Internal combustion engines using liquid fuels will continue to be the main power sources for road and air transportation for decades, likely throughout most of this century. The global demand for liquid transportation fuels from petroleum and other fossil resources has grown immensely. This will render the economy unsustainable, becoming infeasible only in the distant future [1] . The use of petroleum oil in transportation significantly contributes to environmental deterioration through emissions such as nitrogen oxides (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO), particulate matter (PM), and carbon dioxide (CO₂). These pollutants accumulate in the atmosphere, contributing to the greenhouse effect, global warming, and climate change. Oxygenates, including alcohols, have emerged as alternatives for improving the octane number and oxygen content in conventional fuels (gasoline, diesel) [2] .

Studies, such as one by (Dogan 2011) on compression ignition engines, investigated engine performance and exhaust emissions where alcohol fuels, namely methanol and ethanol, were added to diesel fuels [2] . Lower alcohols, such as methanol and ethanol, are commonly used. Their production can be cheaper and more environmentally friendly than that of petroleum fuels and biodiesel [3] [4] . Methanol can be produced from coal or petrol-based fuels at a low cost, though it has limited solubility in diesel fuel. Conversely, ethanol, a renewable fuel, is produced through the alcoholic fermentation of sugars from vegetable materials like corn, sugar cane, sugar beets, barley, sweet sorghum, cassava, and molasses, among others. Additionally, agricultural residues such as straw, feedstock, and waste wood can also be used, utilizing existing and proven technologies [5] . The use of alcohol as additives in diesel fuel presents several challenges. The primary issue is their solubility with diesel, leading to phase separation. This instability, influenced by the diesel and alcohol composition and temperature, has been extensively researched. For instance, ethanol blends with diesel show two phases below 10˚C. Furthermore, the alcohol percentage and its water content significantly affect solubility. To mitigate this, additives are introduced to improve solubility. However, incorporating alcohol alters the fuel’s properties, including cetane number, viscosity, lubricity, heating value, and ignitability. These changes are also affected by the alcohol’s chain length and structure [6] . Generally, n-butanol has several advantages over methanol and ethanol, including a higher tolerance to water contamination (n-butanol is more hydrophobic than ethanol) and compatibility with the existing fuel distribution infrastructure, allowing the use of current distribution pipelines (Bryan, Kumar et al. 2011), less corrosion to aluminum or polymer components in fuel system, ability to blend in gasoline or diesel at high fraction without modifying vehicles, and better fuel economy due to high energy density. The primary goal of this review paper is to examine the physical and chemical properties, combustion, performance, and emission characteristics in CI engines across different operating conditions. It has been noted that there remains a considerable gap in research concerning the effect of oxygenated fuels, such as n-butanol, on diesel when using specific proven mix ratios. This gap exists largely because n-butanol is primarily utilized as a chemical compound in the food industry, leading to its undeveloped production from non-petroleum sources and higher cost. Consequently, this review aims to analyze various combustion phases and determine n-butanol-diesel ratios that could enhance emission characteristics without adversely affecting engine performance parameters.

Butanol production, one of the oldest fermentation processes known as acetone-butanol-ethanol (ABE), AB, or solvents, is employed for the commercial production of a chemical benefiting mankind. Butanol can be produced through fermentation using various microorganisms, including Clostridium acetobutylicum and Clostridium beijerinckii [7] .

