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CHAPTER 3
AUTOMOTIVE AIR COMPRESSOR
3.1 INTRODUCTION
A machine providing air at a high pressure is called as an air
compressor. Air compressors have been used in industry for well over 100
years because air as a resource is safe, flexible, clean and convenient. These
machines have evolved into highly reliable pieces of equipment that are
almost indispensable in many of the applications they serve. Compressors are
available in a wide variety of different types and sizes. Every compressed-air
system begins with a compressor - the source of air flow for all the
downstream equipment and processes. The main parameters of any air
compressor are capacity, pressure, power and duty cycle. It is known that
capacity does the work; pressure affects the rate at which work is done.
Kazutaka Suefuji and Susuma Nakayama (1980) in their study on hermetic
compressor have quoted that adjusting an air compressor's discharge pressure
does not change the compressor's capacity.
There are a number of basic air compressor designs and variations
in the market today. The three basic types of air compressors are
Rotary Screw
Rotary Centrifugal
Reciprocating
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These types are further specified by
the number of compression stages
cooling method (air, water, oil)
drive method (motor, engine, steam, other)
lubrication (oil, oil-free)
packaged or custom-built
3.2 ROTARY SCREW COMPRESSORS
Rotary air compressors are positive displacement compressors. The
most common rotary air compressor is the single stage helical or spiral lobe
oil flooded screw air compressor. These compressors consist of two rotors
within a casing where the rotors compress the air internally. There are no
valves. These units are basically oil cooled (with air cooled or water cooled
oil coolers) where the oil seals the internal clearances. Since the cooling takes
place right inside the compressor, the working parts never experience extreme
operating temperatures. The rotary compressor, therefore, is a continuous
duty, air cooled or water cooled compressor package.
Rotary screw air compressors are easy to maintain and operate.
Capacity control for these compressors is accomplished by variable speed and
variable compressor displacement. For the latter control technique, a slide
valve is positioned in the casing. As the compressor capacity is reduced, the
slide valve opens, bypassing a portion of the compressed air back to the
suction. Advantages of the rotary screw compressor include smooth, pulse-
free air output in a compact size with high output volume over a long life.
The oil free rotary screw air compressor utilises specially designed
air ends to compress air without oil in the compression chamber yielding true
oil free air. Oil free rotary screw air compressors are available as air cooled
and water cooled and provide the same flexibility as oil flooded rotaries when
oil free air is required.
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3.3 CENTRIFUGAL COMPRESSORS
The centrifugal air compressor is a dynamic compressor which
depends on transfer of energy from a rotating impeller to the air. Centrifugal
compressors produce high-pressure discharge by converting angular
momentum imparted by the rotating impeller (dynamic displacement). In
order to do this efficiently, centrifugal compressors rotate at higher speeds
than the other types of compressors. These types of compressors are also
designed for higher capacity because flow through the compressor is
continuous. Adjusting the inlet guide vanes is the most common method to
control the capacity of a centrifugal compressor. By closing the guide vanes,
volumetric flows and capacity are reduced. The centrifugal air compressor is
an oil free compressor by design. The oil lubricated running gear is separated
from the air by shaft seals and atmospheric vents.
3.4 RECIPROCATING AIR COMPRESSORS
Reciprocating compressors are used in commercial automotives
with air brake system. They are in use for more than six decades.
Development of a compressor requires an insight into the design parameters
and their effects on performance, cost and life of the compressor.
Reciprocating air compressors are positive displacement machines
that they increase the pressure of air by reducing its volume. This means they
are taking in successive volumes of air which is confined within a closed
space and elevating this air to a higher pressure. The reciprocating air
compressor accomplishes this by a piston within a cylinder as the
compressing and displacing element. Single-stage and two-stage reciprocating
compressors are commercially available. Single-stage compressors are
generally used for pressures in the range of 500 kPa to 900 kPa. Two-stage
compressors are generally used for higher pressures in the range of 900 kPa to
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1800 kPa. The reciprocating air compressor is single acting when the
compression is accomplished using only one side of the piston. A compressor
using both sides of the piston is considered double acting. Load reduction is
achieved by unloading individual cylinders. Typically, this is accomplished
by throttling the suction pressure to the cylinder or bypassing air either within
or outside the compressor. Capacity control is achieved by varying the speed
in engine-driven units through fuel flow control. Reciprocating air
compressors are available either as air-cooled or water-cooled in lubricated
and non-lubricated configurations, may be packaged, and provide a wide
range of pressure and capacity selections.
