FUELS | GASOLINE AND DIESEL | ALTERNATIVE FUELS

1.11 FUELS

1.11.1 Gasoline and Diesel

Crude oil is the primary source or feedstock used today to produce

transportation fuels, accounting for more than 95% of transport energy. The

gasoline (or petrol) and diesel fuel produced from crude oil are the

overwhelmingly dominant energy carriers used in SI and CI (or diesel)

engines, respectively. Crude oil contains a large number of different

hydrocarbons, and these compounds range from gases to liquids to waxes. In

a refinery, the crude oil is physically separated by distillation into various

fractions. Portions of these fractions are then chemically processed into fuels

and other products. 32 Figure 1.37 shows the layout of a typical refinery. 33

The crude oil is first separated into various fractions (each with a higher

boiling point range) referred to as naphtha, distillate, gas oil, and residual

oil. The terms light, middle, or heavy break these out further by volatility.

The terms virgin or straight run indicate that no chemical processing has

been done to the fraction. As shown, virgin naphtha can be used directly as

gasoline. Figure 1.37 also shows the different chemical processes typically

used: alkylation, reforming, coking, and catalytic cracking. The refinery

products are gasoline, kerosene and jet fuel, diesel, heating oil, burner fuel,

residual fuel oil, coke, chemical feedstocks, and asphalt. On average, a

refinery will refine 25 to 45% of the input crude oil into gasoline, 25 to 40%

into diesel, jet fuel, and heating oil, 5 to 20% into heavy fuel oils, and the

remaining 20% into other products.

Figure 1.37 Schematic layout (simplified) of crude oil refinery process

ṣGasoline is a blend of hydrocarbons with boiling points ranging from

about 25 to 200°C; diesel fuel is a blend of hydrocarbons with boiling points

ranging from about 160 to 350°C. Chemical processing is used to convert

one fraction into another to upgrade a given fraction so the refinery output

meets each fuels’ specifications. Alkylation is used to increase the fuel’s

molecular weight and knock resistance or octane number, h by adding alkyl

radicals to gaseous hydrocarbon molecules to create branched paraffinic

compounds (alkanes). Light olefin gases are reacted with isobutene in the

presence of a catalyst. This process consumes relatively little energy.

Catalytic cracking breaks up heavy oil molecules to produce a range of

lighter products including aromatics and olefins for use in gasoline. The

products of catalytic cracking go into high octane gasolines. The catalytic

cracking reactions occur at high temperatures so considerable energy is

consumed in this process. The reformer changes the molecular structure of

specific streams such as naphtha to yield higher octane gasoline (e.g.,

conversion of paraffins into aromatic hydrocarbons, and straight chain

hydrocarbons into branched hydrocarbons) in the presence of a catalyst.

Considerable hydrogen is produced that is used elsewhere in the refinery.

The coker converts the heavy reduced crude fraction to naphtha and distillate

fractions. The reduced crude is heated in an oven where the molecules

undergo pyrolytic decomposition and recombination. While the average

molecular weight of the fraction remains the same, a greater spectrum of

components are produced. The heaviest component is coke, a solid largely

carbonaceous material.

Gasoline, or petrol, is the dominant SI engine fuel. It is a blend of light

distillate hydrocarbons (paraffins, naphthenes, olefins, aromatics). Like all

liquid hydrocarbons, it has a very high chemical energy density—energy per

unit mass or volume. This chemical energy is stored in the carbon-hydrogen

and carbon-carbon bonds and is released as thermal energy when these

elements are oxidized by burning the fuel with air. In the United States, fuel

requirements for SI engines are defined by the American Society for Testing

and Materials in ASTM 4814. 35 The equivalent European Standard is EN

228. 36 The average molecular composition of a typical gasoline is close to

C 7H 13; it includes hydrocarbon molecules containing between four and ten

carbon atoms. The C:H atom ratio in commercial gasolines varies from about

1.6 to 2.2.

In terms of SI engine operation, the most critical gasoline properties are

its volatility and its resistance to autoignition (or knock) during the latter part

of the combustion process. A gasoline’s volatility is characterized by its

distillation curve, the volume fraction evaporated at atmospheric pressure as

a function of temperature. Typically 10% of gasoline vaporizes below about

50°C (T 10), 50% evaporates below 100°C (T 50), and 90% below 170°C (T

90). Winter gasolines are more volatile than summer gasolines. The fuel must

contain enough highly volatile components to ensure rapid and low-emission

cold starts. The lower temperature end of the distillation curve affects the

fraction of the fuel injected into the port that vaporizes under these

conditions. It also affects evaporative hydrocarbon emissions from the

vehicle’s fuel system. The upper end of the distillation curve is controlled to

reduce hydrocarbon emissions and lubricating oil dilution with fuel.

