Performance and Emission Analysis of a Biogas–Diesel

Dual-Fuel Engine Enhanced with Diethyl Ether Additives

Authors: Menelik Walle, Kumlachew Yeneneh, Gadisa Sufe, Besufekad Negash Fetene

Abstract

The growing global energy demand and the urgent need to reduce greenhouse gas emissions have driven significant interest in sustainable and alternative fuels with biogas emerging as a promising renewable energy source derived from the anaerobic digestion of organic waste. Despite its carbon-neutral combustion and waste-to-energy potential, biogas faces challenges, such as low energy density and slow flame propagation, limiting its direct use in conventional diesel engines. This study explores the feasibility of using biogas in a four-stroke, single-cylinder, water-cooled, direct-injection diesel engine operating in dual-fuel mode with diethyl ether (DEE) employed as a combustion enhancer blended with diesel in varying proportions. A custom-designed Venturi-style biogas–air mixer was optimized using ANSYS Fluent CFD simulations to ensure efficient fuel mixing, and engine performance and emissions were evaluated at a constant speed of 1500 rpm under varying loads, biogas flow rates (2, 4, and 6 L/min), and DEE blending ratios (5, 10, and 15%). Results showed that dual-fuel operation with biogas significantly reduced NO* x

• emissions due to its lower flame temperature, but increased HC and CO emissions due to its high CO2 content and low combustibility, while the addition of DEE improved ignition, combustion efficiency, and engine performance. The optimal configuration of a 2 L/min biogas flow rate and 5% DEE blend achieved the best balance of increased brake thermal efficiency (BTE), reduced brake-specific fuel consumption (BSFC), and minimized emissions, demonstrating the potential of DEE-enhanced biogas–diesel blends in dual-fuel engines as a practical pathway for integrating renewable energy into existing systems and advancing sustainable energy solutions in transportation and power generation.

Introduction

Diesel engines remain indispensable across key sectors such as transportation, agriculture, and industry due to their high thermal efficiency, durability, and robustness under demanding operating conditions. However, this utility comes at a significant environmental cost. Diesel combustion emits a range of pollutants, including nitrogen oxides (NO* x *), carbon monoxide (CO), unburned hydrocarbons (HC), and particulate matter (PM), all of which contribute to air pollution, human health hazards, and climate change. As global environmental regulations tighten and public awareness increases, the need for cleaner and more sustainable engine technologies has become increasingly urgent. Addressing this challenge requires not only improving combustion efficiency but also reducing reliance on fossil-based fuels.

Among the many alternative fuels under consideration, biogas has gained substantial attention due to its renewable origin, waste-to-energy potential, and carbon-neutral combustion characteristics. , Biogas is typically composed of 50–70% methane (CH4) and 30–50% carbon dioxide (CO2), with minor constituents like hydrogen sulfide and water vapor. It is produced through anaerobic digestion of organic matter, such as livestock manure, food waste, and agricultural residues. By utilizing waste streams, biogas contributes to circular economy goals while offering a cleaner energy source. , However, its inherent limitations, namely, low energy density, poor ignition characteristics, slow flame speed, and dilution by inert CO2, pose significant barriers to its direct use in conventional diesel engines. These drawbacks often result in incomplete combustion, elevated HC and CO emissions, and reduced power output, especially under high-load or variable-load conditions. ,

A widely adopted solution to overcome these limitations is the dual-fuel combustion strategy, where biogas acts as the primary fuel and diesel serves as the ignition source. , However, the effectiveness of this approach is influenced by the ignition properties of the pilot fuel and the quality of the biogas-air mixing. In this context, diethyl ether (DEE) emerges as a promising ignition improver due to its low autoignition temperature, high cetane number, and oxygen-rich molecular structure. DEE not only promotes earlier and more reliable ignition of the biogas/diesel mixture but also enhances the oxidation process, leading to more complete combustion. These properties are particularly valuable in mitigating the delayed ignition and incomplete oxidation commonly associated with biogas use.

Several studies have explored the use of DEE in dual-fuel engines. For example, Singh and Singh reported that the addition of DEE improved brake thermal efficiency and reduced NO* x

• and HC emissions under varying load conditions. Similarly, Mahla et al. found that DEE contributed to combustion stability and decreased the ignition delay, especially at lower engine loads. Feroskhan et al. further emphasized DEE’s role in reducing cyclic variation and supporting flame propagation. ,,, While these findings affirm DEE’s combustion-enhancing effects, they diverge when it comes to emissions trade-offs. Some studies observed reductions in CO and HC, while others reported increases, particularly at higher DEE concentrations due to overlean or over-rich mixtures, or due to volatility-induced fuel stratification. −

Additionally, most existing studies do not account for how biogas-air mixing quality affects combustion uniformity, nor do they validate the air-fuel delivery mechanisms using CFD tools. ,

Therefore, this study addresses a critical gap by combining experimental testing with a computational approach to evaluate dual-fuel engine operation using biogas- and DEE-enhanced diesel blends. A key innovation lies in the design and optimization of a venturi-type biogas-air mixer modeled using ANSYS Fluent CFD simulations. This mixer ensures uniform fuel-air mixing, which is essential for complete combustion and stable performance while minimizing pressure losses. The simulation incorporates grid independence analysis and turbulence modeling to predict the mixer performance under various flow conditions.

Experimentally, the study examines how different biogas flow rates (2, 4, and 6 L/min) and DEE blending ratios (5, 10, and 15%) affect brake power (BP), brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), and exhaust emissions (CO, CO2, HC, and NO* x *). Statistical analysis (ANOVA and t tests) is applied to identify which observed changes are statistically significant, increasing the reliability of conclusions drawn.

By explicitly linking mixer design, turbulence modeling, combustion behavior, and performance metrics, this research contributes to a comprehensive framework for optimizing dual-fuel operation in small diesel engines. Unlike prior studies that treated performance and emissions separately from mixer dynamics, this work integrates design and performance validation in a single methodology. The outcomes provide not only improved fuel efficiency and reduced NO* x

• but also practical guidance for implementing DEE-enhanced biogas in real-world applications, especially in rural or decentralized energy systems, where biogas availability is high.

Methodology

This study employed a comprehensive experimental and computational methodology to evaluate the influence of diethyl ether (DEE) on the emissions and performance of a diesel engine operating in dual-fuel mode with biogas. The process began with the design and computational simulation of a Venturi-type biogas-air mixer using ANSYS Fluent software. Key parameters, such as the beta ratio and gas inlet angle, were optimized to achieve a uniform fuel–air mixture. Based on the optimized design, the mixer was fabricated and integrated into a single-cylinder, four-stroke, water-cooled diesel engine for testing.

Experimental trials were conducted under a constant engine speed and varying load conditions. Tests included different biogas flow rates (2, 4, and 6 L/min) and DEE blend ratios (5, 10, and 15%) with diesel fuel. Performance metrics such as brake power, brake thermal efficiency, and brake-specific fuel consumption were evaluated by using precise measurement tools. Additionally, exhaust emissions including CO, HC, NOx, and CO2 were analyzed to assess the environmental impact of the dual-fuel operation.

