Evaluating the Inﬂuence of Cetane Improver Additives on the Outcomes of a Diesel Engine Characteristics Fueled with Peppermint Oil Diesel Blend

: This paper aims to evaluate the impact of cetane improvers on the combustion, performance and emission characteristics of a compression ignition engine fueled with a 20% peppermint bio-oil/diesel blend (P20). It is hypothesized that the low viscosity and boiling point of peppermint oil could improve the atomization characteristics of the fuel. However, the usage of peppermint oil is restricted due to its low cetane index. To improve this, Diethyl Ether (DEE) and Di- tertiary Butyl Peroxide (DTBP) are added to the P20 blend. The tests are performed in a single-cylinder naturally aspirated water-cooled diesel engine and results indicate that NOx emission for P20 + DEE and P20 + DTBP is decreased by 10.4% and 9.8%, respectively, when compared to P20 at full load condition. Among these two cetane improvers, DTBP is more effective in reducing the CO, HC and smoke emission and the performance of the engine was reported to be higher for P20 + DTBP blends.


Introduction
Rapid industrialization and increased transportation had a significant impact on global warming and climate change. In countries like India, diesel engines are widely used for transportation and most industrial applications due to the higher efficiency and torque produced by diesel engines when compared with petrol engines [1]. These diesel engines also emit large amount of exhaust pollutants like carbon dioxide, hydrocarbon, nitrogen oxide, and soot particles. Among these pollutants, carbon dioxide emission plays a significant part in inducing global warming/climate change [2]. Life cycle emission of CO 2 is lower for eco-friendly biofuels, which can help to decelerate global warming [3]. Many researchers tried various biofuels, since the oil obtained from waste fatty acids, animal fat, vegetables and other biodegradable products have physical and chemical properties very similar to diesel fuel [4]. However, direct vegetable oil usage in diesel engines is not gaining importance due to its unfavourable properties such as high viscosity and low boiling point. Despite the use of viscosity-improvers having been advocated for [5], and hydro-treating gaining considerable attention [6], transesterification continues to be the main-frame process to reduce the high viscosity vegetable oil into low viscosity biodiesel [7]. This pertains to considerably lower infrastructural investments required to produce fatty acid methyl esters (FAME), yet tradeoffs with insufficient chemical properties to meet most automotive fuel quality standards as a stand-alone fuel. Due to these reasons, 7-20 percent of biodiesel blended with diesel is considered an acceptable solution, depending on the application [8].
Many researchers investigated alcohols such as methanol, ethanol [9,10] and higher alcohols such as isobutanol, n-butanol and pentanol as compression ignition (CI) engine fuels [11,12]. Among alcohol fuels, n-butanol has superior stability and vapour lock characteristics. Even though the alcohol fuels have lower cetane number and lower calorific value, it is highly admired for its lower boiling point and viscosity which aids to better atomization and fuel-air mixing [13]. Researchers also tried blending different types of fuels with diesel to form ternary blends to improve the fuel characteristics [14][15][16][17]. However, no significant improvement was noticed due to the blending of fuels with different physiochemical properties. Similar to alcohols, essential oils which are well known for their lower viscosity such as pine oil, eucalyptus oil, lemongrass oil and lemon peel oil have also been tried as a potential biofuel in diesel engines [18][19][20][21][22]. Even though both alcohols and essential oils have a lower cetane number, they can be used in CI engine by altering their ignition properties. Fuels' cetane number can be increased by reducing the fuel aromatic content through hydro-treating or by adding cetane improvers. The cetane number further depends on the composition of the fuel and its molecular structure [23]. Cetane improver can be added with low cetane fuels to improve the ignition qualities and reduce NO X emissions [24]. It will form free radicals by promoting the initiation reaction. Many cetane improvers like Di Ethyl Ether (DEE), Ethyl Hexyl Nitrate (EHN), Di Tertiary Butyl Peroxide (DTBP) are tried by many researchers for low cetane fuels in CI engine. Investigators Vallinayagam et al. examined the impact of cetane improvers: Iso Amyl Nitrate (IAN) and DTBP on 50% pine oil and 50% diesel blends in a diesel engine. They reported that NO X emission is decreased for DTBP blends by 6.4% more than IAN blends and also that performance was improved for DTBP blends [25]. Researchers Abhishek Paul et al. reported the effect of 5% and 10% DEE on Ethanol diesel blends in a diesel engine. NO X and particulate matter emission were reduced for the DEE-Ethanol blend. Particularly, a 25.95% reduction in NO X emission for 10% DEE blends was achieved [26]. Zhang et al. observed that the soot emission decreased by 80% for EHN addition into Dimethyl furan diesel blends and maximum pressure increases with an increase in the proportion of EHN. It was concluded that NO X emission increases slightly but THC emissions were drastically reduced [27]. Thiyagarajan Subramanian et al. (2018) examined the effects of Diglyme as a cetane enhancer in camphor oil diesel blends. They reported the improvement of ignition characteristics with the addition of 10% Diglyme. It was concluded that the cold starting problem and higher NO X emission were overcome by adding Diglyme to Camphor oil without affecting the performance [28]. Purushothaman et al. examined the addition of DEE to orange oil blends on evaluating emission. They reported that DEE is a potential additive for NO X emission reduction but it also influences the increase in the HC emission [29].
The possibility of running diesel engines with lower cetane fuels has encouraged us to investigate the use of peppermint oil in a CI engine through the performance, emission and combustion characters of the engine. The tests were conducted for different load values at a constant speed of 1500 rpm. From our previous study with peppermint oil in a CI engine, 20% peppermint oil mixture is taken as the optimum blend at higher load condition [30]. The P20 blend affects the fuel ignition and increases the NO X emission due to a lower cetane value of the peppermint oil. The use of 20% peppermint oil blended with two different ignition improvers, namely, DEE and DTBP, is considered in this study to reduce the emissions.

