Effect of Intake

Air Preheating on Performance and Emissions of a Diesel Engine Using Diesel–Biodiesel–Ethanol Blends

Authors: Kumlachew Yeneneh, Elias Wakshume, Besufekad Negash Fetene

Abstract

With growing concerns over fossil fuel depletion, environmental pollution, and the need for sustainable energy solutions, alternative renewable fuels have gained significant attention in the transportation sector. Biodiesel and ethanol are promising biofuels that can reduce dependence on conventional diesel and lower harmful emissions. However, challenges such as fuel compatibility and combustion inefficiencies limit their widespread adoption in compression ignition engines. This study investigates the combined effects of intake air preheating and diesel–biodiesel–ethanol ternary fuel blends on the performance and emission characteristics of a single-cylinder compression ignition engine. Cottonseed biodiesel, an underutilized nonedible feedstock abundant in regions like Ethiopia, was blended with ethanol and conventional diesel in three B20E10, B30E10, and B40E10. Experiments were conducted under ambient (25 °C) and preheated (51 °C) intake air conditions to evaluate brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), and gaseous emissions (CO, HC, NO~ x , and CO2~). Results showed that intake air preheating enhanced combustion efficiency and reduced BSFC by up to 11.11% while increasing BTE by as much as 4.28%. CO and HC emissions were significantly reduced across all blends, though NO~ x ~ emissions increased due to higher in-cylinder temperatures. Among the tested blends, B20E10 exhibited the best balance between performance, efficiency, and emissions, delivering brake power comparable to that of diesel with the lowest BSFC and substantially cleaner exhaust. While particulate matter was not directly measured, existing literature supports the expectation of reduced PM due to the oxygenated nature of the fuels. The study highlights intake air preheating as a cost-effective strategy to enhance the viability of renewable fuel use in existing diesel engines. These findings offer a practical pathway toward cleaner combustion and energy diversification, particularly in low-resource settings, where local biofuel adoption is critical for energy security and environmental sustainability.

Introduction

The accelerating global demand for energy, driven by rapid population growth, urbanization, and industrial development, has led to an increased reliance on fossil fuels. , These conventional energy sources currently account for more than 80% of the world’s total energy supply, with petroleum products dominating the transportation sector. Within this sector, road transport alone is responsible for approximately 80% of energy consumption, accounting for nearly 60% of total oil demand globally. Diesel engines are the dominant propulsion system in this domain, powering over 99% of commercial and freight vehicles due to their superior fuel efficiency, torque characteristics, and durability. However, the environmental implications of diesel usage are increasingly alarming. Diesel combustion produces high levels of nitrogen oxides (NO~ x ~), particulate matter (PM), and greenhouse gases (GHGs), which contribute significantly to climate change, urban air pollution, and adverse public health outcomes. The urgency to transition toward cleaner alternatives is therefore both environmentally and economically imperative. ,

While modern diesel engines are engineered to comply with stringent emission standards, they still rely on expensive and complex exhaust after-treatment systems, such as diesel particulate filters (DPFs), selective catalytic reduction (SCR), and exhaust gas recirculation (EGR). , These technologies add weight, cost, and maintenance burden to the vehicle. Moreover, diesel fuel itself remains a finite, nonrenewable resource whose price and availability are subject to global market fluctuations and geopolitical instability. Consequently, researchers and policymakers are increasingly advocating for the adoption of renewable and low-carbon fuels that can seamlessly integrate into existing engine platforms while meeting performance and environmental benchmarks.

Among the many alternative fuels investigated, biodiesel and alcohol-based fuels such as ethanol and methanol have emerged as leading candidates due to their renewability, oxygen content, and potential to reduce harmful emissions. , Biodiesel, typically derived from vegetable oils or animal fats through transesterification, has a higher cetane number than diesel and contains inherent oxygen, promoting more complete combustion and reducing carbon monoxide (CO), hydrocarbons (HC), and smoke emissions. It is biodegradable, nontoxic, and sulfur-free. However, numerous studies have consistently reported that biodiesel combustion tends to increase NO2 emissions due to higher combustion temperatures and extended ignition delay. This NO2 penalty has spurred researchers to explore fuel modification techniques and blending strategies to mitigate adverse effects. ,,

One such strategy is the use of ethanol, an alcohol derived from biomass such as sugarcane, corn, or lignocellulosic agricultural residues. , Ethanol possesses a high latent heat of vaporization, low viscosity, and a high oxygen content, making it a viable additive to improve combustion efficiency and reduce CO, HC, and particulate emissions. Its inclusion in diesel or biodiesel–diesel blends has shown promise in moderating peak combustion temperatures, thereby suppressing NO2 formation. For instance, studies by Ghareeb et al. and Ellappan et al. demonstrated that ethanol–biodiesel–diesel blends significantly reduced smoke opacity and CO emissions compared to neat diesel. , Moreover, ethanol has a high octane number and supports more homogeneous combustion, especially when it is used in dual-fuel or blend configurations.

Nonetheless, the blending of ethanol with diesel presents challenges related to fuel miscibility and stability. Ethanol is polar and hydrophilic, while diesel is nonpolar and hydrophobic, leading to phase separation, especially under cold or humid conditions. To overcome these issues, biodiesel has been proposed as a cosolvent that improves blend homogeneity and long-term storage stability. Research by Ghadikolaei et al. and Yeneneh et al.. confirmed that ternary blends comprising diesel, biodiesel, and ethanol exhibited enhanced physical compatibility, better atomization, and improved combustion behavior. , Furthermore, the presence of biodiesel enhances the lubricity of ethanol-containing blends, offsetting ethanol’s inherently low lubricating properties, which are critical for injector and fuel pump longevity.

Even with these advances, ethanol-blended fuels suffer from ignition difficulties due to their high autoignition temperature and low cetane index. Combustion initiation becomes especially problematic during cold starts and low-load operations. This has led to investigations into thermal enhancement strategies, most notably intake air preheating. By elevating the temperature of incoming air, the evaporation and mixing of fuels improve, reducing ignition delay and enhancing combustion stability. Doppalapudi et al. and Wang et al. demonstrated that moderate preheating of intake air improved brake thermal efficiency and reduced CO and HC emissions in diesel engines fueled with oxygenated biofuels. , Moreover, preheating facilitates more uniform combustion, which is particularly advantageous in partially premixed combustion regimes.

Despite the promising results, few studies have systematically explored the combined effect of intake air preheating and ternary fuel blending using cottonseed biodiesel, a nonedible, locally available feedstock with favorable fuel properties. Cottonseed oil is abundant in regions such as Ethiopia and represents a viable biodiesel source that avoids competition with food crops. However, existing research has primarily focused on soybean, palm, jatropha, or waste cooking oils, with limited attention to cottonseed biodiesel in practical engine applications. , Even fewer studies have investigated its behavior in combination with ethanol and diesel under preheated intake air conditions and variable engine loads.

