Petroleum Laboratory

Gasoline’s vapor pressure is the pressure exerted by gasoline vapor in thermodynamic equilibrium with its liquid phase at a given temperature in a closed system. The vapor pressure is an indication of the gasoline’s tendency to evaporate and thus, a very important physical property of volatile gasoline products. Fuels that vaporize too readily in pumps, fuel lines, carburetors, or fuel injectors will cause decreased fuel flow to the engine, resulting in rough engine operation or vapor lock, especially in mountain driving. In contrast, fuel that does not vaporize readily may cause hard starting and poor warm-up drivability and acceleration. The vapor pressure of gasoline (see Table 1) and ethanol fuel blends (see Table 2) is regulated by various government agencies to ensure fuel products of suitable volatility performance throughout seasonal variations in temperatures.

Table 1 – Vapor Pressure Requirements for Gasoline within the State of Colorado

EPA: Environmental Protection Agency
Note: The Environmental Protection Agency (EPA) allows 1.0 psi higher vapor pressure for gasoline-ethanol blends containing 9 to 15% of ethanol. Reformulated gasoline is excluded from this waiver during the ozone season (June to September 15th).

¹Maximum Vapor Pressure at 37.8 °C (100 °F) for Reformulated Gasoline required in the severe ozone nonattainment areas (Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, Jefferson, parts of Larimer and Weld counties). Figure 1 depicts the severe ozone nonattainment areas in which these specific vapor pressure limits have to be met to improve air quality.
 

Figure 1 - Severe Ozone Nonattainment Area in Colorado

Table 2 – Vapor Pressure Requirements for Ethanol Fuel Blends (containing 51-83% ethanol) within the State of Colorado] 

Note: See Figure 2 for a 105° Longitude Map

Figure 2 - Colorado’s East and West of 105° Longitude

The Colorado State Fuel Quality Laboratory follows ASTM D5191-22, in which a chilled sample is introduced into a thermostatically controlled, evacuated test chamber (see Figure 3). The test specimen is then allowed to reach thermal equilibrium at the test temperature, 37.8 °C (100 °F). The resulting rise in pressure in the chamber is measured using a pressure transducer sensor and indicator.

Figure 3 – Automated Vapor Pressure Instrument used at the Colorado State Fuel Quality Laboratory for the determination of the gasoline’s vapor pressure

The vapor-liquid ratio of fuel indicates the tendency of a fuel to vaporize in automotive engine fuel systems. This ratio is the temperature at which gasoline forms a ratio of vapor volume to liquid volume that is equal to 20: T(V/L = 20). Automotive fuel specifications generally include T(V/L = 20) limits to ensure products have a suitable volatility performance. For high temperatures, a fuel with a higher T(V/L = 20) value, which has a lower tendency to vaporize, is specified (Table 1). Conversely, for low temperatures, a fuel with a lower T(V/L = 20)   value is specified (Table 1). 

Table 1 – Minimum Temperature [°F] for a Vapor/Liquid Ratio of 20.

The Colorado State Fuel Quality Laboratory follows ASTM D5188-16, which covers the determination of the temperature at which the vapor formed from a selected volume of volatile petroleum product (saturated with air at 0 °C to 1 °C (32 °F to 34 °F) produces a pressure of 101.3 kPa (one atmosphere) against vacuum. The instrument used in the Colorado State Fuel Quality Laboratory is depicted in Figure 1.

Figure 1 – Automated Vapor Pressure Instrument used at the Colorado State Fuel Quality Laboratory for the determination of the gasoline’s T(V/L = 20)

The test method of determining the boiling range of a petroleum product by performing distillation has been in use for as long as the petroleum industry has existed. It is one of the oldest test methods under the jurisdiction of ASTM. The distillation (volatility) characteristics of hydrocarbons have an important effect on their safety and performance, especially in the case of fuels. The boiling range gives information on the composition, the properties, and the behavior of the fuel during storage and use. Volatility is the major determinant of the tendency of a hydrocarbon mixture to produce potentially explosive vapors. The distillation characteristics are critically important for automotive gasoline and diesel fuels, affecting starting, warm-up, drivability, dilution of the engine oil, fuel economy, carburetor icing and tendency to vapor lock at high operating temperature or at high altitude, or both. The distillation temperatures are used to characterize the volatility of gasoline and diesel fuels and to determine whether the distillation properties meet the ASTM specifications (Table 1, Table 2, Table 3).

