Introduction

The oil reserve has been continuously depleted and this leads to increase in pollution and increase in the cost of the fossil fuel. A tremendous increase in vehicle population leads to demand for petroleum fuels. The usage of petroleum fuel increases the global warming, which leads to serious problems such as climate change and rises in the sea level (Bragadeshwaran et al. 2018). Exhaustion of petroleum fuels and enlargement of environment pollutions are the main problems to search other sources for the diesel engines. Reports of various researchers, published in the last 3 years on fossil fuels, show fast falls in its reserves which strongly suggests for various alternatives. A diesel engine is widely used as a power source in the agricultural sector, passenger vehicles, and mobile and immobile generators (Panahi et al. 2019). With an increase in the population, the usage of engine increases and the demand for fossil fuel also increases. The emission due to the burning of fossil fuel also increases; greenhouse gas also increases. The usage of biofuel produces a nullified effect on carbon dioxide because of the lesser cycle period. This leads to the search for alternate fuels which reduces the usage of fossil fuel (Ramalingam et al. 2020). Today, the world has been using petroleum products like petrol and diesel to meet the energy demands in transportation, agriculture, and industrial sector. All the economic activities are in a way related to energy in the present-day society (Janakiraman et al. 2020). The use of energy from fossil fuel leads to an environmental hazard. At the same time, the demand for energy is also growing at an alarming rate. The world is turning towards renewable sources such as vegetable oil, biogas, and alcohol. These fuels are easily available, biodegradable, non-toxic, and eco-friendly (De Menezes et al. 2006). Compression ignition engines are widely used as a power source for transportation vehicles and to generate electricity; it’s because of its higher efficiency than SI engine and durability. However, they are considered a major source of air pollution in urban areas due to their black smoke, oxides of nitrogen, hydrocarbon, carbon monoxides, carbon dioxide emissions, and particulate matter (Torres-Jimenez et al. 2011). Alcohol fuels are alternative for petroleum-based fuels. Alcohol is made from renewable resources like biomass such as sugarcane, sugar beet, and wood and from waste products like waste paper, grass, and tree. It can be used as fuel for both compression ignition engine and spark ignition engine. Ethanol has advantages in agricultural feedstocks, because it is highly biodegradable, renewable, and a promoter of rural economy (Guo et al. 2011; Hariz and Takriff 2017). Direct usage of pure ethanol cannot be implemented in CI engine. Blends of ethanol and diesel can be used as fuel. Many researchers have done their research on ethanol-diesel blend but none of the researchers has done their research in the usage of additive to increase homogeneity and the cetane number. Some of the reviews on the usage of alcohol, diesel, and biodiesel are done and summarized below.

Yesilyurt and Aydin (2020) had done a research on blends of cottonseed oil biodiesel, diethyl ether, and diesel. The different fuel and its blend ratios used for the test purpose were D100, B20, D77.5B20DEE2.5, D75B20DEE5, D72.5B20DEE7.5, and D70B20DEE10. The peak BTE is achieved at the load of 1000 W; further increase in the load decreases BTE. Because of higher viscosity of biodiesel, the combustion quality gets reduced, which leads to reduction in BTE. With the increase in the percentage of diethyl ether in the blend, BTE increases. Diesel produces the highest unburnt hydrocarbon (UBHC) when compared with other test fuels, followed by B20. With the increase in the percentage of diethyl ether (DEE) in the blend, UBHC emission decreases. This is because of the increase in the percentage of oxygen content with the addition of DEE. The authors capped the percentage of DEE to 10% of the volume in the blend; this is because the further addition of DEE stops the engine to operate at higher loads. With the increase in the concentration of DEE, CO2 emission decreases, which may be due to the increase (decrease) in oxygen (carbon) content in the blend. Addition of diethyl ether in the blend reduces the CO emission; this is because of higher cetane rating of DEE which increases combustion temperature. Gnanamoorthi and Devaradjane (2015) have analyzed ethanol concentration varied from 0 to 40% in the blends of diesel. One percentage of ethyl acetate and diethyl carbonate was used to make the blend stable. The engine was tested with various compression ratios ranging from 17.5 to 19.5. A higher compression ratio leads to improved combustion quality. At a higher compression ratio, say 19.5:1, the blend of ethanol 40% and diesel 60% produces a higher BTE than diesel. This is because high compression provides higher compression temperature before the combustion process, which increases the ease towards igniting the fuels that have a high auto-ignition temperature. With the increase in compression ratio, HC and CO emissions decrease; the presence of inbuilt oxygen is yet another factor for the reduction of HC and CO emission. Górski and Przedlacki (2014) have analyzed the physical and thermal properties of DEE blended with diesel. Miscibility test and cetane number analysis were also done on the blend. With the increase in the percentage of DEE in the blend, the calorific value, viscosity, and density decrease. The cetane number of the DEE is higher than that of diesel, which leads to an increase in the cetane rating of blends, thus resulting in a lesser ignition delay. The blend of DEE and diesel showed better performance characteristics than that of diesel. These blends also proved to have good combustion characteristics when the engine had a higher cylinder pressure and heat release rate (HRR). Liu et al. (2019) have done an experiment on a CI engine fueled with blends of polyoxymethylene dimethyl ethers, ethanol, and diesel. Polyoxymethylene dimethyl ethers have inbuilt oxygen content and high cetane number which act as a cetane improver in the ethanol-diesel blend. Ethanol-diesel blend containing polyoxymethylene dimethyl ether showed outstanding performance. This blend also exposed lesser HC and CO emission when compared with diesel.