To address these gaps, the present study aims to experimentally evaluate the performance and emission characteristics of a single-cylinder diesel engine operated with diesel–ethanol–cottonseed biodiesel ternary blends, with and without intake air preheating. Three blend ratiosB20E10, B30E10, and B40E10were tested under controlled engine loading and thermal intake conditions. The novelty of this research lies in its integrated unlike previous investigations that addressed either fuel blending or thermal enhancement in isolation, this study concurrently applies both strategies to achieve improved combustion and emission trade-offs. This dual-focus framework is particularly relevant to countries such as Ethiopia, where resource constraints and energy access challenges necessitate locally adaptable, low-cost clean energy solutions. The findings contribute to a broader discourse on sustainable fuel transitions and inform future engine design and fuel policy in developing regions.

Materials and Methodology

Materials

The experimental study utilized a custom-designed setup comprising a fabricated shell-and-tube heat exchanger and biodiesel derived from cottonseed oil. The materials used in the fabrication and testing phases include mild steel sheets, copper tubes, thermocouples, and various machining tools. Ethanol (97% purity) and analytical-grade potassium hydroxide (KOH, 85% purity) were procured from FIRMAMENT TRADING PLC. Laboratory equipment, such as stand funnel separators, beakers, and digital balances, supported biodiesel production and fuel blending. Comprehensive lists of materials used for engine testing are provided in Table

Methodology

This study followed a structured experimental approach comprising the design and fabrication of a counterflow heat exchanger, production of cottonseed biodiesel, and preparation of ethanol–biodiesel–diesel fuel blends. The experimental test rig was assembled and calibrated to evaluate the engine performance and emission characteristics under varying load and intake air temperature conditions. The research workflow was executed sequentially, as outlined in the process flowchart presented in Figure .

Heat Exchanger Thermal Analysis and Design

Heat exchangers are thermal systems that enable energy exchange between two or more fluids at different temperatures without direct mixing. They are fundamental components in numerous industrial applications, including power generation, chemical processing, food production, electronics cooling, HVAC systems, environmental engineering, and aerospace technology.

In the present study, a recuperative-type intake air preheater was integrated into the experimental engine setup. Its function is to elevate the temperature of the intake air before it is mixed with fuel and subsequently enters the combustion chamber. A counterflow shell-and-tube heat exchanger configuration, depicted in Figure , was selected due to its superior thermal efficiency. In this configuration, the hot exhaust gases and cold intake air flow in opposite directions, enabling a higher mean temperature difference and improved heat transfer performance.

A critical component in developing the intake air preheater is thermal analysis of the heat exchanger, which includes both rating and sizing procedures. The rating process involves determining the outlet temperatures and the heat transfer rate based on known inlet conditions and fluid properties. In this study, the Kern method, a widely used and robust iterative technique, was applied to conduct thermal performance analysis. This method incorporates trial-and-error procedures to systematically solve the governing heat transfer and fluid flow equations, as illustrated in (eqs –).1(ΔTLM)cc=(Thi−Tco)−(Tho−Tci)ln(Thi−TcoTho−Tci) 2Q=ṁaCp,c(Tao−Tai) 3Q=ṁaCp,a(Tao−Tai)=ṁgCp,g(Tgo−Tgi) 4(VTLM)cc=(Tgi−Tao)−(Tgo−Tai)](Tgi−Tao)/(Tgo−Tai) 5R=(Tgi−Tgo)(Tao−Tai)=1.08S=(Tao−Tai)(Tgi−Tao)=0.32 6A=QU×LMTD×Ft=0.195⁡m2 7A=N×π×do×LL=400⁡mm 8Rei=(Ms×Di)μ 9Nui=0.023×Rei0.8×Pri0.3×(μμw)0.14 10hi=Nui×Kid 11De=4×(pt2−πdo2/4)πdo 12Reo=De×Msμ 13Nuo=0.364×(Reo0.55)×(Pro0.33) 14ho=Nuo×Ko/do 15Ut=A×(1hi+1ho)−1 16f=0.0014+0.125×Rei−0.32 17Δpi=4f×Ldi×Ms22ρ×[μμw]−0.14 18Δpo=f×(NB+1)×DiDe×Ms22ρ×(μwμ)0.14 19ρblend=ρD×D+ρBD×BD+ρEtOH×EtOH 20HVblend=(νD×HVD×ρD+νB×HVB×ρB+νE×HVE×ρE)ρblend

The thermophysical properties of the intake air were evaluated at the average temperature between its inlet and outlet conditions. These include a specific heat capacity of 1005 J/kg·K, a thermal conductivity of 0.02756 W/m·K, a Prandtl number of 0.669, a density of 1.128 kg/m^3^, and a dynamic viscosity of 19.12 × 10^– 6^ Ns/m^2^. These properties were critical for determining convective heat transfer coefficients and flow regime classifications.

Similarly, the properties of the exhaust gases were assessed at their corresponding mean temperature. The exhaust gas exhibited a specific heat capacity of 1068 J/kg·K, a thermal conductivity of 0.03128 W/m·K, a Prandtl number of 0.69, a density of 0.95 kg/m^3^, and a viscosity of 20.38 × 10^–6^ Ns/m^2^. These values demonstrate notable deviations from ambient air, particularly in terms of thermal conductivity and specific heat, reflecting their higher energy content and fluid dynamic behavior.

The shell-and-tube heat exchanger designed for this application features a compact geometry with robust mechanical specifications suitable for sustained thermal performance. The preheater consists of a shell with an outer diameter of 124 mm and a wall thickness of 3.24 mm. It accommodates five tubes, each with an outer diameter of 31 mm and a wall thickness of 2.62 mm, arranged with a tube pitch of 38.75 mm, to facilitate effective heat transfer and mechanical clearance. The tube length is 400 mm, with six baffles uniformly spaced to ensure optimal shell-side fluid agitation. The central baffle spacing is 55 mm, while the baffle cut is sized at 31 mm to reduce the pressure drop and maintain flow distribution. The design incorporates a single pass on both the shell and the tube sides, conforming to standard engineering practices for compact heat exchangers.

This configuration was selected to balance thermal efficiency, pressure drop constraints, and ease of fabrication, ensuring that the system meets the operational demands of intake air preheating under engine conditions.

Heat Exchanger Manufacturing Process

The intake air preheater heat exchanger was constructed by using a combination of mild steel and copper tubes.

This section details the procedures used to build the heat exchanger according to the design specifications.

Initially, mild steel sheets were prepared by cutting them into circular and rectangular segments, which were then used to fabricate key structural components, such as baffle plates, tube sheets, and the outer shell. The baffle plates and tube sheets were drilled with 31 mm diameter holes arranged in a square pitch layout to accommodate the copper tubes, ensuring accurate alignment and optimal flow distribution.

Subsequent rolling operations were performed to shape the mild steel sheets into cylindrical and conical shell segments. The cylindrical section had a uniform diameter of 110 mm, while the conical section featured a gradual taper from 110 to 40 mm, enabling smooth integration into the engine intake system. All components were joined through gas metal arc welding (GMAW), ensuring structural integrity at the interfaces among the baffles, shell, and tube assemblies.

Following assembly, the welded joints and surface irregularities were finished by using fine-grade glass paper, producing smooth contact surfaces to minimize pressure loss and reduce the risk of thermal resistance or fouling during operation. The fabrication steps are illustrated in Figure A–D, highlighting key stages from raw material preparation to final assembly.

