Engine Combustion: Flames & Processes

Characterization of Flames

• Combustion inside an engine cylinder is key to power, efficiency, and emissions
• Combustion phenomena vary between spark-ignition and diesel engines
• In spark-ignition (SI) engines, fuel mixes with air in intake ports or cylinders
• An electrical discharge starts combustion, creating a flame that spreads from the "kernel"
• Flames can be "quenched" or extinguished at combustion chamber walls
• Undesirable spontaneous ignition of fuel-air mix ahead of the flame can occur
• Autoignition leads to engine knock which can damage engine
• Diesel engines inject fuel into high-pressure, high-temperature air
• Vaporized fuel ignites and spreads rapidly
• Fuel-air mixing is a key control in diesel combustion
• Chapters 3 and 4 focus on the thermochemistry of combustion
• Chapters 9 and 10 handle the phenomenological aspects of engine combustion

Combustion Process Basics

• Combustion is a fast, exothermic gas-phase reaction where oxygen is essential
• A flame exists where combustion takes place, propagating subsonically
• Propagating flames exist in zones small in thickness to the combustion chamber
• Spatial propagation results from strong coupling between: chemical reaction, mass diffusion and heat conduction, and fluid flow
• Heat, active species, and fresh reactants being balanced are important for steady-state flames
• Flames are classified by composition and gas flow
• Premixed flames occur when fuel and oxidizer are uniformly mixed
• Diffusion flames occur when fuel and air mix in the reaction zone
• Gas flow can be laminar or turbulent
• Mixing and transport in laminar flow results from molecular processes.
• Laminar flow occurs at low Reynolds number
• Mixing and transport in turbulent flow are enhanced by macroscopic motion of eddies
• Flames are also classified by steady or unsteady structure and according to the initial phase of reactants (gas, liquid, or solid)
• Engine flames are unsteady and turbulent
• Turbulent convection helps mix, burn, and propagate flames quickly

Spark-Ignition and Diesel Flame Structures

• Spark-ignition flame is a premixed unsteady turbulent flame with gaseous fuel-air mixture
• Thin flame sheet (0.2 mm thick) dominates laminar processes with molecular diffusion
• Combustion reactions that release the fuel's chemical energy occur in the flame's downstream part (zone II)
• Thermal energy conducts upstream into zone I (preheating) and is carried downstream to zone III
• Unburned mixture must reach ignition temperature (Ti) for reaction and energy release
• In spark-ignition engines, the flame moves into the unburned mixture
• The structure is a turbulent premixed flame
• Turbulent fluid motion convects, distorts, and stretches, it produces a "laminar-like wrinkled flame"
• Diesel engine combustion is unsteady, turbulent diffusion flame with liquid fuel
• Diffusion flames have fuel and air initially separate.
• Fuel vapor flows inside the flame and air flows from the outside
• Chemical reaction occurs where temperatures is greatest with the required air to fully release fuel
• Combustion products diffuse away from the flame out to the surrounding air and within the spray/flame boundary
• Diffusion flame surrounding each fuel spray is turbulent, forming the irregular, wrinkled flame

Ideal Gas Model

• Gas species in internal combustion engines (O2, N2, fuel vapor, carbon dioxide, water vapor, etc.) behave as ideal gases
• Ideal gas law: pV = mRT = (m/M)RT = nRT (where p is pressure, V is volume, m is mass, R is the gas constant, T is temperature, R is the universal gas constant, M is molecular weight, and n is the number of moles)
• Relations for specific internal energy (u), enthalpy (h), entropy (s), and specific heats (cv and cp) are developed for ideal gases and ideal gas mixtures

