The working of diesel engine aided by common rail direct injection system with fuel as MME20 (80% diesel fuel + 20% Mahua methyl ester), Ternary fuel (pentanol10%+ MME20%+ diesel 70%), Ternary fuel 40 (Ternary fuel mixed with iron oxide additives of 40 ppm), Ternary fuel 80 (Ternary fuel mixed with 80 ppm iron oxide additives) and Ternary fuel 120(Ternary fuel mixed with iron oxide additives of 120 ppm) blends and was observed to be pretty clean for the entire rated load and no difficulties were faced. BSFC, BTE and emissions (nitrogen oxide emissions, HC, CO) and density of smoke are plotted against load. With the available data regarding combustion, HRR and cylinder pressure was plotted against crank angle.
Engine Combustion characteristics
In this section, heat release rate, in-cylinder pressure and ignition delay are discussed for all the fuels. These are the important parameters that help to understand the behavior of the fuel blends in the combustion chamber. Average temperature of the gas in the cylinder and the heat release rate is calculated using the “Enginesoft/LabView” software that uses the 0D model of a single combustion zone with constant specific heat ratio.
Figure 3 illustrates the variation of inside cylinder pressure w.r.t to crank angle for all the test fuels. It is observed from the plot that MME20 at 53.42 bar pressure is the least point, whereas the highest point of cylinder pressure prevails for D100 at 55.45 bar. The fuel-air combination generation is fairly constant which results in increased pressure when the cetane number is improved and viscosity of fuel remains low. The air-fuel mixture becomes uniform and pressure gets increased due to lesser viscosity and better cetane number. Atomization and evaporation rate, the characteristics of air-fuel interaction with lower cylinder pressure are affected by higher density, viscosity, and lowered calorific value of MME20 as compared to D100 and this can be illustrated through Table 2. Cylinder pressure improved to 56.33 bar from 53.42 bar when ternary fuel blend was used instead of MME20 blend. Improved calorific value and lower viscosity are the main attributes. Pentanol’s presence in ternary fuel helps the oxygen molecules present in it for effective combustion with improvement in cylinder-pressure. Cylinder pressure improves when ternary fuel is mixed with iron oxide nano additives as compared to TFB and MME20. For TF40 cylinder pressure was found at 55.99 bar, for TF80 it is 54.82 bar and for TF120, it is 57.11 bar. The reason for this increase in cylinder pressure is the higher surface area to volume ratio of iron oxide nano additives. This in turn improves the cylinder pressures along with efficient combustion and higher premixed combustion. When working with C.N.T. emulsified biodiesel in diesel engine, comparable trends were discovered by the scientists. Cylinder Pressure of TFB120 is found better, which is 57.11 bar measured amongst various concentrations and it is 2.33% greater from diesel.
Heat Release Rate
Figure 4 illustrates the variation of heat release rate w.r.t to crank angle for all the test fuels at full load condition. When compared to other fuels, heat release rate of D100 is observed to be as 110.36 J/deg-CA this is due to the fact that the accumulation of diesel fuel at the primary combustion phase which is in the premixed stage. More heat also gets generated because of fuel ‘s lower cetane number as and when compared to other blends. Because of typical fuel properties, i.e., high density and viscosity due to which evaporation rate (premixed stage) is affected, the lowest heat release rate of 69.214 J/deg-CA gets exhibited for MME20. Diffusion combustion is prolonged as oxygen molecules present in it affects majorly at later stages of combustion. Also, MME20 liberates heat at prior crank angles completely before the Top dead Center (TDC) because of better cetane number properties. Heat release rate of ternary fuel blend (TFB) is observed to be 90.25 J/deg-CA which is almost 21.92% higher than that of MME20. The reason behind the hike could be pentanol’s presence and lower relative amount of biodiesel, which when blended lowers down the fuel accumulation in the primary phase of combustion. TF40 exhibits heat release rate of 94.22 J/deg-CA, for TF80 it is 96.22 J/deg-CA and for TF120, it is 98.66 J/deg-CA and it is higher than ternary fuel blend having HRR of 90.25 J/deg-CA. The cumulative heat release rate phenomenon represents a substantial variance in ID (ignition delay period) due to excess fuel deposition in premixed combustion. It also reflects a higher heat release rate and the lowest cumulative heat release rate. Thus, here we can conclude that combustion can be improved with the use of TF80 as compared to other fuels.
Engine Performance Characteristics
In this section, the effect of ternary fuel with different combinations on brake thermal efficiency and brake specific fuel consumption is discussed. The two parameters explain the energy conversion efficiency of the engine.
