Effect of Butanol Blends on Nano Particle Emissions from a Stationary Conventional Diesel Engine

In this study, combustion characteristics and nano-size soot particle emissions from a stationary conventional diesel engine have been experimentally investigated using butanol/diesel blends. Experiments were conducted on a single cylinder stationary diesel engine at a constant speed of 1500 rpm for neat diesel and butanol/diesel blends (i.e., 10%, 20% and 30% butanol on volume basis) at different engine loads. Piezoelectric pressure transducer installed in the engine combustion chamber was used for measuring cylinder pressure data. In-cylinder pressure data for 2000 consecutive engine cycles was recorded and averaged data was used for the analysis of combustion characteristics. Butanol/diesel blends show higher rate of heat release in comparison to neat diesel and heat release rate increases with butanol percentage in the blend. Opacity meter and exhaust particle sizer were used for analyzing smoke opacity, size and mass distributions of soot particles respectively at different engine operating conditions. Soot particle distribution from 5 nm to 1000 nm was recorded at each test condition. Results show that total particle concentration decreases with an increase in engine operating loads. It was found that butanol/diesel blends have lower total particulate concentration and the surface area.


INTRODUCTION
Depletion of petroleum reserves and future stringent emission legislation limits mainly governs the development of an efficient internal combustion engines producing lower emissions.A study conducted by the International Transport Forum suggests that by 2050 global transport could be double or even quadruple (OECD/International Transport Forum, 2013), which will increase fuel consumption leading to higher emissions.In present scenario, mostly stationary and automotive engines are running on fossil fuels (gasoline and diesel).Typically, diesel engines are used in heavyduty vehicles and gensets due to their higher fuel efficiency and power output.However, diesel engine emits high level of nitrogen oxides (NO x ) and particulate matter (PM) in the exhaust due to the combustion of heterogeneous mixture in the cylinder at higher combustion temperatures (Heywood, 2011;Pundir, 2012).NO x and PM have adverse effect on human health (Afroz et al., 2003) and the environment.In order to meet the stringent emission legislation limits, diesel particulate filters, lean NO x trap, selective catalytic reduction (advanced after-treatment devices) are typically used to reduce the engine emissions (Johnson, 2010).However, after-treatment devices have penalty effect on the fuel efficiency of the engine (Johnson, 2008;Johnson, 2009).Utilization of alternative fuels in combustion engine is one of the ways to improve engine fuel conversion efficiency and reducing exhaust emissions.Alternative fuels like methanol, ethanol, hydrogen, liquefied petroleum gas (LPG) are high octane gasoline fuels typically used in spark ignition engines.However, these fuels are not suitable for compression ignition (CI) engines due to their lower cetane number.Several studies (Ramadhas et al., 2006;Raheman and Ghadge, 2008;Rounce et al., 2012;Ye and Boehman, 2012;Raheman et al., 2013;Dhar and Agarwal, 2014) summarized that biodiesel can be used as alternative fuel in CI engines and it also lower the emissions.
However, biodiesel fuel is not prominently used in vehicles because of higher NO x emissions and durability issues (like deposition formation, carbonization of injector tip, fuel filter plugging).In recent decades, butanol attracted the attention of researchers as an alternative fuel for CI engine due to its inimitable properties.Butanol has a higher cetane number in comparison to methanol and ethanol and also easily miscible with diesel.
Several studies investigated the performance, combustion and emission characteristics of diesel/butanol blends in CI engine (Rakopoulos et al., 2010;Doğan, 2011;Imtenan et al., 2015;Şahin and Aksu, 2015).Rakopoulos et al. (2010) experimentally investigated the performance and gaseous emission characteristics of diesel/butanol blends on diesel engine for 8%, 16% and 24% diesel/butanol blends at a constant speed of 2000 rpm and three different load conditions.They found that smoke density, NO x and carbon mono oxide (CO) emission were reduced with butanol blends while hydrocarbon (HC) emissions increase with butanol blends as compared to neat diesel.Rakopoulos et al. (2010) also observed that the brake thermal efficiency increases with butanol blends with slightly lower exhaust gas temperature.Doğan (2011) also investigated the effect of diesel/butanol blends on the performance and emission characteristics of diesel engine and found similar results.Imtenan et al. (2015) investigated the performance and emission characteristics of diesel-jatropha biodiesel blend in addition with n-butanol and diethyl ether in a diesel engine.They found that CO, HC and smoke opacity were reduced with jatropha biodiesel/diesel blend while NO x increases with biodiesel/diesel blends as compared to neat diesel.Şahin and Aksu (2015) investigated the effect of 2%, 4%, 6% butanol/diesel blends on the turbocharged diesel engine on combustion characteristics.However, these studies are mainly focused on the performance and regulated gaseous emissions.Several studies were also conducted to investigate the concentration of particle number and its distribution for both diesel and oxygenated fuels (Argachoy and Pimenta, 200;Chien et al., 2009;Petrović et al., 2011;Srivastava et al., 2011;Zhang et al., 2013;Wei et al., 2014;Cho et al., 2015).Chien et al. (2009) investigated the PAH and particle emission from diesel engine fuelled with biodieseldiesel blends.This study used micro-orifice uniform deposit impactor and nano-orifice uniform deposit impactor to measure the particulate matter size distribution.Their results indicate that particulate and PAH emission decreases with an increase in the fraction of biodiesel in the blended fuel.Study also shows that increasing the biodiesel fraction to 60%, the nano and ultra-fine particles were increased.Srivastava et al. (2011) experimentally investigated the concentration and distribution of nano-particle emissions from nonroad diesel engine at different load condition at a compression ratio (CR) of 17.5 and fuel injection pressure of 200-205 bar.Study concluded that the size and mass distribution of particles depend upon the engine operating load.Liu et al. (2012) experimentally investigated the emission characteristics