Effects of Diesel–Biodiesel–Ethanol–TiO2 Nanoparticle Blended Fuels on the Performance and Emissions

of a Direct Injection Diesel Engine

Authors: Kantanat Hemtanon, Jarernporn Thawornprasert, Krit Somnuk

Abstract

This study aimed to assess the emissions and performance of direct injection diesel engines operating with fuel blends consisting of diesel–biodiesel–ethanol–TiO2 (DxByEzTi). The blend proportions were determined by analyzing the phase behaviors and fuel stability of DxByEzTi blends by using a phase behavior diagram. The emission characteristics and performance of the diesel engines were examined at varying speeds ranging from 1100 to 2300 rpm and different engine loads of 25, 50, and 75% using the following four remaining fuel D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti. In terms of engine performance at 50% engine load and 2300 engine speed, the results indicated that the brake specific fuel consumption of all fuel blends was higher than that of diesel but lower than that of biodiesel. This was attributed to the oxygenated additives TiO2 and ethanol, which assisted in improving the atomization of the fuel blends. The brake thermal efficiency of D20B70E10Ti was 1.23% higher than that of diesel. The addition of oxygenated metal-based TiO2 nanoparticles with a high surface area to volume ratio resulted in better fuel oxidation. In terms of engine emissions under the same engine conditions, D20B70E10Ti exhibited reduced CO, CO2, NO~x~, and smoke opacity levels by 7.44, 37.12, 66.29, and 69.43%, respectively, compared to diesel. Consequently, the results of this study suggest that the D20B70E10Ti blend is viable for use in unmodified diesel engines, enhancing their BTE and reducing emissions.

Introduction

As the global population continues to grow, there is an increasing concern about the depletion of fossil fuel supplies. This is a pressing global issue as energy consumption increases with population growth.^1^ In addition, the combustion of fossil fuels, including coal, crude oil, and natural gas, releases significant amounts of greenhouse gases, including carbon monoxide (CO) and carbon dioxide (CO2). These gases are responsible for global warming and must be reduced to prevent irreversible climate damage.^2^ The most effective method for minimizing carbon dioxide and equivalent emissions is to decrease the consumption of fossil fuels.^3^ One solution to address this issue is the widespread adoption of biofuels. By 2023, the global consumption of biofuels reached 960,000 barrels of oil equivalent, marking a nearly 30% increase from the 751,000 barrels consumed in 2020.^4^ Many studies have reported that blending biodiesel fuel, which is characterized by a high cetane number and a high oxygen content, with diesel fuel can improve certain engine emission characteristics, such as lower levels of CO2, CO, and hydrocarbons (HCs).^5^ However, due to the high density and viscosity of biodiesel, some studies have also reported lower engine performance and noted an increase in NO~x~ emissions when high concentrations of biodiesel are blended with diesel.^6−8^

The increasing demand for renewable energy, including biodiesel, is crucial for reducing emissions and promoting sustainable industrial development in the future. The Thai energy ministry has set a target to surpass the current B10 standard by increasing the concentration of palm oil methyl ester (B100) in diesel. Their goal is to implement a 23% blend of B100 and diesel (B23) by 2037 for large vehicles, pick-ups, and trucks.^9^ In Malaysia, the transport sector currently uses a B10 biodiesel blend, but the Malaysian Palm Oil Board (MPOB) is planning to introduce B30 palm biodiesel by 2025.^10^ Similarly, the Indonesian government intends to reduce fossil fuel consumption by increasing the concentration of palm oil methyl ester in diesel to 40% (B40) for agricultural machines, ships, and trains by 2025.^11^

In terms of research on diesel engine performance and emissions, attention has been given to certain oxygenated additives in petroleum diesel including biodiesel, ethanol, and nanoparticles. In particular, the addition of nanoparticles to fuel is seen as an alternative approach to enhance the thermal conductivity, surface-to-volume ratio (S/V ratio), and chemical properties of diesel.^12^ Many studies have shown that including nanoparticles in fuels has led to significant reductions in engine emissions and improvements in engine performance,^13^ including brake power (Pb), brake specific fuel consumption (BSFC), and brake thermal efficiency (BTE). Some properties of nanosized particles, such as their chemical reactivity and thermal conductivity, have helped improve the heat and mass transfer rate of diesel fuel, with a larger S/V ratio increasing the rate of fuel oxidation during combustion.^14^ Most researchers have focused on examining the effects of adding nanoparticles such as carbon nanotubes (CNTs), alumina oxide (Al2O3), titanium oxide (TiO2), nanosilver particles (NSPs), and silicon oxide (SiO2) into diesel fuel on engine emissions and performance.^15−17^ For example, Radhakrishnan et al.^18^ investigated the emission and performance characteristics of a diesel engine fueled by biodiesel (B100) derived from cashew nut shell oil and nanosized alumina oxide (Al2O3) mixed with biodiesel (B100A) as compared to diesel fuel. They conducted studies at various loads and observed reductions in CO, HC, NO~x, and smoke emissions by 5.3, 7.4, 10.23, and 16.1% for B100 and 8.8, 10.1, 12.4, and 18.4% for B100A, respectively, compared to diesel fuel. This indicates that adding Al2O3~ nanoparticles in B100 can decrease the NO~x, CO, and HC emissions. In terms of engine performance, the BTE decreased by 1.1 and 2.3%, while the BSFC increased by 3.8 and 5.1%, for B100A and B100, respectively, when compared to diesel at 9 kW full load. Furthermore, Raju et al.^19^ investigated the effects of using different nanoparticles blended in fuel, such as Al2O3~ and CNTs mixed with tamarind seed methyl ester (TSME), on diesel engine performance and emissions. They found that adding nanoparticles to TSME blends can reduce the levels of CO, HC, and NO~x~ by 15–51, 24–68, and 7–9%, respectively. Moreover, they concluded that nanoparticle additives performed better in terms of the BTE compared to TSME blends and were able to lower the BSFC when compared to diesel. Gad and Jayaraj^20^ blended nanoparticles of Al2O3, TiO2, and CNTs with jatropha methyl ester (JME) to analyze their effects on performance and emissions in diesel engines. The results indicated that CNTs at a concentration of 50 ppm could reduce the level of CO emissions by 35% and that of NO~x~ emissions by 52%. When 100 mg of Al2O3, 25 mg of TiO2, and 50 mg of CNT nanoparticles were added to diesel fuel containing 20% JME (B20), the engine performance improved and emissions were reduced.

