A Study of Performance and Emission Characteristics of Diesel-Palm Oil Mill Effluent Gas on Dual-Fuel Diesel Engines Based on Energy Ratio

Abstract

Biogas from palm oil mill effluent (POME) is a promising fuel that has many advantages as an alternative fuel. The methane content in biogas derived from POME is up to 75% and can be used as an alternative fuel in an internal combustion engine. One of the technologies for utilizing biogas in compression ignition engines is the Diesel Dual-Fuel (DDF) technique due to the different characteristics of fuel and the impact on the environment due to significantly reducing emissions. This study aims to find the effect of biogas POME composition and energy ratio on the DDF engine’s performance and emissions. The simulations using AVL BOOST software were confirmed by experimental engine parameters. The modeling was conducted on the biogas energy ratio (20%, 40%, 60%, and 75% POME) and biogas POME composition (55% and 75% methane). The results showed that the fuel consumption of diesel fuel was reduced by up to 69%, and NO_x and soot emissions were reduced by up to 92% and 80%, respectively, with dual-fuel mode operation. Meanwhile, the value of brake mean effective pressure (BMEP) and efficiency was reduced by up to 18%, volumetric efficiency decreased by up to 4%, the increase in brake specific energy consumption (BSEC) was up to 23%, and brake specific fuel consumption (BSFC) was up to 155%. The optimum of the engine’s performance and emission was 40% biogas ratio with 75% methane content.

1. Introduction

The energy sector plays a major role in contributing to the greenhouse effect, primarily through industrial activities, transportation, and power generation [1,2]. A significant portion of these emissions stems from combustion engines that rely on petroleum-based fuels [3]. The combustion process produces various harmful exhaust gases, including carbon oxides (CO_x), nitrogen oxides (NO_x), hydrocarbons (HC), sulfur oxides (SO_x), and particulate matter [4,5]. In response to these environmental concerns, numerous studies have been conducted to develop strategies for reducing emissions from human activities that drive global warming.

Among various renewable energy alternatives, biogas derived from palm oil mill effluent (POME) has gained attention, particularly in palm oil-producing countries such as Indonesia and Malaysia [6,7]. POME is a liquid waste byproduct generated during the palm oil production process. It contains high levels of biological oxygen demand (BOD) and chemical oxygen demand (COD), making it a potential environmental pollutant if left untreated. Notably, POME emits substantial amounts of methane (CH₄) during decomposition [8]. Methane is a potent greenhouse gas with a global warming potential (GWP) approximately 21 times greater than that of carbon dioxide (CO₂) [9]. Through anaerobic digestion, POME can be processed into biogas, offering a cleaner alternative to fossil fuels. Despite this potential, in palm oil-producing countries like Indonesia—with approximately 640 palm oil mills—less than 10 percent currently process POME into biogas [10].

One promising application of POME-derived biogas is in Diesel Dual-Fuel (DDF) engines, which operate by blending conventional diesel with gaseous fuels like biogas. This dual-fuel configuration can significantly reduce harmful emissions and overall environmental impact [11,12]. However, when gaseous fuels are used in naturally aspirated diesel engines, several technical constraints arise. Introducing gas into the intake displaces part of the air charge, which reduces volumetric efficiency and lowers engine power output [13]. Because gaseous fuels such as biogas have lower heating value than diesel, engines typically experience power derating and reduced brake thermal efficiency (BTE), with higher brake-specific fuel consumption (BSFC), especially at low loads [14].

At low loads, combustion tends to be incomplete, leading to higher emissions of unburned hydrocarbons (HC) and carbon monoxide (CO). At very high gas substitution ratios, combustion may become unstable or even result in knocking. [14,15]. Moreover, naturally aspirated systems lack forced induction; hence, the air–fuel mixture control is more difficult. Therefore, careful optimization of pilot-diesel quantity and injection timing is required to maintain stable ignition and acceptable performance [15].

