Experimental Investigation of Compression Ignition Engine Combustion, Performance, and Emission Characteristics of Ternary Blends with Higher Alcohols (1-Heptanol and n-Octanol)

Abstract

Concerns about the depletion of petroleum reserves and rising pollution led researchers to search for alternate and environmentally compatible fuels for compression ignition engines. As an excellent alternative fuel additive to biodiesel–diesel blends, higher alcohol exhibits outstanding fuel properties (such as high energy content and cetane number) and can operate in diesel engines without requiring engine changes. This study focuses on investigating the ternary blends comprising higher alcohols, namely 1-heptanol and n-octanol, in hybrid biodiesel (animal fat oil–rice bran oil–cottonseed oil) and diesel on compression ignition engine characteristics. The performance, combustion, and emissions of a diesel engine fuelled with mono (D100), binary (B20), and ternary fuel blends (B20H10, B20H20, B20O10, and B20O20) were analysed at a constant engine speed of 1500 rpm. The test fuels met the American Society for Testing and Materials standards for fuel properties and exhibited stable behaviour during testing. Experimental results showed that at 100% load, the least brake-specific fuel consumptions for diesel fuel, B20, B20H10, B20H20, B20O10, and B20O20 were 254.1 g/kWh, 302.14 g/kWh, 281.25 g/kWh, 310.94 g/kWh, 292.8 g/kWh, and 313.80 g/kWh, respectively. Meanwhile, the maximum brake thermal efficiency values were obtained as 38.65%, 37.01%, 37.76%, 36.84%, 37.12%, and 36.38%, respectively. At 100% load, the peak heat release rates for diesel, B20, B20H10, B20H20, B20O10, and B20O20 were found to be 64.65 J/deg, 59.07 J/deg, 62.34 J/deg, 56.12 J/deg, 57.95 J/deg, and 51.9 J/deg, respectively. The addition of 1-heptanol and n-octanol as oxygenated additives into the ternary blend resulted in decreased carbon monoxide and unburned hydrocarbon emissions while increasing carbon dioxide and nitrous oxide emissions compared to diesel fuel. Overall, the study concludes that ternary blends with 1-heptanol and n-octanol as additives improve performance and combustion behaviour and reduce exhaust emissions compared to binary blends.

1. Introduction

Energy has become a crucial topic in both the global economy and human life, with its advancement and usage significantly impacting the sustainability of the human community [1]. Unfortunately, the escalating demand for conventional diesel fuel is rapidly depleting the world’s finite petroleum reserves [2,3]. Diesel engines, being highly efficient powertrains, play a vital role in powering various sectors of the global economy, including industry, agriculture, and transportation [4,5]. However, the utilization of fossil fuels in these engines releases substantial amounts of hazardous pollutants like soot and nitrogen oxides (NO_x) [6]. Consequently, the pressing issues of climate change, acid rain, global warming, and increased greenhouse gas emissions have prompted governments and stakeholders to tighten pollution regulations, driving the academic community to seek long-term solutions for securing future energy supplies [7].

In response, researchers have conducted numerous studies on alternative fuels over the last few decades. Among these options, biodiesel is derived from a range of sources, including both edible and non-edible oilseeds. Edible oilseeds, such as rice bran oil, soybean, linseed, rapeseed, sunflower, safflower, peanut, cashew nut, and coconut, serve as viable feedstocks for biodiesel production [8]. Additionally, biodiesel can also be obtained from waste cooking oil and animal fat, like meat tallow [9,10]. In the case of non-edible oilseeds, feedstocks such as mahua, neem, cottonseed, Karanja, and jatropha can be utilized to produce biodiesel [11]. These diverse sources provide ample options for the production of biodiesel to meet various energy needs for traditional diesel fuel in the energy mix [5]. Transportation uses a lot of fossil fuels, mostly in internal combustion engines. Diesel engines are preferred for their combustion efficiency, dependability, cost-effectiveness, and flexibility [12]. Due to their widespread use, harmful exhaust gas emissions have increased. Vehicle and engine use in cities are rising, causing environmental degradation [2,4]. Vehicles emit large volumes of unburned hydrocarbons (HCs), carbon monoxide (CO), and NO_x in densely populated cities [1]. Pollutants harm humans and the environment [13]. As a result, experts throughout the world are searching for options to reduce pollution and achieve energy independence [14].

In general, biodiesel has a greater cetane number than conventional diesel fuel, resulting in a shorter ignition delay period, implying that the engine starts quickly and runs smoothly [10,15,16,17]. Biodiesel is a beneficial fuel as it possesses a higher flash point and cetane number than diesel [15]. Butanol proportions of more than 10% in a biodiesel blend reduce the cetane number and increase the water content [16]. Alcohols (n-propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, and so on) contribute to next-generation biofuels possessing excellent fuel characteristics, resulting in reduced CO and NO_x emissions and ignition delay with n-butanol addition to mahua biodiesel [17]. Several researchers [18,19,20] reported that biodiesels frequently have lower cetane numbers (CNs) than the minimal limitations established in standards such as EN 14, 214 (minimum 51) and ASTM D6751 (minimum 47). This lower CN can be due to biodiesels’ greater quantities of unsaturated fatty acids. Lower CN causes an increase in ignition delay and NO_x emissions [18]. When compared to regular diesel fuel, biodiesel with alcohols reduces NO_x emissions and improves the combustion properties [19]. The inclusion of pyrogallol enhances the cetane number in Moringa oleifera biodiesel, improving BTE and lowering NO_x emissions [20]. Much research has explored how biodiesel affects diesel engine combustion and emissions. Yesilyurt et al. [21] examined the effects of biodiesel with diesel fuel at various fractions. Biodiesel increased exhaust gas temperature, NO_x, and carbon dioxide (CO₂) while decreasing brake thermal efficiency, CO, and smoke emissions. The authors [22,23] examined diesel engine performance and emissions using biodiesel/diesel blends with different biodiesel fractions. B100 had the lowest torque [24]. Due to biodiesel’s oxygenation, blends produced more NO_x and less CO and smoke. Ge et al. [25] compared diesel and canola oil biodiesel emissions. Biodiesel had decreased peak pressure, thermal efficiency, CO, and PM emissions but higher NO_x emissions [26,27]. Further, biodiesel to diesel fuel increases brake-specific fuel consumption (BSFC), NO_x emissions, HC emissions, CO emissions, CO₂ emissions [28,29], and particulate matter (PM) emissions [30,31]. Due to decreasing energy content, brake thermal efficiency (BTE) has changed to a certain limit [32]. Biodiesel’s high viscosity, poor low-temperature behaviour, and low volatility reduce combustion efficiency and quality, especially at low temperatures. Biodiesel’s low volatility and high viscosity also affect fuel atomization during injection [33,34].