Transesterification of Cottonseed Oil

The transesterification of cottonseed oil into biodiesel was carried out through a well-defined and structured process. The procedure began with the purification of cottonseed oil, which served as the primary source of triglycerides. At the same time, a methoxide catalyst solution was prepared by dissolving 5 g of potassium hydroxide (KOH) in 100 mL of methanol, ensuring a methanol-to-oil molar ratio of approximately 20:1. This solution was stirred thoroughly to achieve complete dissolution of the catalyst, resulting in a homogeneous base-catalyst system.

Subsequently, 500 mL of the purified cottonseed oil was preheated to 60 °C. The prepared methoxide solution was then gradually introduced into the preheated oil. The transesterification reaction was initiated under continuous stirring at 1000 rpm for 60 min, while maintaining the reaction temperature at 60 °C. To minimize methanol evaporation and maintain consistent reaction conditions, the reaction vessel was covered with aluminum foil.

During the reaction, triglycerides in the oil were converted to fatty acid methyl esters (biodiesel) and glycerol. After approximately 40 min, a visible phase separation was observed, with glycerol settling at the bottom and the lighter biodiesel phase forming on top. Following the completion of the reaction, the mixture was transferred to a separating funnel and allowed to settle undisturbed for 24 h, facilitating complete phase separation. The denser glycerol layer was then drained from the bottom, leaving the crude biodiesel as the upper layer.

To purify the biodiesel, it was washed repeatedly with hot distilled water (preheated to 55 °C) and an amount equivalent to approximately half that of the biodiesel. Washing was continued until the rinsewater became nearly transparent, indicating the effective removal of residual glycerol, soap, unreacted methanol, and catalyst impurities. The washed biodiesel was subsequently dried on a hot plate at approximately 140 °C for one h to remove any remaining moisture and alcohol. This yielded a clear, refined biodiesel product suitable for subsequent analysis or direct application as an alternative fuel.

A schematic representation of the entire biodiesel production process, from oil purification through transesterification and final drying, is illustrated in Figures A–F) and A–C.

All reagents used in the transesterification process were of analytical grade. Methanol (99.5% purity) and potassium hydroxide (KOH, 85% purity) were purchased from FIRMAMENT TRADING PLC, Ethiopia, and used without further purification. The catalyst solution was prepared by dissolving 5 g of KOH in 100 mL of methanol, maintaining a methanol-to-oil molar ratio of approximately 1. Ethanol used for fuel blending was 97% pure and also supplied by FIRMAMENT TRADING PLC.

Biodiesel Blend Preparation

Biodiesel, typically obtained through the transesterification of vegetable oils, can be blended with petroleum diesel in various proportions. Commercially accepted blends such as B5 (5% biodiesel and 95% diesel) and B20 (20% biodiesel and 80% diesel) have demonstrated satisfactory performance in conventional diesel engines without the need for engine modifications or additives.

In this study, biodiesel derived from cottonseed oil was blended with petroleum diesel and ethanol to formulate ternary fuel mixtures. The specific blends prepared were B20E10, B30E10, and B40E10, corresponding to 20%, 30%, and 40% biodiesel contents, respectively, combined with 10% ethanol and the remaining portion of petroleum diesel. These three blend ratios were selected based on fuel stability tests and literature precedence. B20E10 is widely studied and commercially viable; B30E10 provides an intermediate benchmark, and B40E10 was the maximum blend tested without observing phase separation or significant operational issues during preliminary trials.

To address potential miscibility challenges between ethanol and diesel, blending was performed at ambient temperatures above 20 °C. The mixtures were stirred using a magnetic stirrer for 20 min to ensure homogeneity, followed by visual inspection to assess phase stability. Biodiesel served as a cosolvent, promoting the miscibility of ethanol in the petroleum diesel matrix. No evidence of phase separation was observed under laboratory conditions. To ensure stability, the blends were prepared and tested at ambient temperatures above 20 °C, and they were stored in sealed containers to prevent moisture ingress. Biodiesel, acting as a cosolvent, enhanced ethanol miscibility and prevented phase separation. No separation was observed during the testing. However, in colder climates or longer storage durations, phase separation remains a concern and warrants further investigation. For quality assurance, all blends were stored in sealed containers and utilized within 24 h to minimize the effects of temperature fluctuations and ensure consistency during engine testing.

Experimental Setup and Procedure

The experimental investigation was conducted using a single-cylinder, four-stroke, naturally aspirated, direct injection diesel engine (Model TBMC3-02), integrated with a computer-controlled data acquisition system, and coupled to an eddy current asynchronous motor dynamometer (Model AM-1) for precise load application. The test rig also included essential subsystems, such as a fuel delivery system, a water-cooling unit, a lubrication system, and an emission measurement unit, enabling comprehensive performance and combustion evaluation. The engine has a rated maximum power output of 2.2 kW and a peak torque of 7.4 N·m. Both schematic and photographic representations of the experimental setup are presented in Figures and , respectively.

The system enabled real-time monitoring of engine performance parameters, such as brake power (Pb), brake-specific fuel consumption (BSFC), and brake thermal efficiency (BTE). Emission characteristics, including carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), and nitrogen oxides (NO~ x ), were measured using the SV-50 automotive emission analyzer. CO and CO2~ concentrations were recorded as volume percentages, while HC and NO~ x ~ levels were measured in parts per million (ppm). The experimental program was designed to evaluate three ternary fuel B20E10, B30E10, and B40E10. The compositions and key physicochemical properties of the tested blends of these fuels are summarized in Table . Each blend was tested under two different intake air temperature conditions. Initially, the engine was fueled with pure diesel and operated under ambient intake air conditions to establish baseline data.

The test engine, TBMC3-02, was used in its stock configuration with all factory sensor calibrations and ECU parameters intact. It has a compression ratio of 1, a nozzle opening pressure of 190 bar, and an injection timing of 15° before the top dead center (bTDC). Detailed technical specifications of the engine are presented in Table

Testing began with pure diesel operation at a constant speed of 1500 rpm under no load for a few minutes to achieve thermal stabilization. Baseline performance and emissions data were then collected by incrementally increasing the engine load from 0% to 80% in 20% steps, as detailed in Table . After the baseline tests were completed, the engine was run on the ternary fuel blends (B20E10, B30E10, and B40E10) under identical conditions to ensure consistency and comparability.

Fuel consumption was measured using a calibrated buret, while performance metrics such as BSFC and BTE were calculated using eqs and . Emission data were recorded in real time using the SV-50 analyzer. After concluding the ambient temperature tests, a counterflow shell-and-tube heat exchanger was installed between the engine’s intake and exhaust manifolds to raise the intake air temperature. In this configuration, hot exhaust gases passed through the inner tubes, while the intake air flowed through the shell in the opposite direction, promoting effective heat transfer. The heat exchanger design is previously illustrated in Figure

Following the installation of the test apparatus, the engine was restarted using the biodiesel–ethanol blends and operated until the intake air temperature stabilized at approximately 51 ± 2 °C, significantly higher than the ambient baseline of 25 °C. This preheating temperature was deliberately selected based on literature reports indicating that it facilitates optimal ethanol vaporization, enhances combustion initiation, and minimizes the formation of nitrogen oxides (NO2). As reported, , the intake air temperatures above 55 °C yield diminishing returns in combustion efficiency while raising potential safety concerns. Therefore, a target range of around 51 °C was considered ideal for balancing combustion performance with operational safety.

