Combustion in c.i. engines

By :ABHISHEK DHAWAN

SG9904 MECHANICAL 4TH SEM

Combustion in CI EngineIn 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 process1

cm

Air flow

In-Cylinder MeasurementsThis graph shows the fuel injection flow rate, net heat release rate and cylinder pressure for a direct injection CI engine.

Start of injectionStart of combustion

End of injection

Combustion in CI EngineThe 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 mixedwith 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.

Four Stages of Combustion in CI Engines

Start ofinjection

End ofinjecction

-10 TC-20 10 20 30

CI Engine Types

Two basic categories of CI engines:i) Direct-injection – have a single open combustion chamber into which fuel is injected directlyii) Indirect-injection – chamber is divided into two regions and the fuel isinjected into the “prechamber” which is connected to the main chamber via anozzle, 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.

Direct Injectionquiescent chamber

Direct Injectionmulti-hole nozzleswirl in chamber

Direct Injectionsingle-hole nozzleswirl in chamber

Indirect injectionswirl pre-chamber

Combustion CharacteristicsCombustion occurs throughout the chamber over a range of equivalenceratios dictated by the fuel-air mixing before and during the combustion phase.

In general most of the combustion occurs under very rich conditions within the head of the jet, this produces a considerable amount of solid carbon (soot).

Shadowgraph

Backlitphoto

1o

ASI5o

ASI

Diffusion flame

Fuel vapour High sootLiquid fuel

Ignition DelayIgnition delay is defined as the time (or crank angle interval) from whenthe fuel injection starts to the onset of combustion.

Both physical and chemical processes must take place before a significantfraction of the fuel chemical energy is released.

Physical processes are fuel spray atomization, evaporation and mixing of fuel vapour with cylinder air.

Good atomization requires high fuel pressure, small injector hole diameter,optimum fuel viscosity, high cylinder pressure (large divergence angle).

Rate of vaporization of the fuel droplets depends on droplet diameter,velocity, fuel volatility, pressure and temperature of the air.

Chemical processes similar to that described for autoignition phenomenonin premixed fuel-air, only more complex since heterogeneous reactions (reactions occurring on the liquid fuel drop surface) also occur.

Factors Affecting Ignition Delay TimeInjection timing – At normal engine conditions the minimum delay occurs with the start of injection at about 10-15 BTC.

Earlier or later injection timing results in a lower air temperature and pressure during the delay period increase in the ignition delay time

Injection quantity – For a CI engine the air is not throttled so the load is varied by changing the amount of fuel injected.

Increasing the load (bmep) increases the residual gas and wall temperature which results in a higher charge temperature at injection decrease in the ignition delay.

Intake air temperature and pressure – an increase in ether will result in a decrease in the ignition delay, an increase in the compression ratio has thesame effect.

Compression ratio limitations in CI engine

•The higher the combustion pressure, the higher the sealing pressure.•The higher the sealing pressure, the higher the friction loss.•The higher the compression ratio, the higher the combustion pressure, sealing pressure, and friction loss.

ROLE OF COMBUSTION CHAMBER ON ENGINE PERFORMANCE•Thedieselengineperformanceisgreatlyaffectedbythephen

omenaoccurringinsidethecombustionchamber,whichdependsmainlyonthepistonbowlconfiguration.•The piston bowl configuration is closely to swirl ratio of the engine.•InordertomaintaintheglobalstandardofDIengineperformance,multidimensionalflowsimulationisusedasaneconomicaltoolfortheoptimumdesignofDIengine.•SwirlisgeneratedduringcompressionprocessinDIengineandsubsequentlyitplaysavitalroleinmixingairandfuelinsidethecylinder.

•Modeling of combustion cylinder and prediction of in-cylinder flow is essential to achieve better performance of a DI engine.

TYPES OF COMBUSTION CHAMBER

1.OPEN TYPE COMBUSTION CHAMBER•Fuel is injected directly into the upper portion of the cylinder(i.e. combustion chamber).This type depends little on turbulence to perform the mixing.•High injection pressures and multi–orifice nozzles are required.•Itwasusedearlieronlowspeedengines,butwithavailabilityoffurtherhigherpressures,beingusedevenforhighspeedengines.

