Alcohols as Biofuel for a Diesel Engine with Blend Mode—A Review

Abstract

In the era of decarbonization driven by environmental concerns and stimulated by legislative measures such as Fit for 55, the industry and transportation sectors are increasingly replacing petroleum-based fuels with those derived from renewable sources. For many years, the share of these fuels in blends used to power compression ignition engines has been growing. The primary advantage of this fuel technology is the reduction of GHG emissions while maintaining comparable engine performance. However, these fuel blends also have drawbacks, including limited ability to form stable mixtures or the requirement for chemical stabilizers. The stability of these mixtures varies depending on the type of alcohol used, which limits the applicability of such fuels. This study focuses on evaluating the impact of eight types of alcohol fuels, including short-chain (methanol, ethanol, propanol) and long-chain alcohols (butanol, pentanol, hexanol, heptanol, and octanol), on the most critical operational parameters of an industrial engine and exhaust emissions. The engines being compared operated at a constant speed and under a constant load, either maximum or close to maximum. The study also evaluated the effect of alcohol content in the mixture on combustion process parameters such as peak cylinder pressure and heat release, which are the basis for parameterizing the engine’s combustion process. Determining ignition delay and combustion duration is fundamental for optimizing the engine’s thermal cycle. As the research results show, both the type of alcohol and its concentration in the mixture influence these parameters. Another parameter important from a usability perspective is engine stability, which was also considered. Engine performance evaluation also includes assessing emissions, particularly the impact of alcohol content on NO_x and soot emissions. Based on the analysis, it can be concluded that adding alcohol fuel to diesel in a CI engine increases ignition delay (up to 57%), p_max (by approximately 15–20%), HRR_max (by approximately 80%), and PPR_max (by approximately 70%). Most studies indicate a reduction in combustion duration with increasing alcohol content (by up to 50%). For simple alcohols, an increase in thermal efficiency (by approximately 15%) was observed, whereas for complex alcohols, a decrease (by approximately 10%) was noted. The addition of alcohol to diesel slightly worsens the stability of the CI engine. Most studies pointed to the positive impact of adding alcohol fuel to diesel on NOx emissions from the compression ignition engine, with the most significant reductions reaching approximately 50%. Increasing the alcohol fuel content in the diesel blend significantly reduced soot emissions from the CI engine (by up to approximately 90%).

1. Introduction

For several years, the automotive industry has been grappling with the problem of excessive exhaust emissions from engines. Compounds emitted during the combustion of the fuel–air mixture in the cylinder of a piston engine are toxic, negatively affecting the health of living organisms, plant vegetation, and the cleanliness of the air we breathe. These include primarily nitrogen oxides, unburned hydrocarbons, carbon monoxide, and soot [1]. Some of the emitted substances are not toxic but still have a negative impact on our environment. An example is carbon dioxide, known as a greenhouse gas, which contributes to global warming and disrupts the natural environmental balance. The problem of excessive emissions from piston engines also affects the energy sector. In industrialized countries, energy production increasingly relies on distributed energy systems, where internal combustion engines are used to drive generators that often combine the production of electricity, heat, and sometimes cooling. The extensive market for industrial engines (both mobile and stationary), which power machinery and equipment used in various industries as well as in daily life, should also be noted [2]. A significant part of engines used in energy and automotive sectors are compression ignition (CI) engines, which, compared to spark ignition engines, have better performance characteristics, higher efficiency, and increased durability. Unfortunately, these engines are mainly powered by diesel fuel, a fossil fuel derived from crude oil, which generates significant amounts of harmful substances during combustion. The current trend of moving away from fossil fuels is gradually phasing out the widespread use of traditional internal combustion engines powered by petroleum-based fuels. Among the fuels that could replace the currently used energy sources in engines are renewable fuels, both gaseous (such as propane-butane, methane, producer gas, or hydrogen) and liquid (such as alcohols and ammonia). The most promising renewable gaseous fuel is, of course, hydrogen, which has favorable combustion properties and is characterized by zero emissions [3,4]. Due to challenges in obtaining, storing, and transporting hydrogen, especially in its liquid state, fuels containing a significant proportion of hydrogen are increasingly valued. An example of such a fuel is ammonia, which can also be produced from renewable sources [5,6]. Biodiesel is considered a valuable substitute for diesel fuel in compression ignition engines, as it has properties similar to diesel and, due to its oxygen content in the molecular structure, is a more environmentally friendly fuel [7,8]. Biodiesel can be produced through the chemical processing of vegetable oils and animal fats, both edible and non-edible. Among renewable liquid fuels, special attention should be given to alcohols (CnH2n + 1OH), which have energy properties similar to diesel and gasoline and can be used in traditional engines without major issues.