Biodiesels have poor low-temperature characteristics, causing crystalline formations and fatty acid solidification [12]. Blending biodiesels with alcohols of different chain lengths can solve these concerns. Alcohols contain more oxygen than biodiesel and diesel fuel, which can ameliorate their drawbacks. Biomass fermentation produces liquid biofuels including methanol, ethanol, and butanol. They are essential spark ignition and compression ignition (CI) engine fuel alternatives [35,36]. Cheaper and more environmentally friendly than other methods are alcohol fermentation [37]. Alcohols have been extensively researched as engine fuels in recent decades. This adoption reduces air pollution and promotes renewable energy [19]. Methanol and ethanol have been extensively studied for biodiesel production in micro-reactors [38]. Methanol and ethanol are mixed with diesel fuel, biodiesel, or biodiesel/diesel fuel mixes to run the diesel engines [39,40,41]. NO_x emissions are decreased when ethanol is added to a biodiesel–diesel blend [39]. Blends of biodiesel, diesel, and ethanol lower the levels of NO_x, CO, HC, and smoke opacity [40]. However, CO₂ emissions are increased. The thermal efficiency of diesel fuel tends to be improved by the addition of methanol and ethanol, although higher NO_x emissions are a trade-off [41]. The major drawbacks of lower alcohols (ethanol and methanol) are low cetane number and flash point and difficulty in handling and storage [19]. To limit the disadvantages of lower alcohols, the higher alcohols with biodiesel–diesel blends are of significant relevance.

Alcohol-treated fuels reduce engine torque, CO, and smoke emissions while increasing BSFC. Oxygen-rich alcohols improve premixed and diffusive combustion [26]. Recent research has concentrated on higher alcohols, rather than lower chain alcohols, as diesel fuel substitutes in terms of combustion. Higher alcohols (alcohols with long chains of molecules), like C4–C8 and C4–C20 alcohols, are receiving more attention because they have more benefits than lower alcohols [42]. Higher alcohols are better diesel alternatives since they have similar physicochemical features [43]. Higher alcohols are long-chain alcohols with molecular weights and carbon atoms over three such as butanol, pentanol, hexanol, heptanol, octanol, decanol, and phytol [44]. Higher alcohols offer promising capabilities for diesel engines such as higher energy content, the ability to mix well with diesel (without any phase separation), superior density, lubricity, better ignition, less corrosion due to less water content, safer handling, and a broad range of flash points [45,46]. CI engines benefit from higher alcohols with long-chain carbon atoms. Higher alcohols have more diesel engine potential than lower alcohols [26]. Heptanol has a longer carbon chain structure, making it a better diesel engine fuel [21,47,48]. Heptanol has an increased cetane number and calorific value. Its decreased latent heat of vaporization hinders cold-start operations [6]. Across all load levels, heptanol in diesel fuel reduces CO and HC emissions but increases CO₂ and NO_x emissions in a single-cylinder diesel engine [21]. Nour et al. [40] evaluated the performance of a single-cylinder diesel engine operating at various loads and speeds while using heptanol at a volume ratio of 10% to 50% [49]. Alcohols and their blends with diesel can run at lower fuel-to-air ratios due to the oxygen in their structure [5]. They also have a higher octane rating, leading to a delayed ignition that produces more premixed combustion and less soot and smoke [44]. Most studies on higher alcohol diesel blends have focused on combustion studies, while fuel properties of higher alcohol diesel blends have received less attention [47,49,50]. Beta oxidation reversal, expanding the 1-butanol route, rerouting branched-chain amino acid biosynthesis, and microorganisms like Escherichia coli and Clostridium species can create octanol isomers [51,52]. n-octanol’s fuel properties including cetane number, calorific value, lower vapour pressure, and flash point outperform those of other higher alcohols (<C8), making it appropriate for biodiesels. Due to its 0.08 mm Hg vapour pressure, n-octanol is safe to store, handle, and transport [53]. Directly fuelling a diesel engine with biodiesel and alcohol causes combustion difficulties, rendering it unsuitable. Researchers have investigated binary mixtures such as diesel–biodiesel, biodiesel–alcohol, and diesel–alcohol. Recently, ternary blends of biodiesel–alcohol–diesel have been studied to minimize diesel fuel use. Ternary fuels possess a tendency to enhance performance features and minimize exhaust pollutants [46,54]. Recent investigations have studied higher alcohols in ternary fuels [52,55,56,57]. Higher-alcohol-based decanol was added to neem biodiesel and diesel to enhance engine performance and reduce emission characteristics without engine modification [55]. Ternary blends (mustard seed oil biodiesel, diesel, and alcohol: butanol and pentanol) showed improved performance in reducing emissions without compromising performance with brake power and torque [56]. Calophyllum inophyllum biodiesel, diesel, and additives (nanoparticles, alcohols, and antioxidants) forming ternary blends showed improved performances in diesel engines [57]. Not much research effort has been made with the use of higher alcohols (1-heptanol and n-octanol), biodiesel, and diesel as ternary blends to evaluate engine combustion, performance, and emission characteristics.

Motivation, Novelty, and Objective of the Study

According to the literature review, there is a significant research gap regarding the performance, combustion behaviour, and environmental characteristics of diesel engines running on 1-heptanol and n-octanol blends, especially at low mixing proportions of up to a 20% volume. To the best of the author’s knowledge, there is limited research on 1-heptanol and n-octanol-blended hybrid biodiesel (animal fat oil–rice bran oil–cottonseed oil)–diesel fuel blends in CI engines to assess performance, emissions, and combustion characteristics at different engine loads (25%, 50%, 75%, and full load) and a fixed engine speed of 1500 rpm. This experimental study is unique in that it bridges the research gap by thoroughly evaluating engine performance indicators (BSFC and BTE), combustion characteristics (in-cylinder pressure and net heat release rate), and exhaust emissions (CO, CO₂, HC, and NO_x emissions). The research adopted a single-cylinder, four-stroke, water-cooled, common rail direct injection (CRDI) computerized diesel engine that ran on diesel fuel, pure hybrid oil biodiesel, diesel/hybrid oil biodiesel, diesel/hybrid oil biodiesel/1-heptanol blends and diesel/hybrid biodiesel/n-octanol blends. Lower amounts of 1-heptanol and n-octanol were used in this study for a variety of reasons. For starters, a modest quantity of 1-heptanol (10% and 20% by volume) and n-octanol (10% and 20% by volume) can be blended with diesel/biodiesel without requiring engine modifications, saving money on engine and component modifications. Secondly, 1-heptanol and n-octanol have a lower manufacturing capacity and a greater cost than regular diesel fuel. As a result, the emphasis was on assessing lower concentrations of 1-heptanol and n-octanol. The results of the 1-heptanol and n-octanol blends were compared to those of higher alcohol blends, and thorough explanations were offered. Overall, this study fills a research gap and provides valuable insights into the performance, emissions, and combustion characteristics of diesel engines fuelled by 1-heptanol and n-octanol blends, particularly at lower mixing proportions, adding to existing knowledge on alternative diesel fuel options.

