Introduction

Alternative fuels and green fuels have supported the substantial research in modern era to improve the combustion properties of conventional fuels and efficiency and reduce environmental exhaust emissions for internal combustion (IC) engines. Due to better fuel efficiency and stability, compression ignition (CI) engines are more proficient than spark ignition (SI) engines, but suffered from elevated exhaust emissions (Franco and Mock 2015; Yao et al. 2017). Exhaust emission may be reduced by modification in engine design parameters, i.e., compression ratio (CR), injection timing (IT), and injection pressure (IP), etc. (Roberts 2003; Wang et al. 2011), or using exhaust treatment devices, i.e., particulate filter, catalytic converter, and exhaust gas recirculation system (EGR), etc. (Divekar et al. 2016; Ramalingam et al. 2018; Tang et al. 2017; Wang et al. 2017) or using alternative fuels such as biodiesel (Palash et al. 2013; Mohsin et al. 2014; Ramalingam et al. 2018), vegetable oil (Agarwal and Agarwal 2007; Corsini et al. 2015; Bayindir et al. 2017; Jain et al. 2017), compressed natural gas (CNG) (Senthilraja et al. 2016; Srivastava and Agarwal 2018), and additives (Can et al. 2004; Lapuerta et al. 2008; Sayin 2010; Sayin et al. 2010; Chen et al. 2012; Luo et al. 2012; Sukjit et al. 2012; Giakoumis et al. 2013; Kumar et al. 2013; Putrasari et al. 2013; Geng et al. 2016) with diesel. The use of additives in appropriate proportion with diesel fuel is a potential method for improving performance of engine and reducing the emissions simultaneously for latest as well as older engines without any structural changes (Curran et al. 2001; Rahman et al. 2013; Moghaddam and Moghaddam 2014; Khalife et al. 2017).

Oxygenated additives have strained more attention among all additives due to their better combustion characteristics because of high oxygen content available in their molecular structure (Choi and Reitz 1999; Rajasekar et al. 2010; Moghaddam and Moghaddam 2014; Rakopoulos et al. 2014). Oxygenated additives are renewable in character and support their oxygen in reaction for better combustion (Chen et al. 2012; Jang et al. 2012; Tutak 2014). Alcohols (i.e., methanol, ethanol, n-butanol, etc.) and nitro paraffin (i.e., nitromethane, nitroethane, etc.) are bio-oxygenated compounds with high oxygen content, low viscosity, and high volatile characteristics which make them appropriate oxygenated additives for CI engines (Desjardins et al. 2008; Leclerc 2008; Nabi and Hustad 2010; Kumar et al. 2014; Datta and Mandal 2016; Fayyazbakhsh and Pirouzfar 2017).

As per recent review studies (Khalife et al. 2017; Kumar et al. 2018b, c), incorporation of alcohols into diesel fuel at less than 10% vol. has reportedly led to improved brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC) values. Addition of alcohols into diesel fuel has also resulted in a decrease in particulate matter (PM) and hydrocarbon (HC) emissions due to presence of higher levels of oxygen in the combustion region; however, carbon monoxide (CO) and nitrogen oxide (NOx) emissions increased.

Methanol

Among alcohols, methanol (CH3OH) is one of the most promising additive fuels for long-term, widespread replacement of conventional fossil-based fuels (Nichols 2003; Celik et al. 2011; Zhen et al. 2013; Kumar et al. 2018a). Methanol is renewable, sustainable, and low-cost fuel additive, as coal is main raw material for its production (Yao et al. 2007; Yao et al. 2008; Zhen and Wang 2015; Datta and Mandal 2016; Brusstar et al. 2002). From several years, a number of researchers have used the methanol as an alternative to conventional fuels for CI engine due to following advantages associated with it:

  • Methanol concentration in the fuel blend increases the heat release rate in premixed burning phase, oxygen content, fraction of premixed combustion phase, and diffusive combustion phase, which may be responsible for improvement in engine performance (Huang et al. 2004).

  • At higher methanol substitution ratio, pilot injection could improve stability in combustion and fuel economy (Wei et al. 2017).

  • Methanol has great potential to reduce NOx and soot emissions simultaneously due to its high oxygen content and latent heat of evaporation (Yao et al. 2017).

  • Using methanol, exhaust temperature could also be lowered along with emission reduction (Najafi and Yusaf 2009).

