16 Cetane number, cetane index and diesel index

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.



606



17.17



The Motor Vehicle



Tendency to deposit wax



In cold weather, a surprisingly small wax content, even as little as 2%, can

crystallise out and partially gel a fuel. These crystals can block the fuel

filters interposed between the tank and injection equipment of the engine,

and ultimately cause the engine to stall. In very severe conditions, the pipelines

can become blocked and a thick layer of wax may sink to the bottom of the

fuel tank. Paraffins are the most likely constituents to deposit wax which,

because they have high cetane numbers, is unfortunate.

The various measures of the tendency of a fuel to precipitate wax include

cloud point, which is the temperature at which the wax, coming out of

solution, first becomes visible as the fuel is cooled (ASTM test D 2500).

Another test (ASTM 3117) is for the wax appearance point, which is that at

which the wax crystals become visible in a swirling sample of fuel.

Then there is the pour point, which is the temperature at which the quantity

of wax in the fuel is such as to cause it to begin to gel (ASTM D97). To

establish the pour point, checks on the condition of the fuel are made a 3°C

intervals by removing the test vessel from the cooling bath and tilting it to

see if the fuel flows. Another criterion, the gel point, is that at which the fuel

will not flow out when the vessel is held horizontal. In practical terms, this

translates roughly into a temperature 3° above that at which it becomes no

longer possible to pour the fuel out of a test tube.

Another way of estimating operational performance of a fuel is to combine

the cloud point, CP, and the difference between it and the pour point, PP, to

obtain a wax precipitation index, WPI. The formula for doing this is as

follows—

1



WPI = CP – 1.3(CP – PP – 1.1) 2



Other tests include the cold filter plugging point (CFPP) of distillate fuels,

IP 309/80 and the CEN European Standard EN116: 1981. This the lowest

temperature at which 20 ml of the fuel will pass through a 45 µm fine wiremesh screen in less than 60 s. However, since paper element filters are now

widely used, the relevance of wire-mesh filters is open to question so, at the

time of writing, the CEN is debating the desirability of introducing a test

called the simulated filter plugging point. In the USA, the low temperature

fuel test (LTFT) is preferred to the CEPP test. The LTFT is the temperature

at which 180 ml of fuel passes through a 17 µm screen in less than 60 s. All

these and other tests are described in detail in Automotive Fuels and Fuel

Systems, Vol. 2, T.K. Garrett, Wiley.



17.18



Density



Because injection equipment meters the fuel on a volume basis, any variations

in density, because it is related to energy content, will affect the power

output. The greater the density of the fuel the higher will be both the power

and smoke. Fortunately, however, hydrocarbon fuels differ relatively little as

regards their densities.

Density is measured by the use of a hydrometer with scales indicating

specific gravity or g/m3. The sample should be tested at 15°C or the appropriate

correction applied. Figures for density differ from those for API gravity, or

deg API, in that the higher the number in deg API gravity, the lighter is the

fuel.



Fuels and their combustion



17.19



607



Volatility



The volatility of the fuel influences many other properties, including density,

auto-ignition temperature, flashpoint, viscosity and cetane number. Obviously,

the higher the volatility the more easily does the fuel vaporise in the combustion

chamber. Low-volatility components may not burn completely and therefore

could leave deposits and increase smoke. Within the range 350 to 400°C,

however, the effects of low volatility on exhaust emissions are relatively

small. The mix of volatilities is important: high-volatility components at the

lower end of the curve in Fig. 17.8 improve cold starting and warm-up while,

at the upper end components having volatilities that are too low increase

deposits, smoke and wear.



17.20



Viscosity



The unit of kinematic viscosity is the stoke, which is the time taken for a

certain volume of fuel at a prescribed temperature and a constant head to

flow under the influence of gravity through a capillary tube of a prescribed

diameter. That of absolute velocity is the poise, which is the force required

to move an area of 1 cm2 at a speed of 1 cm/s past two parallel surfaces that

are separated by the fluid. For convenience, the figures are usually expressed

in centipoises and centistokes (cP and cSt). The two are related in that cP =

cSt × Density of the fluid. The SI units are m2/s, and the CGS or stoke’s units

are cm2/s (see also Section 18.4).