of heavy duty diesel engine fuelled with wastecooking-oil-biodiesel/diesel blends.Study demonstrated that lower CO, PM, PAH and HC emissions could be achieved with waste-cooking-oil-biodiesel/diesel blends while having higher NO x and CO 2 emissions in comparison to neat diesel.Zhang et al. (2013) experimentally investigated the effect of fumigated methanol with diesel on the combustion characteristics and particulate emission of diesel engine at different engine operating load conditions in a modified naturally aspirated, 4-cylinder, water-cooled diesel engine with ECU controlled fumigated methanol injection system.Experiments were performed with 10%, 20% and 30% fumigated methanol injection with diesel at different engine load conditions.Zhang et al. (2013) achieved lower peak pressure for lower and medium loads with fumigated methanol while higher peak pressure achieved for higher load condition.Particulate mass and concentration decreases with fumigated methanol from medium to higher load condition.Zhang et al. (2013) also observed that the concentration of nucleation size particles increases with fumigated methanol from medium to higher load conditions.Wei et al. (2014) experimentally investigated the effect of diesel and di-methyl ether (DME) on PM emissions (particle distribution and soluble organic fraction) in the modified automotive diesel engine.The diameter of the plunger was increased from 8.5 mm to 10.5 mm; injection timing was retarded from 25°BTDC to 22°BTDC while injection pressure was reduced from 19.1 MPa to 18 MPa to run an engine efficiently with DME.The study shows that particle numbers decrease with engine speed and increases with engine operating load for both the fuels.They also found that particles are larger and accumulation size particles are higher for diesel combustion (Wei et al., 2014).Several studies (Sukjit et al., 2012;Zhang and Balasubramanian, 2014a, b;Choi and Jiang, 2015) also investigated particulate emission distribution for diesel/biodiesel/butanol blend.Sukjit et al. (2012) experimentally investigated the effect of addition of methyl esters in the blend of ethanol and butanol/diesel on combustion and emission characteristics of naturally aspirated diesel engine (compression ratio 15.5).They found that 15% methyl esters are sufficient to avoid the phase separation for ethanol and butanol/diesel blends and butanol has lower soot, CO, and total hydrocarbon emissions as compared to ethanol.Zhang and Balasubramanian (2014a) experimentally investigated the effect of butanol/diesel/ biodiesel blends on particulate emission characteristics at different load in a diesel engine.Experimental data were recorded with neat diesel, Bu5, Bu10, Bu15, and Bu20 for 25%, 50%, and 75% engine load condition and particle size measured in the range of 5 to 560 nm.They found that PM 2.5 concentration, total volatile and non-volatile concentration, reduce with butanol/diesel blends.Choi and Jiang (2015) investigated the effect of butanol/diesel blend on HC, CO, NO x , PM and un-regulated emissions at different operating load and speed conditions on a four cylinder, turbocharged, common rail direct injection diesel engine for butanol blend ratio upto 20%.Their results indicated that at lower load condition, ethylene and benzene emissions were higher and increases with butanol fraction in the blend.Lopez et al. (2015) experimentally investigated the effect of alcohol fumigation (i.e., hydrous ethanol and n-butanol) on the performance, combustion and emission characteristics of an automotive diesel engine.Their results show that both the alcohol fuels have faster combustion rate and a lower peak in-cylinder temperature as compared to neat diesel.Study also concluded that n-butanol was better alternative fuel for fumigation as compared to hydrous ethanol on the basis of performance characteristics.Tse et al. (2015) experimentally investigated the trade-off between the particle mass and total particle number concentration and NO x emissions from an automotive diesel engine fueled with diesel, diesel-biodiesel-ethanol blends and biodiesel.Study shows lower particulate emission (both in terms of particle mass and particle number) found with dieselbiodiesel-ethanol blends.Mwangi et al. (2015) experimentally investigated the effect of various fuel blends (butanol, diesel, water and microalgae biodiesel) on the performance and emission (both gaseous and particles) characteristics of an automotive diesel engine.Their result shows that all the blended fuel has higher brake thermal efficiency as compared to conventional diesel while having lower PM and PAH emissions.Zhang et al. (2016) experimentally investigated the effect of butanol and pentanol addition with diesel on the performance and particle emission characteristics of diesel engine.Study shows that both the blended fuels have lower solid particles, elemental carbons emission and particle mass as compared to diesel.Results also indicated that the concentration of total particle emissions, reduce with both the alcohol blended fuels due to reduction in the concentration of particles larger than 50 nm.
Formation of nano-size soot particles depends upon several engine parameters including compression ratio, injection pressure, injection timing, engine speed and load.Reviewed studies show that researchers investigated the effect of various alternative fuels on nano particle emissions (in size ranges of 5 nm-560 nm) for mainly automotive diesel engine.Published studies lack the detailed studies on particle emission on relatively older mechanical fuel injection type direct injection diesel engines.This study is focused on reduction of particulate emission from a stationary diesel engine (typically used for small genset/pumpset in India) at optimized engine conditions using butanol.In the coming future, stringent legislation from government for non automotive engines also makes this study more relevant to this kind of engines.Previous study (Saxena and Maurya, 2016) optimized the compression ratio (18:1) and fuel injection pressure (200 bar) for higher efficiency and lower particle and gaseous emissions using diesel.These optimized conditions may be valid for similar engine with mechanical low pressure fuel injection system.In this study, experiments were conducted on optimized condition.This study mainly focused on the combustion characteristics and particle distribution (in the size range 5 nm to 1000 nm) with neat diesel and butanol/diesel blends (i.e., 10%, 20%, 30% butanol by vol.) at different engine operating load conditions (0%, 25%, 50%, 75% and 100% load).

