Use Of Petroleum In The Internal Combustion Engine

One of the major uses of petroleum as an energy source is in the automobile internal combustion engine. Most automobiles are driven by a four-stroke-cycle piston engine (Figure 6-10). Gasoline is mixed with air in the carburetor and drawn into the cylinder on the first stroke. The oxygen-gasoline vapor mixture is compressed on the second stroke and ignited by the spark plug. The expanding gases formed by combustion then drive the piston down on the third stroke, the stroke that provides the power to drive the car, and the combustion products are forced out of the cylinder on the fourth stroke.

The first automobiles had low-compression engines that could use the gasoline fraction distilled from crude oil (Figure 6-1), but as the automobile was improved, more powerful engines were developed. These high-compression engines required a more sophisticated fuel than straight-run gasoline. In high-compression engines, the hydrocarbons of simple petroleum distillates preignite before the top of the stroke of the compression cycle. Preignition decreases engine power because it results in an increase in the pressure on the piston before it has completed the upward stroke of the compression cycle. The preignition usually occurs explosively (knocking), which jars the engine and hastens its eventual demise.

The octane rating of gasoline is a measure of its tendency for preignition and detonation. The higher the research octane number (RON), the less likely that

Stroke 1

Stroke 2

Stroke 3

Stroke 4

Stroke 1

Stroke 2

Stroke 3

Stroke 4

FIGURE 6-10 Schematic of a four-stroke-cycle engine.

knocking will occur. The octane rating is an empirical number that is measured on a standard test engine. It relates to the performance of the automobile engine when it is operating at low to medium speeds. The RON values of some representative compounds are listed in Table 6-2. The values of 0 for n-heptane and 100 for isooctane were established as arbitrary standards.

A variation of the octane scale designated motor octane number (MON) is also used to rate gasolines. The MON scale was established when it was observed that there was not good agreement between the RON determined in a test engine and that of the motor run under road conditions. The higher speeds and heavier weight of contemporary automobiles resulted in lower octane ratings in road tests than were observed in the test engine. The same test engine is used for both the MON and RON, but the operating conditions for MON determination give octane ratings that correlate more closely with those observed with an engine running at medium to high speed. The octane rating listed on gasoline pumps is an average of the RON and MON. It should be emphasized that octane ratings do not measure the energy released on combustion—all hydrocarbons release about the same amount of energy per gram when burned completely—but instead are a measure of the tendency for preignition and explosive combustion.

A number of methods have been developed to increase the RON rating (55-72) of straight-run gasoline. It was discovered in 1922 that the RON of straight-run gasoline could be increased to 79-88 by the addition of 3 g of "lead" per gallon. The "lead" or "ethyl fluid" that is added to gasoline is a mixture of about 60% tetraethyl lead [Pb(CH2CH3)4], and/or tetramethyl lead [Pb(CH3)4], about 35-40% BrC^C^Br and ClCH2CH2Cl, and 2% dye,

TABLE 6-2

Research Octane Numbers (Octane Ratings) for Some Representative Hydrocarbons

TABLE 6-2

Research Octane Numbers (Octane Ratings) for Some Representative Hydrocarbons

Compound

RON

n-Octane

-19.0

n-Heptane

0"

n-Hexane

24.8

n-Pentane

61.7

2,4-Dimethylhexane

65.2

Cyclohexane

83

n-Butane

93.8

2,2,4-Trimethylpentane (isooctane)

100"

2,2,4-Trimethylpentene-1

103

Benzene

106

Toluene

120

"Standards for RON measurement.

"Standards for RON measurement.

solvent, and stabilizer. The lead prevents preignition by binding to hydroper-oxyl free radicals, reactive intermediates with unpaired electrons, formed in the combustion process:

This quenching decreases the combustion rate and prevents preignition. Leaded gasoline is no longer used in the United States and is discussed further in Section 6.7.4.

Catalytic re-forming, the conversion of aliphatic compounds to aromatic compounds, is the process of isomerization of linear aliphatic hydrocarbons to cyclic derivatives that are then dehydrogenated to aromatics as shown in reaction (6-6). The reaction pathway is determined by the nature of the binding of the hydrocarbons to the surface of a platinum or platinum-rhenium catalyst that is used. A product with a RON of 90-95 is obtained. In reaction (6-6), n-heptane is used to illustrate the cyclic compounds proposed as intermediates in the catalytic re-forming process.

CH3(CH2)5CH3-

ch2ch3

Ethanol and other oxygenated organic compounds that enhance the octane hydrocarbon mixtures are discussed in Section 6.7.4. Apparently when compressed in the cylinder of the automobile engine, the substituted aromatics and oxygenated organics also react with hydroperoxyl and other free radicals to inhibit the preignition of gasoline vapors.

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