Thermal and chemical exhaust gas recirculation potential of punnai oil biodiesel-fuelled diesel engine for environmental sustainability

Major energy production all over the world depends on fossil fuels. Recent research on alternative energy sources has raised major concerns about environmental impacts, future availability, and cost. Pollution from diesel engines also affects the environment negatively. As a result, there is a worldwide concern about reducing the pollutants emitted by diesel engines. In comparison to diesel fuel, biodiesel combustion produces reduced carbon monoxide (CO2) and unburned hydrocarbon (UHC) emissions but higher nitrogen oxide (NOx) emissions. The current study aims to investigate the thermal and chemical effects of exhaust gas recirculation (EGR) on the features of a diesel engine for environmental sustainability. The punnai oil was produced from kernels of punnai seeds and transesterified in two phases using alcohol with the existence of a catalyst. The higher viscosity of punnai oil biodiesel is diluted by mixing it with diesel fuel. Our previous investigation indicated that neat punnai oil biodiesel is a potential fuel; however, the findings showed that the addition of diesel is necessary to obtain acceptable engine performance. In this study, punnai oil biodiesel was mixed at a rate of 20% with diesel (B20) and run in a diesel engine with varied EGR rates under five different engine loads. This combined impact enhanced the maximum heat release rate (HRR) and maximum combustion pressure, according to the findings. The premixed burning fractions were commonly higher at all engine loads, whereas the diffusion combustion fractions were lower. When the centre of the HRR changed toward the top dead centre (TDC), combustion durations remained rather constant. The experimental results revealed the B20 blend at a 10% EGR flow rate produced 6.57% lower BTE, 37.04% higher BSEC, 2.47% higher EGT, 5.13% lower CO, 31.11% higher CO2, 3.13% higher UHC, 8.36% lower NOx, and 4% higher smoke opacity when compared with diesel in a standard diesel engine.


