Influence of Fuel Volatility on Emissions and Combustion in a Direct Injection Spark Ignition Engine

The purpose of this work was to investigate the influence of fuel parameters on emissions, combustion and cycle to cycle IMEP variations in a single cylinder version of a commercial direct injection stratified charge (DISC) spark ignition engine. The emission measurements employed both conventional emission measurement equipment as well as on-line gas chromatography/mass spectrometry (GC/MS).

Four different fuels were compared in the study. The fuel parameters that were studied were distillation range and MTBE (Methyl Tert Buthyl Ether) content. A European certification gasoline fuel was used as a reference. The three other fuels contained 10% MTBE. The measurements were performed at a low engine speed and at a low, constant load. The engine was operated in stratified mode. The start of injection was altered 15 crankangle degrees before and after series calibration with fixed ignition timing in order to vary mixture preparation time.

The measurements showed small differences between the fuels regarding hydrocarbon, nitrogen oxides and carbon monoxide emissions. In the chosen standard point for start of injection, the measurements show about 12% difference between the highest and the lowest HC emission levels for the tested fuels. The GC/MS quantified and identified the organic compounds with the highest concentrations in the exhaust. An estimated 65-75 % of the identified compounds consisted of components also present in the fuels. The HC emissions from this DISC engine mainly consist of aromatics and alkanes. Significant amounts of MTBE were detected in the exhaust for the fuels containing MTBE.