Characterisation of semi-volatile hydrocarbon emissions from diesel engines

Abstract

Diesel exhaust emissions from vehicles have been receiving global attention, due to the potential human health and negative environmental effects associated with exposure to emitted pollutants. Air pollutants emitted by diesel engines include hydrocarbons which, despite very low concentrations in the exhaust, may act as precursors in the formation of secondary pollutants such as photochemical ozone and secondary organic aerosols, which play a vital role in photochemical smog pollution.

The environmental impact of diesel engines is poorly quantified and understood, with much debate on the role played by emitted semi-volatile organic compounds (SVOCs). Research shows that while short chain hydrocarbons (HCs), typically emitted by petrol vehicles, are easier to characterise and have been successfully reduced in many cities, longer chain semi-volatile hydrocarbons, such as those emitted by diesel vehicles, are typically not considered as part of air quality control strategies, and a limited range of these hydrocarbons have been studied.

Development of an ideal method for collection, analysis and characterisation of SVOCs emitted by diesel engines is thus necessary to determine the link between these emissions and photochemical smog. The implementation of increasingly stringent emission limits also brings about the need for a method to determine the effect of emissions control technology, i.e. fuel formulation, catalytic systems and engine technology. An emissions monitoring campaign was therefore conducted in this study under controlled laboratory conditions, using a Euro 3 compliant 1.6 L passenger car diesel engine, operating over a standard test cycle typical of urban driving conditions. Cold and hot start emissions tests were performed using three different fuels (a paraffinic diesel, a South African market diesel, and a European reference diesel). Changes in emissions at different speed phases as well as the role played by exhaust aftertreatment technology on emissions control was also investigated.

During testing, a portion (1/100) of the exhaust was diluted with compressed air within a partial flow dilution system, and simultaneous sampling of the diluted gaseous and particulate exhaust emissions was achieved by means of portable denuder sampling devices consisting of a quartz fibre filter sandwiched between two multi-channel polydimethylsiloxane traps. Instrumental analysis of the traps was performed by thermal desorption-two dimensional gas chromatography-time of flight mass spectrometry and targeted analysis of 43 compounds (30 aromatic HCs and 13 n-alkanes) was conducted.

It was found that the South African market diesel had the highest total n-alkane emissions (26.91 – 255.71 mg/km from the extra high to the low phase), followed by the paraffinic diesel (34.77 – 162.05 mg/km) and the European reference diesel (21.63 – 63.97 mg/km). Qualitative analysis of the fuels’ alkylbenzene emissions was conducted and the European reference diesel had the highest emission factors followed by the South African market diesel and paraffinic diesel, respectively. In addition, for the fuels containing aromatic compounds (SAM10 and EUR10), 1-methyl-3-ethylbenzene, 1-methyl-4-ethylbenzene, 1,3,5-trimethylbenzene, 1-methyl-2-ethylbenzene, 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene, 1-methyl-3n-propylbenzene and n-butylbenzene were found in high abundance in the emissions, which was strongly reflective of the fuels which contained 6% and 4% of C9 and C10 compounds respectively. A decrease in emissions was observed with increasing periods of engine operation (low to extra high speed phases). The presence of exhaust aftertreatment technology, such as a diesel oxidation catalyst and diesel particulate filter, resulted in lower hydrocarbon emissions, particularly in the high and extra high speed phases. The observed changes in emissions could be correlated to the fuel composition, fuel physical properties and engine operating conditions.

The developed method illustrated the suitability of denuder samplers and thermal desorption-two dimensional gas chromatography-mass spectrometry for the collection and analysis of photochemical smog forming SVOCs in dilute exhaust emissions. Emission factors were successfully calculated and it was illustrated how the ozone formation potential of emissions can be estimated from the calculated emission factors, which is critical in understanding elevated ozone levels in urban environments.