Brake Thermal Efficiency
For ternary fuel (MME20 blended with pentanol) and nanoparticle additives blended ternary fuel, it was evident that the BTE has increased with increase in load as illustrated in the figure 5. The brake thermal efficiency of nanoparticle additives blended ternary fuel (TF80) was much greater when compared with other blends and also conventional diesel. Brake thermal efficiency of MME20 is minimum (at all loads) with higher levels of viscosity and density of esterified methyl ester. As compared to D100, the viscosity of MME20 is 52.82% higher resulting in minimum brake thermal efficiency of 24.1%. It also shows partial burning of MME20 as compared to diesel. Brake thermal efficiency at all loads is seen to increase because of the presence of pentanol in Ternary Fuel. This is mostly due to sufficient oxygen molecules in pentanol resulting in better combustion. As brake thermal efficiency of ternary fuel is still lower, enhancement in the same can be sought with the addition of iron oxide nano additives in ternary fuel. When nanoparticle additives were mixed with ternary fuel (40, 80 and 120 ppm) blend an increase of 1.58%, 1.62% and 2.34% was observed in brake thermal efficiency. This can be assigned to the finer combustion characteristics of nanoparticle additives, due to the reason that there might have occurred a refinement in the catalytic activity of nanoparticles. Simultaneously, as the evaporation rate is increased and physical delays are reduced which leads to improved BTE along with an increase in combustion efficiency (Raju et al. (2018), Kao et al. (2007) & Shaafi & Velraj (2015)) wherein the key attributes seem to be nanoparticles catalytic activity promoting primary droplets micro-explosion. More contact surface area of the NPs with high potential to store reactivity and energy, as explained by De Luca et al. (2005) and Yetter et al. (2009) which is also one of the main reasons for rise in brake thermal efficiency. Another reasonable cause for improvement in the engine is the positive effect of NP additives on HTR (heat transfer rate) due to its improved heat/mass transfer, radiative and conductive properties demonstrated by Tyagi et al. (2008). In comparison with TF40 and TF80, better thermal efficiency is observed at TF120 at various engine loads. These results are in accordance with the iron oxide nano additives inclusion in the blend with the increased thermal efficiency as mentioned in the open literature.
Brake Specific Fuel Consumption (SFC)
Figure 6 represents the variations in BSFC (specific fuel consumption) for MME20, ternary fuel blended with iron oxide nanoparticle additives and conventional diesel with load. When compared to conventional diesel fuel MME blend has less calorific value and higher viscosity leading to lesser fuel droplets atomization and vaporization rate so in order to maintain the same output power, more fuel is being consumed during the operation of mahua methyl ester blend 20. So, when compared with conventional diesel fuel Mahua methyl ester blend uses more fuel for the operation. The performance of the engine can be improved by the addition of nanoparticle additives to ternary fuel. As compared to MME20, BSFC for ternary fuel blended nano additives (TF40, TF80 and TF120) is found to be lower for all loads whereas, in case of plain diesel, it is higher. To have excellent combustion characteristics and better atomization results in blended fuels having iron oxide nanoparticles and thus observes the BSFC which is improved. Due to the inclusion of nanoparticles, there will be abundance oxygen to finish entire burning and due to the physical properties of the fuel, there will be lessening of BSFC owing to constructive outcomes of nanoparticles. Fuel consumption will be reduced as a result, if the friction power of the cylinder will decrease due to the reduction in the formation of carbon deposits by NP’s. The BSFC becomes 0.34, 0.33 and 0.31 kg/kW-hr for TF40, TF80 and TF120 respectively when a fraction of 40 ppm, 80 ppm and 120 ppm along with ternary fuel is added with nanoparticles if the load is maximum and fuel consumption of MME20 becomes 0.355 kg/kW-hr. Similar to BTE profile, the energy consumption profile also tends to go down slightly after iron oxide nano additives in ternary fuel blend. This reduction may be due to high cetane number, less physical delay and high contact surface area of iron oxide nanoparticle. Also, TF120 consumed the least energy among all other blends. Probably due to the high heat released from complete combustion prompted through nano additives, improved spray penetration, lowered ID and higher evaporation rate, this reduced BSFC.
Engine Exhaust Emission characteristics
In this section, the effect of ternary fuel with different combinations on CO, HC, NOx and smoke opacity are discussed. The exhaust emissions are the reflection of combustion characteristics of the fuel in the engine. The appropriate combustion exhibits higher NOx, lower HC, CO and smoke emissions.
