Influence of Methanol–Biodiesel Blends on the Particulate Emissions of a Direct Injection Diesel Engine

Abstract

In this study, Euro V diesel fuel, biodiesel, and methanol–biodiesel blends were tested in a 4-cylinder direct-injection diesel engine to investigate the combustion characteristics and particulate emissions of the diesel engine under five engine loads at the maximum torque engine speed of 1800 rpm. Compared with Euro V diesel fuel, biodiesel gives lower and earlier heat release rate. For the blended fuels, the peak heat release rate becomes higher and retarded. With regard to particulate mass concentration, biodiesel generates less than Euro V diesel fuel, while the blended fuels result in significant reduction especially at high engine loads. Compare with Euro V diesel fuel, the total particle number concentration of using biodiesel is always higher while the geometric mean diameter (GMD) of the particles is lower. With the blended fuel, the total number concentration and GMD decrease in comparison with pure biodiesel. Further analysis shows that the difference between the total number concentration of biodiesel and Euro V diesel fuel is in particles smaller than 50 nm rather than in the larger particles. The use of methanol–biodiesel blends, compared with biodiesel, could reduce the number concentration of all sizes. A comparison between the particulate mass emission and total particulate number concentration with the mass of fuel burned in the diffusion mode show that they are strongly related to each other, even for the blended fuel.

1. INTRODUCTION

Biofuels, such as alcohol and biodiesel, which could partly or wholly replace diesel fuel, have been considered as potential alternative fuels for diesel engines. Biodiesel is renewable and non-toxic. Its properties are similar to those of diesel fuel, thus there is no need to modify the diesel engine when it is fueled with biodiesel or its blends. A number of literatures have investigated the combustion characteristics and emissions of biodiesel and found that biodiesel gives lower and earlier heat release rate, lower particulate emission but higher NOx emission (EPA 2002; Canakci 2007; Ozsezen et al. 2009), compared with diesel fuel. The increase in NOx emissions and the poor flow characteristics in low temperature (Bhale et al. 2009) are considered as major impediments of biodiesel application.

The combustion process in a direct injection diesel engine can be divided into three consecutive phases: ignition delay period, premixed combustion and diffusion combustion. Diesel fuel is injected into the engine cylinder and atomized into fine droplets. During the ignition delay period, the droplets are heated up by the hot air and partially vaporized and mixed with hot air to form a combustible mixture. During the premixed combustion phase, the combustible mixture is ignited and combusted rapidly. The amount of fuel burned in the premixed mode depends in part on the ignition delay. The longer the ignition delay, the more fuel is prepared for premixed combustion. Combustion of the remaining diesel fuel is diffusion controlled, depending on the subsequent rate of mixing between oxygen and the fuel. The fuel burned in the diffusion mode is related with soot production because soot can be formed on the fuel-rich side of the diffusion flame where local temperatures are sufficiently high (Dec 1997). Hence, the amount of soot and particulate matter formed is related to the amount of fuel burned in the diffusion mode and the oxygen available for supporting combustion in the diffusion mode.

Methanol, when used as a fuel additive, can provide higher oxygen content in the blended fuel and increase the heat of evaporation of the blended fuel, which are advantageous for reducing both NO_x and soot. Moreover, alcohols could help to improve the poor low temperature flow characteristics of biodiesel (Bhale et al. 2009). Thus, it is meaningful to examine the effect of methanol–biodiesel blends on the combustion and emission characteristics of a diesel engine.

The literature shows that there have been some investigations on ethanol–biodiesel blends (Kumar et al. 2006; Shudo et al. 2007; Bhale et al. 2009; Lebedevas et al. 2009) but very few on methanol–biodiesel blends (Kumar et al. 2003; Cheng et al. 2008). It is found that the effect of ethanol–biodiesel blend on engine performance and emissions include increase of peak pressure, ignition delay period, and heat release rate of the premixed combustion; while there is reduction of NOx, smoke, and particulate. Cheng et al. (2008) compared the emissions of biodiesel and a 10% methanol–biodiesel blend. They found that the disadvantage of increased NO_x emission of biodiesel could be overcome by using 10% methanol in the blended mode, and at the same time the blended fuel was effective in reducing particulate both by mass and by number. However their study was limited to 10% methanol blends and did not include combustion characteristics.

