NOx emission reduction in low viscous low cetane (LVLC) fuel using additives in CI engine – An experimental study

doi:10.21203/rs.3.rs-3916528/v1

This study examines the combustion properties of pine oil (PO), which is classified as a low viscosity, low cetane (LVLC) fuel. It highlights the superior performance of pine oil in comparison to diesel fuel, but acknowledges that its low cetane index causes a delay in combustion initiation, which consequently results in elevated NOx emissions. Fuel atomization, evaporation, and air/fuel mixing is enhanced by the reduced viscosity and boiling point of PO in comparison to diesel. Nevertheless, the low cetane index of PO restricts its applicability as a diesel fuel substitute in CI engines. Because of the significant heat release that occurs subsequent to an extended ignition delay, NOx emissions tend to rise with less viscous and low cetane (LVLC) fuels. A range of cetane improvers, such as diethyl ether (DEE), benzyl alcohol (Bn), diglyme (DGE), and methyl tert-butyl ether (MTBE), have demonstrated effectiveness in mitigating nitrogen oxide (NOx) emissions upon introduction into pine oil. All the cetane improvers were added 5 % and 10 % by volume with pine oil.

A twin-cylinder tractor engine operating at a constant speed of 1500 revolutions per minute was utilized in this testing. In order to achieve a warm-up condition that would enable the smooth operation of PO, the engine was initially operated on diesel fuel. At maximum load condition, NOx emission of PO was higher by 8% in comparison to diesel. NOx emission was significantly reduced with addition of cetane improvers. Maximum reduction of 7% was observed with PO + MTBE 10% in comparison to PO which is in par with diesel. An increase in HC and CO emission was observed with all cetane improver addition with PO.

There is a growing concern regarding the environmental impact of the escalating emissions from conventional diesel engines. As a consequence, crude oil and petroleum products are anticipated to become exceedingly scarce and expensive. In particular, the country's energy security has deteriorated as a result of a significant depletion of crude oil reserves caused by an increase in consumption; in reality, a substantial proportion of crude oil must be imported from nations that control the larger oil fields. Numerous researchers are currently engaged in efforts to enhance the efficiency of conventional diesel engines while minimizing emissions through the use of alternative renewable fuels, as evidenced by the sudden increase in petroleum prices and demands that have already been felt. This is in stark contrast to the predictions made by research studies on alternative fuels, which collectively indicate an unprecedented demand for petroleum fuels by 2030 (Gao et al. 2018).

Biofuel is an oxygenated fuel and diversified as alcohols esters, ethers, carbonates and acetate compounds and derived from biomass namely plant, algae or waste. Additionally to being renewable and biodegradable, biofuels have significantly decreased emissions in comparison to diesel (Chauhan et al. 2016). In recent times, low viscous fuels namely pine oil (Vallinayagam et al. 2013), eucalyptus oil (Thiyagarajan et al. 2016), orange oil (Thiyagarajan et al. 2019), camphor oil (Subramanian et al. 2018) and lemon oil (Ashok et al. 2017) were identified and tested as fuel in CI engine. The utilization of less viscous fuels in diesel engines without requiring any modifications has been associated with several benefits, according to previous research: improved air-fuel mixture, enhanced atomization, and enhanced engine characteristics. However, low cetane number of these fuels lead to poor cold starting and higher NOx emissions. As an alternative to diesel fuel in CI engines, pine oil is utilized as the primary fuel in this study.

1.1 Pine oil

Pine oil is a renewable fuel and it is obtained by the distillation of oleoresins of pine tree. Pine oil is differentiated from other refined pine products, including rosin, the viscous residue remaining after turpentine distillation, and turpentine, the low-boiling fraction obtained from pine fluid distillation. (Rodrigues‐Corrêa et al. 2012). Pine oil primarily comprises α-terpineol and additional cyclic terpene alcohols from a chemical standpoint. Terpene, hydrocarbons, ethers, and esters might also be present. The precise composition is contingent upon a multitude of factors, including the tree part utilized and the variety of pine from which it is derived. Pine oil is a renewable fuel with viscosity and cetane number lower than diesel. Pine oil contains intrinsic oxygen, as evidenced by its lighter molecular weight and shorter carbon chain length in comparison to biodiesel and diesel. Pine oil possesses lower viscosity, boiling point, and flash point values in comparison to diesel fuel. These properties contribute to the enhancement of fuel dispersion, atomization, and evaporation capabilities (Vallinayagam et al. 2014a).

By implementing a double biofuel strategy, Vallinayagam et al. (2014b) were able to entirely eliminate the use of diesel in a diesel engine. In a stationary diesel engine, experiments were carried out utilizing KME (kapok methyl ester), which is a biodiesel compounded with pine oil in different proportions (B25P75, B50P50, and B75P75). At maximum load, they discovered that the thermal efficiency of the brakes was 8% higher for B25P75 compared to diesel. In contrast to diesel, which emits 14.9%, 43.2%, and 33.4% less HC, CO, and smoke, respectively, B25P75 has a higher NOx emission due to its elevated heat release rate, which is caused by its low cetane number. It was determined through analysis that B50P50 emits smoke, HC, and CO at 12.5%, 8.1%, and 18.9% reductions, respectively, in comparison to diesel, while NOx emissions are comparable to diesel. The emission reduction capability of pine oil, a biofuel derived from plants, was examined by Vallinayagam et al. (2014c). The fumigation technique was implemented in a diesel engine with a single cylinder. Pine oil is said to have comparable low viscosity, boiling point, and flash point characteristics to other fuels derived from plants when used as an oxygenated fuel. For the purpose of enhancing its ignition characteristics, pine oil with a lower cetane number is fumigated and then evaluated in a single-cylinder diesel engine in this research endeavor. Pine oil has been found to have a 36% diesel replacement efficiency at maximum load and a 60% diesel replacement efficiency at low load, according to experimental findings. A significant reduction of 64.2% in smoke emissions has been observed compared to typical diesel operation under maximum load conditions. Additionally, it was discovered that CO (carbon monoxide) and HC (hydrocarbon) emissions were reduced by 67.5% and 47.8%, respectively, compared to diesel at maximum load. However, there have been reports of an increase in NOx emissions. Pine oil, despite being a viable substitute for diesel in CI engines, is associated with elevated NOx emissions and cold-starting difficulties due to its low cetane number, according to a small number of other researchers [Thiyagarajan et al. 2019, Thiyagarajan et al. 2017]. By combining cetane improvers and aromatic alcohol, it is possible to circumvent the issue of increased NOx emissions caused by the operation of pine oil.

