CI Engine Knocking

INTERNAL COMBUSTION ENGINES-II Combustion

University of Petroleum & Energy Studies

Dehradun

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Air Motion & Combustion

• Fuel - Air Mixing and Combustion :

-The air motion so organized inside the combustion

chamber and the turbulence of the air passes across

the fuel jet tears it into fine particles.

- A mixture of air and fuel forms at some location in

the spray envelope and oxidation starts

- The liquid fuel droplets initially evaporate and then

get heated up , and A/F ratio ( local ) within

combustible range , autoignite.

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• Combustion in CI Engines

- Stages/ Phases of combustion

1. Ignition delay period ;

Physical delay and chemical delay

2. Rapid combustion period / rapid pressure rise

3. Controlled combustion period / mechanically

controlled period

4. After-Burning period

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• Combustion in CI Engines

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Stages/ Phases of combustion

1. Ignition delay period ;

- Preparatory phase:some fuel admitted,not ignited

- Period : start of injection to start of combustion

(p-t curve separates from motoring curve)

- Important : combustion, design, performance,

emissions

- Fuel does not ignite immediately and total delay

period can be explained as; Physical delay and

Chemical delay. Details are shown in the figure.

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• Combustion in CI Engines : Ignition Delay

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Physical delay :

- period ; from beginning of injection to attainment

of chemical reactions.

- process; atomization, vaporization, mixing of fuel

& air, raising to self-ignition temperature

- depends on; fuel,air motion,combustion chamber

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Chemical delay :

- Period ; from beginning of chemical reactions to

auto-ignition

- process ; pre-reactions in the mixture taking place

up to auto-ignition that occurs at a local

excess-air factor of 0.5 < λ < 0.7

- depends ; chemical reactions are faster at higher

temperature of the surroundings and accordingly

the chemical delay becomes shorter.

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2. Rapid combustion period / rapid pressure rise

- This is the phase of combustion when the rate of

pressure rise is maximum and is also called

uncontrolled combustion phase.The rate of heat

release is maximum during this phase.

- period ; from end of delay or beginning of

combustion to the point of maximum pressure on

indicator diagram

- process ; most of the fuel would have formed

combustible mixture with air and preflame reactions

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completed so that the layers of fuel vapours burn as

fast as they can find fresh oxygen to continue the

combustion process

- depends ; It may be noted that the maximum

pressure reached during this phase depends upon

the delay period. Longer the delay the higher is the

rate of pressure rise as more fuel gets accumulated

during the delay period.

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3. Controlled combustion period / mechanically

controlled period

- This is the phase of combustion when very steep

rate of pressure rise has changed to much slower

rate of pressure rise.

- process ; This is the second phase of fuel injection

when injected fuel finds more difficult to seek out

and react with fresh oxygen in the remaining

unburnt air mass.

- period ; from the point of maximum pressure to the

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point of maximum cycle temperature.

- depends ; crank-angle duration of fuel injection as

per load requirements (higher and full load power

out-put ). Therefore, this is mechanically controlled

by the injection pump out put characteristics.

Prolongation of this variable period is determined

by the amount of additional fuel supplied above that

needed for no- load running.

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Air Motion & Combustion 4. After-Burning period : fuel with the air

- Combustion does not cease with completion of fuel

injection.The unburnt and partially burnt fuel particles

start burning as soon as they come in contact with

oxygen. This continue for a certain duration called

after- burning period.

- period ; starts from the point of max.cycle temp. and

continue over the part of expansion process.

- depends ; rate of after burning depends upon velocity

of diffusion and turbulent mixing of unburnt and

partially burnt fuel with the air

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• Factors Affecting the Delay Period :

Many design and Operating factors affect the delay

period. Some of the significant factors are;

- Compression ratio

- Engine speed

- Output

- Atomization of fuel and duration of injection

- Injection timing

- Quality of fuel

- Intake temperature and pressure

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- Compression ratio (CR )

The effect of CR on compression temperature of air

and the minimum auto ignition temperature of fuel

at the end of compression is shown below

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- It can be seen that the comp. temp. increases and

the auto ignition temp.decreases due to increased

density of air resulting in closer contact between

molecules of air and fuel, thereby reducing the time

of reaction and consequently the delay period.

- The peak pressure is only marginally affected due

to lower rate of pressure rise whereas mechanical

efficiency decreases.

- Optimum CR required considering cold starting &

light loads at high speeds.

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- Engine speed :

The effect of engine speed on ID ( time ) is shown

below

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- It can be seen that delay period in milliseconds

decreases with increase in speed. With increase in

speed, the heat loss during compression decreases

resulting in rise in temp. & press. and therefore

reducing the delay period.

- However, in ºCA the delay period increases as

engine operate at higher rpm. The amount of fuel

injected during delay period increases (as fuel pump

is geared to engine) resulting in high rate of press.

rise during second stage of combustion.

