Scavenging in 2 Stroke Engine Final

8/3/2019 Scavenging in 2 Stroke Engine Final

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By Nitin ParwaniKumar Saurav

Bhavesh DarjeeMayank Kumar


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For a better combustion of fuel oil inside a 2-stroke engine, an adequate supply of fresh airis needed. The method by which sufficientamount of air is provided to the engine's

cylinder is known as scavenging.

During the process, the increased amount ofair helps to clear the cylinder of the gases of

combustion. VIDEO


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POSITION : In Two-stroke during the latterpart of the down stroke (expansion) and theearly part of the upstroke (compression).

In a 4-stroke cycle engine, scavenging takesplace when the piston is nearing and passingTDC during the latter part of an upstroke(exhaust) and the early part of a downstroke(intake).

Provided by the engine's turbo-chargingsystem.

Turbochargerer to supply a consistent flow offresh air inside the main engine.


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The more efficient the scavenging, thebetter is the fuel combustion and poweroutput of the engine.

The passage of scavenge air will also assistcooling of the cylinder, piston and valves. Time available for scavenging process in 2stroke engine is less than 4 stroke engines.


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Scavenging Efficiency

Trapping Efficiency


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CROSS SCAVENGING LOOP SCAVENGING

UNIFLOW SCAVENGING


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CROSS FLOW


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After ignition of the fuel the piston travelsdown the liner uncovering firstly theexhaust ports. The exhaust gas at apressure above atmospheric is expelled.This is often referred to as blowdown andits effect can be seen on the power card forall the types of scavenging as a rapid drop

in cylinder pressure towards the end of thecycle


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CYLINDER PRESSURE VARIATION


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The method of loop scavenging is similar tothe cross flow except the exhaust andscavenge ports may be found on the sameside


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Some 2 stroke engines do not haveexhaust valves; As well as scavengeports in the cylinder liner, they arefitted with exhaust ports located justabove the scavenge ports. As thepiston uncovers the exhaust ports onthe power stroke, the exhaust gas

starts to leave the cylinder. When thescavenge ports are uncovered,scavenge air loops around thecylinder and pushes the remainingexhaust gas out of the cylinder. Thistype of engine is known as a loopscavenged engine. Note that thepiston skirt is much longer than thatfor a uniflow scavenged engine. Thisis because the skirt has to seal thescavenge and exhaust ports whenthe piston is at TDC.


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Although simpler in construction with lessmoving parts, these engines are not asefficient or as powerful as uniflow scavengedengines. The scavenging of the cylinder is not

100%, and thus less fuel can be burnt perstroke.

All modern large 2 stroke crosshead enginesnow being built are of the uniflow scavenged

type.


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2 stroke engineswith an exhaustvalve mounted inthe cylinder head

are known asuniflow scavengedengines. This isbecause the flow

of scavenging airis in one (uni)direction.


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The main difference between the two types is

that uniflow requires an exhaust valve orpiston to operate. Loop or cross flow relieson the piston to open and close exhaustports


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The scavenge air enters through the scavengeports in the lower part of the cylinder liner,

the exhaust gas is expelled through thecentrally mounted exhaust valve in thecylinder cover. The scavenge ports are angledto generating a rotational movement of the

rising column of air.Air is forced out of the cylinder by the risingpiston leading to low flow resistance, theeffect is often compared to squeezing thecontents out of a tube.


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Advantages of the Uniflowmethod


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Opposed Piston ScavengingMethod

The opposed piston :-design is in favour


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The opposed piston :-design is in favourdue to its inherent dynamic load balancingadvantage:-

~ little requirement for scavenging by

dilution.

~As the air flow is symmetrical rising upthe liner the thermal influencing on theliner walls, cylinder covers and piston

crowns is also symmetrical.

~The reduced number of ports (no exhaustports), and reduced size of the scavengeports

~reduces the problems of liner lubricationallowing reduced oil consumption.disadvantage-

mechanical complexity.


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Importance of ScavengingScavenging affects -

Overall efficiency

Fuel consumption per unit power

Incomplete scavenging results in poor

combustionLubricating oil contamination

Wear piston rings and liner

Mean temperature of cylinder high


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Scavenging is not the same for both two strokeand four stroke engines. This is mainly becausein the four- stroke engine there is adequateoverlap between the opening of theinlet valve and closing of the exhaust valve. Butin a two-stroke engine this overlap is limited andfor this reason a slight mixture of exhaust gasesand incoming air occurs inside the cylinder.

Also, in a four stroke engine the air is induced

during a downwards stroke of one of the twocycles, i.e per power stroke, and the exhaustgases are removed in the preceding stroke.
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But in a two stroke engine there is no specificcycle of scavenging. Air is introduced duringthe end of power stroke when the exhaustgases are removed. The time of scavenging is

too less in two stroke engines, as greater theexhaust valves stay open lesser the time thepiston gets for compression. To prevent theeffects due to this phenomenon, more

amount of fresh air is supplied. Thus, greaterthe scavenging better the combustion of fuel.


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Two big disadvantages of SI engine- fuel loss andidling difficulty. CI does not face such problems. SoCI engine is better in case of two stroke.

In SI engine using carburettor ,scavenging is done

with fuel-air mixture and only the fuel mixed withretained air is used for combustion. To avoid fuelloss,fuel Injection just before the exhaust portclosure may be used instead of a carburettor.

The two-strokeSI engine picks up only graduallyand may even stop t low speeds when MEP reducesto about 1.2 bar. This is due to mixing of largeamount of residual gas with small amount ofcharge.


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Backfiring at lower speeds due to slowburning rate.

Fuel injection improves idling and alsoeliminates backfiring as there is no fuel

present in the inlet system.

In CI engines there is no loss of fuel as thecharge is only air and there is no difficulty atidling because of fresh charge (air) is notreduced.

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Scavenging in 2 Stroke Engine Final