Qi et al. (2011) tested stability of the blend of biodiesel 70 %, diethyl ether 5%, diesel 15%, and ethanol 10%. With the addition of ethanol and diethyl ether to biodiesel, smoke and CO emission reduced, but HC emission increased. The additives diethyl ether and ethanol do not have appreciable influence over the cylinder peak pressure. Ibrahim (2018) has assessed the impact of oxygenated additive with biodiesel on CI engine. With the addition of five percentage DEE, the BTE is higher when compared with biodiesel and diesel blend. High LHV and low calorific value of DEE reduce the combustion quality that results in the reduction of the BTE. Higher oxygen content in biodiesel and DEE leads to higher HRR than diesel. The author tested the characteristics of the engine running on different blends of soybean biodiesel–diesel–water containing acetone–butanol–ethanol (ABE). Water containing ABE solution showed a BTE of 7.88% higher than that of neat biodiesel blend. At peak load condition, diesel, BD50, and BD75 produced almost an equal amount of BTE. During high load conditions, the lubrication property of biodiesel led to producing more BTE, while during low load conditions, its high viscosity had created poor atomization and large droplets, thus leading to lower efficiency. The lower cetane number aided the fuel to remain in the combustion chamber for the slightly extended duration. In addition, the low viscosity also helped in creating better atomization, besides the better combustion due to oxygen content. ABE-biodiesel-diesel blends emitted very less amount of NOx than diesel. The blend which has water containing ABE emits less NOx than the blends without it. The amount of NOx emitted is inversely proportional to the percentage of water containing ABE in the blend, although water containing ABE has similar properties to that of biodiesel (rich oxygen and low cetane number). This resulted in a lesser amount of NOx than biodiesel. This is due to ethanol’s higher latent heat of vaporization and low heating value, which reduced the cylinder temperature. The lower cetane number of waters containing ABE had led to a higher ignition delay and also increased the duration of peak temperature. Water containing ABE emits less particulate matter since it is having rich oxygen content. It emits just 69.3% of particulate matter emitted by diesel (Chang et al. 2014).

