Impact of Split and Re-Entrant Type Piston Bowl Geometry Fuelled with Pre Heated Diesel and Biodiesel on a Compression Ignition Engine Characteristics

The present investigation is carried out on biodiesel practicability over the existing non-renewable fuel due to its environmental dilapidation effect and oil crisis. Biodiesel was extracted from crude oil by transesterication, and its properties have been compared with those of neat diesel according to ASTM standards. Then, the blends of biodiesel are prepared for experimental analysis. Experimental results from our previous research study, the best blend was optimized. Then, the standard CI engine with Hemispherical Piston Bowl Geometry (HPBG) is modied to Toroidal or Split type Piston Bowl Geometry (TPBG) and Re-Entrant Piston Bowl Geometry (RPBG). Experimental works were carried out for preheated optimized blend, neat diesel with modied Piston Bowl Geometries. The engine characteristics results were compared with these altered conditions. The modied PBG with preheated biodiesel blend resulted in better Performance and Combustion characteristics. The preheated biodiesel blends indicated signicant depletion in the emission of harmful particulate matter such as CO, NO x , and unburnt Hydrocarbons.


Introduction
Recent investigations and surveys in the eld of fuels have shown that the usage and exploitation of nonrenewable fossil fuels have increased rapidly. With the ever-growing industries and use of motor vehicles, the world will soon need an alternate energy source form. The existing fuels such as diesel and petrol are depleting at an alarming rate, and the use of these fuels adds up enormously to the atmosphere's pollution levels. Researchers have been looking for an alternate source whose performance is comparable to diesel, keeping in mind the extinction and pollution aspects. One such alternate source which was found promising by the researchers is biodiesel [1][2][3].
Biodiesel is extracted from plants and animals and is found to be compatible with existing diesel engines. Biodiesel contains long-chain fatty esters and is obtained by reacting waste vegetable oils, animal fats with alcohol [4]. Generally, diesel is mixed with a proportionate amount of biodiesel known as Blends. Investigations have revealed that 20% blend is compatible with existing engines with very few modi cations to be carried out. The history of biodiesel dates back to 1853, when Patrick Duffey conducted transesteri cation of vegetable oil. In the year 1937, G. Chavanne of University of Brussels was granted a patent for transforming vegetable oils for their uses as fuels. This is the rst signi cant production of what is called biodiesel today [5]. Investigations revealed that biodiesel, when used as a fuel, signi cantly reduced the CO, HC, PM emissions into the atmosphere. In contrast, in a few cases, the NO x emissions seemed to have increased along with increased fuel consumption. Biodiesel has a higher cetane number, a higher ash point, better lubrication properties, better biodegradable properties than diesel. Some of biodiesel's concerns are higher fuel consumption, engine wear, and a viscosity [6][7][8].
Further, it has been seen higher viscosity and biodiesel density even though after esteri cation, this intended to focus on preheating of fuel before sending into the engine cylinder to improve the characterization and engine performance [9,10].
Speci c fuel consumption and engine characteristics depend on the air-fuel mixture's movement in the combustion chamber. The burning of charge in the chamber is greatly in uenced by the piston bowl's pro le leading the effective ow movement to the air-fuel mixture. Swirl movement of air is the movement around the circumference of the piston bowl geometry; as it moves, the increase in velocity leads to better air-fuel mixing to result in incomplete combustion and reduced emissions. The standard hemispherical PBG is modi ed into the split (toroidal) and re-entrant type [11][12][13][14].
Jaichandar et al [15]. Investigated the effect of two modi ed PBGs namely toroidal PBG and shallow depth PBG on a diesel engine fuelled with Pongamia Oil Methyl Ester (POME). The results showcased that the higher brake thermal e ciency and lower speci c fuel consumption with the B20 blend compared to diesel. The emission characteristics indicated a signi cant reduction in carbon monoxide, particulate matter, and unburnt hydrocarbons, but the NO x emission level increased.