2. Materials and Methods

2.1. Preparations of the Fuels

Ternary fuel samples were prepared using the splash blending method, incorporating conventional diesel fuel (D100) as a reference. To prepare binary blend B20 and ternary blends B20H10, B20H20, B20O10, and B20O20, the volume percentages of 1-heptanol and n-octanol were increased from 10% to 20%. The properties of higher alcohols are presented in

Table 1. The properties of hybrid biodiesel, mineral diesel fuel, and higher alcohols.

Properties	Units	D100 a	HBO	1-Heptanol b	n-Octanol c
Density	kg/m3	828	880.4	822	827
Viscosity at 40 °C	mm2/s	2.578	4.33	5.75	7.59
Heating Value	MJ/kg	42.54	38.91	39.92	38.53
Cetane Number		40–55	53	23	39
Flash Point	°C	65–70	159	70	81
Cloud Point	°C	−11.9	−8	−5 to −10	-
Pour Point	°C	−15.8	−15	-	-
Molecular formula		C14–C25	--	C7H15OH	C8H17OH
Molecular weight	wt.	198.4	--	116.19	130.21
Carbon	wt.%	87.05	77.2	72.16	73.72
Hydrogen	wt.%	12.95	11.4	13.71	13.82
Oxygen content	wt.%	6.72	11.3	14.13	15.7
Boiling point	°C	180–370	--	175	195
Latent heating at 25 °C	kJ/kg	270	--	574	315
Autoignition temperature	°C	210–260	275	253	270

^a Values adapted from Refs. [58,59,60]; ^b Values adopted [19,47]; ^c Values adopted [49,61].

. The binary and ternary blends were selected after conducting pilot experiments at the research laboratory. Magnetic stirring was employed to ensure blend quality. The fuel samples were stored in sealed bottles under laboratory conditions for one week at 27 °C without phase separation as shown in Figure 1.

2.2. Testing of the Fuels

The high-quality hybrid biodiesel derived from ternary oils (TOs) of animal fat–rice bran–cottonseed was produced by employing a single-stage transesterification method (SSTM) in a laboratory-scale biodiesel processor in the Biofuel Laboratory of the Department of Mechanical Engineering at Visvesvaraya Technological University in Mysuru, Karnataka, India. Prior to the SSTM, animal fat–rice bran–cottonseed oils were mixed to produce TOs. The SSTM was undertaken by employing esterified TOS in the presence of NaOH as an alkali catalyst. The physicochemical properties of the hybrid biodiesel, mineral diesel fuel, and higher alcohols are presented in Table 1.

2.3. Test Engine and Facilities

The engine performance, exhaust gas emissions, and combustion characteristics of the prepared fuel blends were evaluated at the Biodiesel Research and Demonstration Centre, located at Vidyavardhaka College of Engineering in Mysuru, Karnataka, India. The testing was conducted on an engine test rig using a single-cylinder, four-stroke, naturally aspirated, water-cooled computerized engine. This engine had a bore of 87.5 mm, a stroke of 110 mm, and a capacity of 661 cc. The overall dimension of an engine is W 2000 × D 2500 × H 1500 mm. Detailed technical specifications of the engine can be found in

Table 2. Technical specifications of the test engine.

Description	Specification
Product	Code 244, CRDI-VCR research engine test setup
Type	Kirloskar make, engine one-cylinder, 4-stroke
Cooling Type	Water-cooled
Power	3.5 kilo Watt
Varying CR	12:1 to 18:1
Common Rail	With pressure regulation and pressure sensor
Dynamometer	Eddy current with loading
ECU	Nira i7r with programmable and solenoid injector driver
Fuel Injector	Solenoid-driven
Airbox	With an orifice meter (MS-fabricated) and manometer
Fuel Tank	With fuel metering pipe of glass (15-L capacity)
Calorimeter	Pipe-in-pipe
Data Acquisition Software	Engine soft. performance and analysis software
Rotameter	Cooling (40–400) Iph, calorimeter (25–250) lph, Eureka make
Propeller Shaft	Universal joints, Hindustan Hardy make

. The experimental set-up and measurement system are depicted in Figure 2, while a photograph of the test rig is provided in Figure 3.

Cylinder pressure (CP) and net heat release rate (NHRR) measurements are used to determine the combustion characteristics of a CI engine running on various fuel types. A typical injection timing of 23° before the top dead centre (TDC) was used for experiments.

Table 3. Specifications of Sensors and Transmitters in the Test Rig.

Sensor/Transmitter	Measurement Range	Make	Relative Uncertainty
Piezo pressure sensor	0 to 350 bar	PCB USA make	±1% FS
Crank angle sensor	Resolution 1 Deg, speed 5500 RPM with TDC pulse	Kubler Germany make	±0.06
Temperature sensor	RTD Type and Type K thermocouple	Radix make	±1 °C
Load sensor	0–50 kg	VPG Sensotronics make	±0.85
Temperature transmitter	Range 0–100 °C, Output 4–20 mA	Abustek USA make	±1 °C
Fuel flow transmitter	range 0–500 mm WC	Yokogawa Japan make
Air flow transmitter	Pressure transmitter, range (-) 250 mm WC	Wika Germany make	±1% FS
Overall dimensions	W 2000 × D 2500 × H 1500	Kirloskar	-

provides a brief overview of integrated sensors and transmitters. It includes key details like measurement range, manufacturer, and relative uncertainty for each component. It further provides insights into equipment capabilities and precision in the experimental setup.

Table 4. Specifications of AVL Exhaust Gas Analyser.

Particulars	Measurement	Resolution
CO	0–15% Vol	0.001% Vol
HC	0–20,000 ppm Vol	1 ppm (0–2000 ppm)
CO2	0–20% Vol	0.1% Vol
O2	0–26% Vol	0.01% Vol
NO	0–6000 ppm	1 ppm Vol
Engine Speed	400–6000 rpm	1 rpm
Oil Temperature	0 °C–125 °C	1 °C
Lambda	0–9.999	0.001
Description	Details
Display	LCD display
Interface	USB
Operating Voltage	100–300 V AC, 50–60 Hz
Power Consumption	Max. 10 W
Dimensions (w × h × l).	270 × 85 × 320 mm
Weight	4.5 kg

summarizes the AVL exhaust gas analyser specifications, a crucial tool in the test rig. It outlines measurements, resolutions, and descriptions of parameters assessed: CO, HC, CO₂, NO_x, and lambda. In addition, it concisely presents the analyser’s capabilities, enhancing the comprehension of its role in the experiment.