The design of experiment chosen by researcher is in a such way that it has both direct and indirect injection facility, i.e., it has injectors placed on both head and throttle body. The engine was being operated at three test conditions: in all three operating conditions, B30D70 was directly injected in to cylinder. The first operated condition did not have any additive at throttle body injection; the second operating condition had ethanol at throttle body injection; and the third operating condition had the blend of di-tert-butyl-peroxide-ethanol at throttle body injection. The engine running on biodiesel blend with ethanol injection showed a higher inlet and exhaust temperature than while running without ethanol injection. This is because the ethanol injected along the manifold had reduced the inlet and exhaust temperature. Mere B30D70 emitted more NOx. B30D70 with ethanol injection emitted less NOx than without ethanol injection, since ethanol is having a high LHV (840 kJ/kg) than diesel (200 kJ/kg) and soybean biodiesel (270 kJ/kg). This is due to the reduction in the peak temperature. When the 15% energy share, ethanol with B30D70 emitted 190 ppm of NOx, while without ethanol injection, the same fuel emitted 481 ppm of NOx (Ferreira et al. 2013). Di-tert-butyl peroxide is having a high cetane–improving property. Due to this, the cylinder temperature and pressure are increased. The fuel with additive emitted more NOx but less CO. The emission of HC is directly proportional to the percentage of ethanol injected into the inlet manifold. The engine, while running on 70% diesel–30% soybean biodiesel without the injection of ethanol, emitted 11 ppm of HC, whereas the same engine with the same fuel with 15% ethanol injection emitted 75 ppm of HC. Traces of the ethanol injected in the manifold get trapped within the cervices and escape from combustion; due to this, emission of UBHC increases, another reason for the emission of more UBHC being the absence of oxidation (Shaafi and Velraj 2015). The author has tested various blend ratios of diesel–biodiesel–butanol and keeping diesel as a reference fuel. For all the fuels used, there was a decrease in BSFC wherever there was an increase in the load. The SFC produced by diesel is lower than that produced by used cooking oil biodiesel and used cooking oil biodiesel–butanol blend. This is because diesel is having a higher calorific value than biodiesel and butanol. When there was an increase in the load, the exhaust gas temperature (EGT) increased, irrespective of the fuel used. The EGT of diesel was found to be higher than that of biodiesel, biodiesel–butanol blend, and biodiesel–diesel blend. HC emission was found to be lesser when the engine was running on vegetable oil. Butanol was found to have a higher latent heat of vaporization than cooked vegetable oil biodiesel and diesel. Due to this phenomenon, combustion was incomplete and the emission of HC was higher. Increasing the load decreased the emission of CO, irrespective of the fuel used. When the engine was running on 46% of load, it emitted more CO because there was an increase in the concentration of butanol. When there is an increase in the ratio of butanol in the blend, emission of NOx decreased, since it is having a high LHV (Yilmaz et al. 2015). The main inspiration of focusing on DEE-enriched ethanol-diesel blend was to substantially minimize NOx emission without compromise performance characteristics. The removal of high temperature zone in the chamber owe it to the reduction of NOx emission. The primary reason for formation of smoke emission is high secondary combustion phase in the cycle. Moreover, this lower combustion temperature results in a rise in CO and HC due to post-combustion oxidation regions. In addition, fresh charger impingement on cylinder wall is also a reason for the rise in the HC emission. Based on that, the aim of the present study is (1) to investigate the conventional CI engine to be operated with DEE-enriched ethanol-diesel blend as a fuel; and (2) to study the stability of blends to ensure prolonged homogeneity with different additives such as isopropanol, oleic acid, and ethylene acetate.

Materials and methods

Selection of fuels and blends stability

Ethanol is a renewable and sustainable source of energy for SI engine. It could not use a sole fuel in the CI engine. This is mainly due to lower density and high self-ignition temperature leading to incomplete combustion and knocking in CI engine. Various methods have followed ethanol used as fuel in CI engine. A small fraction of ethanol blend with diesel can be used as a fuel in an unmodified CI engine. This blend may exhibit slightly lower performance than diesel fuel but drastically reduce the NOx emission. Ethanol and diesel do not blend and form a homogeneous mixture at normal temperature. To make the blend homogenous, different additives were used and then the stability was analysed. The additives used for the test purpose were isopropanol, oleic acid, and ethylene acetate. It was found that the oleic acid was the best additive which produces a better homogenous mixture of ethanol and diesel. One percentage of oleic acid is used as an additive to blend ethanol and diesel. The different blends of ethanol and diesel used were 10% ethanol–90% diesel (D90E10), 20% ethanol and 80% diesel (D80E20), and 30% ethanol-70% diesel (D70E30). Figure 1 shows the different ratios of ethanol-diesel blend without additive and with different additives. It was noticed that the blend of ethanol-diesel is more homogenous with oleic acid. In all the blends of ethanol and diesel, one percentage of oleic acid is added. Table 1 shows the stability of ethanol-diesel blend with different additives. The stability of ethanol and diesel has been analyzed for 90 days. The additives used to analyze the test blends were isopropanol, oleic acid, and ethylene acetate. The different ratios of ethanol and diesel blend used for the test purpose were (i) 10% ethanol–90% diesel (DE10%), (ii) 20% ethanol and 80% diesel (DE20%), and (iii) 30% ethanol-70% diesel (DE30%). One percentage oleic acid and one percentage diethyl ether were blended with of ethanol-diesel blend for all blend ratios. The mixtures’ stability was analysed visually and was tabulated as shown in Table 1. Table 2 shows the physical and thermal properties of diesel, diethyl ether (DEE), isopropanol, ethanol, and oleic acid. After the analyses of different ratios, we selected the combination of the blends with the best additives to investigate the performance analysis of diesel engine by using 1% oleic acid as a surfactant.

