Research of Energy and Ecological Indicators of a Compression Ignition Engine Fuelled with Diesel, Biodiesel (RME-Based) and Isopropanol Fuel Blends

: This article presents the results of a study of energy and ecological indicators at di ﬀ erent engine loads ( BMEP ) adjusting the Start of Injection ( SOI ) of a Compression Ignition Engine fuelled with blends of diesel (D), rapeseed methyl ester (RME)-based biodiesel and isopropanol (P). Fuel blends mixed at D50RME45P5, D50RME40P10 and D50RME30P20 proportions were used. Alcohol-based fuels, such as isopropanol, were chosen because they can be made from di ﬀ erent biomass-based feedstocks and used as additives with diesel fuel in diesel engines. Diesel fuel and its blend with 10% alcohol have almost the same thermal e ﬃ ciency ( BTE ). In further examination of energy and ecological indicators, combustion parameters were analysed at SOI 6 CAD BTDC using AVL BOOST software ( BURN subprogram). Increasing alcohol content in fuel blends led to a reduced cetane number, which prolonged the ignition delay phase and intensiﬁed heat release in the premixed combustion phase. Higher combustion temperatures and oxygen content in the fuel blends increased NO x emissions. Lower C / H ratios and higher O 2 levels a ﬀ ected by RME and isopropanol reduced smoke emissions. methodology, A.R., S.M.R. and J.M.; software, S.M.R. and A.R.; formal analysis, A.R. and J.M.; validation, A.R. and S.M.R.; writing—original draft preparation, A.R. and J.M.; writing—review and editing, A.R. and J.M.; supervision, A.R., and J.M.; project administration, J.M. All authors have


Introduction
The rising demand for fossil fuels and the environmental issues concerning their use are the biggest challenges that people face today. The transportation sector alone contributes up to 30% of the world's harmful emissions [1]. As the demand for fossil fuels continues to rise faster than its supply, fossil fuels deplete, which in turn drives the price of such fuels as diesel and petrol up [2][3][4]. The absence of a replacement for vehicles that run on liquid fuels, and a high price of electric vehicles made the automotive industry devote its resources to finding alternative fuels as a replacement and to decreasing the emissions concerned with environmental problems [5][6][7]. Recent studies revealed that greenhouse gases and harmful combustion chamber emissions can be significantly reduced by using fuels, such as alcohol and biodiesel, as primary alternatives [8][9][10][11][12][13][14][15][16].
In order to beat the fossil fuel deficiency and control the increasing demand of natural gas, opportunities for using alternative fuels in internal combustion engines have been searched [17]. The ease of handling and storing alcohol and biodiesel makes them promising substitutes for fossil fuels [9,[18][19][20][21].
When using 25% isopropanol in petrol blends with combustion control, there is a direct relationship between octane number and combustion parameters of isopropanol. Start of combustion (SOC) is delayed by increasing isopropanol content in the blend because isopropanol is more resistant to jerky engine operation [39]. By reducing spark time in a petrol engine, isopropanol-gasoline blends (with isopropanol content up to 30%) had lower NO x emissions than those found at the initial spark time [40].
Oxygen content is the key parameter that differs in fossil fuels and biodiesel [41][42][43]. Environmentally friendly biodiesel produces clean and renewable energy, thus making it the alternative hope [44][45][46]. A study found that rapeseed methyl ester (RME) produced lower CO 2 emissions due to a lower carbon-to-hydrogen ratio as compared to diesel [47]. Methanol, ethanol are most commonly used in fuel blends with biodiesel, and currently the second ACB wave will make butanol cheaper [48]. The use of proponol in fuel blends with biodiesel is a rare case, determined by the greater development of other alcohol production technologies [20,49]. Lower heating values for B90Pr10 (90% biodiesel and 10% propanol fuel mixtures) and cetane number increased BSFC and fuel gas temperatures while reducing BTE [50]. Table 1 lists the properties of pure diesel, RME and isopropanol [51]. The aim of the research is to reveal the performance, combustion and emission characteristics of IC engines using pure diesel and fuel blends with different proportions of diesel, rapeseed methyl ester-based biodiesel and isopropanol.