2. Blend and Fumigation as Dual-Fuel Operation in Diesel Engine

Fuels that are difficult or even impossible to ignite spontaneously cannot be used in mass-produced diesel engines. Alcohols are such fuels. Initiating the ignition of these fuels in the cylinder of a CI engine requires the use of a small amount of highly reactive fuel prone to autoignition, such as diesel fuel. There are two commonly used methods for fueling an engine with diesel and alcohol fuel. The first is the technology of premixing both fuels and directly delivering them to the combustion chamber using the original injection system. The second is alcohol fumigation, where diesel is delivered to the cylinder through direct injection, and the alcohol fuel is supplied to the intake manifold, where it forms a homogeneous mixture with the air (Figure 1). The alcohol can be delivered to the manifold through an additional injection system or using a carburetor [9,10]. The premixing fuel technology is simpler and cheaper because it does not require modifications to the engine design [11]. However, it has a significant drawback, which is the phase instability of the resulting mixture, which tends to become cloudy and separate. To avoid this issue, stabilizers in the form of various additives, including emulsifiers, are used. The main disadvantage of the fumigation technology is the necessity of engine modifications and the use of an additional fuel supply system. However, this method does not have the problem of mixture separation, and it also allows for precise dosing of both fuels and changing their proportions during continuous engine operation [12].

This paper presents a literature review of studies on compression ignition (CI) engines powered by diesel and selected alcohols, operating in the blend mode (Figure 1a). The impact of eight types of alcohol fuels, both light, short-chain (methanol, ethanol, propanol), and long-chain (butanol, pentanol, hexanol, heptanol, and octanol), on combustion, performance, stability, and emissions of diesel engines was analyzed. The compared engines had compression ratios ranging from 15.5 to 21.47, operated at a constant rotational speed, and under constant maximum or near-maximum load. However, it is important to mention that the comparative analysis had a limitation due to different types of engines operating under various conditions, with different, though constant, rotational speeds and different, though near-maximum, loads.

The study focused on evaluating the operational parameters of industrial engines, such as:

-

efficiency and specific fuel consumption: these are of primary concern to engine users, as higher efficiency results in lower operating costs;

-

combustion process parameters: this aspect is mainly of interest to researchers working on similar topics, as controlling the combustion process is crucial for achieving high engine efficiency, which leads to lower fuel consumption;

-

engine stability: this is particularly important for engines powering electric generators that are connected to the power grid;

-

exhaust emissions: nowadays, this is arguably the most critical aspect of engine performance evaluation, particularly in terms of assessing GHG emissions.

3. Characteristics of Alcohol Fuels

Alcohol fuels, commonly used in spark ignition (SI) engines, are increasingly being utilized in compression ignition (CI) engines as well. In many countries around the world, especially in South America, alcohol fuels, most commonly ethanol, are the primary fuel for SI vehicle engines. Diesel engines often use alcohols in industrial applications, particularly in stationary engine setups. There are many types of alcohol fuels that can be practically used as energy sources for internal combustion engines. Most alcohols are liquid under normal conditions, making them easy to transport and store, and they can be used in conventional, mass-produced engines without complex processing or engine modifications. According to Zhang et al. [13], alcohols such as methanol and ethanol have similar transportation and storage requirements to diesel fuel and gasoline. Alcohols can be used as fuels for transportation and as raw materials in the chemical industry. They can be effective fuels for both gasoline and diesel engines. The following alcohol fuels have practical applications in internal combustion engines: methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH), butanol (C₄H₉OH), pentanol (C₅H₁₁OH), hexanol (C₆H₁₃OH), heptanol (C₇H₁₅OH), and octanol (C₈H₁₇OH).

