Experimental investigations on the performance and emissions characteristics of dual biodiesel blends on a varying compression ratio diesel engine

The present work discusses the performance and emissions characterization of dual biodiesel sample blends on a varying compression ratio diesel engine. The dual biodiesel blends were obtained by blending two biodiesels (Mahua and Jatropha) in equal proportions volume (1:1, v/v) with mineral diesel. The sample blends were obtained on a ‘percentage by volume’ basis and named B10, B20, B30, and B40 (B10 was a blend of 5% each biodiesel with 90% mineral diesel and similarly for all other sample blends). All the experiments were performed at a constant engine speed of 1500 rpm, 50% loading conditions (2.6 kW), and varying compression ratios of 13.5:1, 14.5:1, 15.5:1, and 16.5:1. The results revealed that the sample blends had slightly higher brake power and mechanical efficiency with sample blends B10 to B40 had (0.15–1.58%) higher brake power and (1.07–12.42%) higher mechanical efficiency as compared to mineral diesel at a compression ratio of 16.5:1. The In-cylinder peak pressure and exhaust gas temperature were observed to be lower than mineral diesel for the sample blends B10 to B40 by 0.15–0.36 bar and 11.1–69.8 ℃, respectively. Also, the emissions of carbon monoxide and hydrocarbons were lower by 33–62%, respectively, for the sample with the highest blend percentage. However, the carbon dioxide emissions were found to be higher by 42.85% than mineral diesel. From the overall performance and characterization, it is concluded that B20 had optimum properties and blend percentage to be a better substitute fuel for mineral diesel among all the tested samples.