TiO2 nanoparticles blended in diesel have been recently highlighted due to their improved thermal properties when compared to other nanoparticles.^21,22^ El-Seesy et al.^23^ evaluated the effects of adding 25 and 50 mg/L TiO2 additives into jojoba methyl ester (JME)–diesel–n-hexane (JME–D–H) blends on diesel engine emissions and performance. Their study was carried out under varying engine loads of 0, 3, 6, 9, 12, and 13.5 N·m at a constant speed of 2000 rpm. The results showed that adding 25 and 50 mg/L TiO2 nanoparticles to J30D5H (30 vol % JME, 65 vol % diesel, and 5 vol % n-hexane) resulted in improved engine performance. The BTE of J30D5H50TiO2 was 15% higher compared with J30D5H under a load of 13.5 N·m. Additionally, the BSFC of J30D5H50TiO2 was 12% lower than that of J30D5H under similar load conditions. The significant increase in the S/V ratio of TiO2 additions led to a higher release of combustion enthalpy and energy density, hence improving the combustion efficiency. The addition of TiO2 to the J30D5H blend resulted in a 20% reduction in CO and HC emissions, while NO~x~ emissions increased by 15% at a similar load. This was attributed to the improvement in the combustion process, which increased the combustion temperature and the NO~x~ generation. Örs et al.^24^ examined the effects of TiO2 nanoparticles added to waste cooking oil methyl ester (WOME) and n-butanol (C4H9OH) on emissions and diesel engine performance. They observed that adding TiO2 to a 20% WOME-diesel blend (B20) increased the braking engine torque by 10.20% compared to B20. The addition of TiO2 to fuel blends improved engine performance and combustion by increasing the surface area of the nanomaterial, thus improving heat transfer rate during combustion.^25^

Waste products, cellulosic feedstocks, and starch-based crops, such as grass, wood, corn, and sugar cane, can be used to produce ethanol. This ethanol can then be blended with diesel to test its performance and emissions in a diesel engine. The blending of ethanol in fuel blends offers several benefits, including a high latent heat of evaporation and a high oxygen content, which enhance fuel combustion efficiency in diesel engines.^26^ Additionally, ethanol reduces emissions and particulate matter (PM), particularly NO~x~ emissions, by lowering combustion temperatures.^27^ These studies showed that fuel chemistry optimization provided by fuel blend technology was an effective way to increase engine performance, as also stated by a recent study.^28^ In this study, biodiesel and ethanol were selected as diesel additives due to their high oxygen content and potential to minimize greenhouse gas emissions. Additionally, the performance and emissions of these oxygenated components are examined when they are blended with TiO2 nanoparticles (TNPs) and operated in a diesel engine at varying speeds and loads.

This study examines the physical properties of blended fuels including density, viscosity, pour point, cloud point, and heating values. The stability of fuel blends and their separation time were determined through phase separation analysis. The aim of this study was to investigate the effects of diesel–biodiesel–ethanol–TiO2 fuel on diesel engine performance (BSFC, BTE) and emissions (O2, CO, CO2, NO~x~, smoke opacity) without any modifications at engine speeds ranging between 1100 and 2300 rpm and at 0, 25, and 50% engine loads.

Material and Methods

Materials

TNPs, which are a fine white powder with a particle size of 25 nm and a trace metal basis of 99.7%, were purchased from Sigma-Aldrich in the United States. The properties of TNPs include a surface area of 45–55 mm^2^/g, a density of 3.9 g/mL at 25 °C, and a mean molecular weight of 79.87 g/mol. High-resolution topographical images of the TNP surfaces were captured by using a field emission scanning electron microscope (FE-SEM, Apreo, FEI, Netherlands), as shown in Figure 1a. The FE-SEM was equipped with an energy-dispersive X-ray spectrometer (EDS), which was used to perform elemental analysis of the TNPs, as depicted in the EDS spectrum in Figure 1b and the EDS color-coded mapping of the TNPs for Ti Kα, O Kα, and C Kα in Figure 1c–e, respectively. Diesel B10, the standard diesel fuel in Thailand that contains 10% biodiesel and 90% petroleum diesel, was obtained from petrol stations. A baseline of B10 fuel’s performance and emissions was used to evaluate different diesel–biodiesel–ethanol–TiO2 fuel blends. B100 fuel was produced from used cooking oil by the Specialized Research and Development Center for Alternative Energy from Palm Oil and Oil Crops, Faculty of Engineering, Prince of Songkla University. Ethanol was used to add fuel mixtures of diesel blended with biodiesel, which were obtained from a chemical sales company in Bangkok, Thailand.

