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While oxygen sensors provide the means by which changes in exhaust gas AFR (air-to-fuel ratio) are monitored and controlled in three-way catalyst systems, the chemistry of the exhaust gas in contact with this solid state electrochemical sensor can exert a substantial influence on its AFR control performance. Such interactions have been examined in a fundamental study on commercial oxygen sensors (unheated and heated), firstly using simple gas mixtures, and then simulated exhaust gas mixtures of progressively increasing complexity. The work confirms that diffusion effects at the sensor surface are centrally important in determining sensor response, but indicate that the effects of H2 (the smallest species present) do not necessarily dominate the observed behaviour. The results allow the development of a relationship that can be used to estimate the extent of the expected overall lean or rich shift for the sensor as a function of the exhaust gas composition.

As a first attempt to assess the scope for reducing the emissions from gasoline fuelled vehicles in the European market via fuel changes, an experimental scouting programme has been conducted to examine the effects of fuel composition and properties on both regulated emissions and detailed exhaust gas composition. This 4 fuels/4 vehicles programme involved exhaust emission tests on a chassis dynamometer, and was carried out mainly on non-catalyst vehicles, using two commercial gasolines and two (“reformulated”) test gasolines representing “intermediate” and “extreme” changes of the base gasoline design. The fuel characterization includes analysis by hydrocarbon type and carbon number; the exhaust analysis comprises both regulated emissions and hydrocarbon speciation measurements. In the case of regulated emissions, the fuel effects are, as expected, relatively small in comparison with the effect of installing exhaust catalysts.

Following a small scouting programme to examine the scale of emissions benefits achievable by different degrees of gasoline base fuel redesign (SAE 930372), a larger programme has been initiated to investigate more systematically the influence of individual fuel parameters on tailpipe emissions. This coordinated study has been spread across five participating Shell Group laboratories, using a set of common fuels specifically designed and centrally blended for this purpose. Additionally, subsets of these fuels have been used for detailed systematic examination of selected topics within the overall programme scope. This paper summarises the plan for the integrated study. It describes the composition and properties of the fuels and their blending. The results covered here are those of chassis dynamometer-based regulated emissions studies conducted on a composite fleet designed to represent a range of vehicle technologies, using a variety of regulatory driving cycles.

A fuels matrix with aromatics and mid-range volatility (T50) independently varied was applied to 2 fleets (catalyst and non-catalyst) consisting of vehicles currently driven in Europe. For the catalyst fleet, reducing aromatics or T50 gave lower HC/CO. After catalyst light-off, decreasing aromatics gave more NOx sufficient to determine the direction of the composite cycle response. This is fully consistent with recent EPEFE results (future technology vehicles), confirming the general applicability of the EPEFE conclusions. Mostly, HC/CO responses from the non-catalyst fleet were directionally similar, though statistically less robust. However, at high volatility, reducing aromatics increased HC/CO. NOx was reduced by lowering aromatics and, to a lesser extent, mid-range volatility.

An acknowledged consequence of utilising oxygenates such as MTBE as a gasoline component is known to be a lowering of CO exhaust emissions from mature technology vehicles due to the “natural” leaning effect that the inclusion of MTBE can provide. A small decrease in THC is also commonly seen in these circumstances, while the effect of MTBE on NOx emissions is more variable and not usually beneficial. The present paper describes the results of recent studies in the European arena, covering the effects of fuel oxygenates (notably MTBE) on regulated emissions for non-catalyst and catalyst car fleets examined in in-house programmes. It looks at emissions effects according to the broad classification of the onboard vehicle technology employed. It further cites experimental work that has featured MTBE replacement in gasolines by a single saturated hydrocarbon (2,3-dimethyl butane) that is isoelectronic with MTBE. Some related work conducted concurrently on splashblending is also described.