Post on 22-Aug-2021
Combustion Basics
Dr. Md. Zahurul Haq
ProfessorDepartment of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh
zahurul@me.buet.ac.bdhttp://zahurul.buet.ac.bd/
ME 417: Internal Combustion Engines
http://zahurul.buet.ac.bd/ME417/
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 1 / 25
Combustion Basics Combustion Classifications
Combustion
• Combustion of fuel-air mixture inside engine cylinder is one of the
processes that controls engine power, efficiency and emissions.
• Combustion commonly observed involves flame, which is a thin
region of rapid exothermic chemical reaction.
• Flame propagation is the result of strong coupling between
chemical reaction, transport processes of mass diffusion, heat
conduction and fluid flow.
• Conventional spark-ignition (SI) flame is premixed unsteady
turbulent flame, and the fuel-air mixture through which it
propagates is in the gaseous state.
• Diesel engine (CI) combustion process is predominantly an
unsteady turbulent diffusion flame, and the fuel is initially in the
liquid phase.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 2 / 25
Combustion Basics Combustion Classifications
Classifications of Flames
1 Premixed Flame: fuel and oxidizer are essentially uniformly mixed
prior to combustion. It is a rapid, essentially isobaric, exothermic
reaction of gaseous fuel and oxidizer, and flame propagates as a
thin zone with speeds of less than a few m/s.
2 Diffusion Flame: reactants are not premixed and must mix
together in the same region where reactions take place. It is
dominated by the mixing of reactants, which can be either laminar
or turbulent, and reaction takes place at the interface between the
fuel and oxidizer.
1 Laminar: flow, mixing and transport are by molecular process.
2 Turbulent: flow, mixing and transport are enhanced by
macroscopic relative motion of fluid eddies of turbulent flow.
• Steady / Unsteady
• Solid phase / Liquid phase / Gaseous phase.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 3 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Combustion Stoichiometry
CαHβOγNδ︸ ︷︷ ︸fuel
+ as (O2 + 3.76N2)︸ ︷︷ ︸air
−→ n1CO2 + n2H2O + n3N2︸ ︷︷ ︸complete combustion product
• as ≡ stoichiometric molar air-fuel ratio
• (A/F )s ≡ stoichiometric air-fuel ratio
as = α+ β4 − γ
2 =⇒(
AF
)
s=
(
FA
)−1
s=
28.85(4.76as )12α+β+16γ+14δ
• φ ≡ fuel-air equivalence ratio, simply equivalence ratio
• λ ≡ relative air-fuel ratio or excess-air factor
φ = λ−1 =(A/F )s
(A/F )a=
(F/A)a
(F/A)s: φ
< 1 : lean mixture
= 1 : stoichiometric mixture
> 1 : rich mixture
• (A/F )a ≡ actual air-fuel ratio
Homework: Heywood: Ex. 3.1
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 4 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Heating Values of Fuels
T296 T
U
H
or
Reactants
Products
−(U)V,To
−(H)P,To
or
or
Tod001
• The heating value is the heat release per unit mass of the fuel
initially at 25oC reacts completely with oxygen (or air) and the
products are returned to 25oC.
