6 Boiling point, vapour lock and ice formation in induction systems

598



The Motor Vehicle



reduces the boiling point of the fuel, so heat transmitted to these components

by, for example, the exhaust manifold causes it to vaporise. Because pumps

are designed to deliver liquids, they cannot cope with vapour; consequently,

the supply of fuel to the engine is interrupted, causing it, at best, to run

roughly and, at worst to stop.

Vapour lock is most likely to occur after the vehicle has stopped, especially

after a slow climb to the top of a steep hill in hot weather. In these circumstances,

both the forward speed of the vehicle and the rotational speed of the engine

are generally low, so also therefore are the rates of flow from both a

mechanically driven fan and the water pump, and the engine therefore overheats. This heat is then conducted out to the surrounding parts, such as the

carburettor, fuel pump and pipe lines, in which some or all of their contents

vaporise. Consequently, the fuel pump may cease to function efficiently, if at

all, and even vapour, instead of fuel, may be delivered to the carburettor float

chamber or injection system and, with a carburettor, the fuel may have

boiled out of the float chamber. In either event, the engine cannot be restarted.

Indeed, if ambient temperatures are high and the float chamber or a pipeline

unshielded from and is too close to the exhaust manifold, the engine may

even stop while the vehicle is ascending a long steep hill.

Fuel-lift pumps usually deliver the fuel over a weir which, when the

engine has been switched off, retains some fuel within the pump to keep it

primed ready for restarting. However, this helps only marginally if the suction

line is full of vapour, and a long time may elapse before the fuel vapour has

condensed and the engine can be started again. The process can be speeded

considerably by pouring cold water over the pump and suction line. Vapour

can be difficult to clear from fuel-injection equipment so, on modern cars,

fuel pumps are usually electrically driven and installed inside the fuel tank,

so that they do not have to overcome suction heads.

Ice formation in both carburettors and with single-point injection is due to

moisture in the atmosphere wetting throttle valves and barrels and freezing

on them. This happens because of a drop in temperature of these parts,

arising from the latent heat of vaporisation of the fuel. It tends to occur when

the ambient temperature is slightly above freezing point and the relative

humidity high. If the moisture is already frozen before it enters the air intake,

it is more likely to bounce past the throttle and on into the cylinders. In severe

cases, the engine can be stopped by this build up of ice. Subsequently, after

the ice has been melted by heat conducted from the surrounding hot parts,

the engine can be restarted.



17.7



Composition of fuel for spark ignition engines



In the early nineteen-twenties, the term ‘petrol’ was originally introduced as

a trade name by an English company, Carless, Capel and Leonard, which

still exists. The American term ‘gasoline’ is, however, beginning to gain

ground in the British motor industry. Other terms that have been used include

‘motor spirit’ and ‘petroleum spirit’, the latter perhaps being the most

appropriate for what is basically a complex mixture of distillate from petroleum

(crude oil).

Because the contents of crude oils differ widely according to the part of

the world from which they come, the major oil companies often have to

refine crude stocks from different geographical sources and blend the distillates



Fuels and their combustion



599



to produce a petrol suitable for use in motor vehicles. The actual blend

depends also on the season in which it is to be used, and a fuel for a hot

country must of course be less volatile than one for use in a cold climate. A

typical leaded petrol for use in the UK would have a range of volatilities

similar to that in Table 17.2. With the introduction of unleaded petrols, there

has been a trend towards increasing the proportions of lighter fractions, most

of which have higher octane numbers. Such fuels might contain between 24

and 45% aromatics, and from zero up to 26% olefins. The balance would be

made up of naphthenes and alkane saturates.

Because benzene, an aromatic having a high octane number, has been said

to be a carcinogen, legal restrictions in some countries limit it to about 5%

by volume. In general, emissions regulations are now so stringent that most

can be satisfied only by fuel injection and closed-loop control. With injection,

fuel-delivery pressures are higher than with carburation, and of course there

is no evaporation from float chambers. All this has strengthened the trend

towards fuels with a higher proportion of light fractions and therefore providing

good cold starting.



