THERMAL ENGINEERING: MODULE V ©Compiled by AVK

THERMAL ENGINEERING: MODULE V ©Compiled by AVK

Dept of ME, UEC, Vallivattom

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MODULE V

COMBUSTION IN IC ENGINES,

POLLUTION & REMEDIES

AMAL V K

Asst. Professor

UEC, Vallivattom

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Combustion in SI Engines

• Conditions necessary for combustion totake place are

– Combustible mixture

– Means to initiate combustion

– Stabilization and propagation of flame incombustion chamber

2Thermal Engineering

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IGNITION LIMIT

• Ignition limit is the range of air fuel ratiob/w lean and rich mixture beyond which thefuel wont burn even if the spark is fired.

• In case of hydrocarbon fuel, stoichiometricair fuel ratio is 15:1 and the ignition limit isb/w 7:1 and 30:1.

Thermal Engineering 3

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Stages of combustion in SI Engines


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• A is the point of passage of spark.

• B is the point at which beginning of pressurerise can be detected.

• C is the attainment of peak pressure.

• AB is first stage known as Ignition Lag inwhich growth and development of selfpropagating nucleus of flame takes place.

• Ignition lag is the time interval b/w instant ofspark and instant where there is noticeable risein pressure due to combustion.

• It depends on temp, pressure, nature of fuel.


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• BC is second stage known as Flame

Propagation stage which deals with spread of

flame throughout combustion chamber.

• Starting point of second stage is the point

where first measurable rise of pressure is seen.

• This can be seen from the deviation from

motoring curve.


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• Starting point of third stage, After Burning,is usually taken as the point at whichmaximum pressure is reached.

• Flame velocity decreases during this stage.

• As expansion stroke starts before this stage,there can be no pressure rise during thisstage.


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Flame Front Propagation


• For efficient combustion, rate of propagation offlame within cylinder is critical.

• It depends on reaction rate and transpositionrate.

• Reaction rate is the result of chemical reactionbetween flame and unburned mixtures.

• Transposition rate is due to the physicalmovement of flame relative to cylinder walland is the result of pressure difference b/wburned and unburned gases.

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Factors Influencing Flame Velocity

• Speed with which flame travels is known asFlame velocity.

• Flame velocity affects combustion phenomena,rate of pressure rise in the cylinder and thepower produced.

• Turbulence:

– Increase in turbulence results in intermingling ofburned and unburned particles, thereby increasingrate of contact.

– This results in increase in flame velocity11Thermal Engineering

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• Fuel-Air ratio:

– Highest flame velocities are obtained at richermixture nearer to stoichiometric ratio.

– If mixture is made leaner or richer, the flamespeed decreases.

– Lesser thermal energy is released in case of leanmixtures, resulting in lower flame temperature.

– Very rich mixtures lead to incompletecombustion which again results in release of lessthermal energy.


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• Temperature and Pressure:– Flame speed increases with increase in temp and

pressure.

– Higher initial pressure and temp help to form a betterhomogeneous air-vapor mixture which helps inincreasing flame speed.

• Compression ratio:– Higher compression ratio increases pressure and

temp.

– Higher compression ratio decreases clearance volumeand hence increases flame velocity


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• Engine speed:

– Flame speed increases with engine speed sinceincrease in speed increases turbulence.

• Engine size:

– In large engines time required for completecombustion is more because the flame has totravel a longer distance


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Abnormal combustion in SI Engines

• Abnormal combustion deviates from normalcombustion resulting in loss of performanceand physical damage to engine.

• This can be due to

–Pre-ignition

–Knocking or Detonation


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Pre-ignition

• High temperature carbon deposits formed insidethe combustion chamber ignite air-fuel mixturebefore normal ignition occurs by spark plug.

• This ignition due to hot carbon deposits is calledPre-ignition.

• Pre-ignition results in increase in pressureduring compression stroke.

• The work to compress the charge will beincreased and hence net power output will bereduced.


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Auto Ignition

• If the pressure and temperature of air-fuelmixture is increased beyond a limit, it resultsin a chemical reaction to initiate combustion.

• This type of self-ignition in the absence offlame is known as Auto-ignition.

• Temperature at which self ignition takesplace is known as self ignition temperature.

• Auto ignition leads to detonation.


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Detonation or Knocking in SI Engine

• When spark occurs, combustion of fuel nearspark plug commences.

• Flame travels through combustion chamberwith high velocity.

• High pressure and temp gases produced bythis ignition compresses fresh unburnedcharge in front of moving flame.