2.4. Experimental Procedures

In order to establish steady conditions, the process begins with a 15 min engine warm-up, utilizing just 100% diesel fuel. After achieving consistent conditions across all tested fuels, trials occurred within a temperature range of 25–27 °C and a humidity range of 55–60%. Diesel fuel was used as the reference. Engine performance, emissions, and combustion were evaluated under different loads (25%, 50%, 75%, and 100%) at a steady speed of 1500 rpm. The CP data were meticulously collected for each working cycle, spanning a 720° crank angle with precise 1° intervals for all loads. Averaged pressures for tested fuels were derived from a minimum of 25 cycles, using adept filtering. The “Engine Soft” software enabled comprehensive engine assessment, with each test conducted thrice for accuracy. Data were collected on an assortment of parameters, including current, voltage, temperatures, pressures, air and fuel mass flow rates, and the composition of exhaust gases. The same approach described above is used to test the binary (B20) and ternary blends (B20H10, B20H20, B20O10, and B20O20). After each fuel blend test, the fuel line was cleansed with pure diesel. Key variables such as brake power, thermal efficiency, fuel consumption, and combustion characteristics were computed using an in-depth analysis of the collected data. Throughout, strict adherence to safety protocols ensured the reliability and legitimacy of outcomes. The methodology employed to perform experiments and analysis is illustrated in Figure 4.

2.5. Error and Uncertainty Analysis

Generally, experimental measurements carry inherent errors and uncertainties [62]. Within this research, the error associated with the measured parameters originates from the random error anticipated by the analytical technique [63]. It was quantified using Equation (1).

$$\underset{\begin{array}{c}measurement\\ error\end{array}}{\underbrace{{\Delta }_x}}=\frac{e_{\min }}{x_{\min }}=\frac{minimumsizeoftheexperimentalinstrument}{minimumvalueofthemeasuredvalue}$$

(1)

The uncertainty linked with computational parameters is derived based on the root mean square principle [64]. In this context, the uncertainty for the computational parameters can be expressed using Equations (2) and (3).

$$U_R=\left\{{\left[\left(\frac{\partial R}{\partial I_1}\right)u_1\right]}^2\right.{\left.+{\left[\left(\frac{\partial R}{\partial I_1}\right)u_2\right]}^2+\dots \dots +{\left[\left(\frac{\partial R}{\partial I_n}\right)u_n\right]}^2\right\}}^{1/2}$$

(2)

$$R=\left\{I_1,I_2,I_3,\dots \dots \right.\left.I_n\right\}$$

(3)

where R is a function dependent on independent variables from $I_1,I_2,I_3\dots . I_n$. Let u₁, u₂, …, u_n be uncertainties associated with these independent variables, while U_R stands for the resulting uncertainty.

The detailed measurement range and precision of the instruments employed in this research are summarized in

Table 5. Experimental uncertainty of various measuring units.

Parameter	Measurement Range	Accuracy	Uncertainty (%)
Engine speed	400–6000 rpm	±1 rpm	±0.06
Brake power	0–3.5 kW	±0.3 kW	±0.8355
Engine load	0–12 kg	±0.1 kg	±0.85
Mass of the fuel	-	-	±1.553
temperature	0 °C to 125 °C	±1 °C	±1
Time	-	$\pm 0.1 \mathrm{s}$	$\pm 0.2$
BSFC	-	±5 g/kWh	1.7641
Burette system	0–100 cc	±0.1 cc	±0.1
Brake thermal efficiency	-	±0.5%	±1.76
CO	0–15% vol	±0.001%	±1.026
HC	0–20,000 ppm	±1 ppm	±1.478
CO2	0–20% vol	±0.1%	±0.907
NOx	0–6000 ppm	±1 ppm	±0.837

. Uncertainty analysis is carried out to know the accuracy obtained from the experiments. Each set of experiments is repeated thrice and recorded the response measurement data. The resulting uncertainty corresponding to each factor is presented in Table 5.

The total uncertainty derived from the experiments can be computed using Equation (4).

$$U_R=\sqrt{B{P}^2+MC{F}^2+BSF{C}^2+BT{E}^2+C{O}^2+C{O}_2{}^2+H{C}^2+NO{x}^2}$$

(4)

$$U_R=\sqrt{{0.835}^2+{1.553}^2+{1.7641}^2+{1.76}^2+{1.026}^2+{0.907}^2+{1.478}^2+{0.837}^2}$$

$$U_R=\pm 3.752\%$$

The overall uncertainty in experiments was found to be ±3.752%. The reproducibility of results is indicated by the less than 5% uncertainty related to experiments and equipment.

3. Results and Discussion

The study analyses the engine performance, exhaust emissions, and combustion behaviour of different fuel samples in a diesel engine. The tested fuels include diesel, B20, B20H10, B20H20, B20O10, and B20O20. The experiments were conducted at varying engine loads (25%, 50%, 75%, and full load) while maintaining a constant engine speed of 1500 rpm. Comparisons between binary and ternary biodiesel–diesel blends and conventional diesel were made when 1-heptanol and n-octanol were added to diesel fuel, as well as their effects.

3.1. Combustion Analysis

3.1.1. In-Cylinder Pressure

CP was examined in order to comprehend the mechanism underlying the combustion of the composite biodiesel in ICEs. The amount of fuel used during uncontrolled combustion has a negative effect on the CP. Figure 5 shows the trends of the pressure in the cylinder changes for different fuel samples when the engine is run at full load. At 100% engine load, the highest pressure in the cylinders was 66.18 bar for diesel fuel, 63.34 bar for B20, 65.42 bar for B20H10, 62.73 bar for B20H20, 64.51 bar for B20O10, and 61.82 bar for B20O20. At the TDC, when the crank angle was between 5° and 8°, these highest pressures were observed. The highest peak CP was achieved with diesel fuel (DF), and this can be attributed to its superior calorific value and efficient fuel atomization. Diesel fuel consistently displayed the highest in-cylinder pressure profiles in all engine load situations because of its higher calorific value (42.54 MJ/kg) and superior fuel atomization characteristics (Table 1). The trends of the peak in-cylinder gas pressure at the lower alcohol fractions slightly decreased when 10% of 1-heptanol and n-octanol was added to the diesel/biodiesel fuel blend. This may be due to 1-heptanol having a lower auto-ignition temperature than n-octanol (270 °C), as shown in Table 1.