Sato et al. (1997) investigated the combustion and emission characteristics of methanol-diesel blend in DI engine using EGR supercharging and observed higher BTE, reduced the NOx, and unburned HC emissions with heavy EGR during supercharging. For prevention of oxygen shortage and high BTE achieved, premixed combustion even under the full load condition was observed as responsible reason which involved high injection quantity. Popa et al. (2001) used the methanol-diesel blend and found significant reduction in NOx and smoke emissions at all engine loads. Huang et al. (2004) investigated the outcomes of methanol-diesel blends on CI engine and found increment in BTE and decrement in BSFC due to increased fraction of premixed combustion phase, diffusive combustion phase, and oxygen content. They also found significant reduction in CO and smoke emissions, no considerable change in HC, and increment in NOx emissions.

Najafi and Yusaf (2009) performed the experiments on CI engine using methanol-diesel blends and found that the engine performance, i.e., effective power, torque, BTE, and BSFC for methanol-diesel blends were higher as compared to diesel fuel at almost all test conditions. Also, lower exhaust temperature was obtained with DM10 as compared to pure diesel. Wei et al. (2015) examined the effect of methanol fumigation on a turbocharged inter cooling diesel engine and found increment in CO and HC exhaust emissions but NOx and soot were reduced significantly. The fumigation of methanol seems to be a promising method to solve the difficulty of the direct application of methanol in diesel engine, (Udayakumar et al. 2004; Wang et al. 2015).

The blending effects of methanol additives on performance and emission characteristics of diesel engine are summarized in Table 1.

Table 1 Performance and exhaust emission characteristics of methanol-diesel blends
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Recently, effects of pilot injection on combustion and emission characteristics of diesel-methanol dual fuel (DMDF) engine were investigated by Wei et al. (2017) to optimize combustion process at high methanol substitution ratio (M0, M10, M30, and M50) and low load condition. They reported that application of pilot injection could improve combustion stability and fuel economy at high methanol substitution ratio, and it can also reduce regulated emissions HC, CO except NOx, and unregulated emissions tested in this study except CO2 on M0 and M10 mode and toluene on M50 mode when compared with single injection cases.

Nitromethane

Nitro paraffin compounds (i.e., nitromethane (NM), nitroethane (NE), etc.) are considered as oxygenated additives due to high oxygen content in their molecular structure, which may improve the combustion characteristics such as flash point (Moghaddam and Moghaddam 2014) and cetane number (CN) (Fayyazbakhsh and Pirouzfar 2016) of additive-fuel blends. Lower boiling point of nitro paraffin compounds improves atomization and spray quality of the blended fuel which may help in obtaining the higher brake thermal efficiency (Moghaddam and Moghaddam 2014). These additives also improve the engine performance via increment in thermal energy output and combustion product alteration (Corporan et al. 2004; Boyer and Kuo 2007). Nitro paraffin compounds help in soot reduction due to improved combustion in engine and increased cylinder temperature; however, CO and NOx emissions may increase.

Performance characteristics of NM-diesel blends were evaluated in our previous studies (Kumar et al. 2014, 2015) using 4-stroke, single cylinder, DI, water cooled, naturally aspirated, variable compression ratio (VCR) CI engine. The results showed that BTE was increased with NM-diesel blends at CR 17.5 as compared to pure diesel at CR 17.5 and NM-diesel blends at CR 16.5. The maximum BTE (25.15%) was achieved at full load condition. NOx emission reduced using NM-diesel blends as compared to pure diesel at CR 17.5. NOx emission reduced to a great extent for all loads while reducing the compression ratio (CR 16.5).

In the reported literature, very few studies are available on nitro paraffin compound additives blends. The blending effects of nitro paraffin compound additives on performance and emissions of CI engine are exhibited in Table 2.

Table 2 Performance and emission characteristics of diesel-nitro paraffin compound additive blends
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Recentely, Fayyazbakhsh and Pirouzfar (2016) used nitromethane (NM) as tertiary additive with the blends of n-butanol-diesel/nano-particle in a four-cylinder, turbocharged diesel engine with EGR and common rail fuel injection system. They reported that n-butanol in blend made reduction in CN, but CN increased by adding NM (1%).

As per the reported literature (Khalife et al. 2017; Kumar et al. 2018b, c), methanol and nitromethane have superior combustion characteristics due to high oxygen content, low boiling point, high latent heat, etc. Therefore, in the current study, diesel-methanol-nitromethane (D-M-NM) ternary blend was used in CI engine to investigate the performance and emission characteristics of engine experimentally at various concentrations, loads, and compression ratios, as the performance of this ternary blend has not been investigated till date according to available literature.