1.2 Fuel Additives

In this study, cetane improvers namely DEE, Diglyme, MTBE and aromatic alcohol namely benzyl alcohol were identified from the literature. Diethyl ether (DEE) is an ether-class organic compound denoted by the formula (C2H5)2O. Mahla et al. (2012) investigated the performance characteristics of acetylene gas when added to DEE in a dual-fuel engine. In order to conduct experiments in dual fuel mode, acetylene gas was aspirated through the inlet manifold while a diesel-DEE blend was used as the ignition source. Experiments were carried out at varying loading while a fixed quantity of acetylene gas was aspirated and diesel and DEE blends (DEE10, DEE20, and DEE30) were added. Based on the comprehensive analysis, it was determined that the DEE20 blending ratio provides superior performance in comparison to diesel and alternative mixtures. In comparison to operation with pure diesel, dual fuel operation with the addition of DEE produced a greater thermal efficacyThe impact of exhaust gas recirculation and nano-emulsion on engine characteristics using a blend of pure lemon grass oil, diesel, and DEE was investigated by Sathiyamoorthi et al. (2017). An increase in NOx emissions was observed with a blend of diesel and lemon grass oil as compared to pure diesel, owing to the low cetane number of lemon grass oil. In comparison to base fuels, the combined effect of DEE, nanoemulsion, and EGR reduced NOx emissions and haze by 30.7% and 11%, respectively. Kaimal and Vijayabalan (2016) investigated the effect of DEE in conjunction with waste plastic oil in a CI engine through experiments. Variable load experiments were conducted using DEE blended with waste plastic oil in the following proportions: 5%, 10%, and 15%. It was noted that an increase in the proportion of DEE resulted in a corresponding decrease in brake specific energy consumption, which in turn led to an improvement in brake thermal efficiency. They discovered that a mixture of DEE and waste plastic oil decreased NOx and haze emissions while increasing HC emissions marginally. Experiments were carried out by Jothi et al. (2007) on a homogeneous charge CI engine fueled with LPG and DEE as an ignition enhancer. By adding between 58 and 28.8 percent DEE on a mass basis, it is feasible to operate the DI compression ignition engine with stable combustion on pure LPG across the entire load range. Due to the greater evaporation of DEE, which cools the intake charge and lowers the temperature of the cylinder gas, the brake thermal efficiency in LPG mode is approximately 23% lower at full load than in diesel operation. Patil and Thipse (2015) evaluated the performance, emission, and combustion characteristics of a CI engine using a kerosene-diesel blend containing cetane improver (DEE). It was observed that DEE exhibits miscibility in all proportions with diesel and kerosene. Additionally, they noted a decrease in the blend's density, kinematic viscosity, calorific value, and cetane number as the oxygen content increased, and DEE incorporation decreased the cetane number. The results of an experimental study indicated that diesel, petroleum, and DEE blends decreased NOx emissions while marginally increasing smoke emissions.

Diglyme is an additive which has high cetane number and high boiling point. The impact of diesel-oxygenate mixtures on the emission and combustion characteristics of diesel engines was examined by Ren et al. (2008). It was noted that diglyme, in comparison to other oxygenate mixtures, possesses a higher cetane number. This characteristic resulted in a notable decrease in smoke concentration, while the NOx emissions and engine thermal efficiency were not significantly impacted. Additionally, they noted a decrease in CO and HC emissions as the oxygen mass fraction in the mixtures increased. The impact of combining Diglyme and ETBE with diesel was investigated by Barrios et al. (2014). A total of seven additive mixtures were utilized in the experiments, each comprising 5%, 10%, or 15% of the additive. Experiments were carried out using a CI engine under conditions of variable speed and load. It was noted that particle emission exhibits a direct proportionality with mixture ratio. It was determined that there was a significant reduction in particulate emissions, while NOx emissions were not significantly impacted. Varuvel et al. (2018) conducted experiments utilizing mixtures of rubber seed oil (RSO) and babassu oil (BSO) in conjunction with diglyme as an additive in a commercial CI engine. Poor combustion characteristics were observed in vegetable oil mixtures, which resulted in increased smoke emissions. Due to the increased oxygen content, the addition of diglyme facilitated the NOx-smoke tradeoff. Additionally, a notable decrease in HC and CO emissions was noted when compared to the blends of base vegetable oils. Methyl tertiary-butyl ether (MTBE) is a volatile, flammable, and colorless liquid that is sparingly soluble in water. MTBE is a gasoline additive, used as oxygenate to raise the octane number. The impact of a ternary blend of MTBE, calophyllum biodiesel, and diesel on the performance, emission, and combustion characteristics of a single-cylinder CI engine was investigated by Bragadeshwaran et al. (2018). A mixture of diesel and biodiesel was supplemented with MTBE in a volumetric proportion of 5% to 20%. With a marginal increase in fuel consumption, they discovered that the addition of MTBE decreased thermal efficiency. Ternary mixtures contributed to the reduction of NOx, HC, and CO emissions. Incorporating MTBE resulted in a marginally diminished combustion characteristic profile and a prolonged ignition delay. The impact of MTBE oxygenate incorporated with gasoline was investigated by Topgul (2015). By blending MTBE in proportions of 5%, 10%, 20%, and 30% by volume, the performance and emission characteristics of a single-cylinder SI engine operating at variable speeds were evaluated. It was noted that the incorporation of MTBE resulted in enhanced thermal efficiency and decreased energy consumption. The addition of MTBE to the mixture resulted in a decrease in exhaust emissions, specifically CO and HC.