Introduction
Using large volumes of petroleum derivatives like diesel causes them to go extinct in the next 30-40 years (Karthickeyan et al. 2019b). Because of their high thermal efficiency, diesel engines have an extensive choice of uses 1 3 do not include harmful particles like sulphur or lead, resulting in less hazardous discharges to the environment (Zhong et al. 2021). Straight vegetable oils have operational and long-term durability issues as an effect of the low volatility, high viscosity, and polyunsaturated nature of vegetable oils (Fernández-Tirado et al. 2021). While some biofuels have higher performance and lower emissions, their high costs have made them unpopular (Jahirul et al. 2021). Furthermore, there are certain drawbacks, like increased viscosity, decreased calorific value, and gum formation, when using these oils (Rajesh et al. 2021). Various biofuels generated have been used in diesel engines by several researchers (Karthickeyan et al. 2020). A few biofuels have received more attention since their greater potential for renewable energy . To bring about changes, it appears that further research into biodiesel fuels is still required (Ashok et al. 2015;Bhowmick et al. 2019;Karthickeyan et al. 2019a). In recent years, various important studies using biodiesel fuel have been presented in the literature. Rakopoulos et al. (2006) evaluated the use of biodiesels of cottonseed, soybean, sunflower, and olive in a diesel engine. There is a low smoke density, CO emissions, and a minor decrease in NO x emissions. It was reported that the lesser temperature produced by the low heating value of the fuel was the cause of the increase in HC, whereas NO x dropped when biodiesel content was raised. Sahoo et al. (2007) experimented with the effect of polanga biodiesel blends on CI engines and concluded that the smoke and NO x emissions were also better than diesel. It was concluded that it performed somewhat better than diesel in terms of BTE and BSEC due to the additional oxygen concentration in biodiesel. Devan and Mahalakshmi (2009) measured the impact of poon oil biodiesel in a diesel engine. The smoke, UHC, and CO emissions were lesser, and higher NO x emissions were reported. Besides, shorter ID was noted for all fuels tested in comparison with diesel. Nabi et al. (2009) experienced diesel engines with cottonseed oil and observed that the BTE was slightly lower than diesel owing to its low calorific value, high viscosity, and higher density. Furthermore, biodiesel blends emit less CO, PM, and smoke due to the trace of oxygen in the chemical structure and reduced aromatics, with a minor increase in NO x emissions. Murugesan et al. (2009) examined the possibilities of Pongamia oil in diesel engines. While utilising biodiesel, the development of peak pressure, analysis of HRR, and engine vibration were examined. They concluded that the diesel engines exhibited low UHC, CO, NO x , and particulates. Puhan et al. (2010) discussed the monoalkyl esters of vegetable oil with different molecular weights and double bonds and their effects on the diesel engine. For unsaturated biodiesel, the HC, CO, smoke, and NO x pollutants are high when compared to highly saturated biodiesel. The biodiesel with high unsaturated linolenic fatty acid produces more NO x and low thermal efficiency. Ng and Gan (2010) evaluated the diesel engine performance using cottonseed biodiesel at various engine loads. They recorded that the smoke and hydrocarbon emissions of the B25 blend were slightly higher and the CO and NO x emissions for B25 were 4% lower at full load. Higher peak combustion pressure and smoother combustion were recorded when operating with higher blends. Roy et al. (2014) inspected diesel engines using three series of canola biodiesel blends at high idling operations. They observed that the biodiesel blends emit considerably less CO and HC than conventional diesel, whereas NO x emissions were either reduced or maintained at the same level when diesel fuel contained up to 5% canola oil biodiesel. At high idling circumstances, Rahman et al. (2017) did a detailed study on diesel engines powered by biodiesel-diesel blends of palm and Calophyllum. In their results, BSEC increased, and CO and UHC emissions for both biodiesel blends were lesser. Can (2014) investigated the waste cooking oil biodiesel blends in CI engines. In his results, a marginal reduction in the combustion pressure, a maximum HRR, and a rise in combustion duration were reported. It was found that the improvement in the concentration of biodiesel increases the oxides of nitrogen and CO 2 emissions and reduces the smoke and hydrocarbon emissions at the full load. Ong et al. (2011) examined the diesel engine with three different biodiesel blends at full throttle load. The 10% biodiesel blend showed better torque, power, fuel consumption, and BTE and a substantial drop in CO 2 , CO, and smoke emissions with a minor rise in NO x emissions. Muthukumaran et al. (2015) tried Calophyllum oil prepared by fly ash catalyst in a diesel engine and noted that the BTE of B25 performed closer to diesel and was reduced for higher combinations. However, NO x emissions were reduced and other emissions like UHC, CO, and smoke were comparable with diesel. Ashok et al. (2016) experimentally evaluated the parameters of Calophyllum biodiesel and observed lower BTE with low CO and HC with a significant consequence in NO x emissions. However, the in-cylinder pressure, ID, and HRR were closer to diesel. Yadav et al. (2016) studied Kusum, oleander, and groundnut oil-fuelled diesel engines and showed the meagre performance of diesel. However, the CO, HC, and smoke emissions of all biodiesel were lower and NO x emission was high.
Biodiesel is made from a variety of sources, one of which is punnai. Punnai is a non-edible oilseed in the Clusiaceae family of evergreen trees (Arumugam and Ponnusami 2019). Though it grows occasionally inland at high altitudes, the trees are primarily planted near coastline regions and neighbouring lowland woodlands. Asia, Australia, India, and eastern Africa are all home to this species (Hazar and Sevinc 2019). Punnai thrives on sandy, well-drained soils, although it may also grow in clay, rocky, and calcareous soils (Ong 1 3 et al. 2017). A dense canopy of lustrous, robust, glossy leaves; hefty spherical nuts; and fragrant white blooms covers the tree (Arumugam and Ponnusami 2014). When fully grown, the trees grow to be between 8 and 20 m tall, with some reaching up to 35 m (Vigneshwar et al. 2019). The tree grows around 1 m/year in ideal conditions (Jain et al. 2018). The leaflets are glossy and hefty, 10-20 cm in length, and 6-9 cm in width, with a pale green colour throughout growth and a dark green colour at maturity . Because of the following factors, biodiesel made from