The acetone-butanol-ethanol (ABE) fermentation process was discovered by French microbiologist Louis Pasteur in 1861. Research into butanol was revived during World War I due to the high demand for acetone, [8] leading to a patent for the process being filed and tested in the United Kingdom. However, there was a need for an organism to increase acetone production. Strange and Chaim Weizmann succeeded in isolating Clostridium acetobutylicum from garden soil for ABE fermentation, which could produce large amounts of acetone. During World War I, Britain needed to produce large quantities of acetone as it was a crucial chemical for making cordite, an alternative to gunpowder. Following the war, the demand for butanol increased, leading to the establishment of the first large-scale industrial plants in Canada and the USA. After 1936, ABE production industries were set up in the Soviet Union, Japan, China, South Africa, and Egypt. By 1945, at the beginning of the Second World War, Japan started producing butanol from sugar plants, primarily as airplane fuel [9] . In the 1950s, a petrochemical route for n-butanol production emerged, primarily involving the aldol condensation of acetaldehydes, followed by dehydration and hydrogenation of crotonaldehyde. This rapid industrial advancement, known as the Oxo synthesis, led to the abandonment of fermentation processes [10] . By the 1960s, most industrial ABE fermentation facilities had closed due to the lower cost of oil, which favored chemical production methods. The last such factory closed in 1986 in South Africa. However, this chemical production method was short-lived, as a rise in crude oil prices led to a resurgence of industrial ABE fermentation facilities, particularly in China and Brazil. Today, numerous plants have been established worldwide, including in the USA, Slovakia, France, and the UK, to produce bio-butanol as fuel [11] .

Ethanol, a colorless alcohol with a distinctive aroma, evaporates at a relatively low temperature and is highly hydrophilic, meaning it can dissolve indefinitely in water and many other organic and inorganic substances. Its high latent heat results in a cooling effect on intake air, allowing more fuel-air mixture to be filled into the cylinder when used in engines. This, combined with ethanol’s high octane number, enhances its anti-knocking properties, enabling a higher compression ratio and improved engine efficiency. Recently, ethanol has been increasingly used as a blending fuel in internal combustion engines, especially in countries like Brazil with abundant ethanol sources. However, its lower flash point compared to conventional diesel fuel and its corrosive nature require careful handling and storage. Ethanol, like methanol, is part of the lower alcohols group and shares the same chemical formula as di-methyl-ether (C₂H₆O), though it exhibits different thermodynamic behavior [3] [13] . The use of ethanol in engines has been extensively researched due to its potential as an alternative to petroleum-derived fuels. It is a renewable resource that can be produced through the fermentation of sugar- or starch-rich vegetable materials such as corn, barley, molasses, sugarcane, sugar beets, sorghum, etc. [14] [15] . Additionally, other agricultural or municipal residues, such as straw, wood waste, and byproducts from food and paper processing, are utilized to produce ethanol using demonstrated processes [16] [17] . The high polarity of these lower alcohols results in poor miscibility with diesel when directly mixed. To achieve high substitution ratios, techniques such as dual fuel injection and alcohol-diesel emulsions are necessary, which increases the cost and complexity of the diesel engine system [18] . Meanwhile, properties such as low cetane number, low energy density, and high latent heat of vaporization make these lower alcohols less suitable for diesel engines [19] .

Ethanol solubility in ordinary diesel is mainly influenced by temperature and the water content of the blend. At ambient temperatures, dry ethanol readily mixes with diesel fuel. However, below about 10˚C, the two fuels separate—a common occurrence in many parts of the world for much of the year. To prevent this separation, two methods can be employed: i) adding an emulsifier to suspend small ethanol droplets within the diesel fuel, or ii) using a co-solvent that acts as a bridging agent to ensure molecular compatibility and bonding, resulting in a homogeneous blend [20] [21] . Emulsification typically involves heating and blending to produce the final mixture, while co-solvents enable “splash-blending,” simplifying the process. Both emulsifiers and co-solvents have been tested with ethanol and diesel fuel. Moses et al. (1980) studied micro-emulsions of aqueous ethanol (5% water) and diesel fuel using a commercial surfactant [21] . They reported that the blends formed spontaneously, requiring only minor stirring and appeared transparent, indicating that the dispersion sizes were smaller than a quarter of a wavelength of light, making them “infinitely” stable, i.e., thermodynamically stable with no separation even after several months. Approximately 2% surfactant was needed for every 5% aqueous ethanol added to diesel fuel. Boruff et al., 1982, developed formulations for two micro-emulsion surfactants, one ionic and the other non-detergent. Blends of these surfactants with aqueous ethanol and diesel were transparent and stable at temperatures as low as 15.5˚C. Researchers in Sweden tested a blend of 15% aqueous ethanol (5% water) with diesel, containing DALCO, an emulsifying agent developed in Australia [22] . Early studies by Letcher (1980), Meiring et al. (1981), and Letcher (1983) identified tetrahydrofuran, sourced affordably from agricultural waste, and ethyl acetate, inexpensively produced from ethanol, as effective co-solvents. Ternary liquid-liquid phase diagrams in Figure 1 and Figure 2 illustrate how moisture content and temperature impact blend stability, showing the need for increasing amounts of co-solvent with higher moisture and temperature to maintain a single-phase liquid. Letcher (1983) determined that a consistent ratio of ethyl acetate to ethanol of 1:2 is required for complete miscibility down to 0˚C. Gerdes and Suppes (2001) found that the solubility of ethanol in diesel, and thus the effectiveness of emulsifiers and co-solvents, is also influenced by the diesel’s aromatic content. The polar nature of ethanol allows it to interact strongly with aromatic molecules, which remain compatible with other hydrocarbons in diesel, acting as bridging agents and co-solvents. Reducing the aromatic content in diesel fuels impacts the miscibility of ethanol in diesel and affects the required amount of additive for a stable blend [20] [21] .