However, increasing the percentage to 20% in the diesel/biodiesel fuel blend led to a reduction in CP. These combustion trends are mainly due to the difference in energy and oxygen contents in 1-heptanol (14.13%) and n-octanol (15.7%), as shown in Table 1. The position of the in-cylinder peak pressure indicates the speed at which this energy is released. It is essential to mention that the rate of energy released depends on the presence of oxygen in the fuel structure (which accelerates combustion), viscosity (which affects fuel atomization and vaporization), and latent heat (which directly affects the ignition delay and combustion cooling). Combining these factors can explain the behaviour of CP for B20H10 and B20O10 compared to D100 and ternary blends of higher proportions of fuel blends. The binary blend of biodiesel/diesel (B20) exhibits lower CP compared to B20H10 and B20O10 and higher CP than B20H20 and B20O20, likely because the high-quality hybrid biodiesel blend with mineral diesel fuel is not significantly affected by CP. Similar trends were observed by Ramesh et al. [65] who studied a diesel engine powered by Calophyllum Inophyllum methyl ester and its blends with diesel fuel and hexanol. They found that increasing alcohol content reduced peak pressures, and their observations were ascribed to the higher latent heat of evaporation and relative specific temperatures of hexanol. Kumar et al. [66] investigated the effects of adding 30% higher-order alcohols to diesel fuel in a diesel engine. They ranked peak CP as follows: Isobutanol > n-pentanol > n-hexanol > n-octanol > ultra-low sulfur diesel fuel. Nanthagopal et al. [67]. discovered that blending n-octanol with Calophyllum Inophyllum biodiesel increased peak in-cylinder gas pressures by raising oxygen content, enhancing combustion efficiency. Emiroğlu and Şen [68] observed that alcohol-blended diesel fuels had higher and earlier peak CP, resulting in a shortened ignition delay interval.

3.1.2. Net Heat Release Rate

Figure 6 illustrates the NHRR at full engine load. The NHRR test was influenced by the test fuels’ calorific value, volatile matter, cetane number, oxygen concentration, viscosity, density, and length of fuel injection [69]. Conventional diesel fuel consistently exhibited the highest peak heat release rate among all fuel samples at various engine loads. This can be attributed to its superior energy content and better fuel atomization characteristics. The highest NHRR values for D100, B20, B20H10, B20H20, B20O10, and B20O20 fuels at maximum engine load were estimated to be 64.65 J/deg, 59.07 J/deg, 62.34 J/deg, 56.12 J/deg, 57.95 J/deg, and 51.9 J/deg, respectively, and the corresponding graphs are displayed in Figure 6. A lower (10%) amount of 1-heptanol and n-octanol in the diesel/biodiesel blend gradually improves some parameters, such as fuel atomization and blending capabilities. Notably, 1-heptanol (39.92 MJ/kg) and n-octanol (38.53 MJ/kg) have higher calorific values than hybrid biodiesel. Because of its lower blending percentage with diesel and specific physicochemical properties, such as cetane number, viscosity, and density, the hybrid biodiesel blend (B20) has lower net heat release rates than unmodified diesel fuel and also low percentage blends of 1-heptanol and n-octanol mixtures (B20H10 and B20O10). However, there is no substantial improvement in heat release rate statistics when the diesel engine is fuelled with higher percentages (20%) of n-octanol to the diesel/biodiesel blend fuels (B20O20). This is mostly due to decreases in net heat release rate (51.9 J/deg) caused by excess oxygen in the structure lowering the temperature inside the cylinder, as well as the previously noted less excellent fuel qualities.

Qi et al. [70] stated that heptanol and octanol blends show improved premixed-phase combustion compared to D100 despite their lower oxygen content. Longer ignition delays due to higher viscosities and lower cetane numbers lead to more fuel accumulation, resulting in enhanced combustion performance. Ağbulut et al. [71] remarked that an excessive amount of oxygen content in n-octanol promotes the enhancement of the combustion reaction. These findings align with studies conducted by various researchers [68,72,73]. However, Nour et al. [19] observed that the C7 and C8 types of alcohols have higher viscosities than diesel, impacting combustion efficiency negatively. Poorer fuel atomization and insufficient fuel/air mixing caused more fuel to burn in the mixing control phase and less fuel to burn in the premixed phase, which might have resulted in incomplete oxidation prior to the exhaust valve opening.

3.2. Engine Performance

Direct injection with a water-cooled diesel engine of a compression ratio of 17:5:1 (CR) at a stable speed of about 1500 rpm employing diesel (D100), binary blends of biodiesel/diesel (B20), ternary blends of diesel/biodiesel/1-heptanol (B20H10 and B20H20), and diesel/biodiesel/n-octanol (B20O10 and B20O20) blends was used for engine performance analysis. The engine power through BTE and BSFC was analysed and discussed.

3.2.1. Brake Thermal Efficiency (BTE)

BTE is a crucial parameter that assesses the combustion quality of an engine; it is the ratio of brake power output to fuel energy consumption [74]. The brake thermal efficiency of diesel engines is the efficiency of converting the chemical energy of fuel into mechanical energy [65]. The BTE was estimated by using Equation (5).

$$BTE(\eta )=\frac{P_e}{m_{fuel}\times Q_{LHV}}$$

(5)

The terms Q_LHV, P_e, and m_fuel depict the lower heat value of a fuel, effective engine power, and mass fuel consumption, respectively. Figure 7 illustrates the variation in BTE with engine load at a constant speed of 1500 rpm for diesel fuel and binary and ternary mixtures. At the maximum load, the BTE values for diesel fuel, B20, B20H10, B20H20, B20O10, and B20O20 were 38.65%, 37.01%, 37.76%, 36.84%, 37.12%, and 36.38%, respectively. According to the findings of other researchers [19,28,59], as the engine load increases, there is a fractional increase in BTE for all fuel samples following a direct proportionality between BTE and engine load. The decline in BTE for diesel/biodiesel and ternary blends of 1-heptanol and n-octanol is due to their reduced energy content and unfavourable combustion characteristics. Table 1 highlights that diesel fuel has a higher heating value (42.54 MJ/kg) than biodiesel (38.91 MJ/kg), 1-heptanol (39.92 MJ/kg), and n-octanol (38.53 MJ/kg). The improvement of fuel atomization and the air/fuel combination, which impacts BTE, is significantly influenced by changes in viscosity, surface tension, and density.