Experimental

Setup and Procedure of Blending Fuel

The blended fuels of diesel–biodiesel–ethanol–TNP were prepared as shown in Figure 2. In this study, TNPs were added to diesel at the start of the process. First, 0.01 wt % of TNPs was slowly added to 900 g of diesel in a beaker and then mixed using a magnetic stirrer (IKA, model: RCT basic, Staufen, Germany). The TNPs were evenly distributed in diesel at 30 °C under mixing conditions of 700 rpm for 30 min. This resulted in the formation of D100TNP, which contained 100 ppm of TNPs in the diesel. This mixture was immediately mixed with biodiesel and ethanol. The D100TNP, biodiesel, and ethanol were each placed in separate glass bottles and blended using a magnetic stirrer at 700 rpm for 10 min. The blended fuels were then labeled as DxByEzTi, where D, B, and E represent D100TNP, biodiesel, and ethanol, respectively, while Ti represents the dispersed TNPs in the fuel blends. Furthermore, x, y, and z indicate the weight percentages (wt %) of D100TNP, biodiesel, and ethanol in the blends, respectively. Each fuel blend in the glass bottle consisted of 10 to 80 wt % of each component. After the fuel blends were prepared, they were stored at 30 °C for 7 days to observe phase stability. Only four fuel blends that exhibited acceptable fuel density and kinematic viscosity—D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti—were selected for diesel engine experiments.

Analysis

of Fuel Properties

The properties of diesel blends D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel were studied. Density measurements were conducted using a hydrometer at 15 °C according to ASTM D1298-12b. Viscosity tests were carried out using a viscosity bath (model: Julabo, Visco Bath ME-16G, Julabo Labortechnik GmbH, Seelbach, Germany) according to ASTM D445-17a. The pour point and cloud point of the fuels were determined using the analysis methods outlined in ASTM D97 and ASTM D2500 with a cloud and pour point analyzer (model: Herzog CPP 97-2 device, Germany). The higher heating value (HHV) and lower heating value (LHV) of all fuels were determined using a CHNS/O analyzer (model: Flash 2000, Thermo Scientific, Italy). In this study, the fatty acid profile of used cooking oil (UCO), and used cooking oil methyl ester (UCOME) was determined using a gas chromatograph with flame ionization detector (GC-FID) (model: 7890, Agilent Technologies, USA).

Diesel Engine Experimental Setup

The study aimed to investigate the effects of various fuels (D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel) on diesel engine performance and emissions and to compare these effects with those of traditional diesel fuel. Experimental tests were conducted using an engine test apparatus, a Kubota RT100 DI mounted on an engine test rig or stationary test set, which was connected with a dynamometer (model: DW16, Jiangsu Lan Ling Test Equipment Co., Ltd., China) to vary engine load at various speeds, as shown in Figure 3. The specifications of the diesel engine and the eddy current brake dynamometer are listed in Table 1. An exhaust gas analyzer (model: Testo 350 XL, Titisee-Neustadt, Germany) was used to measure the engine emissions including CO, CO2, NO~x, and O2~ and exhaust gas temperature (EGT). Additionally, a smoke meter (model: CAPELEC, CAP3201EX-GO, Montpellier, France) was used to measure the smoke opacity of the engine in the range of 0–99.9%. Engine performance parameters, including Pb that was calculated using the engine speed (N, rpm) and engine torque (T, N·m) in the unit of kW, as shown in eq 1, BSFC that was calculated using the fuel consumption rate (FC) and divided by Pb in the unit of kg/kW·h, as shown in eq 2, and BTE that was calculated using eq 3 were tested at 0, 25, and 50% engine loads across various engine speeds of 1100, 1400, 1700, 2000, and 2300 rpm. The emissions of CO, CO2, NO~x, and O2~ were converted to g/kW·h units using eqs 4–7.^29^1234567

Uncertainty

Analysis

The accuracy and data error of the experimental results were verified and evaluated using uncertainty data. The uncertainty of engine emissions included the uncertainties of CO, CO2, NO~x, O2~, EGT, and smoke opacity. The uncertainty of Pb was calculated based on the uncertainty of the engine load and engine speed. The fuel consumption percentage uncertainty was determined using the uncertainty of several instruments, such as a digital stopwatch, a digital balance, and a temperature sensor. The uncertainty of BSFC was determined using the uncertainty of Pb and fuel consumption. The uncertainty of BTE was calculated by considering the uncertainty of BSFC and LHV. Table 2 shows the technique, measurement range, percentage uncertainty, and accuracy of the equipment used in this study. The overall experimental uncertainty was calculated by using eq 8. Therefore, the overall uncertainty of the experiment according to eq 8 was estimated to be approximately ±0.88%.8