After thermal stabilization, the complete test matrix encompassing load variation and performance/emission measurements was repeated for all three ternary fuel blends under preheated intake air conditions. The ethanol employed in the study, with a purity of 97%, was procured from FIRMAMENT TRADING PLC, a reputable Ethiopian supplier of laboratory-grade chemicals. The fundamental fuel properties of diesel and ethanol, including viscosity, cetane index, density, calorific value, and flash point are illustrated in Table . As referenced from ref , the densities and calorific values of the B20E10, B30E10, and B40E10 blends were determined using eqs and . Characterization of the produced biodiesel was conducted to assess its conformity with international fuel quality standards. As shown in Table , neat biodiesel (B100) and its blends with ethanol and diesel met the relevant specifications of ASTM D6751 and EN 14214. Notably, B100 exhibited higher density and kinematic viscosity compared to B20 and B40, though all values remained within the permissible limits defined by these standards. The viscosities of B20 and B40 were closely aligned with those of conventional diesel, thereby supporting favorable fuel atomization and compatibility with existing diesel injection systems.

All fuel characterization tests were certified by the Ethiopian Petroleum Supply Enterprise, providing independent validation of the biodiesel’s compliance. These findings confirm that biodiesel derived from cottonseed oil, along with its ethanol–diesel blends, possesses the requisite physicochemical properties for seamless integration into current diesel engines without requiring any hardware modifications. This underscores the practical applicability and technical viability of the proposed ternary blends for sustainable diesel engine operation.

Uncertainty Analysis

An uncertainty analysis was conducted to ensure the reliability and accuracy of the experimental data by accounting for instrumentation accuracy, experimental repeatability, and computational methodology. Before testing, key instruments, including the dynamometer, emission analyzer, and thermocouples, were calibrated by using certified reference standards, as summarized in Table .

Each test condition was repeated three times to assess the consistency. The standard deviation of engine performance parameters remained below 2%, while emission measurements exhibited deviations of less than 5%. The uncertainties in the derived quantities, such as BSFC and BTE, were calculated using standard error propagation techniques. The combined uncertainties were estimated to be ± 2.1% for brake thermal efficiency (BTE) and ± 3.4% for brake specific fuel consumption (BSFC).

Results and Discussion

This section presents and analyzes the experimental results. Engine performance and emission characteristics are illustrated by using graphical representations for conditions with and without intake air preheating. The analysis includes performance parameters such as brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), and brake power (BP), along with emission parameters including carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NO2), and unburned hydrocarbons (HC). These parameters are evaluated under varying engine loading conditions for both conventional diesel fuel and diesel-biodiesel-ethanol blends, tested at both ambient and elevated intake air temperatures.

Engine Performance without Preheating Intake

Air

Brake Power (P

b)

Figure illustrates the variation in brake power output as a function of the engine load for all tested fuel blends. A consistent increase in brake power is observed with rising engine load across all fuels, aligning with established thermodynamic expectations. Among the fuels tested, conventional diesel and the B20E10 blend exhibit nearly identical brake power profiles, indicating that the partial substitution of diesel with 20% biodiesel and 10% ethanol does not significantly compromise energy delivery or combustion efficiency. In contrast, the B30E10 and B40E10 blends demonstrate a noticeable reduction in brake power output, particularly under higher engine loads. This decline is primarily attributed to the lower calorific values of these higher biodiesel-ethanol blends, which directly reduce the total energy available for conversion into mechanical work. Additionally, the elevated viscosity and density of higher biodiesel content blends tend to impair fuel atomization and spray characteristics, leading to suboptimal air–fuel mixing and incomplete combustion.

Quantitatively, the average brake power reduction relative to diesel is 1.97% for B20E10, 4.61% for B30E10, and 4.76% for B40E10. These findings suggest that while B20E10 offers a promising compromise between performance and sustainability, further increases in biodiesel content may result in diminishing engine output, necessitating optimization of blend ratios or potential engine calibration adjustments.

Overall, the data support the feasibility of B20E10 as a viable alternative fuel blend for compression ignition engines, delivering performance comparable to that of diesel with reduced reliance on fossil fuels and associated emissions.

Brake Thermal Efficiency (BTE)

Brake thermal efficiency (BTE) progressively increased with rising engine load across all tested fuel types, as shown in Figure . This trend aligns with thermodynamic principles, wherein higher engine loads enhance combustion stability and reduce relative heat losses, thereby improving the overall efficiency of the energy conversion. The biodiesel–ethanol blends, particularly at lower blend ratios, exhibited BTE values comparable to those of conventional diesel. This can be attributed to the inherent oxygen content in both cottonseed biodiesel and ethanol, which promotes more complete combustion. Furthermore, the lower boiling point of ethanol improves the volatility and atomization of the fuel blend, leading to enhanced air–fuel mixing and more efficient combustion, especially in fuel-rich regions. Ethanol’s relatively lower flame temperature also contributes to reduced in-cylinder heat losses, further boosting thermal efficiency.

However, an incremental decline in BTE was observed with an increasing biodiesel concentration. This decrease is mainly due to the higher viscosity and density of cottonseed biodiesel, which impair atomization quality and combustion efficiency. Compared with pure diesel, the average reductions in BTE were 1.17% for B20E10, 4.57% for B30E10, and 6.20% for B40E10.

These findings suggest that while B20E10 maintains a favorable balance between engine efficiency and renewable fuel content, higher biodiesel blend ratios may lead to performance compromises. To harness the environmental benefits of higher blends without sacrificing efficiency, further engine optimization and fuel formulation improvements may be required.

Brake Specific Fuel Consumption (BSFC)

The variation in brake specific fuel consumption (BSFC) with engine load for diesel and various cottonseed biodiesel–ethanol blends is shown in Figure . The results reveal a clear BSFC increases with a higher biodiesel content in the blend. This behavior is primarily attributed to the lower calorific value and higher specific gravity of biodiesel relative to those of conventional diesel fuel. Since BSFC is inversely related to the fuel’s energy content, a lower heating value necessitates greater fuel mass to produce the same power output, thereby increasing BSFC. Among the tested fuels, conventional diesel exhibited the lowest BSFC across all load conditions, confirming its superior energy density and combustion efficiency. In comparison, the B20E10 blend displayed a modest increase in BSFC, yet maintained a relatively favorable performance. This can be ascribed to its comparatively higher heating value, lower kinematic viscosity, and reduced density, which enhance atomization, fuel–air mixing, and overall combustion quality. Furthermore, the inherent oxygen content in both biodiesel and ethanol improves the combustion efficiency by supporting more complete oxidation of the fuel.

Conversely, B30E10 and B40E10 demonstrated significantly higher BSFC values. The increased biodiesel content in these blends introduces higher viscosity and density, which impairs spray atomization and leads to suboptimal fuel–air mixing. These effects contribute to incomplete combustion and lower the thermal efficiency. Quantitatively, BSFC increased by approximately 11.76%, 23.53%, and 32.35% for B20E10, B30E10, and B40E10, respectively, when compared with baseline diesel fuel.