Engine Testing Equipment
The engine used for testing is a 1.9 Turbocharged Direct Injection (TDI) diesel engine with a VP37 (BOSCH, Stuttgart, Germany) electronic controlled distribution type fuel pump. The start of the injection (SOI) was controlled by the Engine Electronic Control Unit (ECU) and it was a single injection strategy. Figure 1 presents a detailed image of the tested engine and its parts, while research conducted by other authors [52][53][54][55] and Table 2 lists the test engine parameters.
The brake torque M B (Nm) was determined on a load bench with a measurement error of ±1.2 Nm. Hourly fuel consumption B f (kg/h) was found using electronic scales SK-5000 with a 0.5% measurement error. The pressure in the cylinder was measured using a piezoelectric sensor GG2-1569 mounted on a glow plug with a sensitivity of 15.8 ± 0.09 pC/bar. Cylinder pressure values were recorded using the LabView Real software at an interval of 0.176 CAD. The pressure in the engine intake manifold was measured using an OHM HD 2304.0 pressure gauge (Delta, Padova, Italy) with a measurement error of ± 0.0002 MPa. The intake air and the exhaust gas temperature were measured using K-type thermocouples IR 8839 accurate to ± 1.5 • C. The exhaust gas concentration was determined using a DiCom 4000 gas analyzer (AVL, Graz, Austria). CO 2 measurement accuracy was 0.1% vol., CO-0.01% vol., HC-1 ppm, NO x -1 ppm, and smoke absorption coefficient-0.01 m −1 .   Statistical calculations of type A uncertainties were used to measure exhaust. Type A uncertainties were used to determine the standard deviation for repeated measurements, where u(x) is uncertainty, n-repeatability of measurements, and s(x)-reliability [55,56]: where x is the mean repeated value; s(x) is a standard deviation; s(x) is a standard deviation of the mean. Uncertainty ranges u(x) of exhaust components are presented in Table 3.

Fuels and Test Conditions
Tests were conducted using 100% pure diesel fuel and fuel blends prepared using different proportions of diesel (D), rapeseed methyl ester (RME)-based biodiesel and isopropanol (P). The first blend contained 50% diesel, 30% rapeseed methyl ester and 20% isopropanol (D50RME30P20), the second blend had 50% diesel, 40% rapeseed methyl ester and 10% isopropanol (D50RME40P10), and the third one 50% diesel, 45% rapeseed methyl ester and 5% isopropanol (D50RME45P5). The properties like density, mass fraction and lower heating value of the blends were calculated using the following formula: properties of fuel blends = [(percentage of fuel blend × property)] Table 3 presents a comparison of the calculated properties of the fuel blends with the standard diesel fuel. Uncertainties were calculated according to the model B [55]. Standard uncertainties were calculated according to the formula: where u c 2 (f) is the total uncertainty dispersion.
The uncertainty calculations for each fuel blend are presented in Table 4. Engine tests were carried out at the engine speed of n = 2000 rpm and engine brake torque M B was 30, 60 and 90 Nm, which corresponds to the Brake Mean Effective Pressure (BMEP) 0.2 MPa, 0.4 MPa and 0.6 MPa in the first experimental tests step. These are the loads of a city car running of the ≈ 50 km/h, ≈ 80 km/h and ≈ 100 km/h speeds. During load-changing tests, fuel Start or Injection Timing (SOI ≈ 2 CAD BTDC) was controlled by the engine electronic control unit. There were (BMEP = 0.3 MPa) injection timing was adjusted (SOI = 0 . . . 16 CAD BTDC) by modulating the SOI control signal in the second experimental test step. Injection timing was adjusted to determine the variation of engine performance using fuel mixtures of different properties under different combustion conditions. The Energy Indicators (Hourly Fuel Consumption B f , Brake Specific Fuel Consumption BSFC, Brake Thermal Efficiency (BTE)) and Ecological Indicators (emission of carbon monoxide CO, carbon dioxide CO 2 , nitrogen oxides NO x , hydrocarbons CH, and smoke) were measured and calculated at different engine loads and by adjusting start of injection. The results of all the tested fuel blends will be analysed comparing them with results of diesel fuel. Experimental tests were performed to ensure repeatability of the experiments. Several experiment design parameters were singled out, such as fuel type used, engine rotation speed, engine load torque, and fuel injection angle.
The density difference between the fuels was found to decrease with increasing alcohol concentration as seen in Table 4. Since the molecular mass of alcohols is lower than that of diesel and biodiesel [57][58][59][60], with the alcohol content increasing from 5% to 10%, the density was found to decrease by 1.45%, and with an additional increase from 10% to 20%, the density decreased by nearly 0.17%. When compared to diesel, fuel blends with a 5 and 10% alcohol content tended to be denser, and the D50RME30P20 blend tended to have a lower density compared to conventional fuel as calculated and presented in Table 4.
A lower heat value was found to differ less with an increase in the alcohol percentage share as seen in Table 4. The lower heat value difference increased to 6.5% when increasing the fuel concentration from 5% to 10%, and a further change of the concentration from 10% to 20% led to the difference increasing by nearly 7.7%. The lower heat value highly depends on carbon and hydrogen content in fuel [61][62][63]. So, with a relatively higher carbon and hydrogen content in diesel compared to their content in other fuel blends, the lower heat value of conventional fuel was found to be higher than that of other fuel blends, which declined with increasing alcohol content as seen in Table 4.
The difference in the cetane number decreased with increasing alcohol content as seen in Table 4. The conventional diesel fuel is known for being rich in paraffin, which helps it achieve a higher cetane number compared to other fuel blends [64]. When increasing the alcohol content from 5% to 10%, the difference was found to increase to 37.5%, and a further increase in alcohol content to 20% led to an increase in the difference of nearly 45%. As seen in Table 4, the cetane number steadily decreased with alcohol content compared to that of the conventional fuel. The fuel stability was ensured by producing fuel blends right before testing and feeding them to the engine.