Introduction
Indigenously available energy resources play a significant role in the economic growth of a country. These resources are also important for the sustainability of a developing country. According to a report given by the Department of Energy, USA, the demand for crude oil will reach its peak by the end of this century and the fossil fuel reserves might reduce to a fractional value of their current estimates. Lately, the extensive use of petroleum products due to the rise in industry and the human population has led to the exhaustion of crude oil reserves (US Department, https:// www. eia. gov/). The higher demand leads to a rise in the higher price of fossil fuels [10]. Globally, people consume more than 11 billion tons of oil from fossil fuels each year. The reserves for crude oil are declining at a very high rate of 4 billion tons per year approximately, and at this rate, the stocks of oil reserves may get depleted in just another four to five decades. Highly populated countries like China and India are consuming the maximum amount of aviation fuel [50]. Further, the emissions of harmful gases like carbon monoxide, hydrocarbons, carbon dioxide, and particulate matter are the direct result of the worldwide combustion of petroleum products. The use of fossil fuels contributes about 25% to the total greenhouse gas emissions [25]. Due to the above-mentioned reasons, finding alternate energy resources has become a necessity for researchers all over the world.
Many researchers around the globe did intensive research on biodiesels that has the potential to provide a possible way out to substitute conventional diesel at lower prices [30]. Biodiesel can be obtained from edible or non-edible vegetable and animal fat oil. It is produced by various methods like pyrolysis, emulsification, blending, and trans-esterification. However, the most commonly used method for producing biodiesel is transesterification since it is the most economical method among all [3,6].
Mahua and Jatropha raw seed oil have prominent free fatty acid (FFA) content, which is around 19-23% [13]. It is possible to reduce the FFA of raw oil with a suitable pre-treatment process. The production of biodiesel from raw oil depends upon various parameters like reaction temperature, category of catalyst used, the ratio of oil to alcohol used, and purity of the reactants [1,5].
The physicochemical properties such as iodine value, acid value, fire point, and flash point, kinematic viscosity, density, etc. make it suitable to be used as biodiesel, but its cold flow properties are still a matter of concern [20]. Contrary to Mahua biodiesel, Jatropha biodiesel has good cold flow properties. Mahua and Jatropha biodiesel also have better combustion and emissions properties. To enhance the physicochemical properties of individual biodiesel, dual biodiesel can be prepared by the blending of these two. It was reported that preheating of Jatropha oil can help to reduce its kinematic viscosity and enhance its fuel atomization for biodiesel [3,8,21]. Preheating of raw palm oil (up to 90 ℃) improved its density and viscosity, which in turn improves its flow properties and reduces the clogging problem of fuel injectors and fuel filters [12]. It has been reported by many researchers that the blending of biodiesel in diesel fuel can help to reduce the engine emissions to a great extent in comparison with the emissions given by the combustion of currently used diesel fuel [40,51]. Chauhan et al. have stressed the recommendation of biodiesel prepared from Jatropha oil as a fuel and reported its engine performance, variation in combustion, and emission properties [15]. Solaimuthu and Govindarajan [53] examined Mahua oil biodiesel blend with diesel and observed that 25% blend had the lowest hydrocarbons and nitrogen oxides emissions with 100% loading conditions. A review of the literature of some researchers is presented in Table 1.
From the review of the works of literature, it is evident that extensive research has been done to determine the efficiency of engine and emissions of exhaust gases for various blended biodiesels with mineral diesel at constant CR, but only a few articles are available on the diesel engine output parameters using dual biodiesel blended with mineral diesel at VCR [2,3,21,27].
The studies mentioned above were carried out in engines with different specifications and using biodiesels prepared from various first-generation or second-generation vegetable oils. In these studies, the effect of varying parameters has been emphasized by all researchers on engine combustion, engine performance, and emissions characteristics [26,43,54,61]. However, it is worth noting that studies on dual biodiesel prepared by mixing different biodiesels with mineral diesel were found to be limited. All previous researches on the physical and chemical characterization of dual biodiesel prepared by mixing of different biodiesel with mineral diesel in various proportions revealed that these sample blends might become a suitable candidate to replace mineral diesel [16,18,29,37]. Therefore, it is highly significant to measure the different characteristics of dual biodiesel on a VCR engine to check its emission properties and engine performance parameters.
Although biodiesel has a strong potential to replace conventional diesel, it has many underlying problems associated with it such as high viscosity, choking of nozzles in fuel injectors, carbon deposition inside the engine cylinder, free fatty acid content, etc. [21]. To overcome  results show that spirulina microalgae emulsions fuel reduces  the SPM, smoke emissions, and soot except for NOx & CO 2 compared with diesel  fuel  2 Not mentioned The author did a detailed review on trending modeling methods such as [44] Analytical methods Regression methods The review recommends combining advanced statistical methods and emerging popular machine learning algorithms to engine research for deriving comprehensive pragmatic models as an empirical compromise 3 Tamanu Methyl Ester (TME) The optimum IAT was found to be 90℃, so further experiments were carried out using the IAT 90℃ at various EGR ratios [38] The EGR-associated HCCI engine produces ultra-lean NOx emissions, slightly lower BTE, cylinder pressure, and heat release rate than the engine operating without EGR. The results also showed that a rise in IAT above 100℃ leads to short HCCI combustion, inclining to knock, thus leading to unstable operation 4 Citronella oil and biodiesel The biodiesel was prepared with diesel on a volume basis, namely B20, B40, and B60, and examined on a 1-cylinder CI diesel engine [41] The BTE of citronella oil and its blend were found lower than diesel, and compared to all blend samples, the B20 blend had better efficiency. The B20 (20% Citronella oil + 80% Diesel) appealed 1.47% lower in engine BTE and 2.7% higher fuel consumption than diesel fuel The smoke emissions of biodiesel lowered by 31% than diesel fuel, with a negligible rise in NOx and CO 2 emission by 12.5% and 7.1%, respectively. Besides that, CO and HC were 16.34% and 22.2% lower than diesel fuel when the engine was running at higher load conditions 5 Karanja and Roselle biodiesel The engine performance at different engine loads at constant speed was tested for prepared biodiesel. It was observed that In-CPP, BTE, EGT, ignition delay, and indicated thermal efficiency were reduced by 4.93, 4.3, 1.2, 2.47, and 0.71% for LA20 (20% Roselle + 80% Diesel) sample blend when compared with mineral diesel [50] BTE, EGT, NO X , and smoke emissions were reduced by 1.84%, 1.62%, 3.84%, and 13.53%, respectively, while BSFC was escalated up to 8.6% for KB20 (20% Karanja + 80% Diesel) 6 Karanja oil and Roselle oil biodiesel blends (KB10 and RB10) The dual biodiesel was prepared by blending with diesel fuel [49] The engine performance showed that the BTE was reduced and BSFC was increased for both RB10 and KB10 compared to diesel 7 Roselle biodiesel The author analyzed B20, B40, and B100 sample blends of Roselle biodiesel on a diesel engine by changing the injection pressure during fuel injection and engine loading and compared it with mineral diesel [48] It was found that smoke emission, thermal efficiency, ignition delay period, and indicated efficiency were reduced while mean effective pressure, In-cylinder peak pressure, CO 2, , and NO X emissions increased at high fuel injection pressure 8 Palm biodiesel and diesel The indicated mean effective pressure, brake power, mechanical efficiency, and emissions of exhaust gas by varying compression ratios on palm biodiesel blended with diesel were investigated [36] The study revealed that for higher compression ratios, 20% blend sample produces around 6% higher brake power and 14.6% higher mechanical efficiency compared with diesel. EGT was found to be lower for all blends. At 100% loading conditions, HC and CO content in exhaust gas was reduced by increasing the compression ratio and blend percentage. However, CO 2 content in exhaust gas was found to be more than diesel  [52]. However, only few researches are available on the production of dual biodiesel by mixing two different biodiesels with mineral diesel. This field has good scope for research to work on [2,13]. Dual biodiesel can decrease the emissions of harmful gases and improve engine performance [39,60], which have been the focus of major research work worldwide. Only a few researchers have worked on the biodiesels prepared from Jatropha and Mahua blend with mineral diesel [2,42], even though the properties of both Jatropha and Mahua oil biodiesel could be used to prepare dual biodiesel with improved physicochemical properties.
There are many articles available on the investigations of emissions properties and engine performance parameters of individual Jatropha and Mahua oil biodiesel blends with mineral diesel, but the exploration of the possibilities of a dual blend of Mahua and Jatropha biodiesels with diesel fuel is still wide open. It is evident from the literature survey that researchers have been trying to obtain a substitute fuel to the presently used mineral diesel by preparing blended dual biodiesel with enhanced engine combustion and emission properties [1,31,45,56]. Further, a few researchers have also tried in the recent past to varying the engine combustion parameters with different variations of blended dual biodiesel to improve engine output and exhaust emissions [7,22,42,55]. Their studies have revealed that the blending of two different biodiesels has a significant impact on the combustion and emissions properties of fuel [46]. Also, some researchers have lately successfully improved the oxidation stability of biodiesel by preparing dual biofuel with the blending of two different biodiesels [2]. The direct impact of two different biodiesels blending on the engine output parameters and emission characteristics of the dual biofuel is still limited as very few works of literature are available on the same. Therefore, in the present work, we aim to evaluate the parameters of engine output (such as In-cylinder peak pressure, mechanical efficiency, brake power, brake-specific fuel consumption, and exhaust gas temperature) and emissions gases (carbon dioxide, carbon monoxide, and hydrocarbons) using VCR engine of Mahua and Jatropha Biodiesel and mineral diesel blend. The various compression ratios (CR) used in current research work were 13.5:1, 14.5:1, 15.5:1, and 16.5:1 at 50% loading condition.