Figure 3.1 shows the schematic diagram of a single stage single
acting reciprocating air compressor.
A reciprocating compressor consists of a crankshaft (driven by a
gas engine, electric motor, or turbine) attached to a connecting rod, which
transfers the rotary motion of the crankshaft to the piston. The piston travels
back and forth in a cylinder. The piston acting within the cylinder then
compresses the air contained within that cylinder. Air enters the cylinder
through a suction valve at suction pressure and is compressed to reach the
desired discharge pressure. When the air reaches the desired pressure, it is
then discharged through a discharge valve. Desired discharge pressure can be
reached through utilisation of either a single or double acting cylinder. In a
double acting cylinder, compression takes place both at the head end and
crank end of the cylinder. The cylinder can be designed to accommodate any
pressure or capacity, thus making the reciprocating compressor the most
popular in the gas industry.
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Crank Shaft
Crank
Connecting rod
BDC
TDC
Discharged air to
reservoir
Discharge ValveSuction Valve
Air from
atmosphere
Valve Plate
Cylinder
Piston
Figure 3.1 Schematic diagram of a reciprocating air compressor
3.5 SUCTION AND DISCHARGE VALVES
A compressor valve is a device that controls the inward flow of
lower pressure gas at atmospheric conditions and the outward flow of higher
pressure gas from a reciprocating compressor cylinder. Normally these valves
open and close automatically, solely governed by the pressure differential in
the cylinder and the intake or exhaust line pressure. There is atleast one
suction valve and one discharge valve for every compression chamber. Each
valve opens and closes in every cycle. A valve used in a compressor operating
at 1200 rpm for 12 hours a day and 280 days a year, opens and closes 72,000
times per hour or 864,000 times per 12 hours in a day or 241,920,000 times
per year.
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There are essentially two requirements to be met by a valve, (a) the
valve must be efficient, and (b) the valve must be durable and quiet in service.
The above criteria can be refined and can include both the aerodynamic flow
efficiency and the volumetric efficiency. Under durability, the maintenance
free operation for over several thousand hours plus relative ease in servicing
and repair can also be included.
There are different kinds of compressor valves: plate or disc valves,
ring valves, channel valves, feather valves, poppet valves, ball valves, reed
and concentric valves, to name just a few. Each design has a specific criteria
with regard to the sealing element and all the other components are designed
accordingly. Most of the air compressors used in automotive braking system
use reed, disc or ring valves.
In disc valves the plate is operated by a compression ring. The ring
valve is an annular disc valve operated by a spring. Figure 3.2 shows the
opening of disc valve used on suction and delivery sides.
Figure 3.2 Inlet and Delivery disc valve openings
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When the valve is closed, part of the valve plate or valve ring is
firmly set against the seat lands. The sealing element initially lifts off the seat
land slowly but accelerates rapidly towards the guard once spring forces are
overcome.
The factors that account for the initial pressure differential between
cylinder and line pressure at valve opening that is seen on all PV-diagrams are
(i) the cylinder pressure exposed to the entire surface area of the sealing
element (ii) the sticking effect of lubrication or condensate and (iii) the spring
load force.
To lift the sealing element off the seat land, a pressure differential
is required across the sealing element. The difference in area of a sealing
element is normally 15% to sometimes as high as 30% between exposure
underneath (seat side) and exposure on top (guard side). Since there is always
some leakage through the closed valve plate along the seat lands, there is a
certain amount of pressure build-up in this area. Therefore, the actual pressure
differential needed to induce or cause the valve open is only 5% to 15% over
the line pressure. As the sealing element lifts off the seat lands, it accelerates
rapidly against the spring load towards the guard. The sealing element
impacts against the guard causing the opening impact, at this stage the valve
is considered fully open.
