(167g) Effect of an Antioxidant on the Gas-Phase Reactivity:an Experimental Study of 2,4-xylenol

Introduction
Fuel-engine adequacy is the key parameter to increase engine performance while getting a benefit from
an emissions reduction. Specifically, reactivity is one of the most important characteristics of fuels
ensuring the fuel-engine adequacy. Fuels are reactive in both liquid and gas phases. Before being burnt
in the engine, the fuel stability is of the first order. Fuel can oxidize in the liquid phase in several places
such as the tank, the fuel circulation system, or the injector under different pressure and temperature.
The incorporation of biofuels such as fatty acid methyl esters (FAME) can reduce the oxidation stability
of fuel. The autoxidation of fuel results in undesirable products, including deposits. The last compounds
can reduce engine performance by affecting several components of engine such as the pump, the injector
filtering system, etc. Additionally, deposits can affect the combustion of fuels. Kupoyama et al. [1]
observed that deposits are responsible for pre-ignition in a low-speed, high-load SI engine. In the
combustion chamber in engines, the gas-phase reactivity of fuels needs to be carefully controlled. The
fuel employed in Compression-Ignition (CI) engines must be prone enough to self-ignition, i.e. have a
high gas-phase reactivity. On the other hand, to prevent undesired combustion phenomenon such as
knock and/or pre-ignition, the gas-phase reactivity of fuel used for Spark-Ignited (SI) engines is
necessarily low. In order to control fuel reactivity in both gas and liquid phases, several additives can be
used. An additive usually refers to a molecule exhibiting an effect at very low concentration. This doping
level is generally lower than 1% mol. To improve the oxidation stability of fuel, antioxidants are
employed. To adjust the gas-phase reactivity of fuels, two types of additives are employed: cetane
boosters which can enhance the fuel reactivity, and octane boosters which are used to reduce the fuel
reactivity. Similar radical mechanisms are reported for liquid phase oxidation [2] and for lowtemperature
gas phase combustion [3]. This implies that additives dedicated to one phase could impact
the other on several aspects. Literature is limited on these "multi-effects” of additives as the additive
use remains mostly on trial and error approach. This study aims at experimentally investigating the effect
of an antioxidant named 2,4-xylenol [4] on the gas-phase reactivity by measuring auto-ignition delay
times in a rapid compression machine (RCM) and the burning velocity in a heat flux burner (HFB)
respectively. The effect of this additive is compared with 2-ethylhexyl nitrate (EHN) and ferrocene,
which are conventional cetane booster and octane booster, respectively.
Experimental method
To investigate the effect of additives, a mixture of toluene (65% vol.) and n-heptane (35% vol.) was
employed as the surrogate fuel.
Ignition delay time (IDT) measurements were performed in the single-piston RCM of the University of
Orléans in Orléans, France. The compression technique and the crevices geometry of this RCM were
built based on the RCM developed at the Argonne National Laboratory [5]. RCM experiments were
conducted at the pressure at the top dead center (Pc) of 10 bar at both stoichiometric and lean (Φ =
0.5) conditions. The temperature at top dead center (Tc) was calculated thanks to the isentropic relation
and varied from 675 to 1000 K. The doping level of additive was set 0.1% mol.
Laminar burning velocity of fuels was measured in a HFB at Laboratoire Réaction et Génie des Procédés
(LRGP) in Nancy, France. Measurements were conducted at atmospheric pressure. The initial
temperature of the unburnt gas flow was set at 398 K. EHN and 2,4-xylenol are introduced into fuels at
a level of 1% weight basis. As a concentration limitation is applied for the use of ferrocene in fuels [6],
the effect of this additive is examined at a concentration of 0.1% by mass
Results and Conclusions
The measured ignition delay times (IDT) of the neat surrogate fuel and fuel doped with additives are
presented in Figure 1. At the stoichiometry, a negative coefficient temperature (NTC) behavior is
observed from 790 K to nearly 885 K while this phenomenon does not appear in the lean condition (Φ
= 0.5). n-Heptane oxidation enables the NTC behavior as described by Battin-Leclerc et al. [3]. At both
equivalence ratios, two-stage ignitions occur in the lowest temperature range (Tc < 790 K). In lean
conditions, the IDT of the neat surrogate are longer, up to 4 times than those in stoichiometric conditions.
At the investigated experimental conditions (10 bar, Φ = 1), EHN promotes the reactivity of the fuel at
all examined Tc (675 – 960 K). A significant reduction of both 1st-stage IDT and main IDT of the
surrogate fuel is observed in presence of EHN. The NTC behavior of the surrogate fuel is mitigated by
the addition of EHN. The EHN promoting effect is minimum at Tc near 710 K and then increases with
Tc. While ferrocene does not show a remarkable effect on the 1st-stage ignition of the surrogate fuel,
this additive presents a strong inhibiting effect on the main ignition. The latter effect increases with
temperature. The main IDT of the doped fuel is up to four times longer than the one of the neat fuel at
the highest examined temperatures (Tc > 900 K). In addition, ferrocene enhances the NTC behavior of
the surrogate fuel. In lean conditions (Φ = 0.5), the doped fuel presents an NTC behavior from 750 K to
910 K which is not observed in the case of the surrogate fuel. The chemical effects of EHN and ferrocene
have been thoroughly discussed in previous articles [7,8]. Different from EHN and ferrocene, it has been
experimentally found that 2,4-xylenol does not show any remarkable effect on the reactivity of the
surrogate fuel at both doping levels over the whole temperature range.

Figure 2 presents the laminar burning velocity of the neat and doped fuel at atmospheric pressure and Ti
= 398 K as a function of equivalence ratio. The laminar burning velocity of the surrogate fuel varies
from 30 to 60 cm/s and reaches its maximum value at Φ = 1.1 as observed in the case of other
hydrocarbons [9]. Under the investigated experimental conditions, all additives including EHN,
ferrocene, and 2,4-xylenol does not affect significantly the flame speed of the fuel.

Experimental works were performed to investigate the chemical effect of 2,4-xylenol, which is an
antioxidant, on the gas-phase reactivity of the surrogate fuel containing toluene and n-heptane. The
effect of 2,4-xylenol was compared with two conventional additives: EHN and ferrocene. While EHN
and ferrocene show reactivity-promoting effect and reactivity-inhibiting effect, respectively, 2,4-xylenol
does not show any remarkable effect on the measured IDT of the surrogate fuel at Tc < 1000 K. None of
above additives affects the laminar burning velocity of the surrogate fuel.
The obtained experimental results confirm that the utilization of 2,4-xylenol as an antioxidant at doping
level below 1% mol. does not show any additional effects on the fuel gas-phase reactivity. This study
contributes to the fundamental understanding of the effect of additives on fuel reactivity.
References
[1] Kuboyama, T. et al., SAE Int. J. Engines 8.2 (2015) 529–537.
[2] Bacha, K., PhD Thesis, Université de Haute Alsace, 2016.
[3] Battin-Leclerc, F., Progress in Energy and Combustion Science 34.4 (2008) 440–498.
[4] McCormick, R.L. et al., Energy Fuels 29.4 (2015) 2453–2461
[5] Bourgeois N. et al., Proceedings of the Combustion Institute 36 (2017) 383–391.
[6] Danilov, A.M. et al., Chemistry and Technology of Fuels and Oils 37.6 (2001) 444–455.
[7] Le et al., Combustion and Flame (2020), Submitted for publication.
[8] Le et al., Proceedings of the Combustion Institute (2020), Accepted for publication.
[9] Dirrenberger, P., PhD Thesis Université de Lorraine, Nancy, 2014.