Chap 3 Combustion

8/2/2019 Chap 3 Combustion

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Combustion of the fuel-air mixture

(based on H.Heywood Internal combustion engine fundamentals

and DieselNet website)

3.1 Essential features of the combustion process

Combustion of the fuel-air mixture inside the engine cylinder is one of the processes that

controls engine power, efficiency and emissions. Some background on relevant combustion

phenomena is therefore a necessary preliminary to understanding engine operation. These

combustion phenomena are different for the two main types of engines spark-ignition and

diesel (compression-ignition).

In spark-ignition engines, the fuel is normally mixed with air in the engine intake system.

Following the compression of this mixture, an electrical discharge initiates the combustion

process. The flame originates in the area around the spark plug and propagates across the

cylinder to the combustion chamber walls. At the walls, the flame is quenched or

extinguished as the heat transfers. The destruction process of active species at the chamber

wall become the dominant processes. An undesirable combustion phenomenon spontaneous

ignition of a substantial mass of fuel-air mixture (called end-gas) can also occur. This auto-

ignition or self-explosion combustion phenomenon is the cause of spark-ignition engine knock

which, due to the high pressure generated, can lead to engine damage.

In diesel engines, the fuel is injected into the cylinder, which contains air at high pressure

and temperature, near the end of the compression stroke. The auto-ignition or self-ignition of

a portion of the developing mixture of already injected and vaporized fuel with this hot air

starts the combustion process, which spreads rapidly. Burning then proceeds as fuel and air

mix to the appropriate composition for combustion to take place. Thus, mixing plays a

controlling role in the diesel combustion process.In general, the combustion process is a fast exothermic gas-phase reaction, where oxygen

is usually one of the reactants. A flame is a combustion reaction which can propagate subs

conically through space motion of the flame relative to the unburned gas is the important

feature. Flame structure does not depend on whether the flame moves relative to the

observer. The existence of flame motion implies that the reaction is confined to a zone which

is small in thickness compared to the dimensions of the combustion chamber. The reaction

zone is called the flame front. This flame characteristic of spatial propagation is the result of

a combination of chemical reactions, the transport process of mass diffusion and heat

conduction and fluid flow. The generation of heat and active species a accelerate the

3


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Internal combustion engines

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chemical reactions a supply of fresh reactants, governed by the convection velocity, limits

the reaction. When these processes are in balance, a steady-state flame results.

3.1.1 The classification of flames

Flames are usually classified according to the following four characteristics.

The first concerns the composition of the reactants as they enter the reaction zone. If the

fuel and oxidizer are essentially uniformly mixed together, the flame is designated as

premixed. If the reactants are not premixed and must mix together in the same region where

reaction takes place, the flame is called diffusion flame, because the mix must be

accomplished by a diffusion process.

The second means of classification relates to the basic character of the gas flow through

the reaction zone: whether it is laminar or turbulent. In laminar (streamlined) flow, mixing

and transport are done by molecular processes. Laminar flows only occur at low Reynolds

numbers (density x velocity x length scale/viscosity and means ratio of inertial to viscous

forces). In turbulent flames (high Reynolds numbers), mixing and transport are enhanced by

the macroscopic relative motion of eddies or lumps of fluid which are the characteristic

feature of a turbulent flow.

The third area of classification is where the flame is steady or unsteady. This depends on

changing structure and motion with time.

The fourth and final characteristic has to do with the initial phase of the reactants; that

is, whether they are a solid, a liquid, or a gas.

Flame in a combustion engine is a consequence of the engines operating cycle. Flames are

usually unsteady and turbulent, and it is only with substantial augmentation of laminar

transport processes by turbulent convection processes that mixing and burning rates and

flame-propagation rates be made fast enough to complete combustion within the time

available.