At all engine loads, diesel fuel has a higher BTE than binary and ternary blends due to their comparable calorific values, with B20H10 following in close second. The injection of the biodiesel/diesel blend (B20) has equivalent combustion chamber effects, with B20O20 having the lowest BTE (36.38%) across all engine loads. Compared to octanol, the blended fuel with a lower percentage of heptanol has a higher BTE (37.76%) owing to its lower heating value. The distinction from n-octanol can be attributed to the fuel samples’ calorific value and oxygen content. When more 1-heptanol and n-octanol are added to the diesel/biodiesel blend fuels (B20H20 and B20O20), the BTE values are lower compared to the lower percentage of higher chain alcohol in B20H10 and B20O10. The slightly higher enthalpy of evaporation (see Table 1) in heptanol (574 kJ/kg) compared to octanol (270 kJ/kg) may contribute to achieving the highest BTE. Various parameters like density, viscosity, and flash point also play a role in achieving good combustion efficiency, which improves the BTE. Ibrahim [36] detected that adding 20% (by volume) butanol to the B50 fuel blend caused the peak thermal efficiency to drop from 30.7% to 29.4%. Similarly, Candan et al. [75] reported that BTE levels were typically decreased while running a diesel engine with a diesel/methanol blend (DMB) and attributed their findings to the alcohol’s calorific value of the DMB. On the other hand, Yesilyurt et al. [56] maintained that 1-butanol and n-pentanol-infused fuel samples exhibited allowable alterations in BTE values, indicating the potential of these higher alcohols as oxygenated fuel additives in CI engines. In contrast, Wei et al. [76] reported that the addition of n-pentanol to diesel fuel showed no significant difference in BTE values. Enhancing BTE on second-hand engines can be carried out in a number of ways. Jianqin Fu [77] for instance, demonstrated that increasing the compression ratio of a liquefied methane engine improved engine torque and thermal efficiency. Additionally, Silitonga [74] revealed similar outcomes for BTE with the addition of bioethanol to diesel fuel and biodiesel. They mentioned that the lower energy content of the alcohol-added blends led to lower BSFC and, consequently, lower BTE.

3.2.2. Break-Specific Fuel Consumption (BSFC)

BSFC measures the fuel needed to produce 1 kW of power per hour in internal combustion engines (ICEs). It is related to effective engine power and mass fuel consumption, and, interestingly, it has an inverse relationship with BTE for the fuels used [78]. BSFC plays a crucial role in fuel selection for engine evaluation. Figure 8 illustrates the results of BSFC for all tested fuel samples at varying engine loads. At 100% engine load, the BSFC values were 254.1 g/kWh for diesel fuel, 302.14 g/kWh for B20, 281.25 g/kWh for B20H10, 310.94 g/kWh for B20H20, 292.8 g/kWh for B20O10, and 313.80 g/kWh for B20O20. The BSFC decreases with increasing engine loads. Diesel fuel exhibits a lower BSFC compared to binary blends and ternary blends of 1-heptanol and n-octanol due to its higher energy content and BTE (Figure 7).

The heptanol-blended fuel shows a lower BSFC than the octanol-blended fuel due to its higher viscosity. Heptanol has a viscosity of 5.75 mm²/s, while octanol has a viscosity of 7.59 mm²/s (refer to Table 1). This higher viscosity negatively impacts fuel atomization and fuel/air mixing, resulting in sub-optimal fuel fractions burned in the premixed phase and increased fuel burned in the mixing control phase. Moreover, fuels with higher proportions of 1-heptanol and n-octanol (B20H20 and B20O20) exhibit higher BSFC values compared to B20H10 and B20O10. The lowest BSFC may have been achieved because heptanol (574 kJ/kg) has a larger enthalpy of evaporation than octanol (315 kJ/kg) (see Table 1). The latent heat of evaporation, calorific value, viscosity, and density are some of the variables that affect how well an engine performs and how fuel burns. Alcohols (heptanol at 175 °C and octanol at 195 °C) vapourize more quietly than diesel fuel (180 °C to 360 °C) due to attained boiling temperatures. During the experiments, the engine loads led to an excess of fuel being injected into the combustion chamber due to the increased mixing ratio in the fuel blends. This resulted in the highest observed BSFC when the engine was fuelled with B20H20 (310.94 g/kWh) and B20O20 (313.80 g/kWh), while the lowest BSFC was recorded in pure diesel fuel (254.1 g/kWh) under the same loads. The peak BSFC values for all tested fuels occurred at 25% engine load, with a slight reduction beyond this under engine loads of 50%, 75%, and 100% compared to other tested fuels. Atmanli et al. [79] found that fuel blends with a lower energy content of higher alcohols and biodiesel led to increased BSFC in their experimental research. Diesel engines consumed more fuel with blends containing higher alcohols to compensate for the brake power loss. In another study, Atmanli et al. [80] investigated diesel fuel, vegetable oil, and n-butanol blends in a diesel engine and revealed the significant impact of cetane number on BSFC. Lower cetane numbers in fuel blends with higher alcohols caused delayed combustion and increased BSFC. Rahimi et al. [81] provided reasons for increased BSFC, attributing it to lower cetane numbers in fuel blends with alcohol and the influence of water content derived from the nature of alcohol. Yilmaz [82] observed that BSFC values increased with higher concentrations of C-1 alcohol (methanol) and C-2 alcohol (ethanol) in the blend. The researcher suggested improving fuel vapourization at higher engine loads and increasing air temperature before the suction process to address the issues. Sayin and Canakci [83] suggested that the increase in BSFC with rising engine loads is due to engine power increasing at a higher ratio than the consumed fuel. Similar findings and explanations have been reported by other researchers [67,68].

3.3. Exhaust Emissions

3.3.1. HC Emission

Unburned hydrocarbon (HC) emissions in internal combustion engines are influenced by critical combustion parameters such as air–fuel ratio, fuel spray characteristics, fuel properties, and engine operating conditions [84]. Figure 9 illustrates the HC emissions of various tested fuel samples with engine load variation, investigating advanced combustion characteristics. The graph shows a consistent trend of increasing HC emissions for all fuels as the engine load rises from 25% to 100%. Conventional diesel fuel, lacking native oxygen content in its atomic structure, exhibited the highest HC emissions at all engine loads due to its high efficiency, absence of oxygen molecules, and higher carbon content. In contrast, the ternary blend of n-octanol showcased lower HC emissions than diesel fuel for all loads. However, the proportion of n-octanol in the blend influenced the HC emission levels, with higher n-octanol percentages leading to overlean mixtures and increased oxygen content. Introducing 1-heptanol to biodiesel/diesel blends resulted in decreased HC emissions compared to pure diesel fuel; yet, they remained higher than those of the n-octanol-blended ternary fuel.