punnai oil is being considered a potential alternative to traditional diesel (Ramesh Bapu et al. 2017). Punnai trees have a longer lifespan and are extremely susceptible to cold and fire. With a planting density of 400 trees per hectare, these trees may flourish on any type of land. With an aggregate production of 4680 kg of punnai oil per hectare of land, each tree can give oil at a rate of 11.7 kg. Punnai seed oil output is discovered to be higher (about 4680 L-oil/ ha) than the oil yield of Jatropha curcas (approximately 1892 L-oil/ha). It has strong solubility with diesel and is also easy to lubricate (Marso et al. 2017). Due to its high heat value, it complies with most biodiesel specifications (Mohamed Shameer and Ramesh 2017). The mix derived from punnai has a greater ignition pressure and temperature, as well as a shorter ID; additionally, BTE, peak HRR, and power output are equivalent and similar to those of normal diesel (Kshirsagar and Anand 2017). Reformulation of biodiesel fuel compositions has been investigated in certain research as a strategy to reduce increasing NO x emissions (Venkatesan et al. 2021). When exhaust gases are recirculated, a portion of the fresh air is supplied with CO 2 from the exhaust gas (Vallapudi et al. 2018). Because there is less oxygen present, the combustion temperature is lowered, which minimises the quantity of NO x (Liang et al. 2021). This allows all types of diesel engines to considerably reduce NO x emissions (Thangaraj and Govindan 2018). CI engines running on biodiesel emit more NO x due to the improved burning of oxygenated biodiesel. Biodiesel causes advanced injection time, which enhances NO x emissions (Bakthavathsalam et al. 2019). The biodiesel-powered engine's fuel injection timing and combustion behaviour are impacted by its increased viscosity, and bulk modulus is identified as a primary contributor to NO x emissions. Abd-Alla et al. (2001) explored the impact of EGR on the diesel engine. They thought that the admission of diluents reduces the NO x emission, which might have resulted due to a rise in inlet temperature. Even though it was a result of the shorter ID, the combustion parameters and NO x emissions improved as UHC decreased. According to their research, mixing exhaust gas with air intake lowers combustion temperatures by raising the specific heat, which substantially reduces engine performance. In another study, Zheng et al. (2004) conveyed that the reduction in flame temperature and the oxygen content by admitting EGR to the airflow decreases the NO x and increases the PM emissions. However, the rise in the EGR rate increases carbonaceous emissions and reduces power. In addition, the increased use of exhaust gases in the fresh charge has caused an imbalance in the engine framework. Finally, they concluded that a lower EGR percentage resulted in lower NO x emissions and better fuel economy. Agarwal et al. (2011) used EGR to lessen the oxygen content in the cylinder to raise the heat capacity of the suction air and reduce the temperatures of the flame. The thermal efficiency slightly increased at low and peak loads owing to low flame temperatures and reduced content of oxygen. Also, they concluded that the UHC, CO, and smoke were improved with EGR, but NO x emission decreased due to lower EGT. However, higher carbon deposits and greater piston ring wear were noticed. According to the findings of Yasin et al. (2015), diesel engines using palm biodiesel with EGR lowered the BP and torque, improved fuel consumption, lowered NO x , and had a very little rise in CO 2 , CO, and PM emissions. Can et al. (2016) thought that the combination of biodiesel with EGR leads to an increase in the maximum HRR and maximum CP by delaying the SOC timing with longer ID. However, the CD was almost stable when the centre of HRR shifted toward TDC due to an increase in the EGR rate. It was reported that the BSFC increased by 6%, and the BTE was reduced by 3% with a 15% EGR. Furthermore, there was a simultaneous improvement of 55% NO x and 15% smoke at full load. Sakhare et al. (2016) calculated the EGR effects on cottonseed biodieselfuelled diesel engines and reported that the ID was shorter with B20 fuel due to higher oxygen and CN. In their results, a rise in NO x emissions is reported owing to the increase in the premixed burning and a decrease in diffusive burning with the B20 blend. It was concluded that the NO x emission was reduced by a smaller quantity of EGR gas, but a higher volume of EGR gas showed deterioration in diesel engine performance. From the results, Yasin et al. (2017) reported that the EGR causes a substantial drop in the NO x emissions and EGT but raises the fuel cost. With the rise in EGR%, Kumar et al. (2018) found a rise in BSFC and a lower BTE. It was observed that the rise in the CO and UHC emissions was correlated with the rise in EGR%. However, the 10% EGR decreases NO x emissions drastically.
EGR is the method often utilised in diesel engines, and it plays a key role in attempts to minimise rising NO x emissions (Sogbesan et al. 2021). It also forced the authors to think about the effect of EGR on combustion, engine performance, and exhaust emissions. Punnai oil biodiesel has been utilised by several researchers due to its great production and low price (Bibin et al. 2019). Normally, it was widely grown in many nations. Several researchers have also recommended using punnai oil as a biofuel ). Many studies on the influence of IT and IP utilising punnai oil biodiesel in blended form have been conducted Ashok et al. 2017c). However, all of these experiments focused on improving the engine's performance. However, although enhancing the engine's performance, the NO x emissions produced represent a serious threat to the environment.
As evidenced by the literature, the application of EGR is an extensively utilised technology in diesel engines and plays an essential part in attempts to reduce rising NO x emissions. In this study, a fuel mixture such as dieselpunnai oil biodiesel (B20) was employed as an alternative fuel in a diesel engine without requiring any engine changes. This mixture was chosen based on the previous research of the author on the same engine. According to the earlier experimental work of the author, the B20 blend might partially remove the use of diesel while maintaining optimal performance and emission characteristics (Bibin et al. 2020). The impacts of EGR on engine performance must be taken into account and evaluated. The goal of this study is to give thorough insights into the influence of combined punnai oil biodiesel and EGR on combustion, engine performance, and pollutant emissions, as well as a comparison to diesel fuel operation and find certain distinguishing characteristics. Some unique features about them have been found.