Lingesan et al [16]. Studied the diesel engine characteristics when chlorela emersonni was used as the fuel. Blends of 10, 20, 30, and 100% were prepared. The ndings indicated that B20 showed the most e cient results. The emissions showed a reduction in carbon monoxide, unburnt hydrocarbons, and smoke levels, whereas NOx levels increased.
Vedharaj et al [17]. Investigated the modi ed combustion bowl geometry of CI engine fuelled with kapok biodiesel. Different mix from 20% to 100% of biodiesel were prepared. The standard PBG was modi ed into trapezoidal type PBG and toroidal type PBG. The results indicated that toroidal type PBG showed higher performance and fewer emission characteristics than trapezoidal and hemispherical PBGs. Venkata et al [18]. Examined the consequences of shallow depth, toroidal and hemispherical PBGs on the performance and emission characteristics. The results revealed that toroidal PBG showed better characteristics when compared to the other two. The brake thermal e ciency increased by 10% with toroidal PBG when the B20 blend was used. The emissions of carbon monoxide, unburnt hydrocarbons, and opacity were lessened by 13%, 11%, and 10%, respectively. Sankar Ganesh et al [19]. Investigated the effect of standard PBG modi cation into a deep bowl and toroidal PBGs fuelled with grape seed oil methyl ester. It was observed that deep bowl PBG showed better performance and emission characteristics when compared to the other. It was also noted that the NO x emission levels also decreased.
Dilip Kumar Bora1 et al [20]. Extracted biodiesel by mixing equal proportions of polongha oil, jatropha oil, and Karanja oil and investigated its effect on CI engine's performance and emission characteristics. It was observed that up to the B40 blend, the brake thermal e ciency increases but above that proportion, the thermal e ciency decreases.
Prakash T et al [21]. Examined the impact of mixing castor oil and pine biodiesel on the combustion characteristics of a diesel engine. The result analysis's optimum indicated was 30% of pine oil and 70% of castor oil. This blend resulted in increased thermal e ciency and reduced emissions. The NOx levels were also observed to have decreased.
Channappagoudra et al., [22] examine the engine traits of toroidal piston bowl geometry (TPBG). In this study, they modi ed HPBG to TPBG for dairy scum biodiesel CI engine. The results showcased that the BTE, the rate of heat energy released, the pressure rise rate, and in-cylinder pressure were increased and less BSFC for both diesel and B20 fuel than typical PBG values. Further, HC emissions were decreased, and shorter ignition delay and less combustion duration were observed for the TPBG operated CI engine.
Kumar., [23] reported the in uence of Toroidal shape with a peripheral cut on the piston top's circumference, Hemisphere shape with Spherical Arc on Circumference of the piston top on CI engine characteristics. The results inferred that the modi ed geometries gave an improved performance, combustion, and fewer emissions except for NOx than that of standard PBG.
Ganji et al., [24] studied the different PBG's CFD models for a CI engine. They created the hemispherical, shallow depth and toroidal shape piston bowl. Then they examined the effects of different PBG's on the CI engine. The results inferred that TCC geometry gave a better swirl motion than the other geometries.
Further, TCC geometry gave an improved performance and fewer emissions with that of the baseline design.
Lalvani et al., [25] investigated the effect of novel design of PBG with adelfa biodiesel at 20% blend on CI engine. The results inferred that improved performance and combustion traits and fewer exhaust emissions except for NOx for A20 blend the novel designed PBG compared to HPBG. Ramesh Bapu et al., [26] examine the suitability of modi ed hemispherical piston bowl geometry [MHCC] for variable compression ratio CI engine. They have carried out a simulation study on MHCC by using Ansys Fluent software. The results concluded that the MHCC has better swirl motion than HCC and the MHCC was a better choice for the conventional engine at all operating conditions. Varun Singh et al., [27] reviewed and discussed the consequence of various piston bowl design on CI engine. They showcased a de ciency in performance when biodiesel was used in the CI engine, which could be overcome by modifying the combustion chamber geometry. Finally, from elaborate discussions, they concluded that small modi cations in the piston crown enhanced the performance and combustion attributes and fewer emissions than conventional piston bowl design.