Increasing viscosity reduces the cone angle of the injected spray and the

distribution and penetration of the fuel, while increasing the size of the

droplets. It will therefore affect optimum injection timing. An upper limit for

viscosity has to be specified to ensure adequate fuel flow for cold starting.

Lucas Diesel Systems quote a figure of 48 cSt at –20°C as the upper and, to

guard against loss of power at high temperatures, 1.6 cSt at 70°C as the

lower limit. BS 2869 calls for a maximum value of 5 cSt and a minimum of

2.5 cSt at 40°C. In Fig. 17.9, all these points are plotted and a viscosity

tolerance band established. The viscosity of the average fuel lies approximately

mid-way between the upper and lower limits.

Too high a viscosity can cause excessive heat generation in the injection

equipment, owing to viscous shear in the clearances between the pump plungers

and their bores. On the other hand, if it is too low, the leakage through those

clearances, especially at low speeds, can be so high that restarting a hot

engine can become impossible. This, however, is because the increase in

temperature of the fuel locally due to conduction of heat to the injection

system during a short shut-down period further reduces the viscosity of the

fuel in the pump prior to starting.



17.21



Smoke



When the engine is started from cold, white smoke comprising tiny droplets

of liquid, mainly fuel and water, increases as cetane number and volatility of

the fuel are reduced. It persists until the temperature has risen to the point at

which the droplets are vaporised in the engine and remain so until well after

they have issued from the exhaust tail pipe. The reason why the fuel droplets,

although surrounded in the combustion chamber by excess oxygen, remain



608



The Motor Vehicle



48.0



40



Kinematic viscosity of fuel, cSt



20



10



5.0

4



2.5

2



1.6



–20 –10



0



10

20

30

40

50

Temperature of fuel, °C



60



70



Fig. 17.9 A typical tolerance band for fuel viscosity is illustrated here. At 40°C, the

limits are from 2.5 to 5.0 cSt



unburnt is that, in the cold environment, not only do they not evaporate but

also their temperature never rises to that of auto-ignition.

Cetane number increases with the density and volatility, and varies with

composition of the fuel. Fuels having high cetane numbers are principally

the paraffinic straight-run distillates. However, because these have both high

cloud points and low volatility, a compromise has to be struck between good

ignition quality and suitability for cold weather operation.

Another product of combustion can be black smoke. This is formed because

the hydrogen molecules are oxidised preferentially so, if there is insufficient

oxygen in their vicinity, it is the carbon atoms that remain unburnt. High aromatic

content, viscosity and density and low volatility all increase the tendency to

produce black smoke.

Although aromatics tend to produce smoke, they also make a major

contribution to the lubricity of the fuel. Consequently, their removal can give

rise to abnormally high rates of wear of injection pumps, especially in

distributor-type pumps in which all the work is done by only one or two

plungers and, perhaps, a single delivery passage might be subject to severe

erosion.

As previously explained, the volatility factor is misleading. Because the

more volatile fuels have high API gravities (indicating low specific gravities),

and their low viscosities allow more to escape back past the injection pumping

elements, the weight delivered to the cylinder per injection is smaller. For

any given power, a certain weight of fuel must be injected, so a greater

volume of the more volatile fuel and a longer injection period are therefore



Fuels and their combustion



609



required, and this might affect droplet size. Depending on the air movement

and other conditions in the combustion chamber, such changes could have

either a beneficial or adverse effect on combustion and therefore smoke

generation.



17.22



Particulates



Even though more than 90% of the total mass of particulates in the exhaust,

Fig. 17.10, is carbon, sulphur compounds are a problem too. Moreover, as

advances in engine and injection equipment design lead to reduced carbon

particulates from both the fuel and lubricating oil, sulphur could become a

significantly higher proportion of the total. Removal of sulphur from the fuel

is a costly, though necessary, process.