Over the past few years, diesel particle number concentration and size distribution have attracted public attention. The diesel particles are mainly submicron in size. They can diffuse easily in the ambient air and could be breathed into the lung (Donaldson et al. 1998). Many studies have been conducted on investigating the particle number concentration and size distribution when the engine is fueled with biodiesel (Tsolakis 2006; Lapuerta et al. 2008a; Di et al. 2009). They concluded that the mean diameter of particles for biodiesel tended to decrease compared with diesel fuel, while results on the total particle number concentrations reported by these studies are different from each other. Tsolakis (2006) and Di et al. (2009) reported that the use of biodiesel could increase the total number concentration and reduce the geometric mean diameter (GMD), while Lapuerta et al. (2008a) reported that both the mean diameter and mean number concentration decreased with biodiesel, compared with reference diesel fuel. However, there is nearly no literature focusing on the particle number concentration and size distribution of methanol–biodiesel blends.

Thus, the aim of this study is to compare the combustion characteristics and particulate emissions of a diesel engine fuelled with methanol–biodiesel blends, pure biodiesel, and Euro V diesel fuel, under different engine loads.

2. TEST ENGINE AND FUEL PROPERTIES

The experiments were carried out on a naturally aspirated, water-cooled, 4-cylinder, direct-injection diesel engine. The specifications of the engine are shown in Table 1. The engine was connected to an eddy-current dynamometer and a control system for adjusting the engine speed and engine torque. The fuels used in this study include Euro V diesel fuel, biodiesel and biodiesel blended with methanol. The major properties of Euro V diesel fuel, biodiesel, and methanol are shown in Table 2. Biodiesel used in this study was produced from waste cooking oil by Dunwell Petro-Chemical Ltd. The methanol–biodiesel blended fuels, or simply the BM blends, were prepared with 5%, 10%, and 15% by volume of methanol in biodiesel, and are identified as BM5, BM10, and BM15 fuels. Since methanol has higher oxygen content, thus the BM blends contain more oxygen compared with biodiesel. On the other hand, since methanol has very low cetane number, the blended fuels will have lower cetane number compared with biodiesel and hence lead to longer ignition delay.

TABLE 1 Engine specifications

Model	Isuzu 4HF1
Type	In-line 4-cylinder
Maximum power	88 kW/3200 rev/min
Maximum torque	285 Nm /1800 rev/min
Bore × stroke	112 mm × 110 mm
Displacement	4334 /cc
Compression ratio	19.0 : 1
Fuel injection timing (BTDC)	8°
Injection pump type	Bosch in-line type
Injection nozzle	Hole type (with 5 orifices)

TABLE 2 Properties of Euro V diesel fuel, biodiesel, and methanol

Property	Euro V diesel	Biodiesel	Methanol
Cetane number	52	51	< 5
Lower heating value(MJ/kg)	42.5	37.5	19.7
Density(kg/m^3)@20°C	840	871	792
Viscosity(mPa·s)@40°C	2.4	4.6	0.59
Heat of evaporation(kJ/kg)	250–290	300	1178
Carbon content (% mass)	86.6	77.1	37.5
Hydrogen content (% mass)	13.4	12.1	12.5
Oxygen content (% mass)	0	10.8	50
Sulfur content (mg/kg)	< 10	< 10	0

3. EXPERIMENTAL SETUP AND MEASUREMENTS

The experimental system is shown in Figure 1. In this study, in-cylinder pressure signals were used to calculate the heat release rate and analyze the combustion process. The cylinder pressure was measured by a Kistler piezoelectric sensor type 6056A and the pressure signals were amplified with a Kistler charge amplifier type 5011B. The amplified pressure signals were recorded and analyzed with a combustion analyzer (DEWETRON, DEWE-ORION-0816-100x). A crank-angle encoder was employed for crank-angle signal acquisition at a rate of 720 signals per engine revolution. Particulate mass concentration was measured with a tapered element oscillating microbalance (TEOM, Series 1105, Rupprecht & Patashnick Co., Inc.). Particle number concentration and size distribution was measured with a scanning mobility particle sizer (SMPS, TSI Model 3934). Before passing through the TEOM and SMPS, the exhaust gas was diluted with a Dekati mini-diluter, which contains two stages of dilution. The dilution ratio (DR) was calculated based on the following equation:

where [CO₂]_exhaust is the CO₂ concentration in the engine exhaust before dilution, [CO₂]_diluted is the diluted CO₂ concentration, and [CO₂]_background is the CO₂ concentration in the background.