Test Fuels
Peppermint oil is an essential-oil based, low cetane fuel obtained from the peppermint leaves using the steam distillation process. The significant components of peppermint oil being eucalyptol, menthone, and menthyl acetate. The cetane improvers DEE and DTBP were procured from the sigma Aldrich. Additionally, diesel fuel and peppermint oil were purchased from the local suppliers. Five percent DEE is mixed with twenty percent peppermint oil denoted as P20 + DEE and 2% of DTBP was added with P20 denoted as P20 + DTBP, respectively. Blends are mixed using an ultrasonic agitator to create thorough mixing. The properties of test fuels, additives and the blends prepared are listed in Table 1.

Experimental Setup and Test Measurements
A water-cooled single-cylinder diesel engine coupled with an eddy current dynamometer was used in this study. The schematic of the experimental setup is depicted in Figure 1. The engine compression ratio is set at the standard value of 17.5:1, with the necessary swirl motion created using a hemispherical combustion chamber. The specifications of the test engine are mentioned in Table 2. An inductive pickup sensor was used to sense the engine speed and indicated the speed through a digital rpm indicator.

Test Fuels
Peppermint oil is an essential-oil based, low cetane fuel obtained from the peppermint leaves using the steam distillation process. The significant components of peppermint oil being eucalyptol, menthone, and menthyl acetate. The cetane improvers DEE and DTBP were procured from the sigma Aldrich. Additionally, diesel fuel and peppermint oil were purchased from the local suppliers. Five percent DEE is mixed with twenty percent peppermint oil denoted as P20 + DEE and 2% of DTBP was added with P20 denoted as P20 + DTBP, respectively. Blends are mixed using an ultrasonic agitator to create thorough mixing. The properties of test fuels, additives and the blends prepared are listed in Table 1.