The OH group of benzyl alcohol is connected to the benzene group, making it an aromatic alcohol group. This effect contributes to an increased rate of hydrogen abstraction, which increases the amount of fuel involved in the combustion process (Godwin et al. 2019). Benzyl alcohol blending in CI engine was done by only few researchers. Raj et al. (2018) evaluated the performance, emission, and combustion characteristics of simarouba glauca biofuel blends (B20 and B40) at variable loads by incorporating benzyl alcohol and 1-pentanol into the blends. Diesel was compared to blends of both alcohols at 5% and 10% by volume with B20 and B40. They observed that the addition of benzyl alcohol improved the unsatisfactory performance of B20 and B40. In contrast to the 1-pentanol blend, the inclusion of benzyl alcohol resulted in a marginal rise in NOx emissions while HC, CO, and particulate emissions were diminished. Zhou et al. (2014) examined the impact of aromatic oxygenates on CI engines in order to determine how the position of the functional oxygen group in relation to the aromatic ring affected the engine. When comparing diesel to all oxygenates, they discovered that the NOx-soot tradeoff and performance improved. They concluded, benzyl alcohol was the most effective oxygenate with or without EGR, as indicated by its efficiency.

To optimize engine parameters, lot of experimentation is required and sometimes huge costs are incurred. To reduce the experimentation time and costs involved it is necessary to reduce the number of experiments. Among other methods, Response Surface Methodology (RSM) can accurately predict the thermal efficiency, energy consumption and emission parameters for a particular fuel blend ratio and engine parameters (Sathyanarayanan et al. 2022). The optimal biodiesel-diesel ratio was determined in one study by combining RSM and central composite design (Parida et al. 2019). The engine performance parameters were predicted with an accuracy of 95% or greater by the model. An additional investigation (Uslu. 2022) utilized RSM to optimize the operational parameters of a diesel engine powered by mixtures of diesel and palm oil. The model consistently predicted engine performance parameters with R² values exceeding 0.9 in all instances.

According to the research, despite the fact that pine oil exhibits superior combustion and performance characteristics, emissions, and performance in comparison to diesel, its low cetane index causes a delay in the initiation of combustion, which results in an increase in NOx emissions. NOx emissions have been demonstrated to be effectively reduced through a variety of methods, including the incorporation of additives into pine oil. Dimethyl benzyl alcohol, diglyme, and MTBE are also effective cetane improvers, although research on their use is more limited than that of DEE. Furthermore, no experimentation has been conducted regarding the application of RSM to maximize engine performance using pine oil and alcohols. As a result, minor amounts of DEE, Diglyme, MTBE, and benzyl alcohol were added to pine oil in this study to mitigate the impact of increased NOx emission. The investigation also examined the impact on additional performance, emission, and combustion attributes. The properties of the fuel and additives are detailed in Table 1. At variable loads, experiments were conducted with 5% and 10% by volume additions of DEE, DGE, MTBE, and Bn to PO. Finally, an engine performance model was constructed utilizing RSM and the provided engine parameters.

Table 1. Fuel Properties

Experiments were performed on a twin-cylinder CI engine manufactured by Simpsons; the engine's specifications are detailed in Table 2. Engine loading was accomplished utilizing an eddy current dynamometer. Concentrations of HC, CO, and NOx in the engine effluent were determined using an AVL 5 gas analyzer. The experimental configuration is illustrated schematically in Figure 1. The AVL 432c smoke meter was employed to determine the opacity of the engine exhaust smoke utilizing the light extinction technique. A two-position, three-way directional control valve was employed to facilitate the swift transition from diesel to the experimental fuel.

A piezoelectric pressure sensor that was affixed to the cylinder head of the engine was utilized to ascertain the in-cylinder pressure. The crank angle was determined with a resolution of one degree by employing an optical shaft position encoder. With the use of an analog to digital converter and a data gathering system, the signal from both sensors was collected and saved on a personal computer. Combustion characteristics of the engine was calculated based on collected data.

With the load varying in 25% increments, the experiments were carried out at a constant speed of 1500 rpm. A U-tube manometer was employed to determine the volumetric air flow rate, while a 50cm³ burette/stopwatch was utilized to ascertain the fuel flow rate. Using the three-way valve, the fuel was changed from diesel to test fuel once the engine had stabilized after being initiated with diesel. The engine's performance was assessed in relation to emission parameters, exhaust gas temperature, and brake thermal efficiency. The average values of all three iterations of each test were utilized for analysis.

Table 2. Engine Specifications

Figure 1. Schematic Diagram of Test Engine

1. Air box; 2. Fuel Tank; 3. Fuel Pump; 4. Fuel injector; 5. Engine; 6. Eddy current dynamometer; 7. Control Panel; 8. Emission analyzer;

All investigations were carried out at a consistent rotational speed of 1500 rpm under different load conditions. The fuel injection pressure and timing were both maintained at 200 bar and 23obTDC, respectively. Every test was performed under steady-state conditions, with no alterations made to the engine's physical structure. In order to mitigate the possibility of errors, each test was conducted five times, and the mean value was utilized for analysis. Cold-starting issues were caused by the PO's low cetane index. Diesel was initially used to power up the engine before the complete transition to PO. Lubricating agent (Lubrizol 539M) is added at a concentration of 500 ppm to PO and its mixtures in order to prevent the malfunction of fuel injection equipment, as PO has inferior lubrication properties. Uncertainty is the result of error-induced deviation from the measured value. The uncertainty associated with the different instruments utilized in the present study is detailed in Table 3. Experimental repeatability ensures that only instrumental errors are accounted for, thereby preventing the random errors.