17.8



Additives



Additives are substances introduced, in small proportions, into fuels to enhance

their performance or to offset the effects of certain undesirable properties. It

all started in the early nineteen-twenties when the demand for fuel was

expanding rapidly and could no longer be satisfied by straight distilled

hydrocarbons. Oil companies started to crack the heavier fractions, to break

down their molecules into lighter ones and thus increase the supply of petrol.

Cracked products of that time tended to be unstable, reacting with oxygen to

form gummy deposits causing problems such as blocked carburettor jets and

filters. Consequently the first additives were anti-oxidants.

Because of the impending increasingly strict legal requirements regarding

exhaust emissions and fuel economy, additive technology is now being taken

more seriously than hitherto. Even so, at the time of writing, only three oil

companies in the UK, handling no more than 30% of the fuel sold there, are

marketing additive fuels.

Table 17.2



PROPERTIES OF A TYPICAL PREMIUM GASOLINE

FUEL FOR THE UK



Property

Specific gravity

Octane No.

Reid vapour pressure, kN/m2

Initial boiling point, °C



Summer

0.734

97

13.5

34



Winter

0.732

97

7.7

30



10% fraction boils off at, °C

25%

50%

75%



55

74.4

104.8

139.2



51

63.5

92.8

129.4



Final boiling point



184



185



600



17.9



The Motor Vehicle



Lead compounds



Also in the early 1920s, Midgley, in the USA, discovered that adding tetraethyl

lead, Pb(C2H5)4, in small quantities to the fuel would inhibit detonation.

Subsequently it was found that compounds called scavengers, mainly 1,2dibromoethane and 1,2-dichloroethane, mixed with the lead, would prevent

it from forming hard deposits in combustion chambers and on valve seats.

By 1930, mixed in at a rate of about 0.6 g/l, tetraethyl lead (TEL) was

widely used to increase octane number. Today, any engine that is not designed

for running on unleaded fuel will suffer rapid wear of its exhaust valve seats

with a fuel having less than about 0.3 g/l of TEL. The reason is that the

combustion process leaves a coating of lead bromide compounds on the seats

and these inhibit welding of the peaks of the surface texture of the seats to

those of their mating faces on the valves.

By about 1960, tetramethyl lead (TML) began to be used. This has a

lower boiling point than TEL, so it evaporates with the lighter fractions of

fuel and therefore is drawn preferentially together with them into the cylinders.

At one time it was not uncommon for a mixture of TEL and TML to be used

as an anti-knock compound. During combustion, the lead additives form a

cloud of metal oxide particles. These, because the lead molecules are heavy

and the oxides chemically active, interrupt the chain-branching reactions

that lead to detonation. Incidentally, sulphur in the fuel reduces the effectiveness

of lead additives.

By the 1950s, huge resources were being poured by interested parties into

research to prove that burning lead additives in fuel produces toxic exhaust

fumes. True, in large concentrations over extended periods, it can adversely

affect brain development but, so far, no one has proved it can do so in the

concentrations that enter the atmosphere from automotive exhausts, even if

deposited on food crops. The reason for the abandonment of lead additives

has been that they adversely affect the performance of the catalysts in the

converters incorporated in vehicle exhaust systems. Lead additives, though

obsolescent, remain the most economical way of increasing octane number.



17.10



Lead-free fuels



One way of producing satisfactory lead-free fuels is to use oxygenate additives,

either alcohols or ethers, though these are costly. Alcohols include ethanol,

methanol, tertiary butyl alcohol (TBA), methyl tertiary butyl ether (MTBE),

tertiary amyl methyl ether (TAME) and ethyl tertiary butyl ether (ETBE).

Their octane numbers range from 104 to 136, and the octane numbers of the

fuels in which they have been blended range from 111 to 123. Even so, they

have tended to fall out of favour because, under certain conditions, they

break down and form hydroperoxides, which are corrosive and, combined

with other substances in the fuel, can produce other corrosive compounds.

