2 Stroke Cycle, marine engineering

7/26/2019 2 Stroke Cycle, marine engineering

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The two stroke cycle is so called because it

takes two strokes of the piston to complete

the processes needed to convert the energyin the fuel into work. Because the engine is

reciprocating, this means that the piston

must move up and down the cylinder, andtherefore the crankshaft must revolve once.

TWO STROKE CYCLE


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The two stroke cycle is completed in two strokesof the piston or one revolution of the crankshaft.

In order to operate this cycle successfully, specialarrangements are required such as:

The fresh air must be forced in under pressureand useful to cleared out e!haust gases.

Instead of valve holes, known as "ports#, areused which are opened and closed by the sides ofthe piston as its moves .


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The crankshaft is revolving

clockwise and the piston ismoving up the cylinder,

compressing the charge of air.

Because energy is being

transferred into the air, itspressure and temperature

increase. By the time the piston

is approaching the top of the

cylinder $known as Top %ead

&enter or T%&' the pressure is

over ()) bar and the temperature

over *))+&


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ust before T%& fuel is in-ected

into the cylinder by the fuel

in-ector. The fuel is atomised

into tiny droplets. Because they

are very small these droplets heat

up very quickly and start to burn

as the piston passes over T%&.The e!panding gas from the fuel

burning in the o!ygen forces the

piston down the cylinder, turning

the crankshaft. It is during thisstroke that work energy is being

put into the engine/ during the

upward stroke of the piston, the

engine is having to do the work.


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0s the piston moves down thecylinder, the useful energy

from the burning fuel is

e!pended. 0t about (()+ after

T%& the e!haust valve opens

and the hot e!haust gas$consisting mostly of

nitrogen, carbon dio!ide,

water vapour and unused

o!ygen' begin to leave thecylinder.


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0t about (1)2 after T%& the

piston uncovers a set of portsknown as scavenge ports.

3ressurised air enters the

cylinder via these ports and

pushes the remaining e!haust gas

from the cylinder in a processknown as scavenging.

The piston now goes past

Bottom %ead &entre and starts

moving up the cylinder, closingoff the scavenge ports. The

e!haust valve then closes and

compression begins.


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( 4 &ompression

4 5 6uel In-ection

5 1 3ower

1 * 7!haust Blowdown

* 8 9cavenging

8 ( 3ost 9cavenging

(. appro! (()2 BT%&

4. appro! ()2 BT%&

5. appro! (42 0T%&

1. appro! (()2 0T%&

*. appro! (1)2 0T%&

8. appro! (1)2 BT%&

The two stroke cycle can also be illustrated on a timing

diagram.


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1

2

3

4 5 6

7

8

PISTONP

OSITIO

N

PRESSURE1

2

3

4 5

6

7

8


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(4 9cavenging period, both e!haust and inlet portsopened.

45 9cavenge stroke ends. 7!haust port remain opens toensure only fresh air remain in the cylinder.

51 &ompression stroke take place. Both ports wereclosed. The air then be compressed by the movement of the pistonupward.

1*8 6uel in-ector in operational to supply fuel oil.

8 %ue to e!pansion of gasses piston moveddownward. $ 3ower stroke'

;


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Rotation

Fuel

injection

begins

Fuel

injection

ends

SCAVENGE

COMPRESSION P

OWERSTROKE

EXHUST

Sca!enge

"o#tso"en

Sca!enge

"o#tsclose

E$%aust

"o#ts

o"en

E$%aust

"o#ts

close

TWO STROKE TIMING DIAGRAM


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In the 4 stroke trunk piston engine, the side thrust caused by the

angularity of the connecting rod is transmitted to the liner by the

piston skirt or trunk. It is therefore known as a 4 9troke Trunk3iston 7ngine. The skirt of the piston also acts to seal the

scavenge air ports when the engine is at T%&. This prevents the

scavenge air from pressurising the crankcase.

=erein lies the disadvantage of this type of engine: although it

has a low overall height, lubricating oil splashed up from the

crankcase to lubricate the liner can find its way into the scavenge

space, causing fouling and a risk of fire. There is also the

likelihood of liner and piston skirt wear, allowing air into the

crankcase. This can supply the required o!ygen for an e!plosionshould a hot spot develop. The crankcase oil must have additives

which can cope with contamination from products of

combustion, and the acids formed during combustion due to the

sulphur in the fuel.

This design of two stroke is generally only used for the smaller

lower powered 4 stroke engines up to about *)))k< for a >(8

engine with a 4;)mm bore and 54)mm stroke.

