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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

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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.