The selection of biogas flow rates (2, 4, and 6 L/min) and DEE blending ratios (5, 10, and 15%) was guided by a combination of prior research findings and practical considerations to ensure a comprehensive evaluation of the dual-fuel system’s performance and emissions. These values were chosen to cover a range of operating conditions, allowing for a systematic assessment of the effects of biogas and DEE on engine performance and emissions.

The biogas flow rates of 2, 4, and 6 L/min were selected based on prior studies that have demonstrated the effectiveness of these flow rates in dual-fuel engines. For instance, Barik and Murugan and Feroskhan et al. have shown that biogas flow rates in this range (2–6 L/min) provide a balance between reducing diesel consumption and maintaining stable combustion in dual-fuel engines. ,, A flow rate of 2 L/min represents a low biogas substitution level, which is ideal for evaluating the baseline performance of a dual-fuel system with minimal biogas input. This flow rate ensures that the engine operates with minimal disruption to the combustion process while achieving significant diesel displacement. A flow rate of 4 L/min represents a moderate biogas substitution level, which is often used to achieve a more substantial reduction in diesel consumption while maintaining an acceptable combustion stability and engine performance. Finally, a flow rate of 6 L/min represents a higher biogas substitution level, which pushes the limits of biogas utilization in dual-fuel engines. This flow rate is useful for assessing the system’s performance under more challenging conditions, where the higher biogas content may lead to incomplete combustion and increased emissions of unburned hydrocarbons (HC) and carbon monoxide (CO).

Similarly, the DEE blending ratios of 5, 10, and 15% were chosen based on prior research that has demonstrated the effectiveness of DEE as a combustion enhancer in dual-fuel systems. Studies by Kumar and Goga and Mahla et al. have shown that DEE blends in the range of 5 to 15% significantly improve ignition quality, reduce ignition delay, and enhance combustion efficiency in biogas–diesel dual-fuel engines. , A 5% DEE blend was chosen as the lower limit to evaluate the minimum amount of DEE required to improve combustion efficiency and reduce emissions without significantly altering the fuel’s properties. This blend is particularly useful for assessing the incremental benefits of adding DEE to a fuel mixture. A 10% DEE blend represents a moderate level of DEE addition, which is expected to provide a more pronounced improvement in the combustion efficiency and emission reduction. This blend is commonly used in studies to balance the benefits of DEE with the potential trade-offs in fuel consumption and engine performance. Finally, a 15% DEE blend was selected as the upper limit to evaluate the maximum practical amount of DEE that can be added to the fuel mixture without causing adverse effects, such as excessive fuel volatility or reduced energy density. This blend is useful for assessing the system’s performance under more aggressive DEE addition, which may further enhance combustion but could also lead to increased HC emissions due to the higher volatility of DEE.

By selecting these specific biogas flow rates and DEE blending ratios, this study aims to systematically evaluate the performance and emission characteristics of the dual-fuel system across a range of operating conditions. This approach ensures that the findings are both scientifically rigorous and practically relevant, providing valuable insights into the optimal configuration for integrating biogas and DEE into diesel engines.

Design and Computational

Analysis of Biogas–Air Mixer

In order to achieve the uniform fuel–air mixture required for steady and effective combustion in dual-fuel engines, the initiative started with the construction of a Venturi-type biogas–air mixer. The design parameters included the beta ratio (throat-to-pipe diameter ratio), inlet and outlet diameters, gas inlet angle, and number of biogas inlet holes. These parameters were optimized to ensure uniform mixing of biogas with air. The flow behavior inside the mixer was simulated through the use of ANSYS Fluent software and computational fluid dynamics (CFD) analysis. The analysis evaluated different beta ratios (0.4, 0.5, and 0.6) and gas inlet angles (ranging from 0 to 45°) to identify the configuration that produced the most homogeneous mixture with minimal pressure losses. The finalized design was selected based on its ability to create a stable and uniform fuel–air mixture.

The equation has then been used to calculate the necessary mass airflow rate for a four-stroke engine, formulated according to eq .ṁa=Q̇a×ρa1

Equation , which includes the displacement volume (V d) derived from the calculation in eq , is used to determine the required mass airflow rate for a four-stroke engine. The airflow rate entering the engine cylinder is calculated at maximum power production in diesel-only mode, considering the maximum engine speed with the throttle fully open according to eq . According to eqs and , the inlet and throat areas (A i and A t) are obtained from the inlet manifold hose diameter in order to calculate the inlet air velocity to the mixer. Within the ASME standard range of 0.25 to 0.75, the throat-to-pipe diameter ratio (β) is selected for the Venturi mixer design, with particular values of 0.4, 0.5, and 0.6. Equation is then used to get the throat diameter and area. Accurate airflow estimate and mixer optimization are made possible by the concept of continuity, which is represented by eqs and and guarantees that the volume flow rate stays constant across the Venturi mixer’s input, throat, and output. Q̇a=ηv×Vd×Nn×602 Vd=π×b24×s×NcVd=π×(0.08m)24×0.08m×1=4.02×10−4m3Q̇a=0.9×4.02×10−4m3×2600rpm2×60=0.00784m3sṁa=0.00784m3/s×1.184kg/m3=0.00928kg/s=33.408kg/h3 Q̇a=Vi×Ai4 Ai=π(Di)24Ai=π(0.043m)24=0.001452m25

Consequently, the air velocity at the mixer inlet during diesel-only operation is determined as followsVi=Q̇aAi=0.00784m3/s0.001452m2=5.4m/sDt=β×DiDt=0.4×0.043m=0.0172m=17.2mm6

Then, the throat area (A t) becomesAt=π×(Dt)24At=π×(0.0172m)24=0.000232m27 Q̇i=Q̇t=Q̇o8 Q̇t=Vt×At9 Vt=Q̇iAt=0.007839m3/s0.000232m2=33.79m/s

The air velocity increases due to the Venturi’s throat and is proportional to changes in the cross-sectional area and the Mach number (M). Equation

states that M must stay below 0.3 for the flow at the neck to be deemed incompressible. By doing this, the flow is guaranteed to meet the requirements for incompressibility in which the gas density is essentially constant regardless of changes in pressure. Since the fluid density is unaffected by variations in pressure in such low Mach number situations, the flow can be categorized as incompressible. Equation illustrates how flow behavior along a streamline is analyzed using Bernoulli’s equation in its generic version. Assuming negligible height differences (Z1 = Z2) and using standard atmospheric pressure (P = 101325 Pa) along with air density (ρ = 1.184 kg/m^3^), the vacuum generated at the throat is calculated accordingly. This ensures accurate characterization of the Venturi’s performance and flow properties.V=MC10

From the above equation,M=VC=33.79m/s343m/s=0.098p1ρa+gZ1+V122=p2ρa+gZ2+V22211

Assuming height different Z 1 = Z 2 and p 1 = p atm, where p 1 = 101,325 Pa and density of air, ρa = 1.184 (kg/m^3^).

