FUELS_FOR_SI_ENGINES_B.pdf

7/29/2019 FUELS_FOR_SI_ENGINES_B.pdf

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FUELS FOR S.I. ENGINES

CONVENTIONAL


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Types of FuelsFuels for engines are typically

1. Originally solid but now hardly used.2. Liquid, the most commonly used form.

3. Gaseous, the most common alternative.

Fuels may be

1. Naturally available or

2. Artificially derived


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Conventional Liquid Fuels

These may be obtained from:

1. Conventional sources crude oil

2. Non-conventional sources coal, bio-mass, shale, tar sands.

Today, the major source is crude oil butscenario may change when crude oil

becomes expensive or scarce.


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Types of Liquid Fuels

Liquid Hydrocarbon fuels may be

1. Paraffins: straight chain compounds like methane,ethane, propane, etc. or branched chain compounds(isomers) like iso-butane, iso-heptane (like 2,2,3 tri-

methyl butane or triptane) and iso-octane (like 2,2,4tri-methyl pentane).

2. Olefins: Open chain unsaturated hydrocarbons with adouble bond like ethene or propylene which also havestraight and branched chain compounds.

3. Diolefins: These are olefins with 2 double bonds.

Both types of olefins produce gum when reacted withoxygen which can block fuel filters.


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Types of Liquid Fuels

(Continued)4. Alkynes: Unsaturated hydrocarbons with a

triple bond. A typical example is acetylene orethyne.

5. Napthenes or Cycloparaffins: Have same

general formula as monoolefins but aresaturated compounds with a ring structure.Examples are cyclopropane, cyclobutane etc.

6. Aromatics: Ring structured unsaturated

hydrocarbons with double bonds but morestable than the parafffinc double bondhydrocarbons. Examples are benzene, toluene,naphthalenes, and anthracenes.


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General Formulas for Organic

Compounds found in Crude Oil

Family Formula Structure

Paraffins (alkane) CnH2n+2 Straight andBranched

Paraffins (alkene) CnH2n Straight andBranched

Paraffins (alkyne) CnH2n-2 Straight andBranched

Naphthenes(cyclanes)

CnH2n Ring

Aromatics

(Benzenes)CnH2n-6 Ring

Aromatics

(naphthalene) CnH2n-12 Ring


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Composition of typical crude oil

Carbon: 80-89%

Hydrogen: 12-14%

Nitrogen: 0.3-1.0%Sulfur: 0.3-3.0%

Oxygen: 2.0-3.0%

Plus oxygenated compounds like phenols,fatty acids, ketones and metallic elements

like vanadium and nickel.


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Typical Refinery Products

Product Boiling Range, oC

Liquefied Petroleum Gas -40 to 0

Motor Gasoline 30-200

Kerosene, jet fuel (ATF) 170-270

Diesel Fuel 180-340

Furnace Oil 180-340

Lube Oils 340-540Residual Fuel 340-650

Asphalt 540+

Petroleum Coke Solid


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Refinery processes

1. Distillation: Continuous, Atmospheric,and Vacuum.

2. Cracking: Thermal, Catalytic and Hydro.

3. Reforming: Thermal, Catalytic and Hydro

4. Polymerization

5. Alkylation

6. Isomerization

7. Hydrogenation


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reforming, in chemistry, processing technique

by which the molecular structure of a

hydrocarbon is rearranged to alter its

properties. The process is frequently applied to

low-quality gasoline stocks to improve their

combustion characteristics alkylation, in petroleum refining, chemical

process in which light, gaseous hydrocarbons

are combined to produce high-octanecomponents ofgasoline.
http://www.britannica.com/EBchecked/topic/388236/moleculehttp://www.britannica.com/EBchecked/topic/454440/petroleum-refininghttp://www.britannica.com/EBchecked/topic/226565/gasolinehttp://www.britannica.com/EBchecked/topic/226565/gasolinehttp://www.britannica.com/EBchecked/topic/454440/petroleum-refininghttp://www.britannica.com/EBchecked/topic/388236/molecule


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ADDITIVES USED FOR GASOLINE

1. Anti-knock Additive: Required to eliminate knock.

2. Deposit-modifiers: Used to modify the chemical character of combustionchamber deposits and so reduce surface ignition and spark plug fouling.