These findings underscore the importance of optimizing the blend composition to balance renewable content with combustion efficiency. While B20E10 emerges as a promising candidate offering a viable compromise, higher biodiesel concentrations may necessitate further engine calibration or additive strategies to mitigate fuel efficiency losses.

Exhaust Gas Emissions without Preheating Intake

Air

Emission standards are regulations that establish specific limits on the number of pollutants that can be released into the environment. While many of these standards primarily target emissions from automobiles and other motorized vehicles, they also apply to industrial sources, power plants, and diesel generators. In this study, the emission outputs of various fuel samples were measured, specifically carbon monoxide (CO), carbon dioxide (CO2), unburned hydrocarbons (HC), and nitrogen oxides (NO2), and compared with the emissions produced by conventional diesel fuel.

Exhaust Emissions of Carbon Monoxide

The variation in carbon monoxide (CO) emissions with engine load for conventional diesel and biodiesel–ethanol blends is depicted in Figure . Across all operating conditions, the biodiesel blends consistently exhibited lower CO emissions compared with pure diesel. This reduction is primarily attributed to the higher inherent oxygen content of both cottonseed biodiesel and ethanol, which promotes more complete oxidation during the combustion process, thereby minimizing the level of CO formation. CO emissions were highest under low-load conditions for all fuel types, a result of reduced in-cylinder temperatures that hindered complete oxidation and favored incomplete combustion. As engine load increased to moderate levels, CO emissions decreased due to enhanced combustion efficiency facilitated by improved vaporization and in-cylinder turbulence. At high loads, however, a secondary increase in the CO emissions was observed. This rise is likely due to elevated fuel injection rates that create locally rich zones and reduce the residence time, both of which limit the extent of oxidation.

Notably, B30E10 and B40E10 demonstrated the most significant reductions in CO emissions, especially under higher engine loads. This enhanced performance is attributed to the combined effect of ethanol and biodiesel, both of which improve oxygen availability and air–fuel mixing, supporting more efficient combustion under challenging conditions.

On average, CO emissions were reduced by approximately 4.65% for B20E10, 11.63% for B30E10, and 20.93% for B40E10 relative to diesel. These findings underscore the critical influence of blend composition and engine loading on emission characteristics and confirm the potential of biodiesel–ethanol blends as cleaner alternatives to fossil diesel in compression ignition engines

Exhaust Emissions of Carbon Dioxide

Carbon dioxide (CO2) emissions were found to increase progressively with rising engine load across all tested fuels. This trend is consistent with the higher fuel consumption and combustion intensity required at elevated loads, which promotes more complete oxidation of the carbon content in the fuel, as shown in Figure . When comparing the biodiesel–ethanol blends to conventional diesel, a general increase in the CO2 emissions was observed. This is primarily attributed to the elevated oxygen content in both cottonseed biodiesel and ethanol, which enhances the combustion efficiency and facilitates the oxidation of carbon monoxide (CO) into carbon dioxide (CO2). Among the tested blends, B20E10 exhibited the highest CO2 emissions, reflecting its enhanced oxidative performance and improved air–fuel mixing characteristics.

Interestingly, although B20E10 showed the most pronounced increase, the magnitude of the CO2 emission increment slightly declined as the biodiesel content increased from 30% to 40%. This subtle reduction may be due to the interplay between improved combustion and the thermal and physical properties of higher biodiesel blends, such as a lower heating value and increased viscosity, which could moderately suppress combustion intensity under specific conditions.

On average, the increases in CO2 emissions relative to diesel were approximately 7.83% for B20E10, 5.34% for B30E10, and 2.85% for B40E10. These findings underscore the dual effect of renewable fuel while CO2 emissions may rise due to more complete combustion, significant reductions in other pollutants, such as CO and unburned hydrocarbons, may offer an overall environmental advantage.

Exhaust Emissions of Hydrocarbons (HC)

Hydrocarbon (HC) emissions were observed to increase with a rising engine load across all tested fuel types. This trend is attributed to shorter combustion durations and the formation of localized fuel-rich zones at higher loads, which hinder complete oxidation of unburned fuel, as depicted in Figure . Compared with conventional diesel, biodiesel–ethanol blends generally exhibited higher HC emissions. This increase is primarily linked to the higher viscosity and lower volatility of biodiesel, which can impair atomization and vaporization, leading to incomplete combustion. Nonetheless, the extent of this increase varied with the blending ratio. Notably, B40E10 demonstrated the lowest HC emissions among the blended fuels. This reduction is likely due to its higher oxygen content, which promotes the more efficient oxidation of unburned hydrocarbons during combustion.

Although HC emissions were elevated for blended fuels overall, the emissions tended to decrease with increasing biodiesel concentration. This indicates that higher oxygen availability in blends such as B40E10 can offset some of the negative combustion characteristics of biodiesel, contributing to improved combustion efficiency and cleaner exhaust profiles.

On average, the increases in HC emissions relative to pure diesel were approximately 16.79% for B20E10, 8.00% for B30E10, and only 2.90% for B40E10. These results suggest that optimizing biodiesel content in ethanol–biodiesel blends may offer a viable pathway to reducing hydrocarbon emissions while leveraging the renewable nature of alternative fuels.

Nitrogen Oxide Exhaust Emissions

Nitrogen oxide (NO2) emissions exhibited a distinct upward trend with increasing engine load for all tested fuels, as shown in Figure . At lower loads, NO2 emissions remained relatively low due to reduced in-cylinder temperatures and slower combustion kinetics, which limited the thermal formation of NO2. The lowest recorded emission value was 166.29 ppm at 20% load for the B30E10 blend. As the engine load increased, NO2 emissions rose substantially across all fuel types. This increase is primarily driven by higher combustion temperatures and accelerated heat release rates, which are further intensified by the additional oxygen content present in biodiesel and ethanol. These conditions enhance combustion completeness but also favor the thermal NO2 formation mechanism, particularly during the premixed combustion phase. The highest observed NO2 emission was 798.54 ppm at 80% load for B40E10, reflecting the impact of the elevated load and oxygen availability on the peak flame temperature.

Interestingly, despite the higher NO2 values at elevated loads, the average NO2 emissions across the full load range for the biodiesel–ethanol blends were marginally lower than those of conventional diesel. Average NO2 reductions were approximately 11.13% for B20E10, 8.65% for B30E10, and 0.19% for B40E10 relative to diesel. These reductions suggest that while biodiesel–ethanol blends tend to increase peak combustion temperature, other properties, such as lower flame propagation rates, fuel-bound oxygen behavior, and altered ignition delay, can contribute to mitigating total NO2 output over the complete engine operating cycle.

These findings underscore the complex interaction among combustion chemistry, fuel formulation, and engine operating conditions in determining NO2 emissions, and they point to the potential for optimizing blend ratios to balance combustion efficiency with emissions control.

