RevisionC-Developmentofconceptofcompressorforcommercialvehicle

velocity impact forces, valve flutter, late closing, and other life deteriorating developments. Once
an acceptable valve lift is defined, the rest of the valve geometry can be selected to balance the
ratios of seat and guard area to free lift area. The diverse applications result in a variety of valve
concepts. For example, slow-speed applications favour wide-ported seats and guards and high
valve lifts, while high-speed applications require narrow ports and lower lifts would be applied.
2.4.5 Back flow during discharge and suction
Whenever the valve closes, there will be a flow of some discharged air into the cylinder.
This phenomenon is called “Back flow during discharge’ and this reduces the mass of air
discharged. Similarly, whenever the valve closes, there will be a flow of some drawn air from the
cylinder to the atmosphere. This phenomenon is called “Back flow during suction’ and this
reduces the mass of air drawn in.
2.4.6 Head Volume
The volume just above the valve plate is called ‘head volume’. It is also called plenum
chamber volume. There are two compartments in the head, suction and discharge plenum
chambers.
Discharge Head: The air is discharged into the receiver through the head volume. The
pressure in the head will not be constant, because, the mass going out of head per degree of
crank rotation is not equal to the mass coming into the head from the cylinder. There will be a
pressure fluctuation in the head and this will affect the discharge of air from the cylinder. Driving
force for flow of air from the cylinder is proportional to (p – pd) in theoretical case and is
proportional to (p – ph) in actual case, where, p is the cylinder pressure, pd is the discharge
pressure and ph is the head pressure.
Suction Head: The air enters the cylinder during suction through the suction head. Driving
force for flow of air from the cylinder is proportional to (pa – p) in theoretical case and is
proportional to (ph – p) in actual case, where, p is the cylinder pressure, pa is the ambient air
pressure and ph is the head pressure.
Flow of air through the valve resists velocity changes because of its mass. The flow in
compressor manifold is intermittent. When a discharge valve opens, the gas flowing from the
cylinder has to push the gas already present in the manifold. This is a problem which increases
with the compressor speed. At 3600 rpm, the time available is only 1/60 s per revolution and only
a small fraction of this is available for the gas mass in the cylinder to be emptied into the manifold,
accelerating in turn the air already present in the manifold. The result is the development of a
back-pressure against which the compressor has to work and the losses can be significant. In
reality, pressure surge will be occurring in the manifold.
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