Mathanol contains 49.9% oxygen, but MTBE and TAME contain only 18.2

and 15.7% respectively. The ethers, whose oxygen contents are lower than

those of the alcohols, are nevertheless an attractive alternative.

A widely used method of producing high-octane hydrocarbon fuel is to

isomerise the distillate to form mostly light, high-octane derivatives. Such

processing, however, is not only costly but also consumes energy that has to

be taken from the oil being processed. It therefore increases emissions of



Fuels and their combustion



601



CO2, NOx and SO2 into the atmosphere. Moreover, there is a limit to the

proportions of light components that can be blended into a motor fuel.



17.11



Detergent additives



Detergents were introduced initially in the early 1960s, in response to

driveability problems arising from the formation of deposits in carburettors.

‘Driveability’ is a term used mainly for describing the smoothness of the

response of the engine to movements of the accelerator pedal.

By the late 1960s and early 1970s, the introduction first of positive crankcase

ventilation and then exhaust gas recirculation led to the appearance of deposits

in all the passages from air filter to inlet valves and even on the valves

themselves. Again, the result was poor driveability. Consequently, it became

necessary to develop detergents that would be effective not only in the

carburettor but also throughout the system. Shell, with its ASD (Additive

Super Detergent) fuel, was first in the field, in the late 1960s and early 1970s,

and was using these second generation detergents in higher concentrations

than hitherto. A significant new feature was the use of carrier fluids, mainly

mineral oils, or polymers such as polybutene or polyetheramine, to take the

additives right through the induction system.

With both carburettors and throttle body injection, deposits are particularly

likely to be formed on hot spots in the induction manifold and any other area

in which heat soak increases local temperatures after the engine has stopped.

The oily additives partially or completely dissolve these deposits, which are

subsequently swept away and burnt in the combustion chambers. Too high a

content of such additives, however, can cause valve sticking and increase

combustion-chamber deposits, leading to higher octane requirements.

From approximately 1970 to 1980, induction-system temperatures increased

significantly, partly as a result of induction air heating and the other measures

for overcoming emissions problems. The situation was exacerbated by the

trends towards use of leaner air : fuel ratios and higher temperatures, for

improving thermal efficiency and hence fuel economy. An outcome was that,

because problems such as injector nozzle fouling arose as a result of heat

soak, oily additives became no longer adequate alone and therefore detergents

had to be used with them. However, the high temperatures involved called

for a different type of detergent additive, so polymeric dispersants and amine

detergents were then introduced.

In general, detergent additive molecules comprise an oleofilic chain-like

tail with a polar-type head, Fig. 17.7. The free arms of the head attach to the

particulate deposit and carry it away in the liquid fuel in which these detergent

molecules are dissolved.



17.12



Corrosion inhibitors



These additives are particularly desirable with injection systems since, without

them, malfunction will be caused by corrosion debris blocking the fine filters

used and the injector nozzles. Corrosion can also cause fuel tanks to leak,

even though they are protected internally by a corrosion-resistant coating.

Most corrosion inhibitors react with the acids that form in the fuels and

some, like the detergents, have polar heads and oleofilic tails, but the heads

latch on to the molecules of the metal surfaces, over which their tails form

a protective coating.



602



The Motor Vehicle



Detergent

molecule

latched on

to carbon

particle



Carbon

particle

Free

detergent

molecule

Wall of manifold



Fig. 17.7 Showing how detergent additive molecules latch on to the dirt particles to

carry them away in solution



The fuel itself can also oxidise, causing the formation of gums that can

lead to difficulties both in storage and in the engine. Fuels containing high

proportions of cracked products are, as previously mentioned, particularly

susceptible to gum formation. Additives inhibiting the oxidation of the fuel

are therefore used, but mainly in storage.



17.13



Spark-aider additives



To satisfy emission control regulations, engines have to be operated on weak

mixtures, so good driveability can be difficult to achieve. As has been indicated

previously, cleanliness can help, but more important are the rapidity with

which the engine warms up and the consistency, from cycle to cycle, with

which the flame develops and spreads through the combustion chamber.