Tools for Numerical Analysis of the Combustion Process
Due to a significant change in the cetane number in diesel and other prepared fuel blends, analysing changes in performance of the engine by calculating combustion characteristics and comparing them with those of diesel is necessary. AVL BOOST software was used in calculating the combustion characteristics with the help of BURN subprogram. BURN analysis was conducted having created a digital model of the 1.9 TDI diesel engine used in the experiment as seen in Figure 2. The digital model of the engine was constructed by selecting the required elements from a displayed element catalogue in AVL BOOST. The analysis of combustion requires general data on engine parameters, fuel and data describing the operating point. Engine parameters and experiment results were uploaded in the BURN subprogram and run to get the combustion characteristics. Changes in the in-cylinder pressure and temperature, heat release rate (ROHR) and the mass fraction burnt (MFB) were calculated in this research: , > where -total fuel heat input; α-crank angle; -combustion shape parameter; -start of combustion; -combustion duration.

Results and Discussion
The experimental tests were carried out in two steps: (a) changing the engine load BMEP (0.2; 0.4 and 0.6 MPa); (b) changing SOI (0…16 CAD BTDC), and BMEP = 0.3 MPa. After plotting the graphs from the (b) experiment results, a polynomial curve was drawn with a degree of 2 for all the energy and ecological parameters to get a change trend.

Energy Indicators
Brake Specific Fuel consumption (BSFC) of diesel fuel was low at all loads compared to other fuel blends, as observed in Figure 3a. When increasing alcohol content, fuel consumption tended to increase with D50RME30P20 being at the maximum. Having replaced 50% of diesel by a blend of biodiesel and propanol and increased the concentration of propanol up to 20% (D50RME30P20), BSFC increased ~9% due to a 7.7% reduction in LHV (Table 4) and a change in combustion process. With BMEP = 0.3 MPa, the analysis of BSFC from the perspective of injection timing revealed a higher consumption of all the fuel blends (7-10%), and with advancing angle, the value tended to decrease and further increase after 8-12 CAD BTDC as seen in Figure 3b. Engine parameters and experiment results were uploaded in the BURN subprogram and run to get the combustion characteristics. Changes in the in-cylinder pressure and temperature, heat release rate (ROHR) and the mass fraction burnt (MFB) were calculated in this research: where Q-total fuel heat input; α-crank angle; m v -combustion shape parameter; α SOC -start of combustion; α CD -combustion duration.

Results and Discussion
The experimental tests were carried out in two steps: (a) changing the engine load BMEP (0.2; 0.4 and 0.6 MPa); (b) changing SOI (0 . . . 16 CAD BTDC), and BMEP = 0.3 MPa. After plotting the graphs from the (b) experiment results, a polynomial curve was drawn with a degree of 2 for all the energy and ecological parameters to get a change trend.

