Kuldeep Dubey

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SEMINAR REPORT

REPORT ON COMBUSTION TECHNIQUE

SESSION: 2011 - 12IN GUIDENCE OF:

MR. KAMAL YADAV

PRESENTED BY:

KULDEEP DUBEY

B.TECH THIRD YEAR

ROLL NO: 0938140023


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COMBUSTIONCombustion or burningis the sequence ofexothermic chemical

reactions between a fuel and an oxidant accompanied by the production of

heat and conversion of chemical species .

CH4 + 2 O2 CO2 + 2 H2O + energy

CONDITION FOR COMBUSTION

1. COMBUSTIBLE MIXTURE

2 . SOME MEANS OF INITIATION COMBUSTION

3. PROPAGATION OF FLAME
http://en.wikipedia.org/wiki/Exothermichttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Oxidanthttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Oxidanthttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Exothermic


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Normal & Abnormal Combustion in SI Engine

Knock is the term used to describe a pinging noise emitted from a SI engine

undergoing abnormal combustion.

The noise is generated by shock waves produced in the cylinder when

unburned gas autoignites.


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THREE PHASES OF

COMBUSTIONThere are three distinct phases or parts to thecombustion in a diesel engine.

Ignition delay.

Rapid combustion.

Controlled combustion.


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Cyclic Variation of Pressure Vs Crank Angle


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Spark Timing

Spark timing relative to TC affects the pressure development and thus the

engine imep and power.

Ignite the gas before TC to center the pressure pulse around TC.

The overall burning angle is typically between 40 to 60o, depending on engine

speed.

motored


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Fuel Injection

Fuel injection is thepreferred method of

metering the fuel in

modern engines.

Cost has limited usefor small gas engines.

BOSH has developed

a system.


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(a) Multi-point injection uses separate

injectors for each cylinder, and each

injector is located close to the inlet

valve of the engine.

(b) Single-point injection uses only

one injector, which discharges fuel

into the: air stream at the point used

by a carburetor.


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CARBURETOR

the engine component that provides the

required air-fuel mixture to the

combustion chamber based on enginespeed and load.


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CARBURETOR OPERATING

PRINCIPLE A pressure difference is also needed to cause the fuel to flow from the fuel

bowel into the air stream.

This is accomplished using a venturi, Bernoullis principle and a tubeconnecting the mouth of the venture to the fuel bowel.

This is a functioning carburetor and it will operate an engine as long as it

has a constant load and constant speed.


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A carburetor is basically a tube.

There is an adjustable plate acrossthe tube called the throttle

plate that controls how much aircan flow through the tube.

At some point in the tube there isa narrowing, called the venturi,and in this narrowing a vacuum iscreated.

In this narrowing there is a hole,called a jet, that lets the vacuumdraw in fuel.


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IGNITION

SYSTEM The ignition system provides a high voltage spark in the combustion

chamber at the proper time.

Two types of ignition systems Battery

Magneto

Battery

Battery systems transforms the battery voltage and fires the sparkplug at the correct time.

Magneto

Magneto systems must produce the current, transform the voltage

and time the spark plug.

Most small engines use the magneto system Two types of magneto systems:

Breaker point ignition

Solid state (electronic) ignition


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Magneto Ignition system

As long as the flywheel is rotating and the ignition switch is on, the

spark plug fires every time the magnets move past the magneto.


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Magneto Ignition System- Firing Spark Plug

When the breaker points open the magnetic field produced by the

current in the primary winding collapses.

The collapsing magnetic field flows across the secondary coil whichinduces a current in the secondary coil.

Because there is a 60:1 ratio of windings in the two coils, the voltage istransformed to the 10,000 and 15,000 volts needed to fire the spark

plug.


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BATTERY IGNITION SYSTEM


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Detonation An undesirable engine condition in which pockets of fuel start to burn at

about the same time as the spark plug fires.

Multiple pressure fronts collide

Sometimes called knocking, spark knock or pinging. Causes large pressure differentials in the combustion chamber.

Can cause engine damage.

