The octane rating refers to the antiknock characteristic of the gasoline. Various hydrocarbons present in gasoline have different antiknock characteristics. The most important compounds that give gasoline higher octane ratings and differentiate gasoline from other light petroleum products are isoparaffins, especially isooctane (2,2,4-trimethylpentane), and aromatic hydrocarbons. In general, the higher the relative content of isooctane and aromatics, particularly toluene, the higher the octane rating of the fuel (Hoffman and McKetta, 1992). The bulk PIANO composition, showing relative contents of major hydrocarbon groups in the fuel, provides a very useful cumulative parameter for fuel type differentiation. Examples presented in Table 2 demonstrate differences in bulk PIANO composition among light-end hydrocarbon fuels with different octane grades. Average values and ranges of bulk PIANO composition for dispensed gasolines are shown in Table 3.

Table 2. Bulk PIANO composition of three newly dispensed gasolines,
JP-4 fuel and aviation gasoline
Reference Fuel
Paraffins
Isoparaffins
Aromatics
Naphthenes
Olefins

Relative %
87 Octane Gasoline
9.59
38.34
38.61
6.10
7.36
89 Octane Gasoline
9.06
38.13
43.36
3.84
5.60
92 Octane Gasoline
7.48
39.68
43.36
3.26
6.22
JP-4 Jet Fuel
29.34
31.02
23.90
13.16
2.58
Aviation Gasoline
3.33
74.22
21.92
0.51
0.01


Table 3. Average and range of bulk PIANO composition of dispensed gasolines
Grade of Gasoline* Paraffins Isoparaffins Aromatics Naphthenes Olefins

Relative %
Regular leaded (5) Average 10.7 32.0 45.0 7.8 4.6
Range

10.4 - 11.0

31.7 - 32.5

44.1 - 45.3

7.5 - 8.0

4.3 - 5.0
Regular unleaded (12) Average 10.2 37.6 38.7 6.0 7.1
Range

8.4 - 11.3

32.5 - 40.3

32.9 - 43.7

3.2 - 9.7

1.5 - 13.7
Unleaded plus (8) Average 10.8 37.0 41.5 5.6 5.1
Range

8.7 - 13.3

34.5 - 40.0

40.0 - 42.9

3.6 - 6.6

2.9 - 9.6
Premium unleaded (12) Average 9.4 34.9 48.4 2.9 3.9
Range

7.1 - 13.6

26.2 - 39.7

43.4 - 59.8

1.2 - 5.3

1.0 - 7.3
Overall average 10.3 35.4 43.4 5.6 5.2

*Number in parenthesis denotes number of sample analysis averaged.

The measurement of the gasoline range hydrocarbon composition in environmental samples provides an essential set of chemical characteristics, which are important in: (1) determining source relationship and octane grade and (2) differentiating automotive gasoline from aviation gasoline or wide-cut jet fuel (mixture of gasoline range hydrocarbons with kerosene).

An example of this application for automotive gasoline, aviation gasoline, and jet fuel (JP-4) is shown in Figure 2. When unaltered, these fuels can easily be distinguished from one another by plotting their PIANO contents as a star diagram, where points on each of the five axes representing the relative content of corresponding compound classes are connected by straight lines. In addition to being a useful fingerprint tool, the distribution pattern of gasoline range hydrocarbons is also an important index reflecting the degree of environmental alteration of escaped fuel.


Fig 2. Star diagram showing comparison among automotive gasoline, aviation gasoline and jet fuel JP-4 based on bulk composition of C3 to C10 hydrocarbons.

Although gasoline manufactured by various refineries exhibit compositional differences, the overall composition of a finished gasoline is different from that of other fuels and is specific enough to be recognized as such even in altered environmental samples. Based on the average gasoline composition, a set of ratios, which allow for estimation of alteration by evaporation, water washing, and biodegradation (weathering ratios), can be established (Kaplan, et al., 1996). These ratios for two newly dispensed gasoline samples and average values for more than 50 groundwater and soil samples are shown in Table 4.

Table  4. Examples of changes in degradation ratios and bulk composition of gasoline

Parameter

87 Octane Gasoline

92 Octane Gasoline

Water Sample

Soil
Sample
Evaporation
n-pentane/n-heptane
2.1
1.4
0.8
0.5
2-methylpentane/2-methylheptane
7.2
4.6
2.0
0.8
Waterwashing
benzene/cyclohexane
4.3
9.6
19.3
3.2
toluene/methylcyclohexane
10.8
20.5
13.0
3.9
aromatics/total paraffins
0.8
0.9
4.6
4.2
aromatics/naphthenes
6.3
13.2
16.0
35.0
Biodegradation
(C4-C8 paraffins+isoparaffins)/C4-C8 olefins
6.7
8.1
5.2
13.9
3-methylhexane/n-heptane
1.6
1.5
1.1
1.2
methylcyclohexane/n-heptane
0.6
0.5
1.6
1.2
isoparaffins+naphthenes/paraffins
4.6
5.7
4.1
4.9
Octane rating
2,2,4-trimethylpentane/methylcyclohexane
2.2
5.1
0.4
2.3
PIANO composition
% Paraffinic
9.6
7.5
4.1
3.5
% Isoparaffinic
38.3
39.7
12.1
15.0
% Aromatic
38.6
43.4
75.1
76.9
% Naphthenic
6.1
3.3
4.7
2.2
% Olefinic
7.4
6.2
4.0
2.4