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• Thus temp and pressure of fresh charge isincreased beyond a limit which leads to selfignition of fresh charge.

• Both the flames get collided resulting inhigh pressure waves striking the cylinderwalls, cylinder head and piston with loudpulsating noise known as Knocking orDetonation or Pinking.



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Effects of engine variables on Detonation

1. Density Factors 2. Time Factors 3. Composition Factors

•Compression ratio•Mass of inductedcharge•Inlet temp of mixture•Temp of combustionchamber walls•Retarding spark timing•Power output of engine

•Turbulence•Engine speed•Flame traveldistance•Engine size•Combustionchamber shape•Location ofspark plug

•Fuel-Airratio•Octanevalue of fuel


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(Density Factors)

• Compression ratio– Increase in CR increases temp & pressure at the end

of compression stroke thereby decreasing ignitionlag & increasing tendency for knock.

• Mass of inducted charge– Increase in mass of charge increases pressure

thereby increasing tendency for knock

• Inlet temp of mixture– Increase in inlet temp further increases

compression temp thereby increasing tendency forknock


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(Density Factors)

• Temp of combustion chamber walls– Increase in temp of combustion chamber walls

increases chance for hotspots & hence increasestendency for knocking

• Retarding spark timing– By retarding spark timing closer to TDC, peak

pressure occurs only during expansion stroke whichreduces knock.

• Power output of engine– Decrease in output of engine decreases temp and

pressure of cylinder thereby decreasing tendency toknock.


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(Time Factors)

• Turbulence

– Increase in turbulence increases flame speed & thusreduces time available for fresh charge to autoignite & hence decreases tendency to knock.

• Engine speed

– Increase in engine speed increases turbulence &hence decreases tendency to knock.

• Flame travel distance

– Knocking tendency decreases with decrease inflame travel distance


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(Time Factors)

• Engine size– Flame requires longer time to travel across

combustion chamber of larger engine. So largerengine have greater tendency to knock

• Combustion chamber shape– More compact the combustion chamber shape,

shorter will be the flame travel distance & hencedecreases tendency to knock.

• Location of spark plug– Spark plug is located in such a way to reduce the

flame travel distance across combustion chamber &hence to decrease tendency to knock.


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(Composition Factors)

• Fuel-Air ratio

– Maximum tendency to knock takes place forfuel-air ratio which gives minimum reactiontime

• Octane value of fuel

– As octane value of fuel increases, tendency toknock decreases.


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Effects of Detonation

• Noise and roughness

• Mechanical damage

• Carbon deposits

• Increase in heat transfer

• Decrease in power output and efficiency

• Pre-ignition


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COMBUSTION CHAMBER

• Important consideration in design ofcombustion chamber

– Turbulence

– Location of spark plug

• High turbulence improves rate of combustion

• Suitable location of spark plug shorten the pathof flame travel.

• Charge near the chamber walls don't burnbecause the cold chamber wall cools the charge


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Combustion Chamber Design for SI Engines

• On the basis of valve arrangement

• T- head

• L- head

• I- head

• F- head

Automobile Engineering 30

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T- head

• Oldest type

• Both valves are operatedby separate camshaft

• Complicated mechanism

• Shape of combustionchamber provides poorcombustion and lowengine performance


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L- head

• Both valves are arranged in asingle row & operated fromsame camshaft.

• Low engine height

• Low production cost

• Quiet operation

• Ease of lubrication

• Inefficient on the account ofcombustion chamber design.

• More prone to detonation


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I- head


• Extensively used

• Higher volumetric efficiency

• Higher compression ratio

• Leaner air-fuel mixture can be burnt

• Rocker arm leverage makes itpossible to have desired cam profilelift.

• Not precise operation at higherengine speed

• Vibration

• Noisy operation

• Greater maintenance required due tomore wear at more joints

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F-head

• Overhead valve forinlet & side valvemechanism for exhaust

• Uses larger inlet valves

• Less efficient

• More expensive


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Combustion Chamber Design for SI Engines

• Side Valve type

• Wedge type

• Inverted Bath Tub type

• Flat Head type

• Hemispherical type

• Multi Valve type

• Split Level type

• Twin Spark Plug type

Automobile Engineering 35

•On the basis of combustion chamber shape

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1. Side Valve Type

• Simpler

• Cheap

• Less efficient fromcombustion point ofview

• Used only for lowercompression ratioabout 7:1

• No longer in use


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2. Wedge Type


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2. Wedge Type

• Single row valves tilted to accommodate slopingroof.

