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Effects of Fuel Properties and Composition on Low-Load Gasoline Compression Ignition Strategies
Abstract:
The autoignition qualities of five so-called Fuels for Advanced Combustion Engines (FACE) gasolines with specific variations in fuel properties and two pump gasolines are evaluated in a Gasoline Compression Ignition (GCI) engine to study autoignition quality at low operating loads. A minimum intake pressure (MIP) metric was used in addition to traditional metrics such as the Research Octane Number (RON) and sensitivity. The results showed that a lower RON correlates with improved autoignition quality at low intake temperatures. At higher intake temperatures, the correlation between the RON and autoignition quality is seen to be poor. The effects of octane sensitivity were dominated by the general reactivity of fuel as characterized by RON. The Octane Index (OI) was calculated and a good correlation was seen between the OI and the intake pressure requirement. Statistical analysis showed that only RON had a statistically significant impact on the autoignition quality of the fuel. Analysis of the effects of fuel composition on the intake pressure requirement showed no statistically significant trends. Analysis of the stable operating window showed that both RON and sensitivity describe stability reasonably at low temperatures. In general, a fuel with a higher RON is predicted to have a smaller stable injection window than a lower RON fuel. Additionally, it was found that for a given RON, a wider operating window can be achieved with a lower sensitivity fuel. The intake temperature dependence was studied by analyzing the combustion event, and a strong correlation between stability and the presence of low-temperature heat release (LTHR) was found at low intake temperatures; at high intake temperatures, LTHR was seen to be reduced/completely absent for all the fuels studied. LTHR was found to not explain instability due to retarded injections. Additionally, pump gasolines showed optimal combustion stability at low loads, highlighting a possible pathway to on-road GCI.