Exploring and Modeling the Chemical Effect of a Cetane Booster Additive in a Low-Octane Gasoline Fuel

Increasing the internal combustion engine efficiency is necessary to decrease their environmental impact. Several combustion systems demonstrated the interest of low temperature combustion to move toward this objective. However, to ensure a stable combustion, the use of additives has been considered in a several studies. Amongst them, 2-Ethylhexyl nitrate (EHN) is considered as a good candidate for these systems but characterizing its chemical effect is required to optimize its use. In this study, its promoting effect (0.1 - 1% mol.) on combustion has been investigated experimentally and numerically in order to better characterize its behavior under different thermodynamic and mixture. Rapid compression machine (RCM) experiments were carried out at equivalence ratio 0.5 and pressure 10 bar, from 675 to 995 K. The targeted surrogate fuel is a mixture of toluene and n-heptane in order to capture the additive effect on both cool flame and main ignition. A kinetic model was developed from literature data assembly and validated upon a large set of variations including species profiles and ignition delays of pure compounds as well as mixtures. At the experimental conditions, it was found that the EHN reduces the ignition delay time (IDT) of the surrogate fuel in the whole temperature range. EHN effectiveness tends to be minimum around 705 K and increases with temperature. The results also indicate that EHN effect increases nonlinearly with EHN doping levels. Numerical analyses revealed that the EHN effect is linked to NO₂-NO loops, which enhances fuel reactivity. The methodology proposed here enables to simulate the EHN effect with simple compounds rather than the full EHN chemistry set. This strategy could simplify the consideration of additive effect when computational fluid dynamics (CFD) simulations are performed on engine. Finally, the study presents the EHN effectiveness on several thermodynamic conditions as well as equivalence ratios. The objective is to assess its performance upon large operating conditions which appears to be of interest with novel combustion systems targeting low temperature as well as lean combustion.