Composition of Air and Fuels

• Engines burn fuels with air
• Dry air composition: 20.95% oxygen, 78.09% nitrogen, 0.93% argon, & trace gases
• Oxygen is the reactive component in combustion
• Atmospheric nitrogen contains traces of other species
• It is sufficiently accurate to consider air as 21% oxygen and 79% inert gases (nitrogen)
• There are 3.773 moles of atmospheric nitrogen for each mole of oxygen in air
• Density of dry air can be calculated using the ideal gas law
• Actual air contains water vapor where proportion depends on temperature and saturation
• Water vapor content is measured using a wet- and dry-bulb psychrometer
• Relative humidity is the ratio of vapor pressure present to saturation pressure
• Fuels commonly used in internal combustion engines are blends of hydrocarbons
• Fuels are about 86% carbon and 14% hydrogen by weight
• Diesel fuels may contain up to 1% sulfur
• Other fuels of interest: alcohols (contain oxygen), gaseous fuels (natural gas, liquid petroleum gas), and single hydrocarbons (methane, propane, isooctane)
• Different classes of organic compounds are important for combustion
• Alkyl compounds are single-bonded open-chain saturated hydrocarbons
• Larger alkyl molecules have isomers which can be straight-chain (normal) or branched-chain (iso)
• Examples of alkyl compounds: methane (CH4,), ethane (C2H6), propane (C3H8)
• Radical molecules are deficient in one hydrogen atom (methyl, ethyl, propyl, etc.)
• Single-bond ring hydrocarbons are unsaturated examples include cyclopropane (C3H6) , cyclobutane (C4H8), and cyclopentane (C5H10)
• Open-chain hydrocarbons containing a double bond are unsaturated like ethene (C2H4) and propene (C3H6)
• Diolefins contain two double bonds
• Open-chain unsaturated hydrocarbons have one carbon-carbon triple bond and an example is acetylene (H–C≡C–H)
• Aromatics are based on benzene ring structure (C6H6) where the ring structure is stable and accommodates expansion
• Examples of aromatics include: toluene (C7H8) and xylene (C8H10)
• Alcohols have one hydroxyl (-OH) group substituted for one hydrogen atom
• Methane becomes methyl alcohol (CH3OH) and ethane becomes ethyl alcohol (C2H5OH)

Combustion Stoichiometry

• Relations are between reactant and product composition
• Relations depend on conservation of mass of each chemical element
• Hydrocarbon fuel can be completely oxidized when oxygen is sufficient and carbon converts to CO2
• Hydrogen converts to water (H2O)
• The Equation for complete combustion of one mole of Propane: C3H8 + aO2 = bCO2 + cH2O
• Air contains nitrogen, this isn't significantly affected where the product is left at relatively low temperatures
• The complete combustion equation for hydrocarbon fuel with air: CaHb + [a + (b/4)](O2 + 3.773N2) = aCO2 + (b/2)H2O + 3.773[a + (b/4)]N2
• Equation defines stoichiometric proportions or theoretical proportions for fuel and air.
• [A/F]s = [1 + (y/4)][32 + (3.773 x 28.16)] / [12.011 + 1.008y]
• [A/F]s = [34.56(4 + y)] / [12.011 + 1.008y]
• Molecular weights of oxygen, atmospheric nitrogen, atomic carbon, and atomic hydrogen are 32, 28.16, 12.011, and 1.008
• In fuel-air mixtures fuel can be burned with either more or less stoichiometric air
• Extra appears in the products unchanged in excess or lead combustion
• Insufficient oxygen makes it so that fuel C and H cannot to oxidize fuel to both CO2 and H2O
• Ratio of actual fuel/air ratio to stoichiometric ratio is a more informative parameter
• The Fuel/Air equivalence ratio is defined as φ where the (FIA)actual / (F/A)s
• For Fuel lean mixtures that means Φ<1, λ>1
• For Stoichiometric mixtures Φ=λ=1
• For Fuel rich mixtures Φ> 1, λ<1
• To determine the overall combustion of fuels with alcohol, fuel oxygen counts toward oxygen balance
• Stoichiometric combustion for Methyl alcohol(methanol), CH3OH : CH3OH +1.5(02 +3.773N2) = CO2 + 2H2O+5.66N2 and A/F = 6.47
• Stoichiometric combustion of Ethyl Alcohol (ethanol), C2H5OH: C2H5OH+3(O2+3.773N2) = 2CO2+3H2O+11.32N2 and A/F is 9.00
• Appropriate oxidation product needs to be found in fuel mixtures with a significant about if sulfur this is determined using their amount of Stoichiometric fuel and air proportion to sulfur dioxide (SO2)
• The Stoichiometric for Hydrogen Fuel : H2 + ½(O2 +3.773N2) = H2O +1.887N2
• The stoichiometric A/F for H2 = 34.3
• Actual composition of products may not occur in practice.
• Significant loss due to dissociation of CO2 and H2O can occur under normal combustion temperatures