Experimental Setup and Test Measurements
A water-cooled single-cylinder diesel engine coupled with an eddy current dynamometer was used in this study. The schematic of the experimental setup is depicted in Figure 1. The engine compression ratio is set at the standard value of 17.5:1, with the necessary swirl motion created using a hemispherical combustion chamber. The specifications of the test engine are mentioned in Table 2. An inductive pickup sensor was used to sense the engine speed and indicated the speed through a digital rpm indicator.   The 10 cc fuel consumed by the engine was calculated on a volumetric basis using a stopwatch and a two-way cock burette is used to calculate the flow rate of the fuel. The in-cylinder pressure was measured using a pressure transducer fitted on the cylinder head and the crank angle was recorded using an encoder fixed at the end of the crankshaft. The crank angle and pressure values are observed in the personal computer with the help of the data acquisition system which is used to process the signals from different sensors. At each crank angle, pressure data were recorded for 100 consecutive cycles and averaged. By using Engine soft software, HRR was computed from the measured cylinder pressure data. Emissions such as HC, CO and NOx are measured using AVL Di gas analyzer and smoke is measured using AVL 437C Smoke meter.
In the beginning, the engine was operated to run for 15 min in no-load condition to attain the steady-state operating condition. Additionally, after the change of each of each fuel blend, the engine was operated for 10 min in 50% load condition. The fuel mixtures filled in separate cans are introduced into the engine through a by-pass tube present in the top of the fuel level indicator unit. Engine speed is maintained at 1500 rpm during the experiments. Readings are taken for different load values from 0% load condition to 100% load condition in 25% increments. Exhaust gas emissions such as HC, CO, and NOx are measured using AVL digas analyser using a small probe at the tail pipe exhaust section of the engine. The analyser works under the nondispersive infrared (NDIR) principle to quantify these emissions. Smoke emission is measured using AVL smoke meter which uses opacimeter principle which is connected through a separate probe. HC and NOx emissions are measured in parts per million (ppm), whereas, CO and Smoke are measured in % vol. basis. Tests are repeated thrice to get higher reliability and the averaged values are used to determine the characteristics. Uncertainty indicates deviation in measured value based on errors. The uncertainties and accuracy of measuring instruments used in this experiment are listed in Table 3. The total uncertainties that prevailed in this experiment were calculated using the equation as stated in [31] and it was found to be 2.7%.

Results and Discussion
The combustion, performance and emission outcomes of the single-cylinder constant speed engine when operated with blends of peppermint oil are given in this section.

Combustion Analysis
This section presents the combustion results such as cylinder pressure, heat release rate, ignition delay and combustion duration obtained for different peppermint oil blends. Figure 2 indicates the in-cylinder pressure variation for P20, P20 + DEE, P20 + DTBP for 50% load value ( Figure 2a) and 100% load value ( Figure 2b) with respect to crank angle. In-cylinder pressure indicates the behavior of the engine operation. In CI Engine, the peak pressure of the cylinder relies on the amount of fuel burned in the premixed phase of combustion [32]. Among the various fuel blends, P20 experiences higher cylinder pressure during both the load conditions. The increase in peak cylinder pressure is due to a prolonged premixed combustion phase caused by higher ignition delay due to the lower cetane value of the biofuel, which facilitates more fuel to be accumulated and better fuel-air mixing process. Thus, more fuel is combusted in a shorter period leading to the increase in cylinder pressure. At 100% load, for P20 + DEE blend the peak in-cylinder pressure is 3.2% lesser than P20 but it is still higher than diesel. Similarly, at 50% load condition the difference between the two blends is 5.8%. Researchers reported that ignition delay was increased when DEE was added with diesel [12]. Contrary to this observation, ignition delay was slightly reduced while adding DEE with P20 blends in both the cases. This may be due to the reactions with diesel aromatics being suppressed by the chemical composition of peppermint oil and thus contributes to the drop in peak in-cylinder pressure. P20 + DTBP on the other hand exhibits a reduced cylinder pressure due to the reduction in ignition delay period due to its self-accelerated thermal decomposition nature [33].

Results and Discussion
The combustion, performance and emission outcomes of the single-cylinder constant speed engine when operated with blends of peppermint oil are given in this section.

Combustion Analysis
This section presents the combustion results such as cylinder pressure, heat release rate, ignition delay and combustion duration obtained for different peppermint oil blends.
3.1.1. Cylinder Pressure Figure 2 indicates the in-cylinder pressure variation for P20, P20 + DEE, P20 + DTBP for 50% load value ( Figure 2a) and 100% load value (Figure 2b) with respect to crank angle. In-cylinder pressure indicates the behavior of the engine operation. In CI Engine, the peak pressure of the cylinder relies on the amount of fuel burned in the premixed phase of combustion [32]. Among the various fuel blends, P20 experiences higher cylinder pressure during both the load conditions. The increase in peak cylinder pressure is due to a prolonged premixed combustion phase caused by higher ignition delay due to the lower cetane value of the biofuel, which facilitates more fuel to be accumulated and better fuel-air mixing process. Thus, more fuel is combusted in a shorter period leading to the increase in cylinder pressure. At 100% load, for P20 + DEE blend the peak in-cylinder pressure is 3.2% lesser than P20 but it is still higher than diesel. Similarly, at 50% load condition the difference between the two blends is 5.8%. Researchers reported that ignition delay was increased when DEE was added with diesel [12]. Contrary to this observation, ignition delay was slightly reduced while adding DEE with P20 blends in both the cases. This may be due to the reactions with diesel aromatics being suppressed by the chemical composition of peppermint oil and thus contributes to the drop in peak in-cylinder pressure. P20 + DTBP on the other hand exhibits a reduced cylinder pressure due to the reduction in ignition delay period due to its self-accelerated thermal decomposition nature [33].