Experimental setup
A single-cylinder diesel engine was used in this experimental investigation. Table 1 lists the test engine specifications. Figure 1 shows a schematic layout for experimental research with EGR. For measuring brake power, an eddycurrent dynamometer and an electronic torque exciter with a diesel engine are used. At 1500 rpm, the eddy-current dynamometer has a rated BP of 4.4 kW. The temperature of the exhaust gas is recorded using a digital temperature metre with thermocouples. Emissions are recorded using an exhaust gas analyser. The CO, CO 2 , UHC, O 2 , and NO x were observed using a five-gas exhaust analyser. A smoke meter is an instrument that measures smoke emissions. This smoke meter measured smoke by measuring the amount of light absorbed in a certain length of exhaust gas column. The filter paper method was used to calculate the filter smoke number. This instrument is suitable for both heavy-and lightduty engines, regardless of their generation. The pressure within the cylinder was measured using a transducer. On the crankshaft, a crank angle encoder was installed, and the in-cylinder pressure was calculated by crank angle. Table 2 lists the properties of B20 punnai oil biodiesel fuel.

Experimental procedure and uncertainty
Experimental trials were achieved at a 1500-rpm engine speed with five engine loads. In the studies, a punnai oil biodiesel comprising 20% by volume was injected with varied EGR rates and the results obtained were compared to diesel. EGR causes a rise in BSFC as well as HC, CO, and smoke emissions. As a result, although EGR reduces NO x emissions, it also increases smoke emissions. Engines operating with oxygenated fuel and EGR reduce NO without compromising BSFC or emissions. The recirculated exhaust gas quantity and the entire intake mixture may be used to compute the EGR percentage. To minimise errors, sufficient time was given to achieve a steady state while the emissions were being measured. Experimental repeatability eliminates random errors, leaving only instrumental faults to be evaluated. The range, accuracy, and uncertainty of the instruments are provided in Table 3. The overall uncertainty was computed analytically using the following expression: The percentage of uncertainty values of TFC, BP, BTE, BSEC, EGT, UHC, CO, NO x, smoke opacity, CO 2 , and pressure pick up were mentioned as X 1 , X 2 , X 3 , X 4 , X 5 , X 6 , X 7 , X 8 , X 9 , X 10 , and X 11 , respectively.

Results and discussions
The engine was initially run on diesel, which served as a reference fuel. The baseline values were tested using diesel fuel at 1500 rpm speed for several load spectrums. The fuels were supplied at 200 bar pressure with a 23°bTDC injection time. The fuel was then changed to B20. Then fresh air entering was displaced by exhaust gas. The pressure of exhaust gas was less than that of scavenging air, so the EGR blower forced the recirculated exhaust gas. The quantity of EGR admitted was recorded by the orifice. Before being supplied into the combustion chamber, the entering air and recirculated exhaust gas were thoroughly mixed in the mixing chamber. The EGR rate was increased through the gradual opening of the EGR valve. Various flow rates, such as 5%, 10%, and 15% EGR, were admitted for the estimation of the performance of the diesel engine. However, the flow rate of EGR was not increased beyond 15%. During the investigation, the in-cylinder pressure, HRR, EGT, and emission parameters HC, CO, CO 2 , NO x , and smoke density were recorded. Lastly, all the results attained from the diesel engine operated with a 20% biodiesel blend with various EGR rates were compared with baseline diesel. The performance, combustion, and emission characteristics of a diesel engine are demonstrated in the following sections with the help of graphs.