Ongoing research involving both modeling and experimental studies is exploring the effectiveness of DDF engines running on POME biogas, showing promising results in emission reduction and performance. Beyond POME, biogas from other organic waste sources such as food waste, livestock manure, sewage sludge, and landfill gas have also been widely studied for dual-fuel diesel engine applications. For example, an experimental study using biogas from food waste in a small diesel engine revealed that optimizing pilot injection timing could mitigate efficiency losses, improve combustion stability, and maintain brake thermal efficiency (BTE) close to pure diesel operation, while still reducing NO_x emissions significantly [16]. Similarly, research on agricultural tractors running on raw biogas from livestock manure demonstrated stable dual-fuel operation with up to 40% substitution ratio, resulting in slightly lower BTE and BSFC increase, but notable reductions in smoke and particulate emissions [17]. Another study using sewage-derived biogas for dual-fuel marine and stationary engines reported that although CO and HC emissions increased due to incomplete combustion, overall soot and NO_x emissions were reduced, and engine operation remained feasible after minor intake adjustments [18]. In addition, tests using landfill gas (LFG) highlighted performance challenges due to its lower methane content, but optimization strategies such as fuel blending and advanced combustion control significantly improved efficiency while maintaining low emissions [19]. These findings indicate that biogas from multiple waste sources can be effectively utilized in dual-fuel diesel engines to enhance sustainability, reduce reliance on fossil fuels, and maintain acceptable engine performance.

The utilization of fuel-based gas in a diesel engine has proven to have a positive impact on the environment [20,21]. The emission of diesel dual-fuel (DDF) engines, such as particulate matter and dangerous chemical fuel reactions, is reduced when diesel fuel is used with environmentally friendly alternative energy [22,23]. Sekar et al. [24] studied a blend of chlorella vulgaris microalgae and diesel fuel, assisted by biogas and nanoparticles to improve the performance of a diesel engine. The study found that the addition of biogas to the intake manifold and the use of nanoparticles in the fuel blend resulted in improved combustion efficiency, reduced emissions of pollutants such as NO_x, CO, CO₂, and HC, and increased engine power output. Bora et al. [25] and Mohite et al. [26] demonstrated that the utilization of biogas and biodiesel-based Mahua oil as a fuel on dual-fuel diesel engines has the potential to improve waste-to-energy prospects, enhance engine performance, and reduce emissions, thereby contributing to sustainability and environmental conservation. According to Das et al. [27], the utilization of waste plastic oil and 20% of biogas in dual-fuel mode operation showed that load, compression ratio, and fuel blend had a substantial impact on the engine performance and emissions. Alabas and Ceper [28] investigated the effect of oxygen enrichment on the combustion characteristics and pollutant emissions of kerosene–biogas mixtures on a mini jet engine combustion chamber. The results showed that increasing the oxygen composition in the combustion air by 3% resulted in a significant increase in the maximum flame temperature. Moreover, Legrottaglie et al. [29] demonstrated the application to micro-cogeneration of an innovative dual-fuel compression ignition engine running on biogas. This study has shown that the new dual-fuel unit running on self-produced biogas can cut the cost of the fuel and considerably reduce the soot emissions.

This study aims to investigate the lack of information on the effects of biogas composition and energy ratio from POME on the performance and emission of the Diesel Dual-Fuel (DDF) engine by modeling on one-dimensional combustion software. Modeling is conducted to predict the behavior or response of the system, complete an understanding of the process to be carried out, and identify control variables of the system.

2. Methods

The research was conducted by modeling based on experimental engine parameters. One-dimensional combustion software by AVL Boost is used to predict the behavior or response of the system and identify control variables of the system. Meanwhile, the start of combustion parameters, combustion duration, heat release, and NO_x post-processing multiplier are estimated from the engine specification [30].

The experiment is carried out to validate the result of modeling by software on actual engine conditions. The validated data are the pressure of the combustion chamber and the NOx emission. After the simulation model is validated, it will continue with the addition of gas fuel in the form of POME biogas with different amounts and compositions to observe the effect of POME biogas on the performance and emissions of the diesel engine. The required data in dual-fuel mode included the composition of biogas POME, as shown in

Table 1. Energy variation and composition of biogas POME.

No.	Variation	Energy Ratio of Biogas POME (QPOME)	Composition of Biogas POME
			CH4	CO2
1	0% POME	-	-	-
2	20% POME55	0.2Qf	55%	45%
3	40% POME55	0.4Qf	55%	45%
4	60% POME55	0.6Qf	55%	45%
5	75% POME55	0.75Qf	55%	45%
6	20% POME75	0.2Qf	75%	25%
7	40% POME75	0.4Qf	75%	25%
8	60% POME75	0.6Qf	75%	25%
9	75% POME75	0.75Qf	75%	25%

. Table 1 consists of the scenario that was conducted in this study. The percentage of variation shows the different biogas energy ratio values. The Q_f means the fuel energy ratio from the total fuel used in the test, which will be used as a reference and guideline; therefore, there is no need to specify units. The Q_POME shows the fuel energy ratio derived from POME biogas. The diesel fuel mass and the biogas POME mass flow rate are shown in

Table 2. The diesel fuel mass and the biogas POME mass flow rate.