The physicochemical and combustion properties of ethanol are indicated in Table 2 below.

Butanol has garnered significant research interest as a second-generation biofuel in recent years, being considered as a potential alternative to ethanol for replacing gasoline and diesel [25] . Until recently, it was less studied than ethanol or methanol, mainly due to its use in the food industry, undeveloped production from non-petroleum sources, and higher costs. However, interest in butanol has surged [6] .

Butanol (C₄H₉OH) is a higher-chain alcohol with a four-carbon structure and exists in different isomers, which vary in physical properties based on the location of the hydroxyl (OH) group and the carbon chain structure [26] . These are 1-Butanol (also known as n-butanol), 2-butanol (also known as sec-butanol), isobutanol, and tert-butanol. 1-Butanol has a straight-chain structure with the alcohol (OH) group at the terminal carbon. 2-Butanol has the hydroxyl group at an internal carbon. Isobutanol is a branched isomer with the OH group at the terminal carbon, and tert-butanol is a branched isomer with the OH group at an internal carbon. The different structures of butanol isomers directly impact their physical properties [26] . Laminar velocities decrease in the following order: 1-butanol, sec-butanol, iso-butanol, and tert-butanol. Gu, Huang et al. (2010) state that the laminar flame speed, or fuel burning velocity, is a crucial parameter of a combustible mixture. This physicochemical property influences combustion duration and performance in spark ignition engines, affecting the fuel burning rate in internal combustion engines, thus impacting efficiency and emissions. The molecular structure of combustible mixtures, particularly the isomers of butanol, affects their laminar flame stability. 1-butanol exhibits the highest flame burning velocity. Functional groups significantly influence laminar burning velocity; branching (-CH3) decreases it, while the hydroxyl functional group (-OH) attached to terminal carbon atoms increases laminar velocity. Terminal C-H bonds possess higher bond energies than those of inner C-H bonds. High bond energies hinder the H-abstraction reaction, leading to a lower reaction rate. Consequently, n-butanol, which mostly contains inner C-H bonds, has the highest laminar flame velocity, while tert-butanol has the lowest [2] .

Higher alcohols, such as butanol and pentanol, have properties closer to diesel fuel than lower alcohols [27] . Compared to commonly used alcohols like ethanol and methanol, butanol offers a higher heating value, meaning it contains more energy and can reduce fuel consumption. It also has a higher cetane number, making it a more suitable diesel additive than ethanol and methanol. Additionally, its lower vapor pressure decreases the likelihood of cavitation issues. Butanol’s lower autoignition temperature improves cold start ignition problems. Being less hydrophilic, it is less corrosive and less prone to separation in diesel blends when contaminated with water [13] [27] [28] . Despite possessing properties similar to diesel fuel, the use of butanol alone is incompatible with certain components of diesel engines, and its heating value remains lower than that of conventional diesel fuel [28] .

