Experimental and modeling study of fuel interactions with an alkyl nitrate cetane enhancer, 2-ethyl-hexyl nitrate

Abstract

This study investigates the autoignition behavior of two gasoline surrogates doped with an alkyl nitrate cetane enhancer, 2-ethy-hexyl nitrate (2EHN) to better understand dopant interactions with the fuels, including influences of accelerating kinetic pathways and enhanced exothermicity. A primary reference fuel (PRF) blend of n-heptane/iso-octane, and a toluene reference fuel (TRF) blend of n-heptane/iso-octane/toluene are used where the aromatic fraction of the latter is set to 20% (liquid volume), while the content of n-heptane is adjusted so that the overall reactivity of the undoped fuels is similar, e.g., Anti-Knock Index (AKI) of ~91, Cetane Number (CN) ~25. Doping levels of 0.1, 1.0 and 3.0% (liquid volume basis) are used where tests are conducted within a rapid compression machine (RCM) at a compressed pressure of 21 bar, covering temperatures from 675 to 1025 K with stoichiometric fuel-oxygen ratios at O 2 = 11.4%.At the experimental conditions, it is found that the doping effectiveness of 2EHN is fairly similar between the two fuels, though 2EHN is more effective in the aromatic blend at the lowest temperatures, while it is slightly more effective in the non-aromatic blend at intermediate temperatures. Kinetic modeling of the experiments indicates that although some of the reactivity trends can be captured using a detailed model, the extents of predicted Cetane Number enhancement by 2EHN are too large, while differences in fuel interactions for the two fuels results in excessive stimulation of the non-aromatic blend. Sensitivity analysis using the kinetic model indicates that the CH 2 O and CH 3 O 2 chemistry are very sensitive to the dopant at all conditions. The rate of 2EHN decomposition is only important at low temperatures where its decomposition rate is slow due to the high activation energy of the reaction. At higher temperatures, dopant-derived 3-heptyl radicals are predicted to play an important role stimulating ignition. Finally, nitrogen chemistry becomes important only at the highest doping levels, primarily through the formation of methyl and ethyl nitrite, and nitric acid.

Influences of ignition improver additive on ternary (diesel-biodiesel-higher alcohol) blends thermal stability and diesel engine performance

“…Cetane number indicates the autoignition quality of fuels [23,34]. The cetane numbers of PB20P5 and PB10P10 enhances after mixing EHN with them as shown in Table 3.…”

“…Although the nitrogen molecules in the chemical structure of EHN accelerate the NO production, the resultant increment of cetane number increases and decreases the ignition delay by shortening the time of premixed combustion phase. As a result, the combustion temperature as well as thermal NO X reduced [34,35,49]. Moreover, increase in cetane number accelerates the free radical production, which promotes the formation of free radicals [50], which may divest hydrogen atoms of the blends restricting the oxidation of hydrocarbon and nitrogen atoms hence NO decreases [21].…”

A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines

“…This is due to the fact that the reactive-species-scavenging reactions discussed above are less likely for the highly-branched tert-butanol molecule [176], so that it acts like an inert compound, rather than an ignition retarder, in the mixture. The combustion properties of alcohol fuels, or their blends with diesel, can be improved by adding ignition promoters, such as alkyl nitrates [273][274][275][276]. More research into the chemical mechanisms behind these ignition promoting additives and studies on engine performance and emissions are needed.…”

Semidetailed Kinetic Model for Gasoline Surrogate Fuel Interactions with the Ignition Enhancer 2-Ethylhexyl Nitrate

“…The products from 2EHN decomposition interact with the fuel and other gases to accelerate the ignition process, and the early reactions often have some exothermicity and thus can provide a thermal stimulant to supplement the accelerating kinetic pathways. 21 Table 1 shows the added reactions with associated rate constants for the 2EHN subset where reaction steps involving 3-heptyl radicals have been added. The subsequent reactions of NO 2 and CH 2 O are already contained in the TRF base mechanism.…”

“…This was also suggested by Goldsborough et al using a detailed kinetic model. 21 When the fuel sensitivity increases, the sensitivity coefficients for chain branching reactions decrease in value. Figure 19a,b,h shows that, the higher the value of S, the more sensitive the fuel becomes to R622 (HO 2 + NO → NO 2 + OH), R625 (NO 2 + H → NO + OH), and R729 (HONO(+M) → OH + NO(+M)).…”