Energy Indicators
Brake Specific Fuel consumption (BSFC) of diesel fuel was low at all loads compared to other fuel blends, as observed in Figure 3a. When increasing alcohol content, fuel consumption tended to increase with D50RME30P20 being at the maximum. Having replaced 50% of diesel by a blend of biodiesel and propanol and increased the concentration of propanol up to 20% (D50RME30P20), BSFC increased~9% due to a 7.7% reduction in LHV (Table 4) and a change in combustion process. With BMEP = 0.3 MPa, the analysis of BSFC from the perspective of injection timing revealed a higher consumption of all the fuel blends (7-10%), and with advancing angle, the value tended to decrease and further increase after 8-12 CAD BTDC as seen in Figure 3b.  At 8 CAD BTDC, D100 fuel was at its lowest on average. The lowest consumption of D-RME-P blends was achieved with injection timing of 10...12 CAD BTDC, as the cetane number of this fuel decreased to 23.06.
BTE of D50RME45P5 and D50RME40P10 fuel blends at BMEP 0.2 and 0.4 MPa was close to that of diesel, and BTE of D50RME30P20 was 3.5% (at BMEP 0.2 MPa) to 1.8% (at BMEP 0.4 MPa) lower as seen in Figure 3a. With a BMEP = 0.6 MPa, BTE of all fuels was slightly different and reached up to 0.35%.
Observing the dependence curve of efficiency on injection timing in Figure 3b, at BMEP = 0.3 MPa, BTE of the D50RME30P20 fuel blend was ~1.8% lower than that of diesel. BTE of fuel blends with 5% and 10% isopropanol content was lower (2.5…1.2%) at SOI = 0…6 CAD BTDC, but at SOI = 8…12 CAD BTDC, BTE of D50RME45P5 and D50RME40P10 was the same as BTE of D100.

Ecological Indicators
Carbon dioxide comparative emissions (g/kWh) were found to decrease for all the fuels with increasing load as seen in Figure 4a as BTE increased and BSFC decreased. At a low load (BMEP = 0.2 MPa), CO2 emissions of the blends containing isopropanol were 0.8…1.9% higher compared to diesel but increasing the load to BMEP = 0.6 MPa resulted in ~0.8% lower CO2 emissions for the D50RME40P10 fuel blend. At 8 CAD BTDC, D100 fuel was at its lowest on average. The lowest consumption of D-RME-P blends was achieved with injection timing of 10...12 CAD BTDC, as the cetane number of this fuel decreased to 23.06.
BTE of D50RME45P5 and D50RME40P10 fuel blends at BMEP 0.2 and 0.4 MPa was close to that of diesel, and BTE of D50RME30P20 was 3.5% (at BMEP 0.2 MPa) to 1.8% (at BMEP 0.4 MPa) lower as seen in Figure 3a. With a BMEP = 0.6 MPa, BTE of all fuels was slightly different and reached up to 0.35%.