A fuel’s resistance to knock is defined by its octane rating. Two standard

tests (see Sec. 9.6.3 ) define the research octane number and motor octane

number of a fuel. The number displayed on gasoline pumps in the United

States, the antiknock index, is the average of these. A typical regular

(standard) gasoline would have a research octane number of 92, a motor

octane number of 82, with an antiknock index of 87. Historically, this

antiknock index correlated the knock resistance behavior of fuels in engines

in vehicles on the road. Modern engine antiknock requirements are better

correlated by the research octane number, which is the fuel’s octane number

used in Europe. At least two quality gasolines are usually marketed: regular

and one or more higher octane grades. Premium gasolines have higher

antiknock ratings (up to 98 RON) and often higher levels of additives that are

used to control deposit build-up tendencies in various parts of the engine and

fuel system. Lead alkyl additives were once widely used to improve the

antiknock resistance of gasolines. Since lead is highly toxic, and the catalytic

converters used in engines’ exhaust systems to reduce emissions (and the

oxygen exhaust sensors used for modern engine control) are poisoned by

lead, unleaded gasolines are now required in the developed world. While

leaded gasoline is still available in some countries, their number is steadily

decreasing.

Environmental authorities are imposing increasingly stringent regulations

for fuels (gasoline and diesel) to ensure low hydrocarbon evaporative

emissions and exhaust pollutant emissions. Thus reformulated fuels are now

widely used. The main characteristics of reformulated gasolines are reduced

vapor pressure (i.e., reduced low-end volatility), lower concentrations of

aromatic compounds and benzene, and a lower final boiling point (T 90—

temperature at which 90% of fuel is evaporated). In the United States,

additives that inhibit deposit formation in the engine intake are also required.

These reformulated gasolines usually contain oxygenated organic compounds

[historically, methyl tertiary-butyl ether (MBTE), now–ethyl alcohol]. In

some parts of the United States, these oxygenates are required in winter

gasolines (at least 2.7% oxygen) to reduce engine carbon monoxide

emissions. Year round, to encourage the use of biofuels, gasolines with up to

5, 10, or 15% ethanol can be marketed (amount depending on country or

region). Both MTBE and ethanol have higher octane numbers than the base

gasoline so they improve the fuels knock resistance. However, since MTBE

can contaminate drinking water supplies, its use in gasolines has been

discontinued. The sulfur levels in these clean unleaded gasolines are being

reduced from historical levels [some 300 parts per million (ppm) by mass]

to levels approaching 10 ppm. Sulfur is a catalyst poison that degrades the

effectiveness of exhaust emission-control catalyst systems.

Diesel fuels contain many individual hydrocarbon compounds with

boiling points ranging from about 180 to 370°C. As shown in Fig. 1.37, they

are a primary product of the crude oil distillation process. To meet the

growing demand for diesel fuel, refineries are adding increasing amounts of

conversion products through cracking and coking of heavy oil. Diesel fuel is

a mixture of paraffins, naphthas, olefins, and aromatics; these have higher

molecular weights (and different proportions) than these types of

hydrocarbons in gasolines. The average molecular composition is (CH 1.8) n,

and average molecular size is C 13H 24. The molecular weight range of the

diesel hydrocarbons is about 170 to 200. The density of diesel fuel is

important because fuel-injection systems are designed to deliver a specified

volume of fuel whereas the combustion characteristics depend on the fuel/air

mass ratio. The average diesel fuel density in Europe and the United States is

about 0.84 kg/liter; the density, in other major geographic regions, varies

between 0.81 and 0.86 kg/liter. Because the density of diesel fuel is higher

than gasoline, the chemical energy content (heating value) per unit volume of

fuel is about 10% higher. Other properties related to density are volatility

and viscosity. Fuel volatility is defined by the distillation curve and the flash

point. The distillation curve effectively defines the vaporization of the fuel

inside the engine cylinder. The flash point is the minimum temperature to

which the fuel must be heated to produce vapor that ignites in the presence of

a flame. The viscosity of diesel fuel affects the fuel’s performance in the

fuel-injection system; it affects both the fuel pump and injector behavior, and

the atomization process in the fuel injector nozzle holes. The low temperature

flow characteristics of the fuel in the fuel system (especially the fuel filter)

are a critical issue in regions with low winter temperatures where waxes can

precipitate out.

Diesel fuel must have appropriate autoignition characteristics—that is,

spontaneously ignite fast enough within the developing fuel sprays (see Sec.