EXPERIMENTAL SETUP ANDMETHODOLOGY
A four stroke, single cylinder, direct injection, variable compression ratio diesel engine coupled with an eddy current dynamometer was used for the present study.The specifications of the test engine are provided in Table 1.The schematic diagram of the experimental setup used in this study is shown in Fig. 1.The experiments were conducted at a compression ratio of 18 and fuel injection pressure of 200 bar.Fuel is injected at 23°BTDC (before top dead center) into the cylinder.All the tests are conducted at constant speed of 1500 rpm.Fuel consumption was measured using an electronic fuel transmitter installed in the fuel line between the fuel tank and the fuel pump of the engine.Piezo-electric pressure transducer was mounted in the cylinder head to measure the cylinder pressure.The crank angle encoder (resolution 1 crank angle degree (CAD)) was used for measuring crankshaft position.Cylinder pressure of 2000 consecutive engine cycles was measured and average cylinder pressure was used for heat release analysis.The rate of heat release (ROHR) is used for calculation of combustion parameters.
The rate of heat release is calculated by using Eq. ( 1) where, Q is the heat release, γ is the ratio of specific heat, P(θ) and V(θ) is the pressure and volume as a function of crank angle position θ.
Volume as function of θ is calculated by using engine geometry and presented in Eq. ( 2)  where, V c is the clearance volume, L is the length of the connecting rod, R is the radius of the crank and B is the bore of the engine cylinder.Smoke Opacity was measured by using an opacity meter (Manufacturer: AVL Dismoke 480 BT).Engine exhaust particle size and particle number distribution were measured by a fast response, particle spectrometer (Manufacturer: Cambustion, UK; Model: DMS 500).Particles of 38 different sizes in the range of 5 nm to 1000 nm can be measured by the instrument (DMS 500).It is real time, equipment, which measures particulates on the basis of electric mobility diameter (Cambustion Ltd., 2015;Gonzalez-Oropeza, 2009).Exhaust gas samples from the engine exhaust tail pipe passing through the cyclone in primary dilution stage (exhaust gases diluted with hot/warm air).Diluted gases from cyclone orifices pass through heating line towards the high ratio diluting rotating disc for the second stage of dilution.The optimal dilution ratio controlled by PC-based user interface and measured particulate concentration are corrected for the total applied dilution.Diluted exhaust gas samples passes through the unipolar corona charger and soot particles are charged (proportional to surface area).Charged soot particles then passed through strong electrified field classifier column.Soot particles are deflected towards downward direction electrometer rings by repulsion from a central high voltage rod.On the basis of electric mobility particles are detected at different distances during downward motion in the column.The charge is induced on the soot particles from electrometer rings and the resulting currents (output signals) are processed in real time by the user-interface into particle number (Cambustion Ltd., 2015).
Typically, soot particles having size lower than 50 nm are considered as nucleation mode particles and particles having size between 50 nm to 1000 nm are considered as accumulation mode particles (Price et al., 2006).In this study, nucleation and accumulation mode particles are calculated by fitting lognormal curve (Cambustion DMS06).Total particulate numbers and mass of the particles is calculated by using following Eqs.( 3) and ( 4 where, is the diameter of the particle.

