UNIT- 2 :

MECHANISM OF POLLUTANT FORMATION IN ENGINES

INTRODUCTION

NITROGEN OXIDES..

Nitric oxide is the major component of NOx (NO+NO2) emissions from the I C

engines. NO2 accounts 1 2 % only of the total NOX emissions in the S I engines,

while substantial amount of NO2 are emitted by the C I engines. Its

concentration is high in diesel engines than that of spark ignited engines. These

oxides of N2 are formed during combustion at high temp. in an internal

combustion engine.

It has an very harmful affect over our nervous system. And of course it is toxic.

NO is formed during combn. in the following three ways :

(i) Formation of thermal NO by oxidation of atmospheric nitrogen at

high temperatures.

(ii) Oxidation of fuel-bound nitrogen at relatively low temperatures to

form fuel NO.

(iii) NO formed at the flame front by a mechanism other than the earlier

two mechanisms, prompt NO.

Thermal NO is the dominant source of nitrogen oxides in I C engines. NO is

formed in the high temperature burned gases behind the flame front. The rate of

formation of NO increases exponentially with the burned gas temperature.

Fuel NO is formed by combustion of the fuels with chemically bound nitrogen.

The species or the intermediate nitrogen and the reactive like HCN, NH3, CN, NH

etc are oxidized to NO by the O2 containing species.

Prompt NO is the significant amount of NO, formed rapidly in the front of the

flame. It is formed in the flame by reaction of intermediate chemical species of CN

group with O and OH radicals.

CH + N2 HCN + N

CH2 + N2 HCN + NH

C + N2 CN + N

KINETICS OF NO FORMATION.

The following three principal reactions govern the formation of thermal NO.

O + N2 NO + N

N + O2 NO + O

N + OH NO + H

The rate of formation of NO using the three reactions can be expressed by the

following equation,

d/dt [NO] = + k1[O][N2] K I[NO][N] + K2[N][O2] K -2[NO][O] + K3[N][OH]

- K -3[NO][H]

Where, k1 , k2 and k3 are rate constants for the forward reaction respectively,

the (-)ve sign denotes the rate constants for the reverse reactions.

FORMATION OF NO2 ..

Nitrogen dioxide in the exhaust gases of the S I engines is negligibly small

compared to nitric oxide. In S I engines the amount of NO2 is generally less than

2%, whereas in diesel engines it is 10 30 % of nitrogen oxides. NO2

concentration is significantly higher in diesel engines than spark ignition

engines.

NO2 is rapidly formed in the combustion zone by reaction of NO with HO2 radial.

Subsequently, in the post flame region NO2 is converted back to NO and O2 on

reaction with atomic O2. However, if the high temperature burned gases rapidly

mix with colder air, or air fuel mixture responsible for conversion of NO2 back to

NO. Such a situation would result in relatively high NO2 concentrations.

NO FORMATION IN S.I. ENGINES :-

For estimation of NO formation in SI engines, thermodynamic state and

equilibrium composition of the burned gases must be known. Temperature of

the burned gases can be calculated from the measured cylinder pressure-time

history or from the calculated pressure-time traces using empirical burn rates or

more fundamental combn models.

In one combustion model combustion model, after combn the post flame

burned gas can be assumed to mix instantaneously with the gases burned

earlier so that all the burned gas at a given instant (a very short time), is

uniform in composition and temperature. This is commonly to as the fully mixed

model.

Another combn model at the extreme is an unmixed multi-zone model where no

mixing occurs between the mixture elements that burn at different instants in

the cycle. In the unmixed model, each burned gas element maintains its

separate identity and undergoes isentropic compression and expansion as the

cylinder pressure changes. The elements that burn early get compressed to peak

pressure after combustion and reach higher temperatures than those burning

later.

Using unmixed model, kinetically formed NO increases initially with time. In the

early burned elements as the peak temperature is reached due to compression

to peak cylinder pressure; the rate controlled NO reaches to near equilibrium

value. Later as the temp. starts falling due to expansion NO starts decomposing.

The rate of decomposition is controlled by the backward reaction of NO kinetics

until the NO chemistry freezes due to expansion. Mass fraction of NO in the

exhaust can be calculated by summing up of the frozen mass NO fraction over

all the burned gas elements as below:

Where , x is a function of crank angle

NO FORMATION IN C.I. ENGINES:-

In the compression ignition engines, rapid combustion in pre-mixed phase is

followed by , diffusion combustion process. The rate at which fuel and air are

mixed controls the diffusion combustion process. The reaction kinetics leading

to form NOx . As the fuel is injected in the hot compressed air, the fuel spray

entrains air and non-uniform fuel distribution exists in the combn chamber. Air-

fuel ratio varies widely from very rich at the core of spray to very lean at the

spray boundaries. A fuel spray injected radically outward in swirling air is shown

in figure. Spray core contains mostly liquid fuel and very rich mixture exists in

the nearby of spray core. Thus, NO is formed at varying rates depending upon

the local air-fuel ratio and temperature.