Ecological Indicators
Carbon dioxide comparative emissions (g/kWh) were found to decrease for all the fuels with increasing load as seen in Figure 4a as BTE increased and BSFC decreased. At a low load (BMEP = 0.2 MPa), CO 2 emissions of the blends containing isopropanol were 0.8 . . . 1.9% higher compared to diesel but increasing the load to BMEP = 0.6 MPa resulted in~0.8% lower CO 2 emissions for the D50RME40P10 fuel blend.  This was due to a 2.1% decrease in the C/H ratio (Table 4) and the fact that at higher loads the BTE of all fuels was similar. Checking the dependence of emissions on injection timing revealed that all the fuels showed a gradual decrease in emissions with advancing injection timing as seen in Figure  4b, BTE increased, and smoke emissions decreased (Figure 5b). Diesel was found to have similar CO2 emissions compared to emissions of other fuel blends. CO2 emissions of the D50RME40P10 fuel blend at various SOIs were only ~0.2% lower than emissions of D100. Even though RME and C/H ratio of isopropanol is lower, increased fuel consumption increases CO2 emissions.
Hydrocarbon emissions of all the fuels were found to decrease with increasing load as seen in Figure 4a, as the combustion temperature increased [65,66]. Having the lowest emissions as compared to all the other fuels, diesel also tended to have a steady decrease pattern of ~38%, ~45% and ~60% (compared to the D50RME45P5, D50RME40P10 and D50RME30P20 fuel blends in hydrocarbon emissions. When increasing the alcohol content, fuel blends tended to have higher hydrocarbon emissions due to increasing alcohol base of the fuel, however, HC emissions were low compared to petrol engine [52]. The observation of the dependence of hydrocarbon emissions on injection timing revealed that with an increase in alcohol content, emissions tended to increase as seen in Figure 4b. HC emissions of D50RME45P5, D50RME40P10 and D50RME30P20 increased by ~15%, ~28% and ~58% compared to D100 at SOI ≈ 6 CAD BTDC. With the engine running on all the fuels, hydrocarbon emissions showed a slight growth trend when injection timing was advanced more than 6 CAD BTDC. This was due to a 2.1% decrease in the C/H ratio (Table 4) and the fact that at higher loads the BTE of all fuels was similar. Checking the dependence of emissions on injection timing revealed that all the fuels showed a gradual decrease in emissions with advancing injection timing as seen in Figure 4b, BTE increased, and smoke emissions decreased (Figure 5b). Diesel was found to have similar CO 2 emissions compared to emissions of other fuel blends. CO 2 emissions of the D50RME40P10 fuel blend at various SOIs were only~0.2% lower than emissions of D100. Even though RME and C/H ratio of isopropanol is lower, increased fuel consumption increases CO 2 emissions.
Hydrocarbon emissions of all the fuels were found to decrease with increasing load as seen in Figure 4a, as the combustion temperature increased [65,66]. Having the lowest emissions as compared to all the other fuels, diesel also tended to have a steady decrease pattern of~38%,~45% and~60% (compared to the D50RME45P5, D50RME40P10 and D50RME30P20 fuel blends in hydrocarbon emissions). When increasing the alcohol content, fuel blends tended to have higher hydrocarbon emissions due to increasing alcohol base of the fuel, however, HC emissions were low compared to petrol engine [52]. The observation of the dependence of hydrocarbon emissions on injection timing revealed that with an increase in alcohol content, emissions tended to increase as seen in Figure 4b. HC emissions of D50RME45P5, D50RME40P10 and D50RME30P20 increased by~15%,~28% and~58% compared to D100 at SOI ≈ 6 CAD BTDC. With the engine running on all the fuels, hydrocarbon emissions showed a slight growth trend when injection timing was advanced more than 6 CAD BTDC.
Nitrogen oxide emissions for D50RME45P5, D50RME40P10, D50RME30P20 at a low load (BMEP = 0.2 MPa) were ~10%, ~12%, and ~21% higher compared to those of D100 fuel as seen in Figure 5a. This was mainly due to the increased ignition delay due to a low cetane number of isopropanol (see the Dependence of the rate of heat release and the mass burnt fraction on the crank angle degree figure bellow.) and the increased oxygen concentration in the fuel blend (Table 4). With an increase in the load (BMEP = 0.6 MPa), the difference in NOx emissions was reduced to ~4%, ~7% and ~10% as the effect of ignition delay on different fuels was reduced. While analysing the dependence of nitrogen oxide emissions on the injection timing, emissions were found to rise steadily with increasing injection timing as seen in Figure 5b, because combustion occurred at a lower volume and higher temperatures [67]. All the fuels were found to follow a similar pattern, but after a more careful analysis, diesel emissions were found to be lower (1…4%) compared to emissions of other fuel blends. An earlier injection timing reduced the difference between NOx emissions of diesel and fuel blends [68][69][70][71].
The smoke level of fuels tended to increase with increasing load as seen in Figure 5a, as fuel mass increases per cycle and the air-to-fuel ratio decreases. Increasing the isopropanol concentration in fuel blends reduces smoke level, and this effect is more intense with an increasing engine load [72]. At a low load (BMEP = 0.2 MPa), the D50RME45P5, D50RME40P10 and D50RME30P20 fuel smoke emissions decreased by ~6%, ~10% and ~14%, respectively, in comparison to pure diesel. Higher Nitrogen oxide emissions for D50RME45P5, D50RME40P10, D50RME30P20 at a low load (BMEP = 0.2 MPa) were~10%,~12%, and~21% higher compared to those of D100 fuel as seen in Figure 5a. This was mainly due to the increased ignition delay due to a low cetane number of isopropanol (see the Dependence of the rate of heat release and the mass burnt fraction on the crank angle degree figure bellow.) and the increased oxygen concentration in the fuel blend (Table 4).
With an increase in the load (BMEP = 0.6 MPa), the difference in NO x emissions was reduced tõ 4%,~7% and~10% as the effect of ignition delay on different fuels was reduced. While analysing the dependence of nitrogen oxide emissions on the injection timing, emissions were found to rise steadily with increasing injection timing as seen in Figure 5b, because combustion occurred at a lower volume and higher temperatures [67]. All the fuels were found to follow a similar pattern, but after a more careful analysis, diesel emissions were found to be lower (1 . . . 4%) compared to emissions of other fuel blends. An earlier injection timing reduced the difference between NO x emissions of diesel and fuel blends [68][69][70].
The smoke level of fuels tended to increase with increasing load as seen in Figure 5a, as fuel mass increases per cycle and the air-to-fuel ratio decreases. Increasing the isopropanol concentration in fuel blends reduces smoke level, and this effect is more intense with an increasing engine load [71]. At a low load (BMEP = 0.2 MPa), the D50RME45P5, D50RME40P10 and D50RME30P20 fuel smoke emissions decreased by~6%,~10% and~14%, respectively, in comparison to pure diesel. Higher oxygen concentrations and lower C/H ratios resulted in lower D50RME30P20 smoke emissions. Increasing the load to BMEP = 0.6 MPa increased the smoke reduction effect to~12%,~16%. The analysis of the dependence of smoke levels on injection timing revealed that the levels tended to decrease with advancing the injection timing as seen in Figure 5b. Interestingly, throughout the SOI study range (0...16 CAD BTDC), the combustion performance resulted in the largest reduction in diesel smoke emissions from~7.25 m −1 to~4.25 m −1 (~40%), while the D50RME30P20 smoke emissions decreased from~5.7 to~5 m −1 (~12%). Therefore, at SOI = 0...8 CAD BTDC (low advanced injection timing), D50RME30P20 had the lowest smoke emissions, and at SOI = 10...16 CAD BTDC (high advanced injection timing), smoke emissions of D100 fuel were the lowest.