Fig. 1
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Different ratio of ethanol-diesel blend with and without additive

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Table 1 Stability of ethanol-diesel blend with different additive
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Table 2 The physical and thermal properties of diesel, DEE, ethanol, and oleic acid
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Table 2 shows the physical and thermal properties of diesel, diethyl ether (DEE), isopropanol, ethanol, and oleic acid. After different ratios of analysis are done, we selected the combination of the blends with the best additives to investigate the performance analysis of diesel engine by using 1% oleic acid as a surfactant. Form the fuel properties, it was found that ethanol has a very lesser cetane number when compared with diesel. This reduces the overall cetane number of the blend. Thus, from the literature review, it was found that diethyl ether has a cetane number of 125, so one percentage of it has been added to all the ethanol-diesel blends.

Experimental setup and procedure

The engine used for the test purpose was a single-cylinder, naturally aspirated, water-cooled, compression ignition engine. The injection pressure used was 220 bar. The detailed specification of the engine used for the test purpose is shown in Table 3. The engine used was operated at a constant speed of 1500 rpm, and the load was varied from 0 to 100%. Figure 2 shows the schematic diagram of the experimental setup. Figure 3 shows the valve timing of the engine. Before conducting the experiment, the engine was overhauled and a new lubricating oil was used. The loading device and emission analyzer were serviced and calibrated before the experiment. The engine tests were carried out at a constant speed of 1500 rpm at different load conditions of 25%, 50%, 75%, and 100%. The fuel flow rate was adjusted according to the load conditions by the governor. After every fuel test, the fuel tank was cleaned, and the engine was operated with diesel for 15 min to clean the fuel system. For each set of load condition, the engine was allowed to stabilize, and 100 ml of fuel was used to take one set of reading. Three sets of reading were taken for one test condition.

Table 3 Specification of the test engine
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Fig. 2
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Schematic diagram of the engine test rig

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Fig. 3
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Valve timing diagram used for this research

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The engine speed was controlled by the governor. Water was supplied through the engine block and cylinder head sockets to cool the engine. The piezoelectric pressure transducer was fitted in the cylinder head to measure the in-cylinder pressure. Eddy current dynamometer was coupled with the engine to give load on the engine. At the beginning, the engine was run without load to attain steady-state conditions. The next stage of experimental work was carried out with various inlet air temperatures ranging from 80 °C to 100° C without EGR. AVL Digas analyzer was used to measure engine exhaust emissions like UBHC, NOx, CO2, CO, and excess oxygen in the exhaust. Non-dispersive infra-red (NDIR) technique was used to measure the gases like CO, CO2, and UBHC. The technical specifications of the AVL Digas analyzer are given in Table 4.

Table 4 Gas analyzer specification
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Uncertainty analysis

The present investigation contains the use of different instruments to measure the physical parameters. The precision of types of equipment can be influenced by many environmental factors. The accuracy of measurement should be considered for all uncertainty and error analysis. Uncertainty analysis could be done by each physical parameter and zero calibration. It also considered the fixed and random error of measurement. The precision of the measured variable and calculated variable is mentioned in Table 5 (Parthasarathy et al. 2020).

$$ \mathrm{Uncertainty}\ \mathrm{of}\ \mathrm{measured}\ \mathrm{physical}\ \mathrm{parameter}\ \mathrm{is}\ \mathrm{given}\ \mathrm{by}\ \Delta {x}_i=\frac{2{\sigma}_i}{x_i}\times 100 $$
(1)
$$ \mathrm{Let}\ E\ \mathrm{be}\ \mathrm{the}\ \mathrm{calculated}\ \mathrm{quantity}\ E=\left(x1,x2,x3,.., xn\right) $$
(2)
Table 5 The accuracy of the measured variable and calculated variable
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The calculated value of uncertainty of experimental work was:

$$ {\displaystyle \begin{array}{c}\Delta E=\sqrt{{\left\lfloor \frac{\partial E}{\partial {x}_1}\ \Delta {x}_1\right\rfloor}^2++{\left\lfloor \frac{\partial E}{\partial {x}_2}\ \Delta {x}_2\right\rfloor}^2+{\left\lfloor \frac{\partial E}{\partial {x}_3}\ \Delta {x}_3\right\rfloor}^2+\dots +{\left\lfloor \frac{\partial E}{\partial {x}_n}\ \Delta {x}_n\right\rfloor}^2}\\ {}=\sqrt{{\left[N\right]}^2+{\left[T\right]}^2+{\left[g\right]}^2+{\left[\mathrm{UBHC}\right]}^2+{\left[ CO\right]}^2+{\left[\mathrm{Smoke}\right]}^2+{\left[\mathrm{CP}\right]}^2+{\left[\mathrm{NOx}\right]}^2+{\left[\mathrm{BSEC}\right]}^2+{\left[\mathrm{BTE}\right]}^2}\\ {}\begin{array}{c}=\sqrt{{\left[1\right]}^2+{\left[0.5\right]}^2+{\left[0.1\right]}^2+{\left[1.0\right]}^2+{\left[0.7\right]}^2+{\left[1.1\right]}^2+{\left[0.3\right]}^2+{\left[1.4\right]}^2+{\left[0.5\right]}^2+{\left[0.7\right]}^2}\\ {}=2.6\%\end{array}\end{array}} $$

