Impact of Fuel Composition and Intake Pressure on Lean Autoignition of Surrogate Gasoline Fuels in a CFR Engine

The critical compression ratio (CCR) criterion (defined as the minimum compression ratio at which the fuel shows initial signs of autoignition) was examined for various gasoline surrogate fuels in a motored engine. This investigation builds on the concept of CCR which is a good indicator of a fuel's autoignition characteristics, to study the fuel compositional effects with increasing intake manifold pressure. The blends consisted of binary and ternary mixtures of n-heptane and/or isooctane, and a fuel of interest. These fuels of interest were higher octane components; toluene, ethanol, and iso-butanol. A lean condition (phi = 0.25) with varying intake pressure (atmospheric to 3 bar, abs) and at a constant intake temperature of 155 degrees C was used to investigate the ignition behavior of all the blends. Two sets of blends consisted of varying percentages of fuels of interest, formulated to approximately have research octane numbers (RON) at 80 and 100. For comparison, neat iso-octane was selected as the representative RON 100 fuel, and (Primary Reference Fuel) PRF 80 blend (20% n-heptane, 80% iso-octane, %v/v) was selected as the representative RON 80 fuel. The results were deduced based on engine-indicated data and exhaust emissions. It was observed that the blends with a higher percentage of n-heptane showed a stronger tendency to autoignite at lower intake pressures. However, as the intake pressure was increased, the lower reactivity components (in this study the highoctane components toluene, ethanol, and iso-butanol) hindered the radical formation in the low-temperature regime and/or delayed the onset of high-temperature heat release. The heat release analysis revealed that the higher-octane components in the blends reduced the low-temperature reactivity of n-heptane and iso-octane as the intake pressure was increased. In addition, distinctively different low-temperature heat release patterns were observed for blends consisting of alcohols and toluene as the intake pressure was increased, confirming distinctively different reaction mechanisms as well as inter component interactions in the blends.