Preparation of various sample blends for dual biodiesel
The individual biodiesel of Jatropha and Mahua oil was prepared using a two-step trans-esterification process. The trans-esterification reaction was carried out at a temperature of 65℃ (± 1) using a water bath, and the reaction time was 30 min. During the reaction, the temperature of the water bath was constantly maintained at 65 ℃ (± 1). After every 5 min, the solution was stirred using a glass rod. In the end, the obtained solution was drained into a separating funnel and it was kept aside for the next 24 h. Finally, the biodiesel was separated, and the average yield of biodiesel was observed to be 93% for Jatropha and 89% for Mahua oil, respectively. Figure 1 illustrates the prepared sample of biodiesel using the trans-esterification process. Table 2 shows the comparison of physicochemical properties of biodiesels used in the present work. The various physicochemical properties (density, acid value, kinematic viscosity, iodine value, flash point, fire point, cloud point, and pour point) of these synthesized Waste cooking oil and mineral diesel The fuel combustion characteristics of blends of diesel in various proportions with biodiesel prepared from the second-generation waste cooking oil through a two-step trans-esterification process [24] A 4-stroke VCR; a 1-cylinder engine was used for the experimentation research work. The results revealed that increased CR (from 14 to 18) gave higher BTE (up to 27.48%), reduced HC and CO (52% and 37.5%) in the exhaust. However, the percentage of CO 2 and NOx emissions came out 36.84% and 14.28% higher, as compared to mineral diesel 10 Cottonseed oil Authors worked on cottonseed oil biodiesel prepared by preheating and transesterification at various temperature ranges (like 50, 70, and 90℃). It was reported that all the major physicochemical properties were better than earlier [28] The outcomes revealed that the preheating of raw oil up to 90℃ enhances the engine efficiency and fuel consumption of fuel by the engine at higher loads in comparison with mineral diesel    Table 3 represents the comparison of physicochemical properties of biodiesel blends with mineral diesel used in the present work.
The physical and chemical properties of these sample blends (dual biodiesel) were examined as per the standards of biodiesels given by various agencies (National and International) and were found to be within limits.
The above-mentioned dual biodiesel sample blends were tested for their emission characteristics and engine performance parameters using the VCR engine at varying CR during the current research work.