The maximum unburned HC emissions were recorded for different fuel samples: D100, B20, B20H10, B20H20, B20O10, and B20O20, with values of 0.156 g/kWh, 0.138 g/kWh, 0.136 g/kWh, 0.12 g/kWh, 0.124 g/kWh, and 0.108 g/kWh, respectively. These findings align with prior investigations involving various higher alcohols, where their presence in the fuel samples facilitated specific reactions within the combustion chamber due to increased temperatures, impacting HC emissions. In conclusion, HC emissions were substantially higher for all tested fuels compared to B20O20 fuel, which comprised 20% n-octanol, 20% biodiesel, and 60% diesel fuel. However, they showed slightly lower HC emissions than traditional diesel fuel at nearly all engine loads (refer to Figure 9). These findings highlight the potential benefits of certain fuel blends for reducing HC emissions and improving overall engine efficiency. Deep et al. [85] also observed a comparable trend when blending 1-octanol with diesel in volumes ranging from 10% to 40%. In a study by Mahalingam et al. [86], the addition of 10% and 20% pentanol to neat biodiesel resulted in a decrease in HC emissions of 2.1% and 3.6%, respectively, compared to B100 at all engine loads. Similarly, Babu et al. [87] found that a blend of 85% biodiesel, 5% diesel fuel, and 10% n-pentanol exhibited the lowest HC emissions among other ternary blends. At 100% load, this blend released 74 ppm, which was 39.4% lower than diesel fuel emissions.

3.3.2. NO_x Emission

The term oxides of nitrogen refer to NO_x, including many different nitrogen compounds, such as nitrogen monoxide (NO), nitrogen dioxide (NO₂), nitrous oxide (N₂O), dinitrogen dioxide (N₂O₂), dinitrogen trioxide (N₂O₃), dinitrogen tetroxide (N₂O₄), and dinitrogen pentoxide (N₂O₅). However, it is widely recognized that more than 90% of NO_x emitted from a CI engine consists of NO gas [88]. The air/fuel ratio, compression ratio, temperature, fuel injection pressure, engine speed, and load are few factors that affect the formation of NO [71]. Diesel fuel has lower NO_x emissions because of combustion efficiency indices like a higher combustion chamber temperature, a faster flame, better thermal efficiency, and fewer oxygen molecules, as well as a higher energy content [89].

Figure 10 depicts the variations in brake-specific NO_x emissions for diesel fuel and binary and ternary fuel blends at different engine loads. Unmodified diesel fuel showed the lowest NO_x emission levels among all fuel samples and engine loads. The addition of n-octanol to the biodiesel/diesel fuel blend (B20O20) increased NO_x emission values due to the higher oxygen content in the blends. This led to weaker combustion conditions at lower combustion chamber temperatures, inhibiting NO_x formation that requires higher temperatures for nitrogen and oxygen molecules to react. At 100% load, NO_x emissions for D100, B20, B20H10, B20H20, B20O10, and B20O20 were recorded as 10.4 g/kWh, 13.68 g/kWh, 12.67 g/kWh, 13.1 g/kWh, 13.85 g/kWh, and 14.1 g/kWh, respectively. Notably, the ternary blend of 1-heptanol/biodiesel/diesel (B20H10) exhibited the lowest NO_x emission levels among the fuel blends. A fuel with a low cetane number results in a longer ignition delay period, leading to a higher end temperature of combustion and increased in-cylinder gas pressure. Although the NO_x emissions for (B20H10) were still higher than those of diesel fuel, at full engine load, the NO_x emission of (B20H10) increased by 21.8% compared to diesel fuel.

Nour et al. [19] observed reduced NO_x emissions for all tested blends under various conditions. The lowest NO_x concentration was achieved when blending butanol, octanol, and heptanol with diesel fuel. Nanthagopal et al. [67] found that fuel samples with added alcohol (hexanol and decanol) exhibited improved oxygen content, leading to decreased NO_x emissions with pure biodiesel and increased emissions with diesel fuel. Balamurugan et al. [90] conducted experiments with diesel fuel containing 4% and 8% n-propanol and n-butanol, resulting in decreased operating temperatures and subsequently reduced NO_x emissions than diesel fuel. Nanthagopal et al. [91] found that using higher alcohols like butanol and pentanol with Calophyllum inophyllum biodiesel in CI engines reduced NO_x emissions due to the high latent heat of alcohols in the blend. Soloiu et al. [92] reported that using oxygenated fuels like alcohol or fatty acid methyl ester helped reduce emissions due to a lower carbon-to-oxygen ratio and relative A/F ratio.

3.3.3. CO₂ Emission

Higher oxygen content supports complete combustion leading to increased CO₂ emissions. The presence of oxygen atoms promotes complete reactions and raises CO₂ emissions in the exhaust. The hydroxyl radical OH, an essential oxidizing agent, converts CO to CO₂ gas when sufficient oxygen is available [91]. Figure 11 illustrates the variation in CO₂ emission values in g/kWh with engine load at a constant speed of 1500 rpm. As engine load increases, CO₂ emissions also rise significantly, resulting in reduced BTE in the tested fuel samples. The B20O20 fuel blend exhibited the highest mean CO₂ emissions at 181.11 g/kWh among the tested fuels owing to its native oxygen atoms that readily react with CO to produce CO₂ gas, significantly influencing the generation of CO₂. The CO₂ emissions for D100, B20, B20H10, B20H20, B20O10, and B20O20 were recorded as 173.23 g/kWh, 176.17 g/kWh, 176.51 g/kWh, 178.53 g/kWh, 177.04 g/kWh, and 181.11 g/kWh, respectively. Blends containing higher alcohols (1-heptanol and n-octanol) showed similar CO₂ emissions, with a difference of approximately 7 to 8%.

Therefore, the addition of alcohol resulted in reduced CO₂ emissions than the B20 blend. 1-Heptanol, with oxygen molecules in its chemical bonds, readily reacts with CO molecules to form CO₂. This has a big effect on how CO₂ emissions are made. Baseline diesel fuel, with a higher calorific value and lower kinematic viscosity (refer to Table 1), improves atomization and BTE, resulting in reduced CO₂ emissions. The characteristic of lower-percentage alcohols, with inherent oxygen molecules, also significantly impacts CO₂ emissions in exhaust gases. Babu et al. [87] found that CI engines running on used frying oil biodiesel/alcohol/diesel fuel blends released more CO₂ emissions than diesel and biodiesel blends as a result of the complete combustion process. Ramesh et al. [65] used hexanol as oxygenated long-chain alcohol in Calophyllum Inophyllum methyl ester/DF blends at various concentrations, resulting in CO gas reacting with the inherent oxygen molecules of hexanol and leading to CO₂ emissions. Akar et al. [93] used butanol/biodiesel/diesel fuel blends and concluded that 1-heptanol-infused fuel blends could be preferred as a significant fuel additive to combat global warming over neat biodiesel fuel.

