A Comparative Morphological Study of Primary and Aged Particles Emitted from a Biodiesel ( B 20 ) vis-à-vis Diesel Fuelled CRDI Engine

A comparative morphological analysis was performed on the exhaust particles emitted from a common rail direct injection (CRDI) sports utility vehicle (SUV) engine for primary and aged particulates. In this study, soot particles were collected from the CRDI engine fuelled with mineral diesel and 20% biodiesel blend (B20). The engine was operated at a constant speed of 1800 rpm for five different loads (0%, 25%, 50%, 75%, 100% rated load) for primary and aged particulate collection. Primary particulate samples were collected on pre-conditioned quartz filter papers from the exhaust stream after partial dilution. For collection of aged particles, the diluted exhaust was passed through a customized photochemical chamber with 2 hour retention time. This allowed photochemical reactions to proceed in presence of ultraviolet light leading to particulate aging in simulated daylight conditions and particulates were collected downstream of the photochemical chamber. Particulate laden filters were observed by scanning electron microscopy (SEM). It was observed that B20 emitted significantly lower primary particulates in comparison to mineral diesel for all the loads. B20 emitted lower particulates at lower load conditions for aged samples. At higher engine loads (75% and 100%), this reduction in aged particulates for B20 compared to mineral diesel was quite significant.