• Heating value at constant pressure ≡ QHV ,P = −(∆H )P ,To
• Heating value at constant volume ≡ QHV ,V = −(∆U )V ,To
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 5 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
T299 T304
QHHV ,P = QLHV ,P +
[
mH2O
mfuel
]
hfg ,H2O
• QHHV ,P ≡ Higher (Gross) Heating Value• QLHV ,P ≡ Lower (Net) Heating Value• mH2O/mfuel ≡ mass ratio of water produced to fuel burned.• hfg ,H2O ,298K = 2.445 MJ/kg for water
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 6 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Example: Methane-Air Combustion
T839
• CH4 + as (O2 + 3.76N2) −→ n1CO2 + n2H2O + n3N2
• as = 2.0, n1 = 1.0, n2 = 2.0, n3 = 7.52
=⇒ LHV = (802.33/16.0) MJ/kg = 50.14 MJ/kg
=⇒ HHV = 50.14 + 2 × 18 /16 ×2.445 = 55.64 MJ/kg
Homework: Estimate HHV of CH4 at constant volume.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 7 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Adiabatic Flame Temperature, Tad
T
U
H
or
Reactants
Products
or
To Tad
UoR H
oR
or
d003
U oR = Uprod(Tad ,V = constant)
H oR = Hprod(Tad ,P = constant)
Adiabatic Flame Temperature is the product temperature in an ideal
adiabatic combustion process. Actual peak temperatures in engines are
several hundred degrees less due to:
• heat loss from the flame,
• combustion efficiency is less than 100%: a small fraction of fuel
does not get burned, and some product components dissociate
(endothermic reaction) at high temperatures.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 8 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Typical Equilibrium Combustion Product
10−4
10−3
10−2
10−1
100
0.2 0.4 0.6 0.8 1.0 1.2 1.4
N2
O2
CO2
H2O
CO
H2
OH
NO
1750 K
Mol
efr
actio
n
φT1054
10−4
10−3
10−2
10−1
100
0.2 0.4 0.6 0.8 1.0 1.2 1.4
N2
O2
CO2
H2O
CO
H2
H
O
OH
NO
2250 K
Mol
efr
actio
n
φT1055
10−4
10−3
10−2
10−1
100
0.2 0.4 0.6 0.8 1.0 1.2 1.4
N2
O2
CO2
H2O
CO
H2
H
O
OH
NO
2750 K
Mol
efr
actio
n
φT1056
iso-octane at 30 bar
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 9 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
T840
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 10 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
• Major products of lean combustion are H2O , CO2, O2 and N2;
while, for rich combustion these are H2O , CO2, CO , H2 and N2.
• Maximum flame temperature is at slightly rich condition
(φ ≃ 1.05) as a result of both the heat of combustion, ∆Hc and
heat capacity of products decaying beyond φ = 1.0.
• Between 1.0 ≦ φ ≦ φ(Tmax ) heat capacities decays more rapidly
with φ than ∆Hc and beyond φ(Tmax ), ∆Hc falls more rapidly
than does the heat capacity.
• Increase in temperature promotes dissociation (endothermic)
reactions and increase in pressure decreases dissociation.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 11 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Auto-ignition & Self-ignition Temperature (SIT)
If the temperature of an air-fuel mixture is raised high enough, the
mixture will self ignite without the need of an external igniter. The
temperature above which this occurs is called the SIT.
e732
• If the mixture temperature is lower than
SIT, no ignition will occur and the
mixture will cool off.
• If mixture temperature is above SIT,
self-ignition will occur after a short time
delay called ignition delay (ID).
• The higher mixture temperature above
SIT, the shorter will be the ID.
• ID depends on initial temperature,
pressure, density, turbulence, swirl,
fuel-air-ratio presence of inert gases, etc.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 12 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
T868
Fuel Symbol (A/F )s as LHV Tad ,P SIT
(MJ/kg) (K) (K)
Hydrogen H2(g) 34.01 0.5 119.95 2383 673
Methane CH4(g) 17.12 2.0 50.0 2227 810
Methanol CH4O(l) 6.43 1.5 19.9 2223 658
Gasoline C7H17(l) 15.27 11.25 44.5 2257 519
Octane C8H18(l) 15.03 12.50 44.4 2266 691
Diesel C14.4H24.9(l) 14.3 20.63 42.94 2283 483
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 13 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Combustion Efficiency in ICEs
• Exhaust gas of an ICE contains incomplete combustion products
(e.g. CO, H2, unburned hydrocarbon, soot) as well as complete
combustion products (CO2 and H2O). The amounts of incomplete
combustion products are small in case of lean mixture, however
these amounts become more substantial under fuel-rich conditions.
e694
ηc =HR(To)−HP (To)
mf QHV
ηc ≡ combustion efficiency
To ≡ ambient temperature
HR ≡ enthalpy of reactants
HP ≡ enthalpy of products
mf ≡ mass of fuel
QHV ≡ heating value of fuel
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 14 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Minimum Ignition Energy
T869
T870
T871
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 15 / 25
Combustion Basics Combustion Chemistry & Thermodynamics
Flammability Limits
• As the combustible mixture gets too rich or too lean, flame
temperature decreases and consequently, flame cannot propagate
when the equivalence ratio is larger than an upper limit or smaller
than a lower limit.