1.8 ) to initiate combustion at the desired point in the engine cycle. This

characteristic of diesel fuels is defined by the cetane number, which

compares the autoignition characteristics of a diesel fuel with those of

defined reference fuels (see Sec. 10.5.2 ). Typical cetane numbers are in the

40 to 55 range; the higher the cetane number, the easier (more rapid) is

autoignition in the engine at around top center, just after injection starts. Fuel

composition is adjusted to provide the appropriate cetane number; it can be

enhanced by ignition improving additives—active oxidizers such as alkyl

nitrates. Diesel fuel property requirements are defined in ASTM D975.

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1.11.2 Alternative Fuels

Alternative fuels to the gasolines and diesel fuels produced from petroleum

are in limited use in IC engines, and are being explored for expanded use in

the future. Important driving forces are the need to reduce transportation’s

dependence on petroleum, and to reduce emissions of GHGs—especially CO

2. LPG and natural gas are in limited use as SI engine fuels in specific

applications. Ethanol, produced from biomass, to date is used primarily as a

gasoline blending agent. Alternative diesel fuels—for example, biodiesel

(methyl esters made from rape seed or soybeans); also dimethyl ether

(DME), Fischer-Tropsch gas-to-liquids fuels made from natural gas (or from

coal)—are being produced and explored. Hydrogen is being examined as a

possible longer-term SI engine fuel largely for its potential, once on the

vehicles, to be a non carbon-emitting energy carrier.

The more important properties of these alternative fuels, along with those

gasoline and diesel, are tabulated in the App. D. An important property of

these fuels relevant to their use in engines is their chemical energy content or

heating value per unit volume of fuel-vapor/air mixture, which has just

enough air for complete combustion (i.e., just enough for full oxidation of the

fuel’s carbon and hydrogen to CO 2 and H2O—the stoichiometric mixture

ratio). The basic objective of the SI engine’s intake process is to fill each

cylinder with such a mixture. In the diesel, this objective is met locally as the

fuel is combusted. While the heating value of these different fuels per unit

mass of fuel varies substantially, the differences in the composition of these

fuels (and thus the amount of air each fuel requires for complete combustion)

bring their heating values per unit volume of stoichiometric (chemically

correct) mixture to surprisingly similar values. Thus SI engine outputs (i.e.,

maximum power per unit displaced volume with other parameters held fixed)

over a wide range of fuels are closely comparable.

Note also that the combustion characteristics of these fuels when mixed

with air in engines are similar (with some important differences in details—

especially their anti-knock ratings). The exception is hydrogen which has a

much higher flame speed than the hydrocarbon fuels.

Liquid petroleum gas , which consists of C3 and C4 hydrocarbons, is

produced either by its removal from natural gas during the gas extraction

process, or from the refining of crude oil. The fraction of LPG used in

transportation varies widely (from a percent or so in the United States to

significantly higher levels in some other countries), as does the composition

of LPG. The dominant component is propane (C 3H 8): this can range from 30

to over 90%. Propylene, butanes, and ethane make up the remainder. In

vehicles, LPG is stored as a compressed liquid at pressures between about

10 and 15 bar, in cylindrical on-board tanks, at ambient temperature. In most

LPG fueling systems, the fuel is injected into the intake manifold in similar

manner to multipoint injection gasoline systems. A common fuel rail and one

injector valve for each cylinder inject the LPG (in liquid or gaseous form)

into each intake manifold, continuously or intermittently. 13

The components of LPG have higher octane ratings or knock resistance

than gasoline, so higher engine compression ratios can be used. However,

since the fuel is a gas with a lower molecular weight than gasoline, when

mixed with air, the volume occupied by the fuel becomes more significant.

LPG fuel therefore displaces more air than does gasoline vapor, and results

in a modest loss in power. Air pollutant emissions from a LPG-fueled SI

engine (prior to any catalyst in the exhaust) are usually lower than from

gasoline-fueled engines.

Natural gas is a primary energy source and carrier. It is largely (80 to

98%) methane, CH 4. The remainder is ethane (1 to 8%), propane (up to

2%), with varying amounts of nitrogen (from a few to 10%). Thus it has a

higher molar hydrogen:carbon ratio, so its CO 2 emissions on complete

combustion is lower than those of gasoline and diesel by about 25%.

However, methane itself is a GHG with a global warming potential about 20

times (per molecule) higher than CO 2. Natural gas leakage and unburned

methane emissions would need to be carefully controlled if natural gas is

used in transportation on a much larger scale.

Methane has a high octane number, or antiknock rating, though the other

hydrocarbon species in natural gas reduce this somewhat depending on their

relative amounts. Its combustion characteristics are comparable to those of

gasoline.