RESULTS AND DISCUSSION
In this section, combustion and particulate emission characteristics of diesel and butanol/diesel blends are presented and discussed.Experiments are conducted at constant speed of 1500 rpm and five different engine operating load conditions, i.e., 0%, 25%, 50%, 75% and 100% load.For each test condition, average in-cylinder pressure data of 2000 cycles were used to reduce cyclic variability.Comparative experimental investigation of performance and gaseous emission characteristics of diesel and butanol/diesel blends at different load conditions were analyzed in previous study (Saxena and Maurya, 2016).

Combustion Characteristics
Combustion characteristics of engines are often explained using heat release curve estimated from direct measurement of cylinder pressure.To reduce the effect of cyclic variability, average of 2000 consecutive engine cycles is used for heat release calculation.Fig. 2 shows the variation of averaged cylinder pressure and rate of heat release (ROHR) curves for diesel and butanol/diesel blends.It can be observed from the Fig. 2 that cylinder pressure and ROHR increases with engine load for all test fuels because of a higher amount of fuel burnt at a higher engine load.It can also be noticed from Fig. 2 that higher ROHR obtained with butanol/diesel blends as compared to diesel and ROHR increases with butanol percentage in the blend.The higher heat release rate in butanol blends is due to increase in delay period, which increases the fraction of fuel burned during premixed combustion phase.Ignition delay is comparatively higher in butanol blends due to lower cetane number of butanol.Fig. 2 also confirms that the lower heat release rate for butanol blends during mixing control phase at the same load conditions.Fig. 3 shows the variation of start of combustion (SOC) and combustion duration with engine loads for diesel and butanol diesel blends.SOC and combustion duration is calculated from the heat release rate.SOC is defined as a crank angle position for 10% heat release and combustion duration as difference of crank angle position between 90% and 10% heat release.It is observed from Fig. 3(a) that SOC advances with the increase in engine loads.It can also be observed from Fig. 3(a) that SOC is delayed for But20 and But30 at all operating load conditions as compared to But10 blend, which is possibly because of the high heat of vaporization of But20 and But30 blends as compared to But10 blend.Moreover, at the higher engine load condition SOC delayed for But30 blend as compared to neat diesel, But10 and But20 blends.It might be due to the lower cetane number and higher latent heat of vaporization is dominated for But30 blend as compared to neat diesel.Fig. 3(b) shows that combustion duration reduced with butanol/diesel blends as compared to diesel fuel and shorter combustion duration achieved as an increase in the fraction of butanol in the blend.Shorter combustion duration shows that the combustion rate increased with butanol fraction in neat diesel due to the higher oxygen content in the fuel.During mixing controlled combustion phase, fuel-oxygen mixing rate dominates the combustion (Chen et al., 2013).Therefore higher oxygen content in the butanol blends will dominate and lead to shorter combustion duration.

