Introduction

The industrial revolution and brisk growth of the world’s population have led to a significant influence on climate change and global warming [1]. Moreover, apart from the exponential growth of energy demand all over the world, the need for a low cost, less air contaminant, and high-quality fuel has remained a concern [2]. Diesel fuel as a cheaper fuel than gasoline is a widely consumed fuel, in sectors such as transportation, agriculture, energy generation, mining, military industry, etc. [3, 4].

The complete combustion would yield carbon dioxide, water, and nitrogen [5]. However this combustion in diesel engines is not always complete and various combustion-related side products, such as nitrogen oxides (NOx), carbon monoxides, air unburned total hydrocarbon, volatile organic compounds (VOC), and particulate matter (PM), would be generated in the course of incomplete combustion [1, 6]. It is noteworthy to outline that the primary air pollutants exhausting from diesel engines include NOx and PM, and the emission of VOC and CO is lower compared with gasoline engines [7].

Many studies have been performed to develop more efficient, reliable and cleaner diesel engines by employing multi-stage injection, turbochargers, exhaust gas recirculation, etc. [8-10]. Although significant developments towards inventing and utilizing new technologies have led to considerably reducing air contaminant emission, the request for improving pollutant emissions behavior of these types of engines has become obligatory [11, 12]. Furthermore, many efforts have been made on improving diesel fuel quality through blending with various additives such as vegetable oils, biofuels alcohols, etc., as an alternative method to meet environmental regulations [13].

One of the most appropriate and efficient fuel additives is fuel system and engine performance improvement agents, which is comprised of injector cleaners, soot formation reducers, fatty additives, and cetane number (CN) improvers [14, 15]. Oxygen additives, nitrogen additives, and oxide/solid nanoparticle additives are CN improvers that have been blended with diesel fuels extensively [16-18]. Oxygen additives are polar compounds which exhibit the significant potential of reducing PM under companion increase of NO_x, through providing required oxygen for complete combustion. Alcohols and methyl/ethyl esters (i.e., biodiesel) are amongst the most studied oxygen additives; however, various investigations have been carried out on the effects of ethers, glycol ethers, and esters on fuel characteristics, pollutant emission, and combustion quality [19].

Different studies have shown that PM emissions would decline as the oxygen content of the fuel is increased. However, some others have outlined that this PM emission reduces when the molecular structure of blended oxygen additive differs [20]. Curran et al., [21] found out that PM emission would decline when the oxygen content is increased, and carbon-carbon bonds are decreased as well. Miyamoto showed that the formation of soot and its associated particles only depends on fuel’s oxygen content regardless of oxygen functional group type [22]. Huang et al., [23] showed that increasing methanol would engender soot and CO formation, but increase NOx prediction, during combustion. Najafi and Yusaf [24] investigated three various mass ratios of methanol: diesel and found that the blended fuel with 10:90 ratio, would generate lower soot temperature and better Brake thermal efficiency (BTE); however, the generated torque of the all synthesized blends was lower than diesel fuel. Some researchers have studied the effects of ethanol and/or butanol addition to the fuel diesel (varies from 2 vol. % to about 50 vol. %) on the contaminant emission, CN, fuel stabilization, power generation, etc. improvement. They showed that increasing alcohol proportion would mostly tend to significant PM reduction, whereas, in some investigations, NOx and CO emission were reduced and in some others increased. Koganti et al., [25] showed that utilizing alcohol would tend to decrease BTE whereas the decline in generated power has been reported.

The application of dimethyl ether (DME) as diesel fuel has attracted great attention due to it’s environmentally, friendly and high-quality combustion [26-28]. Elasi et al., [29] showed that application of DME instead of fuel would produce lower soot amounts and increase nominal engine revolution while no reduction in engine output power and torque were reported. There are many studies about the effects of nitrogen additives on fuel quality and combustion. Starkman [30] showed that nitroparaffins usage as a fuel additive would increase the engine’s revolution without any further changing the motor’s size or application of significant mechanical complexities. He also reported that nitro paraffin addition could enhance spark ignition in lower temperatures due to their temperature-sensitive property, which is regarded as a great advantage. Nitromethane and nitroethane are amongst the most being noticed nitroparaffins owing to high oxygen content, which limits PM [31]. The results obtained by Zhang et al., [32] indicate that the addition of 2-ethylhexylnitrate (ETH) to the diesel fuel consisting of 40 vol. % of dimethylfuran would tend to 80 % of soot emission reduction because of better adaptability of knocking and soot emission.