Experimental facility, material and methods

Fuel preparation and properties

Fuel preparation was the primary step for the experiments in which pure methanol (purity ≥ 99.0%, residue ≤ 0.001%, water ≤ 0.2% by weight) and nitromethane (purity ≥ 99.0%, acidity as acetic acid ≤ 0.1%, water ≤ 0.1% by weight) were mixed with diesel fuel. Table 3 shows the various physical and combustion properties of methanol, nitromethane and diesel separately.

Table 3 Properties of methanol, nitromethane, and diesel (Sayin et al. 2010; Moghaddam and Moghaddam 2014; Tutak et al. 2015)
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Diesel-methanol-nitromethane concentration was taken on v/v percentage for preparation of all D-M-NM blends. First, the diesel fuel was taken (as per blending ratio) in glass container and then methanol (as per blending ratio) poured drop by drop in diesel as well as homogenized by magnetic stirrer. After that, nitromethane (fixed amount 2.5%) was mixed in diesel-methanol blend in the same manner. Above procedural steps (Fig. 1) were repeated for each fuel blend.

Fig. 1
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Fuel blending steps

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Before experimentation, stability of fuel blends was also checked and no settling was observed for about 96 h. The sample was kept at room temperature and observed at regular intervals for any settling. Figure 2a-c shows the pictures of pure diesel, D-M7.5-NM2.5 blend at beginning and after 96 h, respectively.

Fig. 2
figure 2

Photographs of the fuel sample. a Pure diesel. b D-M7.5-NM2.5 blend at beginning. c D-M7.5-NM2.5 blend after 96 h

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The properties of different D-M-NM blends (D-M2.5-NM2.5, D-M5-NM2.5, D-M7.5-NM2.5) were measured by the standard laboratory equipments. For measuring the calorific value of fuel blend, bomb calorimeter (make: Aditya, max power: 1.5 kW, precision: < 0.1% RSD) was used as shown in Fig. 3. The process of a calorimeter involves burning of fuel sample in an oxygen-filled metal bomb which is kept in a water jacket. By observing the temperature rise of the water and knowing the energy equivalent of the calorimeter, the heating energy or calorific value of the fuel can be calculated. Hydrometer (make: Liemco, model: M-50, range: 0.850-0.900 g/ml) as shown in Fig. 4 was used to measure the relative density of fuel blends, which is based on the concept of buoyancy.

Fig. 3
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Bomb calorimeter

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Fig. 4
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Hydrometer

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The properties of different blends were also calculated using Eqs. (1)-(3) as mentioned in previously published studies (Park et al. 2013; Nayyar et al. 2017) and found calculated values approximately same as the measured values. The composition of various D-M-NM blends and their properties are shown in Table 4.

$$ Calorific\ value\ of\ blends\ (CV)\left( kJ/ kg\right)=\left\{\left(\frac{V_m}{V}\times {\rho}_m\times {CV}_m\right)+\left(\frac{V_n}{V}\times {\rho}_n\times {CV}_n\right)+\left(\frac{V_d}{V}\times {\rho}_d\times {CV}_d\right)\right\}/\left\{\left(\frac{V_m}{V}\times {\rho}_m\right)+\left(\frac{V_n}{V}\times {\rho}_n\right)+\left(\frac{V_d}{V}\times {\rho}_d\right)\right\} $$
(1)
$$ Cetane\ No. of\ blends\ (CN)=\left(\frac{V_{\mathrm{m}}}{V}\times {CN}_m\right)+\left(\frac{V_d}{V}\times {CN}_d\right) $$
(2)
$$ Density\ of\ blends\ \left({\rho}_b\right)=\frac{\mathrm{X}}{\frac{X_m}{\rho_m}+\frac{X_n}{\rho_n}+\frac{X_d}{\rho_d}\ } $$
(3)

where

CV :

= calorific value of blend (kJ/kg),

CVm, CVnand CVd:

= calorific value of methanol, nitromethane and diesel,

V:

total volume,

Vm, Vn, and Vd:

= volume percentage of methanol, nitromethane, and diesel,

ρ m , ρ n , and ρ d :

= density of methanol, nitromethane, and diesel,

CN m and CN d :

= cetane number of methanol and diesel,

X :

= total mass,

Xm, Xn, andXd:

= mass fraction of methanol, nitromethane, and diesel.