Piston velocity at top or bottom dead center is zero and increases
gradually to a maximum at the middle of its stroke. Valve velocity follows a
slower path than the piston. The flow of the gas out through the seat keeps the
sealing element open. As the flow diminishes due to the decreasing piston
speed, the springs or other cushioning elements force the sealing element to
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Valve lift
Inlet valve
Cylinder bore
Delivery valveDelivery valve stopper
Cylinder bore
Valve lift
return to the seat lands and close the valve on time. Preferably, the valve is
completely closed when the piston is at or near dead center.
A reed valve is a flow actuated one-way valve. A port in the line is
covered by the free end of a thin and flexible blade whose other end is
fastened so that the port is normally closed. Pressure in the port or vacuum on
the far side, will lift the blade, permitting the flow. If the pressure reverses, it
closes the blade, stopping the flow. Usually the reed valves use a single blade,
but modern versions combine four, six or eight blades, or petals, into tent-like
arrays, fastened to a multi-ported reed cage. Reed valve involves the loss of
pressure, as some pressure difference is required to open the valve. Even with
this limitation, they have excellent versatility. Figure 3.3 shows the inlet and
delivery valves employed in a 160 cc air cooled compressor.
Figure 3.3 Inlet and Delivery Reed Valve openings
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Modern compressors employ reed valves because of the following
features:
1. Number of components required is less. So almost no wear
takes place.
2. The number of holes in the valve plate can be increased which
will increase the flow area. This will reduce the pressure
required to open the valves, and hence lesser pressure drop
across the valves.
3. Lesser assembly difficulties.
3.6 PERFORMANCE PARAMETERS OF COMPRESSOR
The performance of the compressor can be studied by individual
parameters, such as pump up time, delivery air temperature, speed and power.
3.6.1 Pump up time
Pump up time is the time required to develop a delivery pressure in
a reservoir of given volume connected to the compressor air outlet. Pump up
time is important as it indicates the volume flow rate of air inside the
compressor under given operating conditions. Mainly the clearance volume
affects pump time performance in addition to the flow area available in the
cylinder head. The flow area available should not be less than the adapter
inside flow area.
3.6.2 Delivery air temperature
It is the temperature of air after compression measured at the
delivery port of the cylinder head. Delivery air temperature has two issues:
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(i) the degree of heat generated by the compression process and (ii) the degree
of cooling of the compressor after the compression process.
The air from the compressor is led into the air drier (Air processing
unit) which purges the air from most of the moisture. The temperature of the air
that enters the air processing unit is limited to about 70oC. This necessitates the
use of long metallic finned pipelines (nearly 6 m long) in order to allow
sufficient time for cooling of air. A long pipeline complicates assembly issues
on the vehicle. Thus a reduced delivery air temperature would reduce the need
for long pipelines and thereby simplify the problems. A high delivery air
temperature increases oil carryover and thereby further increase in the delivery
air temperature due to the formation of carbon deposits on the piston and the
cylinder head. Carbon deposits on the cylinder head reduce the heat dissipation
capacity of the fins on the inner cavity of the cylinder head. Cylinder head
design has a vital influence on the delivery air temperature.
3.6.3 Power
Power is measured under three conditions:
Loaded power: Loaded power is the power consumed by the
compressor while pumping against a pressure gradient.
Unloaded power: Unloaded power is the power consumed
while pumping to atmosphere (with ideally no pressure
gradient) through the unloaded valve. The unloaded valve
regulates the pressure against which the compressor is
pumping. Unloaded power reflects the power losses at the
unloaded valve due to flow resistance.
No load power: No load power is the power consumed while
the compressor’s delivery is open to atmosphere. No load
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power is indicative of the power losses due to the flow
resistance in the cylinder head of the compressor.
3.7 COMPRESSOR TERMINOLOGY
Various terms related to the compressor specification are shown in
Table 5.1 and the performance analysis are discussed below.
3.7.1 Discharge and suction pressure
Discharge pressure is the pressure of discharged air or theoretically
the reservoir pressure. The pressure of air during suction process is called
suction pressure.