Research on butanol as an alternative fuel is continually growing. Generally, adding butanol to diesel slightly increases brake specific fuel consumption and brake thermal efficiency. Exhaust gas temperatures, nitrogen oxides, and carbon monoxide decrease with lower butanol blending ratios, while unburned hydrocarbons and nitric oxide levels increase. Notably, the reduction of smoke opacity in exhaust emissions is significant [2] [27] .

The properties of n-butanol significantly influence combustion parameters. Its presence impacts the fuel-air mixing process and the development of the injection spray. N-butanol’s lower density and kinematic viscosity compared to diesel fuel result in improved atomization quality for butanol-diesel blends. Moreover, n-butanol’s higher volatility facilitates a quicker evaporation process. Both enhanced atomization and accelerated evaporation help in forming more homogeneous fuel-air mixtures, thereby reducing soot formation [41] . The cetane number is a crucial parameter for combustion quality as it aids in optimizing combustion timing. For n-butanol blends, the peak pressure in the combustion chamber during combustion decreases with higher butanol content due to three main effects: the energy effect, which is the reduction in heating value; the chemical effect, related to the decrease in the equivalence ratio; and the dilution effect, caused by over-dilution and large delay times. Both the chemical and dilution effects reduce flame velocity, thereby increasing heat transfer to the chamber walls during combustion and diminishing combustion quality [52] .

Rakopoulos et al. conducted a comprehensive study on the impact of n-butanol on diesel engine emissions during discrete transient schedules, focusing on n-butanol/diesel blends during acceleration [54] and hot starts of a turbocharged diesel engine. The research found that in all acceleration tests, the n-butanol/diesel blend produced less smoke than pure diesel, albeit with higher NO emissions (in ppm). The study also compared a biodiesel/diesel blend, offering a unique comparison of the effects of two biofuels. Figure 3 exemplifies the findings reported in Ref. [54] . The illustration shows how a six-cylinder, medium-duty, turbocharged, direct injection (DI) diesel engine responds during medium to high-speed acceleration while running on neat diesel fuel, a blend of 25% n-butanol and 75% diesel fuel (Bu25), and a blend of 30% (sunflower-cottonseed) biodiesel with 70% diesel (B30). Both biofuel blends, as shown in Figure 3 , were found to reduce engine smoke emissions during demanding acceleration, with further improvements observed at the final steady-state conditions. The reduction in smoke emissions was significantly more pronounced for the n-butanol blend than for the biodiesel blend, which also had a higher oxygen content. Although fuel injection and spray development slightly differed from

the neat diesel fuel case, it was suggested that this behavior was due to the engine running ‘leaner’ compared to when it used neat diesel fuel. This holds true because the trapped relative air-fuel ratio remained essentially the same, with the combustion being assisted by the fuel-bound oxygen in the biodiesel or n-butanol in locally rich zones, which had the dominant influence. In fact, the higher oxygen mass percentage of the n-butanol blend compared to the biodiesel blend led to its superior smoke behavior. As a result, while the peak opacity value was reduced by 13% for the biodiesel blend, the n-butanol blend (with higher oxygen content) saw a significant decrease of 50% compared to neat diesel fuel operation. Further, results are documented in Figure 4 for hot starting this time. Here, the effect of each biofuel on smoke emissions was found to be contradictory. Specifically, the biodiesel blend increased both the peak soot value and the duration of unacceptable smoke emissions, whereas its n-butanol counterpart (confirmed also by the results of Armas et al. [55] substantially decreased both of them, compared to the neat diesel fuel case; the relative differences were +40% and −69%, respectively, in the maximum opacity value. Furthermore, opacity exceeded 10% for 10, 14, and just 3 engine cycles (or 1.9, 2.5, and 0.5 seconds), respectively, for neat diesel fuel, biodiesel, and normal butanol blends. For the diesel/n-butanol blend, the improvement in smoke emissions was attributed to the engine running leaner. This was due to combustion being aided by the higher fuel-bound oxygen content of n-butanol in the locally rich zones, which appeared to have a dominant influence. During turbocharged diesel engine transients, the extra oxygen available inside the cylinder compensates for the significant air deficiency from the compressor during turbocharger lag, proving crucial for the emission development shown in Figure 4 . Similarly, n-butanol’s lower viscosity and higher volatility compared to biodiesel (and diesel fuel) are expected to reduce soot production. Not surprisingly, the cleaner operation of the engine with added n-butanol, as reported in [54] during warm/hot starting, also resulted in an increase in emitted nanoparticles [55] . The arguments for discrete transient schedules are strengthened by comparing smoke emissions from neat diesel fuel and a Bu10 blend, as shown in Figure 5 . This figure displays the instantaneous smoke emissions during the 1180-second European passenger car New European Driving Cycle (NEDC). The n-butanol blend consistently maintained lower levels of smoke emissions, including during the cold start phase. This was particularly notable at the beginning of each acceleration event, where the effects of turbocharger lag—caused by a sudden shortage of air from the turbocharger compressor—are more significant [13] .