Table 4 Composition of diesel-methanol-nitromethane blends and their properties
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Test engine

The experiments were done on four-stroke, single-cylinder, water-cooled, constant-speed, DI, VCR, CI engine coupled with air-cooled eddy current dynamometer. For varying the CRs, lifting and lowering cylinder block arrangement was used without altering the geometry of combustion chamber and stopping of engine. The schematic layout and pictorial view of experimental setup are as depicted in Fig. 5a, b, respectively. Table 5 demonstrates all technical specification of the engine and other measuring equipments.

Fig. 5
figure 5

a Schematic layout. b Pictorial view of the experimental setup

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Table 5 Equipment specifications and various formulae used to determine different quantities
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Before conducting the experiments, fresh and clean lubricating oil was filled in the engine oil sump. The experimental results were monitored and stored on the pc-based LABVIEW software.

Test procedure and calculation of engine performance parameters

The experimentations on engine were carried out at constant speed of 1500 rpm by varying the load from 0 (no load) to 100% of rated power (full load) using pure diesel and D-M-NM blends. Each test has carried out for half an hour (to get steady-state condition) before taking the final readings. Once a test is completed with one blend, the remaining blend was eliminated from the fuel line and fuel tank for put off mixing and change of genuine ratio of blend. The experiments were performed in two stages. Diesel and diesel-methanol-nitromethane blends were tested on normal engine test conditions (CR 18.5, IP 210 bar and IT 23° CA btdc) in first stage. In the second stage, experiments were performed on different CRs (16.5, 17.5, 18.5, 19.5, and 20.5) to find out the best suitable blend with most favorable engine operating parameters.

For all fuel blends, exhaust emissions (i.e., NOx, CO, and HC) were measured by exhaust gas analyzer (make: AVL-DIGAS, model: 444N), and exhaust smoke emission (smoke density) was measured by a smoke meter (make: AVL Austria, model: 437C) as shown in Fig. 5b. The fundamental principle for measurement of NOx emissions is electrochemical method, whereas non-dispersive infrared radiation (NDIR) method is used for CO and HC emission measurement.

Uncertainty analysis

There were chances of uncertainty/error during taking the experimental data due to engine operating conditions, human errors, precision of measuring equipments, schedule of testing, etc. Therefore, all the instruments used in test set up were properly calibrated and uncertainty analysis was carried out as per the guidelines given in previous studies (Moffat 1988; Plint and Martyr 2007). To obtain precision of given parameters, the root sum square method was used. Uncertainty analysis results for different performance and exhaust emission parameters are as depicted in Table 6. The uncertainty range attained through analysis was found to be quite low and comparable with other published research studies in this area.

Table 6 Uncertainty of various parameters
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Results and discussion

Performance and exhaust emission characteristic of D-M-NM blends

The performance and exhaust emission of engine tested at standard CR of engine 18.5 with different blends of D-M-NM are as depicted in Figs. 6, 7, 8, 9, 10, and 11. Figure 12 shows the values of engine performance parameters and various exhaust emissions at full load condition.

Fig. 6
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Variation in BTE with BP for pure diesel and D-M-NM blends

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Fig. 7
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Variation in BSFC with BP for pure diesel and D-M-NM blends

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Fig. 8
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Variation in smoke with BP for pure diesel and D-M-NM blends

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Fig. 9
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Variation in NOx with BP for pure diesel and D-M-NM blends

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Fig. 10
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Variation in CO with BP for pure diesel and D-M-NM blends

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Fig. 11
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Variation in HC with BP for pure diesel and D-M-NM blends

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Fig. 12
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Various outcomes for pure diesel and D-M-NM blends at 100% rated power

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It was observed that best BTE and BSFC values are obtained with D-M5-NM2.5 blend among all D-M-NM blends and pure diesel. Methanol has lower density (729 kg/m3) as compared to the diesel (829 kg/m3), which helps to make stable solution of blends. Methanol and nitromethane have higher oxygen content as compared to diesel and it helps to complete the combustion of fuel due to which increased BTE and decreased BSFC values are obtained. From Fig. 12, it can be concluded that 13% increment in BTE and 19.5% decrement in BSFC can be achieved using D-M5-NM2.5 blend as compared to diesel at 100% rated power (full load condition). However, D-M7.5-NM2.5 blend showed increment in BSFC as compared to D-M5-NM2.5 due to decrement of lower calorific value and lower cetane number, as indicated in Table 4. In BSFC increment at higher blend, lower cetane number plays a major role, as it increases ignition delay significantly which increases the amount of combustible fuel prepared within the period of the ignition delay (Huang et al. 2004; Nayyar et al. 2017). Lower density and viscosity of methanol are also responsible for increasing fuel consumption (Datta and Mandal 2016).