20% POME			40% POME			60% POME			75% POME
mdiesel (kg/Cycle)	mPOME55 (kg/s)	mPOME75 (kg/s)	mdiesel (kg/cycle)	mPOME55 (kg/s)	mPOME75 (kg/s)	mdiesel (kg/Cycle)	mPOME55 (kg/s)	mPOME75 (kg/s)	mdiesel (kg/Cycle)	mPOME55 (kg/s)	mPOME75 (kg/s)
6.96 × 10−6	4.49 × 10−5	3.29 × 10−5	5.22 × 10−6	8.99 × 10−5	6.59 × 10−5	3.48 × 10−6	1.35 × 10−4	9.88 × 10−5	2.18 × 10−6	1.69 × 10−4	1.24 × 10−4
1.12 × 10−5	7.24 × 10−5	5.31 × 10−5	8.41 × 10−6	1.45 × 10−4	1.06 × 10−4	5.61 × 10−6	2.17 × 10−4	1.59 × 10−4	3.51 × 10−6	2.71 × 10−4	1.99 × 10−4
1.63 × 10−5	1.05 × 10−4	7.72 × 10−5	1.22 × 10−5	2.11 × 10−4	1.55 × 10−4	8.16 × 10−6	3.16 × 10−4	2.32 × 10−4	5.10 × 10−6	3.95 × 10−4	2.90 × 10−4
2.17 × 10−5	1.40 × 10−4	1.03 × 10−4	1.63 × 10−5	2.80 × 10−4	2.06 × 10−4	1.09 × 10−5	4.21 × 10−4	3.09 × 10−4	6.79 × 10−6	5.26 × 10−4	3.86 × 10−4
2.82 × 10−5	1.82 × 10−4	1.33 × 10−4	2.11 × 10−5	3.63 × 10−4	2.66 × 10−4	1.41 × 10−5	5.45 × 10−4	4.00 × 10−4	8.80 × 10−6	6.81 × 10−4	4.99 × 10−4

. Moreover, the diesel (Pertamina Dex) and POME biogas specifications are shown in

Table 3. Diesel (Pertamina Dex) [31,32] and POME biogas specifications [33,34].

Diesel Specifications (Pertamina Dex)		POME Biogas Specifications
Characteristics	Value	Characteristics	CH4	CO2
Cetane number	53	Density [kg/m3]	0.688	1.842
Specific gravity @ 15 °C (kg/m3)	810–850	Lower heating value (LHV) (MJ/kg)	50	-
Viscosity @ 40 °C (mm2/s)	2.0–4.5
Final boiling point (°C)	370
Lower heating value (LHV) (MJ/kg)	47.314

. CH₄ material was obtained from the State Gas Company (PGN), Indonesia, CO₂ was obtained from PT Samator, Indonesia and Diesel fuel was obtained from PT Pertamina Patra Niaga, Indonesia.

Several variations in the biogas energy ratio (REB) and the composition of biogas POME were examined. The simulation results are BMEP, efficiency, BSFC, NO_x emissions, and soot emissions of the model. The Q_f shows the fuel energy from the test results that will be used as a reference. Meanwhile, QPOME shows the fuel energy from POME biogas. The use of the percentage (%) POME term indicates the percentage of POME energy from the total fuel energy. POME55 means biogas POME with a methane composition of 55%, and POME75 is biogas POME with a methane composition of 75% as mentioned by Peter Jacob Jørgensen [35] and Rahayu et al. [9]. In this modeling study, elements below one percent were ignored because they had little influence on the modeling results, especially the combustion results on the biogas, which are based on methane (CH₄), carbon dioxide (CO₂), and other compounds, including highly harmful sulfur compounds. Biogas is widely used in spark ignition (SI) engines, which are fed directly with biogas or filtered with filters designed to remove sulfur compounds, as mentioned by Alfredas Rimkus and Justas Žaglinskis [36].

2.1. Engine Specification and Experimental Procedure

The engine specifications are shown in

Table 4. The engine specifications.