Engine description
The research work was conducted on a VCR four-stroke single-cylinder diesel engine connected with an eddy current dynamometer. The setup was used to determine the results of varying CR in the engine cylinder to the operational and exhaust emissions. The performance parameters were evaluated for the engine (such as mechanical efficiency, brake power, brake-specific fuel consumption, In-cylinder peak pressure, and exhaust gas temperature).
The computerized data acquisition system coupled with the VCR engine is used to perform the experiments and to collect and store data. The experimentation setup used for the study is presented in Figs. 2 and 3. Further, Table 4 presents the various specifications of the VCR engine used for the research work.

Gas analyzer description
Five gas analyzer AVG-500 (AIRVISOR) model was connected to a VCR engine to analyze the emissions. It was provided with an exhaust gas inlet which was put in Table 3 The comparison of properties (physical and chemical) of various dual biodiesel blend samples with the mineral diesel [47] Physical and chemical properties

Experimental methodology
To start and run the engine, ignition self-start switch was used, and initially, it was fueled with mineral diesel. When the operating temperature of the engine is reached, a 50% load is applied to the engine. The warming phase ends when the cooling water temperature shows a stabilized reading of 60 ℃. The engine was tested at a fixed speed of 1500 rpm. During experiments, the readings were measured and recorded digitally in the computer system. The readings for combustion and emission parameters with respect to different CR were evaluated for mineral diesel and dual biodiesel sample blends.

Error and uncertainty analysis
Error and uncertainties in the conduct of experimentation may arise due to many factors such as environmental conditions, the selection, and calibration of instruments and equipment used for research [21]. The accuracy of the experiments performed requires the analysis of uncertainties to be done. The analysis is done as explained by Dubey and Gupta [21] using the formulation given by authors the uncertainty for whole experimentation was calculated and found to be ± 0.58%. The uncertainty ranges for different parameters of VCR engine and exhaust gas analyzer are shown in Tables 5 and 6.