Gvidonas, L. et al. (2021) performed experiments and investigated the effects of biodiesel-n-butanol fuel blends on performance and emissions in a diesel engine. The combustion of neat biodiesel produced the highest total NOx emissions at 2290 ppm. However, the addition of n-butanol to biodiesel did not significantly reduce NOx emissions as expected. At full engine load with a BBu20 blend, nitrogen oxides emissions decreased by 23.5%. At low engine load, the highest carbon monoxide emissions were recorded with neat biodiesel (302 ppm), while the lowest was with the BBu20 blend (241 ppm). The n-butanol oxygenated biodiesel blends BBu10 and BBu20 significantly reduced carbon monoxide emissions at full load. Furthermore, at full load, smoke opacity decreased by 28.5% and 35% with BBu10 and BBu20 blends, respectively, compared to neat biodiesel. Renewable binary biodiesel-n-butanol blends are suitable for powering diesel engines, replacing fossil-origin diesel fuels, reducing climate change, and preserving untouched nature for future generations. Due to the absence of polycyclic aromatic hydrocarbons in these renewable biofuel blends, lower maximum NOx emissions, drastically reduced CO emissions, and decreased smoke opacity (PM) can be expected. This is attributed to the potentially improved homogeneity of the combustible mixture and the benefits to particulate matter (PM) emissions, suggested by the higher n-butanol-oxygen mass content (21.62 wt%) and a much lower carbon-to-hydrogen ratio (4.8) [58] .

Diesel engines are recognized for their high efficiency, high compression ratio, and excellent stability, but have traditionally been associated with significant emissions of particulate matter (PM) and nitrogen oxides (NOx). In recent decades, manufacturers have substantially reduced these harmful emissions by introducing new technologies [116] . B. Mahr et al. [117] investigated the future and potential of diesel injection systems, suggesting that new product engineering, such as new nozzle designs (k-factor, vario nozzle) or newly developed actuators, are key factors for the development of fuel injection systems. With a flexible diesel injection system, it is possible to achieve the optimum rate shaping, injection timing, and multiple injections at each point of the engine map, allowing for the best compromise between emission reduction and fuel consumption. Besides the injection system, other important measures to improve combustion and achieve low exhaust gas raw emissions in heavy-duty engines include the exhaust gas recirculation (EGR) rate, turbocharging, intercooling, four-valve technology, the shape of the combustion chamber, compression ratio, air motion, and the air-fuel ratio. A significant reduction in soot emission can be achieved with higher boost pressure. Suitable engine modifications include higher maximum combustion peak pressure, increased EGR rate, and higher charge air pressures through variable turbine geometry (VTG) or, even more effectively, with two-stage turbocharger systems. X. Zhou et al. [106] investigated the potential of n-butanol/diesel blends for CI engines under post-injection strategy and varying EGR rates. A turbocharged diesel engine was utilized to examine the effects of n-butanol and EGR rates on engine emissions and combustion performance. The results indicate that post-injection has a minimal impact