Heat Release Rate
The rate of heat release for peppermint oil blends is shown in figure 3 for 50% load value ( Figure 3a) and 100% load value (Figure 3b). At both the load conditions, the peak HRR value for P20 blend is higher than diesel. This is due to the increase in ignition delay period associated with the usage of peppermint oil due to its lower cetane value. Extended ignition delay period accumulates more fuel, thus releases more heat energy during the combustion [34]. When DTBP is added with peppermint oil diesel blends, the peroxide group generates free alkaline-oxy radicals during the evaporation process, which leads to

Heat Release Rate
The rate of heat release for peppermint oil blends is shown in Figure 3 for 50% load value ( Figure 3a) and 100% load value (Figure 3b). At both the load conditions, the peak HRR value for P20 blend is higher than diesel. This is due to the increase in ignition delay period associated with the usage of peppermint oil due to its lower cetane value. Extended ignition delay period accumulates more fuel, thus releases more heat energy during the combustion [34]. When DTBP is added with peppermint oil diesel blends, the peroxide group generates free alkaline-oxy radicals during the evaporation process, which leads to better ignition attributes of the fuel blends. Accordingly, early SOC occurs for P20 + DTBP blend and the HRR curve swings away from TDC, thereby tumbling the level of Peak HRR. Thus, the DTBP blend shows the least Ignition delay period and also has the lowest Peak HRR value of 71.2 kJ/m 3 . A similar response is seen for DTBP even at 50% load value, showing a reduction of 7.9% compared with P20 blend. Addition of DEE improves the overall cetane value of the blend and reduces the ignition delay period but the cooling nature of DEE due to its higher latent heat property, slightly delays the start of combustion. P20 + DEE blend has a peak HRR value of 73.24 kJ/m 3 at 100 % load value and 50.84 kJ/m 3 at 50% load value. better ignition attributes of the fuel blends. Accordingly, early SOC occurs for P20 + DTBP blend and the HRR curve swings away from TDC, thereby tumbling the level of Peak HRR. Thus, the DTBP blend shows the least Ignition delay period and also has the lowest Peak HRR value of 71.2 kJ/m 3 . A similar response is seen for DTBP even at 50% load value, showing a reduction of 7.9% compared with P20 blend. Addition of DEE improves the overall cetane value of the blend and reduces the ignition delay period but the cooling nature of DEE due to its higher latent heat property, slightly delays the start of combustion. P20 + DEE blend has a peak HRR value of 73.24 kJ/m 3 at 100 % load value and 50.84 kJ/m 3 at 50% load value.

Ignition Delay and Combustion Duration
The combustion duration and ignition delay period at 50% load and 100% load values are shown in Figure 4a and Figure 4b, respectively. The values were calculated from the rate of heat release (ROHR) for all the four blends (diesel, P20, P20 + DEE, P20 + DTBP) obtained at both the load conditions. Ignition delay is measured from the difference between the crank angle at the start of injection (SOI) and the start of combustion (SOC).

Ignition Delay and Combustion Duration
The combustion duration and ignition delay period at 50% load and 100% load values are shown in Figure 4a,b, respectively. The values were calculated from the rate of heat release (ROHR) for all the four blends (diesel, P20, P20 + DEE, P20 + DTBP) obtained at both the load conditions. Ignition delay is measured from the difference between the crank angle at the start of injection (SOI) and the start of combustion (SOC). better ignition attributes of the fuel blends. Accordingly, early SOC occurs for P20 + DTBP blend and the HRR curve swings away from TDC, thereby tumbling the level of Peak HRR. Thus, the DTBP blend shows the least Ignition delay period and also has the lowest Peak HRR value of 71.2 kJ/m 3 . A similar response is seen for DTBP even at 50% load value, showing a reduction of 7.9% compared with P20 blend. Addition of DEE improves the overall cetane value of the blend and reduces the ignition delay period but the cooling nature of DEE due to its higher latent heat property, slightly delays the start of combustion. P20 + DEE blend has a peak HRR value of 73.24 kJ/m 3 at 100 % load value and 50.84 kJ/m 3 at 50% load value.

