Elsevier

Combustion and Flame

Volume 217, July 2020, Pages 77-84
Combustion and Flame

Impact of lubricating base oil on diesel soot oxidation reactivity

https://doi.org/10.1016/j.combustflame.2020.03.035Get rights and content

Abstract

In this study, the surface functional groups types and concentrations, graphitization degree, and oxidation reactivity of exhaust soot samples generated by a heavy-duty engine when employing neat diesel and four base oil dosed mixtures as the fuel was compared. Fourier transform infrared (FTIR), Raman spectroscopy and Thermal gravimetric analyzer (TGA) were used to characterize particle physicochemical properties. Four lubricating base oils were blended in diesel at a mass ratio of 0.5% and 1.0% for combustion. It was found that although the chemical compositions of these four base oils were different, the aliphatic Csingle bondH functional groups content, graphitization degree and oxidation reactivity of exhaust soot exhibited similar trends. The combustion of base oil was found to change the graphitization degree of emitted soot and lead to soot with a more disordered nanostructure. The soot particles derived from base oil blended fuels had higher aliphatic Csingle bondH groups concentration than those from neat diesel. Soot oxidation reactivity analyses showed that soot generated from base oil blended fuels had lower characteristic temperatures and lower activation energy than that from neat diesel. Besides, there was a clear correlation between the soot graphitization degree, relative concentration of aliphatic Csingle bondH groups and its oxidation reactivity.

Introduction

It is well known that diesel particulate matter (PM) emissions can cause air pollution and has a negative effect on human health [1,2]. In response to these concerns, regulators have promulgated ever-stricter legislation for PM emissions from diesel engines. Euro 6 regulation has even already introduced particle number emission measurements. Thus reducing particle emissions from diesel engines has become an important issue in recent decades. Majority of the attention had been paid to the fuel-related emissions since the fuel is the most important factor to impact the PM emissions. Many different techniques have been developed to control PM emissions, such as engine design [3], fuel quality [4], and advanced after-treatment systems [5,6]. However, there are also many studies [[7], [8], [9], [10], [11], [12]] found that the engine lubricating oil does have an effect on diesel PM emissions, especially on the formation of soluble organic fraction and ultrafine particles.

Lubricating oil affects diesel particulate emissions by involving into the combustion chamber through the evaporation or other ways. Earlier studies concentrated on the contribution and mechanism of lubricating oil to diesel PM emissions. From the work done by Jefferd et al. [13], the part-synthetic engine oil generated over 20% less particulate matter than the other two fully synthetic oils but contributes to about 10% higher NOX emission. Taylor [14] showed that with the continuous removal of light fractions of the oil below 200 to 260 °C, the smoke value and particulate mass emission decreased obviously. Miller et al. [15] investigated the formation of the particles with the influence of lubricating oil, using a modified diesel engine fueled with hydrogen. The engine produced exhaust particles with diameter ranging from 18 to 31 nm. The elemental carbon content was much lower than normal diesel engine and a higher metallic fraction was found by analyzing the exhaust PM. Jung et al. [16] showed that the lubricating oil led to increasing particle number emissions by an order of magnitude and higher oxidation rate of the particles, by using oil-dosed fuel (2% by volume). More recently, studies have focused on the role of different lubricating oil additives in particle formation, such as sulfur and metallic ash content, which is closely related to the regeneration of diesel particulate filters. Neeft et al. [17,18] and Kannana et al. [19] investigated the influence of metal oxides and metal-based additive on soot oxidation characteristics and found that oil additives could affect soot physicochemical properties and increase the ash content of PM. In diesel engines, the combustion of lubricating oil might contribute to PM formation by increasing soluble organic fraction adsorbed on soot. Besides, the sulfur and metallic content may vapor to form nanoparticles [16,20,21].

Engine lubricating oil is composed of base oil and several additives. In general, the base oil is composed of petroleum-derived mineral oils and accounts for about 85% by mass, which is much higher than the additives. The base oil is consumed and burned in the cylinder and may have an impact on diesel particulate emission. To control or reduce the oil-related particle emissions, we need to know much better about how the engine base oil affects the formation of particles and its basic properties, which has not been investigated much yet. The objective of this study is to investigate the influence of base oil on the diesel soot oxidation reactivity. And the correlations between soot oxidation reactivity and graphitization degree and surface functional groups were analyzed and discussed. Particle emissions when four different kinds of base oils were blended into the fuel were studied. Aliphatic Csingle bondH surface functional groups concentration, graphitization degree, oxidation characteristic temperatures, and activation energy of exhaust soot samples generated by a heavy-duty engine were analyzed and compared.

Section snippets

Engine setup

Experiments were performed on a 3.865 L, four-cylinder, turbocharged direct injection diesel engine. The engine specifications are shown in Table 1. A hydraulic dynamometer and a fuel consumption meter were used to measure the engine speed, torque, power and fuel consumption. The engine was fueled by commercial diesel (500 ppm maximum sulfur content). During these experiments, the room temperature was controlled at 20 ± 3 °C by an air conditioner. The cooling water temperature and lubricating

Engine combustion process

To better understand the effect mechanism of lubricating base oil on the combustion process, it is necessary to monitor the combustion process in the chamber. In-cylinder pressures, mean temperatures and apparent heat release rates were compared when applying neat diesel fuel and base oil-blended fuels. The determination of the combustion phase followed the method suggested by Heywood [26].

As shown in Fig. 1, there is a little bit drop of the peak pressure and temperature values when using base

Conclusions

In this study, the aliphatic Csingle bondH group concentrations, graphitization degree, oxidation characteristic temperatures, and activation energy of exhaust soot samples generated by a heavy-duty engine when employing neat diesel and four base oil dosed mixtures as the fuels were compared. The combustion of lubricating base oil has a clear influence on the surface functional groups, nanostructure and oxidation reactivity of exhaust soot samples. Despite the different base oil employed, the trends in

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by National Natural Science Foundation of China (Nos. 51976135 and 51806148). The author also wants to appreciate the support of high-tech Ship Research Program of MIIT (MC-201501-D01-01).

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