• These two limits are referred to as the rich and the lean
flammability limits (RFL and LFL respectively), and they are
often expressed as fuel percentage by volume in the mixture.
T356
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 16 / 25
Premixed Flame
Premixed Flame
• Flame is the result of the self-sustaining chemical reaction
occurring within a region of space called flame front where
unburned mixture is heated and converted into products.
• Flame reaction zone temperatures are around 2800 K. At these
temperatures, flame contains highly reactive atoms and radicals.
• Flame propagation arises from heat transfer and diffusion of active
particles from the hot flame to the relatively cold mixture.
• Flame speeds are significantly increased by turbulence which
distorts flame front to increase burning area and enhances heat
and mass diffusion.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 17 / 25
Premixed Flame
T872
SL ≡ laminar burning velocity
Tig ≡ ignition temperature
δl ≡ laminar flame thickness
HRR ≡ Heat release rate
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 18 / 25
Premixed Flame
T874
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 19 / 25
Premixed Flame Laminar Flame
Laminar Flame Propagation
• Laminar burning velocity, SL is an intrinsic property of a fuel-air
mixture. It is defined as ‘the velocity, relative to & normal to the
flame front, with which unburned gas moves into the front & is
transformed to products under laminar flow conditions’.
SL = 1ρuAf
dmb
dt: Ss = SL + ug
dmb/dt is mass burning rate & Af is flame front surface area.
• Ss ≡ flame speed: space velocity of flame front normal to itself. It
is not a unique property of combustible fuel-air premixture.
• ug ≡ gas expansion velocity & is a function of ρu & ρb .
SL = f (fuel ,T ,P , φ) ∼= SL,0
(
TTo
)α (PPo
)β
• u ≡ unburned mixture, 0 ≡ datum pressure & temperature.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 20 / 25
Premixed Flame Laminar Flame
Flame location at
t = t0t = t1
t = t1
t = t1
unburned
unburned
unburned
burned
burned
burned
ug = −SL
Ss = SL
Ss = 0
Ss = SL + ugd004
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 21 / 25
Premixed Flame Laminar Flame
Effect of Equivalence Ratio, φ
e713
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 22 / 25
Premixed Flame Laminar Flame
Effect of Unburned Mixture Temperature, Tu
300 350 400 4500.250.25
0.3
0.4
0.5
0.6
φ = 1.0
Tu (K)
Iso-octane-air mixture
Methane-air mixture
φ = 0.8
Pu = 0.1 MPa
S L (m
/s)
e731
α is +ve. Increase in temperature increases chemical reaction rates and
dissociations. So, more active radicals are produced to enhance flame
propagation.
© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 23 / 25
Premixed Flame Laminar Flame
Effect of Unburned Mixture Pressure, Pu
0.10.1 0.2 0.4 0.6 0.8 1.01.00.10.1
0.2
0.3
0.4
0.50.60.7
φ = 0.8
Iso-octane-air mixture
Methane-air mixtureφ = 1.0 T
u = 358 K
S L (
m/s
)
Pu (MPa)e730
β is either zero or negative. Increased pressure increases flame
temperature because of less dissociation, and less dissociation means
less active radicals are available to diffuse upstream to enhance flame
propagation.© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 24 / 25
Premixed Flame Turbulent Flame
Turbulent Flame Propagation
T1244
ST = f (fuel ,T ,P , φ, turbulence) =⇒ST
SL
∼= f (turbulence)
In engines, propagating flame fronts are wrinkled by turbulence which
result in higher burning rate. The effect of turbulence is not always
beneficial as too much turbulence can lead to extinction of flame.© Dr. Md. Zahurul Haq (BUET) Combustion Basics ME 417 (2021) 25 / 25