Engine Performance with Preheated Intake Air

The performance and emission characteristics of diesel and biodiesel–diesel blends supplemented with ethanol were experimentally evaluated under two air intake temperature conditions. Initial tests were conducted at a baseline intake air temperature of 25 °C, followed by comparative testing at an elevated intake air temperature of 51 °C to assess the influence of the thermal intake conditions on engine behavior.

Brake Power (P

b)

Brake power (BP) performance under elevated intake air temperature conditions (51 °C) is presented in Figure . Across all fuel types, brake power exhibits a near-linear increase with engine load, indicating that the load remains the dominant factor influencing power output, regardless of fuel composition. Notably, no substantial differences in brake power are observed between the tested fuels at lower loads, suggesting that the effects of fuel properties are more pronounced at higher thermal and combustion intensities. Despite its lower calorific value, the B40E10 blend demonstrated superior brake power at higher loads compared to B30E10 and B20E10. This enhancement can be attributed to the synergistic effects of ethanol’s lower boiling point and higher oxygen content, which promote finer atomization and more complete combustion. At elevated intake temperatures, these benefits are amplified, as preheating the intake air reduces ignition delay and improves fuel vaporization. These conditions facilitate more efficient utilization of oxygen present in both biodiesel and ethanol, thereby enhancing combustion completeness and thermal efficiency.

At lower engine loads, however, the advantage of B40E10 diminishes due to incomplete vaporization and longer ignition delay associated with higher viscosity and lower volatility of biodiesel. Nonetheless, the improved lubricity and preheated intake air compensate for some of these limitations, contributing to moderate power gain.

Quantitatively, intake air preheating resulted in average increases in brake power of approximately 1.34% for B20E10, 2.78% for B30E10, and 3.25% for B40E10 when compared to operation at ambient intake air temperature (25 °C). These findings underscore the role of thermal intake conditioning in enhancing the combustion behavior and performance of biodiesel–ethanol blended fuels in compression ignition engines.

Brake Thermal Efficiency (BTE)

Brake thermal efficiency (BTE) trends for the tested fuels under varying intake air temperatures are depicted in Figure . As expected, BTE increases with the engine load for all fuel types. This behavior is typical of compression ignition engines, where higher loads improve combustion efficiency due to increased in-cylinder temperatures, better atomization, and enhanced turbulence, all of which contribute to more complete fuel oxidation and improved energy conversion. A consistent improvement in BTE is also observed for the biodiesel–ethanol blends (B20E10, B30E10, and B40E10) when the intake air is preheated to 51 °C. Intake air preheating enhances the initial temperature of the air–fuel mixture, promoting earlier ignition and faster flame propagation. This leads to a reduction in ignition delay, improved fuel vaporization, and more efficient mixing with air, especially critical for biodiesel blends, which tend to have higher viscosity and lower volatility. As a result, combustion becomes more complete, thereby increasing the thermal efficiency.

However, as the proportion of biodiesel in the blend increases, BTE shows a gradual decline. This can be primarily attributed to the lower calorific value of biodiesel, which reduces the total energy available per unit mass of fuel. Furthermore, higher biodiesel content may slightly impair spray characteristics and air–fuel mixing due to its greater viscosity, especially under suboptimal combustion conditions. Interestingly, the peak BTE values for all tested fuels were recorded at 80% load with intake air preheated at 51 °C, indicating that this condition offers the best compromise between combustion completeness and energy conversion efficiency. The maximum BTEs recorded were 28.01% for B20E10, 26.00% for B30E10, and 25.00% for B40E10, as illustrated in Figure .

On average, intake air preheating at 51 °C led to BTE improvements of 3.9% for B20E10, 4.1% for B30E10, and 4.28% for B40E10 compared to baseline operation at ambient air temperature (25 °C). These enhancements underscore the potential of intake air thermal conditioning to mitigate the inherent limitations of biofuels, such as lower energy density and higher viscosity, by promoting more complete combustion and improving the overall engine thermal efficiency.

Brake Specific Fuel Consumption (BSFC)

Brake specific fuel consumption (BSFC) trends for biodiesel–diesel blends with ethanol addition at preheated intake air temperatures are presented in Figure . Among the tested fuels, B20E10 exhibits the lowest BSFC, which is consistent with its comparatively higher calorific value relative to those of other blends containing higher biodiesel proportions. This higher energy content allows the engine to extract more useful work per unit of fuel consumed, thereby decreasing the fuel consumption rate. Preheating the intake air notably improves the engine performance across all fuel blends. The elevated intake air temperature facilitates better fuel vaporization and mixing, which reduces the latent heat required for vaporization. This effect shortens the ignition delay period, leading to a more timely and efficient combustion. Consequently, the combustion process becomes more complete, and fuel utilization improves, resulting in a decrease in BSFC.

As the intake air temperature increases from 25 to 51 °C, a clear downward trend in BSFC is observed for all biodiesel–ethanol blends. This trend highlights the positive impact of intake air preheating on fuel economy, especially for blends with higher biodiesel content that typically suffer from poorer atomization and slower vaporization due to their higher viscosity and lower volatility.

Quantitatively, intake air preheating at 51 °C results in average BSFC reductions of 5.2% for B20E10, 9.52% for B30E10, and 11.11% for B40E10 compared to baseline operation without preheated intake air at 25 °C. The larger reductions observed for blends with higher biodiesel content (B30E10 and B40E10) indicate that preheating mitigates some of the combustion inefficiencies associated with biodiesel’s physical properties, enhancing overall engine fuel efficiency.

Exhaust Emissions with Preheated Intake Air

Exhaust Emissions of Carbon Monoxide

Figure illustrates the variation of carbon monoxide (CO) emissions with engine load at a constant speed and an elevated intake air temperature of 51 °C. Across the entire load range, blends with higher biodiesel content generally show lower CO emissions compared to those with lower biodiesel proportions. This reduction is primarily due to the increased oxygen content in biodiesel, which promotes more complete oxidation of CO during combustion and in exhaust gases. CO emissions are strongly affected by the engine load, biofuel composition, and intake air temperature. Preheating the intake air to 51 °C effectively reduces CO emissions for all biodiesel–diesel blends containing ethanol compared to operation at ambient intake temperature (25 °C). The elevated intake temperature enhances fuel vaporization and combustion completeness, thereby decreasing the level of CO formation.

The effect of intake air preheating is especially significant for blends with higher biodiesel concentrations, where the additional oxygen supplied by both biodiesel and ethanol further facilitates the oxidation of CO to CO2. The increased cylinder gas temperature at the elevated intake condition also promotes the chemical reactions that convert CO, leading to an overall decline in the level of CO emissions. Interestingly, at full engine load, the CO emissions are lower for all tested blends regardless of intake air temperature, and no significant difference is observed between preheated and ambient conditions. This suggests that at full load, the engine’s combustion chamber already reaches sufficiently high temperatures and energy levels for complete ethanol vaporization and combustion, making intake air preheating less critical.

On average, preheating the intake air to 51 °C results in substantial reductions in CO emissions-approximately 31.71% for B20E10, 36.84% for B30E10, and 32.35% for B40E10 compared to operation without preheating at 25 °C. These findings highlight the significance of intake air temperature and fuel composition in regulating CO emissions and improving combustion efficiency.