FIG. 1 Schematic diagram of experimental installation.

Experiments were conducted at a steady engine speed of 1800 r/min and at five engine loads, corresponding to the brake mean effective pressures (BMEP) of 0.08 MPa, 0.20 MPa, 0.38 MPa, 0.55 MPa, and 0.70 MPa. The BMEP, which represents the useful work developed by the engine per unit displacement volume of the engine, is a commonly adopted parameter for representing engine load. At each engine operating mode, experiments were carried out for diesel fuel, biodiesel and each of the BM blends. To ensure the repeatability and comparability of the measurements, the cooling water temperature was automatically controlled by a temperature controller to 80°C, and held to within ± 2°C, while the lubricating oil temperature varied from 90 to 100°C, depending on the engine load. Data were recorded after the engine has reached the steady state, as indicated by the lubricating oil temperature and the cooling water temperature. In this study, the diesel engine was not modified during all the tests. Moreover, for minimizing cross contamination of different fuels with each other, before each test, the engine was allowed to operate with the new fuel for thirty minutes to clean the fuel system. The data were recorded continuously for 5 min and average values were presented while the experimental standard errors, as shown in Table 3, have been calculated by the method of Kline and McClintock (1953) and Ames et al. (2000). Each test was repeated twice to ensure that the results are repeatable within the experimental uncertainties. Student's t-test was employed to analyze whether the difference between the results obtained from different fuels are statistically significant at 95% confidence level. In Table 3, the standard error of pressure is evaluated using the peak pressures obtained in the same operating condition. The same level of error is assumed in the heat release parameters which are derived from the pressure data.

TABLE 3 Experimental standard errors

Parameter	Standard error (%)
Mass fuel consumption	1.3
Particle mass concentration	1.7
Total number concentration	2.5
GMD	1.7
In-cylinder pressure	3.2

4. RESULT AND DISCUSSION

4.1. Combustion Characteristics

Diesel particles are composed of soot, soluble organic fraction, and sulfate while soot is formed mainly in the diffusion phase of combustion. Hence it is important to examine combustion characteristics of the different fuels, in particular influence of the fuels on diffusion combustion characteristics. In this study, in-cylinder pressure was measured to investigate effects of methanol in the blended fuels on combustion. The in-cylinder pressure, averaged over 400 cycles, was converted into heat release rate through the DEWETRON combustion analyzer. The methods of heat release rate analysis have been covered in Canakci (2007) and Lu et al. (2004). Basically, the First Law of Thermodynamics is applied to obtain the heat released arising from the fuel burned per crank angle.

The heat release rate curves for the different fuels are compared in Figure 2 for the engine loads of 0.20 MPa, 0.38 MPa, and 0.7 MPa, respectively. The premixed combustion and diffusion combustion phases are identified in the figures. All fuels give a rapid premixed combustion phase followed by a diffusion combustion phase. The diffusion combustion phase becomes more dominant at higher engine load. The peak heat release rate increases with an increase in engine load from the low to the medium, but decreases at high engine load for all test fuels, which is similar to the results of Lu et al. (2004). Moreover, with increase of engine load, the peak heat release rate occurs slightly closer to the TDC.

FIG. 2 Effect of methanol and engine load on heat release rate.

Compared with Euro V diesel fuel, the peak heat release rates of biodiesel are lower and occur earlier, which are similar to the results of Tsolakis et al. (2007) and Leung et al. (2006). For the BM blends, the peak heat release rate increases with the increase of methanol fraction in the blended fuel and occurs further away from the TDC in comparison with biodiesel, indicating that there is a delay in the start of combustion.

To illustrate the influence on the combustion process, the start of combustion and the combustion duration are derived from the heat release rate curves and displayed in Figure 3. The start of combustion is defined as the beginning of heat release. It is an indication of ignition delay; the later the combustion starts, the longer is the ignition delay. The end of combustion is defined as the crank angle where the summated heat release is 95% of the total heat released. The combustion duration is the time interval from the start of combustion to the end of combustion. Compared with Euro V diesel fuel, biodiesel has earlier start of combustion and shorter combustion duration which have been explained in the literature (Leung et al. 2006; Tsolakis et al. 2007; Yu et al. 2002; Ziejewski et al. 1995).