Ignition Delay and Combustion Duration
The combustion duration and ignition delay period at 50% load and 100% load values are shown in Figure 4a and Figure 4b, respectively. The values were calculated from the rate of heat release (ROHR) for all the four blends (diesel, P20, P20 + DEE, P20 + DTBP) obtained at both the load conditions. Ignition delay is measured from the difference between the crank angle at the start of injection (SOI) and the start of combustion (SOC).   The ignition delay was higher in both the load conditions for P20 blends due to their reduced cetane value. With the addition of cetane improvers, ignition delay was significantly reduced. However, the ignition delay of P20 with DEE's addition was not considerably reduced due to the retardation of dynamic injection timing due to its high latent heat and lower viscosity. Combustion duration was computed based on the crank angle difference between 90% cumulative HRR and SOC. The duration of combustion for diesel and P20 at full load condition are 49 and 46 crank angle degree (CAD), respectively. This combustion duration reduction for P20 blends compared with diesel is due to the rapid burning rate of P20 blends as more fuel being burnt due to the higher ignition delay period. Moreover, the blends with peppermint oil enhance the combustion due to the excess oxygen content, and thus a relatively lower combustion duration period than diesel fuel is noticed in both the load conditions. The combustion duration of P20 + DTBP blend is 44 CAD at full load condition and 60 CAD at 50% load condition. This is comparatively lesser than both P20 and diesel. This reduction indicates that a quicker combustion process happens while mixing DTBP with peppermint oil blend.

Performance Analysis
This section presents the results that determine the engine's performance, such as brake-specific fuel consumption and brake thermal efficiency.

Brake Specific Fuel Consumption
BSFC implies the amount of fuel consumed by the engine for producing unit brake power. The BSFC variation for various test fuel blends is indicated in Figure 5. BSFC for the P20 blend is less than diesel at full load condition. This may be ascribed to better fuel properties of peppermint oil, leading to an improved combustion process. Adding 5% DEE to peppermint oil blend decreased the BSFC further at all loads. Even though DEE's calorific value is lower compared to diesel, its higher oxygen content of around 21.6% helps to improve the combustion efficiency, leading to lesser energy requirement. The results obtained match with the previous research work done by Amr Ibrahim et al. [35] for biodiesel DEE blend. BSFC of P20 + DTBP blend is less than P20 due to better burning, which is influenced by an increase in the blends' cetane number. As DTBP undergoes homolysis and efficiently induces the combustion process, more fuel molecules were involved in the combustion, thereby reducing the overall BSFC value. Compared with diesel at full load, 7.47% and 11.37% reduction is noticed for P20 + DEE and P20 + DTBP, respectively.

Brake Thermal Efficiency
BTE indicates the work output to the energy supplied in an engine. Enhanced ignition and combustion process obtained by increasing the Peppermint oil's cetane value using Diethyl ether and di tert butyl peroxide leads to the improvement in BTE. From Figure 6, it can be seen that BTE for P20 + DTBP is superior to P20 + DEE and P20 blends at all load values. The improvement in BTE with the usage of 20% peppermint oil can be attributed to the properties of the oil itself. The higher oxygen concentration with lower viscosity and boiling point helped to improve the atomization of the fuel molecules which leads to better evaporation. This improves the combustion process leading to the increase in BTE of the P20 blends. Though there was an improvement of about 4.31% compared to diesel at full load condition, further improvement might be possible considering the bio oil's lower cetane value. Hence, DTBP and DEE are added to this P20 blend. With the addition of DEE and DTBP, the improvement achieved is shown in the graph. DEE increases the fuel's cetane value, whereas DTBP improves the ignition quality by self-accelerated decomposition and reduces the ignition delay period [36]. Higher brake thermal efficiency for P20 + DTBP may be attributed to higher in-cylinder pressure induced by the blends' enhanced fuel properties. It is also reflected in reduced combustion duration for these fuel blends. Enhancements in the engine's performance as determined with the present study concurs well with the reports of Vedharaj et al., that pine oil diesel blends with DTBP as an ignition enhancer [25]. Energies 2021, 14, x FOR PEER REVIEW 8 of 16

