Article pubs.acs.org/EF
Characteristics of Soot and Particle Size Distribution in the Exhaust of a Common Rail Light-Duty Diesel Engine Fuelled with Biodiesel Xusheng Zhang,*,† Zhijun Wu,‡ and Liguang Li‡ †
Merchant Marine College, Shanghai Maritime University, Shanghai, 201306, China School of Automotive Studies, Tongji University, Shanghai, 200092, China
‡
ABSTRACT: The effect of biodiesel blends (derived from waste oil) on soot and particle size distribution (PSD) was investigated in a light-duty common rail direct injection diesel engine under different load conditions. The experiments included four fuels: baseline diesel (D), B40, B60, and B100. (BXX means a blend of XX vol % biodiesel in diesel). Particle size distribution in exhaust was measured by a scanning mobility particle sizer (TSI SMPS3936) through a partial dilution tunnel. The results showed that as the blend ratio increased, the filter smoke number (FSN) of biodiesel blends decreased significantly. With respect to PSD at the downstream of two diesel oxidation catalysts, an increase of engine loads led to nanoparticle formation in the nucleation mode (NM) and log-modal PSD of diesel and biodiesel blends shifts to bimodal. Biodiesel blends will significantly reduce the particle number of accumulation mode (CM) due to soot oxidation from oxygen content. However, the absence of NM was still found for B100. Nanoparticle formation was dominated by the sulfur content of biodiesel blends in this common rail light-duty diesel engine. This was attributed to the process of S−SO2−SO3−sulfate with the existence of a diesel oxidation catalyst. This conclusion suggested that if the sulfur content of biodiesel blends could not reach to a certain level, the blend ratio will have little influence on the nanoparticle number. Total particle number concentration (TPNC) decreased with the increasing of blend concentration. B100 will result in 2 orders of magnitude reduction of TPNC compared with diesel due to the decrease of particle number in CM and absence of NM. The difference between calculated particle mass and FSN soot mass suggested that it is necessary to consider the effect of sulfate on the effective density of the particle when the exhaust aerosol consists of a large amount of sulfate.
1. INTRODUCTION Biodiesel is a renewable alternative diesel fuel produced through a process called transesterification. This process can convert renewable feedstocks like vegetable oils, used cooking oils, and animal fats into a methyl/ethyl ester which depends on the alcohol used. Biodiesel has a higher cetane number than conventional diesel fuel, no aromatics, almost no sulfur, and about 11% oxygen by weight. These characteristics of the fuel generally reduce the emissions of carbon monoxide (CO), hydrocarbon (HC), and particulate matter (PM) in the exhaust gases compared with petroleum diesel. Particulate matter (PM) is an important emission in diesel engine exhaust. Soot, a major component of PM, is formed during the combustion and is exhausted as a solid agglomerate. The soluble organic fraction (SOF) in the exhaust may be adsorbed onto diesel particles, thus the size and mass of PM may increase. Biodiesel, with about 10% oxygen content, could result in maximum reduction (around 40%) of PM in heavy duty diesel engines.1 The established method to measure particle emissions for type approval tests is gravimetric analysis of filter samples, taken from a full exhaust flow dilution tunnel. Another notable characteristic of the particle is the particle size distribution (PSD) or particle number since the particle diameter has influence on the human health, especially nanoparticles.2,3 There are some differences of PM composition between biodiesel and diesel due to reduction of soot and potential increase of SOF.4,5 The volatile composition of PM has the effect on the particle number, especially the nanoparticle number, since volatile gas could be nucleated to © 2012 American Chemical Society
the particle under supersaturation conditions. Therefore, it is necessary not only to investigate the particle size or particle number but also to understand the nanoparticle characteristics of biodiesel. Recently, some studies have reported the results on the particle size of diesel engine fuelled with biodiesel. The results of PSD or particle number of biodiesel mainly focused on the smaller particles and showed an increase or decrease in the number of smaller particles emitted. 1.1. Increase. Krahl et. al6 investigated the PSD of different fuels including rapeseed oil methyl ester (RME) under CEC R49 cycle in a turbocharged diesel engine. The results showed more particle of RME in the range of 10−40 nm and fewer particles for larger diameters compared with diesel. Schonborn et al.7 studied the effect of molecular structure of individual fatty acid alcohol esters (biodiesel) on the PSD in a singlecylinder engine using the common rail injection system. The results showed that all biodiesels produce larger numbers of small particles (5−30 nm) due to the high boiling point of unburned fuel. Tsolakis et al.8 found the significant increase of smaller particles number (60−100 nm) and a decrease of mean diameter for rapeseed methyl ester. Aakko et al.9 found the increase of particle number in 10−30 nm on a Euro II diesel engine without catalyst as was due to condensates of sulfuric acid, water, and hydrocarbons. Zhu et al.10 found the increase Received: May 13, 2012 Revised: August 5, 2012 Published: August 9, 2012 5954
dx.doi.org/10.1021/ef300823k | Energy Fuels 2012, 26, 5954−5961
Energy & Fuels
Article
of the particle number in NM and reduction in CM when biodiesel blends were fuelled at smaller blending ratios (
