratio obtained by dividing the 'Volume at BDC' by the 'Volume atTDC' (Clearance Volume). It is difficult to start an engine if the Compressionratio is less than 12. Main propulsion engines on board ships, which burn

poor quality heavy fuel oil have Compression Ratio around 14.

swept volume + clearance volume

Compression Ratio =

clearance volume

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Piston is connected directly

to the upper end of the

connecting rod by the

gudgeon pin & bearings

Lower end of connecting

rod is connected to the

crankshaft with bearings

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Piston is fastened to verticalpiston rod

Lower end of piston isattached to crossheadassembly

X-head assembly slides up& down in guides

X-head assembly isconnected to crankshaft byconnecting rod withbearings

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A cycle is a full series of separate steps or eventsessentialfor

efficient engine operation. These steps or events are

Suction of airCompression of air

Injection of fuel

Expansion of gases

Exhausting of gases

There are two types of D.E. cycles 2 stroke & 4 stroke cycles

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Slow speed engines operate on 2-stroke cycle

principle

In two-stroke engine, one working cycle occurs for

every one revolution of the engine crankshaft

Every down stroke of piston is a power stroke

Every upward stroke is a compression stroke

Exhaust of burnt gases & fresh charge of air is taken

during the late period of downward stroke and early

period of upward stroke

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Two Stroke Cycle

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Crosshead type 2stroke diesel engine working principle

Fresh

air

Exhaust gas

Air cooler

PV diagram

Gas turbineRotary aircompressor

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2Stroke Engine

Design Features

100 RPM

Upto 12 cylindersLarge power outputper cylinder (3MW)Uniflow scavenging

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Methods of producing reverse

thrust from propeller1. Direct Reversal, so that propeller turns in astern direction. Slow speed & medium speed

1. Controllable Pitch Propeller, which changes the angle of

blades while the propeller shaft turns in same direction2. A diesel electric system in which the propeller is driven bya reversible electric motor

3. Reverse gear and clutches, which cause propeller to turnin opposite direction

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Medium speed and high speed engines are

generally operate on 4-stroke cycle principle

It takes four strokes of the piston i.e 2 revolutions

of crankshaft to complete one working cycle of

operation

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Four Stroke Cycle

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Gas TurbineRotary air compressor

Fresh

air

Exhaust gas

Air cooler

Inlet valve Exhaust valve

PV diagram

Fly

wheel

Trunk piston 4 stroke diesel engine working principle

Timing diagram

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4 Stroke Engine

Design Features

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Energy Distribution

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Engine Systems

LubricatingCoolingAir for combustionFuel

Air for starting

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Lubricating System

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Cooling System

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Air for Combustion

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Fuel System

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Air for starting

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Medium Speed Engine for

propulsionTypically 4 stroke engine

RPM 300 to 600 RPM

Requires Gear box,

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Medium Speed

EngineDemerits:1. Piston absorbs

generated

by the angle ofconnecting rod2. It is possible for

contaminants toreach crankcase

2

1

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Medium Speed

EngineMerits:1. Smaller engine room, useful for ferries etc.2. Redundancy. If one engine fails

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Need for a Reduction gearThe optimum rotational speed of a mediumspeed engine is 300 to 600 RPM.

This speed is higher than optimum propellerspeed of about 70 RPM

In order to improve the overall performanceof energy conversion system, reduction gears

are fitted to enable each component to workefficiently.

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SummaryTerminology

2 stroke engine

4 stroke engine

Comparison between 2 stroke & 4stroke

Engine ancillary systems