Combustion Characteristics
The analysis of combustion characteristics was conducted with the engine operating at BMEP = 0.3 MPa (n = 2000 rpm). Changing SOI (0 . . . 16 CAD BTDC) allowed comparing energy and ecological performance of the engine running on different fuels (Figures 3b, 4b and 5b) and concluding that with ignition timing being 6 CAD BTDC engine efficiency is close to the maximum, and smoke and NO x emissions are relatively low. The pressure values of all the fuels at SOI = 6 CAD BTDC obtained during the experiment as seen in Figure 6 were uploaded in the AVL BOOST (BURN subprogram) to get the combustion characteristics. Since the result at SOI = 6 CAD BTDC was relatively good, the combustion characteristics of fuel blends, such as in-cylinder pressure, pressure rise, rate of heat release (ROHR) and mass fraction burned (MFB), were analysed and compared to those of pure diesel at that particular degree of ignition timing.
Energies 2020, 13, x FOR PEER REVIEW 11 of 18 oxygen concentrations and lower C/H ratios resulted in lower D50RME30P20 smoke emissions.
Increasing the load to BMEP = 0.6 MPa increased the smoke reduction effect to ~12%, ~16%. The analysis of the dependence of smoke levels on injection timing revealed that the levels tended to decrease with advancing the injection timing as seen in Figure 5b. Interestingly, throughout the SOI study range (0...16 CAD BTDC), the combustion performance resulted in the largest reduction in diesel smoke emissions from ~7.25 m −1 to ~4.25 m −1 (~40%), while the D50RME30P20 smoke emissions decreased from ~5.7 to ~5 m −1 (~12%). Therefore, at SOI = 0...8 CAD BTDC (low advanced injection timing), D50RME30P20 had the lowest smoke emissions, and at SOI = 10...16 CAD BTDC (high advanced injection timing), smoke emissions of D100 fuel were the lowest.