The conventional spark ignition flame is thus a pre-mixed unsteady turbulent flame and

the fuel-air mixture through which the flame propagates is in the gaseous state. The diesel

engine combustion process gives predominantly an unsteady turbulent diffusion flame and the

fuel is initially in a liquid state. Both of these flames are extremely complicated because theyinvolve combinations of complex chemical mechanisms, through which the fuel and oxidizer

Fig. 3.1. Scheme of burning a single drop of fuel at micro-gravity conditions (Heywood)


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Chapter 3. Combustion of the fuel-air mixture

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react to form products, through the turbulent convective transport process. The diesel

combustion process is even more complicated than the spark-ignition one because

vaporization of liquid fuel and fuel-air mixing processes are involved, too.

Combustion in an engine can be considered as the burning of a single drop of fuel,combustion of a group of drops of fuel, and vaporisation of a drop at the hot wall see Figure

3.1 above and Figures 3.2, 3.3 and 3.4 below.

Fig. 3.2. Models of combustion of a group offuel drops (from Heywood).

a) combustion of single drops inside a group,b) beginning of combustion of a group of fuel

drops external drops are burning in anindividual way and internal flames are incontact with one another,

c) partial combustion of group of fuel drops -external drops are burning individually andinternal ones together,

d) critical combustion one flame stars cover all fueldrops and fuel vapour does not diffuse away yet,

e) combustion of a whole group of fuel drops flamediameter is large and far away from drops,

f) combustion of a whole group of fuel drops whendrops are much closer one to another thanbefore- called sheath combustion

Fig. 3.3. The vaporisation of a fuel drop at thehot wall inside the combustion chamber(Heywood)

a) start of vaporisation,b) low temperature boiling,c) point of max. velocity of boiling,d) transient process of vaporisatione) spheroidal vaporisation,f) spheroidal combustion

Fig. 3.4 Vaporisation time of different fuels vs.temperature of wall for a fuel drop with apreliminary diameter of - do (Heywood).

Temperature points a, b, c etc. refer respectively tothe vaporisation mechanisms in the diagram on the

left.


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In an SI engine, following inflammation, a turbulent flame develops, propagates through

this essentially premixed fuel, air burned gas mixture until it reaches the combustion

chamber walls and then extinguishes.

In a diesel (CI) engine, air is only taken in on the intake stroke. The liquid fuel, usually

injected at high velocity as one or more jets, atomises into small drops and penetrates into

the combustion chamber. The fuel vaporizes and mixes with the cylinder air which is at high

temperature and high pressure. Since the air temperature and pressure are above the fuels

ignition point, spontaneous ignition of portions of the already-mixed fuel and air occurs after

a delay period of a few crank angle degrees. Photographs of the combustion processes taking

place in an operating engine are shown in Figures 3.5 and 3.6 below.

Fig. 3.5. Colour photos from high-speed movie of spark-ignition engine combustion process(Heywood)

Fig. 3.6. Sequence of photographs from high-speed camera taken in special visualisation dieselengines (Heywood)

a) combustion of single spry burning under large direct injection (DI) engine conditionsb) combustion of four sprays in DI engine with counter-clock swirl,

c)

combustion of single spry in MAN (M) DI engine,d) combustion in pre-chamber (on the left in photo) and main chamber (on the right) in RicardoComet indirect injection (IDI) swirl chamber diesel


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Normally in engines, fuels are burned with air. Dry air is a mixture of gases that has

a representative composition by volume of 20,95 percent oxygen, 78,09 percent nitrogen,

0,93 percent argon and trace amounts of carbon dioxide, neon, helium, methane and others

see Table 3.1 below.

Gas ppm by volume Molecular weight Mole fraction Molar ratio

O2 209500 31,998 0,2095 1

N2 780900 28,012 0,7905 3,773

A 9300 38,948 0 0

CO2 300 40,009 0 0

Air 1000000 28,962 1,000 4,773

Table 3.1. Principle constituents of dry air (Heywood, Kowalewicz)

In combustion, oxygen is the reactive component of air, nitrogen is inert gas. For each

mole of oxygen in air there are

(1-0,2095)/0,2095 =3,773 moles of atmospheric nitrogen.

