Chemistry fuels and combustion

Knocking Octane and Cetane numbers

Unit 3: Fuels and Combustion

1

www.Vidyarthiplus.com


SRI

Typewritten text

CY2161-engineering chemistry-II

SRI

Typewritten text

year/semester:I/II

Suction Stroke: The mouth of the cylinderopens up letting the fuel vapours with air fromcarburettor to enter into the cylinder andmoves the piston down.

Compression Stroke: The piston moves upagain and compresses the air into a muchsmaller volume before igniting it with the sparkplug. The amount of compression is called thecompression ratio of the engine. A typicalengine might have compression ratio 8 to 1.

2

Knockingwww.Vidyarthiplus.com


Power Stroke: When a gasoline-air mixture isignited by electric spark in the cylinder, itproduces a flame that rapidly andhomogeneously spread throughout the gasolinemixture. The hot gases produced due tocombustion increases the volume and pressureand pushes the piston down.

Exhaust stroke: When combustion iscompleted, the pressure decreases. This drivesthe piston upwards again and pushes out theexhaust gases from the cylinder through theexhaust valve.

3

Knockingwww.Vidyarthiplus.com


In certain situations, the smooth burning isinterrupted by the un-burnt mixture in thecombustion chamber.

The first portion of the fuel burns in a normalway, while the last portion of the charge almostignite instantaneously in the form of anexplosion or detonation.

The cylinder pressure rises dramatically beyondits design limits and damages the engine parts.

This results in a characteristic metallic soundcalled the knocking.

Such a knocking results in loss of efficiency.

4

Knocking - Explanationwww.Vidyarthiplus.com


The ratio of the gaseous volume in the cylinderat the end of the suction-stroke to the volume atthe end of the compression stroke of the pistonis known as the “Compression Ratio”.

5

Compression Ratio - Definitionwww.Vidyarthiplus.com


In certain circumstances, the rate of oxidationbecomes so great that the last portion of thefuel-air mixture gets ignited instantaneously,producing an explosive violence, known as“knocking”.

Knocking is also known as “Pinking” or“Pinging”.

Knocking results in loss of efficiency.

6

Knocking - Definitionwww.Vidyarthiplus.com


The main causes of knocking are: Poor quality of fuel (nature and composition) Poor conditions of engine such as:

Poor design of engine Poor mechanical condition of engine Poor operational manners of engine such

as: Incorrect combustion process Improper cooling of engine Improper exhaust gas re-circulation

7

Knocking - Causeswww.Vidyarthiplus.com


Knocking can be prevented by – Using good quality fuel with higher octane rating Adding anti-knocking agents Increasing the amount of fuel injected or

lowering the air to fuel ratio Reducing the cylinder pressure Improving the combustion chamber design

Correct ignition timings are necessary for betterengine performance and fuel efficiency.

Now-a-days, modern automotives have sensors thatcan detect knock and retard ignition (spark plugfiring) to reduce knocking and protect the engine.

8

Knocking - Preventionwww.Vidyarthiplus.com


The tendency of fuel constituents to knock isgiven in the following order:

Straight-chain paraffins > branched-chainparaffins (i.e., iso-paraffins) > olefins >cycloparaffins (i.e., naphthalenes) > aromatics

Hence, olefins of the same carbon chain lengthpossess better anti-knocking properties than thecorresponding paraffins.

9

Knocking and Chemical Structurewww.Vidyarthiplus.com


In the year 1972, Edger introduced a system ofranking of fuel based on the efficiency of fuel.

It has been found that n-heptane knocks very badlyand hence anti-knocking value of heptane hasarbitrarily been fixed as zero.

On the other hand, iso-octane (2,2,4-trimethypentane) was found to give very little knocking andhence its anti-knocking value was fixed as 100.

CH3-(CH2)5 – CH3

n-heptaneOctane No.=0

10

Octane Ratingwww.Vidyarthiplus.com


The octane Number (or octane rating) of agasoline ( or any other IC engine fuel) is thepercentage of iso-octane in a mixture of iso-octane and n-heptane, which matches the fuelunder test in knocking characteristics.