2.PRE COMBUSTION CHAMBER

It is separated into two chambers.•Thesmallerchamberoccupiesabout30percentoftotalcombustionspace.•Astheprecombustionchamberrunshot,delayperiodisveryshort.Thisresultsintosmallrateofpressureriseandthus,tendencyofDieselknockisminimum,andassuchrunningissmooth.•Productsofcombustionfromprechambermovetomainchamberinaviolentway,whichhelpsinaveryrapidcombustioninthirdstagedue

MEXICAN HAT TYPE CHAMBERMost common Produces desirable turbulenceThe deeper the bowl the greater the turbulenceLower fuel Inj. Pressures possibleShallow bowl less turbulenceHigher fuel Inj. Pressures required

Late model engines use Mexican hat because:Desirable gas dynamicsLow risk of fuel burn-out on the piston below the injector

TYPES OF DIESEL COMBUSTION SYSTEM1.DIRECT –INJECTION SYSTEMS

•Have a single open combustion chamber into which fuel is injected directly.•Used for large size engines.•Additional air motion not required .•As engine size decreases , increasing amounts of air swirl are used to achieve faster fuel –air mixing rates.

2.INDIRECT –INJECTION SYSTEMS

•Chamber is divided into two regions•Fuelisinjectedintoprechamberwhichisconnectedtothemainchamberviaanozzle.•Used in the smallest engine sizes.•Duringcompression,airisforcedformthemainchamberabovethepistonintotheauxiliarychamber,throughthenozzleororifice.Thus,towardtheendofcompression,avigorousflowinauxiliarychamberissetup.

PRIMARY CONSIDERATION IN THE DESIGN OF COMBUSTION CHAMBERS FOR C.I ENGINE•Injection and combustion both must complete in short time in order to achieve the best efficiency.•For best combustion mixing should complete in the short time.•In C.I engine it is evident that fuel air contact must be limited during the delay period in order to limit dp/dt, the rate of pressure rise in the second phase of combustion. This result can be obtained by shortening the delay time.

ToachievehighefficiencyandpowerthecombustionmustbecompletedwhenthepistonisnearertoT.D.C,itisnecessarytohaverapidmixingoffuelandairduringthethirdstageofcombustion.•ThedesignofcombustionchamberforC.Ienginesmustalsotakeconsiderationoffuelinjectionsystemandnozzlestobeused.

COMBUSTION CHAMBER DESIGN CONSIDERATIONSMinimal flame travelThe exhaust valve and spark plug should be close togetherSufficient turbulenceA fast combustion, low variabilityHigh volumetric efficiency at WOTMinimum heat loss to combustion wallsLow fuel octane requirement

KNOCKING AND ITS MechanismExplosive combustion of air-petrol vapor mixture produces shock waves which hit the cylinder wall and piston of IC engine, creating rattling sound is known as knocking.In an internal combustion engine, a mixture of gasoline vapor and air is used as a fuel. After the initiation of the combustion reaction, by spark in the cylinder, the flame should spread rapidly and smoothly through the gaseous mixture; thereby the expanding gas drives the piston down the cylinder. The ratio of the gaseous volume in the cylinder at the end of the suction-stroke to the volume at the end of compression-stroke of the piston is known the ‘compression ratio’. Increase of compression ratio not only increases the efficiency of the engine but also saves the fuel which dependent on the nature of constituents present in the gasoline .

Engine Damage From Severe KnockDamage to the engine is caused by a combination of high temperature and

high pressure.

Piston Piston crown

Cylinder head gasket Aluminum cylinder head

CETANE NUMBERThe knocking tendency of a diesel fuel is expressed in terms of cetane number. Diesel engines works on the principle of compression ignition.Cetane (n-cetane) or hexadecane [CH3-(CH2)14-CH3] is a saturated hydrocarbon, its cetane number is arbitrarily fixed as 100.a-Methyl naphthalene is a aromatic hydrocarbon, its cetane number is arbitrarily fixed as zero.

Definition: Cetane number is defined as the percentage of hexadecane (n-cetane) present in a mixture of hexadecane and 2-methyl naphthalene, which has the same ignition characteristic of diesel fuel in test. Generally diesel fuels with cetane numbers of 70-80 are used.

The method employed to measure CN uses a standardized single-cylinderengine with variable compression ratio

The operating condition is:

Inlet temperature (oC) 65.6Speed (rpm) 900Start of fuel injection (oBTC) 13Coolant temperature (oC) 100Injection pressure (MPa) 10.3

With the engine running at these conditions on the test fuel, the compression ratio is varied until combustion starts at TC ignition delay period of 13o.

The above procedure is repeated using blends of cetane and HMN. Theblend that gives a 13o ignition delay with the same compression ratio isused to calculate the test fuel cetane number.

Cetane Number Measurement