• Spark plug located at thick side of wedge, midwayb/w valves.

• Smooth and uniform combustion.

• At the end of compression stroke, piston reachesquench area & gases squeezed from quench areacauses turbulence.

• Fast and smooth burning.

• These combustion chambers are left withoutmachining, which reduces production cost.


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3. Inverted Bath Tub Type

• Another form ofwedge type.

• Turbulence ispromoted.

• Flame spreadsrapidly due towhich tendencyto detonation isdecreased.


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4. Flat Head Type


• Cylinder head is flat.

• Combustion chamber isplaced in the piston crown.

• High combustion efficiency.

• High compression ratio.

• Large valves may be used inthe large diameter head, stillmaintaining smallcombustion space.

• Heat of combustion is mainlydissipated through piston.

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5. Hemispherical Type

41

• Shape of combustionchamber is close tohemispherical.

• Spark plug is locatedcentrally & valves areplaced in slantedposition.

• Inlet & exhaust valves areon different sides,requiring two camshafts.

• Efficient & compact.

Thermal Engineering

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5. Hemispherical Type

• Combustion rate is slower due to no turbulenceyet flame travel is short.

• High volumetric efficiency.

• High production cost.

• Roughness & noise at lower speed due to rapidburning

• Suitable for small, high speed automotive engines.

• More power & torque with more fuel economy.

• Combustion chambers are completely machined

• High production cost.


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6. Multi-Valve Type

• Increased flow rate.

• Higher volumetric efficiency.

• Better combustion.

• Reduced emissions.

• Better performance.

• More fuel economy.

• High cost.

• Complex construction.


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6. Multi-Valve Type


•Four valve designcreates barrel shapedswirl.•Good turbulence.•Good combustionefficiency.•Uniform mixing forfuel & air.

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7. Split Level Type

• Exhaust valve is deeplyrecessed in a circularchamber.

• High compression ratio.

• No detonation.

• Eliminates most endgases before they getover heated.


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8. Twin Spark Plug Type

• Reduces length of flame travel.

• Helps to reduce rough and harsh runningespecially at light loads.

• Develops more torque at lower speeds.

• Improves fuel consumption at part loads.

• Enables more EGR utilization.


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Stages of combustion in CI Engines


1. Ignition delay period

– Physical delay

– Chemical delay

2. Period of rapid combustion

3. Period of controlled combustion

4. Period of after burning

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Stages of combustion in CI Engines


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• Ignition delay period is the time periodcounted from the start of fuel injection tothe point where there is noticeable rise inpressure due to start of combustion.

• Ignition delay period is divided into

– Physical delay period

– Chemical delay period


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• Physical delay period– It is the time interval b/w start of fuel injection &

attainment of chemical reaction conditions.

– During this period, fuel is atomised, vaporised andmixed with air and is raised to self ignition temp.

• Chemical delay period– During this period chemical reaction b/w fuel & air

takes place depending upon surrounding temp &pressure

– Reaction starts slowly & then accelerate until ignitiontakes place.


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2. Period of rapid combustion

• Period of rapid combustion is counted fromthe end of delay period to the point ofmaximum pressure on indicator diagram.

• During this period combustion isuncontrolled & pressure rise is rapid.

• Rate of heat release is maximum during thisperiod.


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3. Period of controlled combustion

• Period of controlled combustion is countedfrom the point of maximum pressure onindicator diagram to the point of maximumcycle temperature.

• Pressure decreases during this period dueto expansion stroke.

• Fuel injection is continued till the end of thisperiod.


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4. Period of After burning

• Period of After burning starts from the pointof maximum cycle temperature & continuesover a part of expansion stroke.

• During this period, unburned & partiallyburned fuel particles left in combustionchamber starts burning as soon as theycome into contact with oxygen.


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Factors affecting delay period in CI engine

• Compression ratio– Increase in compression ratio increases maximum temp &

pressure thereby decreasing delay period.

• Engine speed– With increase in engine speed, loss of heat during compression

stroke decreases, resulting in increased temp & pressure ofcompressed air, thus reducing delay period in milliseconds.

• Engine output– With increase in engine output, Air-Fuel ratio decreases & hence

operating temp increases thereby decreasing delay period.

• Intake pressure– Increase in intake pressure or supercharging reduces delay

period.Thermal Engineering 55

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Factors affecting delay period in CI engine

• Atomisation of fuel– Higher injection pressure increases degree of atomisation & thus

decreases ignition delay due to higher surface to volume ratio.