The exhaust emission for D-M-NM blends was found to be having considerable reduction in smoke quantity as shown in Fig. 8. It is mainly due to lower density of methanol which makes the blend solution more homogenized; and higher volatility of methanol (boiling temp. 64 °C) and nitromethane (boiling temp. 162 °C) in comparison with diesel (boiling temp. 185-340 °C), both factors improve the combustion quality. Another aspect for better result is due to high oxygen content which provides sufficient oxygen in the fuel-rich region and oxidizes the smoke. At full load condition, 26.47% reduction in smoke emission was found with D-M5-NM2.5 blend with respect to diesel as summarized in Fig. 12.

As the NO from the fuel-nitrogen present in nitroparaffin structure is negligible in comparison with thermal NOx, (Moghaddam and Moghaddam 2014), only thermal NOx was considered in the current study. Although thermal NOx content in emission for all blends was found to be comparatively low with respect to diesel, but this reduction was significant using D-M2.5-NM2.5 and D-M5-NM2.5 as shown in Fig. 9. The NOx reduction could be achieved due to two possible reasons. Firstly, the blending of M-NM with diesel reduces cetane number (Table 4) and thereby increases delay period. The increased delay gives sufficient time to fuel for mixing with air and reducing the cylinder temperature, which may reduce the formation of NOx in emission. Secondly, the blending of M-NM with diesel improves homogeneity and combustion of fuel blend, resulting in higher (peak) temperature for short period due to increased flame speed at the same time.

Long ignition delay due to high concentration of methanol in fuel blend (D-M7.5-NM2.5) resulting in higher peak temperature is mainly responsible for increment in NOx as compared to the other fuel blends (D-M2.5-NM2.5 and D-M5-NM2.5). The maximum reduction (21.66%) in NOx emission was obtained with D-M5-NM2.5 blend as compared to diesel at 100% rated power as depicted in Fig. 12.

Initially at no load condition, CO emission was found to be high with all fuel blends and diesel and decreased with increase in load as shown in Fig. 10. Minimum CO emissions were achieved with pure diesel and after diesel; D-M5-NM2.5 exhibited the best results among all fuel blends even better than diesel at full load condition. Among all fuel blends, highest decline (14.28%) in CO emission was obtained with D-M5-NM2.5 blend with respect to the diesel at 100% rated power as indicated in Fig. 12. The higher value of CO associated with D-M7.5-NM2.5 blend was possibly due to higher latent heat and self-ignition temperature of methanol (Table 3). Due to higher latent heat, addition of methanol content in blend cools the air-fuel mixture at the beginning of combustion and may cause of incomplete combustion. Delay in the beginning of combustion due to higher self-ignition temperature of methanol may also be the cause of incomplete combustion and increased CO content in emission as mentioned by Chao et al. (2001) and Moghaddam and Moghaddam (2014).

Similar to CO emission results, minimum HC emissions were obtained with pure diesel and after diesel, D-M5-NM2.5 exhibited the best results among all fuel blends as shown in Fig. 11. As compared to diesel, higher latent heat and self-ignition temperature; and lower cetane number of D-M-NM blends resulting in longer ignition delay may be the responsible reasons for higher HC content in emission (Chao et al. 2001). Among all fuel blends, increment in HC emission was found lowest (10.71%) using D-M5-NM2.5 with respect to neat diesel at 100% rated power condition as indicated in Fig. 12.

From performance and exhaust emission results as summarized in Fig. 12, D-M5-NM2.5 was found to be most favorable blend among all D-M-NM blends and diesel for CI engine; therefore, further study was focused in line to find out best suitable CR value for this blend (D-M5-NM2.5).

Selection of compression ratio for D-M5-NM2.5 blend

After selection of D-M5-NM2.5 as most favorable blend, the engine was tested at different compression ratios (i.e., 16.5, 17.5, 18.5, 19.5, and 20.5). The performance and exhaust emission of engine using D-M5-NM2.5 blend with different compression ratios are as depicted in Figs. 13, 14, 15, 16, 17, and 18. Figure 19 shows the values of engine performance parameters and various exhaust emissions for D-M5-NM2.5 blend with different CR at full load condition.