Results

and Discussion

Phase Stability of DxByEzTi

The phase stabilities of DxByEzTi blends were observed at room temperature, approximately 30 °C, for 7 days. After the first 6 h, the physical appearance of DxByEzTi blends was categorized into two types of phase a liquid single phase and two liquid phases, as shown in Figure 4a. The liquid single phase represents a homogeneous liquid phase (indicated by black circles in Figure 4a), while the liquid two phases refer to a double-phase liquid system (indicated by white circles in Figure 4a). In terms of the phase behavior of fuel blends in the three-component system, the liquid single phase was observed to be the widest with 10–30 wt % D100TNP, 40–80 wt % biodiesel. Two liquid phases appeared with 40–80 wt % D100TNP, 10–30 wt % biodiesel, as shown in Figure 4a. These DxByEzTi blends were then stored for a total of 7 days at 30 °C for further phase behavior observations. It was concluded that the phase behavior of DxByEzTi fuel blends remained unchanged. Following the above discussion, 16 fuel blends, as shown in Figure 4b, were selected for analysis of fuel properties. The density and viscosity of these blends were also determined. As a result, the viscosity of D10B20E70Ti (white circles in Figure 4b) was measured to be 1.72 cSt at 40 °C. Therefore, 15 fuel blends (black circles in Figure 4b) met the diesel fuel specification, which required a fuel density within the range of 810 to 870 kg/m^3^ at 15 °C and a fuel viscosity within the range of 1.8 to 4.1 cSt at 40 °C, and were considered for testing in a diesel engine. However, ethanol concentrations exceeding 10 wt % have been found to not only decrease the BTE and increase the BSFC of the engine^30^ but also lead to unstable engine vibrations and irregular performance, making it difficult to control the engine.^31^ Excessive ethanol has also been observed to increase the ignition delay of the engine.^32^ Therefore, as shown in Figure 4c, four fuel blends—D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti (black circles in Figure 4c)—were selected for further investigation of performance and emissions in a diesel engine, and the remaining 12 fuel blends (white circles in Figure 4c) were discarded. Table 3 presents the densities and viscosities of the DxByEzTi fuel blends, and the fatty acid profiles of used cooking oil (UCO) and used cooking oil methyl ester (UCOME) are shown in Table 4.

Engine Performance

Brake Power

Figure 5 illustrates the relationship between Pb and various engine speeds of diesel, biodiesel, and DxByEzTi blends (D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti). The engine crankshaft generates the actual power (Pb) based on the maximum torque it produces at the designated engine speed.^34^ It was shown that Pb increased accordingly with engine speed from 1100 to 2000 rpm. The Pb of D20B70E10Ti and D10B80E10Ti decreased significantly at engine speeds of 2000 to 2300 rpm. According to the results, diesel fuel exhibited the highest Pb level followed by biodiesel and the DxByEzTi blends of D30B60E10Ti, D40B50E10Ti, D20B70E10Ti, and D10B80E10Ti at a maximum engine speed of 2300 rpm. The higher viscosity and density of biodiesel and fuel blends compared to diesel, as demonstrated in Table 3, led to poor atomization of fuels, resulting in decreased fuel combustion efficiency. Blended fuels had a decreased LHV, which was one of the factors contributing to the irregular combustion characteristics. Pb results were significantly affected by these properties.^35^ Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on the Pb level of the engine. The results showed that the Pb level of D100B0E0Ti was slightly lower than that of pure diesel. At maximum engine speed, the Pb of D100B0E0Ti was lower than diesel by 4.50%, and the Pb of D0B100E0Ti was lower than biodiesel by 3.50%. When comparing D0B90E10 and biodiesel, the addition of 10 wt % ethanol in biodiesel led to lower Pb of fuel blends at all engine speeds tested. At 2300 rpm engine speed, the Pb of D0B90E10 decreased by 12.60% when compared to biodiesel due to the lower cetane number of ethanol, ranging from 5 to 8.^36−38^ Shirneshan et al.^39^ also found similar results when comparing biodiesel fuel and biodiesel blended with 10% ethanol fuel blend. The addition of ethanol that has a low cetane number in fuel blends also caused a longer ignition delay in the engine and resulted in lower Pb when compared to biodiesel despite the increased LHVs of D40B50E10Ti, D30B60E10Ti, and D20B70E10Ti. According to the results obtained at maximum engine speed, fuel blends containing TNPs had a higher Pb level. In comparison to D40B50E10 and D20B70E10 blends without TNPs, the Pb levels of D40B50E10Ti and D20B70E10Ti were increased by 1.68 and 3.36% at 2300 rpm conditions, respectively.