According to what mentioned before, oxygen (i.e., alcohols) and nitrogen (i.e., nitromethane and EHN) additives, in this regard, are amongst the most important additives due to exhibiting better performances. So, the main aim of this study was to evaluate individual effects of isobutanol, isobutanol + nitromethane, and EHN as well as simultaneous influences of isobutanol + nitromethane + EHN (INE) on diesel fuel properties such as calculated cetane index (CCI), distillation and volatility, flash and pour points as well as density and viscosity. Moreover, the impacts of three different alcohols additives consisted of n-pentanol, n-butanol, and isobutanol were investigated through the combustion process.

Material and methods

In this study, two Euro-IV grade diesel fuels (supplied by Isfahan refinery) were utilized.  Table 1 presents the properties of studied unblended diesel fuels which were determined by the Isfahan refinery. Isobutanol, pentanol, n-butanol, and EHN were purchased from Sigma-Aldrich and nitromethane was obtained from Merck.

Table 1: Properties of studied unblended diesel fuels

The solubility of nitromethane in diesel was found inadequate, so three alcohols (pentanol, n-butanol, and isobutanol) were used to prepare BDF followed by measuring the cetane index (CI) of the obtained samples. To obtain this purpose, a given amount of nitromethane was first solved in each type of alcohol and then diluted by fuel 1 to obtain 5 wt. % of nitromethane in diesel fuel. After selecting the best alcohol to dissolve nitromethane in diesel fuel 1 blend based on samples’ CI, four different blends comprising of 5, 10, 15, and 20 vol. % of nitromethane were prepared followed by property evaluation of each sample. In the next step, EHN was added to the diesel fuel 2 and stirred for a few second to obtain 0.02, 0.05, 0.1, 1, and 2 vol. % mixtures. It is noteworthy that EHN is completely soluble in fuel diesel and a new additive was not required. After finding optimum compositions, the given amounts of alcohol + nitromethane + EHN were added to the diesel fuel 2.

Property measurements

To investigate how BDF would be affected under the influence of different additives, it is necessary to characterize blend fuel diesel through appropriate tests. On the other hand, the combustion process would significantly vary under the effect of fuel properties. The CCI, distillation behavior, flash point, pour point, density, and viscosity are amongst the most critical parameters that should be determined. This result could make a better understanding of the additives' influence on the BDF’s properties [3].

Calculated Cetane Index

CCI is a critical index that plays a significant role in evaluating diesel fuel quality. It indicates the delay time between fuel injection and ignition [5, 33]. Several methods have been proposed to measure CCI such as ASTM D 976, ASTM D 4737 and other methods that are based on the physical properties of diesel fuel. ASTM D 976 is one of the widely used approaches for calculating CCI based on API gravity and mid-boiling point (T₅₀). In this study, CCI was calculated according to the ASTM D 976 through the following formula (1) [34]:

CCI = 454.74-1641.416D+774.74D²-0.554B+97.803(log B)²                                      (1)

where D and B are the density at 15 °C and the mid-boiling point that were calculated employing ASTM D 1298 and ASTM D 86, respectively [34].

Flash point

The flash point is a descriptive fuel diesel property that identifies the flammability of fuels. It is defined as the minimum temperature that fuel will ignite when it is exposed to an ignition source. The shipping and storage classification of fuel would be identified according to the flash point data [35]. The ASTM D 93 as a standard approach, which is also known as Pensky-Martens closed cup method, was employed for calculating diesel fuel flash points [34].

Pour and Cloud points

Samples’ pour and cloud points were calculated according to ASTM D 6749 and ASTM D7683 (MINI POUR/CLOUD POINT TESTER MODEL MPC-601, TANAKA, Japan) respectively.

Density and viscosity

The density of diesel fuel and all of the prepared BDF were measured according to ASTM D 4052 [34]. A viscometer (SVM 3000/G2, Anton Paar, USA) was employed to calculate the viscosity of the samples that works based on ASTM D 7042.

Result and Dissection

Individual effects

Effect of alcohol type on the BDF’s properties

As mentioned before, in order to identify the best type of alcohol for the purpose of incorporation into the diesel fuel, three mixtures consisting of 5 vol. % of nitromethane in three different alcohols were prepared. As it is shown in Table (2), the BDFs, prepared by blending isobutanol and pentanol as the solvents, have the highest and lowest CCI, respectively. In other word, as heavier alcohol was utilized, higher CCI’s was observed owing to the extension of the alcohol’s carbon chain that may lead to the easier decomposition of OH bonds.

