Commercial naphtha blends for partially premixed combustion

Partially Premixed Combustion has shown the potential of low emissions of nitrogen oxides (NOx) and soot with a simultaneous improvement in fuel efficiency. Several research groups have shown that a load range from idle to full load is possible, when using low-octane-number refinery streams, in the gasoline boiling range. As such refinery streams are not expected to be commercially available on the short term, the use of naphtha blends that are commercially available could provide a practical solution. The three blends used in this investigation have been tested in a single-cylinder engine for their emission and efficiency performance. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to legislated emission levels. Conventional diesel combustion benchmarks are used for reference to show possible improvements in indicated efficiency. Analysis of the heat release patterns revealed an interesting and strong correlation between the premixed fraction and the amount of soot produced. To be specific, each of the fuels showed a decrease in this fraction as either fuel pressure was lowered or load was increased, showing a transition from more premixed to mainly mixing-controlled combustion, with the corresponding soot emissions. For one blend, over the whole load range EURO VI PM levels were approached or achieved, combined with a peak gross indicated efficiency of 50% clearly indicating the potential of this concept.