They are usually a phosphorus or boron compound.3. Anti-oxidants: Used to reduce gum formation and decomposition of the lead

compounds. They are usually an amine.

4. Detergents: Used to prevent deposits in the carburetor and manifold. Theyare usually an alkyl amine phosphate.

5. Lubricants: Used to lubricate valve guides and upper cylinder regions. Theyare usually light mineral oils.

6. Metal de-activators: Used to destroy the catalytic activity of traces ofcopper. They are usually amine derivatives.

7. Anti-rust Agents: Used to prevent rust and corrosion due to moisture in theair. They are usually fatty acid amines, sulfonates, or alkyl phosphates.

8. Anti-icing Agents: Used to prevent the freezing of gasoline from water in thefuel and throttle plate icing from moisture in the air. Methanol is added to

gasoline to absorb water- and so prevent ice from forming in the fuel linebetween the tank and carburetor. Isopropyl alcohol or a surface action agent(orsurfactant) like ammonia salts or phosphates is added to prevent icefrom forming or adhering to the throttle plate. The alcohol acts by loweringthe freezing point of the condensate. The surface-action additive forms afilm on the metal, which discourages adhesion of ice. Some of the surface-action additives also have detergent qualities.

9. Dye: To identify the fuel.


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DESIRED CHARACTERISTICS OF GASOLINE


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DESIRED CHARACTERISTICS OF GASOLINE1. KNOCK CHARACTERISTICS. The measure for knock characteristics for a

spark ignition engine fuel is the octane rating. The fuel should have anoctane rating to suit the engine requirements.

2. VOLATILITY. There are four aspects of volatility for a spark ignition engine

fuel. These are:(a) Starting and Warm up Characteristics. If the fuel or a portion of it has a

low boiling point, the engine will start readily.

(b) Vapor lock and Hot Start and Idling Characteristics. The fuel should havea low vapor pressure at the existing fuel line temperatures to avoidvaporization of the fuel in the fuel lines and the carburetor float-bowl.This will help in eliminating the problem of stoppage or reduction in the

flow of liquid fuel.(c) Running Performance or Normal Acceleration. In general, the fuel with

the lowest distillation temperature is the best.

(d) Crankcase Dilution. The dilution of the lubricating oil may occur whenthe fuel condenses or fails to vaporize in the engine. A low distillationtemperature range is desirable.

(e) Other Factors. This includes factors like carburetor icing, evaporationloss and fire hazard.

3. GUM AND VARNISH DEPOSITS. The fuel should not deposit either gum orvarnish in the engine.

4. CORROSION. The fuel and the products of its combustion should be non-corrosive.

5. COST. The fuel should be inexpensive; its price must be competitive.


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VOLATILITY

Volatility is "the temperature at which a given

air-vapor mixture is formed when under

equilibrium conditions at a pressure of one

atmosphere, when a given percentage of thefuel is evaporated".

According to this definition, one gasoline is more

volatile than another for any given percentage

evaporated, if it forms the given air-vapormixture at a lower temperature.


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A simple test for volatility

A flask containing 100 ml of fuel is heated. The vapors arecondensed. The temperatures are recorded when thefirst drop falls into a graduated cylinder, then dropscontaining 5, 10, 15 per cent etc. are condensed. Since

the fuel contains many hydrocarbons, the boilingtemperature continuously rises as evaporation proceeds.At the end of the test, a limiting end point temperaturewill be reached. A residue will condense in the flaskafter cooling. However, some fuel would have been lost

(called loss) which is assumed to be due to the mostvolatile part of the fuel. Thus we get a correctedASTMdistillation curve, where the loss appears at the start andthe residue at the end of the curve.