INTRODUCTION
Atmospheric aerosols are mainly composed of elemental carbon (EC), organic carbon (OC) and inorganic species (Heintzenberg, 1989), whereas diesel particulate matter (DPM) consists mainly of EC, OC, sulphate and trace metals.High temperature and pressure conditions in the combustion chamber during combustion in diesel engine results in formation of numerous organic compounds, which get adsorbed onto the solid carbonaceous core of the particulates.These organic compounds are formed as a result of incomplete combustion in the combustion chamber, and are responsible for grave health concerns.Identification and quantification of these compounds has been quite a challenge so far because it depends on a large number of variables such as type of fuel used, type of lubricating oil used, design of combustion chamber, fuel injection strategies etc.
Diesel particulates can be categorized as primary and aged particles.Primary particles are produced directly by the emission source i.e., diesel engines and aged particles are formed by condensation of gaseous species present in the diesel exhaust over existing available nuclei as a result of complex photochemical reactions in the atmosphere and subsequent transformations (Gupta et al., 2011).DPM lead to poor visibility and have adverse effects on human health, therefore DPM categorized as "Carcinogenic" (Mansfield et al., 1991).Organic species present in the engine exhaust consists of some highly carcinogenic compounds such as nitro-polyaromatic hydrocarbons (nPAHs) (Ravindra et al., 2008).On the other hand, primary diesel particulates undergo complex chemical transformation in presence of sunlight and oxidizing agents in the atmosphere and may get converted into even more toxic secondary species.When exhaust is released into the atmosphere, dilution ratio reaches around 1000 within a couple of seconds and the temperature of the exhaust drops rapidly (Abdul-Khalek et al., 1999).Due to excessively high saturation condition, particle formation is enhanced by formation of sulphate nuclei, upon which, various organic compounds get condensed.These organic compounds are primarily high boiling point hydrocarbons, which are already present in the engine exhaust (Kittelson, 1998).In order to investigate the primary and aged particulates, sampling of diesel particulates needs to be done carefully.Sampling technique should not affect the particle's physical and chemical characteristics.Abdul-Khalek et al. (1999) explained that the particulate characteristics may change with the use of inappropriate dilution technique.Particle number and size distribution are highly dependent on the dilution ratio, dilution temperature, residence time etc.
Researchers have attempted to reduce particulate emissions by using alternative fuels such as biodiesel.In numerous recent studies, it has been shown that biodiesel blends significantly reduce the particulate emissions (Lapuerta et al., 2008;Gangwar et al., 2012).Biodiesel is comprised of various fatty-acid esters, which offer few advantages over mineral diesel such as higher cetane number, higher fuel oxygen content (~11%) (Junget al., 2006), lower aromatics etc. Aromatics and sulphur contents are nearly absent in biodiesel, which results in relatively lower toxic potential.The oxygen content of biodiesel reduces the calorific value of the fuel, therefore higher volume of fuel is consumed for per unit power production, however the fuel oxygen in biodiesel reduces the calorific value but it improves combustion quality.Agarwal et al. (2011) studied physical and chemical aspects of particulate emissions from biodiesel and took SEM images of B100 and B20 particulates and found bigger size grains compared to diesel particulates.
The effect of engine operating conditions on the particle size and nanostructure was studied by numerous research groups (Zhu et al., 2005;Lapuerta et al., 2007;Lee et al., 2011).Stevenson, 1982 investigated the morphology and crystallography of DPM.It was observed that the degree of crystallinity of carbonaceous DPM increases with particle size and are more crystalline in nature near the centre of the agglomerated particle (Stevenson, 1982).Zhu et al. (2005) concluded that most particle size measuring instruments measure the aggregate particle diameters, which are known as aerodynamic and mobility diameters.In reality, most particles have fractal shapes, which can only be observed by direct visualization techniques like SEM (Zhu et al., 2005) or transmission electron microscope (TEM).SEM is also useful for observing the particulate microstructures.Ruiz et al. (2007) designed and fabricated two photochemical chambers in order to generate aged sulphuric acid particles (aged aerosol sampling).They used one of the chambers in the laboratory for aged sulphuric acid particle generation by providing artificial gases; it was observed that the sulphuric acid production increased with the enhanced ozone concentration in the chamber (Ruiz et al., 2007).Other photochemical chamber was used for sampling the diluted exhaust from a coal fired power plant for aged sulphuric acid particle generation for comparison with the laboratory data (Ruiz et al., 2007).Lu et al. (2012) investigated the primary particle size and nano-structure by using TEM for diesel exhaust.It was observed that particulates possessed disorder in size and shape at low loads and high speeds.Lee et al. (2002) investigated microstructure, dimensions and fractal geometries of diesel particulate samples using a high resolution TEM.Their study revealed that the primary particulate samples were amorphous and were mainly composed of soluble organic compounds, while particulate sample at higher engine loads were EC enriched, ordered, graphitic structures.To study the primary particle size, fractal dimensions of the particulates and nano-particle characteristics, studies were conducted using SEM/TEM (Ishiguro et al., 1997;Park et al., 2004).Particulates in the diesel exhaust remain one of major concerns in the context of present emission regulations.In-cylinder and aftertreatment technologies were developed and they need further improvements in order to reduce particulate emission (Majewski and Khair, 2006), in view of globally emerging emission regulations.Therefore, it is necessary to have complete understanding of the particle formation and its physical and chemical transformations after it comes out of the engine tailpipe, followed by its subsequent aging (photochemical oxidation) in the atmosphere.Keeping this in mind, the present study explored the morphological changes in the primary and aged particulates emitted from a modern diesel engine used in a SUV.Furthermore, the effect of blending biodiesel (B20) with diesel on such particulate emissions is also presented here.

EXPERIMENTAL SETUP
A CRDI diesel engine, typically used in a SUV, was used for the comparative study of diesel and B20 (Tata India; Safari Dicor 3.0 L, Euro III).This four cylinder engine was equipped with a CRDI fuel injection system, which was having a maximum fuel injection pressure of 1600 bar.The engine was turbocharged with intercooling.Engine performance was optimized by an electronic control unit (ECU).An eddy current dynamometer (Dynomerk controls, Pune, India, EC300) was coupled with the engine for loading it.Detailed specifications of the test engine are given in Table 1.
This engine does not use any exhaust gas after-treatment device.Mineral diesel was purchased from a local diesel filling station in Kanpur, India.Karanja biodiesel was prepared in laboratory scale biodiesel pilot plant using Karanja oil as feedstock vegetable oil and methanol, which was also procured locally.Details of the production process are provided in our earlier publication (Agarwal et al., 2009).Hoekman et al. (2012) reported that the biodiesel feedstock significantly affects the properties of the biodiesel.Karanja biodiesel blend (B20) was prepared by mixing 20% Karanja biodiesel (V/V) and 80% mineral diesel (V/V).