Additionally, comparing effects of n-butanol and isobutanol showed that CCI was increased as the alcohol was branched which could be related to easier decomposition of OH bond in isobutanol than that in n-butanol. CCI, flash and pour points, and density of studied alcohols are much lower than those of neat diesel fuel, so the addition of alcohols to the diesel fuel could lead to a decline in mentioned properties (Table 2). Moreover, the addition of heavier and branched alcohols showed lower and higher pour points than diesel fuels blended with light and linear alcohols, respectively. It could be said that the heavier alcohols have similar properties to the diesel fuel and result in more paraffin contents and vice versa [36]. Accordingly, the mixture of isobutanol + nitromethane (5 vol. %) in diesel fuel (95 vol. %) was selected as the best blend.

Table 2: Effect of different alcohols on the diesel fuel’s properties

Effect of nitromethane+ isobutanol and EHN on the BDF’s CCI

After selecting isobutanol as the best solvent, influences of isobutanol, nitromethane+ isobutanol, and EHN on the CCI of the BDF were investigated. As shown in Figure (1), the addition of isobutanol from 2 vol. % to 5 vol. %, into the BDF, significantly reduced CCI from 50.8 to 50.3. However, the addition of isobutanol along with nitromethane to the BDF increased CCI, in which, higher proportions of nitromethane in isobutanol and finally in the BDF would tend to a significant enhancement of CCI compared with that of diesel fuel. It could be related to CCI improver properties of nitromethane resulted from its oxygen content [14].

Figure 1: Effect of alcohols and nitromethane addition on CN of the diesel fuel

Figure 2 shows the influence of the EHN addition on CCI of the BDF. As it was illustrated, the increased values of CCI for BDF at lower amounts of ETH was followed by a decline as the further amount was added. In the course of combustion, nitrate molecules of EHN could break down and lead to the generation of free hydroxyl radicals that decompose the diesel fuel [37]. Further increasing EHN concentration in the BDF would limit the ignition due to the high production of NO and NO₂ [32].

Figure 2:Effect of EHN on the CN of the diesel fuel

Effect of isobutanol, nitromethane+isobutanol and EHN on distillation and volatility

The distillation data of diesel fuels 1 and 2, and BDFs blended with pentanol, n-butanol, isobutanol, nitromethane+isobutanol and EHN, are presented in Table 3. The addition of alcohols to the diesel fuel and increasing nitromethane content would tend to raise and reduce boiling points, respectively. Pentanol, n-butanol, and isobutanol addition to the diesel fuel on the concentration of 5 % vol., exhibited 23.81 %, 30.95 %, and 35.71 % reduction in IBP with respect to that in diesel fuel. It could be attributed to the lower IBP of the added alcohols compared with that of diesel fuel [38, 39].

Table 3: Distillation data of the unblended and blended diesel fuels

Moreover, IBP and FBP of the BDFs containing low concentrations of the EHN were higher than those of diesel fuel, and further increasing EHN to above 1 % would tend to a decline in IBP and FBP. Comparing the date of the recovered, residual, and loss contents of distillation data showed that with increasing EHN content, the amount of the formed cock was also raised consistently due to increased reactivity of the EHN [37].

It is noteworthy that the color of the BDF included EHN started to get darker as the temperature raised from IBP to T₁₀, in a manner that at higher contents of EHN, the dark color was accelerated. It could be attributed to released free radicals that have been generated during the decomposition of EHN in the mentioned distillation temperature intervals [1, 2].

Effect of isobutanol, nitromethane+isobutanol, and EHN on flash point

The flash point was evaluated with respect to 2 vol. % and 5 vol. % of isobutanol, 5, 10, 15, and 20 vol. % of dissolved nitromethane in isobutanol as well as 0.02, 0.05, 0.1, 1, and 2 vol. % of EHN. As it was represented in Figure 3a, increasing isobutanol content would reduce flash point more than it for diesel fuel. Moreover, it would be restored slightly by the addition of nitromethane and increasing its concentration in BDF, due to lower flash points of alcohols compared with diesel fuel and CCI inhibitor property of the nitromethane [40]. The addition of EHN to the diesel fuel continuously increased flash point of BDF that could be related to the weak bondings of functional groups of EHN at moderate temperatures [37]. In other word, EHN starts decomposing to more active components that lead to better reacting with oxygen (Figure 3b).