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St t U


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Start Up Limits of Flammability of air-gasoline vapor

mixture are 6:1 to 20:1 by weight.

Rich mixture is used to ensure greater mass offuel available as vapor.

Engine should start in 10 revolutions. Air-vapor

mixture for successful start is given by log10 (AV) = 1.301 2/n

where n is the number of revolutions

For n = 10, AV = 12.62

If 5% (AV=20) fuel evaporates it will start. For safestart it is 7.9% (AV=12.62). For sure start it is16.7% (AV=6).

Fuel with low 10% temperature (T10) would start

easily in cold weather.

St t U (C ti d)


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Start Up (Continued)It is found that percent distilling below 70oCinfluences ease of cold start: the greater the

percentage, the more readily the start.

Table III Effect of percentage distilled below 70oC (ASTMTest D86) on lowest starting temperature.

---------------------------------------------------------------------------------Percentage distilled Lowest ambient

below 70oC(vol.%) temperature foracceptable start, oC

---------------------------------------------------------------------------------

30 -17.825 -12.2

15 - 6.7

10 -1.1

---------------------------------------------------------------------------------

Warm Up


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Warm Up

Length of warm up depends on number of

factors.The higher the percentage distilling below

100oC, the faster the warm up

The lower the temperature at 50% point(T50), the faster the warm up.

Based on a test, in a car at 0oC:

1. Fuel with 42% distilled at 100oC warm updistance was 4.3 km.

2. Fuel with 60% distilled at 100oC warm up

distance was 2.5 km.

Warm Up


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Warm UpWith full choke at (cold) starting, fuel supply is 10

times that needed for normal running.

At warm up, fuel consumption is double thatrequired for fully warmed up engine.

To reduce warming up period we would need

(1) heating of the intake with quick choke off. This

affects drivability or(2) use fuel with increased volatility.

A more volatile fuel will

(1) be easier to start from cold,

(2) give faster warm up,(3) consume less fuel, and

(4) give lower exhaust emissions during earlyphase of engine operation.


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Normal Running

High distillation temperature (low volatility)

causes less fuel to evaporate gives poor

distribution. Requires over-fuelling. Gives

more power but fuel consumption is high.

Low distillation temperature (high volatility)

gives better mixing; may even superheat the

fuel vapor. But vapor displaces the air and

reduces volumetric efficiency and gives less

power. This is similar to using gaseous fuel.


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Acceleration

Fuel lag due to sudden acceleration

Requires an acceleration pump to give aricher mixture or use of a high volatility

fuel. May require manifold heating to avoid

use of the acceleration pump.


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Vapor Lock

Low boiling point components are needed

to give good starting, e.g. Butane.

Boils off causing slugs of fuel in the fuel

line gives rough running and prevents

re-starting after shut down.

Occurs due to presence of high

temperature and low pressure in the lineor due to high ambient temperature.

Reid vapor pressure


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Reid vapor pressure

The vapor lock tendencies of a gasoline are

directly related to the front-end volatility, thatis, 0 to 50% of the ASTM curve (Taylor

specifies 10% only). The Reid Vapor

Pressure (RVP), which is a direct measureof the vapor pressure, is a reliable indicator

of the vapor lock tendency. Gasoline

specifications usually place an upper limit on

vapor pressure, the limit depending upon

type of service and climatic conditions

expected in service.


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Reid vapor pressure

The Reid Vapor Pressure depends upon the

vapor-liquid (V/L) ratio. The V/L ratio is defined

as the equilibrium volume of vapor at a given

temperature and pressure, per unit volume ofliquid, supplied at that temperature.

The higher the RVP, the more volatile the

gasoline.

For Indian fuels it is 35 to 70 kN/m2.


