On-Line Mass Spectrometric Characterization of Hydrocarbons in Engine Exhaust Gases

Mass spectrometry provides a powerful and versatile method for the characterization of the unburnt and also the pyrolysed and partially oxidized gaseous hydrocarbon species present in exhaust gases. Flame ionization detection, the usual analysis method for measuring exhaust hydrocarbons, can give only a total hydrocarbon figure when used on-line. A mass spectrometer can perform the on-line characterization of the individual gaseous hydrocarbon species in the exhaust and can detect any trends in their concentrations. This permits the rapid assessment of experimental approaches for reducing these pollutants.

In the present work exhaust gases have been sampled from the exhaust of a Ricardo E6 research engine fuelled with gasoline and they pass to the analysis equipment via heated sample lines. The gases can be analysed as discrete samples by gas chromatography mass spectrometry (GC–MS) in order to identify the components, or can be continuously monitored by mass spectrometry alone (MS) in order to measure any trends in the component concentrations.

These mass spectrometric analysis techniques have been compared with other gas analysis and general data acquisition methods, and have permitted the collection of much information about engine exhaust emissions. This information has been related to engine operating parameters with special reference to the fuel–air ratio.

The results from this work show that not only does the combustion result in an increase in the relative amounts of NO and CO₂, but also suggest that the substituted aromatic hydrocarbons may be products of combustion. Continuous monitoring of specified exhaust components has been performed mass spectrometrically and related to the air–fuel ratio used for the engine. With lean fuels, the hydrocarbons are not totally combusted, but those that are burnt are combusted with reasonable efficiency. On the other hand, rich mixtures are associated with not only inefficient combustion, but also incomplete oxidation (that is CO–CO₂ ratio is increased), and an increase in the substituted aromatic hydrocarbons. Stoichiometric mixtures have been found to be associated with most efficient combustion (highest CO₂–CO ratio) and minimal hydrocarbon emissions.