The molecular weight of air is as 28,962. Because atmospheric nitrogen contains traces of

other species, its molecular weight is slightly different from that of pure molecular nitrogen

i.e.

MaN2 = (28,962-0,2095 x 31,998)/(1-0,2095) = 28,16

Nitrogen will refer to atmospheric nitrogen and a molecular weight of 28,16 will be used. An

air composition of 3,773 moles of nitrogen per mole of oxygen will be assumed.

The density of dry air can be obtained form of ideal gas law (Clapeyron formula)

pV = mRT

and at 1 atmosphere (1,0133 x 105 Pa) and 25oC is equal 1,184 kg/m3.

Actual air normally contains water vapour, the amount depending on temperature and

degree of saturation. Typically the proportion by mass is about 1% though it can rise to about

4% under extreme conditions. The relative humiditycompares the water vapour content of

air with that required to saturate.

The fuels most commonly used in internal combustion engines (petrol or diesel ) are blends of

many different hydrocarbon compounds obtained by refining petroleum or crude oil. These

fuels are predominantly carbon and hydrogen (typical about 86% carbon and 14% hydrogen by

weight) thought diesel fuels can contain up to about 1% sulphur.


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The relation between the composition of reactants (fuel and air) of a combustible mixture

and the composition of the products is discussed below.

If sufficient oxygen is available, a hydrocarbon fuel can be completely oxidized. The

carbon in the fuel is converted to carbon dioxide (CO2) and the hydrogen to water (H2O). For

example, the overall chemical equation for the complete combustion of one mole of propane

(C3H8) is as follows:

C3H8 + aO2 = bCO2 + cH2O

A carbon balance between the reactants and products gives b = 3. A hydrogen balance gives

2b = 8 means c = 4. An oxygen balance gives 2b + c = 10 = 2 a means a = 5.

Thus, the equation shown above changes to:

C3H8 + 5O2 = 3CO2 + 4H2O

This equation relates the elemental composition and does not indicate the process by

which combustion proceeds, which is much more complex. Air contains nitrogen, but when

the products are at low temperature the nitrogen is not significantly affected by the reaction.

The complete combustion of a general hydrocarbon fuel of average molecular composition

CaHb with air can be shown by:

C3H8 + (a + b/4)(O2 +3,773N2) = aCO2 + (b/2)H2O + 3,773(a + b/4)N2

Equation like above defines the stochiometric (chemical correct or theoretical) proportion

fuel and air i.e. there is just enough oxygen for conversion of all the fuel into completely

oxidized products. The stochiometric air/fuel or fuel/air ratios depend on fuel composition.

A/F = (F/A)-1 = [(1 + y/4)(32 + 3,773 x 28,16)] / [(12,011 + 1,008y)]

Numbers 32; 28,16; 12,011 and 1,008 are the molecular weights of oxygen, atmospheric

nitrogen, atomic carbon and atomic hydrogen, respectively. The air/fuel ratio depends onlyon y which is changed from 1 (e.g. benzene) to 4 (methane).

Fuel-air mixture with more than or less than the stochiometric air requirement can be

burned. With excess air or fuel-lean combustion, the extra air appears in the products in

unchanged form. For example, the combustion of isooctane with 25% excess air or 1,25 times

the stochiometric air requirement gives

C8H18 + 1,25 x 12,5 (O2 +3,773N2) = 8CO2 + 9H2O + 3,13O2 + 58,95N2

With less than the stochiometric air requirement i.e. with fuel-reach combustion, there is

insufficient oxygen to oxidize fully the fuel C and H to CO2 and H2O.


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The products are a mixture of CO2 and H2O with carbon monoxide CO and hydrogen H2 as

well as N2. The product composition not be determined from an element balance alone and

an additional assumption about the chemical composition of the product species must be

made.