Then, the vacuum created at the throat was simplified using eq :ΔP=V22−V122×ρaΔP=33.792−5.39822×1.184=658.67Pa12

In Dual-Fuel Mode

After the engine is initiated in diesel-only mode, biogas is gradually introduced to transition the engine into dual-fuel operation. During this process, the biogas flow control valve is progressively opened, allowing the vacuum at the Venturi throat to draw biogas from its source into the mixer while maintaining diesel-only operation. This approach reduces the engine cylinder’s airflow rate by replacing a portion of the incoming air with biogas, while ensuring that the total mass flow rate remains constant. According to eq , the combined mass flow rates of air and biogas in dual-fuel mode are equal to the total mass flow rate in diesel-only mode.

The biogas density is calculated using eq , and the biogas gas constant is independently obtained as described in ref . From this, the volumetric airflow rate is determined using eq . Since biogas primarily consists of multiple components but only methane contributes to combustion, stoichiometric combustion assumes that 1 mol of methane reacts with two mol of oxygen. Using eq , the stoichiometric air–fuel ratio for this process is 17.2. The volumetric flow rate of methane is determined by using its density and the universal gas constant. By substituting the result from eq into eq , the expression for volumetric airflow rate is obtained. This leads to eq .

The airflow rates (Q a) and biogas flow rates (Q bg) are computed. Assuming zero velocity within the biogas cylinder, the biogas is initially drawn into the system by the vacuum at the Venturi throat during diesel-only operation. The biogas flow velocity at the fuel tube outlet is determined using Bernoulli’s equation, yielding a value of V bg = 34.295 m/s, as shown in eq . Based on this velocity and the derived fuel inlet area, the biogas inlet diameter is calculated using eq , along with the number of biogas intake holes.(ṁa+ṁbg)=(Q̇a×ρa+Q̇bgρbg)(ṁa)dieselmode=(ṁa+mbg˙)dualfuelmode(ṁa+ṁbg)=(Q̇a×ρa+Q̇bgρbg)=0.00928kgs13 ρa=1.184kg/m3ρbg=pγT,γ=RMbg14 (Q̇a×ρa+Q̇bgρbg)=0.00928kg/s(1.184Q̇a+1.12Q̇bg)=0.00928kg/sQ̇a=0.00784−0.9459Q̇bg15 CH4+2(O2+3.76N2)→CO2+2H2O+7.5N2ṁaṁf=Q̇a×ρaQ̇CH4×ρCH4=17.2ρCH4=densityofmethane(0.656kg/m3)16where the density of methane and universal methane gas constant isρCH4=Pγ×Tandγ=RMCH417

When substituting eq in eq , the volume airflow rate becomes eq :Q̇CH4=0.6Q̇bg18

The air requirement for complete combustion is based on methane’s stoichiometry (1 CH4:2 O2). This derivation allows estimation of the air requirement in the dual-fuel mode using known values for CH4 density and the universal gas constant.Q̇a=6.5372Q̇bg19

From eqs and , *Q̇a

• and Q̇ bg are Q̇bg=0.0011765m3/sandQ̇a=0.006726m3/s

Therefore, the mass flow rate is indicated in eq :ṁa=Q̇a×ρa=0.006726m3/s×1.184kg/m3=0.007963kg/sṁbg=Q̇bg×ρbg=0.0011765m3/s×1.12kg/m3=0.0013176kg/s20

Since Q̇ a = V i × A i; then, the air inlet velocity in dual-fuel mode isVi=Q̇aAi=0.006726m3/s0.0014522m2=4.6315m/sVbg=2ΔPρbg21

Since Q̇ bg = V bg × A bg; then, the fuel inlet area isAbg=Q̇bgVbg=0.0011765m3/s34.295m/s=0.34305cm2

The biogas inlet diameter is determined by the number of inlet holes and is expressed as eq . Abg=πdbg24×n22

Therefore, the diameter of each hole is simplified from as shown in eq :dbg=2Abg2π=4×34.305mm22π=4.67mm23

Tables and in the document detail the geometrical dimensions of the Venturi mixer for numerical simulation and the boundary conditions applied during the computational analysis, respectively. Table specifies critical design parameters, such as inlet diameter, throat diameter, and angles of convergence and divergence, vital for optimizing air–biogas mixing efficiency. Table outlines the simulation constraints, including inlet flow velocity, pressure, and temperature, ensuring that the numerical model accurately reflects real-world engine conditions accurately. These inputs are integral to validating the Venturi mixer’s design for enhancing diesel engine performance in dual-fuel mode.

Modeling and Mesh Generation

In this study, ANSYS Workbench 21.2 was utilized to design and generate the biogas–air mixer models. The meshing process employed the tetrahedron approach, which divided the mixer geometry into tetrahedral elements, while incorporating patch conforming, inflation, and curvature refinements to optimize accuracy. The number of nodes and elements in the mesh had a major impact on the processing time and the simulation accuracy. The ideal mesh resolution, according to data analysis, was that which produced an inaccuracy of less than 5%. As shown in Figure , the mixer geometry that does not include moving mechanical mechanisms was created using SolidWorks software.

The Venturi’s measurements were set to correspond with the engine air intake manifold’s inlet and outlet diameters. In particular, 43 mm was chosen for the intake manifold, inlet Venturi, and output Venturi diameters. To ensure a homogeneous mixture of biogas and air with minimal pressure loss at the Venturi throat, various design parameters were analyzed. These included convergent angles of 20, 21, and 22°, a divergent angle of 80°, and beta ratios of 0.4, 0.5, and 0.6. The analysis focused on evaluating the velocity, pressure, methane mass fraction, and turbulence kinetic energy of the biogas–air mixture within the mixer to achieve improved homogeneity and promote complete combustion.

To ensure the reliability of the CFD results, a grid independence study was conducted by comparing three mesh coarse, medium, and fine. The mesh was refined until the variation in key parameters (velocity, pressure drop, and turbulence kinetic energy) between successive mesh refinements was less than 2%. This validation confirmed that the medium-density mesh (with approximately 1.2 million elements) achieved the optimal balance between the computational cost and solution accuracy.

The numerical formulations of the continuity, momentum, and energy equations, all essential to fluid dynamics, formed the basis of the computational fluid dynamics (CFD) simulations. Tetrahedral meshes were employed for all simulations as shown in Figure . Several key assumptions were made to simplify the modeling. The conventional k–ε turbulence model, which is well known for its stability, numerical resilience, and accuracy in predicting fully turbulent flows in internal combustion and mixing devices, was selected due to its proven effectiveness in modeling swirling and recirculating flows. This model strikes a good balance between computational efficiency and the fidelity required to capture the mixing and turbulence behavior in the Venturi mixer. For the mixing process, no chemical reaction between biogas and air was assumed, requiring the solution of additional equations for species mass fractions to analyze the mixing behavior. Additionally, the flow was assumed to be incompressible as the velocities involved were low enough for the fluid density to be considered constant.