Although the technologies for using alcohols in gasoline engines have been known for many years, efforts are still ongoing to achieve efficient and practical applications in diesel engines. Alcohols are attractive alternative fuels to petroleum-based fuels because their production is based on renewable biological resources and they are rich in oxygen. This offers the potential to reduce emissions of CO, particulate matter, and soot from internal combustion engines [14,15]. Alcohols can reduce CO emissions in two ways: by replacing fossil fuels and by recycling CO released during the combustion of biofuel [16]. From an economic point of view, alcohols can help reduce trade deficits and improve energy security. According to Zhang et al. [13], alcohols have significant market potential due to their lower production costs compared to fossil fuels. For instance, the price of ethanol is approximately one-third that of gasoline and diesel fuel [8]. Currently, there is a steadily increasing global production of alcohol fuels, including the simplest and most commonly used, ethanol. The largest producers of bioethanol are the USA, Brazil, and China [17]. The oldest and best-known biotechnological process for ethanol production is alcoholic fermentation. The raw materials for ethanol production include products containing starch or sucrose. The first group includes grains and potato tubers, while the second group includes sugar beets and sugar cane. Ethanol can also be produced from cellulose: wood, straw, grass, or organic waste. The practical challenges of using alcohol fuels in both diesel and gasoline engines include deterioration in the lubrication quality of fuel injection system components, the potential for alcohols to dissolve fuel system seals, and their increased corrosiveness. In case of blending mode, it is also important to consider the difficulties in mixing diesel fuel with alcohols. Alcohols and diesel fuel are not miscible and require cosolvents or emulsifiers to create a homogeneous liquid [18].

Table 1. Literature review of the basic properties of diesel fuel and alcohols.

Fuel	Diesel		Methanol		Ethanol		Propanol
Molecular Formula	CnH1.8n (C8–C20)		CH3OH		C2H5OH		C3H7OH
Molar mass (g/mol)	190–220 142	[23] [24]	32 32.04	[23,25,26] [25,27,28,29,30]	46 46.07	[25,26,31,32,33] [24,29,34]	60.1 60.065	[24,29,35] [36]
Density at 1 bar, 20 °C (kg/m3)	840 830	[23,26,27,30] [25]	790 796	[23,25] [27,30]	780 790	[24] [33]	785 815	[35] [36]
Viscosity at 40 °C (mm/s2)	2.6 2.86	[32] [37]	0.58 0.65	[28,37,38] [30]	1.2 1.1	[32,34] [39]	1.74 2.497	[37,38,40,41,42] [35]
Cetane number	40–55 52	[23] [24,28,40]	2 5	[29,37] [28,38,43]	5–8 11	[25] [29]	<15 12	[24] [29,36,37,38,40,41,42]
Autoignition temperature (°C)	180–240 254–300	[26] [28,40,44]	470 463	[23,25] [27,28]	360 425	[26] [31]	350 399	[29,36,37,38,40] [42]
Stoichiometric A/F ratio	15 14.3	[32] [42]	6.4 6.47	[26] [28]	9 8.97	[32] [45]	10.35 10.4	[36] [42]
Lower heating value (MJ/kg)	44.5 42.49	[24,26] [28,40]	19.5 22.7	[27] [43]	23.8 29.8	[26] [24]	33.6 29.82	[24,37] [40]
Latent heat of evaporation (kJ/kg)	250–290 270–37	[23] [28,40]	1178 1100	[23,24] [26,27]	840 923	[31,32] [24]	790 727.88	[24] [29,37,38,40]
Hydrogen content (wt%)	14 13	[23] [27]	12.6 12.48	[24] [29,44]	13 13.02	[31] [28,29]	13.4 13.31	[24,36] [29,40]
Carbon content (wt%)	86 84–87	[23] [24]	37.5 37.48	[23,27,40] [28,29]	52.2 52.14	[24,31] [28,29]	60 59.96	[24,36] [29]
Oxygen content (wt%)	0	[23,27,40]	50 49.9	[23,27,30] [24]	34.8 34.7	[31,32,33] [24,29]	26.6 26.62	[24,36,42] [29,40,46]

and

Table 2. Literature review of the basic properties of alcohols.

