Dr. Md. Zahurul Haq - Combustion Basicszahurul.buet.ac.bd/ME417/ME417_CombustionBasics.pdf · 2021....

Combustion Basics

Dr. Md. Zahurul Haq

ProfessorDepartment of Mechanical Engineering

Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh

[email protected]://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.


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.


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



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



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



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.



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.



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



T840



• 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.



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.



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



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



Minimum Ignition Energy

T869

T870

T871



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


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.


Premixed Flame

T872

SL ≡ laminar burning velocity

Tig ≡ ignition temperature

δl ≡ laminar flame thickness

HRR ≡ Heat release rate


Premixed Flame

T874


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.



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



Effect of Equivalence Ratio, φ

e713



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.



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

Dr. Md. Zahurul Haq - Combustion Basicszahurul.buet.ac.bd/ME417/ME417_CombustionBasics.pdf · 2021....

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