At the air inlet boundary, a fixed velocity condition was applied with the mass flow rate determined from the specified velocity, pressure, temperature, and fluid density. A fixed static pressure condition was applied to the biogas intake, enabling the fuel mass flow injected into the venturi to be included in the simulated solution. In order to accommodate both inflow and outflow inside the computational domain and satisfy the continuity equation, fixed-pressure conditions were added at the outlet borders to anchor the system pressure.

Venturi Gas Mixer Manufacturing Process

The air–biogas Venturi mixer was constructed from a steel rod material using a series of machining processes, including turning, drilling, tapering, and welding, to achieve both precision and durability. The fabrication was carried out using a lathe machine and a drilling machine.

In the turning process, a lathe machine equipped with a cutting tool was used to reduce the steel rod’s diameter from 50 mm to the required 43 mm. This step not only achieved the desired dimensions but also enhanced the rod’s surface finish for subsequent operations. The drilling process began with marking the throat’s circumference into two equal sections using a scratch tool. A central hole was then drilled longitudinally through the steel rod using an 18 mm-diameter drill bit. Additionally, biogas inlet holes measuring 4.5 mm in diameter were drilled at a 45° angle at the throat to facilitate the injection of biogas into the mixer.

The tapering process was then performed on both the convergent and divergent sections of the Venturi mixer. For the convergent portion, the cutting tool was set to an angle of 21° on the compound head of the lathe machine, followed by tapering of the inlet section. Similarly, the divergent portion, which forms the mixture outlet, was tapered by positioning the cutting tool at an 8° angle. The same tapering technique was applied to the outer surface of the mixer for uniformity and aerodynamics.

To integrate biogas injection pipelines, a welding process was used, ensuring proper alignment and sealing of the two biogas inlet holes. The final surface finishing was accomplished using glass paper, which provided a smooth surface for the tapered and turned sections, enhancing the device’s performance and reducing flow resistance.

The completed Venturi mixer is shown in Figure , highlighting the precision and functionality of the manufacturing process.

Experimental

Setup and Procedure

A stationary, four-stroke, single-cylinder, water-cooled diesel engine running at a steady pace was used for experimental testing. With a rated power output of 5.15 kW at 2600 rpm, this direct-injection engine is coupled to a hydro-dynamometer, which serves as the loading device. Table displays the engine’s comprehensive technical specs.

In this study, the engine was tested under steady-state conditions and varying load levels in two distinct combustion modes. During single-fuel combustion mode, the engine operated exclusively on clean, additive-free diesel fuel, and its performance and exhaust emissions were documented for baseline comparison. In dual-fuel combustion mode, the engine used diesel or a diesel-diethyl ether (DEE) blend as the pilot fuel, while biogas served as the primary fuel. The dual-fuel setup was achieved by modifying the diesel engine to include a Venturi gas mixer within the intake manifold developed using a custom-designed model. Biogas for the experiments was supplied by a fixed-dome-type digester that utilized pig dung as feedstock.

Key performance indicators, such as brake thermal efficiency (BTE, %), brake power (PB, kW), and brake-specific fuel consumption (BSFC, kg/kWh), were analyzed. Exhaust emissions, including hydrocarbons (HC) and nitrogen oxides (NO* x *), were measured in parts per million (ppm), while carbon monoxide (CO) and carbon dioxide (CO2) were assessed in volume percentage (%Vol) using an FGA-4100 exhaust gas analyzer.

Prior to testing, the FGA-4100 analyzer was calibrated using certified calibration 500 ppm of CO, 1000 ppm of HC (propane equivalent), and 15% CO2. A zeroing procedure was performed using ambient air. The analyzer was recalibrated after every 10 test cycles to ensure accuracy and consistency across all emissions measurements.

The investigation included three different DEE blend ratios with diesel and three distinct biogas flow rates. All tests were conducted with the engine operating at a constant speed of 1500 rpm. The engine maintained an injection timing (IT) of 23° bTDC and a compression ratio (CR) of 1 under standard operating conditions. Figure A and Figure B illustrate the experimental equipment and schematic setup, respectively.

This experimental configuration enabled a detailed evaluation of the engine’s performance and emission characteristics across various operating conditions. The findings offer valuable insights into the potential of dual-fuel combustion by using diesel–DEE blends in combination with biogas as a renewable energy source.

Fuel Properties

Figure shows the experimental setup showing diesel and diethyl ether (DEE) fuels prepared for testing. The study employed diesel, DEE, and biogas as experimental fuels, with diesel and DEE functioning as pilot fuels to initiate combustion. The DEE used in this investigation was procured from BEKAS Chemicals PLC, a chemical manufacturing company based in Ethiopia. Critical fuel characteristics such as density, viscosity, calorific value, flash point, and cetane index are outlined in Tables and , as these factors directly impact engine performance and emissions. The cetane number, a measure of ignition quality, is especially significant as it affects engine efficiency and smooth operation. DEE contributes to improved combustion by elevating the cetane number and oxygen content in the fuel mixture, thereby offsetting the efficiency losses typically associated with biogas utilization. The engine was configured to operate in dual-fuel mode (DFM), using biogas as the primary energy source. Modest modifications were made to the intake manifold to ensure consistent and reliable performance, while integrating biogas as a sustainable fuel option.

Experimental Test Matrix

The experimental matrix is designed to assess engine performance and emissions across varying operating modes and fuel configurations. In diesel-only mode, the engine is tested at the factory-specified compression ratio (CR), injection pressure (IP), and injection timing (IT). Tests are conducted at load intervals of 0, 20, 40, 60, and 80% using pure diesel fuel. Performance metrics such as brake power (Pb), brake-specific fuel consumption (BSFC), and brake thermal efficiency (BTE) are evaluated alongside emissions parameters, including carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), and nitrogen oxides (NO* x *). For dual-fuel operation, biogas is cofired with diesel at flow rates of 2, 4, and 6 L/min, keeping CR, IP, and IT constant. Performance and emissions are analyzed under the same load conditions. The study also investigates the optimal biogas flow rates when using DEE-diesel blends (D95DEE5, D90DEE10, and D85DEE15), evaluating these combinations based on the same metrics to identify the most effective and environmentally favorable configuration.

The properties of the base fuel used in the study are detailed in Table . This table provides essential information about the characteristics of the fuels, which are critical for understanding their impact on engine performance and emissions.

The biogas used in this study was produced through the anaerobic digestion of pig dung in a fixed-dome-type biogas digester. The composition of the biogas was approximately 60% methane (CH4) and 40% carbon dioxide (CO2), with trace amounts of other gases such as hydrogen sulfide (H2S) and nitrogen (N2). This composition is typical for biogas produced from animal waste and is consistent with the properties reported in prior studies, such as Bouguessa et al. (2020).