Smoke Opacity and Density
Compression ignition engine emits soot/particulate emissions in the form of black smoke.Soot formation depends on the overall fuel-air ratio.Typically, smoke opacity represents the fraction of transmitting light blocked in the sample of smoke emitted from an engine.Smoke density determines the ability of smoke to conceal the light.Smoke density is a function of the total smoke particle numbers per unit gas volume and the distribution of different size particles.Variations in smoke opacity and density are shown in Fig. 4. It can be observed from Fig. 4 that smoke opacity and smoke density increases with engine operating load.Lower smoke opacity and density obtained with butanol/diesel blends as compared to neat diesel fuel and higher reduction obtained as fraction of butanol increases in the fuel blend.This fact is attributed to lower carbon and higher oxygen content in butanol/diesel blends.Similar trends were also observed by few researchers (Rakopoulos et al., 2010;Doğan, 2011;Chen et al., 2013).

Particulate Emission Characteristics
Fig. 5 shows the variation of size and number distribution for diesel and diesel butanol blends at different engine loads.It can be observed from Fig. 5. that at lower load conditions, smaller size particles (range 10-60 nm) are higher in concentration, while larger size particles are higher in concentration at full load operation.It can also be noticed that higher reduction in particulate concentration obtained with butanol/diesel blend at lower load in comparison to full load conditions.Particle size-number distribution follows the bimodal log-normal distribution curve (Fig. 5).First peak typically represents the nucleation mode particles while the second peak represents accumulation mode particles.Peak number concentration is observed at a mobility diameter of 20 nm.Fig. 5 shows that nucleation size particles decreases with engine load and accumulation size particle concentration increases with increase in engine operating load.With the increase in engine load the overall temperature of the combustion chamber increases (higher amount of fuel burned) leading to increase in the rate of agglomeration and decrease in nucleation mode particles.Most of the particles are in the size range of 5-300 nm for all the test fuels.Particles larger than 500 nm size are also emitted in significant number for this engine (Fig. 5) and these particles are not typically measured by instruments/researcher (Srivastava et al., 2011).Significant concentrations of particles larger than 500 nm size are possibly due to mechanical fuel injection system (fuel injection at lower injection pressure) which results in poor atomization (larger size fuel droplets).Larger fuel droplet leads to poor evaporation and mixing, which results pyrolysis of richer mixture and larger particles are formed in significant number at higher engine load.High fuel injection pressure technologies (such as common rail direct injection) in modern engines lead to better atomization (smaller sauter mean diameter droplets) and efficient combustion resulting typically smaller soot particles.
Fig. 6 shows the variations of total particle concentration with engine load for different butanol blends.Concentration of total particle number decreases with an increase in engine operating loads due to higher combustion temperature at higher loads.Moreover, lower particle concentration obtained with butanol/diesel blends in comparison to diesel fuel and reduction increases with increase in the fraction of butanol due to the higher percentage of oxygen content in fuel and better combustion.It can also be observed from contour plot that butanol blending is more effective in   reduction of particle concentration at lower loads (upto 70% loads).After 25% engine loads, blending of more than 10% butanol is not very effective in particle reduction as contour lines are almost vertical (after But10).
To further explore the reduction in total particle number emissions, nucleation and accumulation mode particles are estimated and discussed (Fig. 7).Fig. 7 shows the variations of the concentration of the nucleation and accumulation mode particles with engine load for butanol/diesel blends.It is observed from Fig. 7 that concentration of nucleation mode particles are higher in comparison to accumulation mode particles at all engine load except full load condition.This trend is similar for both diesel and butanol blends.Fig. 7 also show that concentration of nucleation mode particles reduces with engine load while the concentration of accumulation mode particles increases with engine load.Characteristics of the particle size and their dependence on operating conditions of the engine found in the present study are consistent with previously reported study (Zhang and Balasubramanian, 2014).Fig. 7 show the significant reduction in the concentration of nucleation and accumulation size particulates for diesel/butanol blends.It can also be noticed that the concentration of nucleation and accumulation size particles reduces with butanol/diesel blends at all load conditions.
Mass of total particulate matter emitted from diesel engine is the key factor for stringent emission legislations.Figs. 8  and 9 show the variations of the particle surface area and the mass with different load conditions, respectively for diesel and butanol/diesel blend.Particle surface area and particle mass is dependent on the diameter of the particle and aggregate number of particles of that diameter.The mass of particles is calculated using Eq.(4).
It is observed from Fig. 8(a) that at idle load condition, nucleation size particles have a higher peak surface area while on increasing the load accumulation size particles have higher peak surface area distribution.At lower load nucleation mode particles are in higher number concentration, therefore although nucleation mode particles are in smaller diameter they show higher surface area.At higher loads peak surface area concentration shift towards higher mobility diameter particles (Fig. 8).A similar trend is also observed for other butanol/diesel blends (Fig. 8).Moreover, it can be observed that the surface area of particle reduces with increase in the fraction of butanol in the blended fuel due to lower number concentration.Concentration of nucleation mode particle drastically decreases for butanol blends (Fig. 7).Smaller particles contribute more towards the surface area and large reduction in nucleation mode particle leads to reduction in surface area also.Lower surface area of particles can be related to lower toxicity of particulates.Lower the surface area, lower will be possibility of surface adsorption of the polycyclic aromatic hydrocarbons (PAHs), aldehydes, ketones (which form soluble organic fraction (SOF) for smaller sized particles) and thus lower will be the toxic potential.Smaller particles tend to have significantly higher surface area for the same particle mass compared to larger particle, offering larger surface area for condensation of toxic VOC's and PAH's.Therefore, smaller particles tend to become more hazardous for human health compared to larger particles.
It is observed from Fig. 9 that the mass of the particles depends upon the engine load and mass of the particles increases with an increase in engine load.Furthermore, large reduction in the mass of particles is found with increasing the fraction of a butanol fraction in butanol/diesel blend as compared to diesel due to reduction in particle number concentration (Figs. 6 and 7).The maximum reduction in particle mass is obtained with But30 Blend.It can also be noticed from Fig. 9 that mass of particles in the size range of 700 nm-1000 nm is significant for this kind of engine, which is typically ignored even in some of measuring instruments/studies.Number of particles in this size (700 nm-1000 nm) is significant enough so that they can have mass comparable to higher concentration particles.