17.23



Additives



Until the mid-nineteen-seventies, the diesel fuel generally available in the

UK and Europe was of a very high quality. Subsequently, on account of a

shortage of appropriate crude oils, the quality world-wide showed a tendency

to fall, though not as rapidly as had been expected. Since fuel reserves are

being consumed at an increasing rate, the trend inevitably will remain

downwards unless some economical way of synthesising high quality diesel

fuel in the enormous quantities required can be developed. The trend has

already led to the introduction of additives many of which hitherto had been

entirely unnecessary and therefore not even under serious consideration.

Even so, there is no incentive for the oil companies to use additives unless

they are cost-effective. Some are sold in the after-market but, unless the

purchaser knows in detail the content of his fuel, he could be either paying

for products that do not suit it or simply adding to what is already present at

saturation level, in which event the extra will make little or no difference. A

possible exception is where a commercial vehicle operator has a large residual

stock of fuel bought in bulk during the summer, which he needs to convert

by adding anti-wax additives for winter use.

Residue

Sulphur

dependent

NOx



HC



HC

Soot

Soot



CO



Pollutants



Particulates



Fig. 17.10 Typical analysis of the pollutants in the exhaust from a diesel engine



610



The Motor Vehicle



17.24 The effects of additives on combustion and

performance



Type of operation



One of the early entrants, in 1988, into the multi-additive diesel fuels market

was Shell’s Advanced Diesel. This contains several additives, one of which

has raised its cetane number from the 48 required by British Standards, and

the minimum of 50 for Shell’s base fuel, to typically between 54 and 56.

Among the others are a corrosion inhibitor, anti-foam, cold flow and reodorant additives. The benefits claimed include lower noise, 3% better fuel

economy, Fig. 17.11, 8.4% less black and white smoke Fig. 17.12, a general

improvement in overall engine performance and durability, and a reduction

in vehicle downtime.

Of all the additives available, the most obviously important to the operator

are the cetane improvers and those that help to overcome the tendency to

wax precipitation in winter. Others used include anti-oxidants, combustion

Road tankers

City delivery

Long haul

Long haul

City delivery

Short haul

Road tankers

Bus fleet

Bus fleet

Bus fleet

Commercial delivery

Road tankers

Average saving



5.39%

0.27%

2.2%



0



1



2

3

4

5

Fuel saved, %



6



Fig. 17.11 Considerable scatter of results were apparent in fuel consumption trials

instituted by Shell on several different fleets and types of operation. Among the

variables of course are driver habits

White

smoke



Black

smoke



8.4%



10



% 20

25%

30



Fig. 17.12 Results of tests carried out by Shell to show that their Advanced Diesel

fuel offered a reduction of 25% in white and 8.4% in black smoke as compared with

another commercially available fuel



Fuels and their combustion



611



improvers, cold flow improvers, corrosion inhibitors, detergents, re-odorants,

anti-foamants and, less commonly, stabilisers, dehazers, metal deactivators,

biocides, anti-icers, and demulsifiers. Anti-static additives are used too, but

mainly to benefit the blenders by facilitating storage, handling and distribution.



17.25



Cetane number and cetane improvers



Because cetane number is a measure of the ignitability of the fuel, Fig.17.13,

a low value may render starting in cold weather difficult and increase the

tendency towards the generation of white smoke, Fig. 17.14. It also increases

the ignition delay (interval between injection and ignition). As a result, fuel

must be injected into the combustion chamber earlier. Consequently, more

fuel is injected before ignition occurs, so the ultimate rate of pressure rise is

more rapid, and the engine therefore noisy. Also, because the fuel has less

time to burn before the exhaust valve opens, the hydrocarbon emissions are

increased.

On the other hand, if the cetane number is higher than that for which the

injection system is timed, power will be lost because a high proportion of the

pressure rise will occur when the piston is at or near TDC. Furthermore, the



Fig. 17.13 Combustion sequences photographed through a quartz window in the

piston crown of an engine running on (top) a commercially available alternative fuel

and (bottom) Shell Advanced Diesel. The two part circles are the inlet and exhaust

valve heads and the bright spot to the left of the cylinder is an illuminated pointer over

degree markings on the flywheel, which are too small to be visually identifiable here



Fig. 17.14 Two photographs showing white smoke in the exhaust gases of an engine

running on (left) a commercially available fuel and (right) Shell Advanced Diesel



612



The Motor Vehicle



fuel might ignite before it has mixed adequately with the air, so smoke and

hydrocarbon emissions may be experienced. Consequently, fuels having high

cetane numbers perform best when the injection is retarded. Conversely,

with too much retard, there will not be enough time for complete combustion,

so smoke and hydrocarbon emissions will again be the outcome. Since high

cetane numbers are difficult to achieve, the regulations in most countries

specify only the lowest limit that is acceptable.