Model	Yanmar TF155RDi
Type	Horizontal Single Cylinder Four-Stroke Diesel
Bore × Stroke	102 mm × 105 mm
Displacement	857 cc
Compression Ratio	17.8:1
Cooling system	Radiator

below:

The experiment is carried out on an eddy current dynamometer by Schenk W70 (Schenck Process Group GmbH, Darmstadt, Germany) with gradual load variation from 0 to 40 Nm. The fuel consumption was recorded using a fuel mass meter, AVL 733 (AVL List GmbH, Graz, Austria). The cylinder pressure was collected using a pressure transducer, Kistler 6041C (Kistler, Winterthur, Switzerland), combined with an engine combustion analyzer, DEWETRON DEWE-2600-CA (DEWETRON, Grambach, Austria). Meanwhile, the exhaust emission data were recorded using the exhaust emission analyzer HORIBA MEXA-720 NO_x. (HORIBA, Irvine, CA, USA) The schematic diagram for the engine test is depicted in Figure 1a, and the illustration of the engine can be seen in Figure 1b.

2.2. Compression Ignition Engine 1D Simulation

Simulations were carried out using AVL BOOST software version v2011.1. The model in Figure 2 was created in the 1D simulation step, and the item symbol information was shown in

Table 5. The item symbol of simulation model.

E1	Engine 1
SB1, SB2	System Boundary 1, 2
MP1, MP2, MP3	Measuring Points 1, 2, 3
CL 1	Air purifier
R1, R2	Restriction 1, 2
I1	Injector
C1	Cylinder
PL1	Plenum/Muffler
1, 2, 3, 4, 5, 6, 7	Pipes 1, 2, 3, 4, 5, 6, 7

.

The model is developed according to the real conditions of the engine. The gas fuel in the form of biogas POME was fed into the intake manifold and was modeled as an injector at the inlet plenum. Diesel dual-fuel simulations were carried out using AVL BOOST software version v2011.1. The Woschni heat transfer model was employed to estimate convective heat losses in the cylinder. Pilot diesel injection was modeled with injection pressures of approximately 400–600 bar, while injection timing was derived from experimental data and calibrated using the Vibe combustion model. POME biogas was supplied through the intake manifold using an injection system. The valve timing diagrams for both intake and exhaust were set according to the engine specifications, including flow coefficients. An air purifier was positioned before the intake manifold to facilitate mixing of the intake air with biogas. The cylinder wall and cylinder head temperatures were set within 450–500 K, while the piston temperature was set at 500–600 K. Engine geometry (bore, stroke, compression ratio) and operating conditions (speed, intake air temperature, ambient pressure) were adjusted to reflect actual environmental conditions. For emission predictions, the extended Zeldovich mechanism was applied for NO_x formation, and soot formation was modeled using AVL’s empirical approach. Model validation was conducted against experimental data, specifically cylinder pressure traces and NO_x emissions under neat diesel operation.

To perform modeling with dual-fuel mode, the mass of biogas POME and the mass of diesel fuel data were used in this study. The diesel fuel mass as prime fuel will decrease when the addition of methane occurs, as obtained from experiments by determining the load. The mass of the diesel fuel is adjusted until it reaches the specified load and engine speed. Since the modeling process in one-dimensional modeling only accepts the total energy value, the calculation of the total energy value is based on the same parameters, where, in this case, the parameters used are the lower heating value (LHV) of each fuel. The assumption in this study was that the fuel energy value was considered the same as the fuel energy value in the experiment as the reference value. The fuel energy in the model can be calculated using Equation (1) by entering the value of the mass of diesel fuel ($\dot{m}$_diesel), which is obtained from the experiment results and the calorific value of diesel fuel. If more than one fuel is used, then the energy from the fuel (Q_f) can be calculated as the total energy input from each fuel, with the information 1, 2, and n indicating the 1st, 2nd fuel, and others in sequence.

$$Q_f=m_{f,1}·H_{n,1}+m_{f,2}·H_{n,2}+\dots +m_{f,n}·H_{n,n}\left[J\right]$$

(1)

where Q_f is the total fuel energy (kJ), m_f is the mass of fuel (kg), and H_n is the calorific value of fuel (kJ/kg).

In this study, two fuels were used, namely liquid fuel (diesel) and gas fuel (biogas). There is a parameter called the Biogas Energy Ratio (BER). The Biogas Energy Ratio (BER) shows the comparison of energy produced by biogas with energy produced by all fuels. The calculation of the Biogas Energy Ratio can be observed in Equation (2).

$$BER={\displaystyle \frac{m_{biogas}·H_{biogas}}{Q_f}}$$

(2)

where m_biogas is biogas mass in dual-fuel mode (kg) and H_biogas is biogas calorific value (J/kg). The biogas energy ratio can be expressed as a percentage by multiplying its value by 100%. The test results data and the calculation of fuel energy can be seen in

Table 6. Diesel fuel mass consumption and torque from experiments as the basic information of dual-fuel modeling.