PARTICULATE SAMPLING SYSTEM
Fuel properties of mineral diesel and B20 were determined using appropriate test procedures and equipments.Table 2 shows the fuel properties for mineral diesel and B20 used in this study.SAE 20W-40 lubricating oil was used.Table 3 shows the specification of the lubricating oil used.
The schematic of the experimental setup used in this study is depicted in Fig. 1.To simulate the environmental conditions, a partial flow dilution tunnel was used for collection of primary exhaust particles from diesel and B20 (Dwivedi et al., 2006;Gangwar et al., 2012).A small portion of exhaust was taken from the engine exhaust stream into the dilution tunnel, which was well mixed with preheated and pre-filtered atmospheric air, in order to get desired dilution ratio, according to EPA guidelines (dilution ratio was maintained at 16) (Baumgard and Kittelson, 1985;Abdul-Khalek et al., 1998).Finally, the particulates were collected on a pre-conditioned 47 mm diameter quartz filter paper.Experiments were performed at a constant engine speed of 1800 rpm at five engine loads using diesel and B20 (Fig. 1).In this study, experiments were conducted at steady state condition only; therefore the results may not be applicable exactly for the transient conditions often encountered with the vehicles on road.In actual road conditions, diesel engines operate in transient conditions and the particulate emission charecteristics are quite different.Relatively higher volume of B20 was burnt in the engine to obtain similar power output for similar operating conditions, because B20 has marginally lower calorific value compared to mineral diesel.The carbon dioxide (CO 2 ) concentration was measured for undiluted exhaust, ambient air and diluted exhaust in order to calculate and verify the dilution ratio: (1) For aged particulate collection, a photochemical chamber was used (Gupta et al., 2011;Agarwal et al., 2013).Exhaust was diluted with fresh air and was allowed to pass through the photochemical chamber (smog chamber).In the photochemical chamber, particulates get sufficient time to condense, agglomerate and coagulate (Gangwar et al., 2012).

Field Emission Scanning Electron Microscope (FE-SEM)
For morphological investigation, field emission scanning electron microscope facility at IIT Kanpur was used.Particulate samples were collected on quartz filter papers.Sampling duration was kept 15 min for each operating condition.Aged particulates were collected downstream of photochemical chamber with the aid of a PM 2.5 sampler (Gupta et al., 2011).Samples were collected at 1800 rpm engine speed at five different engine loads (0%, 25%, 50%, 75%, and 100%), fuelled with diesel and B20.Particle laden filters were kept in a desiccator for 24 hours in order to remove the moisture.A small round section was cut from each filter using a 9.5 mm internal diameter punch and these sections were observed in the field emission scanning electron microscope (Carl Zeiss NTS GmbH, Germany; SUPRA 40VP).

RESULTS AND DISCUSSION
The main objective of this work was to perform a comparative morphological study for particulates collected from engine exhaust (diesel and B20) for primary and aged particulate emissions.SEM images were obtained at 5000X magnification.Particulate collection patterns were compared for both the fuels for primary and aged particulates.