Because the composition on the combustion products is significantly different for fuel-lean

and fuel-reach mixture, and because the stochiometric fuel/air ratio depends on fuel

composition, the equivalence ratio of the actual fuel/air ratio to stochiometric ratio is a

more informative parameter for defining mixture composition.

= (F/A)actual / (F/A)s

The inverse of , the relative air/fuel ration (lambda) is much more popular in

combustion engine theory.

= -1 = (A/F)actual / (A/F)s

for fuel-lean mixtures: > 1

for stochiometric mixtures: = 1

for fuel-reach mixtures: < 1

When the fuel contains oxygen (e.g. alcohols), the procedure for determining the overall

combustion equation is the same except that fuel oxygen is included in the oxygen balance

between reactants and products. For example, for methyl alcohol (methanol) CH3OH, the

stochiometric equation is as follows:

CH3OH + 1,5(O2 + 3,773N2) = CO2 + 2H2O + 5,66N2

and (A/F)s = 6,47.

For ethyl alcohol (ethanol) C2H5OH, the stochiometric combustion is defined by:

C2H5OH + 3(O2 + 3,773N2) = 2CO2 + 3H2O + 11,32N2and (A/F)s = 9,0.

If there are significant amounts of sulphur in the fuel, the appropriate oxidation product

for determining the stochiometric air and fuel proportions is sulphur dioxide (SO2).

For hydrogen fuel, the stochiometric equation is

H2 + 0,5(O2 + 3,773N2) = H2O + 1,887N2

and (A/F)s = 34,3.


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Note that the composition of the products of combustion in the equations shown above

may not occur in practice because of ambient conditions. Note too, that at low temperatures,

recombination brings the product composition to that indicated by the overall equations

depending on the rate of cooling of the product gases.

3.2 Characterization of combustion processes from cylinder pressure data

Cylinder pressure changes with crank angle as a result of cylinder volume change,

combustion, heat transfer to the cylinder walls, flow into and out of crevice regions and

leakage. The first two of these effects are the largest. Combustion rate information can be

obtained from accurate pressure data usually measured with piezoelectric pressure

transducers.

This type of sensor contains a quartz crystal. One end of the crystal exposed through a

diaphragm to the cylinder pressure. As the cylinder pressure increases the crystal is

compressed and generates an electric charge which is proportional to the pressure. A charge

amplifier is then used to produce an output voltage signal. Accurate cylinder pressure versus

crank angle data can be measured at the test stand (see Figures 3.7 and 3.9) as shown

diagrammatically below in Figure 3.8. Using existing formulas it is possible to calculate heat

balance inside the engine and to take combustion data.

Fig. 3.7. An engine test bed

Fig. 3.8. Diagrammatic representation of measuring at the test stand.


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Pressure diagram for max performance of SW400 diesel engine

Fig. 3.9. Pressure diagram for max performance of SW400 diesel engine as an example

Using the right formulas, the rest thermodynamic features like temperature and heat flux

can be calculated see subchapters below.

3.2.1 Calculation of burning gas temperature

General Equation:

pV = MRT,

.)(

)()()(

1MRnb

VpT

i

iii

Where p is the pressure,V is the volume,T is the temperature,M is the mass,R is the Gas universal constant, R = 8314,3 J/(kmolK).

)sin11(cos1)(1.(2

11)(

2

2

2

iikil

a

a

lRCVV

Where Vk is the minimum clearance volume,C.R is the compression ratio,l is the connecting rod length,a is the crank radius.

,1

)()(

iib

Where is the real factor of fresh mixture(fuel and air), is the rest of exhaust.


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,)1( 001 nnnn v Where nv is the kg of unburned mixture,

n0 is the kg of fresh mixture,

is as above.