Causes1. Increased compression2. High temperatures3. Lean fuel/air mixture4. Advanced ignition timing5. Lower octane fuels


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Engine Damage From Severe Knock

Damage to the engine is caused by a combination of high temperature and

high pressure.

Piston Piston crown

Cylinder head gasket Aluminum cylinder head


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Engine parameters that effect occurrence of knock are:

i) Compression ratio at high compression ratios, even before spark ignition,the fuel-air mixture is compressed to a high pressure and temperature which

promotes autoignition

ii) Engine speed At low engine speeds the flame velocity is slow and thus

the burn time is long, this results in more time for autoignition

However at high engine speeds there is less heat loss so the unburned gas

temperature is higher which promotes autoignition

These are competing effects, some engines show an increase in propensity to

knock at high speeds while others dont.

Knock


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iii) Spark timing maximum compression from the piston advance occurs at

TC, increasing the spark advance makes the end of combustion crank angle

approach TC and thus get higher pressure and temperature in the unburnedgas just before burnout.

Knock

P,T

T

Ignitionx

xEnd of combustion


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Preignition

Fuel starts to burn before the spark plug fires.

Decreases engine performance and produces and audible pinging orknocking sound in the engine.

Increases the peak combustion pressure in the cylinder.

Increases internal temperature.

Will cause engine parts like pistons, connecting rods and crankshafts to

fail.

Causes;

1. An overheated spark plug

2. Glowing carbon deposits

3. Over heated exhaust valve

4. A sharp edge in the combustion chamber or on top of a piston

5. Sharp edges on valves that were reground improperly

6. A lean fuel mixture.


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DIESEL ENGINESIndirect and Direct Injection

FIGURE 4-4 A direct injection

diesel engine injects the fueldirectly into the combustion

chamber. Many designs do not use

a glow plug.


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Combustion in CI Engine

In a CI engine the fuel is sprayed directly into the cylinder and the vaporised

part of the fuel mixes with air and ignites spontaneously.

These photos are taken in a RCM under CI engine conditions with swirl

0.4 ms after ignition 3.2 ms after ignition

3.2 ms after ignition Late in combustion process

1cm

Air flow


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In-Cylinder Measurements

This graph shows the fuel injection flow rate, net heat release rate and

cylinder pressure for a direct injection CI engine.

Start of injection

Start of combustion

End of injection



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The combustion process proceeds by the following stages:

Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of

the compression stroke. The liquid fuel atomizes into small drops and

penetrates into the combustion chamber. The fuel vaporizes and mixes with

the high-temperature high-pressure air.

Premixed combustion phase (bc) combustion of the fuel which has mixed

with the air to within the flammability limits (air at high-temperature and high-pressure) during the ignition delay period occurs rapidly in a few crank angles.

Mixing controlled combustion phase (cd) after premixed gas consumed, the

burning rate is controlled by the rate at which mixture becomes available for

burning. The burning rate is controlled primarily by the fuel-air mixing process.

Late combustion phase (de) heat release may proceed at a lower rate well

into the expansion stroke (no additional fuel injected during this phase).

Combustion of any unburned liquid fuel and soot is responsible for this.


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Four Stages of Combustion in CI Engines

Start of

injection

End of

injecction

-10 TC-20 10 20 30


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CI Engine Types

Two basic categories of CI engines:

i) Direct-injection have a single open combustion chamber into which fuel

is injected directly

ii) Indirect-injection chamber is divided into two regions and the fuel is

injected into the prechamber which is connected to the main chamber via a

nozzle, or one or more orifices.

For very-large engines (stationary power generation) which operate at low

engine speeds the time available for mixing is long so a direct injection

quiescent chamber type is used (open or shallow bowl in piston).

As engine size decreases and engine speed increases, increasing amounts

of swirl are used to achieve fuel-air mixing (deep bowl in piston)

For small high-speed engines used in automobiles chamber swirl is not

sufficient, indirect injection is used where high swirl or turbulence is generated

in the pre-chamber during compression and products/fuel blowdown and mix

with main chamber air.


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Direct Injection

quiescent chamber

Direct Injection

multi-hole nozzle

swirl in chamber

Direct Injection

single-hole nozzle

swirl in chamber

Indirect injection

swirl pre-chamber


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