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

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the cylinder cover and down the opposite side, expelling exhaust gas through exhaust ports on that side.This system is obsolete now.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

Uniflow Scavenging

Uniflow scavenging is he 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