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Scavenging in Diesel Engines - Marine Engineering

Scavenging is the process of removing exhaust gases from the cylinder after combustion and replenishing it with fresh air. It is important for efficient combustion and cooling. Scavenging methods include cross-flow, loop, and uniflow systems. Uniflow scavenging provides the highest efficiency by forcing exhaust up and out through a valve, while loop scavenging risks mixing gases but is simpler. Proper scavenging depends on exhaust evacuation before fresh air intake and minimizing air losses.

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Александр Кармалита
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329 views3 pages

Scavenging in Diesel Engines - Marine Engineering

Scavenging is the process of removing exhaust gases from the cylinder after combustion and replenishing it with fresh air. It is important for efficient combustion and cooling. Scavenging methods include cross-flow, loop, and uniflow systems. Uniflow scavenging provides the highest efficiency by forcing exhaust up and out through a valve, while loop scavenging risks mixing gases but is simpler. Proper scavenging depends on exhaust evacuation before fresh air intake and minimizing air losses.

Uploaded by

Александр Кармалита
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Scavenging in Diesel Engines – Marine Engineering

Scavenging in Diesel Engines

Scavenging is the process of removing exhaust gases from the cylinder after
combustion and replenishing the cylinder with fresh air.
 Efficient scavenging is necessary for good combustion of fuel inside the
engine cylinder.
 The passage of scavenge air will also assist cooling of the cylinder, piston
and valves.
 Time available for scavenging process in 2 stroke engine is less than 4
stroke engines.
Scavenging efficiency = (Weight of air enter into the cylinder) / (Weight of air
for stroke volume)

Satisfactory Scavenging Depends on:


1. Efficient evacuation of exhaust gases
2. Exhaust ports or valves open before scavenge ports
3. Scavenge ports close after the process of gas exchange
4. Loss of fresh air through exhaust passage to be minimum

Importance of Scavenging
Scavenging affects –
 Overall efficiency
 Power output per unit weight
 Fuel consumption per unit power
 Incomplete scavenging results in poor combustion
 Lubricating oil contamination
 Wear piston rings and liner
 Mean temperature of cylinder high

Different Stages of Scavenging


1st Stage – Blow Down Period
 Sudden opening accelerates exhaust gases to exhaust manifold.
2nd Stage – Scavenge Period
 Begins after scavenge ports open
 Scavenge air sweeps gases out
 Pressure difference between the scavenge trunking and exhaust manifold is
important
3rd Stage
 Effort made to contain air in the cylinder

1
Cross Flow Scavenging

Cross Flow Scavenging is the gas exchange process in


which the charge air passes is directed upwards, passing
under the cylinder cover and down the opposite side,
expelling exhaust gas through exhaust ports on that side.
This system is obsolete now.

Loop Scavenging

Loop Scavenging is the gas exchange process in which he


charge air passes over the piston crown and rises to form a
loop within the cylinder, expelling exhaust gases through
exhaust ports cut in the same side of the liner above the
scavenge ports.
Major requirement for the loop system
 The inlet air ports are angled to give rotary
swirling effect to the incoming air.
 It also required long piston skirt or exhaust
timing v/v to prevent scavenging air leak into exhaust while
the piston is at top of it’s stroke.
Advantages
 Simplicity, Reliability and cylinder cover is
simplified.
Disadvantages
 Temperature gradient exhaust to scavenge ports is
high
 Exhaust back pressure higher with deposits
 Possibility of intermixing air and gases
 Piston and liner distortion and uneven piston ring
wear due to ports
 Scavenge efficiency is lower than uniflow scavenge
efficiency at all scavenge ratios

2
Uniflow Scavenging

Uniflow scavenging is the gas exchange process in


which the charge air passes straight up through the
length of the cylinder, forcing the exhaust gas through
exhaust valve at the top of the cylinder.

Advantages
Highest scavenge efficiency at all scavenge ratios
Drop in scavenge efficiency lowest with high scavenge
pressure
Most suitable for pressure charging
Greater area for air and gas flow
Possibility of short circuiting and mixing less

Disadvantages
Complicated cylinder head and increased maintenance

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