Performance characteristics
Brake thermal efficiency Figure 2 shows a comparison of BTE to BP for B20 at various EGR rates. The values of BTE at 0%, 5%, 10%, and 15% EGR flow rates were 28.6%, 28%, 27.4%, and 26.9%, respectively, for B20 blend, compared to 29.2% for diesel. EGR reduces BTE at all loads in comparison with diesel and B20 fuel. The rise in EGR flow rates proportionally  decreases the BTE due to the deficiency of oxygen content within the cylinder during combustion when using EGR, which resulted in poor burning (Lamba et al. 2017). The lessening in combustion temperature is due to the higher heat capacity of EGR gases resulting in lower BTE. Figure 3 presents the variation of BSEC to BP for various EGR rates. The BSEC values obtained for diesel and B20 are 10.63 MJ/kW h, and 11.65 MJ/kW h, respectively, whereas for B20 with 5%, 10%, and 15% EGR is 12.9 MJ/kW h, 14.56 MJ/kW h, and 16.22 MJ/kW h at full throttle. When compared to diesel, the B20 blend has a high BSEC. EGR increases BSEC at all loads compared with diesel and B20 blends without EGR. The BSEC for B20 is higher than diesel with and without EGR at all spectrums of loads. This is due to the B20 blend mode's lower burning temperature and greater rate of fuel flow, both with and without EGR. A rise in BSEC was also assisted by lower calorific values (Rangel et al. 2021). Higher combustion temperatures and improved fuel usage result in improved fuel economy at high loads . At full throttle, the BSEC increased by 8.78%, 17.61%, 27.02%, and 34.51% for B20 + EGR 0%, B20 + EGR 5%, B20 + EGR 10%, and B20 + EGR 15%, respectively, in comparison to conventional diesel fuel. Reduced oxygen concentration leads to poorer combustion, resulting in high BSEC with a rise in EGR%. Figure 4 depicts the comparison of EGT versus BP for various EGR rates. It was reported that with the rise in load, EGT also increased. The EGT for standard diesel was 364 °C, and in the case of B20 blend with 0%, 5%, 10%, and 15% EGR flow rates, it was 386 °C, 380 °C, 373°, and 368 °C, respectively, at full throttle. The EGT was lower when the engine was run with EGR than when it was operated without EGR (Subramanian et al. 2018). Owing to the unavailability of oxygen due to the recirculated exhaust gases blended with fresh air in the cylinder for combustion and the greater heat capacity of the admitted mixture, a rise in the rate of EGR correspondingly produced a fall in the EGT (Muniappan and Rajalingam 2018). With the induction of EGR, dilution of the working mixture lowers the peak cylinder temperature, resulting in a drop in EGT.