Brake Thermal Efficiency
BTE indicates the work output to the energy supplied in an engine. Enhanced ignition and combustion process obtained by increasing the Peppermint oil's cetane value using Diethyl ether and di tert butyl peroxide leads to the improvement in BTE. From Figure  6, it can be seen that BTE for P20 + DTBP is superior to P20 + DEE and P20 blends at all load values. The improvement in BTE with the usage of 20% peppermint oil can be attributed to the properties of the oil itself. The higher oxygen concentration with lower viscosity and boiling point helped to improve the atomization of the fuel molecules which leads to better evaporation. This improves the combustion process leading to the increase in BTE of the P20 blends. Though there was an improvement of about 4.31% compared to diesel at full load condition, further improvement might be possible considering the bio oil's lower cetane value. Hence, DTBP and DEE are added to this P20 blend. With the addition of DEE and DTBP, the improvement achieved is shown in the graph. DEE increases the fuel's cetane value, whereas DTBP improves the ignition quality by self-accelerated decomposition and reduces the ignition delay period [36]. Higher brake thermal efficiency for P20 + DTBP may be attributed to higher in-cylinder pressure induced by the blends' enhanced fuel properties. It is also reflected in reduced combustion duration for these fuel blends. Enhancements in the engine's performance as determined with the present study concurs well with the reports of Vedharaj et al., that pine oil diesel blends with DTBP as an ignition enhancer [25].

Emission Analysis
This section presents the results that quantify the emissions produced from the engine, such as carbon monoxide (CO), unburned hydrocarbon (UBHC), nitrogen oxides (NOx) and Smoke.

Carbon Monoxide
Since the diesel engine is operated in a lean mixture range, its CO emission is considerably lower. Usually, the cylinder's temperature, oxidation rate and the fuel spray pattern influence the CO formation [37]. From Figure 7 it is understood that CO emission for

Emission Analysis
This section presents the results that quantify the emissions produced from the engine, such as carbon monoxide (CO), unburned hydrocarbon (UBHC), nitrogen oxides (NOx) and Smoke.

Carbon Monoxide
Since the diesel engine is operated in a lean mixture range, its CO emission is considerably lower. Usually, the cylinder's temperature, oxidation rate and the fuel spray pattern influence the CO formation [37]. From Figure 7 it is understood that CO emission for P20 was higher than diesel fuel at low loads and reduced at higher loads. The main reason might be that the oxidation rate will be lower at low temperatures due to delayed combustion onset, and that CO emission is formed due to the insufficient oxygen molecules; the inherent oxygen concentration in the biofuel and the additives also play a vital role in the reduction in CO emission [38]. Thus, CO emission is reduced with the usage of peppermint bio oil. Compared to diesel, CO emission was reduced by 11.9% and 18.57% when DEE and DTBP are added with P20; however, P20 + DEE has significantly less impact than DTBP. The higher latent heat of vaporization property of DEE makes the blend produce more CO than DTBP. This is because the higher latent heat of vapourisation induces a cooling effect inside the combustion chamber [39]; this affects the combustion itself and results in higher CO emission in the exhaust. Combustion initiation was advanced to be noticed for P20 + DTBP blends causing complete combustion, leading to a further CO emission reduction.

Unburnt Hydrocarbon
Unburned hydrocarbon emissions or simply hydrocarbon emissions are the fuel molecules that escape to the atmosphere without taking part in the combustion process. From Figure 8, it is to be noted that UBHC emission for the P20 + DTBP blend was lower among all fuel blends. With the addition of DTBP to the P20, the blend's combustion quality improves, leading to a decrease in the ignition delay. It promotes the prior onset of ignition, giving more time for oxidation and improved combustion, thus contributing to HC emission reduction. The addition of DEE reduces HC emission by 11.54% and 24.5% more than P20 and diesel fuel, respectively. This is due to the improvements caused in the oxygen concentration, boiling point, viscosity and cetane value of the blend. However, HC emission for P20 + DEE blend was higher than P20 + DTBP blend by 17.95%. This increase in HC emission is due to the reduced in-cylinder temperature caused by the cooling effect of DEE's higher latent heat property. This HC emission reduction was comparable with the results of Samuel et al. for diesel pine oil blends with the addition of cetane improver 1,4 dioxane [40]. HC emission was reduced for P20 + DTBP blend by 35.49% and 24.33% at full load condition compared to diesel and P20 blend.