4.1 Performance Parameters

Fig. 2 shows the variation of brake thermal efficiency (BTE) for diesel, PO and PO with additives at different load conditions. Improved physical properties of PO improved the BTE to 31.8 % at full load condition in comparison to 30.2 % for diesel. PO has very low viscosity and improved volatility which improves the fuel preparation leading to more fuel prepared for the instantaneous combustion. It is observed that a marginal increase in BTE was observed with addition of DEE and DGE. Both, DEE and DGE has higher flame speed which aids in further improvement in combustion process. However, it is observed that the improvement is not significant with 10 % addition of DEE and DGE in comparison to 5 % addition by volume. Similarly, addition of MTBE also showed improvement in BTE for 5 % addition by volume. With addition of Bn5 and Bn10 with PO, BTE was increased to 33 % and 33.5 % respectively. As an aromatic alcohol, benzyl alcohol has an indirect bond between the OH and the aromatic ring. This phenomenon results in an escalation in the rates of hydrogen abstraction, isomerization, and free radical propagation. These unique features of benzyl alcohol aided in improved BTE and maximum improvement was observed in comparison to other additives.

Fig. 3 shows the variation of BSEC for diesel, PO and PO with additives at different load conditions. It is observed that BSEC reduces with load for all the test fuels. At maximum load, BSEC for diesel and PO is 11.9 MJ/kWh and 10.1 MJ/kWh respectively. Due to lower viscosity of pine oil, its atomization is improved thus causing proper mixing of fuel and air. Thereby improving the combustion quality and reducing the energy consumption. By blending diethyl ether, benzyl alcohol and MTBE the reduction in energy consumption was insignificant, whereas a significant decrease in energy consumption at all loads was observed by blending diglyme to pine oil. With 5% blending of diglyme in pine oil maximum reduction in energy consumption of 8.67% was observed at full load in comparison to neat pine oil operation. The probable decrease in energy consumption with diglyme is caused by earlier and higher heat release in the premixed phase as compared to pine oil, such that the peak pressure is close to top dead center and most of the heat is utilized in producing useful power.

Fig. 2 Variation in brake thermal efficiency with brake power

Fig. 3 Variation in brake specific energy consumption with brake power

The modulation in carbon monoxide emission with load for the test fuel samples is depicted in Fig. 4. It is seen that the engine operation with neat pine oil at full load resulted in a 12.5% decrease in CO emission as compared to diesel. The reduction in carbon monoxide (CO) emissions can be attributed to the oxygen-promoting properties of pine oil. Further, the combustion temperature with pine oil engine operation is higher than diesel due to higher heat released resulting in superior oxidation of the intermediate carbon monoxide. Moreover, addition of additives resulted in increase in CO emission except benzyl alcohol which resulted in lower CO emission, but the emission was still higher than neat pine oil engine operation. 10% blending of benzyl alcohol with pine oil resulted in lowest CO emission among the additives. At full load the emission with Bn10 was 14.29% higher than pine oil engine operation and similar to that of diesel. The overall increase in the CO emission with oxygenates is caused by the high latent heat of vaporization. Among the additives benzyl alcohol has relatively lower latent heat of vaporization and as the hydroxyl group is present outside the ring, the oxygen present in the alcohol can be easily utilized for the oxidation of carbon monoxide.

Fig. 5 depicts the variation in unburned hydrocarbon emission with load for diesel, pine oil and various additives at 5% and 10% blending in pine oil. The engine operation with pine oil resulted in lower unburned hydrocarbon emission than diesel. At full load the HC emission with diesel is 19ppm which reduced to 16ppm with pine oil. The decrease in the emission is caused by complete combustion of fuel as can be seen in the heat release curve (Fig. 9). Also, the amount of fuel injected per cycle is lower with pine oil (Fig. 3) hence less amount of hydrocarbon needs to burn for producing the same power. Therefore, there is less chance of fuel escaping combustion and leave the cylinder as unburned hydrocarbon. Further, the addition of additives resulted in higher HC emissions than pine oil. The HC emissions follow the same trend as that of CO emission i.e., the highest HC emission was observed with MTBE followed by diglyme and diethyl ether. Also, higher percentage of additive in the blend resulted in higher emission. However, the trend reversed with benzyl alcohol and lower emission is formed with 10% of benzyl alcohol in the blend which is the lowest emission among the additives. The increase in HC emission with the additives is similar to that of CO emissions i.e., higher latent heat of vaporization of oxygenates. The highest HC emission was observed with MTBE as the energy consumption of the engine with MTBE blends was highest.

Fig. 4 Variation in carbon monoxide emission with brake power

Fig. 5 Variation in unburned hydrocarbon emission with brake power

The NO emission for the test fuels at all load conditions is shown in Fig. 6. It is seen that NO emission increases with increase in load for all the test fuels. The NO emission with neat pine oil engine operation was highest for all the loads and at full load it increased by 7.65% in comparison to diesel. The increase in emission is caused by higher heat release which leads to an increase in combustion temperature. In general, higher combustion temperature causes the nitrogen to split in to atomic nitrogen which has high affinity for the oxygen present in the air, resulting in increase in oxides of nitrogen emission. The nitrogen content inherent in the pine oil is an additional factor contributing to the rise in emissions. Addition of 5% and 10% oxygenates to the pine oil resulted in a slight decrease in NO emission at all loads. As the additives replaces pine oil, the nitrogen present in the charge is reduced as the percentage of nitrogen containing pine oil is reduced. Further, increase in additive percentage further reduces the nitrogen percentage, thereby reducing the emission. Among the additives the lowest NO emission of 1327ppm was observed with Bn10.