Cetane improvers, mainly alkyl nitrates, are substances that decompose

easily and form free radicals at the high temperatures in combustion chambers.

Unfortunately, however, it is the fuels that have the lowest cetane numbers

that tend to respond least to cetane improvers.

Combustion improvers differ from cetane improvers in that they are mainly

organic compounds of metals such as barium, calcium, manganese or iron

and are catalytic in action. Barium compounds could be toxic, so interest

now centres mainly on the others. Although these compounds produce metalbased particulates, they also lower the auto-ignition temperature of the carbonbased deposits in both the cylinders and particle traps, causing them to be

more easily ignited.



17.26



Cold weather problems



A surprisingly small wax content can partially gel a fuel. Different countries

specify cold filter plugging points ranging from about –5°C in the Mediterranean region to –32°C in the far north. In practical terms, the basic consideration

is that the engine shall start at the lowest over-night soaking temperature

likely to be experienced in service. Once it has started, if the filter becomes

partially blocked, the rate of flow could be such that the return flow of warm

fuel to the tank is reduced, and therefore also the rate at which the temperature

of the fuel in the tank is raised.



17.27



Cold weather additives



All additives for cold weather operation modify the shapes of the wax crystals,

which otherwise tend to adhere to each other. They therefore come under the

general heading of wax crystal modifiers (WCMs). From Stoke’s law we

deduce that the rate of settling of crystals is directly proportional to the

square of their diameter times the difference between their density and that

of the fluid, and is inversely proportional to the viscosity of the fluid.

Consequently, small crystal size is the overriding need.

There are three types of modifier: pour-point depressants flow improvers

and cloud-point depressants. Which of these is used depends basically on

local requirements and the type of wax to be treated. The last mentioned is

mainly a function of the boiling range of the distillates and the country of

origin of crude oil. In general, cracked products contain high proportions of

aromatics, which have low cetane numbers, but they also contain less wax

and, moreover, dissolve what wax there is more readily than do straightdistilled fuels. Fuels having narrow boiling ranges form large wax crystals

that are less susceptible to treatment by additives than the smaller and more

regular-shaped crystals formed in fuels having wider boiling ranges.

The earliest WCM additives were the pour-point depressants (PPDs)

introduced in the 1950s. These modify the shape and reduce the size of the



Fuels and their combustion



613



wax crystals. The flat plates of naturally formed wax crystals tend to overlap

and interlock, and thus to gel the fuel. Those formed in the presence of a

PPD, however, are thicker and smaller, and some multi-axial needle crystals

are introduced between the platelets, all of which makes it more difficult for

them to interlock.

More effective are the flow improvers. These cause small multi-axial

needle crystals to form, instead of larger platelets. Moreover, by virtue of the

presence of some additive molecules between the crystals, they tend not to

adhere to each other. Although these crystals will pass through wire-gauze

strainers, they are stopped by the finer filters used to protect the injection

pumps and injectors. Even so, by virtue of their small size and the multiaxial arrangement of the needles, the unaffected liquid fuel still tends to pass

between them.

The small compact wax crystals tend to settle in the bottoms of tanks.

This is more of a problem in storage than vehicles, but wax anti-settling

additives (WASAs) can nevertheless play a useful part in the avoidance of

wax enrichment as vehicle fuel tanks become empty, especially in very cold

climates. These, too, modify the crystal formation, by both forming nuclei

and arresting growth. As the temperature of the fuel falls through the cloud

point, their nuclei form centres on which small wax crystals grow and

subsequently, other additive molecules attach to their surfaces and block

further growth. Primarily, these additives improve cold filterability, but they

also lower the pour point.

