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Thermo-chemistry of Engine Combustion

Thermo-chemistry of Engine Combustion. P M V Subbarao Professor 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.

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Thermo-chemistry of Engine Combustion

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  1. Thermo-chemistry of Engine Combustion P M V Subbarao Professor Mechanical Engineering Department A n Important Clue to Control Rate of Heat Release ….

  2. Real Combustion & Model Testing

  3. 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

  4. Stoichiometry of Actual Combustion • CXHY +e 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 e (X+Y/2) = 2P +Q +2U+V • Conservation of Nitrogen: 2 e 3.76 (X+Y/2+Z-K/2) = T

  5. For every 100 kg of fuel. • CXHY +e 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

  6. 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.

  7. 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 +en 4.76 (X+Y/4) AIR + Moisture in Air → x1CO2 +x6 H2O + x5 N2 + x4 O2 + x2 CO + x7C

  8. nCXHY +en 4.76 (X+Y/4) AIR + Moisture in Air + → x1CO2 +x6 H2O + x5 N2 + x4 O2 + x2 CO + x7C • 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 ne (X+Y/4) = 2x1 +x2 +2x4+x6 • Conservation of Nitrogen: e n 3.76 (X+Y/4+Z-K/2) = x5

  9. nCXHY +en 4.76 (X+Y/4) AIR + Moisture in Air → x1CO2 +x6 H2O + x5 N2 + x4 O2 + x2 CO + x7C + Ash • Re arranging the terms (Divide throughout by n): CXHY +e 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 +e 4.76 (X+Y/4) AIR + Moisture in Air → P CO2 +Q H2O + T N2 + U O2 + V CO + W C

  10. Air-fuel Ratio: Equivalence ratio: Fuel Lean Mixtures : f <1 Fuel-rich Mixtures: f >1

  11. 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:

  12. 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:

  13. Residual Gas Fraction: Effect of Speed Intake

  14. Residual Gas Fraction: Effect of Valve Overlap Intake

  15. Residual Gas Fraction : Effect of Compression Ratio Intake

  16. 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 ra,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 ra,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.

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

  18. 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.

  19. Anatomy of Volumetric Losses Quasi-static Effects Charge/air Heating Flow friction Backflow Choking

  20. 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: The combustion Efficiency:

  21. Variation of Combustion Efficiency with Equivalence Ratio

  22. A Phenomenological Theory • MATtr Theory • Proposed by Dixon • Mixing: Proper Mixing of fuel and air. • Air: Sufficient amount of air. • T : Sufficient temperatures. • t : Sufficient time. • r: Local density of air and fuel.

  23. Realization of MATtr Theory • Mixing: Fuel preparation systems. • Air: Intake and exhaust manifolds &valves. • T : Preheating of fuel through adiabatic compression. • t : Duration of combustion process. • r: Turbulence generation systems.

  24. 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. SI Engine CI Engine

  25. 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.

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