No.	Speed [rpm]	Torque [Nm]	$\dot{\mathit{m}}$diesel [kg/s]	Qfuel [kJ/s]
1	1800	0	1.305 × 10−4	0.412
2	1800	10	2.103 × 10−4	0.663
3	1800	20	3.061 × 10−4	0.966
4	1800	30	4.075 × 10−4	1.285
5	1800	40	5.278 × 10−4	1.665

.

The mass value of diesel fuel (m_diesel) and the mass value of biogas POME (m_POME) were obtained from the energy value of the experimental fuel. In the dual-fuel simulation, the mass of air entering through the intake and fresh air were assumed to be equal to the mass of air in the neat diesel mode (experiment).

2.3. Optimization Analysis on the Performance and Emission of Simulation Result

To determine the best results of the simulation, the data was then analyzed by choosing the optimum value of the performance and emission parameters. Optimal operation of a dual-fuel engine requires selecting the right variation that produces high performance while minimizing emissions. The data from the dual-fuel simulation (diesel and POME biogas) that will be analyzed include BMEP, torque, power, effective efficiency, BSEC, BSFC, diesel fuel consumption, NOx emissions, and soot emissions.

3. Results and Discussion

3.1. Validation of Simulation Model

The base model was built on one-dimensional combustion software by AVL Boost, and the results are validated by comparing them with the experimental results. The comparison of the pressure in the combustion chamber from the simulation and experiment for diesel fuel with a specific load is shown in Figure 3. Meanwhile, the validation of NO_x emissions can be seen in

Table 7. NO_x emission validation.

No.	Speed [rpm]	Torque [Nm]	NOx Emission [ppm]
			Experiment	Simulation
1	1800	0	243	243
2	1800	10	419	419
3	1800	20	790	790
4	1800	30	1036	1036
5	1800	40	1150	1150

. The AVL Boost software provides a feature called the “NO_x processing multiplier”. This parameter allows users to calibrate and fine-tune the model for specific engine configurations and operating conditions, ensuring more accurate NO_x predictions. By using this feature, simulated NO_x values can be adjusted to closely match the experimental data, thereby improving the reliability and robustness of the model.

Based on Figure 3 and Table 7, it can be seen that the amount of NO_x emission in the simulation and experiment is the same. Thus, it can be stated that the simulation model was valid. The validity of the simulation model states that the combustion engine model that was built and used in the simulation is appropriate or close to the actual condition of the engine, such that the model can be used to predict the performance and emission tendencies of the combustion engine with several different parameters. Based on the result of combustion characteristics that were generated from modeling by software, further simulations were carried out with the addition of biogas from POME biogas with different compositions.

3.2. Effects of Biogas on the Performance and Emission of the Engine

According to the modelling result, it is predicted that the use of biogas from POME will decrease in the average brake mean effective pressure (BMEP), as shown in Figure 4. The utilization of biogas POME as a substitution fuel for the diesel dual-fuel engine will decrease the BMEP value. The BMEP value will decrease gradually until it reaches the critical point of the engine, which causes the engine to not be able to work because the amount of energy from the fuel is not able to provide enough pressure in the combustion chamber to push the piston and then rotate the crankshaft. The composition of biogas from POME (POME55 and POME75) does not have any significant impact on BMEP, as shown in Figure 4a–d.

The insufficient energy input from fuel (diesel and POME biogas) to combust in the combustion chamber is shown in Figure 4d. The combustion does not occur for both the 55% and 75% biogas energy ratios. This indicates that when the total fuel energy is 0.4 kJ or less, it is not possible for biogas to contribute 75% of the energy; the maximum contribution from biogas is only 60%. Both on 75% biogas POME55 and biogas POME75, the mass of diesel fuel is very small compared to the POME biogas and fresh air entering the cylinders [37]; the act of diesel fuel as pilot ignition on the diesel dual-fuel engine is not enough to generate combustion to burn the fuel mixture in the combustion chamber. Diesel engines cannot be operated in dual-fuel mode if sufficient diesel fuel is injected into the cylinder [38]. On the condition of lower than 75% POME with fuel input energy higher than 0.4 kJ, the combustion successfully occurred. There are many reasons for this phenomenon, such as low energy density of biogas from POME, CO₂ content, a naturally aspirated intake port mechanism that limits gas insertion into the combustion chamber, and incorrect combustion process.