%etroit diesels manufacture 4 stroke trunk piston engines as do

engine is shown below.


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The 4 stroke crosshead

engine works on e!actly

the same principle andcycle as the 4 stroke

trunk piston engine.
http://www.marinediesels.info/2_stroke_crosshead_engine_access.htm


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The disadvantages of the two stroketrunk piston engine are that although it

has a low overall height, lubricating oil

splashed up from the crankcase to

lubricate the liner can find its way into

the scavenge space, causing fouling

and a risk of fire. There is also the

likelihood of liner and piston skirt

wear, allowing air into the crankcase.

This can supply the required o!ygen

for an e!plosion should a hot spot

develop. The crankcase oil must haveadditives which can cope with

contamination from products of

combustion, and the acids formed

during combustion due to the sulphur

in the fuel.


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The ma-ority of 4 stroke engines encountered at sea are of the crosshead type. In

this type of engine the combustion space $formed by the cylinder liner, piston and

cylinder head', and the scavenge space are separated from the crankcase by the

diaphragm plate.

The piston rod is bolted to the piston and passes through a stuffing bo! mounted inthe diaphragm plate. The stuffing bo! provides a seal between the two spaces,

stopping oil from being carried up to the scavenge space, and scavenge air leaking

into the crankcase.

The foot of the piston rod is bolted to the crosshead pin. The top end of the

connecting rod swings about the cosshead pin, as the downward load from the

e!panding gas applies a turning force to the crankshaft.

To ensure that the crosshead reciprocates in alignment with the piston in the

cylinder, guide shoes are attached either side of the crosshead pin. These shoes are

lined with white metal, a bearing material and they reciprocate against the

crosshead guides, which are bolted to the frame of the engine. The crosshead

guides are located inbetween each cylinder.sing the crosshead design of engine allows engines to be built with very long

strokes which means the engine can burn a greater quantity of fuelCstroke and

develop more power. The fuel used can be of a lower grade than that used in a

trunk piston engine, with a higher sulphur content, whilst high alkalinity cylinder

oils with a different specification to that of the crankcase oil are used to lubricatethe cylinder liner and piston rings and combat the effects of acid attack.


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The most powerful diesel engines in the world are two stroke crosshead

engines. 9ome of these engines have cylinder bores approaching (metre with

a stroke of over 4.* metres. The crankshaft can weigh over 5)) tons, with the

engine weighing in e!cess of 4))) tons


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9&0>7D?ID? 37EIF%

It can be defined as inlet ports and e!haust portswere opened at the same time where:

Eemaining e!haust gas will be e!pelled from the

cylinder through e!haust ports or e!haust valve $if

fitted'.

6resh air which have been collected in the

scavenge manifold rushing entered the cylinder

9cavenging period normally occur when the pistonat B%& or as per maker or engine design or the

location of the ports itself.

Thus creating completed combustion


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9&0>7D?ID? @7T=F%9

&EF99C%IE7&T G 6HF< 9&0>7D?ID?

HFF3 9&0>7D?ID?

DI6HF< 9&0>7D?ID?


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C#oss&di#ect 'lo(

sca!enging

E$%aust

)ani'old

Sca!enge

)ani'old


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*oo" sca!enging

E$%aust

)ani'old

Sca!enge

)ani'old


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9ome 4 stroke engines do not have e!haust

valves/ 0s well as scavenge ports in the

cylinder liner, they are fitted with e!haust

ports located -ust above the scavenge ports.

0s the piston uncovers the e!haust ports on

the power stroke, the e!haust gas starts to

leave the cylinder.


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9H7E EH0 Hoop 9cavenged 7ngine


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Uni'lo( sca!enging

OPPOSE+

PISTON

EXHUST

,*,E


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4 stroke engines with an e!haust

valve mounted in the cylinder head

are known as uniflow scavenged

engines. This is because the flow of

scavenging air is in one $uni'

direction.


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@0D BJ< @& series uniflow scavenged engine


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9&0>7D?ID?

To ensure a sufficient supply of fresh air for combustion byremoving all remaining e!haust gases by blowing withthese fresh air.

9upercharging is a large mass of air is supplied to thecylinder by blowing it in under pressure either byelectrically driven au!iliary blower or e!haust gas driventurbocharger.

The flow path of the scavenge air is decided by the engine

port shape and design and the e!haust arrangements.


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7!haust ?as turbocharger

arrangement

Inlet toscavenge

manifold

Air filter

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2 Stroke Cycle, marine engineering