Statistical regression analysis
The MS Excel Software was used to perform all the statistical regression analysis and to develop a mathematical model. This model is used to develop the linear equations for fuel combustion characteristics and exhaust emission properties of different dual biodiesel sample blends with the changing parameters such as varying compression ratio and the blend percentage of biodiesel with mineral diesel. The developed linear equations along with the coefficient of regression (R 2 ) can be effectively used for statistical analysis and to estimate the parameters at any given compression ratio and percentage of blending for the change in properties for the sample blends used. Further, some other researchers have also tried to develop models on similar grounds either for biodiesel-diesel blends or biodiesel-biodiesel blends [9,59] 3 Results and discussion

Characteristics of engine operation
The current research work evaluated the engine's operational parameters and characteristics of exhaust emissions using various sample blends of dual biodiesel. These sample blends of dual biodiesel were prepared earlier by the mixing of Jatropha and Mahua oil biodiesel in equal proportions of volume (1:1) with mineral diesel. The various sample blends were obtained on the percentage by volume basis B10, B20, B30, and B40 (B10 is a blend of 5% each biodiesel with 90% mineral diesel and similarly for all other sample blends). The dual biodiesel sample blends were tested for engine operational performance characteristics (mechanical efficiency, brake specific fuel consumption, brake power, and tailpipe gases temperature) and emission characteristics (carbon dioxide, carbon monoxide, and hydrocarbons) at different compression ratios (13.5:1, 14.5:1, 15.5:1, 16.5:1) by using the VCR engine. The results for various parameters are summarized in Table 8.

Brake power (BP)
BP is the power generated by the engine at the output. It defines the engine's capacity to do work with the fuel used. The brake power with the variation in CR for different tested dual biodiesel sample blends is presented in Fig. 5.
The results revealed that the brake power escalated with the increase in CR. At a lower compression ratio of 13.5:1, it was found to be highest (0.994 kW) for blend B40 and at a higher CR of 16.5:1, it also came out to be highest (1.283 kW) for the same blend sample. The performance of B40 was better than other fuel blends and it was found to be greater by 1.58% than mineral diesel (1.263 kW) at a compression ratio of 16.5:1. It is worthwhile to note that the brake power also surged as the CR was increased at fixed loading. The other blend samples also had better brake power as compared to mineral diesel at different CR. The increase in brake power was seen to be linear to the compression ratio. Nagaraja et al. [34] also reported an increase in brake power for percentage increase in biodiesel blend while working on palm biodiesel. The reason for the same could be an improvement in the mixing of air-fuel ratio, better atomization-spray characteristics, and proper combustion of fuel due to preheating. Also, another reason can be the density of the B40 blend, which was more in comparison with mineral diesel. As the denser fuel-air mixture goes into the engine cylinder, the engine brake power is improved [33,35].

Mechanical efficiency (ME)
ME indicates overall engine performance with the calorific value given by the combustion of fuel. The trends of mechanical efficiency for various dual biodiesel sample blends at variable compression ratios are shown in Fig. 6. It was noted that the overall ME of the sample blends (dual biodiesel) increased as the CR was increased, that means the ME of the dual biodiesel samples varied directly with the variation of CR. ME was found 19 [4] also reported an increase in engine ME with the rise in blend percentage at a higher CR in the case of Karanja biodiesel blends. This could be due to the better lubrication properties of blended dual biodiesel samples owing to the presence of methyl esters in the fuel samples used for combustion [37].

In-cylinder peak pressure (In-CPP)
The peak pressure produced in the combustion chamber inside the engine cylinder just after the TDC is known as the In-cylinder peak pressure. The trends of In-cylinder peak pressure were found to be increasing with an increase in CR. However, it was revealed to be decreasing with the higher blend percentage of biodiesels in mineral diesel. The trends for various sample blends tested at different CR are presented in Fig. 7.
At lower CR, mineral diesel (D100) had a lower value of In-CPP (1.23 bar) whereas it surged with the increase in CR and was found to be highest (2.14 bar) for mineral diesel at 16.5:1 CR. The results revealed that the dual biodiesel sample blends had a lower value of In-CPP. At CR of 13.5:1, it was found to be low and similar to that of diesel for samples B10, B20, and B30 (1.21, 1.2, and 1.2 bar, respectively), and B40 had an even lower In-CPP of 1.19 bar. However, even though In-CPP of biodiesel samples surged for higher CR, it was still found to be lower in comparison with mineral diesel. The value of In-CPP for mineral diesel (D100) came out to be 2.14 bar, for B30 it was 1.84 bar, and for B40 1.78 bar at the maximum CR (16.5:1). On average, it was evident that among all the tested samples of dual biodiesel, the B40 had the lowest value which was 16.82% lower than mineral diesel. The reason for the same could be the lower calorific values of biodiesel than mineral diesel. Muralidharan and Vasudevan reported a similar decrease in In-CPP during their research on biodiesel produced from the waste cooking oil and diesel blends. [33].