FIG. 3 Effect of methanol and engine load on start of combustion and combustion duration.

For the BM blends, the start of combustion is retarded and the combustion duration is shortened, compared with both biodiesel and Euro V diesel fuel. Several factors could be attributed to the changes in the combustion characteristics. The higher peak heat release rate could be attributed to the longer ignition delay or retardation of the start of combustion because of the higher latent heat of evaporation and lower cetane number of methanol. Thus more fuel is prepared for combustion during the longer ignition delay period, leading to more fuel burned in the premixed mode. Moreover, due to the higher oxygen content of methanol, the diffusion combustion of the BM blends could be improved and the period of diffusion phase is shortened, leading to shorter combustion duration, as shown in Figure 3. Finally, the microexplosion of methanol in oil droplets might permit better atomization (Zeng and Lee 2007), accelerate the diffusion combustion and reduce the combustion duration.

The mass of fuel burned in the diffusion mode of each cycle, which is referred to as the diffusion fuel mass in this article, can be derived from the heat release analysis and the mass of fuel consumed per cycle. From the heat release analysis, the percentage of heat released in the premixed mode and in the diffusion mode can be evaluated. The fuel is assumed to be completely burned and the percentage of fuel burned in each mode is assumed to be equal to the percentage of heat released in that mode. For the blended fuel, it is assumed that the blended fuel is uniformly combusted. The results are shown in Figure 4 together with the total mass of fuel consumed per cycle. For biodiesel and the BM blends, there is an increase of mass of fuel consumed per cycle. This is due to the lower heat values of biodiesel and methanol thus more fuel is consumed at the same engine load. However, the diffusion fuel mass of biodiesel and Euro V diesel fuel are statistically the same and they are higher than those of the BM blends. The diffusion fuel mass in general increases with the methanol content in the BM blends and the maximum difference is about 10% between biodiesel and the BM blends. The difference in diffusion fuel mass of biodiesel and the BM blends are statistically significant at the 95% confidence level. With the increase of engine load, the diffusion fuel mass increases for each test fuel.

FIG. 4 Effect of methanol and engine load on total fuel mass and diffusion fuel mass.

4.2. Particle Mass Concentration and Number Concentration

4.2.1. Effect of Engine Load

The variation of PM mass concentration is shown in Figure 5. In general, for each fuel, PM mass concentration increases with engine load, because more fuel is injected and burned in the diffusion mode. Figure 4 and Figure 5 indicate the strong relationship between PM mass concentration with mass of fuel burned in the diffusion mode, in particularly for the diesel fuel and for biodiesel.

FIG. 5 Effect of methanol and engine load on particulate mass concentration.

The particle size distributions were measured at five engine loads, using the SMPS, for particles from 15 nm to 750 nm. A typical particle number concentration and size distribution curve obtained for the engine load of 0.38 MPa is shown in Figure 6, while the variation of total number concentration and the geometric mean diameter (GMD) are shown in Figure 7 for all the five engine loads. For each fuel, with increase in the engine load, there is an increase in both total number of particles and GMD. With an increase in the engine load, more fuel is injected into the combustion chamber, which could increase the mass of fuel burned in the diffusion mode, leading to the increase in particles. Moreover, because of the longer period of diffusion combustion, there is less time for the soot particles to be oxidized after the end of the diffusion combustion period, and also there is less oxygen available for soot oxidation (Tsolakis 2006). This is supported by Figure 8 which plots the total number concentration again the diffusion fuel mass. Figure 7 and Figure 8 resemble each other indicating the strong relationship between total number concentration with mass of fuel burned in the diffusion mode, for all the fuels.

FIG. 6 Effect of methanol and engine load on particulate number concentration and size distribution of 0.38 MPa.

FIG. 7 Effect of methanol and engine load on total number concentration and GMD.

FIG. 8 Variation of total number concentration with diffusion fuel mass.