Fuel	Butanol		Pentanol		Hexanol		Heptanol		Octanol
Molecular Formula	C4H9OH		C5H11OH		C6H13OH		C7H15OH		C8H17OH
Molar mass (g/mol)	74 74.123	[32] [34]	88.15	[32,38,40,47,48]	102.18	[28,29,33,38,47]	116.2	[29,44,47,49]	130.23	[28,29,44,47]
Density at 1 bar, 20 °C (kg/m3)	794 810	[34] [37,50,51]	814.8 815	[37,40,48] [41,52]	821.8	[28,29,38,47,53]	818 824	[29,44,47,49] [42]	827 820	[28,29,38] [54]
Viscosity at 40 °C (mm/s2)	2.28 3.64	[44] [34]	2.89	[28,37,40,41] [48]	4.64 3.32	[28] [53]	5.75 6.9	[44,49] [42]	7.59 10.2	[44] [54]
Cetane number	<15 17–25	[28] [51]	18.2–20 20	[38,40] [28]	23 42	[28,29,38,47,53] [33]	39 23	[42,55] [38]	37 39.1	[28,54] [44]
Autoignition temperature (°C)	415 343	[28] [51,56]	300	[29,37,40,44,47]	285	[28,29,38,47,53]	275 270	[29,44,47] [38]	270 253	[28,29,44,47] [54]
Stoichiometric A/F ratio	14.2 11.13	[44] [45]	11.76	[28]	12.15	[28]	14.41 12.47	[44] [49]	14.45 12.7	[47] [54]
Lower heating value (MJ/kg)	33.64 33.08	[28] [45]	32.16 35.06	[29,47] [48]	36.4 39.1	[28] [29,33,38,47,53]	39.92 34.65	[29,44,47] [55]	37.53 52.94	[28] [29,38,47]
Latent heat of evaporation (kJ/kg)	684 581.4	[28] [34]	647.1 305	[28] [52]	603 486	[28,53] [29,33,38,47]	575 408	[49] [42]	315.1 550	[44] [54]
Hydrogen content (wt%)	13.49 13.64	[28,29,47] [44]	13.61	[28,29,40,47]	13.70	[28,29,47]	13.71 13.88	[29,44,47,49] [42]	13.8 13.91	[28] [44]
Carbon content (wt%)	64.82 64.8	[28,29,44,47] [34,51]	68.13	[28,29,47]	70.52 70.53	[28,29] [47]	72.16 73.80	[29,44,47,49] [42]	73.72 73.73	[28,29] [47]
Oxygen content (wt%)	19.54 21.62	[44] [50]	18.15	[28,29,40,47] [52]	15.7	[28,29,33,47]	14.13 12.32	[44] [42]	12.29 12.41	[28,29,47] [44]