Brake

Specific Fuel Consumption

The term BSFC refers to the number of kilograms of fuel consumed by the engine per hour for every 1 kW of Pb produced by the engine. Figure 6 shows the variation in BSFC for diesel, biodiesel, and DxByEzTi blends at different engine loads and engine speeds. It was observed that all tested fuels exhibited a decrease in BSFC as the engine load increased, as the percentage increase in fuel consumption was less than the percentage increase in Pb.^40^ When different fuel types were compared, biodiesel and DxByEzTi blends were found to have higher BSFC values than diesel due to their lower LHV, requiring more fuel to achieve the same Pb values. At an engine speed of 2300 rpm and 50% load, biodiesel showed the highest BSFC compared to diesel and DxByEzTi blends, which was attributed to the effects of lower LHV and higher viscosity of biodiesel.^41^ This caused issues with fuel atomization, resulting in larger droplets during fuel injection, which inhibited fuel evaporation and led to higher BSFC values.^42^ At 25% load and 2300 rpm engine speed, biodiesel, D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti exhibited BSFC values that were 17.25, 12.67, 15.36, 17.52, and 19.68% higher than those of diesel, respectively. At 2300 rpm speed and 50% engine load, the BSFC values of biodiesel, D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti were 19.78, 12.95, 13.31, 13.67, and 16.24% higher than those of diesel. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on the BSFC value of the engine. The results showed that at 25% load and 2300 rpm engine speed, D100B0E0Ti exhibited a BSFC value that was 1.96% higher than that of diesel. The D0B100E0Ti had a BSFC level 2.80% lower than biodiesel, while D0B90E10 had a BSFC level 7.22% higher than biodiesel. It was found that the BSFCs of D40B50E10 and D20B70E10 blends without TNPs were 2.15 and 4.11% higher than those of D40B50E10Ti and D20B70E10Ti, respectively. At 50% load and 2300 rpm engine speed, D100B0E0Ti exhibited a BSFC value that was 1.06% higher than that of diesel. D0B100E0Ti had a BSFC level that was 3.77% lower than that of biodiesel, while D0B90E10 had a BSFC level that was 3.42% higher than that of biodiesel. Similar results were obtained in a study conducted by de Oliveira et al.,^43^ who found that the addition of ethanol in blends led to an increase in BSFC due to the lower LHV of alcohol compared to other fuels. The BSFCs of D40B50E10 and D20B70E10 blends without TNPs increased by 1.91 and 3.25% when compared to fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively.

Brake Thermal Efficiency

Figure 7 illustrates the BTE of all fuels tested under various engine conditions. The engine operating at higher speeds exhibited lower thermal efficiency because of increased frictional losses from the internal engine components operating at high speeds and temperatures.^44^ At 25% load and 2300 rpm speed, diesel demonstrated a higher BTE compared to biodiesel, D40B50E10Ti, D30B60E10Ti, and D20E70E10Ti by 0.39, 0.20, 1.19, and 1.86%, respectively. Conversely, D10B80E10Ti showed a higher BTE of 1.97% compared with diesel under the same engine conditions. When the load was increased to 50% with an engine speed of 2300 rpm, the BTE of D40B50E10Ti and biodiesel was lower than that of diesel by 0.55 and 2.80%, respectively, due to the lower LHV of biodiesel. However, D30B60E10Ti, D20E70E10Ti, and D10B80E10Ti blends exhibited BTE higher than that of diesel by 0.32, 1.23, and 4.88%, respectively. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on the BTE level of the engine. At 25% load and 2300 rpm speed, D100B0E0Ti demonstrated a slightly lower (by 0.43%) but comparable BTE level to diesel. The BTE levels of D0B100E0Ti and D0B90E10 were 4.24% higher and 2.51% lower than biodiesel, respectively. D40B50E10 and D20B70E10 showed a decrease in BTE level when compared to D40B50E10Ti and D20B70E10Ti by 3.32 and 2.59%, respectively. At 50% load and 2300 rpm speed, D100B0E0Ti demonstrated a slightly higher BTE level (by 0.31%) when compared with diesel. The BTEs of D0B100E0Ti and D0B90E10 increased by 5.46 and 1.24%, respectively, when compared to biodiesel. For fuel blends without TNPs, D40B50E10 and D20B70E10 showed that the BTE levels when compared to fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti were lower by 3.22 and 1.69%, respectively. This finding is consistent with the study conducted by Pullagura et al.,^45^ where the engine performance of diesel–biodiesel–dimethyl carbonate with TiO2 addition was investigated. The addition of nanoadditives TiO2 with a high S/V ratio to B20T50DMC10 resulted in higher BTE compared to B20 and diesel. These results confirmed that the higher S/V ratio and the presence of oxygen in the TNPs resulted in better fuel oxidation during combustion,^23^ causing an increase in the BTE levels in DxByEzTi fuel blends.

Engine Emissions

Oxygen

Gas

Figure 8 shows the relationship between the O2 content and various engine loads and speeds. An increase in the engine load led to a decrease in O2 emitted in exhaust gas emissions. This is attributed to the engine operating at a richer fuel-to-air ratio under higher load conditions, requiring more O2 for complete combustion.^46^ At an engine speed of 2300 rpm and no load, it was observed that the D20B70E10Ti blend, D10B80E10Ti blend, and biodiesel emitted 0.54, 1.37, and 1.95% more O2, respectively, compared to diesel. Conversely, the D40B50E10Ti and D30B60E10Ti blends emitted 2.47 and 1.22% less O2, respectively. Furthermore, at 25% load and 2300 rpm speed, the O2 emissions from D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel blends were 3.24, 4.49, 7.32, and 5.54% higher than diesel, while the D40B50E10Ti blend emitted 1.03% less O2. The trends for the emissions of aqueous O2 at 50% load and 2300 rpm were similar to those at 25% load. The D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel fuels emitted 5.24, 16.34, 17.70, and 5.13% more O2, respectively, compared to diesel. Only the D40B50E10Ti blend was found to emit less aqueous O2 (0.86% less) when compared to diesel. Notably, blending B100, which had a high O2 level, with the highest biodiesel content in the D10B80E10Ti blend (80% biodiesel) resulted in the highest levels of O2 emissions. Ağbulut et al.^47^ also stated that the addition of metal-based oxide nanoparticles in fuels played a crucial role in increasing the oxygen content during the combustion process. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on O2 emissions. The results showed that at an engine speed of 2300 rpm and no load, it was observed that D100B0E0Ti emitted 3.14% more of the resulting O2 compared to diesel. D0B100E0Ti and D0B90E10 emitted 4.18 and 1.07% less O2 when compared to biodiesel, respectively. For fuel blends without TNPs, the O2 levels of D40B50E10 and D20B70E10 were 0.26% higher and 1.84% lower when compared to D40B50E10Ti and D20B70E10Ti, respectively. At 25% load and 2300 rpm speed, it was observed that D100B0E0Ti emitted 14.82% more O2 compared to diesel. D0B100E0Ti and D0B90E10 emitted 1.75 and 2.22% less O2 when compared to biodiesel, respectively. The O2 levels of D40B50E10 and D20B70E10 were 3.12% higher and 7.16% lower when compared to D40B50E10Ti and D20B70E10Ti, respectively. At 50% load and 2300 rpm speed, it was observed that D100B0E0Ti emitted 23.98% more O2 compared to diesel. D0B100E0Ti and D0B90E10 emitted 11.35% more and 0.47% less O2 when compared to biodiesel, respectively. The O2 levels of D40B50E10 and D20B70E10 were 4.00 and 13.96% lower when compared to fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. Similar results were obtained in a study conducted by El-Sheekh et al.,^48^ who investigated the engine performance of diesel–biodiesel–ethanol fuel blends at various loads and a fixed speed of 1500 rpm. They concluded that adding 10% ethanol improved the combustion efficiency of the engine. The increased oxygen content in the biodiesel and ethanol blends resulted in higher O2 emissions.

