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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.

The EPEFE study (European Programme on Emissions, Fuels and Engine Technologies), /1/ and other programmes have identified an increase in tailpipe NOx emissions with reduced gasoline aromatics content for modern 3-way controlled catalyst vehicles. This effect occurs with fully warmed-up catalyst under closed-loop operation. In order to understand the reasons for this effect VW and Shell have mechanistically investigated the effects of fuel properties on EMS (engine management system) and catalyst performance. Fuels with independent variation of oxygen, aromatics and mid-range volatility were tested in different VW engines. λ was monitored using sensors located both pre and post catalyst. The results confirmed that reducing gasoline aromatics content reduced engine-out emissions but increased tailpipe NOx emissions. It could be shown that differences in H/C ratio led to differences in the hydrogen content of engine-out emissions which affected the reading of the λ sensor.

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.