If the flame kernel around the spark does not expand rapidly to a certain

critical size, either the mixture will subsequently burn inefficiently or the

flame will die. Even if the engine is cold, the nominally rich mixture supplied

can still be weak in the region of the spark plug. This is partly because, on

the way to the cylinder, the lighter fractions can condense out and be deposited

on cold metal surfaces.

Consistency of combustion can be improved by the use of spark-aider

additives. However, if they are used together with lead additives containing

halogen compounds, they could lead to sticking and deterioration of the inlet

valves. They function by coating the electrodes with a compound facilitating

the passage of the spark and thus allowing more energy to be applied to

ignite the mixture.



17.14



Diesel fuels



Whereas for the spark ignition engine the fuel and air are supplied pre-mixed

to the cylinders, in a diesel engine the fuel is not injected into the air until

shortly before TDC. Consequently, there is considerably less time for completion of the mixing and evaporation processes. Furthermore, the diesel engine,

having no throttle, is controlled by regulating the quantity of fuel injected

per induction stroke. Add to this the fact that ignition cannot occur until the

temperature generated by compression is high enough, and it becomes obvious

that fuel quality is even more important for the diesel than the spark ignition

engine.



Fuels and their combustion



603



Whereas in Europe there is one grade of diesel fuel for road vehicles, in

the USA there are two, ASTM D1 and D2. The European Union defines a

diesel fuel as containing a maximum of 65% distilled off at 250°C and a

minimum of 85% distilled off at 350°C. A UK diesel fuel might have the

following properties—

Specific gravity

Sulphur

Cetane No.

Cold filter plugging point



0.85

0.22%

51

–18°C



Cloud point

Initial boiling point

50% vaporisation

Final boiling point



–5.5°C

180°C

280°C

360°C



Hydrocracking and catalytic cracking are used to convert fractions having

even higher boiling points into hydrocarbons suitable for use as diesel fuels.

However, both hydrocracked and catalytically cracked fuels tend to have

cetane numbers, Section 17.16, in the region of only 10 to 30. Catalytically

cracked fuels, moreover, tend to be slightly unstable in storage.



17.15



Properties required for diesel fuel



A few of the properties required, such as high calorific value (energy content),

are common to both gasoline and diesel power units, but most are much

different. Diesel fuel mostly comprises fractions boiling off from approximately 150 to 355°C, Fig. 17.8, as compared with about 15 to 210°C for gasoline.

As delivered from the fractionating tower, these higher boiling point fractions

400

Final boiling point (FBP)



Temperature, °C



300

Mid boiling point (MBP)



200

Initial boiling point (IBP)



100

0



20



40

60

Fraction, %



80



100



Fig. 17.8 This characteristic distillation curve for diesel fuel is similar in form to that

illustrated in Fig. 17.1, but of course the initial and final boiling points are higher



604



The Motor Vehicle



contain about 20 times more sulphur than those from which gasoline is

derived, so extra attention has to be devoted to removing it during refinement.

The following are the properties that must be controlled when diesel fuels

are blended—

Volatility



High volatility helps with cold starting and obtaining

complete combustion.

Flashpoint

The lower the flashpoint the greater is the safety in handling

and storage.

Cetane number This is a measure of ignitability. The higher the cetane

number the more complete is the combustion and the cleaner

the exhaust.

Viscosity

Low viscosity leads to good atomisation.

Sulphur

Low sulphur content means low wear and a smaller

particulate content in the exhaust.

Density

The higher the density the greater is the energy content of

the fuel.

Waxing tendency Wax precipitation can render cold starting difficult and

subsequently stop the engine.

As in the case of petrol, properties of a diesel fuel depend in the first

instance on the source of the crude oil from which it is distilled. These vary

as follows—

UK and Norway



Mainly paraffinic and therefore of high cetane

number. Calorific value relatively low and cloud

point high. Sulphur content low to medium.