Fig. 6 Variation in NO emission with brake power

The variation in smoke opacity with engine load for diesel, pine oil, and the additives is shown in Fig. 7. At full load, the smoke opacity of the exhaust gas with pine oil operation is 57%, whereas with diesel the smoke opacity is 62%. The decrease in the opacity with pine oil is caused by its low viscosity which improves the mixing of fuel and air such that the thermal pyrolysis of fuel molecules in the absence of oxygen is reduced. Additionally, the reduction of particulate emission is accelerated by the high combustion temperature and the presence of molecular oxygen in pine oil.. The soot emission with all oxygenates was lower than pine oil. At full load, the smoke opacity with DEE5, DEE10, DGE5, DGE10, Bn5, Bn10, MTBE5 and MTBE 10 is 55%, 53%, 56%, 52%, 49%, 46%, 56% and 55%, respectively. An improvement in fuel combustion results in elevated combustion temperatures, which facilitates additional particulate emission reduction when oxygen is present, thereby contributing to the decrease in emissions.

Fig. 7 Variation in smoke opacity with brake power

Fig. 8 depicts the effect of the additives on the combined reduction of NO and smoke emission at full load. It is seen that the addition of oxygenates resulted in a decrease in NO and smoke emission as compared to neat pine oil engine operation. There was a slight decrease in both the emissions with DEE5, DGE5 and MTBE5 addition to pine oil. With 10% addition of MTBE to pine oil the NO emission was lowest, however, the smoke emission was similar to pine oil. Whereas 10% addition of benzyl alcohol resulted in lowest smoke emission and slightly higher NO emission than MTBE5. The combined decrease in NO and smoke emission with Bn10 can be attributed to better combustion quality of the fuel blend.

Fig. 8 NO-smoke trade-off at full load

The NO-smoke trade off shows that addition of 10% of benzyl alcohol to pine oil resulted in the combined reduction in NO and smoke emission. Also, the HC and CO emission with the blend was lowest among additives therefore, the combustion data of only benzyl alcohol was compared with neat diesel and pine oil engine operation. The variation in heat release at full load for the three fuel samples is shown in Fig. 9. It is seen that the heat release with pine oil initiated later than diesel on account of low cetane number of the oil. Therefore, when the combustion started large amount of the accumulated fuel was ready to burn that resulted in higher heat release in the initial phase of combustion. With 10% addition of benzyl alcohol in pine oil the highest heat release rate is observed. The improvement in heat release is caused by the increase in calorific value of the fuel sample that tends to increase the heat release rate. Also, a decrease in ignition delay is observed as compared to pine oil, due to improvement in cetane number of the test sample.

Fig. 9 Variation in heat release rate at full load

The in-cylinder pressure at full load with diesel, pine oil and 10% addition of benzyl alcohol to pine oil is shown in Fig. 10. The peak cylinder pressure with diesel is 84.68bar at 9°aTDC, with pine oil it is 89.19bar at 14°aTDC and with Bn10 it is 93.47bar at 10°aTDC. The rise in peak pressure associated with pine oil can be attributed to enhanced combustion quality resulting from the fuel's increased volatility. Further substitution of pine oil with benzyl alcohol resulted in increase in-cylinder pressure which is caused by higher heat release due to improvement in combustion. It is also seen that the peak pressure with Bn10 reached 4°CA earlier than pine oil due to improvement in cetane number that reduces the ignition delay.

Fig. 10 Variation in in-cylinder pressure at full load

4. Response Surface Methodology Analysis

The experiments carried out can be modelled and analyzed statistically and mathematically using response surface methodology (RSM) (Kumar and Dinesha. 2018) . In this method, critical factors are used to form a model for an output and the model values are also optimized to predict the output (Abdalla.et al. 2019). This technique is effective for a vast array of applications in a variety of industries. Eqs. 1 and 2 illustrate that the function utilized in this methodology to ascertain the correlation between the input and output may be linear or polynomial in nature (Singh et al. 2021).

In this study, brake power, percentage of pine oil and additives were considered as input. The levels of input are shown in Table 3. Central composite design and quadratic models were used for optimizing and predicting the responses. The experimental design, run order, and data for forming the models for each of the output parameters are given in Table 4.

Analysis of variance (ANOVA) statistical technique was applied to understand the relationships between the independent and dependent variables. The experimental data of brake thermal efficiency, brake specific energy consumption, CO, HC, NO and smoke emission was analyzed using ANOVA. The significance of the model was given by F-test & P-test values which are tabulated in Tables 5 and 6. A model is said to be significant, if the p-values are under 0.05, indicating that the probability values are moving away from null hypothesis, else if the p-value is higher than 0.05 then the model is considered insignificant (Rajmohan and Palanikumar 2013). It is seen in Tables 5 and 6 that the model is significant for BTE, BSEC, CO and NO emission while it is not significant for HC and smoke emission. The correlation coefficient ‘R²’, adjusted correlation coefficient ‘Adj. R²’ and predicted correlation coefficient ‘Pred. R²’ were used to find the fit statistics of the model. These values were calculated using Eqs. 3, 4 and 5, respectively (Sathyanarayanan et al. 2023).

The correlation coefficient ‘R²’ evaluates the correctness of the model and its sufficiency. High correlation coefficient shows that the model can accurately predict the output parameters. The R² value of BTE, BSEC, CO, NO, HC and smoke was found to be 0.989, 0.9879, 0.9918, 0.9956, 0.5394 and 0.4829, respectively. It is seen that the accuracy of the model in predicting the unburned hydrocarbon and smoke emission is low. The model has a desirability of 0.583. The operating settings, related responses and evaluation of the model for determining the optimum responses is listed in Table 8. The optimum desirability ramp graph for additive ‘Bn10’ is shown in Fig. 11. Confirmation tests were then carried out to test the accuracy of the model. The error percentages between the experimental data and predicted data for all the additives is tabulated in Table 9. The table shows that the model can predict BTE, BSEC, NO and CO emission within 5 error percentage. However, the model was unable to accurately predict HC and smoke emission as the model failed to pass p-test which is seen in the analysis of variance (Table 6).