Emission characteristics
Carbon monoxide emissions Figure 5 shows a comparison in CO emission to BP for B20 blend and diesel at different EGR rates. It was noticed that CO emissions improved with increasing EGR rates. The heterogeneous mix does not combust completely and produces high CO emissions. The CO emission for standard diesel was 0.41% at full throttle, and the CO emission for B20 blends with 0%, 5%, 10%, and 15% EGR flow rates were 0.35%, 0.37%, 0.38%, and 0.39%, respectively. Incomplete combustion produced by the diluted mixture is most likely to blame for the CO generation (Damodharan et al. 2018). As the EGR% rises, the fresh air is swapped for exhaust gas, and the lack of oxygen may be ascribed to an increase in CO emissions. Another cause of increased CO emission might be a rise in the CO 2 concentration of the injected mixture rather than fresh air (Živković and Veljković 2018). In conclusion, it reduces combustion temperature, resulting in higher CO emissions. However, incomplete combustion owing to a rise in equivalence ratio and lower oxidation temperatures with EGR rise at full throttle might show a substantial rise in CO emissions, specifically at 15% EGR. Figure 6 depicts the comparison of UHC emissions to BP for B20 and diesel at different EGR rates. The UHC emissions improved with an increase in EGR percentages. This could be the result of the drop in oxygen content by the exhaust gas supplied into the cylinder, resulting in partial and incomplete combustion. At full load, UHC emissions for diesel, B20 (EGR 0%), B20 (EGR 5%), B20 (EGR 10%), and B20 (EGR 15%) were 64 ppm, 57 ppm, 61 ppm, 66 ppm, and 68 ppm, respectively. According to the findings, the percentage of UHC emissions increased as EGR increased (Sonthalia 2019). Lean mixtures are more difficult to ignite and create more UHC due to the heterogeneous nature of the mixture (Chinnasamy et al. 2019). UHC emissions increase dramatically as the EGR rate rises, implying that the combustion quality deteriorates as the EGR rate rises (Singh et al. 2019). The air is replaced by EGR in the case of EGR, and the oxygen deprivation associated with a rise in EGR% can be attributed to an improvement in UHC emissions (Jain et al. 2019). Another cause for the increase in UHC emissions might be a rise in CO 2 concentration in the injected mixture rather than fresh air (Kumar et al. 2019). Finally, the drops in combustion temperature might lead to improvements in UHC emissions. Figure 7 illustrates the difference in NO x emissions to BP for B20 and diesel at different EGR rates. A portion of recirculated exhaust gas is supplied into the cylinder to act as a diluent in the inlet charge. The EGR gases replace some volume of fresh air in the cylinder, lowering the oxygen concentration. When EGR% increases, the NO x emission reduces, which may be due to a drop in temperature and oxygen rate. At full load, oxides of nitrogen emissions for diesel, B20 (EGR 0%), B20 (EGR 5%), B20 (EGR Fig. 5 The comparative plot of CO emissions versus brake power at all loads Fig. 6 The comparative plot of UHC emissions versus BP at all loads Fig. 7 The comparative plot of NO x emissions and BP at all loads 10%), and B20 (EGR 15%) were 1516 ppm, 1547 ppm, 1455 ppm, 1399 ppm, and 1325 ppm, respectively. The improvement in the EGR flow rate proportionally reduces NO x emissions. The decrease in NO x emissions at peak loads is high. The increase in heat capacity of the charge by diluting the incoming air with exhaust gases leads to a reduction in the maximum flame temperature on the diffusive combustion lean side, resulting in lowered reaction rates and lower NO x emissions (Viswanathan et al. 2019). It enhances the heat capacity of the charge and lowers the cylinder temperature for the same HRR, resulting in reduced NO x emissions because of significantly greater specific heat of triatomic gases (Bragadeshwaran et al. 2019). Because of the presence of triatomic gases in the EGR, the results showed that NO x emissions are greatly decreased with EGR adoption. The CO 2 absorbs the energy generated by burning, resulting in a lower oxygen mass fraction in the cylinder (Balasubramanian and Lawrence 2019). NO x emissions declined dramatically due to low cylinder temperatures and oxygen levels. Figure 8 presents the comparison of CO 2 emissions to BP for B20 and diesel at different EGR rates. At full throttle, the CO 2 emission for standard diesel was 18%, and in the case of B20 blends with 0%, 5%, 10%, and 15% EGR flow rates, they were 20%, 21.8%, 23.6%, and 25.2%, respectively. At all engine loads, CO 2 emissions increased as a result of combined biodiesel and higher EGR. The proportion of CO 2 added using EGR, as well as the biodiesel concentration, was shown to be controlled by engine load. The greater CO 2 emissions are an indication of the reduced calorific value of B20 fuel (Perumal Venkatesan et al. 2019).

Smoke emissions
The comparison of smoke opacity with BP for diesel and B20 with various EGR rates is depicted in Fig. 9. The rise in the EGR rate resulted in somewhat higher smoke emissions than diesel. The smoke density of diesel and B20 is 50% and 48%, respectively, for normal operating conditions, whereas for B20 with 5%, 10%, and 15% EGR, it was 51%, 52%, and 53%, respectively, at full throttle. There is a proportionate rise in smoke density corresponding to the EGR% flow rate. EGR inclusion causes lower in-cylinder temperature due to the weakening of the oxygen concentration with the rise in heat capacity of the charge, leading to an increase in smoke emission (Ramalingam et al. 2019). The rise in smoke emissions is caused by the fractional replacement of air by exhaust gases, which causes unstable burning owing to the lack of oxygen required for full combustion.

