26749626 types of scavenging

8/2/2019 26749626 Types of Scavenging

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Loop or cross scavenge

Uniflow.



After ignition of the fuel the piston travelsdown the liner uncovering firstly the exhaust

ports. The exhaust gas at a pressure aboveatmospheric is expelled. This is often referredto as blowdown and its effect can be seen onthe power card for all the types of scavengingas a rapid drop in cylinder pressure towardsthe end of the cycle.



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



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 the

piston 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.



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.



2 stroke engineswith an exhaustvalve mounted inthe cylinder headare known asuniflow scavengedengines. This isbecause the flow

of scavenging airis in one (uni)direction.



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.



Disadvantages of the loop/crossflow methodof scavenging

The greatest disadvantage of this system,

and the one that has led to theabandonment of its usage where once itwas widespread is its inefficiency in clearingthe cylinder of all combustion products.

Following the exhaust blowdown thescavenge ports are opened.~ The period available for scavenging is

limited to the recovering of the exhaustports and is only at its most effective until

the closing of the scavenge ports.~Therefore, high air velocities are designedin, the air entering through steeply angledshaped ports.



~The possibility exists for the scavenge air toshortcut directly to the exhaust ports, asituation which worsens with blockage of

the scavenge ports due to carbon build up.~Due to the inefficiencies above there is ahigh volume of scavenge air requirementwith this design.This has led to the complicated underpistoneffect designs to augment the turbobloweroutput with some engine power beingabsorbed dropping cycle efficiency.

~ a problem with having fixed ports is that

difficulty is encountered with port timing.



~On the piston down stroke the exhaust port isopened followed by the scavenge port to makeeffective use of blowdown.

However, the same timing for closing the

ports means that the effective compressionstroke is reduced.

REMEDIES-To try to remedy this differing means of

closing the exhaust ports before the pistoncovered the ports was tried, One suchmethod was by engine driven rotating valveswhich opened and closed the exhaust ports.

All the designs increased complexity and oftenproved unreliable due to the arduousconditions they had to operate in.



1-To prevent exhaust gas entering the cylinderunder the piston as the piston moves up toTDC, extended piston skirts are fitted.

-This adds to the reciprocating mass~ increases load on the crosshead bearing. The

small amount of side thrust not absorbed bythe crosshead is spread over the larger of the

skirt reducing loading and~wear on the liner,

~however, problems of increased lubricationrequirements for the increased surface area

largely negate any advantage.



The requirement for both the exhaust andscavenge ports being fitted into the linermakes for a more complicated design with

increased liner lubrication difficultiesespecially in way of the exhaust ports. Thisregion suffering not only the washing awayeffect of the gas flow but also contaminationfrom combustion products and increased

temperature. Cylinder lubricating oil volumedemand is therefore higher with this design.In an attempt to improve the scavenge

efficiency shaped pistons have been usedwhich produce a combustion chamber shapenot the best for efficient combustion.

Asymmetrical piston designs can also lead toexcessive thermal loading and complicatedstrengthening and cooling designs.



advantage of the Loop method of scavengingis that it does away with the requirement foran exhaust valve or opposed piston.

This means that all the extra running gearassociated with this can be omitted.

That means, simpler cylinder cover design,simpler and less stressed camshaft and

camshaft drive train.Where an exhaust valve is fitted in the cover,

there is increase thermal stressing especiallyin way of the valve where higher

temperatures are encountered.



Additional advantages of the Uniflow methodThe fitting of an exhaust valve does give a

major advantage in that the timing of openingand closing of the valve can be altered whichis used to its fullest with modern designs with'Variable exhaust timing' control fitted.

This means that the effect of the scavenge airinertia entering the cylinder, and optimisingthe closing of the valve to increase the

effective compression (which starts when theexhaust valve is closed) can all be taken intoaccount for the varying loads and enginespeeds.

The increased scavenging efficiency with this

type of scavenging creates greater scope forincreased economy and so all modern designsare based on this design.



The opposed piston :-design once in favour due toits inherent dynamic load balancing has now largelydisappeared due to its increased mechanicalcomplexity.

There is a minimum air demand as the ingressing airpushes the combustion products ahead of it with

little requirement for scavenging by dilution.As the air flow is symmetrical rising up the liner the

thermal influencing on the liner walls, cylinder

covers and piston crowns is also symmetrical.this allows simpler oil cooling of the piston crownThe reduced number of ports (no exhaust ports), and

reduced size of the scavenge ports (due to reducedair volume requirements), this reduces the

problems of liner lubrication allowing reduced oilconsumption.

26749626 types of scavenging

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