From the above widespread literature, it was seen that only a few works had been done on the different piston bowl geometries (PBG) with various biodiesel to evaluate the engine characteristics. And also, there are no many investigations on preheating of fuel and modi ed PBG to improve the performance and reduce the CI engine emissions. Thus, the current study aims to evaluate the impact of SPBG and RPBG on a CI engine fuelled with and without preheated fuels, and the best PBG has to be optimized based on the engine characteristics studies. Therefore, in this study, comprehensive investigations have been done on the modi ed PBG's namely SPBG/TPBG and RPBG, along with preheated fuel, to evaluate the best PBG shape for with and without preheated fuelled CI engine by likening the results of modi ed PBG's with Standard PBG.

Extraction of Biodiesel
Calophyllum Inophyllum non-edible oil was selected in the present study because of its wide availability, low cost, and high oil content. Initially, FFA was tested, and it was found to be more than 2%, which is not acceptable as per ASTM standards. Therefore, the Transesteri cation method was adopted to reduce the FFA by less than 2% and maintain the other properties as per ASTM standards. The detailed biodiesel extraction processes have been discussed in our previous research work [28]. The produced biodiesel is blended in different ratios. CI20 blend is made up of 20% Biodiesel and 80% Pure Diesel, CI100 blend consists of 100% Biodiesel and is entirely free from diesel, making it the purest biodiesel, and 'D' is neat diesel with 0% Biodiesel. These blends have been prepared to carry out the engine test. The photographic view of the Transesteri cation test arrangement and biodiesel blends are portrayed in Figure 1, and 2. Table 1 portrays the biodiesel blends with neat diesel physical and chemical characteristics. These geometries were used to optimize the best performance traits in the CI engine. Figure 3 depicts the standard PBG (hemispherical combustion chamber). Figure 4 and gure 5 are the modi ed piston bowl geometry used for the experiment work. Figure 4 is the toroidal type piston bowl geometry, and Figure 5 shows the re-entrant type PBG. The speci cation table of all the PBGs are depicted in Table 2.

Experimental Set-up and Experimental Tests
In this investigation, CI engine with four strokes, one cylinder setup was used. The engine has a maximum power of 5.2kW and 1500rpm constant speed with varying loads from 0-100 % by the eddy current dynamometer. 'Engine soft' software was used for online data recording, and the engine was fully computerized. The exhaust pipe emissions like CO, HC, and NOx were measured by using ve gas analyzer. A cold trap and sieve elements were attached to the gas analyzer to preclude it from dust particles and moisture from the dissipate emissions. An accelerator smoke meter was used to measure the smoke density. The photographic view of the engine test rig is shown in Figure 6. The engine test rig, dynamometer, gas analyzer, and smoke meter speci cation are illustrated in tables 3, 4, 5, and 6. The engine was run with the standard HPBG, and the baseline reading was recorded with diesel and biodiesel samples. In this experimental work, two modi ed PBGs were used. The two geometries used were toroidal/split type and re-entrant type PBG. The standard hemispherical piston bowl geometry was used without any modi cation to record the baseline readings. The engine achieved steady-state condition by allowing it to run with diesel for about 30 minutes. Further, the engine's steady-state condition was monitor by observing the exhaust water temperature, and it is limited to 50-55 o C. The engine loads various from 0 to 100% at an interval of 25%. To use alternate fuel after completing the engine test with convention fuel, all the fuel used in the previous experiment was completely drained out from the tank and fuel pipe. Then CI fuel was lled into the tank, and the same procedure was followed for each sample. After completing the test for standard PBG, the same procedure was followed for TBPG and RPBG, and the results were recorded. The accuracy of the experimental setup was found out by doing an error analysis.