Carbon monoxide Emissions
Figure 7 illustrates the variation of carbon monoxide emissions for various blends of fuel, clearly indicates that in comparison to plain diesel, nanoparticle additives blended ternary fuel blends outperforms it by emitting a significantly lesser quantity of CO. The use of nanoparticle additives reduces ignition delay by advancing the fuel combustion inside the cylinder through speedy combustion reactions. However, if the nanoparticle additives usage is excluded, the combustion was observed to be partial in contrast to a fuel mixture with a proper ratio of fuel and air which resulted in high-quality complete combustion consequently producing less emissions of carbon monoxide. Apparently, due to the presence of high oxygen quantity, the rate of conversion from carbon dioxide to monoxide gets accelerated while using mahua methyl ester in an engine. When TFB is used for fueling the engine, the CO emissions are also observed to be reduced than MME20 at all load conditions. This reduction is due to the combustion enhancer, pentanol which amplifies the combustion process. The further addition of iron oxide nano additives in ternary fuel makes it more resistant to CO emission. The observations of CO emission by TF40, TF80, TF120 were 7.89%, 11.23%, 23.26% lower than TF. The reason behind this phenomenon is due to the high oxygen-bearing nature of iron oxide nanoparticle additives which helps in increasing the catalytic activity to oxidize the CO molecules. Additionally, the high reactivity of nano additives due to high contact surface area per volume reduces CO emissions through better combustion by lowering the ignition delay period.
Hydrocarbon Emissions (UHC)
At different loads and fuel blends, the variation in emissions of unburnt hydrocarbon (UHC) against load is presented in figure 8. To determine the emission behaviour of the engine UHC is also an important limitation. For all the engine loads, UHC emission is observed to be highest in D100 as compared to MME20 which emitted 6.84%,10.36%,11.31%,9.49%, and 3.54% of unburnt hydrocarbons. This trend matches with CO emission of MME20 with respect to plain diesel. The load rise also affects the UHC emission because of high air-fuel mixture generation in wall films and crevices (cold quench areas) and plenty of available fuel in the combustion zone. By adding nanoparticle additives in fuel blends, emission of hydrocarbon is reduced to a considerable extent (TF40, TF80 and TF120 emitted UHC 5.08%, 6.78%, and 7.56% less than TF). The combustion of fuel is further advanced by the presence of nanoparticle additives in the biodiesel blend due to the fact that it acts as a catalyst (oxidising nature) for enhancing the propagation of flame and hence reducing carbon activation temperature. The presence of sufficient oxygen atoms in ethyl alcohol to be supplied for complete burning of fuel makes it a blend with least HC emission. The UHC emission is restrained by each of these elements from biodiesel blends including nanoparticle additives. But interestingly, due to the high cetane number as well as oxygen content of biofuels, a quantitative rise in iron oxide nanoparticles in their blends were found to produce considerably less HC emission. At full load, ternary fuel shows lowest emission as compared to MME20 and D100 by 3.68% and 10.84% respectively.
The variation of pure diesel, ternary fuel blended with nanoparticle additives and MME20 against load for NOx emission is illustrated in figure 9. NOx generation is a sophisticated process. A number of factors are responsible for NOx formation like working conditions, response time, combustion temperature, features of engine design, fuel properties etc. From the plot, it can be observed that diesel produced lesser NOx emissions than MME20. The reason could be the oxygen molecule presence which amplified the combustion process and thereby raising the combustion chamber temperature. Due to very high temperature inside the cylinder resulting through better combustion, NOx emissions raised for MME20 by 12.46% when compared with D100 when the load is increased to maximum.
However, this property of high oxygen content helps in improving combustion and HRR in presence of high temperature inside cylinder but it’s a threat to our environment as well. This NOx generation further increases due to its bulk modulus and injection of fuel happening earlier than usual. As per Zeldovich reaction mechanism, the NOx emissions increased because of the presence of iron oxide nanoparticle additives in ternary fuel which enhances the fuel burning inside the engine cylinder and it results in high temperature inside it. Thus, during combustion, NOx is higher compared to the conventional fuel. It is clear indications of higher HRR and cylinder pressure. TF40, TF80 and TF120 emitted NOx 7.67%, 9.29%, and 4.89% less than TF as shown in the figure 9. Finally, it can be observed that among all other test blends, minimum NOx emissions are given be TF40 (745ppm) at 100% load which is under standard limits.