Hence, if a fuel’s octane number is 80: It meansthat the fuel is one which has the samecombustion characteristics as a 80:20 mixture ofiso-octane and n-heptane has.

11

Octane Number - Definitionwww.Vidyarthiplus.com


The most common way of expressing theknocking characteristics of a fuel is by octanerating.

It is a system of grading gasoline. It is a measure of suitability of fuel (petrol) for

high compression engines.

12

Octane Number - Significancewww.Vidyarthiplus.com


13

Octane Number of some Hydrocarbons

Hydrocarbon Octane No.

N-Octane -10

N-Heptane 0

Diesel fuel 15-25

2-Methyl Heptane 23

N-Hexane 29

1-heptene 60

1-pentene 84

Cyclohexane 97

Iso-octane (2,2,4-trimethyl pentane) 100

Benzene 101

Toluene 112

The Octane Number :Increases with increase in compactness, double bonds and cyclic structures.



A US gas station pump offering five different octane ratings

14



http://en.wikipedia.org/wiki/File:Gas_Station_Pump_Five_Octane_Ratings.jpg

By definition, the maximum octane number is100, i.e., 100% of iso-octane characteristics.

But, some fuels are more knock resistant thaniso-octane, and hence the octane number isextended to allow for numbers greater than100.

The octane performance of certain substanceshave higher shock resistance and has beenassigned with numbers greater than 100.

Octane Boosters such as tetra ethyl lead (TEL),toluene, etc, also increases the octane numberof gasoline.

15

Octane Number – Above 100www.Vidyarthiplus.com


Aviation gasoline has hydrocarbons in lowboiling range (40-170oC), which shows highoctane number.

During ‘take-off’, an aircraft engine developsincreased power which is provided by anincrease of fuel supply to give a rich mixture ofair and fuel.

Using higher octane number fuel in aircraftengines, increases the power to take-off.

A change in fuel from 90 octane no. to 100octane no. leads to 10-30% increase in poweravailable for ‘take-off’, due to increase in CR.

16

Octane Number – Aviation Gasolinewww.Vidyarthiplus.com


Efficiency of engine cycle may be defined by anexpression:

E = 1 - (1/CR)0.258

Consider the octane number of an aviation fuelis increased from 90 to 100 and thecompression ratio changed from 7.8 to 9.5, letus calculate the increase in efficiency of theengine.

Initial efficiency, E1 = 1 – (1/7.8)0.258 = 0.411 Subsequent efficiency, E2= 1 – (1/9.5)0.258 = 0.441 Increase in efficiency = 0.441 – 0.411 = 0.030 % increase in efficiency = (0.030/0.411) = 7.29%

17

Octane Number – Aviation Gasolinewww.Vidyarthiplus.com


Ignition quality order among hydrocarbonconstituents of a diesel fuel is as follow:n-alkanes > naphthalenes > alkenes > branchedalkanes > aromatics

Thus, hydrocarbons which are poor gasoline fuelsare good diesel fuels.

18

Diesel Engine Fuelswww.Vidyarthiplus.com


The suitability of a diesel fuel is determined bycetane number or cetane value.

Definition of Cetane number:Cetane number is the percentage of hexadecane in amixture of hexadecane and 2-methyl naphthalene,which has the same ignition characteristics as thediesel fuel under test.

CH3-(CH2)14-CH3

Hexadecane or cetane

Cetane no.: 100

19

Cetane Ratingwww.Vidyarthiplus.com


The cetane number of a diesel fuel can be raised bythe addition of small quantity of certain compoundslike ethyl nitrate, acetone peroxide, iso-amyl nitrite,etc. Such compounds are called “pre-ignitiondopes”.

20

Cetane Numberwww.Vidyarthiplus.com


CN is a significant expression for diesel fuel quality It is a measure of fuel’s ignition delay, the time

period between the start of injection and start ofcombustion (ignition) of the fuel.