The reduction in the BMEP trend might be happening due to the lowest energy source being input to the piston cylinder, which was then compressed and combusted in the combustion chamber. Biogas from POME is a fuel-based gas; the characteristic of fuel-based gas is lower energy density than liquid fuel. Since the parameter on the modeling process is restricted to the cylinder volume boundary, the energy content of fuel substituted for the combustion process becomes a major parameter of the base energy being taken.

The decreasing BMEP trend for utilization of biogas from POME compared to neat diesel fuel is shown in Figure 5. Both POME55 and POME75 biogases have similar decreasing percentage values of BMEP. These reasons have become major parameters that affect the decreasing pressure on the combustion chamber (BMEP average value). The average BMEP decreases in POME biogas utilization for the 20% load is 5% lower, 40% load is 10% lower, 60% load is 15% lower, and 75% load is 18% lower than the utilization of neat diesel fuel.

Figure 6 shows the BSFC values due to the variations in the amount and composition of biogas POME. The specific fuel consumption or brake specific fuel consumption (BSFC) in the combustion engine occurs at its smallest when the diesel engine is run with neat diesel fuel (without any addition of biogas POME). Fuel consumption will increase along with the increase in biogas POME consumption, because fuel consumption is an indication of the engine efficiency for generating power from POME consumption [37]. The variation in the composition of biogas showed significant results in BSFC. The biggest increase in BSFC is shown in Figure 6, which occurred in the use of 25% diesel–75% POME55 with an increase value of 155.46%. At the same time, the smallest increased value occurred in the 80% diesel–20% POME75, which was 9.75%.

Based on Figure 7, the prediction shows the increasing trend of BSFC by modeling. The trend is similar but occurs at higher fuel consumption levels, which may be due to the energy content of the POME biogas. Biogas POME55 contains 55% CH₄ and 45% CO₂; meanwhile, POME75 contains 75% CH₄ and 25% CO₂. Based on that composition, it is known that the energy content of POME75 is higher than that of POME55. The higher hydrogen content of POME75 affects more volatile matter that can be combusted when the pilot combustion by diesel fuel occurs. Meanwhile, the CO₂ contained in the fuel does not react in the combustion chamber due to its stable structure, so it is generally released directly as emissions. The effect of this phenomenon is the lower fuel consumption from using 25% Diesel + 75% POME75 (75% CH₄ + 25% CO₂) compared to 25% Diesel + 75% POME55 (55% CH₄ + 45% CO₂), as shown in Figure 7.

The trend of NO_x emission based on simulation results can be observed in Figure 8. Figure 8 shows that NO_x emissions will decrease by increasing the ratio of biogas energy used. The use of biogas POME55 showed a greater reduction in NO_x than biogas POME75. This was evidenced by the use of only a 20% biogas energy ratio; POME55 can reduce 46.64% of NO_x emissions. This percentage is almost the same as the decrease produced by the use of 40% POME75, which is 47.82%. The use of 40% POME55 resulted in an emission reduction of 75.39% which is greater than 75% POME75, which is reduced to 68.48% of NO_x emissions. In fact, 75% POME55 can reduce NO_x emissions up to 91.81%.

Figure 9 shows that the emission of soot tends to be stable until the input of fuel energy is about 1.665 kJ. Overall, the addition of biogas POME can reduce soot emissions, especially at high biogas energy ratios. However, at a fuel energy of 1.665 kJ, the utilization of POME55 tends to increase the resulting soot (except at 75% POME). The highest percentage of soot reduction was achieved with the 25% diesel–75% POME75, which was 79.99%.

The observed reductions in NO_x and soot emissions can be explained by the lower combustion temperatures during dual-fuel operation. In general, NO_x formation becomes significant only when the peak in-cylinder temperature exceeds approximately 2100 K [5]. In the present model, the replacement of intake air with POME-derived biogas reduces the available oxygen for combustion, while the high CO₂ fraction in the biogas further dilutes the mixture and lowers its heating value. As a result, the energy released during combustion decreases, leading to a reduction in in-cylinder temperature and suppressed thermal NO_x formation. At the same time, the gaseous nature of POME biogas improves the homogeneity of the mixture, which facilitates more complete combustion and helps suppress soot formation. Similar findings have been reported in previous studies, which showed that CO₂ enrichment or biogas substitution reduces peak combustion temperature and NO_x emissions while simultaneously promoting cleaner combustion with lower soot levels [13,39].