Exhaust Emissions of Carbon Dioxide

Figure depicts the variation of carbon dioxide (CO2) emissions with engine load for fuels tested under preheated intake air conditions (51 °C). As expected, CO2 emissions rise progressively with increasing engine load for all fuel blends. This trend reflects the greater fuel consumption and intensified combustion processes needed to meet the higher power demands. Among the tested blends, B40E10 consistently shows the highest CO2 emissions throughout most of the load range. This is primarily due to the higher oxygen content in biodiesel, which enhances the oxidation of carbon-containing compounds during combustion, thereby increasing the level of CO2 formation. The increased oxygen availability promotes more complete combustion, converting a larger fraction of the fuel’s carbon into CO2 rather than incomplete combustion products.

Furthermore, preheating the intake air leads to a noticeable increase in CO2 emissions compared with operation at ambient intake temperature (25 °C). This can be explained by improved fuel vaporization and better mixing of air and fuel, which enhance combustion efficiency inside the cylinder. More complete combustion results in higher CO2 emissions, as carbon is fully oxidized rather than partially oxidized or emitted as unburned hydrocarbons or CO.

Interestingly, the average CO2 emission increases at 51 °C intake air temperature are approximately 24.3% for B20E10, 22.14% for B30E10, and 21.24% for B40E10 compared to operation without intake air preheating. While B40E10 has higher absolute CO2 emissions, the relative increase due to preheating is slightly lower as biodiesel’s combustion characteristics interact with the thermal conditions in complex ways. This behavior may also be related to the lower carbon-to-hydrogen (C/H) ratio in biodiesel compared with conventional diesel. A lower C/H ratio means relatively less carbon per unit of hydrogen in the fuel, which influences the amount and nature of the combustion products. With more hydrogen present, combustion tends to produce more water vapor and less carbon dioxide for the same energy release, but the enhanced combustion completeness and oxygen content with biodiesel lead to net higher CO2 emissions.

In summary, the trends in Figure highlight that higher loads, biodiesel content, and intake air preheating collectively promote more complete fuel oxidation, thereby increasing the level of CO2 emissions. While this indicates efficient combustion, it also reflects the inherent trade-offs when using oxygenated biofuels and elevated intake temperatures.

Exhaust Emissions of Hydrocarbons

Figure illustrates the variation of hydrocarbon (HC) emissions with engine load for different test fuels under intake air preheating conditions (51 °C) compared to ambient temperature (25 °C). The intake air temperature significantly influences HC emissions by affecting the combustion quality and timing. Among the fuel blends, B40E10 consistently shows the lowest HC emissions across the entire load range. This can be explained primarily by the higher cetane number of biodiesels, which leads to a shorter ignition delay and an earlier combustion start. Early combustion allows more complete fuel oxidation within the combustion chamber, reducing the number of unburned hydrocarbons emitted. Additionally, biodiesel’s higher oxygen content improves combustion efficiency by supplying more oxygen molecules directly to the fuel-air mixture. This enhanced oxygen availability promotes a more thorough oxidation of fuel, further reducing HC emissions.

Preheating the intake air to 51 °C also contributes to lowering HC emissions by increasing the combustion temperature. The elevated temperature facilitates better fuel vaporization and faster chemical reaction rates, which help to minimize fuel-rich zones, where incomplete combustion typically occurs. At 80% load, the maximum HC emissions observed are 75.22 ppm for B20E10, 67 ppm for B30E10, and 65 ppm for B40E10, while at 20% load, the minimum values are 26, 23.2, and 24 ppm, respectively. These values reflect the expected increase in HC emissions with load due to shorter combustion durations and richer mixtures but also demonstrate how biodiesel blends and intake air preheating mitigate this effect.

In terms of percentage reduction, preheating the intake air leads to HC emission decreases of 11.49% for B20E10, 13.23% for B30E10, and 12.29% for B40E10 compared to operation without intake air preheating. This highlights the benefit of intake air heating in enhancing combustion completeness and lowering pollutant emissions.

Nitrogen Oxide Exhaust Emissions

Nitrogen oxide (NO2) emissions remain one of the most regulated and environmentally detrimental pollutants generated by diesel engines, comprising predominantly nitric oxide (NO) and nitrogen dioxide (NO2). Their formation is inherently linked to in-cylinder thermochemical conditions, particularly peak combustion temperatures, oxygen availability, and the heat release rate (HRR) during the combustion process.

Figure presents the variation of NO2 emissions as a function of engine load for various diesel–biodiesel–ethanol blends under preheated intake air conditions. The data reveal a consistent increase in NO2 emissions with rising engine load across all fuel blends. This trend is attributed to elevated in-cylinder temperatures at higher loads due to increased fuel input and combustion intensity, which promote thermal NO2 formation via the extended Zeldovich mechanism. Notably, among the tested blends, B40E10 (40% biodiesel, 10% ethanol, and 50% diesel) consistently exhibited the highest NO2 emissions at all load levels as indicted Figure . This behavior is linked to the higher HRR observed during the premixed combustion phase of this blend, resulting in pronounced peak cylinder pressures and temperatures. The higher oxygen content inherent to both biodiesel and ethanol enhances combustion completeness and air–fuel mixing, which collectively amplify thermal NO2 generation.

Additionally, the presence of ethanol, an oxygenated additive with a high latent heat of vaporization, may induce localized charge cooling during injection. However, this cooling effect appears to be offset by the net increase in the combustion efficiency and the resulting rise in average gas temperatures. The enhanced atomization and vaporization characteristics of ethanol-blended fuels further facilitate more uniform air–fuel distribution, especially under high-load conditions, thereby intensifying peak combustion temperatures and NO2 formation.

The role of the intake air temperature was also investigated by comparing engine operation under preheated (51 °C) and ambient (25 °C) intake conditions. Across all blends, NO2 emissions were found to be significantly higher with preheated intake air. The elevated intake temperature reduces the ignition delay, accelerates combustion kinetics, and shortens the premixed phase, which leads to an increase in peak cylinder pressure and combustion temperature. This thermal enhancement directly contributes to higher NO2 levels. Specifically, the average percentage increase in NO2 emissions due to intake air preheating was quantified as 14.13% for B20E10, 8.68% for B30E10, and 9.82% for B40E10. These results are consistent with the findings of Menelik and Niranjan (2018), reinforcing the conclusion that preheating enhances combustion efficiency at the expense of increased thermal NO2 production.

In summary, the observed trends in NO2 emissions are the result of complex interactions among the combustion temperature, oxygen availability, fuel reactivity, and intake air conditions. Blends with higher biodiesel and ethanol content promote complete combustion and higher HRR, but they also elevate NO2 due to increased oxygen supply and in-cylinder temperatures. Preheating intake air further amplifies these effects, emphasizing the critical need to balance combustion efficiency with emission control strategies, such as exhaust gas recirculation (EGR) or selective catalytic reduction (SCR), when utilizing alternative fuel blends in compression ignition engines

Result Outcomes

The experimental investigation revealed that the use of diesel–biodiesel–ethanol ternary blends, especially when coupled with intake air preheating, significantly influenced engine performance and exhaust emissions. Across all blends testedB20E10, B30E10, and B40E10brake thermal efficiency (BTE) and brake power (BP) improved with engine load. Intake air preheating at 51 °C further enhanced these metrics by reducing the ignition delay and improving fuel vaporization, particularly for higher-viscosity blends. Quantitatively, intake air preheating increased BTE by 3.9% for B20E10, 4.1% for B30E10, and 4.28% for B40E10, while also reducing brake-specific fuel consumption (BSFC) by 5.2%, 9.52%, and 11.11%, respectively. These improvements highlight the role of thermal conditioning in overcoming the combustion limitations of oxygenated biofuels. Among the tested blends, B20E10 demonstrated the closest performance to conventional diesel with only a 1.97% reduction in brake power and the lowest BSFC, making it the most balanced and efficient formulation.