Uncertainty Calculation
Before conducting regular experiments, it is essential to know the instrumentation error to get accurate results. Therefore, the preliminary experiment has repeated a minimum of 10times at the standard operating conditions, and results were recorded. Then average values were taken. Further, the percentage uncertainty of various devices was calculated by the root mean square method. The total uncertainty of the instruments used was calculated as follows The total improbability of the instrument used is 1.48%. The comprehensive Uncertainty, Accuracy, and Range of various measuring instruments are shown in Table 7. In this study, the energy share rate of different proportions of fuel has been calculated to know its contribution of energy in each load varying conditions. The same has been tabulated in Tables 8, 9, 10, 11, 12, and 13. This happened might be due to the lower viscosity of preheated fuels leading to the e cient vaporization of fuel, which gave higher BTHE than cold fuel [29][30][31][32].   .7 and 71.9% respectively. It indicates the ME of preheated CI20 fuel increased by 1.13% for TPBG than the other PBGs and fuels. Furthermore, it has been inferred from Figure 9 (a) & (b) that the preheated fuels have more ME than those of non preheated fuels. It might be happened due to effective vaporization of fuel results the complete combustion causes for the higher ME [29][30][31][32].
The in uence of different PBG's on IP for with and without preheated fuels at varying load conditions are depicted in Figures 10 (a) and (b). Refers to Figure 10(a), the IP results of without preheated fuels for D-HPBG, CI 20-HPBG, CI100-HPBG, D-TPBG, CI20-TPBG, CI100-TPBG, D-RPBG, CI20-RPBG, CI100-RPBG noted as 7, 6.83, 6.78, 6.86, 6.83, 6.78, 6.99, 7.06 and 6.87 kW respectively. The results indicated that there is an improved IP in RPBG than those of TPBG values, and for the CI20 blend, the IP is almost 3.36% higher than the TPBG at 100% engine load. It happened might be due to high combustion pressure and -RPBG are 6.85, 6.95, 6.89, 6.76, 6.8, 6.88, 6.98 and 6.83 kW respectively. From these results, it is clear that preheated fuels have higher IP than cold fuels for different PBG's. The main reason for this variation is the e cient vaporization of fuel, which gave high combustion pressure [29][30][31][32].  than to other blends and geometry. Due to improved atomization and better fuel was mixing for preheated fuels resulting in the decreased BSFC [29][30][31][32]. .05 o C respectively. It has been noticed that for TPBG the EGT is less than that of other geometries at preheated conditions. This might be due to improved combustion, and better atomization of fuel particles results in the lower EGT [29][30][31][32].

In uence of Split and Re-Entrant
Type Piston Bowl Geometry with diesel and CI20 biodiesel (with and without preheated) on Engine exhaust emissions.
The variation of CO emission with and without preheating for different PBGs are depicted in Figure 14  The results show that CO emission is maximum at full load for D-TPBG and is lowest for CI20-HPBG. Thus indicating biodiesel helps in the reduction of emission of CO. The emission of CO for biodiesel blends is slightly lesser than conventional diesel. The reduction in emission of CO for biodiesel blends is due to the higher oxygen content of biodiesel that will help in complete oxidation combustion. In this study, observed that at lower load, CO emissions are considerably less compared to the higher load emissions. Because increasing load will increase the gas's in-cylinder temperature, thus resulting in the increased CO emission. Then for modi ed geometries, it is seen that they have higher CO emissions with calophyllum inophyllum biodiesel, which is quite interesting as normally modi ed geometries are expected to reduce the emission of CO, with better air-fuel movement. Modi ed PBGs will have better turbulence and squish than the standard diesel engine. Still, CO emissions can sometimes be unpredictable as many minor aspects determine it depending on the biodiesel and geometry. It is seen from the results that CO emission is maximum at full load for D-50 o C TPBG and is lowest for D-50 o C HPBG. Preheating pure diesel will reduce CO emissions compared to diesel at room temperature even though the reduction in CO is almost the same as the diesel at room temperature in the case of TPBG. But it has been observed a considerable amount of reduction in CO in HPBG and RPBG with preheating. This may be due to the better combustion and oxidation of diesel, as it is heated before getting inducted into the engine. The same trend is observed for the CI20 blend preheated in comparison to the CI20 blend without preheating. The study outcomes show that the emission of CO for biodiesel blends is slightly lesser than conventional diesel [29][30][31][32].  Figure 15 (b) shows that HC emission is maximum at full load for CI20-50 o C HPBG and is lowest for CI20-50 o C TPBG. It is clear that the preheating emission of HC reduces considerably for a standard diesel engine. Further, with preheating, it was noted that HC's emission is reduced considerably for preheated diesel compared to the diesel at room temperature. Preheating ensures proper combustion; hence it is seen that the HC emissions were reduced. Even the Biodiesel blends follow the same trend compared to the biodiesel at room temperature.
