Handbook of Turbomachinery Second Edition Revised - Ventech!

speeds. The sound is caused by the surging or pulsing intake and discharge
at both the inlet and discharge manifolds, repectively, as the lobes pass the
delivery port. This noise can be reduced substantially by using three or more
lobe rotors. Another, more satisfactory approach uses rotors with a helically
shaped rotor. The helix layout is along the longitudinal axis. This
configuration reduces the noise pulsations because the air delivery is
smoother with less pressure spikes. The twisted rotor design provides nearly
constant inlet and outlet volumes and pressures while minimizing losses in
volumetric efficiency. In addition to twisted rotors, attention must be paid
to the shape of the inlet and outlet ports to obtain optimum noise reduction.
Besides reducing noise, twisted designs can also have higher efficiency.
The volumetric efficiency of a naturally aspirated engine is defined as
the ratio of the actual volume of air drawn into the cylinder during one
induction stroke to the geometric or theoretical swept volume of the
cylinder. Losses in volumetric efficiency can be attributed to filters,
carburetor or throttle valves, manifold ducting, port and valve restrictions,
plus residual exhaust gases left from the previous cycle. Supercharging and
turbocharging can improve volumetric efficiency, even above 100% in some
cases.
Normally, boost pressure in mechanical superchargers such as the
Roots-type blower is controlled through use of a clutch that can be
disengaged or has controlled slippage. A bypass valve can also be used to
relieve excessive pressures. The disadvantage of the latter technique is that it
dissipates some of the energy that went into moving the air, representing a
loss in efficiency and a waste of fuel. However, proper valve design can
minimize this loss.
Rotary Piston Compressors
Rotary piston machines are generally true compressors in which the volume
of a chamber changes between inlet and outlet resulting in an internal
pressure rise. Many rotary piston compressors have been proposed, usually
with an intersecting axis rotary piston configuration where the axis of the
moving elements and the housing are not parallel and may operate with an
oscillating motion. However, very few have been used in automotive
applications to any extent, mainly because they are usually quite complex
and require precise machining, leading to high manufacturing cost.
Providing continual sealing during the complete operation cycle is one of
the main design challenges.
Probably the most successful rotary piston design is the Kuhnle, Kopp
&Kausch Ro-charger (Fig. 8). The design uses atwin-lobed piston that
rotates eccentrically with respect to the unit’s main axis of rotation. A three-
Copyright © 2003 Marcel Dekker, Inc.