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Reid vapor pressure

For a single constituent fuel, the vapor-liquid ratio is amulti-valued one since the extent of vaporization doesnot change the vapor-pressure at a fixed temperature.On the other hand, for the usual multi-constituent fuels,the vapor pressure depends on the extent ofvaporization. Thus, if the vapor volume is made larger,the vapor decreases. Hence for commercial gasoline,specifying a temperature and a pressure also specifies adefinite vapor-liquid ratio.

The Reid method specifies a V/L ratio of 4 and atemperature of 100oF or about 38oC.


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Vapor-locking Tendency

Four Equations to measure this:1. Vapor Forming Index: (ASTM Standards on Petroleum

Products, Part 17):

VFI = RVP + 2 (slope of ASTM 10% point)

2. Front End Volatility Index :FEVI = RVP + 0.13(percent evaporated at 70oC ASTMdistillation curve)

3. General Motors Vapor Pressure:

Vapor Pressure Measured at a V/L of 25, at 100oF

(approximately 38o

C)4. Modified General Motors Vapor Pressure:

Vapor Pressure Measured at a V/L of 25, at 55oC.

Vapor lock can be eliminated by fuel


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Vapor lock can be eliminated by fuel

system design that achieves the

following conditions:

1. Fuel handling with minimum temperature rise: fuel lines,pumps, carburetors, etc., must be located away from theheat of the engine and exhaust. Insulating gaskets arerequired for mounting the pump and carburetor.

2. Minimum pumping requirement from the pump to thecarburetor: the pump should be amply over-sized or ofadequate capacity so that the presence of vapor will notreduce the flow demanded by the engine.

3. Minimum fuel line elevation and cross-sectional area

changes.4. There must be proper flow of air through and past the

fuel system by adequate design of the fan and ventilationlouvers.


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Hot Start and Idling

Percolation: This is due to fuel being forced

through the main jet when pressure in the

float bowl increases due to restriction in

the venting.

It will cause (hot) starting problems because

a very rich mixture enters the engine. It

may be due to high volatility of the fuel.Idling will be rough.


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Drivability

Drivability of a vehicle is the extent to which a car,when the engine has cooled to ambienttemperature starts and operates smoothly andreliably during the first few kilometers of

operation.Drivability malfunctions, which occur during thisperiod, include stall, hesitation, stumble, surge,backfire, and idle roughness or quality, andstretchiness.

Drivability can be improved by increasing the fuelvolatility in the 40 to 90% range of the ASTMcurve.


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The drivability index (DI)

DI = 1.5T10 + 3T50 + T90

The lower this value, the more volatile the

fuel, and the better the drivability,

particularly in cold weather after a cold

start.


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Crankcase Dilution

Condensation of vapor on the cylinder walls

or the presence of liquid fuel in the air will

result in mixing of the fuel with the

lubricating oil.

The upper portion of the distillation curve

must be low to ensure that all the fuel will

vaporize.Crankcase ventilation will reduce dilution.


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Carburetor Icing

This is due to condensation of the water

vapor in the air on to the throttle plate.

Can be reduced by using manifold heating

or by use of additives.

Volatility also plays an important role here.


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Evaporative Loss

The lower the 10% temperature (T10) thehigher the loss. Loss is greater from a tankthat has less fuel. The loss from a tank

full was 60% more than from a tank thatwas full.

This is also a pollution hazard.

Can be controlled by venting the tank into acharcoal canister which adsorbs anddesorbs fuel vapors.


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Fire Hazard

The explosive range for gasoline vapor is

between 65oC and 20oC.

Gasoline vapor gives a highly combustible

atmosphere in a closed space.

Fires develop a great intensity almost

immediately.

A fuel of higher volatility is a greater fire

hazard.


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Calorific Value and Volatility

Motor gasoline is sold by volume, so a fuel

of high calorific value per unit volume is

necessary.