In a particular diesel engine, higher cetane fuels willhave shorter ignition delay periods than lowercetane fuels.

21

Cetane Number - Significancewww.Vidyarthiplus.com


SummaryKnockingOctane NumberCetane number

22



Gaseous Fuels

23



Gas fuels are the most convenient - because they require the least amount of handling and are used in the simplest and most

maintenance-free burner systems. Large individual consumers do have gasholders

and some produce their own gas. Water Gas Producer Gas CNG LPG

24

Gaseous Fuelswww.Vidyarthiplus.com


Types of gaseous fuel

(A) Fuels naturally found in natureNatural gas (NG)Methane from coal mines

(B) Fuel gases made from solid fuelGases derived from coalGases derived from waste and biomassFrom other industrial processes

(C) Gases made from petroleumLiquefied Petroleum gas (LPG)Refinery gasesGases from oil gasification

(D) Gases from some fermentation

25



Advantages of Gaseous Fuels

1. Can be produced at a central location and cleangas can be distributed over a wide area.

2. Nuisance of smoke production and ash disposaleliminated at point of fuel utilization.

3. Greater control of variation in demand,conditions of combustion and nature of flameand heating atmosphere possible.

4. Greater economy by use of efficient heatexchange methods possible.

5. Gaseous fuels require far less excess air forcomplete combustion.

26



Drawbacks in using gaseous fuel

1. Its high specific volume results in displacementof air in premixed combustion systems.

2. Hence power produced with gaseous fuels isless when compared to solid and liquid fuels.

3. Due to its high specific volume, gaseous fuelcontainers are much larger than those forliquid fuels.

27



Natural Gas

Natural gas may be used as

1. Liquefied Natural Gas (LNG).

2. Compressed Natural Gas (CNG).

“Natural” gas when made artificially it is calledsubstitute, or synthetic or supplementalnatural gas (SNG).

28



CNG compared to LNG

CNG is often confused with liquefied naturalgas (LNG). While both are stored forms of naturalgas, the key difference is that CNG is gas that isstored (as a gas) at high pressure, while LNG is inuncompressed liquid form.

CNG has a lower cost of production and storagecompared to LNG as it does not require anexpensive cooling process and cryogenic tanks.

29



CNG compared to LNG

CNG requires a much larger volume to store thesame mass of gasoline or petrol and the use ofvery high pressures (3000 to 4000 psi, or 205 to275 bar).

CNG can be stored at lower pressure in a formknown as an ANG (Adsorbed Natural Gas) tank, at35 bar (500 psi, the pressure of gas in natural gaspipelines) in various sponge like materials, suchas activated carbon and metal-organicframeworks (MOFs).

30



Liquefied Petroleum Gas (LPG) is obtained as a by-product, during the cracking of heavy oils or fromnatural gas.

Major constituents of LPG are n-butane, iso-butane,butylene and propene.

LPG is dehydrated, desulphurized and traces ofodorous organic sulphides (mercaptans) are addedto give warning of gas leak.

LPG is supplied under pressure in containers underthe trade names like Indane, Bharat gas, etc.

Its calorific value is about 27,800-28,000 kcals/m3.

31

LPG (or bottled gas or refinery gas)www.Vidyarthiplus.com


Water gas is essentially a mixture of combustiblegases, CO and H2, with a little non-combustiblegases, CO2 and N2.

It is a synthesis gas, containing CO and hydrogen.

