Hydrocarbon Speciation

In emissions research, hydrocarbon speciation is a powerful tool used to understand the combustion process and to observe the effects of varying gasoline and alternate fuel composition, fuel additives, and emission control hardware. The procedures, which identify and quantify exhaust constituents, can also be used to evaluate evaporative emissions.

Southwest Research Institute (SwRI) performs hydrocarbon speciation of gasoline, reformulated gasoline, and alternative fuel exhaust, identifying approximately 160 individual hydrocarbon components.

Using speciation data, Institute scientists evaluate the relative reactivity of exhaust and evaporative emissions. Different hydrocarbons in urban air form differing levels of ozone (smog) depending on the reactivities of these individual components. To evaluate hydrocarbon impact on ozone formation, complete speciation is required. The procedure also determines the presence and level of toxic emissions from automobiles and other sources.

Using novel sample collection and analysis techniques, SwRI also conducts hydrocarbon speciation for diesel exhaust, which is far more challenging than gasoline exhaust speciation. Diesel exhaust is comprised of hundreds of constituents, far more than are found in gasoline-derived exhaust.

Diesel Speciation

Diesel exhaust consists of heavier hydrocarbons, reflecting the composition of diesel fuel. While gasoline exhaust components typically range from C₁ to C₁₂, diesel fuel produces C₁₀ to C₂₂. Because hydrocarbons from C₁₂ to C₂₂ are too heavy to remain in a gaseous state at room temperature, traditional means of capturing exhaust gas samples are inadequate.

To collect samples, SwRI passes diesel exhaust through a cartridge packed with a solid sorbent that efficiently adsorbs hydrocarbons heavier than C₁₀ at moderate temperatures. Samples are subsequently desorbed into an analytical instrument by heating the cartridge while passing helium through it.

Heavy hydrocarbon speciation technology represents an important new development for automotive emissions researchers. The relative reactivity of diesel exhaust emissions can now be compared to those of other fuels. The technology will result in more data related to the effects of diesel fuel reformulation, fuel additives, and exhaust aftertreatment.

These screens represent steps in the speciation process for heavy hydrocarbons using mass spectrometry. At the top, a total ion chromatogram shows the intensity of response vs. elution time. Each peak represents an individual compound. For each compound, there is a unique mass spectrum, as depicted on the middle screen. The mass spectrum is compared to thousands of spectra in a reference database; when a match is found, the sample spectrum, its match, and the compound structure are displayed, as shown in bottom screen.

Hydrocarbon Speciation Applications

The Institute has conducted partial speciation of automotive exhaust since 1975 and full speciation since 1989. Capabilities now include speciating exhaust from engines and vehicles operating on a variety of fuels, including:

• Methanol and ethanol
• Alcohol-gasoline blends
• Compressed natural gas
• Liquefied petroleum gas
• Reformulated gasoline
• Diesel fuel

Using data from hydrocarbon speciation, SwRI investigates the effects of fuel properties on automotive exhaust and evaporative emissions to determine relative reactivities and/or toxicities. Hydrocarbon speciation data generated by SwRI helped establish California’s Phase II gasoline requirements and were used in the development of the EPA complex model.

Using speciation data, SwRI researchers have made a number of nonproprietary observations about the relationships between fuel property modifications and automotive emissions. While specific fuel reformulations can reduce hydrocarbon emissions from modern, catalyst-equipped vehicles, they can actually increase emissions from older vehicles. Some reformulations decrease the total mass of hydrocarbon emissions, but increase the mass of toxic emissions or increase the overall emission reactivity (ability to form ozone).

SwRI uses hydrocarbon speciation to evaluate catalyst efficiencies and emissions from various advanced-technology vehicles. This prototype emission control device is evaluated for its ability to reduce reactive hydrocarbons from vehicle exhaust.

The Institute has unparalleled hydrocarbon speciation equipment and facilities, including nine gas chromatographs like this one, each equipped with flame ionization detectors and columns selected for specific compounds and concentration ranges.

Sampling and analysis for aldehydes, ketones, and alcohols are conducted using SwRI-developed and CRC Auto/Oil methods.

The Institute’s liquid chromatographs are equipped with ultraviolet detectors to analyze aldehydes and ketones.

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