Table 3. Engine input level

Table 4. Experimental Design Data

Table 5. Engine performance ANOVA result

Table 6. Engine emissions ANOVA result

Table 7. Fit statistics

Table 8. Optimization setup and model evaluation

Table 9. Validation of predicted data and experimental data

Fig. 11 Optimum desirability ramp graph for Bn10 additive

4.1. Interaction effects

4.1.1. Interaction effects of thermal efficiency and specific energy consumption

Fig. 12 shows the interaction between pine oil percentage, additives, brake power and thermal efficiency. It is seen that overall, 10 percentage of any of the additives and 90 percentage of pine oil resulted in higher improvements in thermal efficiency at all the engine loads. While lowest thermal efficiency was observed when engine was operated with only pine oil. Addition of alcohol lead to increase in oxygen content of the charge which aided the combustion process, thus more useful work was produced. Similar trends were observed with brake specific energy consumption as shown in Fig. 13. The actual factors of brake thermal efficiency and brake specific energy consumption is given in Eqs. 8 and 9, respectively.

BTE = 28.77+3.98*A-1.81*B+1.58*C[1]+0.4408*C[2]+0.1613*C[3]+1.14*C[4]+0.1508*C[5]-0.8473* C[6]-1.12*C[7]+0.0064*AB-0.0374*AC[1]-0.0254*AC[2]-0.213*AC[3]+0.1425*AC[4]+0.1435* AC[5] +0.061*AC[6]-0.1581*AC[7]-0.9575*A² (8)

BSEC=10.7-1.6*A-0.1251*B+0.6961*C[1]-0.5895*C[2]+0.2125*C[3]+0.2823*C[4]+0.122 * C[5]-0.7146*C[6]-0.0053*C[7]-0.0649*AB-0.0217*AC[1]+0.1005*AC[2]+0.0766*AC[3]-0.0438*AC[4] -0.0842*AC[5]+0.0356*AC[6]+0.0462*AC[7]+0.608*A² (9)

4.1.2. Interaction effects of carbon monoxide emission

The interactions between alcohol-based additives, pine oil and brake power on carbon monoxide emission is depicted in Fig. 14. Overall, the emissions were lower with additives as compared to pine oil due to improvement in combustion ability caused by enrichment of oxygen and wide flammability. Eq. 10 shows the actual factor of carbon monoxide emission.

CO=0.0966-0.0352*A+0.0413*B-0.09*C[1]-0.0088*C[2]-0.0162*C[3]-0.0312*C[4]+0.037*C[5]+ 0.0475 *C[6]+0.04*C[7]+0.0082*AB-0.018*AC[1]-0.0007*AC[2]-0.0052*AC[3]-0.0082*AC[4] +0.0037*AC[5]+0.0075*AC[6]+0.009*AC[7]-0.0006*A² (10)

Fig. 12 Interaction effect of pine oil, additives and brake power on thermal efficiency

Fig. 13 Interaction effect of pine oil, additives and brake power on energy consumption

4.1.3. Interaction effects of unburned hydrocarbon emission

Fig. 15 shows the effect of brake power, pine oil percentage and additive percentage on unburned hydrocarbon emission. The emissions were reduced with additives due to improvement in combustion caused by higher flame speed of alcohols. The actual factors for unburned hydrocarbon emission are shown in Eq. 11.

HC=15.2-1.3*A-1.08*B+2.17*C[1]-0.67*C[2]+1.58*C[3]-2.17*C[4] +1.08*C[5]-0.25*C[6]+0.5*C[7] +1.55*AB-3.7*AC[1]+0.7*AC[2]+2.95*AC[3]-0.2*AC[4] -1.85*AC[5]-0.15*AC[6]+1.2*AC[7]-0.0625*A² (11)

4.1.4. Interaction effects of oxides of nitrogen emission

The interaction effect of additive and pine oil percentage and brake power on oxides of nitrogen is shown in Fig. 16. The emissions were found to be lower with addition of alcohols as they have a cooling effect due to high latent heat of vaporization resulting in lowering of combustion temperature, thereby reducing the NO emission. The actual factor of the emission is mentioned in Eq. 12.

NO=1231.99+297*A+93.42*B-66.08*C[1]+3.33*C[2]-18.42*C[3]-51.92*C[4] -15.17*C[5]+65.25* C[6]+42.5*C[7]-3.9*AB+6.15*AC[1]+2.85*AC[2]+1.2*AC[3]+17.7*AC[4]-0.15*AC[5] -7.35*AC[6] +7.5*AC[7]-112.63*A² (12)

Fig. 14 Interaction effect of pine oil, additives and brake power on emission of carbon monoxide

Fig. 15 Interaction effect of pine oil, additives and brake power on emission of unburned hydrocarbons

4.1.5. Interaction effects of particulate emission

The interaction between brake power & pine oil concentration for all the additives and smoke emission is depicted in Fig. 17. The emission with the additives was higher than neat pine oil at low load condition while the trend reverses at high load condition. The actual factor of the emission is mentioned in Eq. 13.