Thermodynamics and Combustion

• In a combustuion process, fuels and oxidizers react to make products with differen compositions
• Thermodynamics laws can help determine the end states of mixtures going through combustion
• Thermodynamics relates changes in internal energy (or enthalpy) to heat and work transfer interactions
• When applying thermodynamics in chemical composition, take care in dealing with the reference states of zero internal energy and enthalpy for each group of species
• The first law provides AQ-AW= \Delta H
• Positive sign indicates heat transfer into the system and positive work transfer out of the system
• Heat transfer and work transfer happen via normal force displacements of the system boundary.
• Constant volume process means QR-P=UP-UR=(AU)v,T
• An amount of Internal energy that goes through change has an amount of (AU)V,T which can be measured or can be calculated
• A decrease in systems internal energy happens in the exothermic process because the amount of QR-P and (AU)V,T are negative
• Increase in internal at constant volume is express through one mole where (AU)v,T is known
• Heat of reaction at constant volume at temperature T’ or -(AU)v,T’ is known
• Constant pressure process involves QR-p = p(Vp-VR)= UR-UR or QR-p = p(Up - pVR) - UR + PVR - AH- HR = (ΔΗΡτο
• The enthalpy of has a change from amount (ΔΗ)ρ,τ is also calculated.
• Change in Enthalpy at constant pressure is (ΔΗΡ,τ’
• Reactions are classified in internally or externally to shown schematicially temperature plot vs internal energy.

Enthalpies of Formation

• To relate the product and reactants enthalpies, fuel needs to be in hydrocarbon compounds, which helps relate the enthalpies to products and reactants
• Enthalpies of formation or Δhf of a chemical compound is the enthalpy association of forming one mole of the compound
• Elements each include thermodynamic substances that are in standard state at a given temperature
• The standard reference state is on at 1 atmosphere pressure
• Reference states of the select element is assigned zero enthalpy at the datum temperature
• Oxygen can be found reference a state of being oxygen (O2)gas at around 278.15 K
• When a combustion has been given, enthalpies include being at certain standards by following rules
• The enthalpy with the products relative on the standard is then given via Hp = ΣN(Δhf) P
• When relating the reactant to the enthalpy in the internal state we must follow this equation HR= ΣNp(Δhj)
• To calculate enthalpies a great example is the calculation of stoichiometric methane and oxygen at around 298.15K.
• To solve use the following equations. CH4 + 2O2 > CO2 + 2H2O
• HO gas and then the Table in 3.2 = HP= -393.52 + 2(-241.83) = -877.18 Ml / Kmol HO
• The internal energy has a nitrogen that in the excess that it does not change in calculations.

Heating Values

• Enthalpy cant from reactant species, which to be able to solve is heating values
• The fuel is measure, is then heating values if measured with either magnitude is constant or with standard temp of mostly with fuel
• With all the carbon turns in CO2, Hydrogen turns to H2O, is why a complete combustion occurs
• The heating value normally joules pre kliogran where British pounds are unit from British thermals
• To be able to specifies how most fuel and heating values affect one one another, The equation must surpass
• Whether liquid or gaseous, hydrogen contains products that affect heat and reaction
• The Higher heating values are where H2 gas formation where condensed is where it use to change phase
• This is true where water is also in mass and ratio