Now kJ/m3 = (kJ/kg)(kg/m3)

Thus the fuel must have high density and

it means a lower volatility. But we have

seen that it must be volatile also.

Hence a compromise is necessary.

Summary of Volatility Characteristics and


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Front end Middle range Rear end

10-30% 50-70% 90%

Engine Problem:Cold starting Prime Insignificant Insignificant

Evaporation loss Prime Insignificant Insignificant

Vapor lock Prime Intermediate Insignificant

Hot starting Prime Intermediate Insignificant

Hot idle Prime Intermediate Insignificant

Carburetor icing Intermediate Prime InsignificantWarm up Insignificant Prime Intermediate

Acceleration Insignificant Prime Intermediate

Oil dilution Insignificant Insignificant Prime

Cleanliness Insignificant Insignificant Prime

Calorific value Insignificant Insignificant Prime

Summary of Volatility Characteristics and

Effects

The table gives a summary of the various aspects of volatility


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GUM DEPOSITS

Reactive hydrocarbons and impurities in the fuel have atendency, especially when stored during long periods athigh ambient temperatures, to deteriorate due tooxidation and form viscous liquids and solids, which arereferred to as gum.

This can also occur when used in the engine.

These gums can seriously influence the performance of thegasoline.

Mainly due to presence of olefins and diolefins in the fuel.

Require hydrogen treatment or use of anti-oxidant additives

Problems Associated with Gum Formation


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Problems Associated with Gum Formation

1. Sticking of intake valves, piston rings, and automaticchokes, also fuel lines and filters.

2. Carbon deposits in the engine and gum deposits in themanifold; the latter could build up to a sufficient degree toreduce volumetric efficiency.

3. Clogging of carburetor jets and also cause sluggishoperation of the carburetor linkages.

4. Lacquering of the valve stems, cylinders, and pistons.Lacquer is the name assigned to varnish-appearing residuethat the gum leaves when exposed to high temperatures. Ifthe gum is inflamed, it is reduced to a residue of carbon.

This carbon, lacquer, and gum deposits, all result from gumin the liquid fuel.

5. High sludge levels in storage tanks, cloudy gasoline anddeposits in various parts of an engine fuel system that cangive rise to vehicle malfunctions.

Factors which Increase Gum Formation


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Factors which Increase Gum Formation

1. Increased concentration of oxygen in fuel.2. Rise in temperature.

3. Increased exposure to sunlight (this

influence is considerable).4. Contact with metals which act as catalysts;

copper is particularly reactive. Deactivators

must be added to neutralize the catalyticaction of the metal.


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CORROSION

Corrosion in the fuel system of a vehicle can lead to severeproblems.

1. Leaks can develop in automobile fuel tanks

2. Particles of rust can block fuel lines, filters and critical

carburetor orifices such as jets.3. Small amounts of water and dissolved air promote

corrosion of ferrous parts of the fuel system.

4. In addition, alcohols such as methanol or ethanol,present as blend components, can attack many non-ferrous parts, as well as increase the dissolved watercontent of the gasoline.

Major Cause of Corrosion


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j

The major cause of corrosion in fuels is the presence of sulfur.

Hydrocarbons may contain free sulfur, hydrogen sulfide, andother sulfur compounds.

Free sulfur and hydrogen sulfide in the fuel can corrode fuellines, carburetors, and fuel injection pumps.

Sulfur in all forms in the fuel, will form sulfurous and sulfuric aciddue to combustion with oxygen, and the presence of water, at

low temperatures.Though a large portion of the exhaust gases escape through the

exhaust pipe, the gases trapped in the clearance volume(residual gases) contain sulfur dioxide at the appropriatetemperatures to form sulfurous acid.

The sulfur dioxide may combine with other substances to formproducts that cause engine wear. The presence of sulfur inthe fuel can reduce its self-ignition temperature therebypromoting knock in the spark ignition engine.

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FUELS_FOR_SI_ENGINES_B.pdf