CI20-50 o C TPBG has the lowest HC emission due to the modi ed geometry that ensures proper air-fuel movement [29][30][31][32]. respectively. It shows that CO 2 emission is maximum for D-RPBG 50 o C and least for B20-TPBG 50 o C at full load. Even with preheating, there is no considerable change in the emission of CO 2 in both biodiesel blends and pure diesel compared to the fuels at room temperature. This might be due to the carbon content present in the fuel that has to be oxidized to CO 2 . So it is clear that there are no considerable changes in the CO 2 emission using different PBGs [29][30][31][32].
The variation of O 2 emission with and without preheated fuel blends for different PBGs are depicted in  with preheating at 50oC are illustrated in Figure 17  Heating the biodiesel blends and diesel at 50 o C before inducing it into the engine results in better oxygen emission, and less CO is produced as the reaction takes place faster when preheated. Though CO 2 and NO x consumption also increased, it can be reduced by processes and improving emission characteristics. Even here, biodiesel blends have more oxygen content than diesel blends, giving more fuel consumption [29][30][31][32].
The disparity of NO x emission with and without preheated fuels for different PBGs are depicted in Figure   18  show that NO x emission is maximum at full load for D-HPBG and is lowest for CI20-RPBG. The standard diesel engine emits the maximum amount of NO x compared to the others. NO x is formed due to the reaction of oxygen and nitrogen in the chamber. Thus the more oxygen, the more is the NO x formation.
But NO x formation is less for Re-entrant piston bowl geometry (RPBG) when compared to the other two geometries. Because the swirling action in re-entrant helps the reaction to reduce and thus biodiesel, the re-entrant (CI20-RPBG) is the most e cient for reducing emission for NO x . The maximum NO x emission at full load with preheating for diesel and CI20 blends with different PBGs are 1325, 1307, 876, 916,899, 880 and 839 and 841ppm for D50 o C -HPBG, CI20 50 o C -HPBG, D 50 o C -TPBG, CI20 50 o C -TPBG, CI100 50 o C -TPBG, D 50 o C -RPBG, CI2 50 o C -RPBG, CI100 50 o C -RPBG respectively. The NO x emission is maximum at full load for D-50 o C HPBG and is lowest for CI20-50 o C RPBG. It has been observed that with the preheating, the emission of NO x reduces considerably for a standard diesel engine. By preheating the biodiesel and diesel blends at 50 o C, the NO x formation at maximum load is decreased drastically compared to without preheated blends. This is because, at high temperature, the O 2 formation CO 2 takes place more when compared to NO x formation. Thus, it is a better choice to preheat the fuels for better emission characteristics. The re-entrant piston bowl geometry gives better results at full load than the other two piston bowl geometry. At 75% load, the toroidal piston bowl geometry shows the least emission, which concludes that the modi cation in the piston bowl geometry gives better results than the standard geometry [29][30][31][32]. The results show that Opacity is maximum at full load for D-RPBG and is lowest for CI20-HPBG. This indicates that the smoke from D-RPBG is highly opaque, and the least amount of light will pass through it. Opacity means how much smoke is liberated at the end of the fuel consumption process. Figure 19 (a) shows that at full load, the modi ed piston bowl geometry's Opacity is more compared to the standard, but with biodiesel blend, it's comparatively less. Thus, the biodiesel blend of  Figure 19 (b) clari es that Opacity is maximum at full load for D-50 o C RPBG and is lowest for D-50 o C HPBG. So it is clear that heating has the least effect on Opacity of the fuel. When the biodiesel blend and the diesel blend are heated at 50oC, the reaction and the formation of CO2 takes place rapidly, and opacity increases. Thus, for preheated biodiesel blends, the Opacity is more, and it is least for diesel hemispherical piston bowl geometry (D-HPBG). It is highest for diesel re-entrant piston bowl geometry (D-RPBG), but overall it is seen that the biodiesel blends have less Opacity when compared to the diesel blends. Thus, giving better emission characteristics [29][30][31][32].