Smoke=33.17-0.39*A+1.82*B-9.13*C[1]-4.19*C[2]+7.93*C[3]-16.57*C[4] +6.76*C[5]+5.2* C[6] + 14.5*C[7]+15.51*AB-33.72*AC[1]-2.69*AC[2]+11.04*AC[3]-4.86*AC[4]-9.86*AC[5] +8.28*AC[6] +19.2*AC[7]-1.54*A² (13)

Fig. 16 Interaction effect of pine oil, additives and brake power on emission of oxides of nitrogen

Fig. 17 Interaction effect of pine oil, additives and brake power on emission of particulates

Utilizing neat Pine oil as a drop-in fuel in a twin-cylinder CI engine was the concept behind this research. However, the autoignition is impacted by the low cetane index of PO, and increased NOx emissions were anticipated when it was utilized in CI engines. In order to support the auto igniton and to reduce NOx emission, 5% and 10% by volume of fuel additives such as DEE, DME, MTBE and Bn were added with PO. Fuel additives including DEE, DME, MTBE, and Bn were added with PO at volumes of 5% and 10% to promote auto-ignition and reduce NOx emissions. The fuel blends that have been prepared are evaluated in a CI engine operating at a constant speed of 1500 rpm and a fuel injection of 23 bTDC. Combustion was improved with neat PO in comparison to diesel due to delayed start of combustion as a result of longer ignition delay period. With addition of additive, the ignition delay was shortened with reduced combustion duration. NOx was higher with PO in comparison to diesel and was reduced with all the additives. Maximum reduction in NOx emission was observed with MTBE 10. Marginal reduction in smoke emission was also observed with most of the additives in comparison to base fuels. Marginal increase in HC and CO emission were observed with all the additives in comparison to diesel and PO. In general, it is feasible to utilize pure PO as a substitute for diesel in a CI engine by supplementing the fuel with fuel additives, which improves both performance and emissions. The response surface methodology can accurately predict the thermal efficiency, energy consumption, NOx and CO emission. The major drawback with this study was cold starting problem associated with use of PO as a result of lower cetane index.

Author Contribution

ANKIT SONTHALIA - Experimentation, Data analysis, RSM analysis, engaged in writing part of manuscriptEDWIN GEO VARUVEL - engaged in writing part of manuscript, Reviewed the manuscriptS. Thiyagarajan - Experimentation, Data analysis, engaged in writing part of manuscriptNaveen Kumar - engaged in writing part of manuscript, Reviewed the manuscript

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Table 1. Fuel Properties

	Pine Oil	DEE	DGE	MTBE	Bn
Density (kg/m³)	865	715	944	744	980
Viscosity@40℃ (cSt)	3	0.23	2.12	0.409	5.6
Calorific value (MJ/kg)	41.6	33.7	24.5	35.1	34.5
Cetane number	13.6	11.1	126	-	29

Table 2. Engine Specifications

Model	Simpsons S 217 Tractor Engine
Rated Power	12.4 kW @ 1500 rpm
Type / Configuration	Vertical in-line Diesel Engine
Bore x Stroke	91.44 mm x 127 mm
No. of cylinders	2
Displacement	1670 cc
Compression Ratio	18.5:1

Table 3. Engine input level

Engine Input	Code	Levels
Engine Input	Code	-1	1
Brake Power (kW)	A	3.11	12.44
Pine Oil (%)	B	90	100
Additives	C	1	9

Table 4. Experimental Design Data

Std	Run	Brake Power (kW)	Pine Oil (%)	Additives [Code]	BTE (%)	BSEC (MJ/kWh)	CO (%)	NO (ppm)	HC (ppm)	Smoke (%)
7	1	3.11	100	0[1]	23.54	13.65	0.02	838	10	12
13	2	6.22	100	0[1]	27.60	11.64	0.04	1141	13	28
10	3	9.33	100	0[1]	29.10	11.04	0.06	1387	15	43
1	4	12.44	100	0[1]	31.82	10.10	0.07	1421	16	57
24	5	3.11	95	DGE5[2]	24.10	12.29	0.05	824	12	11.5
34	6	6.22	95	DGE5[2]	28.09	10.55	0.08	1107	15	27
25	7	9.33	95	DGE5[2]	30.42	9.74	0.1	1351	17	42
19	8	12.44	95	DGE5[2]	32.11	9.22	0.12	1409	19	56
35	9	3.11	95	DEE5[3]	24.22	13.05	0.05	809	12	11
18	10	6.22	95	DEE5[3]	27.98	11.30	0.07	1090	14	27
2	11	9.33	95	DEE5[3]	29.22	10.82	0.09	1305	16	41
17	12	12.44	95	DEE5[3]	32.18	9.82	0.11	1400	18	55
27	13	3.11	95	Bn5[4]	24.75	13.28	0.04	757	11	6
9	14	6.22	95	Bn5[4]	28.51	11.53	0.05	1050	13	20.8
23	15	9.33	95	Bn5[4]	31.25	10.52	0.08	1285	15	35
28	16	12.44	95	Bn5[4]	33.00	9.96	0.09	1378	18	49.3
15	17	3.11	95	MTBE5[5]	23.65	13.21	0.06	810	13	12
5	18	6.22	95	MTBE5[5]	27.74	11.26	0.09	1102	15	27
4	19	9.33	95	MTBE5[5]	29.90	10.44	0.11	1302	17	42
8	20	12.44	95	MTBE5[5]	32.10	9.73	0.14	1403	19	56
33	21	3.11	90	DGE10[6]	24.53	12.29	0.07	802	14	9
22	22	6.22	90	DGE10[6]	28.60	10.55	0.09	1099	16	24
26	23	9.33	90	DGE10[6]	30.98	9.74	0.11	1265	19	39
11	24	12.44	90	DGE10[6]	32.70	9.22	0.14	1399	21	52
30	25	3.11	90	DEE10[7]	24.88	12.84	0.06	780	13	10
29	26	6.22	90	DEE10[7]	28.21	11.33	0.08	1034	15	25
31	27	9.33	90	DEE10[7]	29.81	10.72	0.11	1275	18	40
21	28	12.44	90	DEE10[7]	32.83	9.73	0.13	1385	20	53
20	29	3.11	90	Bn10[8]	25.44	13.05	0.03	734	10	5
3	30	6.22	90	Bn10[8]	29.43	11.27	0.04	1008	13	18
14	31	9.33	90	Bn10[8]	31.87	10.41	0.07	1235	14	31
16	32	12.44	90	Bn10[8]	33.46	9.92	0.08	1327	17	46
12	33	3.11	90	MTBE10[9]	23.94	13.16	0.07	790	14	11
6	34	6.22	90	MTBE10[9]	28.02	11.25	0.1	1065	16	25
32	35	9.33	90	MTBE10[9]	30.12	10.46	0.12	1289	19	40
36	36	12.44	90	MTBE10[9]	32.27	9.77	0.15	1322	22	55

