Fuel-Air Modeling of Combustion in I.C. Engines

P M V SubbaraoProfessor

Mechanical Engineering Department

Another Step towards Phenomenological Modeling.….

Phenomenological Modeling of Combustion

• Engineering Objective of Combustion:

• To Create Maximum Possible Temperature through conversion of microscopic potential energy into microscopic kinetic energy.

Thermodynamic Strategy for conversion:Constant volume combustionConstant pressure combustion

Engineering Strategy to Utilize A Resource

• Engineering constraint: Both combustion and expansion have to be finished in a single stroke.

• Rapid combustion : Constant Volume combustion– Less time to combustion process.– More time to adiabatic expansion

• Slow combustion : Constant pressure combustion– More time to combustion process.– Less time to adiabatic expansion

2—3 Complete & Adiabatic combustion at constant volume

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2—3 Complete & Adiabatic combustion at constant pressure

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Generalized Theory of Extent of Reaction Possible

Extent of Reaction

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Mathematical Model for Culmination of Reaction

For every fuel, a designer should know all possible reactants !!!Some products will influence the efficiency of reaction.Few other may not influence the efficiency of reaction but severely affect the environment.The optimal parameters for efficient reaction may not be optimal for safe reaction !!There may be a need to use secondary reactor with catalysts.

Catalytic Converter

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Possible extent of reaction:

BS-IV-- Petrol and Diesel

• BS-IV Petrol and Diesel are cleaner fuels as they have low sulphur content vis-à-vis BS-III fuels.

• While the BS-III Petrol and Diesel contain 150 mg/kg and 350 mg/kg of sulphur respectively, the sulphur content in BS-IV Petrol and Diesel respectively.

• Sulpur being a major air pollutant, reduction in sulphur content in auto fuels would go a long way in reducing air pollution in Delhi

8

https://www.araiindia.com/pdf/Indian_Emission_Regulation_Booklet.pdf

How to predict possible products of combustion???

How to control the concentration??

Rapid Reaction to Very Slow Action

Formation of Fossil Fuels

Sorry !!! We Don’t Know How To Use Crudes !!!

Sorry !!! We Don’t Know How To Produce What we Precisely Need !!!

• BOILING RATE # CARBON ATOMS• Refinery Gas <25oC 3• Gasoline 40-150oC 4-10• Naptha 150-200oC 10-12• Kerosene 200-300oC 12-16• Diesel Fuel 300-400oC 16-25• Residual Oil >400oC >25

Other components of Gasoline

Name of Hydro-carbon Sample 1Sample 1 Sample 2Sample 2 Sample 3Sample 3

EthanolEthanol 13.40%13.40% 12.10%12.10% 10.20%10.20%

2-methyl-pentane2-methyl-pentane 10.20%10.20% 4.50%4.50% 4.50%4.50%

HexaneHexane 0%0% 2.10%2.10% 0%0%

BenzeneBenzene 0%0% 0%0% 7%7%

TolueneToluene 0%0% 0%0% 8.20%8.20%

pentanepentane 5.50%5.50% 3.70%3.70%    0%0%

Fuel Model & Ideal Combustion

• Ultimate Analysis of fuel: Gravimetric

• Percentage of carbon : x --- Number of moles, X = x/12

• Percentage of hydrogen : y --- Number of atomic moles, Y = (y-M/9)/1

• Percentage of oxygen: k --- Number of atomic moles, K = (k-8M/9)/16

• Percentage of sulfur: z – Number of atomic moles, Z = z/32

• Equivalent chemical formula : CXHYSZOK

• Equivalent Molecular weight : 100 kg.

• Ideal combustion with oxygen

• CXHYSZOK + (X+Y/4+Z-K/2) O2 → P CO2 +Q H2O + G SO2

• Ideal combustion with air

• CXHYSZOK + 4.76 (X+Y/4+Z-K/2) AIR → P CO2 +Q H2O + R N2 +G SO2

Modeling of Field Level Combustion

• CXHYSZ +4.76 (X+Y/4+Z) AIR + Moisture in Air + Moisture in fuel → P CO2 +Q H2O +R SO2 + T N2 + U O2

+V CO+WCH4

• Exhaust gases: P CO2 +QH2O+R SO2 + T N2 + U O2 + V CO+WCH4 kmols.

• Excess air coefficient: .• Volume fraction = mole fraction. • Volume fraction of CO2: x1 = P * 100 /(P+Q+R + T + U + V+W) • Volume fraction of H2O: x1 = Q * 100 /(P+Q+R + T + U + V+W)• Volume fraction of CO: x2= V * 100 /(P +Q+R + T + U + V+W) • Volume fraction of O2: x4= U * 100 /(P +Q+R + T + U + V+W)• Volume fraction of N2: x5= T * 100 /(P +Q+R + T + U + V+W)

Closure on Analysis of Combustion

Complete & Adiabatic combustion at constant volume

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Complete & Adiabatic combustion at constant pressure

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Design of sub-systems for better combustion needs more detailed analysis of thermo-physical & thermo chemical processes

preceding and during combustion.