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
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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.