Although a cloud-point depressant by itself lowers the cold filter-plugging

point of a base fuel, the opposite effect may be obtained by using it in a fuel

containing also a flow improver. The improvement obtainable by cloudpoint depressants is generally only small, of the order of 3°C, and they are

costly. Therefore they are unattractive, except where the cloud point is included

as part of a diesel-fuel specification and the blender therefore wishes to

lower it.



17.28



Dispersants and corrosion inhibitors



The primary function of dispersant additives is to restrict the size of the

particles formed within the fuel at the high temperatures in the engine and,

additionally, to remove them from the metal surfaces. However, they must be

used continuously, otherwise gum deposits that form when they are not

present are dislodged when they are, and tend to block filters.

There are also dispersant modifiers, or detergents, which keep the surfaces

of the combustion chambers and injection nozzles clean. However, if used to

excess, some can actually cause gums to form.

Finally, there are corrosion inhibitors for protecting fuel-system components

and also bulk storage tanks and barrels.

Fuel surfaces can be oxidised in contact with air, causing the formation of

gums, sludges and sediments. Surface-active additives can help to prevent this,

but must be added immediately after refining the fuel and while it is still warm.



17.29



Detergents and anti-corrosion additives



Detergents are used mainly to remove carbonaceous and gummy deposits

from the fuel-injection system. Gum can cause sticking of injector needles,



614



The Motor Vehicle



while lacquer and carbon deposited on the needles can restrict the flow of

fuel, distort the spray and even totally block one or more of the holes in a

multi-hole injector, Fig. 17.15. The outcome can be misfiring, loss of power,

increased noise, fuel consumption, and hydrocarbon, CO, smoke and particulate

emissions in general, Fig. 17.16. Furthermore, starting may become difficult,

because the fuel droplets have become too large owing to the reduction in

flow rate.

Detergent molecules are characterised by, at one end, a head comprising

a polar group and, at the other, an oleofilic tail. The arms of the polar group

latch on to the metal and particulate molecules, Fig. 17.17. Those attached to

the metal form barrier films inhibiting deposition and, incidentally, offering

a degree of protection against corrosion, while those latched on to the

particulates are swept away with the fuel because their oleofilic tails carry

them into solution in it.



Fig. 17.15 Effects of carbon deposits on diesel injector sprays: (left) a new and (right)

a sooty injector



%

–25

–20

–15

–10

–5

0



CO



HC



(HC + NOx)Particles



Fig. 17.16 The left- and right-hand columns represent respectively the percentage

exhaust clean-up and actual output levels measured after 4000 and 15 000 miles

respectively during road tests with Shell Advanced Diesel fuel. The exhaust emissions

measurements were taken while running with the vehicle on a chassis dynamometer

immediately after it had completed its road mileage



Fuels and their combustion

Detergent

molecule

latched on

to carbon

particle



615



Carbon

particle

Free detergent

molecule

Detergent latched on to

metal molecules

Wall of manifold



Fig. 17.17 Molecules of detergent additives in fuels are held in solution by their tails,

while their polar heads latch on to the molecules of the contaminant. In application to

lubricants, some additives have molecules that also latch on to those of the metal

surfaces, leaving the tails to protect the latter from corrosion. Alternatively, different

additives, functioning in a similar manner but latching only on to the metal, may be

used for protecting it against corrosion



Anti-corrosion additives (perhaps about 5 ppm) are mainly used to protect

pipelines in which diesel fuel is transported, but no more than trace proportions

are likely to remain by the time it reaches the vehicle. Therefore, if vehicle

fuel-system protection is required too, the treatment must be heavier. As in

the case of the detergents, the polar heads attach themselves to the metal, but

the water repellent tails form an oily coating over the surface to protect it

against corrosive attack. In Fig. 17.18, test samples that have been left over

a long period in the base fuel are compared with those left in Shell Advanced

Diesel.



17.30



Anti-foamants and re-odorants



To allow the fuel tank to be completely filled more rapidly, and to avoid

splashing, surfactant anti-foamant additives are used. The inclusion of re-



Fig. 17.18 Specimens subjected to the ASTM D.665A corrosion test: (top) with Shell

Advanced Diesel and (below) in a commercially available alternative fuel