Table 1 – Distillation Temperature [°F] Requirements for Gasoline Fuels at Percent Evaporated

at 101.3 kPa (760 mm Hg)

Table 2 – Distillation Temperature [°F] Requirements for Diesel Fuels at 90 % Evaporated

at 101.3 kPa (760 mm Hg); Minimum 90 % recovered temperature is waived if cloud point is at or below 10.4 °F.

Table 3 – Distillation Temperature [°F] Requirements for Ethanol Fuel Blends at End Point 

The Colorado State Fuel Quality Laboratory follows ASTM D86-23. A 100 mL sample is distilled under prescribed conditions for the specific fuel sample. The distillation is performed in a laboratory batch distillation unit (Figure 1) at ambient pressure. Systematic observations of temperature readings and volumes of condensate are made automatically. The volume of the residue and the losses are also recorded.

Figure 1 – Distillation Unit used at the Colorado State Fuel Quality Laboratory for the quantitative determination of boiling range characteristics of gasoline fuels 

Ethers, alcohols, and other fuel oxygenates can be added to gasoline to increase octane number for greater efficiency and power and to reduce emissions. Types and concentrations of various oxygenates are specified and regulated to ensure acceptable commercial gasoline quality. Drivability, vapor pressure, phase separation, exhaust, and evaporative emissions are some of the concerns associated with oxygenated fuels.

The Colorado State Fuel Quality Laboratory uses ASTM D4815-22 to determine ethanol contents of gasoline samples. However, samples of ethanol fuel blends with high ethanol concentrations (e.g., E85) are analyzed following ASTM D5501-20, and shall contain 51-83 % of ethanol.

In brief summary, a gasoline sample is introduced into a gas chromatograph (Figure 1) equipped with two columns and a column switching valve. The samples are broken down into their components by being washed by a solvent. . The components are then detected by a thermal conductivity detector. The detector response, proportional to the component concentration, is recorded; the peak areas (Figure 2) are measured; and the concentration of each component is calculated.

Figure 1 – Gas Chromatograph
 

Figure 2 – Schematic depiction of various fuel oxygenates peaks as analyzed by a gas chromatograph

The driveability index is a measure of a gasoline’s performance across a number of operating conditions, covering cold start and warm-up operations. Its calculation incorporates elements of the distillation profile for the gasoline (10%, 50% and 90% evaporated distillation temperatures) identified by test method D86-23 as well as the gasoline’s ethanol content quantified by test method D4815-22. 

The driveability index requirements for Colorado are summarized in Table 1.
 

Table 1 - Driveability Index (DI) Requirements for Colorado for gasoline containing 0-15 % of ethanol by volume.

The octane number is a measure of the antiknock performance of a gasoline or gasoline component. The higher the octane number, the greater the fuel’s ability to resist engine knock. There are two types of octane numbers – the Research Octane Number (RON) and the Motor Octane Number (MON), which are based on different engine operating conditions. RON simulates low speed (idling), which affects low to medium speed knock. MON simulates higher speed (acceleration), which affects high-speed knock or ping. Octane number is not a measurement of a fuel’s power, but a measurement of the temperature at which the fuel ignites when compressed with a spark.

Both RON and MON are based on the knocking tendencies of pure hydrocarbons. n-heptane has an assigned value of zero and isooctane a value of 100. The octane number of a fuel is the percentage of isooctane in a blend with n-heptane that gives the same knock intensity as the fuel under test when evaluated under standard conditions in a standard engine.

Antiknock Index (AKI) is the average of RON and MON (or (R+M)/2), which is the number posted on gas station pumps (Figure 1). 

Figure 1 – The Antiknock Index (AKI) posted on gas station pumps

The Colorado State Fuel Quality Laboratory quantifies the knock ratings of engine fuels in accordance with ASTM D2699-21 and ASTM D2700-19. 

RON and MON of a spark-ignition engine fuel are determined using standard test engines (Figure 2) and operating conditions to compare its knock characteristics with those of reference fuel blends of known octane number. Compression ratio and fuel-air ratio are adjusted to produce a standard knock intensity for the sample fuel, as measured by a detonation measurement system.