The purpose of this blog is to introduce
the readers the further details of scavenging methods for better performance of
engine and discuss relative factors which are responsible for the same.
Analysis of two stroke engine is more
difficult than four stroke engine as the overlapping period for the exhaust and
inlet ports is considerably large. The pattern flow is more difficult and
uncertain. Therefore, it is more difficult because of randomness in formation
of pattern during the exhaust and charging the engine cylinder.
Two stroke engine is easy in
construction as there are very few moving parts compared to the four stroke
engine but more difficult for analysis because of overlapping the inlet and
exhaust. As there are only two strokes for performing four basic operations,
the overlapping process cannot be avoided. During the downward stroke,
expansion is carried out during part of the stroke and during remaining part of
the stroke, the exhaust and charging are carried out simultaneously. This
process is also continued during the upward motion of the piston for the part
of stroke, and for the remaining part of the stroke, the compression is carried
out.
Use of two stroke SI engines is
universally adopted with two wheelers because of its simplicity in construction
and maintenance free operation. Two stroke diesel engines are used in bigger
capacity 2000 kW and above for generating the power or they are used for ships.
Scavenging
Process and Scavenging methods:
The method of removing the exhaust gases
with the help of fresh charge in petrol engine or by the inlet air in diesel
engine is known as Scavenging. This process is carried out during the
overlapping of inlet and exhaust ports. The basic requirement of an ideal
scavenging system is to remove exhaust gases without any loss of fresh charge
or air. Such ideal system is practise is impossible but utmost care should be
taken to reduce the loss to minimum.
The best air path is achieved through
scavenging in which air is admitted at one end of the cylinder and exhaust
gases are discharged from the other end. Ideally, this method results in
perfect scavenging in which the incoming air displaces the exhaust gas without
mixing. In cross-scavenging, care must be taken to avoid short circuiting.
Without the hump on the piston top, the incoming air or mixture would have a
tendency to simply go in and out of the cylinder without displacing the exhaust
gases. This is known as Short circuiting. From experiment it is suggested that
the best scavenging that can be achieved via the cross method occurs when there
is perfect mixing in which the fresh air introduced successively dilutes the
residual exhaust gases. If the sufficient air is used, at the end of
scavenging, an acceptable scavenging efficiency can be achieved.
In an ideal scavenging process, the
fresh mixture should push the residual gases without mixing and exchanging heat
and this process should continue until all burned gases are replaced with fresh
charge. At this point, the flow must stop. The pressure and temperature of the
charge in the cylinder should be same as that inlet. The cylinder is expected
to be filled with fresh charge when the piston is at BDC. Also fresh charge
should not escape through the exhaust post.
Scavenging
methods:
1.
Cross-scavenging
2.
Combined
port and valve scavenging system
3.
Auxiliary
exhaust valve arrangement
1.
Loop
scavenging system
a.
Full
loop or MAN
b.
Tangential
loop scavenging system
c.
Combination
of loop and tangential scavenging
2.
Uniflow
scavenging system
3.
Kadency
scavenging
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