Combustion Characteristics
The analysis of combustion characteristics was conducted with the engine operating at BMEP = 0.3 MPa (n = 2000 rpm). Changing SOI (0…16 CAD BTDC) allowed comparing energy and ecological performance of the engine running on different fuels (Figures 3b, 4b and 5b) and concluding that with ignition timing being 6 CAD BTDC engine efficiency is close to the maximum, and smoke and NOx emissions are relatively low. The pressure values of all the fuels at SOI = 6 CAD BTDC obtained during the experiment as seen in Figure 6 were uploaded in the AVL BOOST (BURN subprogram) to get the combustion characteristics. Since the result at SOI = 6 CAD BTDC was relatively good, the combustion characteristics of fuel blends, such as in-cylinder pressure, pressure rise, rate of heat release (ROHR) and mass fraction burned (MFB), were analysed and compared to those of pure diesel at that particular degree of ignition timing. Conventional diesel fuel (D100) showed the maximum pressure of ~7.32 MPa at 367 CAD, while the maximum pressures of D50RME45P5, D50RME40P10 and D50RME30P20 were ~1.0%, ~1.1% and ~1.7% higher as seen in Figure 6. There was also a delayed burning with isopropanol.
A combustion-driven pressure starts to increase the earliest with the engine running on diesel. The maximum pressure rise of ~0.40 MPa/CAD was observed at 361 CAD as seen in Figure 6. The maximum pressure rise increases with an increase in alcohol percentage. ~7.5% (361.5 CAD), ~17% (362 CAD) and ~29% (363 CAD) was the maximum increase for D50RME45P5, D50RME40P10 and D50RME30P20.
The maximum rate of heat release of diesel fuel was lower at 32.0 J/deg than that of other fuel blends as seen in Figure 7. The maximum rate of 34.8 J/deg (~8% higher) was observed with the D50RME4P5 fuel blend, while the maximum rate of D50RME40P10 was 38.1 J/deg (~19% higher) and that of D50RME30P20-46.4 J/deg (~44% higher). Isopropanol reduces the cetane number (Table 4), prolongs the ignition delay phase and significantly increases the maximum ROHR during the Conventional diesel fuel (D100) showed the maximum pressure of~7.32 MPa at 367 CAD, while the maximum pressures of D50RME45P5, D50RME40P10 and D50RME30P20 were~1.0%,~1.1% and 1.7% higher as seen in Figure 6. There was also a delayed burning with isopropanol.
A combustion-driven pressure starts to increase the earliest with the engine running on diesel. The maximum pressure rise of~0.40 MPa/CAD was observed at 361 CAD as seen in Figure 6. The maximum pressure rise increases with an increase in alcohol percentage.~7.5% (361.5 CAD), 17% (362 CAD) and~29% (363 CAD) was the maximum increase for D50RME45P5, D50RME40P10 and D50RME30P20.
The maximum rate of heat release of diesel fuel was lower at 32.0 J/deg than that of other fuel blends as seen in Figure 7. The maximum rate of 34.8 J/deg (~8% higher) was observed with the D50RME4P5 fuel blend, while the maximum rate of D50RME40P10 was 38.1 J/deg (~19% higher) and that of D50RME30P20-46.4 J/deg (~44% higher). Isopropanol reduces the cetane number (Table 4), prolongs the ignition delay phase and significantly increases the maximum ROHR during the premixed combustion phase [58]. The LHV of isopropanol is lower, but this is offset by the higher fuel content (Figure 7), and the diffusion combustion phase produces a similar amount of heat for all fuels. Higher maximum temperatures during premixed combustion phase increase the formation of nitrogen oxides, but allows for a better combustion of soot at the end of the combustion process [69][70][71].
Energies 2020, 13, x FOR PEER REVIEW 12 of 18 premixed combustion phase [58]. The LHV of isopropanol is lower, but this is offset by the higher fuel content (Figure 7), and the diffusion combustion phase produces a similar amount of heat for all fuels. Higher maximum temperatures during premixed combustion phase increase the formation of nitrogen oxides, but allows for a better combustion of soot at the end of the combustion process [70][71][72].
The mass burn fraction diagram in Figure 7 confirms the prolongation of the ignition delay phase in fuel blends with a higher alcohol content. Ignition delay for D100 was ~ 5 CAD, D50RME45P5 -~6 CAD, D50RME40P10-~7 CAD and D50RME30P20-~8 CAD. Although ignition delay is longer for blends with isopropanol [73], oxygen concentration in blends significantly increases due to RME and isopropanol. This significantly accelerates the combustion process during the premixed combustion phase, and 0.5 MBF of all fuels is available at ~ 7.5 CAD ATDC. The diffusion combustion phase MBF intensity is similar for all fuels, although the fuel consumption of D50RME30P20 increased by ~9% (Figure 3), which was offset by increased injection rate due to a lower isopropanol viscosity and faster combustion driven by a higher oxygen concentration. 99% of the D100 fuel mass ends up burning ~410 CAD ATDC, D50RME45P5 -~412 CAD ATDC, D50RME40P10 -~413 CAD ATDC and D50RME30P20 -~414 CAD ATDC.

Conclusions
The analysis of the energy, ecological and combustion parameters of diesel 100, D50RME45P5, D50RME40P10 and D50RME30P20 in a turbocharged direct injection diesel engine at the speed (n) of 2000 rpm and under various loads and injection timings allows making the following conclusions: 1) RME and isopropanol reduce LHV and the cetane number of fuel blends, but increase the oxygen concentration in the blend and lower the C/H ratio. 2) D50RME30P20 brake specific fuel consumption increased by ~9% compared to D100 and BTE decreased by ~1.8% due to a 7.7% reduction in LHV and a change in the combustion process. The maximum BTE of the D50RME40P10 fuel blend was equal to D100 efficiency having advanced the injection timing of the fuel blend ~2 CAD. This offset the increase in the ignition delay due to a low propanol cetane number (~12). 3) Carbon dioxide emissions of all fuels are similar, but the best carbon dioxide effect was obtained with the D50RME40P10 fuel blend. At medium loads, CO2 emissions of this blend declined by ~0.2% compared to diesel, though fuel consumption increased by ~6%, as the C/H ratio of the fuel blend was 2.1% lower. A more advanced injection timing (SOI = 8…10 CAD BTDC) at the minimum fuel consumption allows achieving lower CO2 emissions. The mass burn fraction diagram in Figure 7 confirms the prolongation of the ignition delay phase in fuel blends with a higher alcohol content. Ignition delay for D100 was~5 CAD, D50RME45P5-~6 CAD, D50RME40P10-~7 CAD and D50RME30P20-~8 CAD.
Although ignition delay is longer for blends with isopropanol [72], oxygen concentration in blends significantly increases due to RME and isopropanol. This significantly accelerates the combustion process during the premixed combustion phase, and 0.5 MBF of all fuels is available at~7.5 CAD ATDC. The diffusion combustion phase MBF intensity is similar for all fuels, although the fuel consumption of D50RME30P20 increased by~9% (Figure 3), which was offset by increased injection rate due to a lower isopropanol viscosity and faster combustion driven by a higher oxygen concentration. 99% of the D100 fuel mass ends up burning~410 CAD ATDC, D50RME45P5-~412 CAD ATDC, D50RME40P10-~413 CAD ATDC and D50RME30P20-~414 CAD ATDC.