Figure 3: (a) Effect of different alcohols and nitromethane addition on the flash point, (b) Effects of EHN addition on the flash point

Effect of isobutanol, nitromethane+isobutanol, and EHN on pour point

As illustrated in Figure 4, the pour point of BDFs was declined while the isobutanol content was raised, due to lower pour points of isobutanol. However, the pour point of BDF exhibited no significant influence while the nitromethane concentration was increased. It was owing to the dependency of pour point on the paraffin contents of the fuel. It was also attributed to the small size of nitromethane molecules which could not lead to a significant change in pour point [36].

Figure 4:Effect of alcohols and nitromethane on the pour point of the diesel fuel

Figure 5 represents the influence of the EHN addition on the pour point of the diesel fuel. As could be seen, increasing EHN would steadily reduce the pour point due to the smaller size of EHN molecules [37].

Figure 5: Effect of EHN addition on the pour point of the diesel fuel

Effect of isobutanol, nitromethane+isobutanol, and EHN on density

The addition of isobutanol and increasing its content from 2 vol.% to 5 vol. % reduced the density of the BDF and further increasing nitromethane concentrations resulted in raising the density compared to that for diesel fuel (Fig. 6 -a). Figure (6 -b) shows the influence of EHN addition on the density of the BDF. As could be seen, no significant impact was observed at low concentrations of EHN but further increasing in EHN content would lead to an exponential rise of density owing to the higher density of the EHN compared to that of diesel fuel.

Figure 6: (a) Effect of alcohols and nitromethane on the diesel fuels’ density, (b) Effects of EHN on the density of the diesel fuel

Effect of isobutanol, nitromethane+isobutanol, and EHN on viscosity

The viscosity of BDFs in various temperatures was evaluated and compared with that of the diesel fuel through the addition of 2 and 5 vol. % of isobutanol solution with 0, 10 and 20 vol. % of nitromethane (Table 4). Although the final blend viscosity was dependent on the viscosity of the diesel fuel, the addition of isobutanol to the diesel fuel was likely to yield an overall viscosity in a manner that the more isobutanol content, the more decline in viscosity observed. It could be attributed to the lower viscosity of the isobutanol, which is close to the ASTM minimum, compared with that of diesel fuel, in which acts as a dilutant and reduces the viscosity [2, 34]. However, increasing nitromethane content of the BDFs exhibited a viscosity restoration owing to the higher molecular weight of the nitromethane that resulted in a change the molecular mobility of the overall blend [14].

Table 4: Effect of various amounts of isobutanol and nitromethane on the viscosity of the diesel fuel

As the low concentrations of EHN were added to the diesel fuel, viscosity was reduced slightly and followed by sharp declines as further EHN amounts were added to the diesel fuel (Fig. 7). The density of the diesel fuel was determined based on the bulk property, thus while the small proportions of EHN were added to the diesel fuel, the viscosity could not be affected significantly. Accordingly, the addition of higher amounts (e.g., greater than 1 vol. %) of EHN would tend to exhibit a clear decline in viscosity. This could be related to the complex structure of the EHN, in which, when incorporated into the blend at high concentrations, the proportion of molecules with complex structure and ability of stronger intramolecular bondings is increasing that tend to decline significantly in kinematic viscosity of the BDF [41]

Figure 7:Effect of EHN addition on the viscosity of the diesel fuel

Optimum compositions of BDFs

The optimum composition of BDFs was selected considering all of the properties, which were investigated in previous sections. This selection was performed based on higher CCI, flash point, density, and lower volatility, pour point and viscosity compared with those of diesel fuel. Accordingly, the sample that was blended with 2 vol. % of isobutanol + nitromethane containing 20 vol. % of nitromethane was selected as the optimum BDF of diesel fuel 1. Moreover, the 0.02 vol.  % of EHN was selected as the optimum concentration based on higher CCI, volatility, flash point as well as lower pour point, viscosity.

Simultaneous effects

After investigating simultaneous effects of alcohols, nitromethane, and EHN on diesel fuel properties the influence of the optimized amount of added INE these properties were evaluated.

Effect of INE on CCI

Figure 8 illustrates the effect of INE addition on the CCI of BDF. As perceived from Figure 8, the CCI of the BDF with INE was higher than that of the pure diesel and BDF with EHN (0.02 vol. %). It could be related to CCI improver properties of EHN and nitromethane that restored the CCI decline resulted from isobutanol addition [37].

Figure 8: Effect of INE on CN of the diesel fuel

Effect of INE on distillation and boiling point

The distillation data of the diesel fuel blended with INE is shown in Table (5). A decrement in IBP and boiling points lower than 200 ⁰C, which means increasing volatility of the BDFs, was observed due to the presence of isobutanol.

Table 5: Effect of INE on the boiling points of the diesel fuel compared with unblended diesel fuel