Download - 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 $if
fitted'.
6resh air which have been collected in the
scavenge manifold rushing entered the cylinder
9cavenging period normally occur when the pistonat B%& or as per maker or engine design or the
location of the ports itself.
Thus creating completed combustion
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&EF99C%IE7&T G 6HF< 9&0>7D?ID?
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DI6HF< 9&0>7D?ID?
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C#oss&di#ect 'lo(
sca!enging
E$%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