3.3. Biogas Optimization to Obtain the Best Engine Performance and Emission

To determine the best engine performance and emission when using biogas, POME analysis and optimization calculations were conducted based on the simulation results. Therefore, this section describes in detail the optimization analysis. The analysis begins by summarizing the results of the study as shown in

Table 8. The performance and emission variations data of dual-fuel diesel engines.

Condition	BMEP	Efficiency	BSFC	Diesel Consumption	NOx	Soot
20% POME55	−5.07%	−5.06%	+18.95%	−15.73%	−46.64%	−12.73%
20% POME75	−4.94%	−4.93%	+9.75%	−15.84%	−27.97%	−20.57%
40% POME55	−10.43%	−10.42%	+43.79%	−33.00%	−75.39%	−27.02%
40% POME75	−9.99%	−9.98%	+23.69%	−33.33%	−47.82%	−41.55%
60% POME55	−15.68%	−15.67%	+90.42%	−52.54%	−88.42%	−47.09%
60% POME75	−15.28%	−15.27%	+54.95%	−52.77%	−61.71%	−62.58%
75% POME55	−18.61%	−18.61%	+155.5%	−69.28%	−91.81%	−68.13%
75% POME75	−18.42%	−18.42%	+100.7%	−69.35%	−68.48%	−79.99%

.

Table 8 shows that the average percentage values of BMEP, efficiency, diesel fuel consumption, NO_x, and soot are lower when the diesel engine is operated in dual-fuel mode. The percentage value of BMEP decreases due to the lower total energy of the mixture of diesel fuel and Biogas POME. The lower BMEP produced in the internal combustion engine indicates that the engine is operating in an inefficient condition, where the amount of energy used is not enough to produce higher pressure in the combustion chamber. Based on the simulation results, although POME Biogas has a higher LHV value than diesel fuel, the amount of POME biogas sucked into the piston cylinder cannot replace the same amount of energy as diesel fuel. This may be due to the fuel delivery system selected and used in the simulation. Since POME Biogas is a gas-based fuel, the energy density of the fuel becomes a disadvantage in the natural aspirated diesel engine system selected as a model in the simulation. The two fuels have different energy densities, with diesel fuel having a higher energy density than biogas. The percentage value of BSFC shows an increase when the diesel engine is operated in dual-fuel mode. The increase in the BSFC value may occur because the fuel combustion process in the combustion chamber is not the right process, where the injected diesel will act as a pilot ignition that will ignite the remaining unburned fuel in the combustion chamber.

A higher ratio of biogas energy results in a lower BMEP and diesel fuel consumption, while the BSFC increases. The decreases in BMEP and effective efficiency show quite identical values. The decrease in these achievements tends to be linear from one variation of the biogas energy ratio to another. Meanwhile, in the achievement of BSFC, the increase occurred quite significantly among the variations in the ratio of biogas energy and biogas composition, especially from 20% POME55 to 40% POME55, which were 18.95% and 43.79%, respectively. The reduction in diesel fuel consumption ranges from 15% (at 20% POME) to 69% (75% POME).

The addition of biogas from POME, with a 75% POME variation already calculated, shows that the amount of diesel fuel needed to produce a 0.4 kJ energy change is very small: only 2.176 × 10⁻⁶ kg, or about 2.176 mg. This value is the lowest value compared to the mass value in other variations. The reduced diesel fuel was also affected by the decrease in combustion energy. This happened because the two fuels have different energy densities; diesel fuel has a higher energy density than biogas. The reduction in the combustion of energy decreases the power generated by the engine. In addition, the pressure in the combustion chamber and the effective efficiency also decrease.

The various biogas compositions do not appear to have a significant impact on BMEP and diesel fuel consumption. However, as shown in Table 8, the performance of the engine when using 75% methane gives slightly better results, despite the decreasing percentage. This happened due to the POME75 having a higher heating value than POME55. The level of methane content in biogas greatly determines this value. The composition of biogas with greater methane led to lower BSFC because methane gas contributes to the combustion and energy results, unlike carbon dioxide. The higher the amount of methane in the biogas, the higher the power that can be generated from combustion, thus the BSFC value will also decrease. However, the addition of biogas POME still increases the BSFC value. This happened because the POME biogas, which is injected into the inlet manifold and then into the combustion chamber, is considered as additional fuel. In other words, more fuel was used, but the produced power was lower (compared to the neat diesel mode), resulting in an increase in fuel consumption.