Middle East

Similar, but high sulphur content. Middle East crude

oils are a particularly good source for diesel fuel,

because they contain a high proportion of alkanes

and a small proportion of aromatics.

Nigeria

Naphthenic. Low cetane number, cloud point and

sulphur content. Calorific value medium.

Venezuela and Mexico Naphthenic and aromatic. Low cloud point and very

low cetane number, but low to medium sulphur.

Calorific value high.

Each of the properties in the lists above influences engine performance,

so we need to study them in more detail.



17.16



Cetane number, cetane index and diesel index



Basically, the cetane number is the percentage of cetane in a mixture of

cetane (n-hexadecane) and heptamethylnonane (the latter is sometimes referred

to as α-methylnaphthalene) that has the same ignition delay, generally expressed

in terms of degrees of rotation of the crankshaft, as the fuel under test. There

is a more precise definition but, before we come to it, a brief note on ignition

delay is necessary.

Ignition delay, Section 17.32, is important because, if it is too long, the



Fuels and their combustion



605



whole charge in the cylinder tends to fire simultaneously, causing violent

combustion. With a short delay, ignition is initiated at several points, and the

flame subsequently spreads progressively throughout the charge. On the

other hand, the injection must be timed appropriately relative to the cetane

number of the fuel that will be used: a higher cetane number than that for

which the timing was set can lead to ignition before adequate mixing has

occurred and thus increase emissions.

Cetane number is defined precisely as the percentage of n-cetane + 0.15

times the percentage of heptamethylnonane contents of the blend of reference

fuel having the same ignition quality as the fuel under test. Ignition quality

is determined by varying the compression ratio to give the same ignition

delay period for the test fuel and two blends of reference fuels. One blend

has to be of better and the other of poorer ignition quality than the test fuel,

but the difference between the two has to be no more than five cetane numbers.

The cetane number is obtained by interpolation between the results obtained

at the highest and lowest compression ratios.

Carrying out these laboratory engine tests, however, is not at all convenient,

so two other criteria are widely used. One is the diesel index and the other

the cetane index. The diesel index, which is obtained mathematically, is

computed by multiplying the aniline point of the fuel by its API gravity/100.

The aniline point is the lowest temperature in degrees fahrenheit at which the

fuel is completely miscible with an equal volume of freshly distilled aniline,

which is phenylamine aminobenzene. API, measured with a hydrometer,

stands for American Petroleum Institution, and degrees API = (141.5/Specific

gravity at 60°F) – 131.5 It is a measure of density for liquids lighter than

water.

The cetane index is calculated from API gravity and volatility, the latter

originally taken as represented by its mid-volatility, or mid-boiling point

(50% recovery temperature, T50). Since its introduction, the formula has

been modified from time to time, to keep up with advancing fuel technology,

and is now based on an extremely complex formula embracing the density

and volatility of three fractions of the fuel (those at the 10, 50 and 90%

distillation temperatures T10, T50 and T90, respectively). This formula can be

found in Automotive Fuels and Fuel Systems, Vol. 2, T.K. Garrett, Wiley.

The cetane index is, in general, better than the diesel index as an indication

of what the cetane number of a fuel would be if tested in a CFR engine in a

laboratory, and it is much less costly and time-consuming to obtain. In general,

alkanes have high, aromatics low, and naphthenes intermediate, cetane and

diesel indices.

Values of 50 or above for either the diesel or cetane index indicate good

combustion and ignition characteristics, below 40 are totally unacceptable

and even below 45 undesirable. Low values mean difficult cold starting the

generation of white smoke, and the engine will be noisy.

BS 2869: Part 1: 1988 prescribes minimum limits of 48 for the cetane

number and 46 for the cetane index. In Europe and Japan the minimum

cetane number requirement is 45, and in the USA it is 40, the latter possibly

being because a high proportion of their crude oil comes from Mexico and

Venezuela. A reduction in cetane number from 50 to 40 leads to an increase

in the ignition delay period of about 2° crankshaft angle in a direct and about

half that angle in an indirect injection engine.