Unburnt Hydrocarbon
Unburned hydrocarbon emissions or simply hydrocarbon emissions are the fuel molecules that escape to the atmosphere without taking part in the combustion process. From Figure 8, it is to be noted that UBHC emission for the P20 + DTBP blend was lower among all fuel blends. With the addition of DTBP to the P20, the blend's combustion quality improves, leading to a decrease in the ignition delay. It promotes the prior onset of ignition, giving more time for oxidation and improved combustion, thus contributing to HC emission reduction. The addition of DEE reduces HC emission by 11.54% and 24.5% more than P20 and diesel fuel, respectively. This is due to the improvements caused in the oxygen concentration, boiling point, viscosity and cetane value of the blend. However, HC emission for P20 + DEE blend was higher than P20 + DTBP blend by 17.95%. This increase in HC emission is due to the reduced in-cylinder temperature caused by the cooling effect of DEE's higher latent heat property. This HC emission reduction was comparable with the results of Samuel et al. for diesel pine oil blends with the addition of cetane improver 1,4 dioxane [40]. HC emission was reduced for P20 + DTBP blend by 35.49% and 24.33% at full load condition compared to diesel and P20 blend.

. Nitrogen Oxides
Nitrogen oxides is the combination of two toxic gases such as nitric oxide (NO) and nitrogen dioxide (NO2). These two gases are formed by the combination of nitrogen and oxygen at higher temperatures inside the combustion chamber. Previous studies indicate that low cetane fuels such as Pine oil, Eucalyptus oil, and Methanol emit higher NOX emission when blended with diesel [10,20,41]. The primary cause for this increase in NOX emission is higher peak HRR. In conformity with this, P20 diesel blends produce 10.16% (at full load) higher NOX emission than diesel, as shown in Figure 9. With the addition of DEE in P20 blends, NOX emission was reduced by 2.33% compared to P20 blend. The primary cause for reducing the NOX emission was DEE's cooling effect due to its high latent heat property [42]. These results are well in accordance with the work by Paul et al. for diesel DEE blends [26]. Though NOX emission was reduced for the P20 + DTBP blend compared with P20 blend, it is slightly higher than that of the P20 + DEE blend by 1.14% at full load. The addition of DTBP reduces the ignition delay, leading to a fall in peak heat release rate, causing an overall reduction in cylinder temperature, thereby contributing to NOX reduction compared to diesel and P20 blend. At full load condition, nitrogen oxides emission is reduced by 10.4 and 9.8% for P20 + DEE and P20 + DTBP blends, respectively, compared to the P20 blend.

Nitrogen Oxides
Nitrogen oxides is the combination of two toxic gases such as nitric oxide (NO) and nitrogen dioxide (NO 2 ). These two gases are formed by the combination of nitrogen and oxygen at higher temperatures inside the combustion chamber. Previous studies indicate that low cetane fuels such as Pine oil, Eucalyptus oil, and Methanol emit higher NO X emission when blended with diesel [10,20,41]. The primary cause for this increase in NO X emission is higher peak HRR. In conformity with this, P20 diesel blends produce 10.16% (at full load) higher NO X emission than diesel, as shown in Figure 9. With the addition of DEE in P20 blends, NO X emission was reduced by 2.33% compared to P20 blend. The primary cause for reducing the NO X emission was DEE's cooling effect due to its high latent heat property [42]. These results are well in accordance with the work by Paul et al. for diesel DEE blends [26]. Though NO X emission was reduced for the P20 + DTBP blend compared with P20 blend, it is slightly higher than that of the P20 + DEE blend by 1.14% at full load. The addition of DTBP reduces the ignition delay, leading to a fall in peak heat release rate, causing an overall reduction in cylinder temperature, thereby contributing to NO X reduction compared to diesel and P20 blend. At full load condition, nitrogen oxides emission is reduced by 10.4 and 9.8% for P20 + DEE and P20 + DTBP blends, respectively, compared to the P20 blend.  Figure 10 indicates smoke emission for all the blends in comparison with diesel fuel at different load values. The addition of peppermint oil with diesel fuel improves the oxygen concentration and atomization due to its lower viscosity nature. This helps in better vaporization of the fuel molecules and improves the combustion process, leading to an increased temperature inside the combustion chamber. This helps in better oxidation of soot and reduces smoke considerably compared to diesel fuel [43]. Smoke emission was further reduced by adding DEE with P20 blend. This is due to the increased oxygen contribution, cetane value and reduced viscosity by di-ethyl ether additive, further improving the combustion resulting in better soot oxidation and reduced smoke emission [44]. The smoke emission for the P20 + DTBP blend is only slightly lesser than the P20 + DEE blend. The addition of DTBP to P20 blend increases the premixed combustion phase and contributes to reducing the smoke emission. At full load condition, smoke emission is reduced by 7%, 10.58% and 12.6% for P20, P20 + DEE and P20 + DTBP, respectively, compared to diesel fuel.  Figure 10 indicates smoke emission for all the blends in comparison with diesel fuel at different load values. The addition of peppermint oil with diesel fuel improves the oxygen concentration and atomization due to its lower viscosity nature. This helps in better vaporization of the fuel molecules and improves the combustion process, leading to an increased temperature inside the combustion chamber. This helps in better oxidation of soot and reduces smoke considerably compared to diesel fuel [43]. Smoke emission was further reduced by adding DEE with P20 blend. This is due to the increased oxygen contribution, cetane value and reduced viscosity by di-ethyl ether additive, further improving the combustion resulting in better soot oxidation and reduced smoke emission [44]. The smoke emission for the P20 + DTBP blend is only slightly lesser than the P20 + DEE blend. The addition of DTBP to P20 blend increases the premixed combustion phase and contributes to reducing the smoke emission. At full load condition, smoke emission is reduced by 7%, 10.58% and 12.6% for P20, P20 + DEE and P20 + DTBP, respectively, compared to diesel fuel.

