The Effect of Gasoline Composition on Exhaust Emissions from Modern BMW Vehicles 941867

In a cooperative programme between BMW and Shell, the effects of gasoline properties and composition on regulated emissions (HC, CO, NO_x), CO₂, fuel consumption and catalyst performance have been studied. The objective of the test programme was to investigate the effect of different hydrocarbon groups from typical refinery streams on exhaust emissions with a detailed analysis not only of the tailpipe emissions but also engine out emissions and catalyst performance. In total thirteen fuels with widely varying physical properties and chemical composition were evaluated in a 1991 series production BMW 525i, and a subset of three of these fuels in two other BMW models to verify their sensitivity to fuel quality.

The results for the BMW 525i showed that significant reductions in HC, CO and NO_x emissions were seen for fuels containing splashblended oxygenates and with aromatics replaced by isoparaffins. Similar reductions in HC and CO emissions were seen in the other two vehicles, although the BMW 316i was somewhat less sensitive to fuel changes. It is not clear whether these reductions were due to chemical composition (i.e. MTBE and aromatics) or to associated changes in physical properties (i.e. fuel volatility). This important question will be addressed in the forthcoming European EPEFE programme (European Programme on Emissions Fuels and Engine Technologies). The effect of fuel quality on NO_x emissions varied for the different vehicles. The MTBE-containing, high volatility, low aromatic fuel reduced emissions from the BMW 525i, had negligible effect on the BMW 316i but increased emissions from the BMW 740i. These varying effects can be explained in terms of specific engine/catalyst/lambda effects.

The fuel effects were shown to be partly due to changes in engine-out emissions, but more to changes in catalyst efficiency, especially over the EUDC part of the test cycle where emission rates are relatively low, but the greatest relative fuel effects were seen.

Catalyst rig tests with varying fuel sulphur content between 10 and 100 ppm showed no significant effect on the conversion efficiency of the current platinum/rhodium catalyst. However a prototype palladium based catalyst showed greater sensitivity to fuel sulphur. Chassis dynamometer tests on the BMW 525i with platinum/rhodium catalyst showed a reduction in NO_x emissions when reducing fuel sulphur, but CO and HC emissions were unaffected.

The MTBE containing, volatile, low aromatics fuels gave small reductions in CO₂ emissions within the repeatability of the experiment, but showed significant increases in volumetric fuel consumption.