Centrifugal and Axial Compressors in IC Engine

Applica ons:
1. Introduction

Compressors are crucial components in many engineering applications, especially

in internal combustion (IC) engines where they are used to increase the air
pressure before it enters the combustion chamber. The two main types of dynamic
compressors used are centrifugal and axial compressors. This assignment aims to
explore their construction, operation, analysis, and role in IC engine applications.

2. Classification of Compressors

Compressors can be broadly classified as:

 Positive Displacement Compressors

o Reciprocating
o Rotary
 Dynamic Compressors
o Centrifugal
o Axial

3. Centrifugal Compressors

A centrifugal compressor is a mechanical device that compresses a fluid with the help
of the impeller’s radial acceleration, which is surrounded by the compressor housing. In
a centrifugal compressor, the air or gas enters axially in the impeller and discharges
radially
 3.1 Construc on
A centrifugal compressor typically consists of:

 Impeller: A rota ng disk with blades that imparts velocity to the air.
 Diffuser: A sta onary set of vanes that convert velocity to pressure.
 Volute Casing: Collects the compressed air and directs it to the next stage or
outlet.

3.2 Working Principle

Air enters the impeller axially and is thrown radially outward by centrifugal
force. The high-velocity air enters the diffuser where it slows down, conver ng
kine c energy into pressure energy.

3.3 Velocity Diagrams

[Insert labeled diagrams of inlet and outlet velocity triangles here. Show
components: Absolute velocity (V), Blade velocity (U), and Rela ve velocity

β1 = Angle of the rotor blade at inlet

ADVERTISEMENTS:
β2 = Angle of the rotor blade at outlet
α1 = Angle made by entering air or exit angle of guide blade
α2 = Angle made by the outgoing air from rotor blade
V1 and V2 = Absolute velocity of air at inlet and outlet of rotor, m/s
Vr1 and Vr2 = Rela ve velocity of air at inlet and outlet of rotor, m/s
Vf1 and Vf2 = Velocity of flow at inlet and outlet of rotor, m/s
Vw1 and Vw2 = Velocity of whirl at inlet and outlet of rotor, m/s
u1 and u2 = Mean peripheral velocity of blade p at inlet and outlet, m/s
r1 and r2 = Inner and outer radii of rotor, m
m = Mass flow rate of air, kg/s
α3 = Vaned diffuser inlet angle or vaneless diffuser outlet angle
 At the inlet to rotor, air enters with absolute velocity V1 making an angle α1 to
the direc on of mo on of blade (usually α1 = 90°), without any shock and its
whirl component Vw1 = 0.
Inlet triangle of veloci es is now drawn to scale se ng V1 in the radial
direc on, u1 in the tangen al direc on to inlet periphery. The vector joining
end points of V1 and u1 represents rela ve velocity Vr1 at inlet. The blade p at
inlet has angle β1, i.e., its curvature at inlet lies in the direc on of Vr1.
At the outlet from rotor, air leaves with a rela ve velocity Vr2 at an angle β2,
with the direc on of mo on. Now, as u2 is known, V2 can be found by vectorial
addi on.

In a vaneless diffuser, the flow is assumed to be logarithmic spiral and free-

vortex. Air enters the vaned diffuser with velocity V3 at an angle α3 and leaves
the diffuser with a velocity.

3.4 Work Done and Pressure Ra o Deriva on:

The first law of thermodynamics, which is just the applica on of energy balance,
when applied in the context of steady flow compression gives us the following
rela onship which was derived in Chapter 02 as (42)

W˙input=Q˙loss+m˙(h2total−h1total)W˙input=Q˙loss+m˙(h2total−h1total)

where
W˙input W˙input is the rate of work input to the compressor or Power input
Q˙lossQ˙loss is the rate of heat loss from compressor body, bearing, seals etc.
m˙m˙ is the mass flow rate
h2totalh2total is the total or stagna on enthalpy at outlet condi ons
h1totalh1total is the total or stagna on enthalpy at inlet condi ons
For the sake of brevity alone, we shall write total enthalpies as if they were just sta c
enthalpies. Prac cally, the kine c energy effect might be insignificant, but if we find
that the accuracy has fallen below our expecta on, we can use total enthalpies
instead of sta c enthalpies without altering the structure of these equa ons.

So for the sake of brevity, the first law equa on can be wri en as:

(1)¶

W˙actual=Q˙loss+m˙(h2−h1)W˙actual=Q˙loss+m˙(h2−h1)

or dividing by m˙m˙, we get on per unit mass basis as

wactual=qloss+(h2−h1)wactual=qloss+(h2−h1)

For a perfect gas the enthalpy difference can be arrived in a more simple manner and
we can write as

wactual=qloss+cpavg(T2−T1)wactual=qloss+cpavg(T2−T1)

In most prac cal scenarios, the actual work process is adiaba c in nature, and we can
simplify the above further by taking qloss=0qloss=0 in such cases. Therefore,

wactual=cpavg(T2−T1)wactual=cpavg(T2−T1)

Please note that compressors using interstage cooling or diaphragm cooling should
never use this approxima on as the error would be unacceptable.