Thus, NO is formed at varying rates depending upon the local a/f ratio and

temperature. As the combn progresses, the already burned gases keep on

mixing with colder air and fuel vapor changing its composition and temperature.

Formation of NO occurs mainly in the burned gases produced during premixed

combustion phase in the lean flammable region. These gases are compressed to

a higher pressure and temp. and hence NO formation rates are high.

Formation of NO2 ___

NO2 in the exhaust gases of the spark-ignition is negligibly small generally less

than 60 70 ppm compared to several hundred to thousands of ppm of nitric

oxide. In S.I. engines NO2 is generally less than 2% , while in diesel engines NO2

can constitutes 10 to 30% of the total emissions of nitrogen oxides.

Concentrations of NO2 and NO in the S I and diesel engines exhaust are

compared in the figs. NO2 concentrations in the diesel engine are significantly

higher than for the S I engine.

NO2 is rapidly formed in the combustion zone by reaction of NO with HO2.

However, if the high temp. burned gases rapidly mix with colder air or air-fuel

mixture caused by high turbulence it may quench reactions responsible for

conversion of NO2 back to NO. such a situation would result in relatively high

NO2 concentrations.

CARBON MONOXIDE ___

CO emissions result during combustion of fuel rich mixtures due to deficiency of

oxygen. A two-step process may approximate complete combustion of

hydrocarbon fuel to form finally the CO2.

First step is conversion of hydrocarbons to CO. During this step several

oxidation reactions occur involving formation of intermediate species like

smaller HC molecules, aldehydes, ketones etc.

The second step is conversion of CO to CO2 provided sufficient oxygen is

available. One of the principal reactions for conversion of CO to CO2 is,

CO + OH CO2 + H

The reaction is quite fast and at high temperatures is continuously under

equilibrium. The CO increases sharply as the a/f ratio decreases below

stoichiometric condition. For lean mixtures, CO concentration is very small.

UNBURNED HYDROCARBON emission (HC)___

Unburned HC emissions arise as part of the fuel inducted in to the engine

escapes combustion. The unburned HC are also called volatile organic

compounds.

Most of the fuel HC are present in the exhaust. Thus, fuels containing higher

fractions of aromatics & olefins produce exhaust hydrocarbons also rich in

aromatics & olefins, which are more photochemically reactive. Almost 400

hundred different organic compounds are present in the engine exhaust.

Methane is also present in significant amounts in the exhaust gases particularly

of the SI engines.

FLAME QUENCHING___

For flame to prope, the energy during chemical reactions should keep the

reaction zone at high enough temperature to sustain rapid combustion

reactions. As a flame approaches walls in the combustion chamber, the walls

being at lower temperature than the gases in the flame more and more heat is

lost from the flame and hot gases to the walls. This results in drop in the

temperature of reaction zone, which slows down rections thereby lowering heat

release rate. Finally, this process leads to lowering of gas temperature below

ignition point and flame quenching takes place.

For quenching of flame propagating normal to a single wall, energy balance at

the instant of quenching gives,

k Tc /q = SL h

= SL pb Tf

where,

k = thermal conductivity of the unburned mixture,

TC = characteristics temp. difference for heat transfer,

q = quench distance,

= unburned mixture density,

SL = laminar flame speed ,

h = heat release per unit mass of the mixture burned,

pb = average specific heat of burned gases, and

Tf = temp. rise on combustion in the flame.

Introducing thermal diffusivity, in the above eqn. , we get,

q = / SL . TC / Tf

HC EMISSIONS FROM S.I ENGINES__

In the s.i engines, several processes contribute to unburned hydrocarbon

emissions. Main sources of hydrocarbon emissions in the 4-stroke, homogenous

charge spark ignition engines are :

(i) Flame quenching on the cylinder walls,

(ii) Crevices flame quenching,

(iii) Absorption and desorption in oil film on cylindre walls,

(iv) Carbon deposits in the chamber,

(v) Misfired combustion or bulk gas quenching,

(vi) Liquid fuel in the cylinder,

(vii) Exhaust valve leakage,

(viii) Crankcase blow by.