Natural gas is, of course, a gas and thus storing adequate quantities

storage on board the vehicle is a challenging problem. Storage as high pressure

compressed gas (35 MPa) is expensive and requires a substantial

volume even for limited vehicle range. At these conditions, the volumetric

energy density (MJ/m 3) of compressed natural gas is about one-quarter that

of gasoline. Also, because it is a gas with low molecular weight (methane’s

is 16), when natural gas is mixed with air to combustible proportions, it

displaces about 10% of the air (at fixed pressure and temperature) thus

reducing the torque that can be produced per unit displaced cylinder volume.

Similarly to LPG, natural gas is delivered to the engine intake manifold or

intake port via a pressure regulator low pressure common rail, and injectors

that inject natural gas intermittently into the intake. Because the fuel is a gas,

mixture formation is easier to control since there are no liquid fuel drops and

films to vaporize.

Alcohols, such as ethanol C 2H 6O and methanol CH4O, can be used as

SI engine fuels. Ethanol can be produced from biomass; methanol can be

produced from natural gas. These alcohols contain oxygen, so their chemical

energy content or heating value per unit mass is less than that of

hydrocarbons (0.45 for methanol and 0.61 for ethanol). Heating values per

The unit mass of stoichiometric mixture is 5% less than gasoline.

These alcohols can be used as stand-alone fuels, or blended with gasoline

at high or low fractions. These are designated E100 or M100 for ethanol or

methanol alone. Blends with about 15% gasoline (e.g., E85 or M85) are used

to broaden the volatility or vaporization characteristics of the single

compound alcohol. Ethanol is the more practical of these two fuels. A

The disadvantage of methanol is its toxicity. Ethanol is used extensively as a

transportation fuel in Brazil, where it is made from sugar cane. It is also

made from corn (and can be more efficiently made from cellulosic biomass

sources such as switchgrass and fast growing bushes and trees). Gasolines

blended with up to 10% ethanol are used in the United States and elsewhere

to both meet government requirements for oxygen content in “clean” loweremission

reformulated gasolines, and as an outlet for ethanol fuel.

Both ethanol and methanol have significantly higher antiknock or octane

ratings than gasoline; the value depends on whether the fuels are tested or

used as stand-alone fuels or in gasoline blends (see Table D4, App. D ).

Hydrogen can be produced from natural gas, coal, biomass, and by

electrolyzing water. Hydrogen for industrial use is currently largely made

from steam reforming of natural gas. Storage of hydrogen on board vehicles

is a major challenge. As compressed gas, at 70 MPa (10,000 psi), about the

maximum practical storage pressure, hydrogen has about one-third the energy

density per unit volume of natural gas. Liquid hydrogen, at 20 K, has about

one-fourth the energy density per unit volume of a hydrocarbon fuel. When

mixed with air to stoichiometric proportions, the hydrogen in the fuel-air

mixture occupies about 30% of the mixture volume, compared with less than

2% for gasoline vapor-air mixtures, reducing the power per unit displaced

cylinder volume at constant conditions.

The combustion characteristics of hydrogen are substantially different to

those of hydrocarbons, and thus gasoline. The laminar flame speed (a

fundamental combustion characteristics of fuel-air mixtures) in hydrogen-air

mixtures is several times higher than that of equivalent hydrocarbon-air

mixtures due to the much faster diffusion of hydrogen species within the

flame. Its flammability limits are broad (4 to 75% by volume at standard

atmospheric conditions) and its self-ignition temperature is lower than that of

gasoline. However, it has a higher knock resistance and octane number than

do gasolines.

Several other fuel’s pathways are potential alternatives to petroleum based

diesel fuel. Fuels made via the Fischer-Tropsch process from natural

gas are being produced from natural gas reserves that are difficult to

transport to the major natural gas markets. Fischer-Tropsch F-T fuels can

also be made from coal. The F-T process catalytically combines CO and

hydrogen, usually produced by reforming natural gas, to form CH 2 radicals

that are combined to form larger hydrocarbon chains. These fuels are

completely paraffinic, have very low levels of sulfur, and excellent

autoignition characteristics (a high cetane number). They would thus be

excellent diesel engine fuels.

Other potential diesel fuels are DME C 2H 6O, and biodiesel. DME is a

gas at ambient pressure and temperature; it can be liquefied at about 5 bar

pressure. It burns easily in a diesel engine, and its oxygen component helps

inhibit the formation of soot and diesel smoke. Vegetable oils, such as

rapeseed methyl ester and soybean methyl ester, can be processed to produce

biodiesel fuels. Their potential attractiveness is that they are produced from

biomass and thus are a possible low GHG emitting source of diesel fuel.

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