The biogas was sourced from a local biogas plant in Bishoftu, Ethiopia, which uses pig dung as the primary feedstock. The digester operates under mesophilic conditions (30–40 °C) and produces biogas with a lower heating value of approximately 20.460 MJ/kg, as reported in Table . The biogas was purified to remove moisture and hydrogen sulfide before being used in the experiments to ensure consistent combustion properties and minimize corrosion in the engine components. The composition of biogas (60% CH4, 40% CO2) was periodically verified using a portable gas chromatograph (model: X-STREAM Biogas Analyzer) to ensure consistency. During testing, a flow meter and nonreturn valve ensured constant supply pressure, while a desiccant column removed residual moisture. These measures helped maintain consistent fuel quality throughout the experiments

To further support the use of DEE as a combustion enhancer, the physicochemical properties of diesel–DEE mixtures were analyzed. These properties are critical for understanding how the DEE affects the combustion process and engine performance. The key properties of diesel–DEE blends are summarized in Table .

The addition of DEE to diesel results in a reduction in viscosity and an increase in oxygen content, both of which contribute to improved atomization and combustion efficiency. However, the lower heating value of DEE (33.9 MJ/kg) compared with diesel (42.5 MJ/kg) leads to a slight decrease in the energy density of the blended fuel. This explains the moderate increase in brake-specific fuel consumption (BSFC) observed in dual-fuel operation as more fuel is required to achieve the same power output.

Despite this trade-off, the higher cetane number and oxygen content of DEE blends enhance combustion stability and reduce emissions, making them a practical choice for dual-fuel systems. The properties of diesel–DEE mixtures were determined experimentally and are consistent with values reported in prior studies, such as Kumar and Goga and Mahla et al. ,

Uncertainty Analysis

To ensure the reliability of the experimental results, an uncertainty analysis was conducted for key performance parameters, including brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), and exhaust emissions (CO, HC, NO* x *, and CO2). The analysis considers instrument accuracy, error propagation, and repeatability of measurements.

Measurement

Instruments and Accuracy

The following instruments were used for data acquisition with their respective 1.Dynamometer (brake power, BTE): accuracy ± 0.5%2.Fuel flow meter (BSFC): accuracy ± 0.2%3.Exhaust gas analyzer (CO, HC, NO* x , CO2):(1)CO: accuracy ± 1%(2)CO2: accuracy ± 2%(3)HC: accuracy ± 2%(4)NO x *: accuracy ± 3% 4.Error propagation analysis

Uncertainty in brake thermal efficiency (BTE) and brake-specific fuel consumption (BSFC) was determined through standard error propagation according to eqs and .UBTE=(∂BTE∂PbUPb)2+(∂BTE∂mf˙Umf˙)224

Similarly, for BSFC:UBSFC=(∂BSFC∂PbUPb)2+(∂BSFC∂mf˙Umf˙)225

The uncertainty for each emissions measurement was directly taken from the accuracy of the gas analyzer.

Repeatability

and Statistical Confidence

To ensure repeatability, three independent trials were conducted for each test condition. The standard deviation of these measurements was used to calculate the 95% confidence interval. A summary of all uncertainty values is presented in Table .

The uncertainties fall within acceptable limits, confirming the reliability of the experimental findings. The brake thermal efficiency and fuel consumption results are within ±1.5% uncertainty, ensuring their validity. Similarly, the uncertainty in emissions measurement is within ±3.5%, supporting the accuracy of the reported trends. Future improvements, such as employing higher-precision instruments and increasing the number of trials, can further reduce these uncertainties and enhance the robustness of the results.

Result and Discussion

This section presents the findings of the research, which combined numerical simulations and experimental analysis to assess the effects of biogas–diesel dual-fuel operation enhanced with diethyl ether (DEE) on engine performance and emissions. The numerical study primarily focused on optimizing the design of the Venturi biogas–air mixer using computational fluid dynamics (CFD) simulations conducted with Ansys Fluent software. Various geometric parameters, including the biogas inlet angle and beta ratio, were analyzed to achieve an optimal mixing efficiency and airflow dynamics. The findings showed that the most consistent air–biogas combination was obtained with an inlet angle of 45° and a beta ratio of 0.6. By reducing the turbulence, this arrangement created ideal circumstances for effective combustion. The consistent and reliable mixing process was ensured by the optimized mixer design, which allowed for efficient biogas induction while preserving the required pressure decreases.

Analytical Validation

The pressure decreases at the neck along the mixer’s length, which was constructed with a 21° convergent angle and was examined analytically as part of the validation procedure. Because the pressure drop inside the Venturi mixer has a major impact on the flow velocity, turbulence, and mixing efficiency of the included species, this method was selected. Equation , which describes the link between the velocities at the inlet and neck and the pressure drop at the throat, was established by the validation using Bernoulli’s equation according to eqs and ), ΔP=V22−V122×ρa

For beta ratio 0.4ΔP=33.792−5.39822×1.184=658.67Pa

From the computational result, ΔP = 668.2 Pa. Hence, the value of error with a beta ratio of the 0.5 level is 3.55%.

For a beta ratio of 0.5:ΔP=21.5922−5.39822×1.184=258.748Pa

From the computational result, ΔP = 265.5 Pa. Hence, the value of error with a beta ratio 0.5 level is 2.514%.

For a beta ratio of 0.6:ΔP=14.42−5.39822×1.184=115.95Pa

From the computational result, ΔP = 105.78 Pa. Hence, the value of error with a beta ratio of 0.6 level is 8.77%.

Venturi Gas Mixer Simulation Results

The Venturi gas mixer simulation was performed by using Ansys Fluent to evaluate its performance in terms of velocity distribution, pressure behavior, and turbulence kinetic energy (TKE). These parameters are critical for ensuring a homogeneous biogas/air mixture, which is essential for efficient combustion and optimal engine performance. The velocity analysis revealed that the air velocity increased significantly from 5.4 m/s at the inlet to 33.79 m/s at the throat, as shown in Figure . This dramatic increase, driven by the reduction in the cross-sectional area at the throat, created a strong suction effect that effectively drew biogas into the mixer. Downstream of the throat, the velocity gradually decreased to approximately 12.5 m/s at the outlet, ensuring flow stability and reducing turbulence levels.

The pressure distribution analysis, depicted in Figure , showed a substantial drop at the throat from atmospheric pressure (101,325 to 658.67 Pa), a vacuum sufficient to entrain biogas effectively. Beyond the throat, the pressure recovered smoothly to 99,000 Pa at the outlet, minimizing energy losses and ensuring a stable biogas–air mixture before entering the engine intake manifold. The gradual recovery also supports the prevention of flow separation, which could otherwise disrupt the uniformity of the mixture.

The turbulence kinetic energy (TKE) distribution, illustrated in Figure , highlighted localized peaks of approximately 9.5 m^2^/s^2^ at the throat and the diverging sections. These regions of enhanced turbulence facilitated the effective mixing of biogas and air. The TKE values decreased to around 1.2 m^2^/s^2^ at the outlet, indicating a stabilized flow suitable for consistent combustion. This balance of turbulence and flow uniformity is critical for achieving the desired combustion characteristics while minimizing pressure fluctuations and energy losses.

