If the burnt exhaust gases,in an Internal Combustion Engine(whether it works on spark ignition or compression ignition principle),are made to remain inside the cylinder.These burnt gases,would mix up with the fresh incoming charge thus diluting latter and reducing its strength.
The process of removing burnt exhaust gases from the combustion chamber of the engine cylinder is known as Scavenging.
In four stroke cycle engine,the piston,during its exhaust stroke pushes the burnt gases into the exhaust manifold to escape to atmosphere through exhaust valve.But in two stroke cycle engine,a blast of fresh charge is made to enter at higher velocity into the combustion chamber at the end of working stroke of the piston and thus drives out the burnt exhaust gases.
In case of two stroke spark ignition engine there is a possibility of the fresh charge escaping out with the burnt exhaust gases because there is a small fraction of revolution of the crank during which the exhaust of the burnt gases and the entry of the fresh charge is taking place simultaneously.To overcome this difficulty,the piston crown is manufactured in a particular shape so that the fresh charge is deflected upward after entering the cylinder.
To carry out Scavenging,following are the three basic methods used.
01)Cross Flow Scavenging:-
In this method,the inlet or transfer ports are situated on one side of the cylinder while exhaust ports are situated on the opposite side.The piston crown is designed into a particular shape so that the fresh charge is deflected upward to push out the burnt exhaust gases.
02)Back Flow or Loop Scavenging:-
In this method,the inlet and exhaust ports are situated on the same side of the cylinder.A loop is formed as the fresh charge of air-fuel mixture or pure air enters the cylinder to push out the burnt exhaust gases.
In this method,both the fresh charge and the burnt exhaust gases moves in the same upward direction in the cylinder.The valve closes,as the piston moves upward during the compression stroke.Fresh charge is supplied from the side of the cylinder.
The compression of fresh charge can be achieved by the following way.
Sometimes a separate compressor is provided to compress the charge for being subsequently supplied to the combustion chamber.The compressor may be driven by engine crankshaft or by external power.
In this,the charge is previously compressed by the other side of the piston sliding on the other side of cylinder,which is also closed.It has suction valve to admit the fresh charge.The partially compressed charge is then supplied to the combustion chamber as usual.
This is discussed in "The History And Working Of Two Stroke Petrol Engine. "(Please refer)
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