10.ICEn II CombDelKnok

7/30/2019 10.ICEn II CombDelKnok

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INTERNAL COMBUSTION ENGINES-II

Combustion

Dr. Mukesh Saxena

University of Petroleum & Energy Studies

Dehradun


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I C ENGINES - IIAir Motion & Combustion

Fuel - Air Mixing and Combustion :

-The air motion so organized inside the combustion

chamber and the turbulence of the air passes acrossthe 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 thenget 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 / mechanicallycontrolled 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 delayperiod 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 < < 0.7

- depends ; chemical reactions are faster at highertemperature 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 calleduncontrolled combustion phase.The rate of heatrelease 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 formedcombustible mixture with air and preflame reactions


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completed so that the layers of fuel vapours burn asfast as they can find fresh oxygen to continue thecombustion process

- depends ; It may be noted that the maximumpressure reached during this phase depends uponthe delay period. Longer the delay the higher is therate 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 steeprate of pressure rise has changed to much slowerrate of pressure rise.

- process ; This is the second phase of fuel injectionwhen injected fuel finds more difficult to seek outand react with fresh oxygen in the remainingunburnt 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 powerout-put ). Therefore, this is mechanically controlledby the injection pump out put characteristics.Prolongation of this variable period is determinedby the amount of additional fuel supplied above thatneeded for no- load running.


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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 andartiall burnt fuel with the air


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-Air Motion & Combustion


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Factors Affecting the Delay Period :

Many design and Operating factors affect the delayperiod. 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 fuelat the end of compression is shown below


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- It can be seen that the comp. temp. increases andthe auto ignition temp.decreases due to increaseddensity of air resulting in closer contact betweenmolecules of air and fuel, thereby reducing the timeof reaction and consequently the delay period.

- The peak pressure is only marginally affected dueto lower rate of pressure rise whereas mechanicalefficiency 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 millisecondsdecreases with increase in speed. With increase inspeed, the heat loss during compression decreases

resulting in rise in temp. & press. and thereforereducing the delay period.

- However, in CA the delay period increases asengine operate at higher rpm. The amount of fuelinjected during delay period increases (as fuel pumpis 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 fuelinjected ( at constant rpm ) increases and thereforethe air-fuel decreases and results in increase intemperature 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 ignitiondelay due to higher surface to volume ratio.

- Smaller droplets have low depth of penetration dueto less momentum. Such droplets have less velocityrelative to air and shorter fuel spray path, where ithas to find oxygen after vaporization affecting airutilization factor. Also the aggregate area ofinflammation after ignition will increase resulting in


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higher pressure rise during second stage ofcombustion.

- On the other hand, lower injection pressure givinglarger droplets give lower pressure rise and soother

running.

- An optimum group mean diameter of the dropletshould be attempted with appropriate fuel deliverycharacteristics.


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- Injection timing :

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

for constant injected quantity of fuel on cylinderpressure is shown below :


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- As pressure and temperature at the begning ofinjection are lower for higher injection advance, the

delay period increases with increase in injectionadvance.

- The optimum angle of advance depends on manyfactors. Generally it is about 20 bTDC.

I C ENGINES II


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- Quality of fuel

Self ignition temperature is the most importantproperty of fuel which affects the delay period. Alower self-ignition temperature results in lowerdelay period.

- Fuels with higher cetane number give lower delay.

Other properties of fuel which affect delay periodinclude volatility, latent heat, viscosity and surfacetension.

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- Intake temperature and pressure :

Increase in intake temp. increases compressed airtemp. resulting in reduced delay period. Preheating

may not be desirable as it reduces density of airaffecting volumetric efficiency and power.

- Increase in intake pressure reduces the auto -ignition temp. and hence the delay period. The peakpressure will be higher as compression pressurewill be higher with higher intake pressure.

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increases

___________

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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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I C ENGINES II


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Knock in CI Engines :

- Knocking is characterized by extreme pressuredifferentials and violent gas vibrations evidenced byaudible knock.

- In CI engines knocking occurs in the beginning ofcombustion.

- In order to decrease knocking the actual burningshould start as early as possible after the fuelinjection begins.

- It has been found that provided the rate of press.rise does not exceed 3 bar per CA combustion is

I C ENGINES II


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Knock in CI Engines :

- smooth. Between 3 to 4 bar there is tendency toknock, above this rate of pressure rise the dieselknock will be prominent.

- Normally audible knock is always present in CIengines, when it becomes sever and cause heavyvibrations in the engine , it is said to be knocking. It

is, therefore, a matter of judgment.

- Low ignition temp., ignition delay & speed reduceknock as also higher CR, Inlet Temp. & press.,comb. wall temp. cylinder size


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I C ENGINES - II Air Motion & Combustion

Combustion Chambers for C.I. EnginesCI 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)


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Combustion Chambers for C.I. EnginesCI combustion chamber :

Direct- Injection (DI)

- entire volume of combustion chamber located inmain cylinder where fuel is injected. Also called

open combustion chamber.

Indirect- Injection (IDI)

- Combustion space divided into two parts ;

one part main cylinder & other part in cylinder head


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Combustion Chambers for C.I. EnginesIndirect- Injection (IDI)

- Classified as :

1. Swirl chamber ;compression swirl generated

2. Precombustion chamber ;

combustion swirl induced

3. Air cell chamber ;

compression & combustion swirl induced


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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 & COMBUSTIONCHAMBERS

IV: SUPERCHARGING and TUBOCHARGING

V: DIESEL ENGINE TESTING &PERFORMAMCE

VI: DIESEL ENGINE Emissions AND their

Control Technology

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10.ICEn II CombDelKnok