Carbon

Monoxide Emissions

Figure 9 shows the relationship between CO emissions and different fuels under various loads and speeds in engines. Biodiesel, with its higher oxygen content in comparison to diesel, promoted better combustion, resulting in reduced CO emissions across all of the test loads. Low combustion temperatures can lead to increased CO emissions at lower load conditions, while a short residence time in the combustion process may result in incomplete combustion, causing an increase in CO emissions at the same engine load and different engine speeds.^49^ At 0% load and 2300 rpm speed, biodiesel emitted 22.84% reduced CO emissions compared to diesel. However, the blends D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti exhibited CO emissions that were 44.20, 34.94, 29.25, and 14.30% higher than those of diesel, respectively. At 25% load and 2300 rpm, the D40B50E10Ti blend showed 14.11% higher CO emissions than diesel, while the CO emissions from D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel were reduced by 7.49, 8.42, 10.06, and 21.20%, respectively, compared to diesel. At 50% load and 2300 rpm speed, the CO emissions of D40B50E10Ti increased by 24.23% compared to diesel. On the other hand, the blended fuels of D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel emitted 6.92, 7.44, 13.16, and 17.24% less CO than diesel, respectively. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on CO emissions. At 0% load and 2300 rpm speed, D100B0E0Ti emitted 20.73% reduced CO emission when compared to diesel. D0B100E0Ti and D0B90E10 exhibited CO emissions that were 18.17 and 26.89% lower than those of biodiesel, respectively. For fuel blends without TNPs, D40B50E10 and D20B70E10 showed 3.74 and 37.03% lower CO emissions when compared to D40B50E10Ti and D20B70E10Ti, respectively. The additional experiment also showed that at 25% load and 2300 rpm speed, D100B0E0Ti showed 51.40% reduced CO emission compared to diesel. D0B100E0Ti and D0B90E10 exhibited CO emissions that were 17.74 and 1.45% lower than those of biodiesel, respectively. For fuel blends without TNPs, D40B50E10 and D20B70E10 showed 7.84 and 4.22% lower CO emissions when compared to D40B50E10Ti and D20B70E10Ti, respectively. Moreover, additional results also showed that at 50% load and 2300 rpm speed, D100B0E0Ti showed 19.16% reduced CO emission compared to diesel. D0B100E0Ti and D0B90E10 exhibited CO emissions that were 23.74% lower and 37.52% higher than those of biodiesel, respectively. CO emitted by D40B50E10, and D20B70E10 increased by 6.75 and 36.20% when compared to fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. The addition of the TNPs in blended fuel showed that the high surface area of the TNPs contributed to enhancing the combustion reaction of the diesel engine, resulting in decreased CO emissions.^24^ Furthermore, the oxygen content present in biodiesel and ethanol significantly improves the combustion process, leading to a decrease in CO emissions compared to diesel.^50^