,)1()( 0

3

0

bb

pk

iki

Where b0 is the theoretical factor of gases,

i is the angle at point from data,

p is the angle of start of burning,

k is the angle of end of burning.

,32

O

32

S

4

H

12

C110

tLb

Where is the air / fuel ratio ,Lt- is the theoretical number of kmol of mixture,

C, H, S, O are C - carbon, H - hydrogen, S - sulphur, O - oxygen.

otot

otsv

RTBL

pV

Where v is the efficiency of filling ratio,Vs is the volume,

pot is the ambient pressure,Lt is as above,B0 is the fuel dose per 1 cycle,R is as above,Tot is the ambient temperature.

21,0

32

O

32

S

4

H

12

C

tL ,

300

ni

GB e

Where Ge is the fuel consumption per hour,n is the engine speed (rpm),i is the number of cylinders.

,)1.( otot

spp

0 TpRC

Tp

n

n

v

sv


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Where nv is as above,n0 is as above,

psp, Tsp is pressure and temperature of exhaust gas,pot, Tot is as above,C.R is as above,

v is as above.

.00 BLn t The results of mathematical transformation of measured pressure to temperature in one cycle

is shown on the figure below.

Fig 3.10. Temperature of charge

3.2.2 Calculation of heat flux

Total Heat Flux:

)()()( irikig .

,)()(93,127)( 525,0786,0786,0214,0 iiik TpwD

Where D is the bore diameter,w is the speed of gas in chamber,

p(i) is the pressure from data,

T(i) is the temperature from data.

si

siplilpo

irTT

TTC

)(

)()()(

44

Where C0 is the radiation constant of black body, C0= 5,67108 W/(m2K4),

p(i) is the specific emissivity of flame,

Tp(i) is the temperature of flame,Ts is the average temperature of the combustion wall,

T(i) is the temperature from data.


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,

)(

)(19,01

18,0)(p

if

ifi

Where f(i) is the independent emissivity of flame

))p(10(1)( pi

l

if e

,

Where p(i) is the measured pressure,

lp is the mean path of the flame

,106,32

k

kp

F

Vl

Where Vk is the minimum swept volume,Fk is the chamber area.

,

)()()(1

)()(1)(p

ipit

i

iivi

cL

WT

Where v is the efficiency of filling ratio,W is the calorific value W = 42 700 kJ/kg,

is Wiebes function,

Lt is the theoretical number of kmol, is the molar mass of dry air, = 0,02896 kg/mol,

is the air / fuel ratio,cp is the heat coefficient.

,1)(

7,1

908,6

pk

pi

ei

Where i is the angle at point from data,

p is the angle of start of burning,

k is the angle of end of burning.

)( )(101561)(

103,997)(

60065,0

3

ii

ipTT

c

T(i) is the temperature from data


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2,2733,9485,9377,468,35

1

10233

o

kkk

k

s tD

d

D

d

D

d

D

dT

Where dk is the diameter of the piston chamber in the piston crown,D is the bore diameter,t0 is the temperature at the mid-point of the piston crown, C,

e

e

p

PDfe

RCtt

..002,0

ch016

.38,00,0025243,0

,

Where tch is the cooling temperature, C,C.R is the compression ratio ,

pe is the mean effective pressure,D is as above

,10

045,0136245,0747041,0128 eepp

pnnnfe

Where n is the engine speed (rpm),pe is as above.

w3 wt,

Where wt is the speed of gas in chamber

,30t

Sn

w

Where S is the piston stroke,n is the engine speed (rpm).

The heat flux diagram is presented on the figure below. It shown how big portion of heat

energy is going outside through the chamber walls.

Fig. 3.11Total heat flux from charge


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Review questions

1. What kind of thermodynamic reaction occurs in the combustion process in an engine?

2. What are the differences between the combustion processes in SI and CI engines?

3.What does it mean to have complete combustion, and what would be some of the

implications if motor vehicles operated with complete combustion?

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Chap 3 Combustion

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