Result and discussion

Performance characteristics

Brake specific energy consumption

Figure 4 shows the variation of brake specific energy consumption (BSEC) against the brake power (BP). Irrespective of the fuel used, increase in engine’s load decreased the BSEC. This shows that better combustion takes place at higher load conditions. At all load conditions, diesel produces the least BSEC than other all blends. In general, BSEC of ethanol blends are higher than that of all other blends and diesel. This is mainly due to lower CV value of ethanol which results in consumption of high quantity of fuel. At peak load, BSECs of D90E10, D80E20, and D70E30 were 12.8 mJ/kW.hr, 13.1 mJ/kW.hr, and 13.4 mJ/kW.hr, respectively, which were 6.1%, 8.7%, and 10.3% higher than that of diesel. This may be due to lower CV value and high LHV of ethanol which results in inferior combustion. It was also noticed that with increase in the ethanol concentration in blend, there is an increase in the BSEC. This is because the higher ratio ethanol blend has lower CV value. BSECs of DEE-enriched D90E10, D80E20, and D70E30 were 12.3 mJ/kW.hr, 12.6 mJ/kW.hr, and 12.8 mJ/kW.hr, respectively, which were 0.7%, 1.7%, and 3.1% lower when compared with without DEE ethanol blends of D90E10, D80E20, and D70E30 respectively. In addition, BSEC of DEE-enriched ethanol blend D90E10 was 0.7%, 2.6%, and 6.6% lower than without DEE ethanol blend D90E10, D80E20, and D70E30 respectively at rated brake power condition. It was noticed that BSEC of DEE-enriched ethanol blend was slightly lower than that of the one without it. This reliable decrease in BSEC was due to the improvement in the chemical reaction between the molecules into constructive work. BSEC of DEE-enriched D90E10 was very close to diesel. This is mainly due to low density, inbuilt O2, and high CV value of DEE that improves the homogeneity of charge mixture which results in superior combustion as well as lesser fuel consumption (Nabi et al. 2019).

Fig. 4
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Comparison of BSEC with BP of an engine at various ethanol blend

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Brake thermal efficiency

Figure 5 exhibits the variations of brake thermal efficiency (BTE) against the brake power for different blend ratios of ethanol and diesel. For all the blends ratios of ethanol-diesel, with an increase in the BP, the BTE increases. This is mainly due to better combustion occurred at higher load condition. Diesel fuel was noticed that highest BTE compared with other ethanol-diesel blends. At full load condition, BTEs of D90E10, D80E20, and D70E30 were 28.7%, 28.1%, and 27.0%, respectively, which were 5.7%, 7.5%, and 11.2% lower than that of diesel. This may be due to high LHV of ethanol and lower volatility. Thus, more amount heat energy is consumed by ethanol to produce flammable vapor in the ethanol blend compared with pure diesel, which results in inferior combustion. It was also noticed that BTE decreases with an increase in the percentage of ethanol in the blend. This may be due to the poor combustion quality of ethanol and higher latent heat of vaporization of ethanol. With an increase in the blend ratios of ethanol in the blend beyond 30%, engine vibration was noticed. This may be due to lesser cetane number and CV of ethanol which result in more fuel being burnt at diffusion phase. Similar result was noticed in previous studies investigated with ethanol blends (Alptekin 2017). On the other hand, BTE of DEE-enriched ethanol blend D90E10 was 0.7%, 2.6%, and 6.6% higher than without DEE ethanol blend D90E10, D80E20, and D70E30 respectively at rated brake power condition. This shows that the addition of diethyl ether has a great influence on the ethanol blend. This is mainly due to superior volatility, inbuilt O2, and calorific value of DEE that enhance the mixing rate of A/F composition which in turn give better combustion efficiency. At peak load condition, the BTEs of DEE-enriched D90E10, D80E20, and D70E30 blends were 0.7%, 1.7%, and 3.1% higher when compared with their receptive blends without DEE. Increase in ethanol concentration in blends decreases the BTE. This is attributed to an account of higher concentration of ethanol which reduce the heat content blends that results in more fuel accumulated in the chamber and produce inferior combustion (De Menezes et al. 2006).

