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CHAPTER 1

INTRODUCTION

The function of an engine is to produce mechanical movement from

any form of energy. When the fuel is burnt, heat is released. This heat energy

is converted into useful work. Diesel engine is an internal combustion engine

that converts chemical energy that reciprocate piston between top dead centre

and bottom dead centre. The pistons are connected to the engine crank shaft

through connecting rod, which change the reciprocating motion into the rotary

motion needed to propel the vehicle wheels. With both gasoline and diesel

engines, energy is released in a series of small explosions called combustion

as fuel reacts chemically with oxygen from the air. For fuel combustion and

consecutive energy conversion is taken in the combustion chamber. The

different chambers and its function are described in the following headings.

1.1 COMBUSTION CHAMBER

A combustion chamber is the part of an engine in which fuel is

burned. The combustion chamber is recessed in the cylinder head and

commonly contains a single intake valve and a single exhaust valve on the top

roof. Some engines use a dish-in-piston and in this case the combustion

chamber can be considered as partly within the cylinder. The combustion

process is symbolically shown in Figure 1.1. At the end of compression stroke

the fuel is injected into the chamber, air at this condition is in high

temperature and pressure. The heat content of air is used to vaporise the diesel

particle and vapour form of diesel is exploded by the process of oxidisation.

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At the end of working stroke or expansion stroke the combustion process is

ceased and air fuel mixture becomes combustion product. It may contains the

Hydro Carbon(HC), Carbon di Oxide (CO2), Carbon mono Oxide (CO),

Oxides of Nitrogen (NOx), Particulate Matter (PM).

Figure 1.1 Combustion Process

1.2 TYPES OF COMBUSTION CHAMBER

A well-designed engine uses a combustion chamber that is designed

for the intended usage. The following combustion chambers are widely used

in the case of diesel engine.

Open Combustion Chamber

Pre-combustion Chamber

Turbulence Chamber

Spherical Chamber

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1.2.1 Open Combustion Chamber

The open combustion chamber is the simplest form of chamber as

shown in Figure 1.2. It is suitable for only slow-speed, four-stroke cycle

engines, but is widely used in two-stroke cycle diesel engines. In the open

chamber, the fuel is injected directly into the space on top of the cylinder. The

combustion space, formed by the top of the piston and the cylinder head, is

shaped to provide swirling action of the air, as the piston comes up on the

compression stroke. There are no special pockets, cells or passages to aid the

mixing of the fuel and air. This type of chamber requires a higher injection

pressure and a greater degree of fuel atomization than is required by other

combustion chambers to obtain an acceptable level of fuel mixing. To

equalize combustion in the combustion chamber, use a multiple orifice-type

injector tip for effective penetration. This chamber design is very susceptible

to ignition lag.

Figure 1.2 Open Combustion Chamber

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1.2.2 Precombustion Chamber

The pre combustion chamber is an auxiliary chamber at the top of

the cylinder shown in Figure 1.3. It is connected to the main combustion

chamber by a restricted throat or passage. The pre combustion chamber

conditions the fuel for final combustion in the cylinder. A hollowed-out

portion of the piston top causes turbulence in the main combustion chamber,

as the fuel enters from the pre-combustion chamber to aid in mixing with air.

The following steps occur during the pre-combustion process:

During the compression stroke of the engine, air is forced into

the pre-combustion chamber and because the air is

compressed, it is hot. At the beginning of injection, the pre-

combustion chamber contains a definite volume of air.

As the injection begins, combustion begins in the pre-

combustion chamber. The burning of the fuel, combined with

the restricted passage to the main combustion chamber,

creates a tremendous amount of pressure in the combustion

chamber. The pressure and the initial combustion cause a

super-heated fuel charge to enter the main combustion

chamber at a high velocity.

The entering mixture hits the hollowed-out piston top, creating

turbulence in the chamber to ensure complete mixing of the

fuel charge with the air. This mixing ensures uniform and

complete combustion. This chamber design provides

satisfactory performance with low fuel injection pressures and

coarse spray patterns because a large amount of vaporization

occurs in the pre-combustion chamber. This chamber is

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susceptible to ignition lag, making it suitable for high-speed

operations.

Figure 1.3 Pre-combustion Chamber

1.2.3 Turbulence Chamber

The turbulence chamber shown in Figure 1.4 is similar in

appearance to the pre-combustion chamber, but its function is different. There

is very little clearance between the top of the piston and the head, so a high

percentage of the air between the piston and cylinder head is forced into the

turbulence chamber during the compression stroke. The chamber is usually

spherical and the small opening through which the air must pass causes an

increase in air velocity, as it enters the chamber. The turbulence speed of air

in turbulence chamber is about 50 times the crankshaft speed. The fuel

injection is timed to occur when the turbulence in the chamber is greatest.

This ensures a thorough mixing of the fuel and air, causing the greater part of

combustion to take place in the turbulence chamber. The pressure, created by

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the expansion of the burning gases, is the force that drives the piston

downward on the power stroke.

Figure 1.4 Turbulence Chamber

1.2.4 Spherical Chamber

The spherical combustion chamber is designed principally for use in

the multi-fuel diesel engine. The chamber consists of a basic open type

chamber with a spherical shaped relief in the top of the piston head as shown

in Figure 1.5. The chamber works in conjunction with a strategically

positioned injector and an intake port that produces a swirling effect, as it

enters the chamber. Operation of the chamber is as follows:

Figure 1.5 Spherical Chamber

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As the air enters the combustion chamber, the shape of the

intake port introduces a swirling effect to it.