Table 5. Engine performance ANOVA result

Source	BTE		BSEC
Source	F- value	p-value	F- value	p-value
Model	84.93	< 0.0001	77.32	< 0.0001
A – Brake Power	1441.39	< 0.0001	1209.62	< 0.0001
B – Pine Oil	29.76	< 0.0001	0.7586	0.3959
C-Additives	4.84	0.0037	13.55	< 0.0001
AB	0.0002	0.9887	0.1132	0.7406
AC	0.1798	0.9860	0.3134	0.9378
A²	29.68	< 0.0001	63.70	< 0.0001

Table 6. Engine emissions ANOVA result

Source	CO		NO		HC		Smoke
Source	F- value	p-value	F- value	p-value	F- value	p-value	F- value	p-value
Model	114.87	< 0.0001	215.09	< 0.0001	1.11	0.4195	0.8821	0.6038
A – Brake Power	1168.94	< 0.0001	3601.65	< 0.0001	7.73	0.0128	2.06	0.1690
B – Pine Oil	188.31	< 0.0001	35.32	< 0.0001	0.2751	0.6067	0.0220	0.8838
C – Additives	88.49	< 0.0001	4.47	0.0055	0.9742	0.4804	1.13	0.3891
AB	4.18	0.0566	0.0342	0.8555	0.3128	0.5833	0.8920	0.3582
AC	3.07	0.0277	0.6181	0.7340	0.7626	0.6254	0.6581	0.7037
A²	0.1537	0.6999	182.55	< 0.0001	0.0033	0.9552	0.0561	0.8156

Table 7. Fit statistics

	BTE	BSEC	CO	NO	HC	Smoke
Std. Dev.	0.4687	0.2031	0.0043	22.23	2.92	17.31
Mean	28.84	11.08	0.0825	1138.28	15.53	31.71
C.V. %	1.63	1.83	5.15	1.95	18.81	54.59
R²	0.9890	0.9879	0.9918	0.9956	0.5394	0.4829
Adjusted R²	0.9774	0.9752	0.9832	0.9910	0.0517	-0.0645
Predicted R²	0.9487	0.9490	0.9641	0.9782	-1.4733	-1.8084
AP	29.2540	29.4718	41.1222	44.6408	3.8639	3.5386

Table 8. Optimization setup and model evaluation

Parameter	Goal	Lower limit	Upper limit	Importance
A: Brake Power	is in range	3.11	12.44	3
B: Pine Oil	is in range	90	100	3
C: Additives	Is equal to (8)	1	9	3
BTE	maximize	23.5403	33.4575	3
BSEC	minimize	9.22285	13.6493	3
CO	minimize	0.02	0.15	3
NO	minimize	734	1421	3
HC	minimize	10	22	3
Smoke Opacity	minimize	5	57	3

Table 9. Validation of predicted data and experimental data

Engine Load	Pine Oil (%)	Additive		BTE (%)	BSEC (%)	CO (%)	HC (ppm)	NO (ppm)	Smoke (%)
3.11	100	0	Exp.	23.57	13.6	0.02	12	837	14
			Pred.	23.64	13.57	0.021	20	847	25
			Error (%)	0.296	0.22	4.96	40	1.18	44
6.22	95	DGE5	Exp.	28.1	10.42	0.08	15	1108	29
			Pred.	27.78	10.68	0.078	14	1122	30
			Error (%)	1.15	2.4	2.56	7.14	0.36	3.33
12.44	95	DEE5	Exp.	32.21	9.79	0.11	19	1402	54
			Pred.	31.75	9.99	0.112	18	1399	51
			Error (%)	1.44	2	1.78	5.55	0.21	5.8
9.33	95	Bn5	Exp.	31.2	10.49	0.07	14	1282	28
			Pred.	31.18	10.5	0.071	13	1272	20
			Error (%)	0.06	0.095	1.4	7.6	0.78	40
9.33	95	MTBE5	Exp.	29.8	10.5	0.12	16	1301	43
			Pred.	30.15	10.32	0.115	15	1303	36
			Error (%)	1.16	1.74	4.34	6.67	0.15	19.4
6.22	90	DGE10	Exp.	28.45	10.72	0.09	15	1096	25
			Pred.	28.28	10.68	0.092	17	1093	39
			Error (%)	0.17	0.37	2.17	11.7	0.27	35.9
3.11	90	DEE10	Exp.	24.91	13.6	0.06	14	772	14
			Pred.	24.68	13.92	0.062	18	760	23
			Error (%)	0.93	2.3	3.21	22.2	1.58	39.13
9.33	90	Bn10	Exp.	31.81	10.48	0.05	13	1233	30
			Pred.	31.78	10.38	0.052	15	1226	26
			Error (%)	0.094	0.96	3.84	13.3	0.57	15.38
12.44	90	MTBE10	Exp.	32.12	9.82	0.15	21	1328	51
			Pred.	32.22	9.78	0.145	17	1339	40
			Error (%)	0.31	0.41	3.44	23.5	0.82	27.5

No competing interests reported.

NOx emission reduction in low viscous low cetane (LVLC) fuel using additives in CI engine – An experimental study

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental Setup

3. Experimental Procedure

4. Results and Discussion

5. Conclusion

Declarations

Author Contribution

References

Tables

Additional Declarations

Status:

Journal Publication

Version 1