provide a literature review of the main properties of eight alcohol fuels: methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, and octanol, as well as conventional diesel fuel. It is evident that these alcohol fuels have slightly lower densities compared to diesel fuel under normal conditions. The density of diesel fuel is approximately 835–840 kg/m³, while the density of alcohols ranges from 790–796 kg/m³ for methanol to 820–827 kg/m³ for octanol. An important characteristic of diesel engine fuel is the cetane number (CN), which reflects the fuel’s tendency to autoignition [19]. For diesel fuel, the CN ranges are from 40 to 55, which are higher compared to alcohols. Among the alcohol fuels, methanol has the lowest CN at 2–5, while octanol has the highest at 37–39.1. Another parameter determining the ignition potential of the fuel mixture in a CI engine cylinder is the autoignition temperature [20]. Diesel fuel has a lower autoignition temperature, ranging from 180 to 254 °C, compared to alcohol fuels. Methanol has the highest autoignition temperature at 385–470 °C, while octanol has the lowest at 253–270 °C. The most crucial property of fuel used for energy production is the lower heating value (LHV) [21]. Diesel fuel is a high-energy fuel with an LHV of 42.49–44.5 MJ/kg. Most alcohol fuels have a lower LHV. Methanol has the lowest energy content per unit mass at 19.58–22.7 MJ/kg, while octanol has the highest at 37.53–52.94 MJ/kg. The latent heat of vaporization of the fuel significantly impacts the combustion process in diesel engines. A high value of this parameter can increase the ignition delay of the mixture, increases pressure and heat release rate, but it also decreases the cylinder temperature, potentially reducing NOx and soot emissions and improving thermal efficiency [22]. The oxidation reactions of the fuel in the engine combustion chamber require an adequate amount of oxygen. Ideal combustion of each fuel demands a different quantity of this oxidizer. The parameter used to define the required theoretical amount of oxygen for the combustion of a specific fuel dose is the stoichiometric air/fuel ratio (A/F ratio). Due to the oxygen content in their molecules, alcohol fuels have lower stoichiometric air requirements compared to diesel fuel. Methanol, composed of 49.9–50% oxygen, has the lowest stoichiometric A/F ratio of 8.97–9.06, while octanol, with 12.29–12.41% oxygen, has an A/F ratio of 12.7–14.45. For diesel fuel, which does not contain oxygen, the A/F ratio is 14.3–15.

Table 3. Parameters of diesel fuel and alcohols specified in European, American, and international fuel quality standards.

Fuel	Diesel	Methanol	Ethanol	Propanol	Butanol	Pentanol	Hexanol	Heptanol	Octanol
Molecular Formula	CnH1.8n (C8–C20)	CH3OH	C2H5OH	C3H7OH	C4H9OH	C5H11OH	C6H13OH	C7H15OH	C8H17OH
Density at 1 bar, 20 °C (kg/m3)	820–845 (EN)	792 (ISO) 791–793 (ASTM)	790–793 (EN) 790 (ASTM)	803 (EN) (ISO) (ASTM)	810 (EN) (ISO) (ASTM)	810 (EN) (ISO) (ASTM)	825 (EN) (ISO) (ASTM)	827 (EN) (ISO) (ASTM)	826 (EN) (ISO) (ASTM))
Viscosity at 40 °C (mm/s2)	2.0–4.5 (EN) 1.9–4.1 (ASTM)	-	-	-	-	-	-	-	-
Cetane number	51 (EN) 40 (ASTM)	-	-	-	-	-	-	-	-
Autoignition temperature (°C)	220–290 (EN) 210–290 (ASTM)	-	-	-	-	-	-	-	-
Lower heating value (MJ/kg)	42.8 (EN) 42.8–43 (ASTM)	19.7 (EN) (ASTM)	23.5 (EN) (ASTM)	27.6 (EN) (ASTM)	28.9 (EN) (ASTM)	29.7 (EN) (ASTM)	29.8 (EN) (ASTM)	30.5 (EN) (ASTM)	31.1 (EN) (ASTM)

presents the parameters of diesel fuel and alcohols specified in European, American, and international fuel quality standards. European (EN), American (ASTM), and international (ISO) standards for alcohol fuels, particularly in the context of their use as biofuels, do not precisely define the technical parameters of these fuels. They only specify requirements regarding the composition and purity of the fuels, including the content of alcohol, water, impurities (such as metals and aldehydes), and sulfur. Meeting these requirements is crucial for ensuring fuel efficiency and minimizing negative impacts on engines and the environment.

