Φ-Sensitivity for LTGC / ACI Engines: Understanding the Fundamentals and Tailoring Fuel Blends to Maximize this Property 2019-01-0961
φ-sensitivity is a fuel property that has important benefits for the operation and control of 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 HRR peak. This allows higher loads without knock and/or CA50 to be advanced for higher efficiencies. Second, CA50 can be controlled by the late-DI settings of a multiple-injection strategy. Finally, experiments show that ITHR (intermediate-temperature heat release) increases with φ-sensitivity, increasing the allowable CA50 retard and stability.
CHEMKIN simulations were performed using a detailed mechanism 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 regime, φ-sensitivity is lower and almost constant with changes in operating condition.
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 4-component blends that fulfill EPA regulations. Fuel mixtures are composed by a combination of olefins (1-hexene), paraffins (2-methyl-hexane and iso-octane), aromatics (p-xylene) and alcohols (iso-butanol, which was recently approved by the EPA for gasoline blending). This study shows that it should be possible to have both high φ-sensitivity and high RON with good octane sensitivity.
Dario Lopez Pintor, John Dec, Gerald Gentz