Fig. 5
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Comparison of BTE with BP of an engine at various ethanol blend

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Emission characteristics

Oxides of nitrogen

Figure 6 shows the deviation of NOx emission against brake power. Irrespective of the fuel, increase in the brake power increases the NOx emission. This is because at higher load condition, the combustion temperature increases and nitrogen reacts at a temperature above 1300 °C. The better combustion efficiency, the higher the NOx emission produced. At full load condition, NOx of D90E10, D80E20, and D70E30 were 765 ppm, 725 ppm, and 680 ppm, respectively, which were 6.9%, 11.8% and 17.2% lower than that of diesel. It is evident that with an increase in the percentage of ethanol in the blend the combustion temperature and NOx emission decrease. This is mainly due to high LHV of ethanol which absorbs considerable amount heat in the combustion chamber, resulting in lower combustion temperature. Thus, the cooling effect of ethanol turns in lower combustion temperature inside the cylinder that results in lower NOx emission. At the same time, NOx of DEE-enriched D90E10, D80E20, and D70E30 were 774 ppm, 755 ppm, and 700 ppm, respectively, which were 5.8%, 8.1%, and 14.8% lower than that of diesel. In addition, NOx emission of DEE-enriched ethanol blend was noticed to be 3% higher when compared with the blend without DEE. Fast combustion of DEE might result in higher combustion temperature that results in higher NOx emission. This may attribute to high cetane rating and volatility feature of DEE which lead to enhance the combustion temperature that results in higher NOx formation. Similar result was observed in previous work with ethyl alcohol-biodiesel blends (Parthasarathy et al. 2014).

Fig. 6
figure 6

Comparison of NOx with BP of an engine at various ethanol blends

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Carbon monoxide

The variation of carbon monoxide (CO) emission across different brake powers is shown in Fig. 7. The CO emissions mainly depend upon the physicochemical properties of the fuel. At full load condition, CO of D90E10, D80E20, and D70E30 was 0.12%vol., 0.125%vol., and 0.129%vol., respectively, which were 5.8%, 9.6%, and 12.4% higher than that of diesel. It was noticed that ethanol blend has shown higher CO emission compared with other blends. This is because the low cetane number and high LHV of ethanol lead to higher ignition delay period, which results in partial combustion. Thus, deterioration of ethanol combustion might be responsible for the production of higher CO emission. In addition, ethanol has lower energy density. Thus, rich-mixture zone could be responsible for incomplete combustion that turns into higher CO formation. At the same time, CO emission of DEE-enriched ethanol blend D90E10 was 2.5%, 6.4%, and 9.3% lower than without DEE ethanol blend D90E10, D80E20, and D70E30 respectively at rated brake power condition. The reduction in CO emission of DEE-enriched ethanol blends could be due to lesser carbon fraction that results in improved combustion. Superior evaporation feature and inbuilt oxygen of DEE might raise the oxygen availability inside cylinder that results in improved oxidation of CO and enhance the combustion efficiency. Moreover, the time period of oxidation of ethanol is the factor controlling the OH radicals, which is encouraging the conversion of CO to CO2 and minimizes the CO formation. Similar result was noticed in previous studies investigated with ethanol blends (Parthasarathy et al. 2019). Moreover, DEE-enriched ethanol blends of D90E10, D80E20, and D70E30 showed 2.5%, 4%, and 4.6% lower CO emission than without DEE ethanol blends of D90E10, D80E20, and D70E30 at rated brake power of 5.2 kW. The availability of O2 molecules in DEE ethanol blends improves the combustion reaction which leads to superior combustion and thereby reduction of emission formation. While increasing the ethanol concentration in blends from 10 to 30%, the CO emission was gradually increased due to lower post-combustion temperature that results in restriction of promotion of CO emission.