Fig. 13
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Variation in BTE of D-M5-NM2.5 blend with BP for various CR

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Fig. 14
figure 14

Variation in BSFC of D-M5-NM2.5 blend with BP for various CR

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Fig. 15
figure 15

Variation in smoke of D-M5-NM2.5 blend with BP for various CR

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Fig. 16
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Variation in CO of D-M5-NM2.5 blend with BP for various CR

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Fig. 17
figure 17

Variation in HC of D-M5-NM2.5 blend with BP for various CR

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Fig. 18
figure 18

Variation in NOx of D-M5-NM2.5 blend with BP for various CR

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Fig. 19
figure 19

Various outcomes for D-M5-NM2.5 blend with different CR at 100% rated power

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Best engine performance with more power (BTE) and fuel economy (BSFC) was obtained at CR 19.5 as shown in Figs. 13 and 14. Enhancement in BTE and reduction in fuel consumption were obtained as 3.8% and 3.4%, respectively, with respect to standard CR (18.5) using D-M5-NM2.5 blend at 100% rated power as indicated in Fig. 19. Higher value of CR than standard value creates more pressure, temperature, and swirl of air at the beginning of combustion, resulting in better fuel-air mixing and less delay period.

The D-M5-NM2.5 blend at CR 19.5 also exhibited best results for smoke, CO, and HC emissions (Figs. 15, 16, and 17) due to better combustion of fuel at higher CR. Higher CR results in shorter ignition delay and better mixing of fuel-air. The reduction in smoke, CO, and HC was observed as 10%, 16.67%, and 61.29%, respectively, at CR 19.5 in comparison with standard CR 18.5 for full load condition as shown in Fig. 19.

It can be observed from Fig. 18 that NOx emission showed the opposite results as compared to other emissions. Using D-M5-NM2.5 blend at higher CR values resulted the higher NOx content in the emission due to higher combustion temperature. An increment of 6.38% in NOx was observed for CR 19.5 with respect to the standard CR 18.5 at 100% rated power as indicated in Fig. 19. Lowest value of NOx was observed at CR 16.5 due to low temperature of combustion chamber.

As per the summary of engine performance and emission results (Fig. 19), best performance of D-M5-NM2.5 blend was observed at CR 19.5.

Comparison of D-M5-NM2.5 blend (at CR 19.5) with diesel (at CR 18.5)

Figure 20 shows the comparison of performance and emission characteristics of D-M5-NM2.5 blend (at CR 19.5) with diesel (at CR 18.5). It can be observed that there is slight improvement in BTE (17.39%) and considerable reduction in BSFC (22.2%), smoke (33.82%), NOx (16.67%), CO (28.57%), and HC (57.14%) with D-M5-NM2.5 blend as compared to pure diesel at 100% rated power.

Fig. 20
figure 20

Comparison of D-M5-NM2.5 blend with pure diesel

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Conclusions

Experimental study was carried out to evaluate the performance and exhaust emission characteristics of VCR stationary CI engine fueled by diesel-methanol-nitromethane blends. Initially, D-M-NM blends were tested at normal engine test conditions (CR 18.5, IP 210 bar and IT 23° CA btdc) and compared with diesel to obtain best suitable blending ratio. D-M5-NM2.5 blend was found to be most favorable blend which provides the better combustion characteristics due to rich oxygen content present in it. This blend improved the engine performance (13% increment in BTE and 19.5% decrement in BSFC) and controlled emissions (smoke, NOx, and CO reduced by 26.47%, 21.66%, and 14.28%, respectively) simultaneously as compared to diesel at full load condition. However, slight increment in HC emission was found by 10.71%.

After getting the best blend (D-M5-NM2.5), experiments were further performed at different CRs to find out the best suitable value of CR for D-M5-NM2.5. Higher compression ratio of 19.5 gave the better results as compared to CR 18.5 (standard) under similar operating conditions using D-M5-NM2.5 blend due to higher pressure and temperature produced. An increased compression ratio also compensates the effect of the long delay period. At CR of 19.5, further improvement in engine performance (BTE increased 3.8% and BSFC decreased 3.4%) and reduction in emissions (smoke 10%, CO 16.67%, and HC 61.29%) were observed as compared to CR of 18.5 at rated power. However, NOx was found to be with negligible increment of 6.38% due to higher combustion temperature. By comparing the D-M5-NM2.5 blend (at CR 19.5) with pure diesel (at CR 18.5), significant level of enhancement in performance (BTE increased 17.39% and BSFC decreased 22.2%) and reduction in emission (smoke 33.82%, NOx 16.67%, CO 28.57%, and HC 57.14%) was found at 100% rated power.