Brake-specific fuel consumption (BSFC)
BSFC indicates the fuel efficiency of an IC engine. It primarily compares the consumption of fuel to the power produced by the engine. BSFC reduced progressively with the increase in CR from 13.5 to 16.5 for all tested dual biodiesel sample blends and a similar trend was also observed for mineral diesel. At CR of 13.5:1, the BSFC was found to be 1.69 (B10), 1.71 (B20), 1.72 (B30), and 1.78 (B40) kg/kW-s. At CR of 16.5:1, it was lower at 1.35 (B10), 1.41 (B20), 1.48 (B30), and 1.59 (B40). It was evident that as the blend percentage increased, the BSFC of the fuel also got higher. The dual biodiesel blend B40 had the highest BFSC at CR of 16.5:1 which was 20.45% greater as compared to mineral diesel.
This possible reason for this could be because of the lower heat content value of B40 due to the higher methyl ester content present in the dual biodiesel sample blends when compared with the mineral diesel. Mathew and Anand reported similar results during their work on biodiesel prepared from fresh and waste cooking oil [32]. The variation of BSFC of dual biodiesel sample blends at different CRs is presented in Fig. 8

Exhaust gas temperature (EGT)
The results of EGT for different dual biodiesel sample blends are shown in Fig. 9. The results clearly show that the temperature of exhaust gases was lower for all the sample blends when compared to EGT of mineral diesel when the CR was raised from 13.5:1 to 16.5:1. The B10 blend, however, had nearly the same values of EGT as that of mineral diesel (D100) which was recorded to be 185.7 ℃ at the lowest CR and 279.4℃ at the higher CR. The B40 blend had the lowest EGT among all tested sample blends. It was found to be 132.1 ℃ and 220.7 ℃ for the lowest and highest tested compression ratios. For the highest sample blend B40, the EGT was 11.1-69.8 ℃ lower than mineral diesel for different CR (13.5:1-16.5:1). The lower EGT recorded for dual biodiesel sample blends could be due to the lower heat value of the biodiesel in comparison with the mineral diesel. The present study as discussed above also showed similar results. The lower amount of exhaust losses could also be attributed to higher brake power and mechanical efficiency [11].

Emission analysis
The emission analysis was done with the help of a 5-gas analyzer to evaluate the exhaust emissions of carbon dioxide (CO 2 ), hydrocarbons (HC), and carbon monoxide (CO) for all dual biodiesel samples. The Indian National standards followed for the analysis of emission are given in Table 7.

Carbon monoxide (CO)
The CO emissions of an engine largely depend upon the quality of fuel used, amount of oxygen present in the fuel, ratio of the A/F mixture, turbulence, and temperature inside the engine cylinder. The trend of CO exhaust for dual biodiesel sample blends at various CR is presented in Fig. 10. The results revealed that the CO emissions were found to be 0.07, 0.06, and 0.06% at CR of 13.5:1 for sample blends B10, B20, and B30, respectively. Further, with the increase in CR, the CO emissions were found to be even lower. It was 0.06, 0.05, and 0.04% for sample blend B10, B20, and B30, respectively. However, the sample blend B40 had the lowest value for CO gas emission with the values of 0.05% at a lower CR of 13.5:1 and 0.04% at a higher CR of 16.5:1. This percentage was even lower than that of mineral diesel. The CO emission of other samples was also less than mineral diesel at various CR's. In comparison with mineral diesel, B40 blend emitted 33.33% lesser CO gases ( Table 8).
The reason behind less emission of CO gas by all dual biodiesel samples is the preheating process that improves the fuel atomization and sprays characteristics [21]. These trends revealed the quality of the combustion of dual biodiesel due to more quantity of oxygen present in these  Further, results showed that as the blend percentage rises, the emission of CO decreases. The increase in the compression ratio reduced the carbon monoxide output due to the higher amount of temperature produced during the air-fuel combustion [19]. The result analysis clearly revealed that a greater blend percentage of biodiesel results in lower emissions of toxic CO. Dubey and Gupta reported lower CO emissions during the analysis of Jatropha and turpentine biodiesel at a constant speed using a diesel single-cylinder engine [21]. Chauhan et al. also reported similar results in the case of diesel and Jatropha biodiesel blends in a diesel single-cylinder engine at constant load. A similar decrease in carbon monoxide emission was also reported by other researchers with the increase in CR and blend percentage during their research work [15].