Based on modelling results of soot emissions shown in Figure 9, the increase in biogas POME consumption will reduce the soot emissions. This occurred due to the increase in the utilization of gaseous fuel and a decrease in the use of liquid fuel. Gaseous fuel tends to be easy to mix with air because it has the same phase [40,41]. This good fuel mixture results in better and cleaner combustion because all the fuel can be burned completely. However, an interesting phenomenon occurred in the simulation results of soot emissions. Soot emissions increased when POME with 55% methane and an input energy of 1.665 kJ was used. This is because, at an input energy level of 1.665 kJ, the mass of diesel fuel consumption is greater than that of other variations in input energy. Meanwhile, a small amount of air was used for combustion. The gaseous fuel at the biogas POME55 variation also tends to be higher than the biogas POME75. The energy input of 1.665 kJ, the mass rate of gaseous fuel at a variation of 40% POME55 is 3.63 × 10⁻⁴ kg/s. While at 40% POME75, the mass rate used is only 2.66 × 10⁻⁴ kg/s. In this case, the amount of fuel is not proportional to the air intake, such that less-than-perfect combustion tends to occur.

In general, the addition of biogas POME has various impacts on the performance and emissions of the combustion engine. The selection of the appropriate variations to be able to operate a dual-fuel motor optimally is required, namely variations that can produce high performance, but can produce low emissions. In this study, it is known that the highest performance occurs in neat diesel operation. Thus, variations in the addition of biogas are required to achieve performance similar to that of neat diesel. For the emissions part, the highest value also occurred in neat diesel operations. Therefore, additional appropriate biogases are required to obtain a greater emission reduction. In Table 8, the decreases in BMEP, effective efficiency, and diesel fuel consumption tend to be the same in each variation; hence, a deeper review was carried out on other parameters, namely BSFC and emissions. The BSFC increase of POME55 tends to be higher than that of POME75. This indicates that the use of POME55 is more fuel-intensive.

Therefore, although the use of 60% and 75% of biogas results in very good emission reductions, a deeper review will only be carried out on the use of biogas energy ratios at 20% and 40%. In terms of NOx emissions, POME55 shows a fairly high reduction, that is 75% (at 40% POME). While at 40% POME75, the reduction in NOx emissions is only 43%. However, the soot reduction on POME55 was not very high, only 27%, while on POME75 it was 41%. It should also be noted that at POME55, with an input energy of 1.665 kJ, the emission of soot increased by 29%. Whereas in general, the resulting soot should be reduced. Therefore, the use of POME75 is recommended. Furthermore, between 20% and 40% biogas energy ratio, the use of a 40% biogas energy ratio is preferable. Overall, the resulting emissions can be reduced by almost half (40–50%). The optimum for dual-fuel engine operation is the 40% of biogas POME75 variation.

4. Conclusions

This study investigated the performance and emission characteristics of a diesel engine operating in dual-fuel mode using biogas derived from palm oil mill effluent (POME) with varying methane compositions (55% and 75%) and energy substitution ratios (20%, 40%, 60%, and 75%). Based on the characteristics of the modeling results that have been carried out in this study, the following conclusions can be drawn:

• Diesel fuel consumption was reduced by up to 69% under the highest biogas substitution, demonstrating the potential for significant fossil fuel savings.
• Brake mean effective pressure (BMEP) and thermal efficiency decreased by up to 18%, primarily due to the lower heating value of biogas and intake air displacement in the naturally aspirated configuration.
• Brake-specific fuel consumption (BSFC) increased substantially, reaching up to 155%, particularly at high biogas ratios, indicating reduced engine efficiency.
• NOx emissions decreased dramatically, achieving reductions of over 90% at 75% biogas substitution, while soot emissions were reduced by up to 80%, highlighting the strong environmental benefit of dual-fuel operation.
• Comparative evaluation showed that biogas with 75% methane (POME75) outperformed POME55, offering better combustion stability, lower BSFC, and greater emission reductions.
• The most optimal operating condition was observed at 40% biogas energy substitution with POME75, providing the best balance between emission reductions and performance retention.

These findings underscore that while dual-fuel operation with POME biogas can significantly reduce harmful emissions and fossil fuel usage, it also introduces performance trade-offs that must be addressed through optimization of substitution levels and fuel composition. Future research should encompass a comprehensive evaluation of POME biogas compositions to establish the optimal blend for maximizing engine performance while minimizing emissions. In addition, incorporating a rigorous techno-economic analysis is imperative to assess the scalability, cost-effectiveness, and overall viability of dual-fuel implementation in practical applications.