Primary Particulates Emission
For primary sampling, particulates were collected directly on to a 47 mm diameter quartz filter paper after diluting the exhaust with clean, pre-conditioned air.There was virtually no significant particulate collection observed on the filter paper at no load condition (Fig. 2).The possible reason for no visual evidence of particulates may be that most of the nuclei mode particulates are formed with very little contribution from EC (Zielinska et al., 2003).Most of the nuclei mode particles were not visible with the magnification used as they may have got trapped deep into the filter fibers and due to dominance of light colored OC, they may not be visible due to poor contrast with the white color filter background.With increasing engine load, particulate collection can be observed clearly for both the fuels (diesel and B20) in primary particulates.As the fuel injection quantity increased with increasing engine load,  (excess air-fuel ratio) decreased, which resulted in higher combustion heat generation, correspondingly leading to higher particulate emissions due to higher degree of heterogeneous combustion.Higher fuel injection quantity led to the increase in number of fuel droplets sprayed in the combustion chamber, which increases the degree of heterogeneity during combustion (Heywood, 1988).
At 25% engine load, flaky particulates were collected.Particulate collection increased with engine load and it reached a maxima at 100% engine load.One important observation was that particulate flakes were relatively dense and bigger in case of 50% load in comparison to lower loads.At 75% and 100% engine loads, particulate collection was so dense that it was difficult to notice the filter fibers, which indicates significantly higher particulate emissions at higher engine loads.Flaky structure near the intermediate engine loads possibly indicates the presence of significant OC and condensed volatile organic materials.However, particulate collection at higher engine load seems more crystalline, which indicated higher EC.Zielinska et al. (2003) reported quite high OC content for low engine loads.At lower engine loads, ultrafine particles are emitted, which are mainly composed of OC, whereas at higher engine loads, these particles are mainly composed of EC (Zielinska et al., 2003).Dowling, 1992reported that fraction of carbonaceous matter in the DPM increases with the increase in engine load and speed.This also suggests presence of higher EC fraction at higher engine loads.Higher in-cylinder temperature at higher engine loads lead to oxidation of majority of OC.EC oxidation is difficult because it requires significantly higher cylinder temperature (~600°C).
As far as fuel is concerned, comparatively higher particulate collection can be observed for diesel than B20 under identical engine operating conditions.A previous study from our group showed that biodiesel blends produced less particulate emissions than diesel (Gangwar et al., 2012).Oxygen content in biodiesel is ~11%, which helps in improving combustion thus lowering particulate emissions (Jung et al., 2006).The possibility of oxidation of biodiesel fuel molecules in B20 increases because of the fuel oxygen present in the biodiesel molecules itself.It leads to rapid oxidation of biodiesel fuel molecules therefore oxidation stability of biodiesel remains a serious issue to be dealt with.