Carbon dioxide (CO 2 )
The trend of CO 2 emission for dual biodiesel sample blends at different CRs is presented in Fig. 11. All sample blends emit higher CO 2 as compared to mineral diesel with increasing CR. The emissions of carbon dioxide were found to be lower for all dual biodiesel sample blends at CR of 13.5:1 (1.09, 1.43, 1.61, and 1.8% for sample blends B10, B20, B30, and B40, respectively). However, the emissions values increased with the increase in compression ratio. At a higher CR of 16.5:1, it was recorded to be 1.7, 1.8, 2.0, and 2.1% for sample blend B10, B20, B30, and B40, respectively. The CO 2 emissions were recorded lowest for mineral diesel. At a lower CR of 13.5:1, it was recorded to be 0.98%, whereas at a higher CR (16.5:1) it was observed to be 1.2%. It was observed that CO 2 emission was 42.85% lower when compared with the highest percentage blend of biodiesel.
The higher value of CO 2 emissions could be due to enhanced air-fuel combustion inside the cylinder due to the greater oxygen content present in the dual biodiesel blends. Higher CO 2 emission as compared to CO is the indication of better fuel combustion in an engine cylinder. The higher quantity of CO 2 is although harmful to the environment, but less toxic than CO and also consumed by various plants for the photosynthesis process. Dubey and Gupta revealed greater CO 2 in the exhaust for Jatropha and turpentine dual biodiesel sample blends tested at a fixed speed in the 4-stroke, 1-cylinder CI engine [23]. Other researchers also reported similar results in case of other different biodiesel blends [21,27].

Hydrocarbons (HC)
The trend of HC emission for different dual biodiesel sample blends at various CR is shown in Fig. 12. All the dual biodiesel sample blends had lower HC emissions than mineral diesel at all CRs for fixed loading conditions. The HC emissions were recorded to be highest (44 ppm) for mineral diesel at the lowest CR of 13.5:1. However, with the rise in CR, the HC emissions were reduced and were recorded to be 32 ppm for mineral diesel. The dual biodiesel sample blends had lower HC emissions as compared to the mineral diesel. Further, HC emission was found to be the lowest for the highest sample blend percentage B40. The emissions of HC have recorded 28 ppm at a lower CR (13.5:1), and it even lower down to 12 ppm at a higher CR (16.5:1) for dual biodiesel sample blends. Also, the other sample blends had lower HC emissions when compared with the mineral diesel at all CRs.
The increased value for CR in the engine resulted in lowering the HC percentage in the exhaust for all tested dual biodiesel sample blends. The lowering of HC emissions could be attributed to the higher amount of temperature inside the engine cylinder with the increase in CR. HC emissions for B10 blend and mineral diesel were 31% and 32%, respectively. Further, B20, B30, and B40 blends had 22%, 18%, and 12% HC emission, respectively, at CR of 16.5:1.
Chauhan et al. also analyzed Jatropha biodiesel blends on the CI engine and reported a decrease in emissions of HC for biodiesel than mineral diesel [15]. Similar results of HC emission were also reported by other researchers in the case of other different biodiesel blends [17,57].
Saravanan et al. [46] prepared a mixture of Mahua and Rapeseed oil biodiesel in the same proportions by volume and performed an experimental analysis. They observed that the performance of the B20 blend was very close to that of mineral diesel during testing. The brake thermal efficiency for sample blend B20 was noted 2.8% lower than that of mineral diesel. Also, 13 [46]. Srithar and Balasubramanian [55] have also prepared a fuel from the combination of two biodiesels blend with diesel and found two samples among them had properties almost similar to that of diesel fuel. The thermal efficiency of DPJ1 (DPJ1 = Diesel 90% + Pongamia Pinnata 5% + Jatropha 5%) and DPJ2 (DPJ2 = Diesel 80% + Pongamia Pinnata 10% + Jatropha 10%) were closer to that of diesel, and their specific fuel consumption was almost equal to that of diesel. From the emissions analysis, the results of DPJ1 and DPJ2 were found to be similar to diesel. The authors recommended these blend fuels as fuel for stationary diesel engines used for agricultural purposes [55].