3.3 In uence of Split and Re-Entrant Type Piston Bowl Geometry with diesel and CI20 biodiesel (with and without preheated) on Combustion attributes.
The disparity of cylinder pressure with and without preheating fuels for different PBGs is depicted in Figure 20 (a) & (b) at 100% engine load. Figure 20 (a) shows that with standard piston bowl geometry, the highest peak is observed, with diesel having a value of 73.67bar compared to CI20, which has a value of 72.41 bar. This may be due to diesel having a higher calori c value as compared to CI20. Also, diesel's lower viscosity compared to CI20 leads to better burning of the fuel in the rapid combustion phase. It is also observed that CI20 RPBG has resulted in the highest cylinder pressure of 74.33bar followed by diesel RPBG of 74.01bar. The RPBG has resulted in the most elevated cylinder pressure, resulting from improved air-fuel mixing with RPBG, resulting in effective burning of charge compared to other type PBGs. Figure   20 (b) observed that due to a decrease in biodiesel's bulk modulus and the increase in fuel temperature, the fuel injection was slightly delayed. Short ignition delay at preheated temperature can also be due to early combustion, resulting in a lower peak pressure of 73.92bar for CI20 RPBG [33][34].
Impact of different PBG's on Cylinder pressure for with and without preheated fuels at 100% Engine load conditions are portrayed in Figure 21 (a) & (b). Biodiesel has a higher stickiness, lesser heating value, and lower capriciousness than diesel, which accounts for the more signi cant diesel pressure rise. The pressure drops progressively after reaching a maximum value during the expansion process. Figure 20 (a) also inferred that the RPBG has the highest rate of pressure rise of 4.52bar/degree crank angle compared to other piston bowl geometries. This resulted from increased heat conduction to the cylinder wall of the RPBG due to extreme turbulences, which lead to hasty con agration temperature and pressure rise rate during burning of fuel at full load. This reason also accounts for toroidal piston bowl geometry with a higher pressure rise rate of 4.49bar/degree crank angle compared to standard piston bowl geometry of 4.22bar/degree crank angle for diesel. Similarly, for CI20, TPBG has a higher pressure rise rate of 4.27bar/degree crank angle compared to HPBG of 4.06bar/degree crank angle. From Figure 21(b), it is observed that even with preheating, the pressure rise for diesel is higher as compared to CI20. This could account for the highly volatile nature of diesel, which releases more energy per unit crank angle [33][34]. and without preheated fuels at 100% engine load conditions. It is observed that during the ignition delay period, there is a slight negative heat release rate, which is due to the cooling effect of vaporization of the CI20 blend and also due to loss of heat from cylinder walls. Diesel has a higher heat release rate than CI20 because of its higher calorie content and lower viscidity. The heat release rate in standard HPBG for diesel was 36.66 KJ/degree higher than CI20 due to better air-fuel mixing and faster evaporation. Toroidal piston bowl geometry gives the highest heat release rate of 42.14 KJ/degree with diesel than other PBGs. This was due to superior chemical reaction and proper air-fuel mixture during the compression process due to high turbulence leading to effective combustion of charge, increasing the heat conduction through the walls of the cylinder. Figure 22 (b) observed that preheating of working uids increases the net heat release rate for both diesel and CI20. This may be because preheating increases the temperature of working uid to a higher temperature compared to not preheating, hence causing it to increase the heat release rate. The highest heat release rate is obtained for toroidal piston bowl geometry is 41.73 KJ/degree for diesel [33][34].