The effectiveness of the Venturi gas mixer in producing a well-mixed air–biogas flow is confirmed by the numerical findings, which include velocity, pressure, and TKE analyses. Together with regulated turbulence levels that improved mixing while preserving the overall flow stability, the design guaranteed a significant pressure decrease and a velocity rise near the throat. The use of the mixer in dual-fuel diesel engines to increase the combustion efficiency and lower emissions is supported by these findings. These results are visibly confirmed in Figures , , and , which offer important information about how well the Venturi gas mixer design performs.

Table provides the quantitative details of these performance metrics, highlighting the trade-offs among pressure drop, velocity, and flow stability at different beta ratios. The analysis confirms that the beta ratio of 0.6, combined with two biogas inlet holes and a 21° divergence angle, offers the best compromise between efficient biogas suction and stable air-biogas mixing. This configuration is recommended for practical applications to enhance dual-fuel engine performance while minimizing energy losses and emissions.

The gas inlet angle significantly influenced the air–fuel mixture within the Venturi mixer, with larger angles leading to increased methane entrainment and a richer mixture. This phenomenon was evidenced by a decrease in the air excess coefficient (λ) from 1.6 to approximately 0.77 as the gas inlet angle increased, indicating a shift toward a fuel-rich condition. Figure illustrates the methane mass fraction distribution along the Venturi mixer for various gas inlet angles, demonstrating a higher methane concentration at the outlet with larger angles under constant convergent, divergent, and beta ratios of 210, 80, and 0.4, respectively. Conversely, Figure , which depicts the air concentration, presents an inverse relationship, reflecting the complementary nature of the air–gas mixture. Given that the total flow volume consistently comprised 100% of the air–gas mixture, the air distribution mirrored the methane distribution. The numerical results obtained from the simulations aligned closely with theoretical calculations, validating the accuracy of the computational model. Analysis of the methane (CH4) and air concentration profiles confirmed that larger gas inlet angles resulted in greater methane suction into the Venturi mixer, ultimately producing a richer mixture at the outlet

Engine

Performance and Exhaust Emission of Biogas–Diesel Dual-Fuel Diesel Engine

The performance and emission characteristics of the biogas–diesel dual-fuel engine were evaluated under varying engine loads and biogas flow rates. Key parameters such as brake power (BP), brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), and exhaust emissions (CO, CO2, HC, and NO* x *) were measured and compared with the baseline performance of diesel-only operation.

Brake power (BP) trends under different engine loads revealed consistent patterns across both diesel-only and biogas–diesel dual-fuel modes. In both cases, BP increased linearly with higher engine loads, reflecting the expected behavior of internal combustion engines. However, in dual-fuel mode with a biogas flow rate of 2 L/min, a slight reduction in BP was recorded, averaging 2.19% less than in a diesel-only operation. This decline is attributed to the lower energy content of biogas compared with diesel, as biogas partially replaced diesel in the fuel mix. The lower calorific value of biogas results in reduced overall energy availability, leading to this marginal drop in the power output.

Consequently, the engine’s output power experienced a slight dip. Despite this, the reduction was relatively minor, showcasing the engine’s ability to maintain functionality under a dual-fuel configuration. The power loss was more noticeable at higher engine loads but remained within acceptable limits for practical applications. Figure graphically represents this decrease in BP, highlighting the power differences between the two modes as the engine load increases. These findings indicate that dual-fuel modes are a feasible option for scenarios where a slight trade-off in power is justified by the environmental and economic advantages of utilizing renewable biogas as a supplementary fuel.

Brake thermal efficiency (BTE), a key metric for evaluating how efficiently an engine converts fuel heat into mechanical work, demonstrated a notable enhancement during dual-fuel operation. This improvement was particularly evident at a biogas flow rate of 2 L/min combined with a 5% diethyl ether (DEE) blend, resulting in an average increase of 2.63% compared to a diesel-only operation. The enhanced BTE can be linked to the inclusion of DEE, an oxygen-rich additive that promotes better fuel oxidation and facilitates a more effective release of energy. The high oxygen content in DEE minimizes the ignition delay, ensuring smoother and more complete combustion of both biogas and diesel fuels, thereby maximizing energy utilization. However, as the biogas flow rate rose to 4 and 6 L/min, BTE exhibited a decline. This reduction is attributed to the increased concentration of inert gases, primarily CO2, within the biogas, which hampers combustion efficiency. These inert gases displace oxygen in the combustion chamber, leading to incomplete combustion and lower thermal efficiency. Figure provides a graphical representation of BTE across different engine loads and biogas flow rates, clearly showing the optimal efficiency achieved at 2 L/min biogas flow with the DEE blend. The findings suggest that controlling biogas flow and optimizing fuel additives such as DEE can significantly improve engine efficiency when operating in a dual-fuel mode.

Brake-specific fuel consumption (BSFC), a key metric for assessing the amount of fuel required to generate a unit of power, plays a crucial role in evaluating the engine performance. In dual-fuel mode, BSFC was slightly higher compared with diesel-only operation. At a biogas flow rate of 2 L/min, the average increase in the BSFC was about 4.6%. This increase stems from the lower calorific value of biogas, which necessitates the use of a larger fuel quantity comprising both diesel and biogas to produce the same power output. While biogas usage reduces diesel consumption, its limited energy content leads to higher overall fuel consumption. However, this limitation is offset by the environmental benefits of integrating biogas, which is a renewable energy source, into the fuel system.

Despite the moderate rise in BSFC, the dual-fuel approach offers significant advantages, such as reduced reliance on fossil fuels and lower CO2 emissions, aligning with sustainable energy objectives. Figure illustrates the BSFC trends, showing a gradual increase in the level of fuel consumption with the introduction of biogas into the fuel blend. These findings emphasize the need to carefully optimize biogas flow rates and blending ratios to minimize fuel consumption impacts while maximizing the environmental and sustainability benefits of dual-fuel operation.

The examination of exhaust emissions during dual-fuel operation highlighted notable differences when compared to diesel-only mode, influenced by the combustion behavior of biogas and its interaction with diesel. Carbon monoxide (CO) emissions, as illustrated in Figure , showed an average rise of 2.75% at a biogas flow rate of 2 L/min relative to diesel-only operation. This increase in the level of CO emissions is primarily attributed to the incomplete combustion of biogas, which is more likely to occur under conditions with limited oxygen availability. Biogas contains a high proportion of inert gases such as CO2, which displaces oxygen and reduces the efficiency of the combustion process. This results in higher levels of CO, which is an indicator of incomplete combustion. However, the increase in CO emissions was relatively small, and dual-fuel operation still provided substantial environmental benefits when compared with conventional diesel engines, particularly in terms of reducing other harmful emissions such as NOx and particulate matter.

Conversely, carbon dioxide (CO2) emissions experienced a notable rise during dual-fuel operation, as depicted in Figure . An increase of 28.84% was observed when compared to that of the diesel-only mode, highlighting the impact of incorporating biogas into the fuel mix. This increase in the level of CO2 is mainly due to the improved combustion efficiency when biogas is used in combination with diesel. Biogas, being a renewable fuel, contributes to the reduction of overall greenhouse gas emissions as the CO2 produced from biogas combustion is part of the natural carbon cycle. Thus, even though the CO2 emissions increased, they are still considered environmentally favorable compared to the CO2 emissions from fossil fuels. The increase in CO2 emissions reflects the higher combustion efficiency in dual-fuel mode, where renewable biogas plays a significant role in displacing fossil fuel usage.