4.2.2. Comparison Between Euro V Diesel Fuel and Biodiesel

Figure 5 shows that, compared with Euro V diesel fuel, PM mass concentration decreases obviously when the engine is fueled with biodiesel, especially at the high engine load. Between the two fuels, it can be observed from Figure 4 that biodiesel in general has a higher mass of fuel consumed per cycle but almost the same mass of fuel burned in the diffusion mode, indicating that there is a larger amount of fuel burned in the premixed mode. Figure 3 shows that the combustion duration of biodiesel is shorter than that of Euro V diesel fuel, which indicates that combustion rate is faster, probably due to the larger amount of fuel burned in the premixed mode. Since soot is mainly formed in the diffusion phase, the use of biodiesel could improve the diffusion combustion, enhance the oxidation and hence reduce the formation of soot particles.

Despite a reduction in particulate mass concentration, however, the total number concentrations are always higher while the GMDs are always lower, for biodiesel than Euro V diesel fuel, indicating that biodiesel increases the formation of smaller particles. This is illustrated in Figure 6 which shows that the particle size distributions of biodiesel are displaced towards smaller size. We have evaluated the number of particles less than 50 nm, PM_{< 50nm} , and those which are larger, PM_{> 50nm} . At each test condition, biodiesel produces larger number concentration of PM_{< 50nm} , and almost equal number concentration of PM_{> 50nm} in comparison with Euro V diesel fuel. The increase in PM_{< 50nm} is statistically significant at the 95% confidence level.

Tsolakis (2006) suggested that the higher viscosity and density of biodiesel will increase the fuel injection pressure, resulting in better atomization and air-fuel mixing in the combustion chamber, and hence producing a larger amount of smaller diameter particles. This is supported by Mathis et al. (2005), which reported a reduction of primary soot particle diameter when increasing the fuel injection pressure. Although Figure 4 shows that the diffusion fuel mass of biodiesel is more or less the same as that of Euro V diesel fuel, the higher oxygen content of biodiesel could improve the combustion process in locally rich diffusion flame, leading to reduction of the total mass concentration and the size of the particles. Muller et al. (2003) demonstrated that oxygen in ester (such as biodiesel) or -O-(C = O)- and -OH structural groups, could improve combustion process and reduce soot formation. Moreover, biodiesel results in advanced fuel injection, which could lead to smaller primary soot particles (Mathis et al. 2005). In addition, compared with Euro V diesel fuel, unburned biodiesel is less volatile and hence is more liable to nucleate and condense into minute particles when the exhaust gas sample is cooled down in the tail pipe, resulting in an increase of nucleation mode particles (Di et al. 2009).

4.2.3. Effect of BM Blends

The BM blends lead to an increase in the mass of fuel consumed per cycle, due to the lower heat value of methanol. Despite that, there is a reduction of mass of fuel burned in the diffusion mode, indicating that there is an increase of fuel burned in the premixed mode, and hence an improvement in combustion, leading to a reduction of the combustion duration. Improvement in combustion and reduction of diffusion fuel mass favors reduction of PM mass concentration.

Figure 5 shows that PM mass concentrations are reduced for the BM blends except for BM15 at low engine load. Furthermore, the difference between biodiesel and the BM blends on PM mass concentration is more obvious at high engine load. Ozaktas et al. (2000) studied the effect of methanol–diesel blend on soot emission. They concluded that the methanol component is relatively effective on the decrease of soot emission.

When the engine is fueled with BM blends, many factors contribute to the decrease of PM mass concentration. Firstly, compared with pure biodiesel, the BM blends have higher oxygen content but lower carbon content, which could improve the combustion process with higher heat release rate both in the premixed and the diffusion phase, suppressing the formation of PM and accelerating the oxidation of PM. Secondly, methanol could reduce the viscosity and density of the blended fuel, leading to better atomization, better air/fuel mixing and hence lower PM emissions. Thirdly, the lower cetane number of methanol will lead to longer ignition delay, as demonstrated by the delay in start of combustion shown in Figure 3. The longer ignition delay period of the BM blends results in more fuel burned in the premixed mode and a reduction of fuel burned in the diffusion mode. Thus less soot and hence less PM is formed.

On the other hand, methanol has a much higher latent heat of evaporation of 1178 kJ/kg compared with both biodiesel and diesel fuel. Thus methanol blended in biodiesel could reduce the combustion temperature due to its cooling effect which deteriorates combustion and increases PM emissions. For all test cases, except for BM15 at light engine load, it is obvious that the higher oxygen content, lower viscosity, and longer ignition delay are the dominating factors. However, for BM15 at the light engine load, PM emissions may be dominated by the cooling effect, causing an increase in PM emissions arising from the condensation of unburned hydrocarbon.