Aged Particulate Emissions
It was also observed that the aged particulate collection was wet in nature compared to primary exhaust particulates collected on the filter paper.There were no particulate traces observed at no load condition (Fig. 3).Significant amount of particulates were collected for low and intermediate engine loads (25% and 50% load).Aged particulates were observed bigger in size in comparison to the primary particles collected under similar operating conditions.Gasto-particle conversion inside the photochemical chamber contributes to this observed significant particle loading of the filter papers.It was observed that the aged particles were relatively denser and larger compared to the primary particles.The possible reason for this phenomenon is the formation of relatively bigger size particles (coagulation and aggregation) followed by adsorption of volatile species on to the PM surface during the photochemical aging process (Agarwal et al., 2013).
For the aged particulate samples collected at 50% engine load, a thick layer of particulates can be observed on the filter paper for mineral diesel.Zhang et al. (2008) also reported that soot particles become denser as a result of atmospheric aging process.At 75% and 100% engine load, surprisingly, lower particulate collection on the filter paper was observed, possibly due to loss of aged particulates in the photochemical chamber itself.This might be due to rapid growth of primary particles well above sub-micron sizes (Baumgard and Kittelson, 1985;Singh et al., 2011).At higher engine loads, more fuel quantity is injected into the engine cylinder, which produces higher quantity of water vapor as a combustion product.This water vapor gets condensed in the photochemical chamber due to low ambient temperature.Condensation of water vapors to liquid form may lead to faster settling of fully grown water droplets in photochemical chamber, which may also remove several other particles by sweeping mechanism,while settling to the bottom of the photochemical chamber (Singh et al., 2011).Ruiz et al. (2007) also reported 21-40% loss for 50-1000 nm size range particles in their photochemical chamber study used for determining the toxicity of coalfired power plant emissions.
As far as fuels are concerned, comparatively lower particulate collection were observed for B20 compared to mineral diesel for identical conditions.Biodiesel has some inherent advantages like no aromatics, absence of sulphur content etc. (Jung et al., 2006).Diesel combustion is generally a lean combustion and there is enough oxygen available to burn every fuel droplet in the combustion chamber.However due to the heterogeneous mixing of fuel and air in the combustion chamber, there are localized regions, where fuel-air ratio becomes richer than stoichiometric, leading to incomplete oxidation of fuel molecules due to pyrolysis.This leads to the formation of pyrolyzed compounds, which are primarily soot and unburnt hydrocarbons.In case of biodiesel, oxygen present in the fuel molecule itself helps in combustion in fuel-rich zones and limits the pyrolysis process.Also, sulphur leads to sulphate (which acts as nuclei for particle growth) formation, which is almost absent in biodiesel.Therefore, B20 leads to formation of lower amount of aged particulates.
In a previous study (Fig. 4) performed in our group by Agarwal et al. (2013), it was observed that the EC and OC emissions were lower at low engine loads, which increased with increasing engine load.However, OC reduced at higher engine loads for mineral diesel.These results are in good agreement with the SEM images obtained for primary and aged particulates.
Thick, black and dense particulates observed in SEM images at higher engine loads indicate higher EC content in mineral diesel exhaust.Also, as noticed from Fig. 4, for both, primary as well as aged particulates, OC content remained fairly constant for B20 whereas it steadily decreased with increasing engine load for mineral diesel.This may very well result in different density as well as morphology of the resultant soot particles.Fig. 5 shows EC/OC ratio for various engine loads for primary and aged particulates fuelled with diesel and B20.EC/OC ratio increased with engine load for primary and aged particulates, when fuelled with mineral diesel (Fig. 5).In case of B20, it became almost flat near 50% engine  load and again increased slightly towards 100% load.It indicates that higher amount of EC is present in mineral diesel and B20 particulates at higher engine loads.One of the important observations was that EC/OC ratio was significantly higher near intermediate loads, when engine Fig. 6.Sulphate determination in primary diesel and B20 particulates (expressed in terms of PM mass collected per unit filter surface area).
was fuelled with B20 (In comparison to EC/OC for mineral diesel).It indicates higher OC contribution in the primary as well as aged diesel particulates.The collection pattern in the SEM images (Fig. 2 and Fig. 3) showed significantly lower particulate collection for B20 compared to diesel particulates (both, primary and aged).Also, mineral diesel particulates were found to be denser and flaky, which was probably due to higher concentration of OC.

Sulphate in Primary Diesel and B20 Particulate
Diesel particulates consist of sulphates, which lead to higher number concentration of emitted particles in engine exhaust.Sulphate content was determined in the primary particulates of diesel and B20 by using Ion Chromatography (Fig. 6).The details about the sample preparation and sulphate determination methodology are provided in Chakraborty and Gupta, 2010.It can be noticed that the variation in sulphate content in particulates was not consistent but it showed an increasing trend with increasing engine load.Sulphate formation was higher for intermediate and higher engine loads in the tested load range (0-75% load) due to higher fuelquantity burnt.Suphates significantly contribute to increasing number of nuclei-mode particulate formation (Henket al., 1992;Majewski and Khair, 2006).

CONCLUSIONS
In primary emissions, particulate collection increased with increasing engine load, which transformed into highly crystalline format higher engine load, indicating higher EC fraction of the particulates.OC contribution to the particulate reduced with increasing engine load because of enhanced oxidation of OC at higher in-cylinder temperatures.Adsorption of organic compounds, and condensation of volatiles at low cylinder temperature resultedin comparatively wetter aged particulate collection.Negligible sulphur content and higher oxygen content in biodiesel led to superior combustion and lower particulate formation for B20 for both, primary as well as aged emissions.It was observed that particulate collection on filter substrates was relatively significantly lower for aged particulates for B20 at 75% and 100% engine loads.It indicated excessive loss of particulates in the photo-chemical chamber itself during photochemical aging and rapid hygroscopic growth.

Fig. 4 .
Fig. 4. EC and OC content in diesel and B20 exhaust for primary and aged particulates (data taken from Agarwal et al., 2013).

Table 1 .
Technical specifications of the test engine.

Table 2 .
Fuel properties of mineral diesel and B20.