Branched versus linear alkane adsorption in carbonaceous slit pores

The presence of carbonaceous deposits on the internal surfaces of a spark ignition engine has been linked in the literature to impaired vehicle performance, as manifested by increased knocking, higher fuel consumption, higher emissions and other adverse effects. One of the proposed mechanisms, in which the deposits affect the processes in the engine, is the adsorption and desorption of fuel components in the pores within the deposit. In this article we investigate this mechanism in more detail by considering single component adsorption of normal and branched alkanes in selected model slit pores representing the structure of the deposits. We further extend these studies to the binary mixture of normal heptane and isooctane, corresponding to a primary reference fuel blend. In particular, we show that in larger pores adsorption selectivity towards isooctane is about 1.2 on average throughout the pressure range. However, in the smaller 10 Å pore selectivity towards isooctane can be in excess of three as a result of packing effects. These results are then placed in the context of engine performance issues.