• Injection timing– As temp & pressure at the beginning of injection are lower for

higher ignition advance, delay period increases with increase inignition advance

• Quality of fuel– Fuels with higher cetane number gives lower delay period &

smoother engine operation.

• Intake temperature– Increase in intake temp increases compressed air temp, resulting

in reduced delay period.Thermal Engineering 56

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Knock in CI Engines

• If the ignition delay is less, fuel starts burning assoon as fuel is injected.

• If the ignition delay period is longer, largeamount of diesel will be accumulated in thechamber during injection.

• Combustion of large amount of fuel may causehigh pressure rise & violent gas vibrationsknown as Knocking & is evidenced by audibleknock.

• In CI engines knocking occurs near thebeginning of combustion whereas in SI engineknocking occurs near the end of combustion.


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Effect of

ignition

delay on

rate of

pressure

rise in

CI

Engine


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Knocking in SI & CI Engine


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Combustion Chamber Design for CI

Engines

• Direct injection or Open type

• Turbulent or Swirl type

• Pre-chamber type


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1. Direction injection type


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1. Direction injection type

• No separate combustion space• Fuel directly sprayed to cylinder• Piston crown contains depression which may be

bowl shaped or torroidal• Space b/w head & piston crown forms combustion

chamber• Multi hole type injectors (30Mpa)• Combustion chamber have lower surface area to

volume ratio, thus reducing heat loss• Low fuel consumption• No auxiliary starting devices are required


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2. Turbulent type combustion

chamber


Vortex Type Comet Type

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2. Turbulent type combustion

chamber


• Designed to introduce turbulence of compressed air

• Single hole type injector nozzle

• In vortex type, air is pushed to the cylindricalchamber in the head at the end of compressionstroke, where fuel is injected.

• Comet type employs spherical combustionchamber

• It is connected tangentially to a cavity in the pistoncrown through a narrow passage called throat

• Compressed air enters spherical chambertangentially through the throat creating turbulence

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3. Pre-chamber type combustion

chamber

• Combustion space is locatedboth in the cylinder as wellas in head.

• Pre-chamber forms 40%combustion space

• Main chamber is formedb/w cylinder head andcavity in piston crown

• Pre-chamber is connectedwith main chamber througha restricted passage calledburner


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3. Pre-chamber type combustion

chamber

• High turbulence

• Smooth running

• Difficult to start without heater plugs

• Require high compression ratio

• Involves heat lose to burner


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Fuel Ratings

• Octane rating is for SI engines

• Cetane ratings is for CI engines

• Higher the octane rating, lesser is knockingtendency in SI engines

• Higher the cetane rating, lesser is dieselknocking tendency in CI engines

• Fuels with higher octane number havelower cetane value


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Octane number

• Octane number of a fuel is the percentage ofoctane in a mixture of octane & n-heptanewhich produces same knocking tendency asthat of the fuel under same conditions.

• Iso-Octane which is very much less prone toknocking have an octane rating of 100.

• N-heptane which is very much prone toknocking have an octane rating of 0.


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Cetane number

• Cetane number of a fuel is the percentage ofcetane in a mixture of octane & alpha methylnaphthalene which produces same knockingtendency as that of the fuel under sameconditions.

• Cetane which is very much less prone toknocking have cetane rating of 100.

• Alpha methyl naphthalene which is very muchprone to knocking have a cetane rating of 0.


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Air pollution from IC Engines

• Following are the major problems caused byair pollution : global warming, acid rain,smog, odours, respiratory & other healthhazards.

• Major causes of these emissions are non-stoichiometric combustion, dissociation ofnitrogen, impurities in fuel & air.


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Pollutants from SI & CI Engines

• Major exhaust emissions are1. Unburned hydrocarbons (UBHC)

2. Oxides of carbon (CO & CO2)

3. Oxides of nitrogen (NO & NO2)

4. Oxides of sulphur (SO2 & SO3)

5. Particulates, soot & smoke

• First 4 are common to both SI & CIengines.

• Last 2 are mainly from CI engines.


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Hydrocarbon emissions

• It is a strong function of air-fuel ratio.

• If air-fuel mixture is too rich, HC emissionoccurs as a result of insufficient oxygen.

• If air-fuel mixture is too lean, HC emissionsoccurs as a result of poorer combustion.

• Incomplete combustion, deposits on walls,oil on combustion chamber walls, valveoverlap are some factors affecting HCemissions


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Carbon monoxide emissions

• Colorless, odorless, poisonous gas.