Carbon Dioxide Emissions

Figure 10 shows the relationship between CO2 emissions and different fuels at various loads and speeds in engines. It was found that at all engine loads tested and at a speed of 2300 rpm, only the D40B50E10Ti blend emitted higher levels of CO2 than diesel. Specifically, at 0% load and 2300 rpm speed, the D40B50E10Ti blend released 17.54% more CO2 compared to diesel. Conversely, the D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel showed reductions in CO2 emissions by 7.25, 12.90, 7.52, and 13.37%, respectively, when compared to diesel. At a load of 25% and a speed of 2300 rpm, the D40B50E10Ti blend emitted 2.31% more CO2 than diesel. In contrast, the D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel had lower CO2 emissions compared to diesel by 20.78, 25.74, 14.57, and 21.15%, respectively. Similarly, at a load of 50% and a speed of 2300 rpm, the D40B50E10Ti blend produced 2.64% more CO2 than did diesel. Conversely, the D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel emitted lower levels of CO2 compared to diesel by 17.36, 37.12, 21.05, and 17.17%, respectively. The results indicate that at maximum speed and maximum load, the D20B70E10Ti blend exhibited the lowest CO2 emissions among all fuels tested. The addition of oxygenated nanoparticles in the fuel blend improved combustion efficiency, leading to more complete combustion and a balance between CO and CO2 emissions.^51^ Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on CO2 emissions. At 0% load and 2300 rpm speed, it was observed that D100B0E0Ti emitted 23.62% less CO2 when compared to diesel. D0B100E0Ti and D0B90E10 emitted 29.83% higher and 35.17% lower CO2 when compared to biodiesel, respectively. For fuel blends without TNPs, the CO2 levels of D40B50E10 and D20B70E10 were 2.21% higher and 18.14% lower than D40B50E10Ti and D20B70E10Ti, respectively. At 25% load and 2300 rpm speed, it was observed that D100B0E0Ti emitted 57.27% less CO2 compared to diesel. D0B100E0Ti and D0B90E10 emitted 2.91 and 4.78% higher CO2 when compared to biodiesel, respectively. The CO2 levels of D40B50E10 and D20B70E10 were 5.70 and 34.99% higher than fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. At 50% load and 2300 rpm speed, it was observed that D100B0E0Ti emitted 41.72% less CO2 compared to diesel. D0B100E0Ti and D0B90E10 emitted 15.16% lower and 21.37% higher CO2 when compared to biodiesel, respectively. The CO2 levels of D40B50E10 and D20B70E10 were 13.83 and 71.79% higher when compared to fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. Similar results were reported by Ranjan et al., where the addition of 30 ppm of oxygenated MgO nanoparticles to pure biodiesel (B100), B20, and B10 resulted in reductions in CO2 emissions by 1.12, 1.7, and 0.71%, respectively. Nanoparticles with enhanced S/V ratio and high catalytic activity helped reduce ignition delay, allowing for increased fuel accumulation in the combustion chamber and improved combustion efficiency, resulting in more complete fuel combustion.^52^

Nitrogen

Oxide Emissions

The emissions of nitrogen oxide compounds, mainly NO and NO2, are collectively known as NO~x. Figure 11 illustrates the relationship between NOx~ emissions and different fuels at various loads and speeds in engines. The results indicate that NO~x~ emissions increased with engine load while they decreased as the engine speed increased. This could be attributed to the higher engine load causing increased fuel combustion and a rise in the temperature of the combustion chamber, both critical factors in NO~x~ formation.^53^ All tested fuels exhibited lower overall NO~x~ emissions compared to diesel. At 0% load and 2300 rpm speed, the D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel emitted lower NO~x~ than diesel by 41.30, 46.46, 48.18, 51.63, and 10.52%, respectively. Similarly, at 25% load and 2300 rpm speed, the emissions of D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel were lower than those of diesel by 39.97, 55.97, 56.64, 54.68, and 27.39%, respectively. At 50% load and 2300 rpm speed, the D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel emitted lower NO~x~ levels than diesel by 26.65, 48.96, 66.29, 51.52, and 54.99%, respectively. The results clearly demonstrate that the high oxygen content of biodiesel and ethanol improves the combustion process of fuel blends. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on NO~x~ emissions. At 0% load and 2300 rpm speed, D100B0E0Ti emitted NO~x~ emissions lower than diesel by 12.05%. D0B100E0Ti and D0B90E10 exhibited NO~x~ emissions that were 6.57% higher and 33.55% lower than those of biodiesel, respectively. NO~x~ emissions of D40B50E10 and D20B70E10 were 2.27% higher and 1.11% lower than fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. At 25% load and 2300 rpm speed, D100B0E0Ti emitted lower NO~x~ emissions than diesel by 53.78%. D0B100E0Ti and D0B90E10 exhibited NO~x~ emissions that were 10.18 and 37.03% lower than those of biodiesel, respectively. For fuel blends without TNPs, NO~x~ emissions of D40B50E10 and D20B70E10 were 2.96 and 17.44% higher than D40B50E10Ti and D20B70E10Ti, respectively. At 50% load and 2300 rpm speed, D100B0E0Ti emitted lower NO~x~ emissions than diesel by 39.17%. D0B100E0Ti and D0B90E10 exhibited NO~x~ emissions that were 58.30 and 21.07% higher than those of biodiesel, respectively. D40B50E10 and D20B70E10 showed 13.74 and 79.33% more NO~x~ emissions when compared to fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. Overall, the results showed that the presence of metal-based oxygenated nanoparticles in fuel blends resulted in a reduction of NO~x~ levels due to their properties of being a heat transfer rate enhancer.^54^ Similar conclusions were drawn by Ağbulut et al.,^47^ who studied the impact of metal-oxide nanoparticles (Al2O3, SiO2, and TiO2) and biodiesel blends on diesel engine emissions. Their results showed that these nanoparticles contribute to the combustion process by donating oxygen, leading to slightly lower NO~x~ emissions from fuel blends with nanoparticles at 2.5, 5, and 7.5% engine loads compared to diesel fuel.