Fig. 7
figure 7

Comparison of CO with BP of an engine at various ethanol blends

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Unburned hydrocarbon

Figure 8 illustrates the difference of unburned hydrocarbon (UBHC) emission with respect to brake power for ethanol-diesel blends. Generally, the formation of UBHC emissions is an effect of inferior combustion; nevertheless, complete combustion is achieved for lower UBHC emission. From the results, UBHC emission drastically increased for ethanol-diesel blends compared with that for diesel at full load condition. At full load condition, UBHCs of D90E10, D80E20, and D70E30 were 54 ppm, 57 ppm, and 61 ppm, respectively, which were 3.7%, 8.7%, and 14.7% higher than that of diesel. This is mainly due to the combined effect of lesser energy content of ethanol and higher latent of vaporization of ethanol which leads to higher mass of the fuel to be injected in the cylinder that results to form a rich-mixture zone. Thus, rich-mixture zone and less CN of ethanol lead to increase in the ID period that results in inferior combustion and formation of higher UBHC emission. In addition, high LHV of ethanol absorbs more amount of heat inside the combustion chamber which could have reduced the combustion temperature, leading to higher UBHC emission. On the other side, UBHC emission of DEE-enriched ethanol blend D90E10 was 1.8%, 7.0%, and 13.1% lower than without DEE ethanol blend D90E10, D80E20, and D70E30 respectively at rated brake power condition. The results noticed that DEE-enriched ethanol blends have slightly reduced the UBHC emission compared with ethanol-diesel blend. The lower UBHC emission for DEE-enriched ethanol blend has an evidence of improved combustion as a result of enhanced oxidation reaction. Thus, DEE might take part in prolonged oxidation, which results in improved performance and lower UBHC formation. Moreover, DEE-enriched ethanol blends of D90E10, D80E20, and D70E30 showed 1.8%, 3.5%, and 4.9% UBHC emission than without DEE ethanol blends of D90E10, D80E20, and D70E30 at rated brake power of 5.2 kW. This is caused by the positive impact of DEE addition and the influence of better oxidation and lowered fuel-rich zone, whereas an increase in ethanol concentration in ethanol blend gradually increase UBHC due to more fuel accumulation rate, rich zone formation, and lower combustion temperature which results in higher UBHC formation (Parthasarathy et al. 2014).

Fig. 8
figure 8

Comparison of UBHC with BP of an engine at various ethanol blends

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Smoke

Smoke emissions of DEE and ethanol blends were found to be slightly increased than that of diesel (see Fig. 9). Increase in load conditions increases the smoke emission for all the test fuels. At peak load condition, smoke of D90E10, D80E20, and D70E30 was 61 HSU, 68 HSU, and 71 HSU, respectively, which was 9.8%, 19.1%, and 22.5% higher than that of diesel. This is mainly due to lesser CV of ethanol and rich fuel zone that result in lesser combustion temperature and more smoke formation. DEE-enriched ethanol blends of D90E10, D80E20, and D70E30 showed 4.9%, 5.8%, and 3.4% lower smoke emission than without DEE ethanol blends of D90E10, D80E20, and D70E30 respectively. Increasing the ethanol concentration tends to increase the smoke emission for all test fuels due to low CV value of ethanol tends to admitted more amount of fuel in the cylinder which results in rich-mixture region and incomplete combustion. Smoke emission of DEE-enriched ethanol blend D90E10 was 4.9%, 14.7%, and 18.3% lower than without DEE ethanol blend D90E10, D80E20, and D70E30 respectively. It could be noticed that DEE-enriched ethanol blends have slightly reduced the smoke emission compared with ethanol-diesel blend. Reduction of smoke emission for DEE-enriched ethanol blend could be attributed to enhancing the combustion temperature and complete combustion. This is due to high cetane number and volatility of DEE, which are known for improving the combustion and minimizing the smoke emission. A similar result was observed in previous work with diesel blends (Qi et al. 2011).

Fig. 9
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Comparison of smoke with BP of an engine at various ethanol blends

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Combustion characteristics

Cylinder pressure

Cylinder pressure is mainly dependent on the quantity of fuel burned throughout the uncontrolled combustion phase (rapid combustion stage) in CI engine. Figure 10 exposes the variation of cylinder pressure for DEE and ethanol blends at peak load conditions. The peak pressure for diesel, D90E10, D80E20, and D70E30 at full load condition was 59.8 bar, 56.9 bar, 55.4 bar, and 53.2 bar, respectively. It was noticed that ethanol-diesel blends showed lower cylinder pressure compared with diesel fuel. This is mainly due to the lower cetane number and high LHV of ethanol compared with that of diesel. This may be attributed to increase the ignition delay period and lower combustion temperature which lead to produce lesser cylinder pressure. Cylinder pressure for DEE-enriched ethanol blends D90E10, D80E20, and D70E30 are 58.2 bar, 56.5 bar, and 54.4 bar, respectively. The DEE-enriched ethanol blend improved the homogeneity of A/F mixture, which results in lesser ignition delay and rapid combustion process. At higher load conditions, DEE-enriched D90E10, D80E20, and D70E30 blends had 2.2%, 1.8%, and 2.4% higher cylinder pressure when compared with the blends without DEE. This is because the adequate oxygen present in DEE enhances the combustion process. In addition, high energy content and cetane number of DEE leads to improvement in cylinder temperature, which results in higher cylinder pressure.

