Thermo-chemistry of Engine Combustion

P M V SubbaraoProfessor

Mechanical Engineering Department

A n Important Clue to Control Rate of Heat Release ….

Real Combustion & Model Testing

Results of Model Testing.

• For a given fuel and required Power & Speed conditions.

• Optimum composition of Exhaust Gas.

• Optimum air flow rate.

• Optimum fuel flow rate.

• Optimum combustion configuration!!!

Molar Analysis of Dry Exhaust Products:• Mole fraction of CO2 : x1 • Mole fraction of CO : x2

• Mole fraction of O2 : x4

• Mole fraction of N2 : x5

Stoichiometry of Actual Combustion

• CXHY + 4.76 (X+Y/4) AIR → P CO2 +Q H2O + T N2 + U O2

+ V CO

• Conservation species:

• Conservation of Carbon: X = P+V

• Conservation of Hydrogen: Y = 2 Q

• Conservation of Oxygen : K + 2 (X+Y/2) = 2P +Q +2U+V

• Conservation of Nitrogen: 2 3.76 (X+Y/2+Z-K/2) = T

• For every 100 kg of fuel.

• CXHY + 4.76 (X+Y/4) AIR + Moisture in Air + Ash & Moisture in fuel → P CO2 +Q H2O ++ T N2 + U O2 + V CO + W C + Ash

• Dry Exhaust gases: P CO2 + T N2 + U O2 + V CO kmols.• Volume of gases is directly proportional to number of moles.• Volume fraction = mole fraction.• Volume fraction of CO2 : x1 = P * 100 /(P + T + U + V) • Volume fraction of CO : x2= VCO * 100 /(P + T + U + V) • Volume fraction of O2 : x4= U * 100 /(P + T + U + V)• Volume fraction of N2 : x5= T * 100 /(P + T + U + V)

• These are dry gas volume fractions.• Emission measurement devices indicate only Dry gas

volume fractions.

• Measurements:• Volume flow rate of air.• Volume flow rate of exhaust.• Dry exhaust gas analysis.• x1 +x2 +x3 + x4 + x5 = 100 or 1• Ultimate analysis of coal.• Combustible solid refuse.

nCXHY +n 4.76 (X+Y/4) AIR + Moisture in Air

→

x1 CO2 +x6 H2O + x5 N2 + x4 O2 + x2 CO + x7 C

nCXHY +n 4.76 (X+Y/4) AIR + Moisture in Air + → x1 CO2 +x6 H2O + x5 N2 + x4 O2 + x2 CO + x7 C

•x1, x2,x3, x4 &x5 : These are dry volume fractions or percentages.

•Conservation species:

•Conservation of Carbon: nX = x1+x2+x7

•Conservation of Hydrogen: nY = 2 x6

•Conservation of Oxygen : nK + 2 n (X+Y/4) = 2x1 +x2 +2x4+x6

•Conservation of Nitrogen: n 3.76 (X+Y/4+Z-K/2) = x5

nCXHY +n 4.76 (X+Y/4) AIR + Moisture in Air → x1 CO2 +x6 H2O + x5 N2 + x4 O2 + x2 CO + x7 C

+ Ash• Re arranging the terms (Divide throughout by n):

CXHY + 4.76 (X+Y/4) AIR + Moisture in Air → (x1 /n)CO2 +(x6/n) H2O + (x5/n) N2 + (x4/n) O2 +

(x2/n) CO + (x7/n) C

CXHY + 4.76 (X+Y/4) AIR + Moisture in Air

→ P CO2 +Q H2O + T N2 + U O2 + V CO + W C

1

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Air-fuel Ratio:

Fuel Lean Mixtures :

Fuel-rich Mixtures: >1

Equivalence ratio: 1

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Partial Pressure of air in Intake System

• In a SI engine, the presence of gaseous fuel , moisture in the intake air and residual exhaust gases reduces the intake air partial pressure below the mixture pressure.

• In a CI engine, the presence of moisture in the intake air and residual exhaust gases reduce the intake air partial pressure below the mixture pressure.

• For a mixture:

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Fraction of air in the Cylinder

• The residual gas fraction in the cylinder during compression is determined by the exhaust and inlet processes.

• Its magnitude affects volumetric efficiency and engine performance directly.

• The residual gas fraction is a function of inlet and exhaust pressures, speed, compression ratio, valve timing, and exhaust system dynamics.

• The residual gas fraction is defined as:

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Residual Gas Fraction: Effect of Speed

Intake

Residual Gas Fraction: Effect of Valve Overlap

Intake

Residual Gas Fraction : Effect of Compression Ratio

Intake

Actual Mass of air per Cycle : Volumetric Efficiency

• Volumetric efficiency a measure of overall effectiveness of engine and its intake and exhaust system as a natural breathing system.

• It is defined as:

• If the air density a,0 is evaluated at inlet manifold conditions, the volumetric efficiency is a measure of breathing performance of the cylinder, inlet port and valve.

• If the air density a,0 is evaluated at ambient conditions, the volumetric efficiency is a measure of overall intake and exhaust system and other engine features.

• The full load value of volumetric efficiency is a design feature of entire engine system.

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Volumetric Efficiency of A Cycle

• The volumetric efficiency is a function of

• Intake mixture pressure pi.

• Intake mixture Temperature Ti.

• Fuel/ air ratio (F/A).

• Compression ratio rv.

• Exhaust pressure, pe.

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Let m is the mass of gas in the cylinder at the end of intake stroke.

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Full Load Overall Volumetric Efficiency

• Overall volumetric efficiency is affected by following variables.

• Intake and exhaust manifold and port design.

• Intake and exhaust valve geometry, size, lift and timings.

• Fuel type, fuel/air ratio, fraction of fuel vaporized in the intake system, and fuel heat vaporization.

• Mixture temperature as influenced by heat transfer.

• Ratio of exhaust to inlet manifold pressures.

• Compression ratio.

• Engine speed.

• The effects of many of above variables are quasi-steady in nature.

• Their impact is either independent of speed or adequately function of speed.

Anatomy of Volumetric Losses

Quasi-static EffectsCharge/air Heating

Flow friction

Choking

Backflow

Combustion Efficiency of Engine

• The fraction of fuel chemical energy not available due incomplete combustion is quantified using combustion efficiency.

• The net chemical energy release due to actual combustion with in the engine is:

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The combustion Efficiency:

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Variation of Combustion Efficiency with Equivalence Ratio

MATt Theory• Proposed by Dixon

• Mixing: Proper Mixing of fuel and air.

• Air: Sufficient amount of air.

• T : Sufficient temperatures.

• t : Sufficient time. : Local density of air and fuel.

A Phenomenological Theory

Realization of MATt Theory

• Mixing: Fuel preparation systems.

• Air: Intake and exhaust manifolds &valves.

• T : Preheating of fuel through adiabatic compression.

• t : Duration of combustion process. : Turbulence generation systems.

Care for Occurrence of Heat Addition

• Occurrence of Heat Addition in SI Engine : A Child Care Event.

• Occurrence of Heat Addition in CI Engine: A Teen Care Event.

CI Engine SI Engine

Type of Fuel Vs Combustion Strategy

• Highly volatile with High self Ignition Temperature: Spark Ignition. Ignition after thorough mixing of air and fuel.

• Less Volatile with low self Ignition Temperature: Compression Ignition , Almost simultaneous mixing & Ignition.