3.3.4. CO Emission

The engine cylinder’s incomplete combustion leads to CO emissions. Long-term combustion and a deficiency of oxygen molecules result in CO emissions. CO is particularly hazardous to humans because it is colourless and odourless [43]. At low engine loads, insufficient air/fuel mixtures result in greater CO emissions. Blends containing fewer carbon atoms, such as heptanol and octanol, are projected to provide lower CO emissions. At the maximum engine load, B20 and B20O20 exhibited CO emissions of 5.25 g/kWh and 4.48 g/kWh, respectively. Figure 12 demonstrates that incorporating higher alcohols into ternary blends of heptanol and octanol significantly reduced CO emissions beyond the B20 blend. The lowest CO emissions measured for D100, B20, B20H10, B20H20, B20O10, and B20O20 were 6.24 g/kWh, 5.25 g/kWh, 5.01 g/kWh, 4.85 g/kWh, 4.91 g/kWh, and 4.48 g/kWh, respectively, at full load. Alcohol addition reduces CO emissions at all engine speeds compared to diesel and B20 blends. The inclusion of alcohol reduces CO emissions at all engine speeds compared to diesel and B20 blends. Ternary blends of 1-heptanol (B20H10 and B20H20) exhibited 19.71% and 22.27% lower CO emissions than pure diesel oil at full load. Similarly, ternary blends of n-octanol (B20O10 and B20O20) showed 21.31% and 28.2% lower CO emissions than pure diesel oil at full load.

The increased oxygen content of n-octanol can be attributed to this phenomenon. Notably, the octanol addition to the blend exhibited superior abilities in lowering CO emissions. Additionally, less CO was formed due to the reduced number of carbon atoms in the chemical bond of alcohols (see Table 1). Similar findings were reported by the vast majority of researchers [21,80,81]. Akar et al. [89] discovered that combining butanol with flaxseed oil biodiesel/diesel fuel lowered CO emissions when compared to normal diesel fuel. Ramesh et al. [65] indicated that the addition of hexanol to B50 fuel to formulate ternary blends reduced CO emissions more than pure biodiesel fuel, but only when the hexanol level was equal to or more than 30% by weight. Li et al. [94] monitored lower CO emissions using (diesel + biodiesel + pentanol) blends. Additionally, Nanthagopal et al. [67] noted that infusing higher-order alcohol (decanol) increased the total oxygen concentration inside the cylinder, resulting in decreased CO emissions. In contrast, Randazzo and Sodré [95] claimed the opposite view: there is a direct correlation between the ethanol proportion in the B20 fuel mix and the steady increase in CO emissions. Wei et al. [76] also demonstrated that adding higher alcohols to mineral diesel fuel resulted in higher CO emissions, with a noticeable impact, particularly at low engine loads.

4. Conclusions

In this experimental study, the influence of binary and ternary blends of diesel/biodiesel with higher alcohols (1-heptanol and n-octanol) on compression ignition engine performance, combustion characteristics, and exhaust emissions was investigated. Tests were conducted at various engine loads, and results were compared with unmodified diesel fuel and B20 blend. Key findings are as follows:

• The introduction of lower percentages of 1-heptanol and n-octanol in the diesel/biodiesel fuel blend resulted in higher peak in-cylinder gas pressure compared to the binary blend. However, increasing the percentage to 20% in the diesel/biodiesel fuel blend led to a reduction in CP.
• The low percentage of heptanol in the diesel/biodiesel provided the highest heat released per cycle at all tested conditions compared to that of the low percentage of octanol fuel blends and B20. Additionally, Heptanol/diesel blends showed the highest combustion efficiency among the tested fuels.
• Diesel fuel demonstrated higher BTE owing to its superior heating value. The introduction of lower percentages of 1-heptanol and n-octanol in the blend resulted in higher BTE compared to binary blends but decreased with higher proportions of these alcohols. The BTE decrease followed the sequence B20O20 < B20H20 < B20 < B20O10 < B20H10. B20O20 experienced the smallest BTE decrease compared to diesel.
• Diesel fuel exhibited a lower BSFC compared to binary blends and ternary blends of 1-heptanol and n-octanol due to its higher energy content and better thermal efficiency. Fuels with higher proportions of 1-heptanol and n-octanol (B20H20 and B20O20) displayed higher BSFC values compared to B20H10 and B20O10. The lower BSFC may be attributed to the higher enthalpy of evaporation in heptanol (574 kJ/kg) compared to octanol (315 kJ/kg). Various factors, including the latent heat of evaporation, calorific value, viscosity, and density, influenced engine performance and combustion characteristics.
• Ternary blends with n-octanol showed lower HC emissions than diesel fuel at all loads. However, the amount of n-octanol in the blend affected HC levels, with higher percentages leading to overlean mixtures and increased oxygen content. Adding 1-heptanol to biodiesel/diesel blends reduced HC emissions compared to pure diesel but remained higher than n-octanol-blended fuel. The maximum HC emissions were in the order of D100 > B20 > B20H10 > B20O10 > B20H20 > B20O20.
• Diesel fuel resulted in the lowest NO_x emissions at all loads. The addition of n-octanol to biodiesel/diesel fuel (B20O20) increased NO_x emissions due to higher oxygen content, resulting in weaker combustion conditions. The ternary blend B20H10 showed the lowest NO_x emissions. However, it still had higher NO_x emissions than diesel fuel, increasing by 21.8% at full load. The maximum NO_x emissions are in the order of B20 > B20O20 > B20H20 > B20O10 > B20H10 > D100.
• The B20O20 fuel blend had the highest CO₂ emissions at 181.11 g/kWh due to its native oxygen atoms readily reacting with CO to form CO₂. CO₂ emissions for other blends ranged from 173.23 g/kWh to 181.11 g/kWh. Blends with higher alcohols showed similar CO₂ emissions, with a difference of around 7 to 8%. The addition of alcohol reduced CO₂ emissions compared to the B20 blend, with 1-heptanol playing a significant role in CO₂ formation. The maximum CO₂ emissions were in the order of B20O20 > B20H20 > B20O10 > B20H10 > B20 > D100.
• CO emissions were reduced significantly more with the addition of alcohol than diesel and B20 blends. Ternary blends of 1-heptanol (B20H10 and B20H20) exhibited 19.71% and 22.27% lower CO emissions than pure diesel oil at full load. Similarly, ternary blends of n-octanol (B20O10 and B20O20) showed 21.31% and 28.2% lower CO emissions than pure diesel oil at full load. The higher oxygen content of n-octanol contributed to this effect, and alcohols’ fewer carbon atoms in the chemical bond resulted in less CO formation. The least CO emissions were in the order of B20O20 < B20O10 < B20H20 < B20H10 < B20 < D100.
• The obtained results with ternary blends ensure an alternate promising next-generation ecofriendly fuel to run in diesel engines without significant modifications. The detailed insights could help to prepare ternary blends at an industrial scale. This research work can be extended in the following directions: 1. Higher alcohols, such as hexanol and decanal, can be investigated as fuel additives for diesel engines. 2. Engine parameters such as injection timing, injection pressure, compression ratio, and engine speed can be altered and analysed for better performance, combustion, and emission characteristics.