Fig. 10
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Variation of cylinder pressure across different ethanol blends

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Heat release rate

As shown in Fig. 11, DEE-enriched ethanol blends had an improved combustion and HRR when compared with the blends without blends DEE. Fuel property has a great influence over the heat release rate and pressure-rise inside the combustion chamber. Diesel produces higher HRR when compared with other test fuels. This is because of better physical and thermal property of diesel which enhances the HRR. The heat release rate for diesel, D90E10, D80E20, and D70E30 at peak load was 69.6, 66.9, 62.5, and 57.0 J/°CA, respectively. With increase in the percentage of ethanol in the blend, HRR decreases as result of increase the ignition delay period. This is because of lesser cetane number and lower energy content of ethanol. At higher load conditions, the HRRs of DEE-enriched D90E10, D80E20, and D70E30 blends were respectively, 2.5 %, 2.2%, and 2.6% higher when compared with their receptive blends without DEE. It was noticed that the combustion of DEE-enriched ethanol blends was superior to other blends. It was also noticed that the start of combustion of DEE-enriched blend was earlier than that of other ethanol blends. This is attributed to the lower LHV and higher cetane number of DEE could attribute to shorter ID. In addition, oxygen-enriched ethanol and DEE blends produce more complete combustion, which results in superior primary combustion phase and higher HRR compared with other ethanol blends.

Fig. 11
figure 11

Variation of heat release rate across different ethanol blends

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Conclusions

Based on the experimental investigation carried out on CI engine fueled with different blend ratios of ethanol-diesel blends, the author tested the blends with different additives to enhance stability of ethanol blends. The important findings of this research work have been notified as follows:

  • Ethanol is immiscible with diesel without additive. Usage of isopropanol, oleic acid, and ethylene acetate improves the homogeneity of the ethanol-diesel blend. It was found that oleic acid produces the best homogeneity among the other additive.

  • Through the addition of ethanol into diesel, the BTE was decreased and BSEC was increased. With an increase in the percentage of ethanol in the blend, the BTE was drastically reduced and the engine consumes more fuel. This is attributed to lower energy density of ethanol that results in the consumption of higher fuel quantity.

  • The BTE of DEE-enriched ethanol blends was higher than that of blends without DEE. This improvement was due to the enrichment in the fuel properties which ensures better combustion efficiency.

  • NOx emission drastically decreased for all ethanol blends compared with diesel operation. This is mainly due to the cooling effect of ethanol which turns to lower down the combustion temperature inside the cylinder that results in lower NOx emission. DEE-enriched blends exhibited higher NOx emission than other ethanol blends. This was attributed to enhance the combustion efficiency which results in higher combustion temperature.

  • The CO and UBHC emissions of ethanol blends were higher compared with those of diesel fuel. In contrast, DEE blends noticed lower CO and UBHC emission compared with that of ethanol blends. This may be due to higher LHV and CN of DEE-enhanced blends which give out complete combustion that results in higher flame temperature.

  • Smoke emission for all ethanol blends increased when the ethanol percentage increased in blends according to diesel. On the other hand, DEE blends were observed to emit low smoke. This is mainly due to high cetane rating of DEE which narrows down the ignition delay period which results in diminishing of the diffusion combustion phase.

  • Blends with DEE showed better combustion characteristics compared with ethanol blends. High energy content and inbuilt O2 with DEE exhibited an excellent cylinder pressure and HRR value. The high CN of DEE resulted in the speed up at the start of combustion (i.e., lesser ignition delay) which results in enhanced premixed combustion and better cylinder temperature. Based on the experimental study, it was concluded that DEE blends showed better performance and combustion characteristics.

Future prospects

In the present study can be implemented in the dual fuel operation for further reduction of smoke emission. Dual fuel mode of operation can be parallelly minimizing the smoke and NOx emission. The dual fuel mode can be investigated fueled with blend diesel-ethanol as a main energy source and DEE inducted along with air as secondary source of energy. This can further enhance the performance of engine. Moreover, experiment studies can investigate the effect of various nanoadditives with ethanol biodiesel blends in CI engine. DEE blends should be tested under different operating conditions in further experiments.