The variation of cumulative heat release rate (CHRR) with and without preheating of fuels for different PBGs are depicted in Figure 23 (a) & (b) at 100% engine load. In Standard operating conditions, with hemispherical PBG, it can be seen that the cumulative heat release rate for diesel fuel is higher than that of the CI20 blend. This accounts for the higher calorie content and lower viscidity of diesel when compared to the CI20 blend. Modi cation of the piston bowl geometry shows the better movement of the fuel inside the chamber. The re-entrant type PBG shows e cient swish swirl movement of air-fuel mixture inside the chamber. When CI20 blend is used as a fuel with re-entrant type PBG along with the e cient movement of the air-fuel mixture, more amount of fuel can be drawn, and hence more heat is released. This statement is evident from Figure 23 as the re-entrant type PBG shows the highest cumulative heat release rate, with a value of 1.19 kJ/crank angle. In toroidal PBG, it results indicate that the cumulative heat release rate of 1.15 kJ/degree when diesel is used as fuel is higher than the CI20 blend gives 1.13 kJ/degree. Preheating the biodiesel increases the fuel temperature. This resulted in higher cumulative heat release from 1.13 kJ/degree to 1.17 kJ/degree in standard operating conditions. In re-entrant PBG, injecting a preheated biodiesel blend has a higher cumulative heat release rate than preheated diesel [33][34]. and without preheated fuels at 100% engine load. It has been inferred from Figure 24 that the 1 o to 2 o early CA of MFB occurred for both preheated and without preheated fuels at Re-entrant PBG than to standard and Toroidal PBG. This was happening due to the adequate mixing of charge and turbulence in fuel ow with that of other geometry [33][34].
The variation of Ignition Delay with and without preheating of fuels for different PBGs are depicted in Figure 25 (a) & (b). Ignition delay is the time period between the start of fuel injection and the beginning of combustion. From Figure 25, it is observed that the ignition delay decreases as the load increases. The reason behind this trend is that as the load increases, the heat inside the chamber increases. This heat aids in faster ignition of the air-fuel mixture and reduces the delay period. The results indicated that the ignition delay decreases when biodiesel blends are used instead of diesel. The reason being, biodiesel has a high cetane number and increased oxygen content. Hence, a reduced ignition delay period is observed. In standard operating conditions with hemispherical PBG, the ignition delay for diesel was 14°CA, whereas for CI20 blend, it was 13°CA. The 14°CA ignition delay period for re-entrant PBG was found to be lower than that of toroidal PBG that is 17°CA. Re-entrant PBG creates higher turbulence in the chamber, which aids in e ciently mixing the air-fuel mixture, leading to less ignition delay. Preheating the working uids tends to increase their bulk modulus. This accounts for the increased ignition delay when preheated diesel and biodiesel CI20 blend is used. The ignition delay for CI20 went up from 13°CA to 15°CA. Hence, the results indicated that preheating the working uids tends to increase the ignition delay period [33][34].
