SCAVENGING

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Scavenging:

Scavenging is the pushing out of the residual gases at the completion of exhaust, with the help of the air intake so that a fresh charge of pure air is available into the cylinder for the following compression.

Ideally, for 100% efficient scavenging, the cylinder must be purged of all residual products of combustion; then filled with uncontaminated fresh air; and in doing so none of the air should be wasted through the exhaust. The ideally scavenged engine, therefore has 100% purity of charge; no loss from short circuiting and the scavenge air, behaving like a piston, pushes all the burned gases out of the cylinder without mixing with them. However, this ideal is unattainable. There is always some loss of fresh air to the exhaust.

In 4-stroke engines, scavenging is easily carried out by careful timing of inlet and exhaust valves and by allowing sufficient valve overlap.

In 2 stroke engines, scavenging is relatively difficult due to very less time available for it and because positive displacement effects of piston cannot be exploited around B.D.C. where two-stroke scavenging takes place.

Methods of scavenging:

1. Crank Case Scavenging:

The crankcase and the underside of the piston act as an air compressor. Partitions between cylinders divide the crankcase into separate compartments. The slight vacuum created by the piston upstroke draws air in through filters and through automatic ring valves in the partition walls. The down stroke of the piston compresses the air slightly. Near the end of the down stroke, the piston uncovers the exhaust ports in the cylinder wall and then the scavenging ports. Air from the crankcase then flows into the cylinder, displacing the burned gas and charging the cylinder for the next cycle.

2. Power Piston (or under piston) Scavenging:

A separate chamber is provided in the space under the piston. The down stroke of the piston compresses the air, which has been previously drawn into the chamber on the upstroke.

3. Pump or Blower Scavenging:

A pump or blower driven either directly from the engine or by an independent source of power usually by an electric motor supplies the scavenging air.

Importance of Scavenging:

It is essential that each cylinder should be adequately scavenged before a fresh charge of air is compressed; otherwise residual exhaust gases from the previous cycle contaminate this fresh air charge.   Further, the cycle temperature is unnecessarily high if the air charge is heated by mixing with residual gases and by contact with hot cylinders and pistons. The scavenging airflow through the cylinder during valve overlap period has a valuable cooling effect; it helps to increase the volumetric efficiency and also to ensure a low cycle temperature. Also, the relatively cooler exhaust allows a higher engine output to be obtained before the exhaust temperature imposes a limitation on the satisfactory operation of the turbine blades.

Effects of Inefficient Scavenging:

1. Less power output per unit weight.
2. Increased specific fuel consumption.
3. Lower mean indicated pressures.
4. Poor combustion.
5. More lubricating oil contamination.
6. More liner and piston rings wear.

Disadvantage:

As work is required to be done to provide scavenge air, mechanical efficiency and hence the specific fuel consumption of the engine are adversely affected, if scavenging air is provided more than necessary. So, scavenging has to be efficient but economical in practice, 0.1 to 0.3 bar gauge scavenging pressure will result in waste of air during scavenging, depending upon the scavenging arrangements, size and speed of the engine.

Scavenging of Two Stroke Engine:

Straightforward designs of the parts dealing with scavenging for a four stroke engine and provision for allowing greater valve overlaps makes four stroke scavenging simple, but as in two stroke engines, the process has to finish in short time at about B.D.C. of piston without having appreciable piston displacement, the system should be really efficient, simple and reliable. Due to obvious advantage of two-stroke engine over four-stroke engine because of less weight per unit power, the scavenging process for it has to be very efficient to consolidate its popularity.