3.5 Advantages and Limita ons:

Advantages:

 High pressure rise per stage

 Compact and robust

Limita ons:

 Lower efficiency than axial types

 Prone to surge

4. Axial Compressors
An axial compressor is a gas compressor that can con nuously pressurize gases. It is
a rota ng, airfoil-based compressor in which the gas or working fluid principally flows
parallel to the axis of rota on, or axially. This differs from other rota ng compressors
such as centrifugal compressor, axi-centrifugal compressors and mixed-flow
compressors where the fluid flow will include a "radial component"

4.1 Construc on:

Components include:
 Rotor Blades: Add energy to the air.

 Stator Blades: Convert velocity into pressure.

 Casing: Houses the rotor and stator stages.

4.2 Working Principle:

Air flows axially through alterna ng rows of rota ng and sta onary blades. Energy is
imparted by rotor blades and pressure is increased in stator blades.
4.4 Work Done and Pressure Ra o Deriva on
h= U(Vw2-Vw1) From the energy transfer per stage:

Total pressure rise is cumula ve over mul ple stages.

4.5 Advantages and Limita ons
Advantages:
 High efficiency
  Suitable for large mass flow rates

Limita ons:
 Complex construc on

 Prone to stall

6. Surge and Stall in Compressors:

A compressor stall is a local disrup on of the airflow in the compressor of a gas
turbine or turbocharger. A stall that results in the complete disrup on of the
airflow through the compressor is referred to as a compressor surge. The
severity of the phenomenon ranges from a momentary power drop barely
registered by the engine instruments to a complete loss of compression in case
of a surge, requiring adjustments in the fuel flow to recover normal opera on.
Compressor stalls were a common problem on early jet engines with simple
aerodynamics and manual or mechanical fuel control units, but they have been
virtually eliminated by be er design and the use of hydromechanical and
electronic control systems such as full authority digital engine control. Modern
compressors are carefully designed and controlled to avoid or limit stall within
an engine's opera ng range.
Causes:
A compressor will only pump air in a stable manner up to a certain pressure
ra o. Beyond this value the flow will break down and become unstable. This
occurs at what is known as the surge line on a compressor map. The complete
engine is designed to keep the compressor opera ng a small distance below
the surge pressure ra o on what is known as the opera ng line on a
compressor map. The distance between the two lines is known as the surge
margin on a compressor map. Various things can occur during the opera on of
the engine to lower the surge pressure ra o or raise the opera ng pressure
ra o. When the two coincide there is no longer any surge margin and a
compressor stage can stall or the complete compressor can surge as explained
in preceding sec ons.

6.1 Surge Phenomenon

Occurs when the flow reverses due to high pressure at low flow rate. Results in
vibra on and noise.

6.2 Stall Phenomenon

Happens due to flow separa on on blades caused by high incidence angles. Leads to
loss of performance.

6.3 Causes and Effects

 Caused by improper blade design, opera ng below design flow.

 Effects include reduced efficiency, mechanical damage.

6.4 Preven on and Control Methods
 Variable inlet guide vanes

 Bleed valves

 Surge margin control

Preven ng surge

In the oil and gas industry the opera on of gas compressors in surge condi ons is
prevented by instrumenta on around the compressor.[15] The measured flow rate of
gas (FT) in the compressor suc on line together with the suc on pressure (PT), and
some mes the suc on temperature (TT) and the pressure (PT) in discharge line is fed
into the surge controller. Algorithms in the controller use the data to establish the
performance of the machine; the data iden fies the opera ng point in terms of the
flow and the developed head. When the compressor’s opera on approaches the
surge point the controller modulates either a flow control valve (FCV) in the recycle
line or adjusts the speed (SC) of the compressor driver. The FCV allows cooled gas
from the discharge to spill back to the suc on of the compressor, thereby maintaining
the forward flow of gas through the machine. The recycle line is ideally located to
take cooled gas from downstream of the compressor a er-cooler and to discharge it
into the feed to the compressor suc on drum.[16]
7. Applica ons in IC Engines:
 Turbocharging in automo ve engines

 Aircra propulsion systems

 Marine diesel engines

These compressors improve volumetric efficiency and power output by increasing the
intake air pressure.

2. Real-world Compressor Maps:

You could include a sample compressor performance map showing surge line, choke
line, and opera ng zones. It helps explain stall/surge visually.

3. Materials Used:
Discuss briefly:

 Common materials for impellers and blades (e.g., aluminum alloys, tanium)

 Why material selec on ma ers (strength, temperature resistance)

4. Cooling and Lubrica on:

Include a paragraph on:

 How heat is managed in high-speed compressors

  Importance of lubrica on in bearings and seals

5. Advancements in Compressor Design:

Add recent trends like:

 Variable Geometry Compressors (VGC)

 Electric Turbochargers

 Use of CFD in blade design

8. Conclusion:
Centrifugal and axial compressors are vital in boos ng the performance of IC engines.
Understanding their opera on, advantages, and limita ons helps in selec ng the
appropriate type for specific applica ons. Managing surge and stall is crucial for safe
and efficient opera on.