32

Water Gas (or Blue Gas)www.Vidyarthiplus.com


Gas Producer

33

1. ASH ZONE2. COMBUSTION OR

OXIDATION ZONE3. REDUCTION ZONE

4. DISTILLATION ZONE

GAS PRODUCER



SummaryGaseous FuelsWater Gas Producer Gas CNG LPG

34



Flue Gas AnalysisOrsat’s Apparatus

Unit 3: Fuels and CombustionPart 11

35u



Flue Gas

• What is Flue Gas ?• …. gases and vapors resulting from the

combustion process

36



Flue Gas Components

• Nitrogen (N2)

• Carbon Dioxide (CO2)

• Oxygen (O2)

• Carbon Monoxide (CO)

• Nitrogen Oxides (NOx)

• Sulfur Dioxide (SO2)

• Unburned Hydrobarbons (CxHy)

• Soot and Particulate Matter

37



Analysis Of Flue Gases• The object of a flue gas analysis is the

determination of the completeness of thecombustion of the carbon in the fuel, and theamount and distribution of the heat losses due toincomplete combustion.

• The quantities actually determined by an analysisare the relative proportions by volume, of

• carbon dioxide (CO2),

• oxygen (O2), and

• carbon monoxide (CO); the determinations beingmade in this order.

38



ORSAT APPARATUS

39



Theoretical air for combustion• Principles of Combustion

• Combustion: rapid oxidation of a fuel

• Complete combustion: total oxidation of fuel(adequate supply of oxygen needed)

• Air: 20.9% oxygen, 79% nitrogen and other

• Nitrogen: (a) reduces the combustion efficiency(b) forms NOx at high temperatures

• Carbon forms (a) CO2 (b) CO resulting in less heatproduction

40



Theoretical air for combustion

• Principle of Combustion

• Oxygen is the key to combustion

41



COMBUSTION

• Combustion is an exothermic chemical reaction,which is accompanied by development of heat andlight.

• For proper combustion, substance must bebrought to its kindling or ignition temperature.

• Definition:

• Ignition temperature is “the minimum temperatureat which the substance ignites and burns withoutfurther addition of heat from outside”.

42



CALCULATION OF AIR QUALITIES

• Following elementary principles are applied, to find theamount of oxygen or air required for combustion of a unitquantity of a fuel.

1. Substances always combine in definiteproportions and these proportions aredetermined by the molecular masses of thesubstances involved and the products formed.

Eg., C (s) + O2 (g) CO2 (g)

Mass proportions 12 32 44

Similarly, 2H2 (g) + O2 (g) 2 H2O (g)

2 x 2 = 4 32 2 x 18 = 36

43




2. 22.4 L of any gas at STP (0oC and 760 mmpressure) has a mass equal to its 1 mol. Thus,22.4 l of CO2 at STP has a mass of 44 g (molarmass of CO2).

3. Air contains 21% of oxygen by volume and 23%by mass.

i.e., 1 kg of oxygen is supplied by:

= (1 x 100)/23 = 4.35 kg of air

similarly, 1 m3 of oxygen is supplied by:

= (1 x 100)/21 = 4.76 m3 of air

44




4. Molecular mass of air is taken as 28.94 g/mol

5. Minimum oxygen required = theoretical oxygenrequired – oxygen present in the fuel

6. Minimum oxygen required should be calculatedon the basis of complete combustion

7. Mass of dry flue gases formed should becalculated by balancing the carbon in the fueland carbon in the flue gases.

8. The mass of any gas can be converted to itsvolume at certain T and P by using the gasequation, PV = nRT

45




9. Hydrogen, present in the combined form (asH2O) is a non-combustible substance and doesnot take part in combustion. The rest ofhydrogen, called available hydrogen only takespart in the combustion reaction.

As, 1 part of hydrogen combines chemically with8 parts by mass of oxygen to form water, the

available hydrogen = mass of hydrogen – (massof oxygen/8)

46




• Theoretical amount of oxygen required for thecomplete combustion of 1 kg solid or liquid fuel:

= [(32/12) x C] + 8[H – (O/8)] + S kg

• Theoretical amount of air required for thecomplete combustion of 1 kg fuel:

= (100/23) [(32/12) x C] + 8[H – (O/8)] + S kg

(since, % of oxygen in air by mass is 23)

where, C, H, S and O are the masses of C, H, Sand O respectively per kg of the fuel.

47



Documents

Chemistry fuels and combustion