• Occurs due to insufficient amount of oxygenduring combustion.


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NOx emissions

• NOx reacts in atmosphere to form ozoneand is one of the major causes ofphotochemical smog.

• Mainly formed from air sucked into theengine cylinder.

• Nitrogen is highly reactive at higher temp &hence NOx is a major pollutant from CIengines.


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Particulate emissions

• Carbon soot particles generated in fuel richconditions in CI engines.

• Occurs mainly at wide open throttle.

• Soot particles are clusters of solid carbonspheres formed mainly due to in sufficientamount of oxygen.


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Methods of emission control

1. Thermal converters

2. Catalytic convertors

3. Particulate traps

4. Exhaust gas recirculation


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Thermal converters

• These are high temp chambers throughwhich exhaust gases flows.

• These promote oxidation of CO & HC.

• For these reaction to occur, temp must beheld above 7000C.

• Major limitation is the insulation problem ofthermal convertors

• NOx emissions cannot be reduced by thismethod.


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Catalytic converters

• Catalytic convertor is a stainless steelcontainer mounted somewhere at theexhaust pipe.

• Inside the container is a porous ceramicstructure through which exhaust gas flows.

• Surface of ceramic passages contains smallembedded particles of catalytic materialthat promote oxidation of exhaust gases.

• Aluminum oxide is base ceramic materialused for catalytic converters


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Catalytic converters

• Platinum, palladium and rhodium are themost commonly used catalyst materials.

• Platinum & palladium promotes oxidation ofHC & CO and rhodium promotes reaction ofNOx.

• CO & HC emissions are oxidized at 250-3000C with the help of catalysts.

• This is the most effective after treatmentmethod for reducing engine emissions.


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Particulate traps

• Filter like systems made of ceramic or metalwire mesh, used at exhaust of CI engines

• Traps typically removes 60-90% ofparticulates in exhaust gas.


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Exhaust Gas Recirculation

• Most effective way of reducing NOx is to reducecombustion chamber temp.

• This can be done by diluting air-fuel mixtureby a non-reacting gas which absorbs heatenergy liberated during combustion.

• This results in lower flame temp.• Non-reacting gas used is exhaust gas.• Amount of exhaust gas used can be as high as

30% of total intake.• Increase in EGR increases HC emissions &

decreases thermal efficiency.


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Blending of fuels

• It is the process of mixing an alternative fuelin a conventional fuel.

• Blending is done so as to reduce theemissions and to make fuel moreeconomical.

• It helps in conservation of fossil fuels.

• Some commonly used blends are B20containing 20% of biodiesel & 80% diesel;E85 containing 85% of ethanol & 15% ofgasoline.


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Alternative fuels for IC engines

• Major alternative fuels used are alcohol,methanol, vegetable oil, biodiesel, hydrogen,natural gas, CNG, LPG.

• Alcohol can be obtained from both natural &can be manufactured.

• Alcohol have higher octane number & lowersulphur content.

• Major limitation of alcohol is poor ignitioncharacteristics, corrosive nature & risk ofstorage.


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• Methanol & ethanol are used alonggasoline & diesel as blends.

• Most common mixtures areM85(85%methanol & 15% gasoline),M10(10%methanol & 90% gasoline),E85(85%ethanol & 15% gasoline),E10(10%ethanol & 90% gasoline) etc.

• M10 have same emission as that of gasolinebut it help to reduce use of gasoline by 10%.


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• With M85 there is measurable decrease in HC& CO emissions.

• Vegetable oil is used as an alternative fordiesel.

• But viscosity of vegetable oil is higher thandiesel which have to be reduced bypreheating or by blending.

• Biodiesel produced by chemically reactingvegetable oil or animal fats with alcohol isalso used as an alternative.


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• Blends of biodiesel with less than 20% biodieselcan be used in diesel engines without anymodifications.

• LPG has higher potential as alternative fuel.

• LPG contains less carbon than petrol & have acost saving of 50%.

• Main limitation is that ignition temp of LPG ishigher than petrol & storage limitations.

• Hydrogen is also used as an alternative fuelbecause of its low emission.


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• Hydrogen have a very high energy content.

• High cost & difficult to refuel are the majorlimitations

• Natural gas is another alternative fuel useddue to its clean combustion.

• Due to low density of natural gas, its ismainly used as compressed natural gas.


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THERMAL ENGINEERING: MODULE V ©Compiled by AVK