HC EMISSIONS FROM CI ENGINES__

Diesel fuel has a higher boiling range and contains hydrocarbons of higher

boiling point and molecular weight compared to gasoline. The five main sources

of HC in diesel engines are :

(i) Over mixing of fuel and air beyond lean flammability limits,

(ii) Under mixing to fuel-air ratios too rich for complete combustion,

(iii) Spray over penetration,

(iv) Bulk quenching of combustion reactions due to mixing with cooler air

or expansion,

(v) Poorly atomized fuel from the nozzle.

OVERMIXING OF FUEL:

Simple diagram of a fuel spray injected radially outward into the swirling air

before combustion.

The leading edge in the downwards swirl stream contains larger numbers of small

droplets, than larger droplets in the core. In this region the a/f ratio is much

higher than it should be in lean limit to cause HC emissions.

UNDERMIXING OF FUEL:

Another cause of high HC emission is undermixing of the fuel with air. This can

happen for the fuel injected later in the cylinder or because of overfueling of the

engine. The fuel left in the injector nozzle holes at the end of the injection gets

fully or partially vaporized during expansion stroke. Therefore, the later injected

fuels get less time to be mixed with air and could not get burnt fully.

SOOT OXIDATION:

Oxidation of soot can occur at the stage of early in the combn. process. Soot can

be oxidized on reaction with O, O2, OH.

CRANKCASE EMISSIONS

Crankcase emissions are made up of water, acids, unburned fuel, oil fumes and particulates.

These emissions are classified as hydrocarbons (HC) and are formed by the small amount of

unburned, compressed air/fuel mixture entering the crankcase from the combustion area

(between the cylinder walls and piston rings) during the compression and power strokes. The

head of the compression and combustion help to form the remaining crankcase emissions.

Since the first engines, crankcase emissions were allowed into the atmosphere through a road

draft tube, mounted on the lower side of the engine block. Fresh air came in through an open oil

filler cap or breather. The air passed through the crankcase mixing with blow-by gases. The

motion of the vehicle and the air blowing past the open end of the road draft tube caused a low

pressure area (vacuum) at the end of the tube. Crankcase emissions were simply drawn out of the

road draft tube into the air.

To control the crankcase emission, the road draft tube was deleted. A hose and/or tubing was

routed from the crankcase to the intake manifold so the blow-by emission could be burned with

the air/fuel mixture. However, it was found that intake manifold vacuum, used to draw the

crankcase emissions into the manifold, would vary in strength at the wrong time and not allow

the proper emission flow. A regulating valve was needed to control the flow of air through the

crankcase.

Testing, showed the removal of the blow-by gases from the crankcase as quickly as possible, was

most important to the longevity of the engine. Should large accumulations of blow-by gases

remain and condense, dilution of the engine oil would occur to form water, soots, resins, acids

and lead salts, resulting in the formation of sludge and varnishes. This condensation of the blow-

by gases occurs more frequently on vehicles used in numerous starting and stopping conditions,

excessive idling and when the engine is not allowed to attain normal operating temperature

through short runs.

EVAPORATIVE EMISSIONS

Gasoline fuel is a major source of pollution, before and after it is burned in the automobile

engine. From the time the fuel is refined, stored, pumped and transported, again stored until it is

pumped into the fuel tank of the vehicle, the gasoline gives off unburned hydrocarbons (HC) into

the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor

was diminished, but not eliminated, from the refinery standpoint. However, the automobile still

remained the primary source of vaporized, unburned hydrocarbon (HC) emissions.

Fuel pumped from an underground storage tank is cool but when exposed to a warmer ambient

temperature, will expand. Before controls were mandated, an owner might fill the fuel tank with

fuel from an underground storage tank and park the vehicle for some time in warm area, such as

a parking lot. As the fuel would warm, it would expand and should no provisions or area be

provided for the expansion, the fuel would spill out of the filler neck and onto the ground,

causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition,

the vehicle manufacturers added overflow plumbing and/or gasoline tanks with built in

expansion areas or domes.

However, this did not control the fuel vapor emission from the fuel tank. It was determined that

most of the fuel evaporation occurred when the vehicle was stationary and the engine not

operating. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large

concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel vapor

emissions would take place, increasing as the temperature increases.

To prevent the vapor emission from escaping into the atmosphere, the fuel systems were

designed to trap the vapors while the vehicle is stationary, by sealing the system from the

atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if

equipped) and the fuel tank when the engine is not operating. When the engine is started, the

storage system is then purged of the fuel vapors, which are drawn into the engine and burned

with the air/fuel mixture.