Hydrocarbon (HC) emissions, as shown in Figure , exhibited an increase of 3.96% during dual-fuel operation, especially at elevated biogas flow rates. This rise is largely due to the slower combustion characteristics of the biogas–air mixture compared to diesel. The lower flame speed and extended ignition delay of biogas contribute to incomplete combustion, resulting in higher HC emissions. This effect becomes more prominent at increased biogas flow rates, where the air/fuel mixture becomes leaner, further reducing combustion efficiency. However, the increase in HC emissions was modest, and it is expected that further optimization of biogas flow rates and fuel blends could mitigate this issue.

Figure illustrates that nitrogen oxide (NOx) emissions experienced an average reduction of 17.3% when operating in dual-fuel mode with a biogas flow rate of 2 L/min. This decline in the level of NOx emissions can be attributed to the lower combustion temperatures facilitated by the presence of biogas. Biogas contains a substantial proportion of CO2, which acts as an inert gas, effectively decreasing the peak temperature within the combustion chamber. This temperature reduction minimized the formation of NOx. Such a decrease in NOx emissions represents a significant advantage of dual-fuel operation, given NOx’s role as a major contributor to environmental issues like air pollution and acid rain. This finding underscores the environmental benefits of utilizing biogas as a sustainable fuel alternative for diesel engines.

In summary, the findings on engine performance and exhaust emissions highlight the promise of dual-fuel modes as a practical substitute for conventional diesel engines. Integrating biogas with diesel and DEE achieves a balance between the operational performance, fuel efficiency, and environmental sustainability. The data presented in Figures through collectively illustrate the detailed patterns in engine behavior and emissions, emphasizing the potential of dual-fuel technology as a pathway toward sustainable energy solutions for internal combustion engines.

Engine Performance and

Exhaust Emission of Diesel Engine Operated with DEE–Diesel Blends in Dual Fuel with Biogas

The study extensively examined the performance and emission characteristics of a diesel engine operating in dual-fuel mode using diethyl ether (DEE)–diesel blends alongside biogas. Various engine loads and DEE blending ratios (5, 10, and 15% DEE with diesel) were evaluated to assess their impact. Key performance indicators, such as brake power (BP), brake thermal efficiency (BTE), and brake-specific fuel consumption (BSFC), were analyzed along with exhaust emissions, including carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), and nitrogen oxides (NO* x *). The investigation aimed to understand how different DEE–diesel mixtures influence engine performance in a biogas-assisted dual-fuel system and to compare these results against the performance of a standard diesel-only configuration.

Brake power, shown in Figure , represents the engine’s ability to produce mechanical output under varying loads and exhibits a linear increase with engine load in both pure diesel mode and DEE–diesel blend operations. However, the introduction of biogas in dual-fuel mode led to a slight decline in BP. Specifically, the 5, 10, and 15% DEE blends showed reductions of 2.19, 4.3, and 5.1%, respectively, compared to diesel-only operation. This reduction can be attributed to the lower energy density of biogas relative to diesel. Although DEE enhances the combustion efficiency of biogas, it cannot entirely compensate for the calorific shortfall introduced by the biogas component. Despite this reduction, the engine maintained a reasonable power output, demonstrating that the dual-fuel system with biogas and DEE could still meet power requirements, albeit with a slight sacrifice in BP. The overall decrease in BP was not substantial, indicating that dual-fuel operation with biogas and DEE blends can be viable for applications requiring moderate power output while offering the environmental benefits of renewable fuel usage.

Brake thermal efficiency (BTE), depicted in Figure , is a critical parameter for assessing how effectively an engine converts fuel energy into a useful mechanical output. Dual-fuel operation with DEE–diesel blends exhibited a significant improvement in BTE. Blends containing 5, 10, and 15% DEE achieved increases of 2.63, 4.1, and 5.2%, respectively, compared to conventional diesel operation. These enhancements are primarily due to the higher oxygen content in DEE, which promotes more efficient and complete combustion of the diesel and biogas mixture. The oxygen-rich nature of DEE also shortens ignition delay, leading to improved combustion and reduced formation of unburned residues. This improvement in combustion efficiency enables the engine to achieve higher thermal efficiency, particularly at lower engine loads.

Additionally, as the proportion of DEE increased, a consistent rise in BTE was observed, underscoring the positive impact of DEE on the combustion quality. However, at higher biogas flow rates and DEE concentrations, a slight decrease in the thermal efficiency was noted. This reduction is attributed to the increased presence of carbon dioxide and other inert gases in biogas, which limit the oxygen available for combustion and slightly impede efficiency. Despite this, the overall improvement in BTE achieved by incorporating DEE as an additive highlights its potential to enhance the engine performance in dual-fuel systems.

Brake-specific fuel consumption (BSFC), illustrated in Figure , represents the fuel required to generate a unit of power. In dual-fuel operation with DEE-diesel blends and biogas, BSFC was higher compared to diesel-only operation. The 5, 10, and 15% DEE blends led to increases of 4.6, 9.1, and 12.3%, respectively. This rise in the BSFC can be attributed to the lower energy density of biogas, necessitating a greater volume of fuel to achieve the same power output. Although biogas helps to reduce diesel consumption, its lower calorific value means that more fuel (a mix of both diesel and biogas) is needed to maintain power output. Additionally, the presence of DEE in the diesel blend does not contribute as much energy as pure diesel does, further increasing fuel consumption. The increase in BSFC was more pronounced at higher concentrations of DEE, which added to the total fuel consumption. While the increase in BSFC is a trade-off for using biogas and DEE as alternative fuels, it is important to recognize that the environmental benefits, such as reduced dependence on fossil fuels and lower carbon emissions, can outweigh the increase in fuel consumption. This trade-off can be minimized by optimizing the biogas flow rate and DEE blending ratios to achieve a balance between fuel consumption and environmental impact.

Exhaust emissions were a crucial part of the analysis as the dual-fuel system utilizing DEE–diesel blends with biogas seeks to reduce harmful pollutants. Figure shows the carbon monoxide (CO) emissions, which increased by 2.75, 3.5, and 4.2% for the 5, 10, and 15% DEE blends, respectively, when compared to the pure diesel operation. This increase in CO emissions is mainly attributed to the incomplete combustion of biogas. The high inert gas content, especially CO2, in biogas results in lower combustion temperatures, which can prevent complete combustion, particularly under partial load conditions. Since CO is a byproduct of incomplete combustion, the higher CO levels indicate a less efficient combustion process when biogas is used. However, CO emissions from the dual-fuel system were still lower than those typically produced by conventional diesel engines. This suggests that with further optimization, the dual-fuel system has the potential to achieve even lower CO emissions.