The variation of combustion duration (CD) with and without preheating of fuels for different PBGs are depicted in Figure 26 (a) & (b). It indicates that the combustion duration increases as the load increases. The reason being, as the load upturns, the amount of fuel entering the chamber enhances, and hence the time taken to burn the fuel increases. Due to the low viscosity of diesel, e cient air-fuel mixing and rapid atomization results in faster combustion. Therefore, the combustion duration of diesel is lesser than the CI20 blend. It can be observed that the combustion duration is the highest for hemispherical PBG (standard condition) when compared to toroidal and re-entrant PBGs. The reason accounted is the low turbulence, which results in increased combustion duration. The higher turbulence levels and high ame velocity in toroidal PBG can be accounted for the lesser combustion duration. Toroidal PBG accounts for a lesser combustion duration of 14°CA when compared to hemispherical PBG has 19°CA CD, and reentrant PBG has 16°CA CD. Preheating the working uids lead to decreased combustion duration. The rate of vaporization of working uids increases on preheating. Due to the increased rate of vaporization of diesel and CI20 blend, the fuel's burning period increases. Across all the piston bowl geometries used, preheated diesel and CI20 blend have lowered the combustion duration [33][34].

Conclusions
In the present research, investigations are carried out on modifying engine piston bowl geometry with CI20 blend as fuel and comparing it with standard HPBG. Following are some conclusions of the results of the present research: There is an improved BTHE in TPBG than those of RPBG values and for CI20 blend, the BTHE is almost 4.08% higher than the RPBG at 100% engine load on without preheating. CI20 gave higher BTHE with that of without preheated fuels for different PBG's.
The CI20 fuel without preheating has approximately 0.32% higher ITHE for TPBG than the RPBG. The ITHE of preheated CI20 fuel has increased by 2.4% for RPBG than the other PBGs and fuels.
The CI20 fuel has approximately 3.79% higher ME for TPBG than the RPBG. The ME of preheated CI20 fuel has increased by 1.13% for TPBG than the other PBGs and fuels.
There is an improved IP in RPBG than those of TPBG values, and for the CI20 blend, the IP is almost 3.36% higher than the TPBG at 100% engine load. Preheated fuels have higher IP than cold fuels for different PBG's. Cylinder pressure increases by 4% for RPBG compared to HPBG when CI20 blend is used as fuel but decreases by 3.6% when TPBG is used.
The rate of pressure rise increases by 5.2% for TPBG and 8.7% for RPBG compared to HPBG.
The ignition delay and combustion duration are decreased by 3.6% and 4%, respectively, for RPBG.
CI fuel produces lesser emissions than diesel with all PBGs except for preheated CI fuel with TPBG.
This may be due to the more inferior air-fuel movement of preheated CI fuel inside TPBG.
The preheated diesel produces lesser CO, HC, and NOx emission with all PBGs compared to the diesel at room temperature. Whereas the release of O 2 is increased, and CO 2 remains almost the same.
Thus, Emission characteristics can also be enhanced by preheating the diesel before inducing it into the engine.
The emission of HC, NO x , CO 2 increases with increasing load. Because of the availability of oxygen is bare minimum at higher loads. The opacity and CO emission doesn't follow a linear trend with increasing weight.
Overall, the results indicate that the modi ed RPBG shows signi cant performance, combustion, and emission characteristics compared to HPBG and TPBG. Hence, RPBG is a suitable alternative to the existing standard PBG. CI20 blend with the modi ed PBGs has resulted in enhanced engine characteristics. Thus, the RPBG fuelled with CI20 blend is a suitable alternative to the conventional CI engine PBG.

Declarations
Ethics approval and consent to participate -Not Applicable

Consent for publication-Not Applicable
Availability of data and materials -Due to the nature of this research, participants of this study did not agree for their data to be shared publicly, so supporting data is not available.

Funding-Not Applicable
Authors' contributions-MDAN has conducted the experiments, and a draft copy of the work has been prepared. SHK, GH, BHB, and JTD have analysed the results and illustrated the conclusions. SN has done tabulation of the results, graphs, and CAD model of the devices used in this work.