During the compression stroke, the swirling motion of the air

continues as the temperature in the chamber increases.

As the fuel is injected, most of the fuel is deposited on the

head of the piston and the remainder mixes with the air in the

spherical combustion chamber.

As combustion begins, the main portion of the fuel is swept

off the piston head by the high-velocity swirl that was created

by the intake and the compression strokes. As the fuel is swept

off of the head, it burns through the power stroke, maintaining

even combustion and eliminating detonation.

1.3 ENGINE VIBRATION

A basic correlation amongst the in-cylinder combustion pressure,

indicting power expression and surface vibration of diesel engine is shown in

Figure 1.6 (Roa and Gupta, 1999). Actually it expresses a mutually dependent

connection amongst the in-cylinder working process, relative transmission

relationship of the mechanical systems and the dynamic performance of the

engine. Obviously, the in-cylinder working process dominates the dynamic

performance of the engine and results in the largest and most significant

impact excitation in it. At the same time, the widespread mechanical and fluid

impacts inside the engine form its high-frequency excitations. A fluid impact

is produced by knocking of gas pressure wave on the cylinder wall. It is

necessary to make an in-depth discussion to them and their influences to the

engine vibration response.

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Figure 1.6 Basic IC engine vibration correlations

1.3.1 The Unstable Combustion Excitation (I)

The in-cylinder high-pressure gas oscillation inside a diesel engine

can be theoretically described as a standing-wave equation in polar

coordinates as follows (Roa and Gupta, 1999):

+ (1.1)

Where is the specific heat constant ratio.

Taking the cylinder as a rigid constraint to the inner-cylinder gas

oscillation, the boundary condition around the cylinder diameter D can be

supposed as:

(1.2)

Generalized solution as

( ) = ( ) (1.3)

Where is the phase lag angle

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Equation (1.1) can be rewritten as:

+ + (1.4)

The frequency composition of the in-cylinder gas oscillation can be

expressed as (Roa and Gupta 1999)

= , ( , … , ) (1.5)

In a theoretical sense, Equation(1.5) expresses a definite description

on the in-cylinder gas oscillation. Firstly, m kinds of solution of the n order

Bessel Equation (1.4) explain the existence of unsteady in-cylinder gas

oscillation with m kinds of frequency composition; it may be single or

complex. Secondly, it is shown that the actual oscillating waveform and

frequency composition are mainly influenced by the structural and/or working

conditions of the engine, such as the geometrical parameters of the cylinder,

the in-cylinder oil and gas pressure and their mixing and combustion status

etc. This explains that the in-cylinder gas pressure with an unstable frequency

composition is a major cause of the abnormal working performance and non-

stationary dynamic process of the engine, and it is also directly related to the

structural and working conditions of the engine.

1.3.2 The Modulated High-Frequency Excitations

In Figure 1.5 segments II and III depict high frequency impulses

modulated by the relative transmission relationship of the sub-mechanical

systems in the engine. These high frequency impulses can be classified into

two groups:

1. Fluid impact induced vibration.

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2. Mechanical impact induced vibration.

1.3.2.1 Fluid Impact Induced Vibration

Fluid impacts are important sources of the high-frequency vibration

and noise inside the engine (Roa and Gupta, 1999). The fluid impact induced

high-frequency vibration in the reciprocating engine has a permanent

frequency composition which does not vary with changes of the engine

operating condition. If the speed is increased, it wouldn’t increase the

temperature and thermodynamic properties. Examples of this are the impacts

caused by combustion, cavitations, and pre-ignition.

1.3.2.2 Mechanical Impact Induced Vibration

The widely existing transient impacts caused by the in-engine

mechanical systems tend to be periodic at the operating condition when they

cause high-frequency resonant responses of the impacting mechanical parts.

Examples of this are the impacts caused by the inlet, outlet valves, piston and

others component failures. The high-frequency resonant response possesses

permanent frequency compositions and successively transforms itself as one

part of the engine vibration. In summary, the transient response caused by the

in-engine mechanical and fluid impacts possesses permanent frequency

compositions. At the same time, the unstable inner-cylinder working process

and relative transmission relationship of the in-engine mechanical systems

irregularly modulate them. In consequence, the practically measured vibration

signal on the engine casing reveals itself as a series of impulses modulated

along the time history.

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Typically, the main sources of excitation that are likely to be

observable from the vibration signal are associated with the following

mechanisms (Liu et al., 2008):

1. Rocking and twisting of the engine block on its supports, due

to the action of inertial forces

2. Impacts due to clearances at links, those at the crankshaft

bearings

3. Piston slap being extremely noisy

4. Closures and openings of valves

5. High-pressure injection of fuel in diesel engines

6. Rapid rising of gas pressure in the cylinders during the

combustion, especially in diesel engines where it has been

compared with a hammer blow.

The impulse responses on a typical engine structure usually last for

a few milliseconds for an engine running at 1500 rpm. This means angular

duration greater than 100 so that within the intervals where impacts occur

close together the vibration signatures will inevitably overlap. This is

typically what happens in the vicinity of the top dead centre, where injection,

piston, combustion and valve impacts occur within few degrees.