In Figure 10, for the various blends (nanoparticle additives blended ternary fuel, biodiesel blend, plain diesel) the smoke opacity variation in relation to load is illustrated. Highest smoke emission is observed in D100 for all the load conditions as compared to other fuels. Excess accumulation of fuel in the cylinder, deficiency of oxygen in the combustion rich areas and poor atomization result in the formation of smoke in CI engines which is mostly due to partial combustion. Thus, it means that the low smoke emissions found in MME20 are due to better fuel oxidation inside the combustion chambers present near the fuel-rich zones. Further, smoke emissions were seen to be nominally reduced by 6.15 %, 6.92 % and 5.37 % at full load by TF40, TF80 and TF120 respectively due to the blending of iron oxide nano additives in the TF as shown in the figure 10. It shows that as compared to diesel, smoke density considerably decreased by the use of ternary fuel blended with nanoparticle additives. It is speculated that this reduction is attributed to the presence of oxygen in the nanoparticle additives which encouraged better burning of the fuel mixture. While the smoke density rises for every blend at given loads there is a decrease in its density after adding more of the nanoparticle additives in the blends. The effect of nanoparticles addition is reflected through better ignition characteristics, shorter ignition delay and high evaporation rate. The lower smoke emission is probably due to the lower delay period due to which before ignition, surplus fuel is collected inside the cylinder making sure that combustion rate is high enough and better fuel-air mix is facilitated.
The delay period, commonly known as the ignition delay period is the time period from the start of fuel injection to the initiation of combustion. It also indicates the chemical as well as the physical delay of the fuel. The ignition delay suggests fuel mixing and atomization at the final stage, while the full pre-combustion process is shown at the initial stage. Depending on engine load for various fuel blends the change in ignition delay is depicted in figure 11. The highest ignition delay is observed in the D100 at all loads. But for lower loads TF40 and TF120 showed a similar delay profile. The reason behind this was mainly due to the less viscosity, high cetane number, density and better fuel mix rate. The next blend having lower delay period was TF80 due to its low compressibility factor, high cetane number and biodiesel composition. Also, the addition of iron oxide nanoparticles in the ternary fuel reduced the delay period as compared to D100. The reason behind low delay period is attributed to better fuel atomization through high surface tension and calorific range of the blend. Also, carbon chain molecule formation is hindered by the addition of iron oxide nano additives and pentanol which tends to rise the latent heat of fuel blends.
The figure 12 illustrates mass fraction burnt deviation w.r.t. crank angle for all blends. Due to the absence of oxygen in diesel, it showed a low range MFB for all crank angles compared to other blends. Irregular MFB trend has been observed for MME20 for all crank angles due to null atomization effect and availability of more fuel-rich zones. The addition of high concentrations of nanoparticle additives and pentanol can be customized for TF with additives and TF to achieve a mass fraction burnt spectrum (customary) in the blend at all crank angles.
Trade-off (BSFC- BTE- NOx)
The detrimental effects of soot and NOx are well known to everyone. They cause a plethora of respiratory illness and degrade the environment by causing global warming through smog formation. Furthermore, a fuel which is consumed in lesser quantity by the engine is one of the factors to choose a fuel; another reason being the depleting fuel reserves. Thus, to get a clearer picture, a trade-off study is required for comparing emission and performance of engines using various fuels with respect to SFC, brake thermal efficiency and NOx emission. It also gives scope for further explanation of intrinsic issues regarding the above. Figure 13 shows the 20% to100% load trade-off for different combinations of fuel (MME20 (80% diesel fuel + 20% Mahua methyl ester), Ternary fuel (pentanol10%+ MME20%+ diesel 70%), Ternary fuel 40 (Ternary fuel mixed with iron oxide additives of 40 ppm), Ternary fuel 80 (Ternary fuel mixed with 80 ppm iron oxide additives) and Ternary fuel 120(Ternary fuel mixed with iron oxide additives of 120 ppm)). It can be noticed clearly that the trade-off shifts to the extreme left corner (minimum fuel consumption) from extreme right corner (maximum fuel consumption). From the graph, the TF120 is seen to push the trade-off to a high-NOx emission zone and BTE with a reduction of BSFC. MME20 fuel operation reduces the equivalent BSFC along with NOx emission. Of the other blends, ternary blend produces lesser NOx and more BTE. When the load is increased to 40%, the TF120 blends the smoke opacity and equivalent-BSFC reduction is seen which is indicated through the shifting of trade-off zone near to origin. Based on the trade-off pattern for the fuel sample considered, the following results can be concluded (1) Whenever the percentage of ternary fuel blended with iron oxide additives got increased, it is seen that BSFC decreases but when BTE rises NOx also goes up (2) On the other hand, TF120 shows high emission of NOx and BTE but relatively low BSFC as portrayed in the top area of the graph (3) Interestingly, TF80 shows the optimum trade-off zone with higher BTE and lower BSFC and lowest NOx emission from the current study.