Coupled with the reduction of PM mass concentration, there is a reduction in both total number concentration and GMD. For the BM blends, the particle size distribution curves become flatter, while there is still a slight shift of the particles towards smaller size. A comparison of PM_{< 50nm} and PM_{> 50nm} between the BM blends and biodiesel shows that the BM blends generate significantly less particles in both size groups, statistically, at the 95% confidence level. Similar comparison between the BM blends and Euro V diesel fuel shows that the BM blend generates similar level of PM_{< 50nm} but much less PM_{> 50nm} .

Several factors can be attributed to the decrease of total number concentration and GMD after adding methanol in biodiesel. First of all, because of the longer ignition delay period, more blended fuel combusted in the premixed mode and less fuel combusted in the diffusion mode, leading to a reduction of particles both in large diameter and small diameter, compared with biodiesel. Also, the microexplosion of methanol in oil droplets might permit better atomization (Zeng and Lee 2007), leading to better air-fuel mixing. Mathis et al. (2005) concluded that improved air-fuel mixture formation led to a reduction in both primary soot particle diameter and total number concentration of particles. Secondly, the methanol with lower viscosity could contribute to the lower injection pressure of BM blends and hence reduce the number of particles, compared with biodiesel. Thirdly, the higher oxygen content of methanol can reduce soot precursor production in the fuel-rich zone, due to an increased concentration of O and OH which promote the oxidation of soot precursors to CO and CO₂, leading to reduction in the number concentration of particles (Tree and Svensson 2007). Finally, the higher volatility of methanol might reduce the possibility of gas-to-particles conversion for the BM blends, resulting in the reduction of particles, in comparison with that of biodiesel. Lapuerta et al. (2008b) studied the effects of ethanol–diesel blends on the particle number concentration, and suggested that the reduction in GMD of ethanol–diesel blends was not caused by an increase in smaller particles but by a decrease in larger particles. While in this study, the reduction in GMD of methanol-biodiesel blends is due to a decrease both in small particles and large particles.

5. CONCLUSION

The combustion characteristics and particulate emissions are investigated and compared for Euro V diesel fuel, biodiesel, and biodiesel–methanol blends under different engine loads. The following conclusions can be drawn from this study:

1. For each test fuel, the peak heat release rate increases from low engine load to medium engine load, and decreases from medium to high engine load. Compared with Euro V diesel fuel, biodiesel gives lower and earlier peak heat release rate in the premixed combustion phase. With the increase of methanol fraction in the BM blends, the peak heat release becomes higher and retarded.

2. The BM blends lead to an increase of mass of fuel consumed per cycle but the mass of fuel burned in the diffusion mode decreases with the BM blends, indicating that there is an increase of fuel burned in the premixed mode, arising from the increase of ignition delay.

3. For each test fuel, the particulate mass concentration and number concentration increases with the engine load due to an increase of mass of fuel burned in the diffusion mode.

4. For each fuel, the particulate mass concentration and the total number concentration increase with the mass of fuel burned in the diffusion mode which supports that particulate is formed in the diffusion mode of combustion.

5. The total number concentration of particles from biodiesel is always higher than that of Euro V diesel fuel, while the GMD is lower, at each test mode, due to an increase of particles smaller than 50 nm.

6. When the diesel engine is fueled with the BM blends, the particulate mass concentration and number concentration decreases obviously, especially at high engine loads, mainly due to the higher oxygen content of the BM blends, and also due to the reduction of mass of fuel burned in the diffusion mode.

7. With increase of methanol in the BM blends, the total number concentration and GMD decrease in comparison with pure biodiesel and even Euro V diesel fuel. Thus biodiesel and methanol have different influence on particle number concentration due to difference in fuel properties.

Thus, it can be concluded that the use of methanol–biodiesel blends is effective in reducing both particulate mass concentration and the total number concentration of particles.

The authors would like to thank the Hong Kong Polytechnic University (GU482), Key Project of Chinese Ministry of Education and Shanghai Jiaotong University for financial support to this project.