on ignition delay time and brake thermal efficiency (BTE), and it reduces the emissions of CO, HC, NO, and soot. Under certain post-injection strategies, the addition of n-butanol (10%, 20%, and 30%) significantly reduced CO and soot emissions. Among these after-treatment systems, a selective catalytic reduction (SCR) system is used to convert NOx emissions to nitrogen (N₂). The NH₃-SCR technology is preferred by manufacturers due to its higher efficiency in NOx conversion, nearly 95% within a temperature range between 200˚C and 350˚C [118] [119] . Rakopoulos et al. conducted a study on exhaust emissions with ethanol or n-butanol diesel fuel blends during transient operation. They identified and discussed the main mechanisms of transient emissions for all exhaust pollutants, noting many of these mechanisms are interrelated with the inherent discrepancies observed during transients, most notably turbocharger lag [13] .

Manufacturers continue to experiment with diesel engines, adding significant extra costs to vehicles and breaking the trend of ever-increasing engine efficiency [116] . The global trend towards stricter vehicle emissions standards requires a deeper understanding of advanced biofuels like butanol. This knowledge is essential for developing strategies to decrease the reliance on non-renewable fossil fuels and minimize the environmental impact of road transport vehicles with diesel engines. Research indicates a promising potential for n-butanol as an advanced biofuel for partially replacing fossil diesel in diesel engines. Further studies have explored the use of n-butanol and conventional diesel fuel blends under the World Harmonized Steady-State Cycle (WHSC) in commercial heavy-duty diesel engines. The combination of modern De-NOx post-treatment devices and the anticipated increase in advanced biofuels within commercial diesel fuels presents a new challenge for engine manufacturers and the biofuel industry. Additionally, there is a significant lack of information on the effects of n-butanol blends with commercial diesel, which will be necessary for future European Union (EU) regulations to ensure the reliability of diesel engines and post-treatment systems under more realistic operating conditions [119] .

The paper reviewed the use of butanol as a biofuel in diesel engines for road freight transportation, tractors, harvesters, and cogeneration. It specifically explored the sustainability of lignocellulosic butanol, the physical and chemical properties of butanol-diesel blends, and their impact on combustion, performance, and emission characteristics in CI engines under various operating conditions. N-butanol, primarily produced through biological processes like acetone butanol ethanol (ABE) fermentation, has significant potential to reduce greenhouse and exhaust gas emissions compared to fossil diesel fuel. Higher alcohols such as butanol have properties closer to diesel fuel than lower alcohols like ethanol. Despite ethanol being the most commonly used alcohol in the transport sector, n-Butanol demonstrates advantages such as higher viscosity, better lubricity, higher heating value, improved blend stability, better cold-flow properties, and higher density. Therefore, N-butanol is a superior renewable component compared to ethanol for diesel blends due to its higher flash point, lower volatility, and less corrosive nature, making butanol blends safer for transportation, fuel handling, and storage. Its properties significantly impact the combustion characteristics of the alcohol blend, affecting temperature, pressure, and apparent heat release rate. Literature shows that 5% and 10% blends increase pressure and apparent heat release rate but may slightly lower temperature, enhancing thermal efficiency, whereas 20% blends reduce pressure, apparent heat release rate, and temperature under all operating conditions.

Studies have shown that adding n-butanol, in blends of 5% to 10%, to conventional fuels can either decrease or increase CO and THC emissions, while sharply reducing particulate matter emissions. However, for NOx emissions, the effect of blending alcohol into diesel fuel is uncertain, with both increases and decreases reported. This variability depends on the specific alcohol percentage, engine calibration, and characteristics of the transient schedule. It has been observed that a 10% butanol blend is suitable for n-butanol-diesel due to its favorable performance and reduction in particulate emissions, without significant changes in NOx gaseous emissions. Furthermore, blending up to 40% butanol with diesel does not require additional engine modifications or ECU recalibrations in diesel engines calibrated for 100% diesel fuel.