Carbon dioxide (CO2) emissions, depicted in Figure , showed a significant increase in dual-fuel mode compared to diesel-only operation. The CO2 emissions increased by 28.84, 35.1, and 38.2% for the 5, 10, and 15% DEE blends, respectively. This increase in CO2 is expected, as biogas, although containing CO2, is considered a renewable fuel with a lower net environmental impact due to its carbon-neutral status. The CO2 produced during biogas combustion is part of the natural carbon cycle, as the CO2 released is offset by the CO2 absorbed during the production of biogas from organic materials. Therefore, while CO2 emissions increased, the environmental impact is still significantly lower than that of fossil-fuel-based diesel combustion. Moreover, the higher CO2 emissions reflect the improved combustion efficiency associated with the use of biogas in the fuel mix.

Hydrocarbon (HC) emissions, depicted in Figure , were found to increase by 3.96, 5.5, and 7.1% for the 5, 10, and 15% DEE blends, respectively, in comparison to diesel-only operation. This rise in HC emissions is mainly attributed to the slower combustion rate of biogas and the ignition delay caused by the biogas–diesel mixture. Biogas, with a lower flame speed than diesel and a high content of inert gases, tends to result in incomplete combustion, leading to higher HC emissions. Although increasing the DEE concentration improved combustion, slightly reducing HC emissions, the overall trend was a rise in HC emissions as the biogas flow rate and DEE concentration increased. These elevated HC emissions indicate that further adjustments in the biogas flow and DEE blending ratios may be required to reduce unburnt hydrocarbon emissions.

Lastly, nitrogen oxide (NOx) emissions, shown in Figure , were significantly reduced in dual-fuel operation by using DEE–diesel blends. The reductions in NOx emissions for the 5, 10, and 15% DEE blends were 20.48, 24.5, and 28.9%, respectively, compared to pure diesel operation. The reduction in NOx is primarily attributed to the lower combustion temperatures associated with biogas, which contains CO2 that acts as a heat sink, thereby lowering peak temperatures during combustion. DEE further contributes to the reduction of NOx by promoting a more uniform combustion process, which helps decrease thermal NOx formation. This decrease in NOx emissions is advantageous, as NOx is a major contributor to air pollution and the formation of smog. Therefore, the dual-fuel system with DEE offers a cleaner alternative to conventional diesel engines.

In summary, utilizing DEE–diesel blends in dual-fuel mode with biogas has shown significant potential to enhance engine performance and reduce emissions, although some trade-offs, such as increased CO emissions and slightly higher fuel consumption, were observed. The notable gains in brake thermal efficiency and the reduction in nitrogen oxide emissions emphasize the viability of this fuel combination as a step toward more sustainable diesel engine operation. The performance and emission trends, as illustrated in Figures through , offer a detailed analysis of the advantages and challenges associated with employing DEE–diesel blends in a biogas-assisted dual-fuel setup. These results suggest that with further refinement and optimization, this approach could serve as an effective strategy for reducing the environmental footprint of diesel engines while maintaining acceptable levels of performance and efficiency.

The findings of this study are broadly consistent with those reported by Barik and Murugan and Mahla et al., who observed NO* x

• reductions between 18 and 35% when using DEE in biogas–diesel dual-fuel configurations. Similar to Singh and Singh, this study confirms that DEE improves combustion stability, especially at lower biogas flow rates, by reducing ignition delay and enhancing the flame front propagation due to its high cetane number and oxygen content. However, while those studies reported minimal changes in HC emissions, our findings showed a 3.96–7.1% increase in HC with higher DEE concentrations. This suggests that excessive DEE volatility may cause localized fuel-rich zones, delaying complete oxidation. These discrepancies may stem from differences in biogas composition, engine geometry, or injection timing. Thus, the combustion enhancement effects of DEE appear more pronounced in leaner biogas mixtures, where it facilitates early ignition and better flame kernel growth.

Statistical Significance of Results

The statistical analysis confirms that diethyl ether (DEE) additives significantly impact certain engine performance and emission parameters in a biogas–diesel dual-fuel configuration. Specifically, the 5% DEE blend at a biogas flow rate of 2 L/min led to a 2.63% improvement in brake thermal efficiency (BTE), with a p-value of 0.023, indicating statistical significance at the 95% confidence level. Similarly, a 20.48% reduction in NO* x

• emissions was observed, supported by a highly significant p-value of 0.008. These effects are further validated by their respective 95% confidence 2.1–3.1% for BTE and 18.5–22.5% for NO* x . In contrast, the 4.6% increase in brake-specific fuel consumption (BSFC) and the 2.75% rise in carbon monoxide (CO) emissions were found to be statistically insignificant, with p-values of 0.12 and 0.09, respectivelyboth exceeding the conventional 0.05 threshold. These findings, derived from one-way ANOVA and paired t tests, indicate that while DEE significantly improves thermal efficiency and reduces NO x
• emissions, its influence on BSFC and CO remains inconclusive under the current experimental conditions.

Conclusions

This study examined the performance and emissions of a single-cylinder diesel engine operating in dual-fuel mode using biogas and diethyl ether (DEE) as the combustion enhancer. A Venturi-type biogas–air mixer was designed, optimized through CFD simulations, and integrated into the intake system to improve fuel–air mixing and ensure stable combustion under varying biogas flow rates and DEE blending ratios.

The most effective configuration was identified as a biogas flow rate of 2 L/min combined with a 5% DEE blend in diesel fuel. Under this setup, the engine achieved a statistically significant 2.63% increase in brake thermal efficiency (p = 0.032) and a statistically significant 20.48% reduction in NO* x

• emissions (p = 0.004) compared to diesel-only operation. Increases in CO (2.75%), HC (3.96%), and BSFC (4.6%) were observed but were not statistically significant (p > 0.05), suggesting they may fall within the range of experimental variability.

Specifically, under biogas-only dual-fuel operation, NO* x

• emissions decreased by 17.3% at biogas flow rates of 2 L/min. However, the addition of 5% DEE resulted in a more balanced outcome, with a 20.48% NO* x
• reduction accompanied by improved combustion efficiency and reduced power loss, confirming DEE’s role in shortening ignition delay and enhancing flame propagation.

Although higher DEE concentrations (10 and 15%) offered additional improvements in thermal efficiency, they also led to increased unburned hydrocarbon emissions and fuel consumption without statistically significant performance gains. This indicates diminishing returns beyond the 5% DEE level.

Overall, the integration of biogas with 5% DEE presents a practical and optimized dual-fuel strategy that reduces NO* x

• emissions and enhances engine efficiency with minimal trade-offs. The CFD-optimized mixer and controlled combustion conditions contributed to these improvements by ensuring better fuel–air homogeneity and reducing turbulence-induced losses.

Future research should explore long-term engine durability, effects under transient load conditions, and economic viability at scale. These findings support the feasibility of DEE-enhanced biogas dual-fuel systems as a transitional pathway toward cleaner, more sustainable internal combustion engine technologies, especially in settings with readily available biogas resources.