Nitrogen dioxide (NO
2
) is an active species
of exhaust
gas recirculation gas, and it has a significant impact on the autoignition
and combustion processes of fuels. This study presented a comprehensive
investigation of the effect of NO
2
on the combustion characteristics
of the
n
-butanol/biodiesel dual fuel. Experiments
were conducted on a single-cylinder engine with 0, 100, 200, and 400
v/v ppm NO
2
addition at two fuel injection ratios. The
findings of the experiments indicated that adding NO
2
resulted
in an earlier start of heat release and an increase in peak in-cylinder
pressure as compared to experiments where no NO
2
was added.
The evolutions of
n
-butanol, biodiesel, and OH radicals
were evaluated using the computational fluid dynamics software coupled
with the
n
-butanol–biodiesel–NO
2
mechanism. The results revealed that when 400 v/v ppm NO
2
was added, the consumption of
n
-butanol
and biodiesel occurred earlier, and the formation of OH radicals was
approximately an order of magnitude higher before the biodiesel was
injected. Furthermore, reaction rate and flux analyses were performed
to understand the effect of NO
2
addition on the reaction
process. When NO
2
was added, 35% of the HO
2
radicals
reacted with NO which converted from NO
2
via the reaction
NO + HO
2
⇌ NO
2
+ OH, promoting the formation
of OH radicals in the reaction system. The addition of NO
2
can also enhance the consumption of CH
3
radicals via
the reaction CH
3
+ HO
2
⇌ CH
3
O + OH.
The contribution of NO 2 to the ethanol ignition delay time was investigated behind reflected shock waves. The experiments were performed at a pressure of 0.20 MPa, temperature range of 1050−1650 K, equivalence ratio of 0.5/1.0/1.5, and ethanol/NO 2 mixing ratios of 100/0, 90/10, and 50/50. The experimental results showed that the addition of NO 2 decreased the ignition delay time and promoted the reactivity of ethanol under all equivalence ratios. With an increase in NO 2 blending, the effect of equivalence ratio on the ethanol ignition delay time decreased, and with an increase in temperature, the effect of NO 2 in promoting ethanol ignition weakened. An updated mechanism was proposed to quantify NO 2 -promoted ethanol ignition. The mechanism was validated based on available experimental data, and the results were in line with the experimental trends under all conditions. Chemical kinetic analyses were performed to interpret the interactions between NO 2 and ethanol for fuel ignition. The numerical analysis indicated that the promotion effect of NO 2 is primarily due to an increase of the rate of production and concentration of the radical pool, especially the OH radical pool. The reaction NO + HO 2 ⇔ NO 2 + OH is key to generating chaininitiating OH radicals.
Reactivity controlled compression ignition (RCCI) engines have a high thermal efficiency as well as low emissions of soot and nitrogen oxides (NOx). However, there is a conflict between combustion stability and harmful emissions at high engine load. Therefore, this work presented a novel approach for regulating n-butanol/methyl oleate dual fuel RCCI at high engine load in attaining lower pollutant emissions while maintaining stable combustion and avoiding excessive in-cylinder pressure. The tests were conducted on a single cylinder engine under rated speed and 90% full load. In this study, n-butanol was selected as a low-reactivity fuel for port injection, and n-butanol/methyl oleate blended fuel was used for in-cylinder direct injection. Combustion and emission characteristics of the engine were first investigated with varied ratios of n-butanol port injection (PFI) and direct injection (DI). Results showed that as the ratio of n-butanol PFI and DI rose, the peak cylinder pressure and heat release rate increased, while NOx and soot emissions reduced, and carbon monoxide (CO) and hydrocarbon (HC) emissions increased under most test conditions. When RNBPI = 40% and RNBDI = 20%, the soot and NOx emissions of the engine were near the lowest values of all test conditions, yet the peak in-cylinder pressure and fuel consumption could not increase significantly. Therefore, the possibility of optimizing the combustion process and lowering emissions by adjusting the pilot injection strategy was investigated utilizing these fuel injection ratios. The results revealed that with an appropriate pilot injection ratio and interval, the peak in-cylinder pressure and NOx emission were definitely reduced, while soot, CO, and HC emissions did not significantly increase.
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