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- Output :

With increase load or output, the amount of fuel

injected ( at constant rpm ) increases and therefore

the air-fuel decreases and results in increase in

temperature thereby reducing delay period.

- The rate of pressure rise is unaffected but the peak

pressure may be high.

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- Atomization of fuel and duration of injection :

Higher fuel-injection press. increases the degree

(fineness) of atomization which reduces ignition

delay due to higher surface to volume ratio.

- Smaller droplets have low depth of penetration due

to less momentum. Such droplets have less velocity

relative to air and shorter fuel spray path, where it

has to find oxygen after vaporization affecting air

utilization factor. Also the aggregate area of

inflammation after ignition will increase resulting in

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higher pressure rise during second stage of

combustion.

- On the other hand, lower injection pressure giving

larger droplets give lower pressure rise and soother

running.

- An optimum group mean diameter of the droplet

should be attempted with appropriate fuel delivery

characteristics.

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- Injection timing :

The effect of injection advance timings ( 9º,18º,27º )

for constant injected quantity of fuel on cylinder

pressure is shown below :

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- As pressure and temperature at the begning of

injection are lower for higher injection advance, the

delay period increases with increase in injection

advance.

- The optimum angle of advance depends on many

factors. Generally it is about 20º bTDC.

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- Quality of fuel

Self ignition temperature is the most important

property of fuel which affects the delay period. A

lower self-ignition temperature results in lower

delay period.

- Fuels with higher cetane number give lower delay.

Other properties of fuel which affect delay period

include volatility, latent heat, viscosity and surface

tension.

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- Intake temperature and pressure :

Increase in intake temp. increases compressed air

temp. resulting in reduced delay period. Preheating

may not be desirable as it reduces density of air

affecting volumetric efficiency and power.

- Increase in intake pressure reduces the auto -

ignition temp. and hence the delay period. The peak

pressure will be higher as compression pressure

will be higher with higher intake pressure.

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• Knock in CI Engines :

- Fuel Injection takes definite interval

- Injection & Combustion take place simultaneously

- Initial droplets while undergoing ignition delay, additional

droplets are being injected

- If ignition delay of fuel is short,

initial few droplets will commence

actual burning

- As a result, mass rate of mixture

burned produce rate of press. rise

for soother combustion as shown

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- As ignition delay is longer , the actual burning of

first few droplets is delayed and accumulation of

greater quantity of fuel take place. When actual

burning take place under such conditions the rate of

pressure rise increases as shown in fig.

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- If ignition delay is quite large and the actual

burning is substantially delayed and the

accumulation of fuel is high when actual burning

take place the rate of pressure rise is almost

instantaneous the knocking begins as shown in fig.

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- Knocking is characterized by extreme pressure

differentials and violent gas vibrations evidenced by

audible knock.

- In CI engines knocking occurs in the beginning of

combustion.

- In order to decrease knocking the actual burning

should start as early as possible after the fuel

injection begins.

- It has been found that provided the rate of press.

rise does not exceed 3 bar per ºCA combustion is

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- smooth. Between 3 to 4 bar there is tendency to

knock, above this rate of pressure rise the diesel

knock will be prominent.

- Normally audible knock is always present in CI

engines, when it becomes sever and cause heavy

vibrations in the engine , it is said to be knocking. It

is, therefore, a matter of judgment.

- Low ignition temp., ignition delay & speed reduce

knock as also higher CR, Inlet Temp. & press.,

comb. wall temp. cylinder size

I C ENGINES - II Air Motion & Combustion

Combustion Chambers for C.I. Engines

CI combustion chamber :

- provide proper mixing of fuel and air in short time

- An organized air movement ;

swirl, turbulence,squish

- produce high relative velocity between fuel

droplets and air

- Shape of combustion chamber control air motion

- CI engines classified as ;

Direct- Injection (DI) & Indirect- Injection (IDI)



CI combustion chamber :

Direct- Injection (DI)

- entire volume of combustion chamber located in

main cylinder where fuel is injected. Also called

open combustion chamber.

Indirect- Injection (IDI)

- Combustion space divided into two parts ;

one part main cyl. & other part in cylinder head



Indirect- Injection (IDI)

- Classified as :

1. Swirl chamber ;

compression swirl generated

2. Precombustion chamber ;

combustion swirl induced

3. Air cell chamber ;

compression & combustion swirl induced

IC ENGINES-II Air Motion & Combustion

Course Outlines ADEG-222 LTP-3 1 1

I: BASIC THEORY

II: FUEL INJECTION SYSTEM

III: AIR MOTION, COBUSTION & COMBUSTION

CHAMBERS

IV: SUPERCHARGING and TUBOCHARGING

V: DIESEL ENGINE TESTING &

PERFORMAMCE

VI: DIESEL ENGINE Emissions AND their

Control Technology

CI Engine Knocking

Engineering

Transcript of CI Engine Knocking