Cylinder pressure
The comparison of CP with the CA for diesel and 20% punnai oil biodiesel with and without EGR is indicated in Fig. 10. During combustion, the in-cylinder temperature was reduced for high EGR flow rates due to the impact of air-fuel mixture dilution and higher specific heat capacity. The most significant effect is the dilution effect on the air and fuel mixture, which increases the ID period and thus permits more mixing of the charge . It was noticed that the reduction in maximum cylinder pressure was owing to increased EGR flow rates for 20% punnai oil Fig. 8 The comparative plot of CO 2 emissions versus brake power at all loads Fig. 9 The comparative plot of smoke emissions versus BP at all loads biodiesel at full load conditions due to the unavailability of oxygen for the burning. The average CP for B20 with 5%, 10%, and 15% EGR is found to be 70.32 bar, 70 bar, and 69.7 bar, respectively, whereas for diesel and B20 without EGR, it is 71.4 bar and 70.9 bar, respectively, at full load.
Because there is less oxygen available for combustion when the exhaust gas is returned to the cylinder for control of NO x emissions, the peak pressure for B20 with EGR is lowered.
Heat release rate Figure 11 indicates the HRR variation with the CA for B20 with the EGR rate at full load. The initial rise in HRR with an improvement in EGR% is due to the exhaust gases being cycled at a higher temperature. However, when the EGR percentage increased, the HRR reduced due to a drop in fresh air. The low oxygen and fewer chemical interactions between the fuel and the oxygen result in incomplete combustion. Because of the dilution effect, which reduces the premixed combustion phase, using a high EGR rate lowers cylinder temperatures during the expansion stroke. At full throttle, the HRR for B20 with 5%, 10%, and 15% EGR is found to be 48 kJ/m 3°C A, 45 kJ/m 3°C A, and 38 kJ/m 3°C A, respectively, whereas for diesel and B20 without EGR, it is 54 kJ/ m 3°C A and 60 kJ/m 3°C A, respectively. With the heat energy absorbed by the exhaust gases during combustion, the HRR decreased (Suski and Mader 2020). The reduction in HRR may be due to the absorption of heat by the exhaust gas during the combustion process, which consequently decreases premixed combustion and hence decreases HRR compared to diesel and B20 blends without EGR at full throttle.

Cylinder peak pressure
The CPP variation with the CA for diesel and B20 with various EGR rates is denoted in Fig. 12. The peak cylinder pressure at a higher percentage of EGR is marginally lower than B20 at full throttle. The reduction in peak pressure for B20 with EGR may be owing to the deficiency of oxygen available for combustion in the exhaust gas and is admitted to the cylinder for the control of the NO x . It may also be possible to improve BTE at part load and lower combustion temperature inside the cylinder at full throttle (Tamilvanan et al. 2021). The cylinder peak pressure for B20 with 5%, 10%, and 15% EGR rates is 63.7 bar, 62.3 bar, and 61.4 bar, respectively, whereas for diesel and B20, it is 68.3 bar and 67 bar, respectively, at full throttle.
Maximum rate of pressure rise Figure 13 shows the difference of the MRPR with BP for all fuels with and without EGR. The MRPR decreased for B20 with a rise in EGR rates. The MRPR obtained for B20 with 5%, 10%, and 15% EGR is found to be 4.0 bar/°CA, Fig. 10 The comparative plot of CP versus CA at full load Fig. 11 The comparative plot of HRR versus crank angle at full load Fig. 12 The comparative plot of CPP versus brake power at different loads 3.8 bar/°CA, and 3.6 bar/°CA, respectively, whereas for diesel and B20 without EGR, it is 4.8 bar/°CA and 4.4 bar/°CA at full throttle. This reduction is reported owing to the lack of oxygen available for burning, which releases less heat energy during the combustion, which is reflected in the lower exhaust gas temperature (Mohan et al. 2019). Figure 14 shows the difference between the MRHR with BP with different EGR rates. The MRHR for B20 with 5%, 10%, and 15% EGR varies from 53 kJ/m 3°C A, 52 kJ/ m 3°C A, and 51.5 kJ/m 3°C A, respectively, and for diesel and B20 without EGR, it is 56 kJ/m 3°C A and 54 kJ/m 3°C A at full throttle due to the absorption of heat by exhaust gas during the combustion process, resulting in decreased premixed combustion and hence decreased HRR compared to diesel and B20 blend without EGR at full throttle (Cai et al. 2021). Figure 15 presents the ID variations with BP for diesel for different EGR rates. At no-load conditions, the ID is higher for all fuels due to the availability of more time during the SOI and the SOC. If no EGR is admitted, then there is a substantial reduction in the ID at all loads due to the replacement of air with EGR, which leads to a decrease of oxygen in the exhaust gases. The ignition delay period becomes longer and the combustion becomes slower (Girardi et al. 2021). The ID for diesel and B20 biodiesel is 17°CA and 16°CA, respectively, at full load, whereas for B20 with 5%, 10%, and 20% EGR, it is found to be 16.5°CA, 17.5°CA, and 18°CA, respectively. Figure 16 shows the CD variation against BP for all fuels with and without EGR at various loads. EGR raises the specific heat of the charge in the cylinder. As a result of improvement in specific heat due to EGR, the charge temperature of the in-cylinder drops, and the ignition is also delayed in the cycle. EGR introduces triatomic molecules like triatomic molecules and also affects the equivalence ratio of in-cylinder. This tends to lower the preignition reactions, resulting in autoignition of the charge and a longer CD (Baweja et al. 2021). The higher EGR rate resulted in a longer combustion duration.