Statistical regression analysis
The variation in engine emission and performance characteristics of all the sample blends of dual biodiesel were compared graphically with the change in CR. The MS Excel software was used for statistical regression analysis to determine the linear equations, correlation coefficient or regression coefficient (R 2 ), and regression equations. The regression coefficient (R 2 ) depicted the trends of various combustion and emission properties with the change in CR for any particular blend. The outcomes showed that all the engine performance parameters were directly proportional to the CR except CO and HC emissions which were found to be inversely proportional to the CR. The mathematical models developed revealed a high coefficient of regression or regression coefficient (R 2 )  Table 9.

Conclusions
In the current experimental work, dual biodiesel blends with diesel in different ratios were evaluated and compared with diesel for engine performance and emission characteristics on a VCR engine. The outcomes of the study revealed that the change in CR leads to the varied engine outputs and emissions characteristics which are comparable with those of mineral diesel in a significant way. The conclusions drawn from the present experimental study are summarized below: • All dual biodiesel samples tested had slightly greater brake power than mineral diesel. It varied from 0.15% to 1.58% more than mineral diesel (for sample blends B10 to B40) at the maximum value of compression ratio and under fixed loading conditions. • The mechanical efficiency increased with an increase in CR at fixed loading conditions for both dual biodiesel samples and mineral diesel. Again, sample blends B10 to B40 had a little higher mechanical efficiency of 1.07% to 12.42% more than mineral diesel. • The In-cylinder peak pressure of mineral diesel had a comparatively higher value than dual biodiesel samples and it lowered among the blend samples; B10 to B40 by 0.15 to 0.36 bar at the highest compression ratio tested. • The increase in biodiesel blending percentage in the dual biodiesel samples resulted in higher brake specific fuel consumption, which was observed to be the highest for sample B40 (26.77% more than mineral diesel) and least for sample B10 (2.27%) when compared with mineral diesel. • The exhaust gas temperature of all the dual biodiesel samples was found to be lower as compared to mineral diesel. B20 recorded 35.9℃, B30, and B40 blend had 53.6℃ & 69.8℃ while B10 had EGT 11.1℃ lower than EGT of mineral diesel at 16.5:1 compression ratio. • The emissions for carbon monoxide were recorded to be lower for all dual biodiesel samples than that of mineral diesel at maximum compression ratios. Further, the exhaust emissions of hydrocarbons were also reduced significantly for all dual biodiesel samples. • The carbon dioxide gas emission was higher for all dual biodiesel samples in comparison with mineral diesel at all compression ratios. • The fuel parameters tested on the engine in terms of combustion, efficiency, and emissions reported better results for dual biodiesel samples.
The current research study revealed that taking into consideration the overall engine combustion performance and emission parameters, the dual biodiesel B20 had superior fuel combustion and emission properties among all tested samples at a compression ratio of 16.5:1 and 50% loading on the engine. From current findings, it may be concluded that the blended dual biodiesel showed comparatively better combustion and emissions properties at

Future work
Further experimental studies can be conducted to check the oxidation stability of the various dual biodiesel sample blends. The sample blends having better oxidation resistance can be further tested for their commercial viability. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.