Conclusions
This study intends to investigate the scope of using peppermint oil influenced by two cetane improver additives in a single-cylinder diesel engine. The following outcomes were observed in this study.

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The peak HRR and NOX emission were higher for P20 blends than diesel due to reduced cetane number, but CO, HC and smoke emissions were reduced significantly.

•
The peak HRR was reduced with the addition of cetane improver DTBP and DEE in the P20 blend. The NOX emission was reduced by 10.4% and 9.8% for P20 + DEE and P20 + DTBP blends compared to P20 blend due to reduced peak HRR. Improved NOX emission reduction was observed for P20 + DEE due to its higher latent heat. • Compared with diesel, P20 + DTBP blend shows improved reduction in CO, HC and Smoke emissions by 18.57%, 35.49% and 12.6% at full load condition.

•
The performance characteristic such as BTE and BSFC were improved when compared with P20 blends.
With cetane improvers, 20% peppermint oil can be a potential alternative fuel for diesel engines and we hope to increase the biofuel percentage in the future. The major advantages in the obtained results are the reduction in the level of nitrogen oxides emission, smoke emission, fuel consumption and the improvement in thermal efficiency. Additionally, this approach can ensure that the fuel blends used in this study can be adopted in most of the engines without the need for any major modifications. Moreover, the usage of biofuels means the reduction in carbon dioxide emission during the overall life cycle analysis starting from the crop plantation to the exhaust emitted from the engine, thereby contributing towards the reduction in greenhouse gases in the long run.

Conclusions
This study intends to investigate the scope of using peppermint oil influenced by two cetane improver additives in a single-cylinder diesel engine. The following outcomes were observed in this study.

•
The peak HRR and NO X emission were higher for P20 blends than diesel due to reduced cetane number, but CO, HC and smoke emissions were reduced significantly.

•
The peak HRR was reduced with the addition of cetane improver DTBP and DEE in the P20 blend. The NO X emission was reduced by 10.4% and 9.8% for P20 + DEE and P20 + DTBP blends compared to P20 blend due to reduced peak HRR. Improved NO X emission reduction was observed for P20 + DEE due to its higher latent heat. • Compared with diesel, P20 + DTBP blend shows improved reduction in CO, HC and Smoke emissions by 18.57%, 35.49% and 12.6% at full load condition.

•
The performance characteristic such as BTE and BSFC were improved when compared with P20 blends.
With cetane improvers, 20% peppermint oil can be a potential alternative fuel for diesel engines and we hope to increase the biofuel percentage in the future. The major advantages in the obtained results are the reduction in the level of nitrogen oxides emission, smoke emission, fuel consumption and the improvement in thermal efficiency. Additionally, this approach can ensure that the fuel blends used in this study can be adopted in most of the engines without the need for any major modifications. Moreover, the usage of biofuels means the reduction in carbon dioxide emission during the overall life cycle analysis starting from the crop plantation to the exhaust emitted from the engine, thereby contributing towards the reduction in greenhouse gases in the long run.
Funding: This research received no external funding, and The APC was funded by University of Vaasa.