3.7.2 Free air delivered (FAD)
The volume of air delivered by the compressor, when the state of
air is reduced to intake (ps, Ts) or atmospheric (pa, Ta) or normal (pa, Tn) or
required (p, T) condition is called FAD.
Let, m1 = Initial mass of air in the reservoir in kg
p1 = Initial pressure of the reservoir in Pa
T1 = Initial temperature of the reservoir in K
m3 = Final mass of air in the reservoir in kg
p3 = Final pressure of the reservoir in Pa
T3 = Final temperature of the reservoir in K
V = Volume of the reservoir in m3
t = Time taken for the pressure to build up from p1 to p3 in
second
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mod = Mass of air discharged into the reservoir in kg
1
11
RT
Vpm (3.1)
3
3
3RT
Vpm (3.2)
The mass added during the interval t at intermediate pressure
p2 = m3 – m1
R
V
T
p
T
p
1
1
3
3 (3.3)
Mass added per cycle at p2 (mod)tNR
V60
T
p
T
p
1
1
3
3 (3.4)
FADf
fod
p
NTRm(3.5)
From Equation (3.4), the FAD is
f
f
1
1
3
3
pt
TV60
T
p
T
p (3.6)
where, pf = Free air pressure in Pa
Tf = Free air temperature in K
N = Compressor speed in rpm
There will be a rise in temperature during filling process at constant
volume. Therefore it is required to measure the temperature at p1 and p3. If
free air temperature is the tank temperature, it is taken as the temperature at
the intermediate pressure p2. This intermediate temperature should be used for
calculating the mass of air discharged.
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3.7.3 Indicated power (IP)
Work energy imparted to the air per unit time is called indicated
power. This power can be obtained from the p-V diagram.
3.7.4 Power consumption
The power available at the compressor shaft to run the compressor
at the desired discharge pressure and speed is termed as the power
consumption. The power imparted to the air in the cylinder is Indicated power
(IP). All the power available at the compressor shaft will not be imparted to
the air in the cylinder. The friction between the moving parts absorbs some
power and it is called friction power (FP). The FP varies with compressor
speed. The load (discharge pressure) on the compressor has a negligible effect
on FP. As the speed increases FP increases.
For power absorbing machines, like compressor,
Mechanical efficiency,BP
IPm (3.7)
If the compressor gets power from electric motor, then the power
required to run the compressorgm
IP (3.8)
where, g = Generator (or) motor efficiency (generally the value lies between
0.85 and 0.95)
If the compressor gets power from I.C engines, it is convenient to
take the power required to run the compressor equal to the brake power (BP)
of the compressor. The mechanical efficiency ( m) of any reciprocating
machine will be around 0.75 to 0.8 at rated speed. For the same speed, the
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power required to run the compressor decreases with decrease in mass of air
handled.
3.7.5 Indicated torque
Torque (or often called a moment) can be thought of as a
“rotational force” or “angular force” which causes a change in rotational
motion. This force is defined by linear force multiplied by a radius.
If a force is allowed to act through a distance, it does mechanical
work. Similarly, if moment is allowed to act through a rotational distance, it
does work. Power is the work per unit time. However, the time and rotational
distance are related by the angular speed where each revolution results in the
circumference of the circle being travelled by the force that is generating the
torque. This means that, torque causes the angular speed to increase in doing
work and the generated power may be calculated as
P = Torque x Angular Velocity
Indicated Power (IP) at a particular crankangle can be estimated
from IP = T (3.9)
From the torque calculation at different crankangles, it is possible
to find the maximum torque and maximum indicated power which the
compressor absorbs in a cycle.
3.7.6 Volumetric efficiency
Analysis of volumetric efficiency ( v) is essential to estimate the
suitability of a compressor for a particular application. The factors affecting
volumetric efficiency are
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Clearance volume (Increase in clearance volume decreases v)
Discharge pressure (Increase in discharge pressure decreases v)
Temperature of cylinder (Heating of the cylinder decreases v)
Compressor speed (Increase in speed decreases the increase in v)
Leakage (Leakage past the piston, decreases v, but this effect
can be neglected)
processsuctionduringcylindertheentercanthatairofvolumepossibleMaximum
processsuctionduringcycletheenteringairofvolumeActualv
n/1
s
dv
p
pkk1 (3.10)
where, k = Clearance ratio = Vc / Vs (3.11)
This expression is valid only for ideal compressors. In an ideal
compressor, the index of expansion and compression are the same and the
discharge and suction pressures are constant throughout the discharge process
and suction process.