Exhaust

Gas Temperature

For all of the fuels tested, higher EGTs were observed at higher engine loads and engine speeds, as shown in Figure 12. Specifically, at 0% load and 2300 rpm engine speed, diesel exhibited a 7.03% higher exhaust gas temperature compared to biodiesel. Fuels of D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends demonstrated lower EGT values than diesel by 19.40, 10.70, 7.30, and 4.75%, respectively. However, the EGT of the D40B50E10Ti blend increased by 8.50% compared to diesel at 25% load and 2300 rpm. On the other hand, blends of D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel showed lower EGT values compared to diesel by 13.05, 6.90, 12.78, and 17.59%, respectively, at the same load and speed conditions. Furthermore, at 50% load and 2300 rpm speed, the D30B60E10Ti, D20B70E10Ti, and D10B80E10Ti blends and biodiesel exhibited lower EGT levels than diesel by 3.81, 12.09, 6.72, and 11.46%, respectively, while the D40B50E10Ti blend showed a 9.98% higher EGT than diesel. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on EGT. At 0% load and 2300 rpm speed, it was observed that D100B0E0Ti demonstrated 9.28% higher EGT when compared to diesel. D0B100E0Ti and D0B90E10 showed 13.74 and 27.91% higher EGT when compared to biodiesel, respectively. For fuel blends without TNPs, the EGTs of D40B50E10 and D20B70E10 were 3.69% lower and 13.63% higher than D40B50E10Ti and D20B70E10Ti, respectively. At 25% load and 2300 rpm speed, it was observed that D100B0E0Ti demonstrated 0.75% lower EGT when compared to diesel. The EGTs of D0B100E0Ti and D0B90E10 were 10.92 and 27.84% higher than those of biodiesel, respectively. The EGTs of D40B50E10 and D20B70E10 without TNPs were 0.61% lower and 9.98% higher than D40B50E10Ti and D20B70E10Ti, respectively. At 50% load and 2300 rpm speed, it was observed that the D100B0E0Ti blend demonstrated higher EGT values compared to diesel by 4.53%. D0B100E0Ti and D0B90E10 showed 11.38 and 24.13% higher EGT when compared to biodiesel, respectively. The EGTs of D40B50E10 and D20B70E10 were 2.42 and 22.12% higher than fuel blends containing TNPs of D40B50E10Ti and D20B70E10Ti, respectively. The increased EGT levels of the tested fuel blends in comparison to diesel can be attributed to the excess oxygen provided by biodiesel,^55^ ethanol, and TNPs, which improved combustion and resulted in higher combustion temperatures.

Smoke Opacity

The relationship between smoke opacity levels at various engine loads and speeds is depicted in Figure 13. At 50% engine load and 2300 rpm speed, the results demonstrated that blends of D40B50E10Ti, D30B60E10Ti, D20B70E10Ti, D10B80E10Ti, and biodiesel exhibited lower levels of smoke opacity compared to diesel fuel by 31.21, 36.30, 69.43, 86.62, and 77.71%, respectively. These findings suggest that a higher concentration of biodiesel in fuel blends leads to a decrease in the smoke opacity. The presence of a higher oxygen content in B100 was observed to have an impact on reducing smoke opacity levels.^56^ Abed et al.^57^ investigated the effects of diesel fuel blends, including jatropha biodiesel, on diesel engine emissions. They observed that as the biodiesel proportion increased, the emitted smoke opacity decreased. This phenomenon was attributed to the high oxygen and lower carbon contents in biodiesel. Besides, the additional fuel blends of D100B0E0Ti, D0B100E0Ti, D0B90E10, D40B50E10, and D20B70E10 were also tested in diesel engines to confirm the effect of ethanol and TNPs on smoke opacity. At 50% engine load and 2300 rpm speed, the results demonstrated that D100B0E0Ti exhibited lower levels of smoke opacity compared to diesel fuel by 80.89%. D0B100E0Ti and D0B90E10 showed levels of smoke opacity that were 77.14 and 54.28% lower than those of biodiesel, respectively. For fuel blends without TNPs, the smoke opacity levels of D40B50E10 and D20B70E10 were 14.81 and 50.00% higher than those of D40B50E10Ti and D20B70E10Ti, respectively. The additional results suggested that adding ethanol into fuels can lead to decreased smoke emitted, while the addition of TNPs showed a significant decrease in smoke opacity. This was confirmed in the study by Ranjan et al.,^52^ who reported the impact of TNPs on smoke opacity reduction. They found that using metal-based oxygenated nanoparticles in WOME could improve the combustion characteristics and contribute to the reduction of smoke opacity.

Conclusions

This study successfully evaluated diesel, biodiesel, and four DxByEzTi blends in a DI diesel engine without modifications. The fuel qualities of the blended fuels were evaluated by using diesel standards. A ternary diagram was used to study phase behaviors of DxByEzTi fuel blends during 7 days of storage at room temperature. The performance and emissions of all fuel blends were analyzed on a DI diesel engine. Test findings recommend using the D20B70E10Ti blend in the engine because of its low environmental impact, while also helping to minimize exhaust gas emissions. Given all of these results, it can be concluded that the TNP in DxByEzTi blends can be considered a promising alternative fuel to be used in a diesel engine. Therefore, adding biodiesel, ethanol, and TNP in diesel blended fuel can improve heat transfer rate, combustion quality, and BTE while reducing engine emissions such as CO, CO2, NO~x, and smoke formation. Further studies require attention to specific variables of the droplet size of TiO2~ blended fuel and operating parameters including multiple injections, injection pressures, and injection timing in modern CRDI engines, besides qualities of fuel blends. These variables have a considerable impact on the performance and emissions of the CRDI engine. Furthermore, more research needs to be done to learn more about how the released TiO2 nanoparticles affect the environment such as particle sizes, physical properties, and chemical elements after combustion.