ONLINE MASS-SPECTROMETRIC CHARACTERIZATION OF HYDROCARBONS IN ENGINE EXHAUST-GASES

Mass spectrometry provides a powerful and versatile method for the cha racterization of the unburnt and also the pyrolysed and partially oxid ized gaseous hydrocarbon species present in exhaust gases. Flame ioniz ation detection, the usual analysis method for measuring exhaust hydro carbons, can give only a total hydrocarbon figure when used on-line. A mass spectrometer can perform the on-line characterization of the ind ividual gaseous hydrocarbon species in the exhaust and can detect any trends in their concentrations. This permits the rapid assessment of e xperimental approaches for reducing these pollutants. In the present w ork exhaust gases have been sampled from the exhaust of a Ricardo E6 r esearch engine fuelled with gasoline and they pass to tire analysis eq uipment via heated sample lines. The gases can be analysed as discrete samples by gas chromatography-mass spectrometry (GC-MS) in order to i dentify the components, or can be continuously monitored by mass spect rometry alone (MS) in order to measure any trends in the component con centrations. These mass spectrometric analysis techniques have been co mpared with other gas analysis and general data acquisition methods, a nd have permitted the collection of much information about engine exha ust emissions. This information has been related to engine operating p arameters with special reference to the fuel-air ratio. The results fr om this work show that not only does the combustion result in an incre ase in the relative amounts of NO and CO2, but also suggest that the s ubstituted aromatic hydrocarbons may be products of combustion. Contin uous monitoring of specified exhaust components has been performed mas s spectrometrically and related to the air-fuel ratio used for the eng ine. 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 com bustion, but also incomplete oxidation (that is CO-CO2 ratio is increa sed), and an increase in the substituted aromatic hydrocarbons. Stoich iometric mixtures have been found to be associated with most efficient combustion (highest CO2-CO ratio) and minimal hydrocarbon emissions.