Conclusions
The analysis of the energy, ecological and combustion parameters of diesel 100, D50RME45P5, D50RME40P10 and D50RME30P20 in a turbocharged direct injection diesel engine at the speed (n) of 2000 rpm and under various loads and injection timings allows making the following conclusions: (1) RME and isopropanol reduce LHV and the cetane number of fuel blends, but increase the oxygen concentration in the blend and lower the C/H ratio. (2) D50RME30P20 brake specific fuel consumption increased by~9% compared to D100 and BTE decreased by~1.8% due to a 7.7% reduction in LHV and a change in the combustion process. The maximum BTE of the D50RME40P10 fuel blend was equal to D100 efficiency having advanced the injection timing of the fuel blend~2 CAD. This offset the increase in the ignition delay due to a low propanol cetane number (~12). (3) Carbon dioxide emissions of all fuels are similar, but the best carbon dioxide effect was obtained with the D50RME40P10 fuel blend. At medium loads, CO 2 emissions of this blend declined bỹ 0.2% compared to diesel, though fuel consumption increased by~6%, as the C/H ratio of the fuel blend was 2.1% lower. A more advanced injection timing (SOI = 8 . . . 10 CAD BTDC) at the minimum fuel consumption allows achieving lower CO 2 emissions. (4) Isopropanol has a greater impact on nitrogen oxide emissions at low loads. NO x emissions of D50RME45P5, D50RME40P10 and D50RME30P20 increased by~10%,~12% and~21% due to a higher oxygen concentration in the blends and a higher combustion temperature. With increasing load, an increase in NO x emissions was lower (~4%,~7% and~10%) as a low isopropanol cetane number had a lesser effect on the ignition delay phase and the heat release rate during the premixed combustion phase. With an early injection timing, NO x emissions increased, but the impact of alcohol was lower. (5) Having replaced diesel with fuel blends at a low load resulted in a~6%,~10% and~14% reduction in smoke and an average load reduction of~12%,~16% and~28%. Smoke was reduced by lower C/H ratios and increased oxygen content in the fuels. As the load increased, the BTE of the fuel blends increased more intensively, which further reduced smoke emissions. In the case of early injection timing (SOI = 8 . . . 16 CAD BTDC), smoke emissions of the fuel blends changed (decreased) less intensively due to changed fuel characteristics compared to pure diesel. (6) At a low engine load (BMEP = 0.3 MPa), the average rotation speed (n = 2000 rpm), the fixed injection timing (SOI = 6 CAD BTDC) and the replacement of diesel by fuel blends with a higher alcohol content (5%, 10% and 20%) resulted in ignition delay changing from~5 CAD tõ 6 CAD,~7 CAD and~8 CAD. A greater ignition delay (accumulates more fuel) and a higher oxygen content in fuel during the premixed combustion phase increased the heat release intensity by~8%,~19% and~44%, which in turn increased the pressure rise by~8%,~17% and~29% in the thermodynamic load of the crank mechanism. During the diffusion combustion phase, the combustion heat release of all the fuels examined was similar. (7) The authors plan to continue research of these three-component blends by increasing the share of alternative fuels in the blends and assessing the impact of the EGR system when using these blends. Acknowledgments: Part of the combustion characteristics reported in this article were obtained using the engine numerical analysis tool AVL BOOST, acquired by signing a Cooperation Agreement between AVL Advanced Simulation Technologies and the faculty of the Transport Engineering of Vilnius Gediminas Technical University.

Conflicts of Interest:
The authors declare no conflict of interest.