CONCLUSIONS
The effect of butanol blends (But10, But20 and But30) on combustion characteristics and particulate emissions has been investigated on a conventional stationary diesel engine at 1500 rpm for different engine loads.Higher rate of heat release was found for butanol/diesel blends in comparison to diesel and heat release rate increases with an increase in butanol percentage in the blend.Start of combustion advances with butanol blends in comparison to diesel.Shorter combustion duration was found for butanol/ diesel blends and combustion duration decreases with the  increase of butanol fraction in the blend.Smoke opacity and smoke density increase with engine load for all test fuels.Total particle number concentration was found to decrease with increase in engine load and lower particle concentration was obtained with butanol/diesel blends in comparison to diesel.It was found that concentration of nucleation mode particulates reduces with engine operating load and the concentration of accumulation mode particles increase with engine load.Concentrations of nucleation and accumulation mode particles are lower for butanol/diesel blends at all engine loads.Higher reduction in particle number concentration was found using butanol blends at lower load in comparison to full load.Nucleation mode particles have higher peak surface area distribution at idle load conditions and accumulation mode particles have peak surface area at higher engine load.It was found that mass of particles increases with an increase in engine load.Larger reduction in mass of particles was found at higher butanol blends.Number of particles in the size range 700 nm-1000 nm is significant and particle mass comparable to higher concentration particles for this type of engine.In summary butanol blending is better alternative for partial replacement of diesel in conventional engine for particulate reduction.

Fig. 2 .
Fig. 2. Variation of cylinder pressure and ROHR for diesel and butanol blends at different engine loads.

Fig. 3 .
Fig. 3. Variation of (a) start of combustion and (b) combustion duration with engine load for diesel and butanol/diesel blends.

Fig. 5 .
Fig. 5. Particle size and number distribution for diesel/butanol blends at different engine loads.

Fig. 6 .
Fig. 6.Variation of total particulate concentration with engine load for diesel and butanol/diesel blends.

Fig. 8 .
Fig. 8. Particle size and surface area distribution for diesel/butanol blends at different engine loads.

Fig. 9 .
Fig. 9. Particle size and mass distribution for diesel/butanol blends at different engine loads.