Φ-sensitivity is a fuel characteristic that has important benefits for the operation and control of low-temperature gasoline combustion (LTGC) engines. A fuel is φ-sensitive if its autoignition reactivity varies with the fuel/air equivalence ratio (φ). Thus, multiple-injection strategies can be used to create a φ-distribution that leads to several benefits. First, the φ-distribution causes a sequential autoignition that reduces the maximum heat release rate. This allows higher loads without knock and/or advanced combustion timing for higher efficiencies. Second, combustion phasing can be controlled by adjusting the fuel-injection strategy. Finally, experiments show that intermediate-temperature heat release (ITHR) increases with φ-sensitivity, increasing the allowable combustion retard and improving stability.
A detailed mechanism was applied using CHEMKIN to understand the chemistry responsible for φ-sensitivity. For fuels with NTC behavior, φ-sensitivity is greatest in the NTC region due to enhanced ITHR reactions, which explains the experimental correlation between φ-sensitivity and ITHR. Under engine conditions, higher intake pressure means lower intake temperature to balance the reactivity, and both effects increase the φ-sensitivity. However, φ-sensitivity remains almost constant if decreased oxygen concentration is used to control the reactivity increase with intake-pressure boost because pressure and oxygen have opposite effects. Finally, for fuels without an NTC region, φ-sensitivity is lower and almost constant as operating conditions vary.
The potential of designing fuel blends that increase the φ-sensitivity compared to RD5-87 (regular E10 gasoline), while maintaining high RON and octane-sensitivity, was investigated. Higher φ-sensitivity and higher RON than RD5-87 can be reached with a 5-component blend that meets U.S. regulations. The fuel mixture is composed of a combination of 1-hexene, n-pentane, iso-octane, p-xylene and iso-butanol (which was recently approved for gasoline in the U.S.). This study shows that it is possible to have both high φ-sensitivity and high RON with high octane-sensitivity.