For practical compressors, the volumetric efficiency is defined in
terms of ‘mass of air’ or FAD.
cycleperindrawnbecouldthatmasspossibleMaximum
cycleperindrawnairofmassActualv
Maximum possible mass = a Vs (3.12)
Actual mass = Mass drawn in (or) Mass delivered out per cycle
where, a = Density of ambient air = pa / (R Ta) (3.13)
Vs = Swept volume
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3.7.7 Clearance volume and stroke volume
Clearance volume (Vc) is the volume that is available after the
piston reaches the TDC. This volume is provided in the compressor for
ensuring free movement of compressor valves. The presence of clearance
volume reduces the volumetric efficiency. Stroke volume (Vs) or swept
volume is the volume corresponding to stroke.
3.7.8 Working volume
It is the volume of air at any crankangle and is obtained using
Equation (4.21). The cylinder volume at various crankangles is shown in
Figure 3.4.
Figure 3.4 Cylinder volume-Crankangle diagram of Compressor 1
3.7.9 Valve Lift
It is the vertical distance travelled by the suction or discharge valve
at any crankangle. Valve lift is governed by the goal to design valves with
acceptable life and uninterrupted service. Since the plate or sealing element
opens and closes with every revolution of the crankshaft, factors such as
rotating speed, operating pressure and molecular weight of the gas determine
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the limits of allowable valve lift. The impact resilience of various materials
used for valve plates (steel, polymers, etc.) also has an influence on maximum
acceptable valve lift. Different valve manufacturers use more or less
conservative guidelines for allowable lift for a given set of operating
conditions. Excessive valve lift can have detrimental effects on valve life, due
to high-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.
3.7.10 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.
3.7.11 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
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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.
Carl et al (1974) stated that the volume directly behind the
discharge valve should be as large as possible, as a minimum it should be
equal to the cylinder volume for high speed compressors, but preferably three
times as large. The same is true for suction valves, since the sudden filling of
the cylinder depletes the supply of gas in the suction manifold and an under-
pressure is created against which the valve has to work. The volume acts like
a collection tank or accumulator of gas, so that an over or under supply of gas
can be stored temporarily.
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Figure 3.5 shows the sectional view of a reciprocating air
compressor used in braking system of heavy automotive vehicles.
Figure 3.5 Sectional view of a typical automotive compressor
The important parts of a reciprocating compressor are piston,
cylinder, connecting rod and suction and discharge valves. The valve plate
accommodates both inlet and delivery valves. The cylinder block houses the
piston with connecting rod. The compressor is run by the engine and receives
49
power through the belt drive at drive end. The compressor is mounted to the
engine using mounting flanges.
3.8 VALVE DYNAMICS
Each compressor valve has to open and close in every compression
cycle. The timing and pattern of the opening and closing events are referred to
as valve dynamics. The valve opening and closing at the right time and
without flutter is important. Compressor valve dynamics are important since
they influence the valve life and compression efficiency. The valve dynamics
can be influenced through proper spring and/or the mass of the moving
components. For proper performance, the valves must be designed for the
specific operating window. Valve flutter is not only detrimental to valve life
because of multi impacting, but it reduces the effective lift area and also flow
efficiency. Delayed closing will especially damage the valve since it is
associated with slamming of the valve against a seat; the resultant back flow
lowers overall efficiency by a substantial margin. Major valve manufacturers
have used valve motion studies to improve valve performance and altered the
design conditions of the valve offered for a specific application to optimise
the performance.
This chapter explained the working and theory of air compressors
used in automotive braking system. The performance parameters like,
volumetric efficiency, free air delivered, power and torque were also
discussed. The development of mathematical model starting from the basic
ideal model is explained in Chapter 4.