Emission analysis showed that biodiesel–ethanol blends substantially reduced carbon monoxide (CO) and hydrocarbon (HC) emissions, particularly under preheated intake conditions. This reduction is attributed to the higher oxygen content in the blends, which supports more complete oxidation and suppresses the formation of partially combusted species. CO emissions dropped by 31.71% for B20E10, 36.84% for B30E10, and 32.35% for B40E10, while HC emissions decreased by 11.49%, 13.23%, and 12.29%, respectively, with intake air preheating. Although carbon dioxide (CO2) emissions increased with both load and biodiesel contents, indicating more complete combustion, this rise is an acceptable trade-off for the significant reduction in toxic pollutants. However, nitrogen oxide (NO2) emissions increased due to elevated in-cylinder temperatures, particularly under preheated conditions, with average increases of 14.13% for B20E10, 8.68% for B30E10, and 9.82% for B40E10. These findings suggest a need for complementary emission control technologies, such as exhaust gas recirculation or selective catalytic reduction, when adopting high-oxygen-content fuels.

Although this study primarily focused on gaseous emissions, particulate matter (PM) was not measured as it was outside the current experimental scope. Nevertheless, the behavior of PM emissions in diesel engines using oxygenated fuels is well established in the literature. Based on the known combustion characteristics of such fuels, it is reasonable to infer that blends with higher oxygen content, such as B20E10, would lead to reduced PM emissions. This is consistent with prior studies demonstrating that the additional oxygen content in biodiesel and ethanol blends enhances the oxidation of soot precursors during combustion, thereby limiting PM formation. Complete combustion is further promoted in diffusion flame zones, where PM typically originates, resulting in cleaner exhaust profiles. However, to validate this inference and provide a comprehensive environmental assessment, future investigations should include direct measurements and detailed characterization of PM emissions, including size distribution and chemical composition. Statistical analysis using one-way ANOVA confirmed the significance of the observed differences in BTE, BSFC, and emissions among fuel blends and intake conditions at a 95% confidence level (p < 0.05), reinforcing the credibility and reproducibility of the findings. The results confirmed that variations in performance and emission parameters across the different blends and intake temperatures were statistically significant. Overall, this study affirms the practical and environmental viability of cottonseed biodiesel–ethanol blends, especially B20E10, as an alternative fuel pathway, and it highlights the value of intake air preheating as a low-cost, performance-enhancing strategy in sustainable engine operation.

Integrating an intake air preheating system offers measurable gains in combustion and emissions, but its economic feasibility must be considered. The heat exchanger used in this study was fabricated from locally available materials at a minimal cost (∼USD 40). Operating costs are negligible since it utilizes waste exhaust heat. However, commercial retrofitting would require design standardization, material durability validation, and integration with engine control units. A preliminary cost–benefit analysis suggests that the reduction in fuel consumption and emissions may offset the one-time investment within a year of operation in high-usage scenarios

Conclusions and Implications

This study establishes that ternary fuel blends composed of diesel, cottonseed biodiesel, and ethanol, specifically B20E10, B30E10, and B40E10, are promising renewable alternatives to conventional diesel in compression ignition engines. When coupled with intake air preheating to 51 °C, these blends yielded notable performance enhancements and emission reductions. Brake thermal efficiency (BTE) improved by 3.9% for B20E10, 4.1% for B30E10, and 4.28% for B40E10, while brake-specific fuel consumption (BSFC) decreased by 5.2%, 9.52%, and 11.11%, respectively, compared with baseline ambient air conditions. These performance gains are attributed to improved fuel vaporization and reduced ignition delay under preheated conditions, particularly for higher-viscosity blends. Among all tested formulations, B20E10 demonstrated the most favorable trade-off, exhibiting near-diesel brake power (only 1.97% lower) and the lowest BSFC, making it the most balanced and viable blend for practical deployment.

In terms of emissions, all biodiesel–ethanol blends significantly reduced carbon monoxide (CO) and unburned hydrocarbon (HC) levels. CO emissions were lowered by 31.71%, 36.84%, and 32.35% for B20E10, B30E10, and B40E10, respectively, while HC emissions decreased by 11.49%, 13.23%, and 12.29% under preheated intake conditions. These reductions are primarily driven by the additional oxygen content in both biodiesel and ethanol, which facilitates a more complete combustion. However, an inherent trade-off was observed with nitrogen oxides (NO2) emissions, which increased due to elevated peak combustion temperatures. Specifically, NO2 emissions rose by 14.13% for B20E10, 8.68% for B30E10, and 9.82% for B40E10, with the highest emission recorded at 798.54 ppm for B40E10 at 80% load. While B40E10 offered the lowest HC output (65 ppm), its NO2 burden emphasizes the need for after-treatment strategies such as exhaust gas recirculation (EGR) or selective catalytic reduction (SCR). Higher biodiesel ratios are to be deployed in environments with strict emissions regulations.

Although the moderate preheating temperature (51 °C) is unlikely to cause immediate mechanical degradation, prolonged use may accelerate wear on components, such as intake valves, piston rings, and lubricating oil, due to elevated thermal stresses. Long-term durability studies and lubricant degradation analyses are recommended for a comprehensive risk assessment. Preheated air at moderate levels (51 °C) is unlikely to induce thermal stress under controlled operating conditions; however, extended use may lead to valve wear, piston ring degradation, and accelerated oil oxidation. Moreover, ethanol’s hygroscopic nature and biodiesel’s oxidative instability may affect fuel line components and combustion chamber surfaces over prolonged durations. While previous studies indicate that properly stored and stabilized cottonseed biodiesel has a shelf life of 6 to 12 months, long-duration engine tests, material compatibility studies, and lubricant degradation analyses are necessary to confirm the durability of engine systems using these blends.

In summary, this research provides strong experimental evidence that diesel–biodiesel–ethanol blends, especially B20E10, can enhance thermal efficiency and reduce harmful emissions when supported by intake air preheating. The results are particularly relevant for resource-constrained regions, such as Ethiopia, where reliance on imported fossil fuels and environmental challenges necessitate locally adaptable, sustainable energy solutions. Future work should prioritize long-term durability assessments, scalable biodiesel production pathways, and NO2 control techniques to advance the practical implementation of these fuel strategies. Additionally, integrating predictive combustion modeling, real-time diagnostics, and life-cycle emission analyses would offer a deeper understanding of system-level impacts and accelerate the transition to cleaner transport technologies.