Conclusions
This work quantifies the effects of B20 blends with various EGR rates like 5%, 10%, and 15% on the performance, emission, and combustion parameters of a standard diesel engine. Preliminary tests have shown that clean punnai biodiesel may quickly corrode engine components, resulting in engine failure. This disadvantage might be considerably mitigated by combining low-content biodiesel. Punnai oil's high volatility might help enhance the fuel/air mixture and, as a result, engine performance, especially under high EGR circumstances. The addition of punnai biodiesel might raise the oxygen content even further, resulting in a reduction in soot emissions. The following is the summary of the results of this experimental investigation: The experimental results revealed B20 blend at a 10% EGR flow rate performed 4.38% lower BTE, 25% higher BSEC, 3.49% lower EGT, 11.43% higher CO, 18% higher CO 2 , 15.79% higher UHC, 10.58 lower NO x , and 8.33% higher smoke when compared to B20 blend in normal diesel engine operation. Also, it was found that B20 blend at a 10% EGR flow rate produced 6.57% lower BTE, 37.04% higher BSEC, 2.47% higher EGT, 5.13% lower CO, 31.11% higher CO 2 , 3.13% higher UHC, 8.36% lower NO x , and 4% higher smoke in comparison to diesel fuel in a standard diesel engine. B20 blend at a 10% EGR flow rate offers 6.57% lower performance and 4.95% higher emission than diesel fuel on normal engine operation without EGR.
Despite the decreased calorific value of the punnai oil biodiesel, the EGR rates of 5% and 10% did not have a noticeable impact on engine performance. When the EGR was raised to a 10% rate, there was a minor improvement in BSFC and a marginal drop in BTE for all engine loads. Because of the generation of a rich mixture due to limited oxygen supply, BSFC increases as the EGR rate goes up. As the EGR rate is improved, the temperature of the exhaust gas slowly decreases. The improvements in UHC emissions were obtained using B20 biodiesel and increasing the EGR rate at low and medium loads. However, at high throttle, degradation occurred when the EGR rate was improved above 5%. The addition of B20 biodiesel and EGR did not result in substantial variations in CO emissions. However, at medium and high engine loads, a 15% EGR rate resulted in minor increases in CO emissions. With biodiesel additives and higher EGR, CO 2 emissions increased marginally for all engine loads.
With the addition of B20 biodiesel and the use of an EGR system, NO x emissions in diesel fuel combustion were significantly reduced at peak loads. Also, at medium, partial, and low loads, smoke emissions were slightly above those of the diesel combustion, but the negative impact on NO x emissions was maintained. The results demonstrate the EGR tolerance on B20 biodiesel and provide a technique for reducing NO x emissions while minimising BSFC and smoke emissions. Lower cylinder temperature results in reduced NO x emissions and greater UHC emissions with EGR owing to the increased heat capacity of intake air and exhaust gas combination and less oxygen existence. The experimental results revealed that, with EGR, B20 (biodiesel 20%) blend at different EGR flow rates performed closer to those of diesel. The B20 blend at 10% EGR produced closer efficiency along with decreased NO x emissions in comparison with diesel. This finding will benefit people in rural areas by replacing the conventional diesel with minimal vegetable oil abundant in their neighbourhood.
Author contribution CB prepared punnai oil biodiesel and also investigated diesel engines. PKD investigated diesel engines and curated data from the study. RS is a contributor to writing original drafts and reviewing and editing the manuscript. SM supervised the experimental study and editing of the manuscript. All authors read and approved the final manuscript.
Data availability Not applicable.

Declarations
Ethics approval Not applicable.

Consent to participate Not applicable.
Consent for publication Not applicable as the data and images given in the manuscript are solely prepared by the authors.

Competing interests
The authors declare no competing interests.

References
Abd-Alla GH, Soliman HA, Badr OA, Abd-Rabbo MF (2001) Effects of diluent admissions and intake air temperature in exhaust gas Fig. 16 The comparative plot of CD versus BP at different loads