4. Methodology

The literature review conducted in the study on the impact of alcohol fuels on the combustion, performance, and emissions of compression-ignition engines focused on one of the well-known technologies for co-combustion of alternative fuels with diesel fuel in such engines. The emphasis was on the blend mode technology, which, along with dual-fuel technology, is the most common method for co-combustion of multiple fuels in piston engines (Figure 2). The literature review was limited to studies on the effects of alcohols on diesel engine performance, although there are many types of renewable fuels that can be combusted with diesel fuel in CI engines (e.g., ammonia, DME, hydrogen, natural gas, or propane-butane). The review covered research on industrial engines operating at a constant rotational speed and under constant maximum or near-maximum load conditions. Despite including both single-cylinder and multi-cylinder engines, as well as air-cooled and liquid-cooled engines, and both naturally aspirated and turbocharged engines, it was considered that comparing these engines would contribute to a deeper understanding of the use of biofuels in internal combustion engines.

The study focuses on evaluating the operational parameters of industrial engines, such as:

• efficiency and specific fuel consumption, which are of primary interest to engine users, as higher efficiency translates to lower operating costs;
• combustion process parameters; this issue is mainly relevant to researchers dealing with similar topics, as controlling the combustion process determines high engine efficiency, i.e., low fuel consumption;
• engine stability; this is a crucial issue, especially for engines powering electricity generators that operate in conjunction with the power grid;
• emissions; nowadays, this is perhaps the most important aspect of evaluating not only piston engine performance but also the assessment of GHG emissions.

5. Effect of Blended Diesel/Alcohol on Combustion, Performance, Stability, and Emissions of a CI Engine

In the available literature, numerous studies focus on the combustion of alcohol fuels in compression ignition engines. Most commonly, studies analyze the combustion process, engine performance, and exhaust emissions. Less frequently, investigations include engine stability analysis [42,57]. This paper compiles studies of various CI engines with compression ratios ranging from 15.5 to 21.47, operating at constant rotational speed and under constant maximum or high load conditions. It compares the impact of eight alcohol fuels (methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, and octanol) blended in varying proportions with diesel fuel on combustion, performance, stability, and emissions of diesel engines. The volumetric fraction of alcohols varied from 0 to 80% depending on the study specifics, with diesel fuel commonly used as the reference fuel. Parameters characterizing the combustion process included ignition delay (ID), combustion duration (CD), peak cylinder pressure (p_max), maximum heat release rate (HRR_max), and maximum rate of pressure rise (PPR_max) [58]. Performance was evaluated based on thermal efficiency (TE) and specific fuel consumption (SFC) values [59]. Stability analysis of engines operating in blend mode utilized the coefficient of variation of indicated mean effective pressure (COV_imep), a commonly used parameter [60,61]. Emission cleanliness assessment considered nitrogen oxides (NO_x), hydrocarbons (HCs), carbon monoxide (CO), carbon dioxide (CO₂), and soot. All analyzed emissions for engines powered by diesel and selected alcohols, operating in blend mode, pertain to raw exhaust gases at the engine outlet. In the measurement of NO_x concentration in the exhaust gases of engines fueled by diesel/alcohol blends, a chemiluminescent detector (CLD) [32,33,56,62,63], or an electrochemical cell [31,44,54] was most commonly used. In rare cases, a zirconia ceramic sensor [34] was employed for NO_x detection. HC measurements were conducted using a flame ionization detector (FID) [32,33,56,62,63] or a non-dispersive infrared (NDIR) detector [31,44,54]. The NDIR technology was also used for measuring CO and CO₂ concentrations [31,32,33,44,54,56,62,63]. The most frequently used method for measuring smoke and soot content in exhaust gases was the filtration method [23,33,40,45,56,63] while the absorption method was used less often [54].

Table 4. Literature review of CI engines with diesel/alcohol blend mode.

Ref.	Engine Type	Fuels	Combustion	Performance	Stability	Emissions
[27] Jamrozik et al.	One-cylinder, naturally aspirated, air-cooled, CR–17, speed of 1500 rpm, full load	diesel/methanol blends (0–40% vol. alcohol)	increased: ID, pmax, HRRmax, and PPRmax; decreased: CD	increased: TE; decreased: SFC	increased: COVimep	increased: NOx; reduced: HC, CO, CO2
[28] Kumar et al.	One -cylinder, naturally aspirated, air-cooled, speed of 1500 rpm, CR–17.5, high load (5.3 bar bmep)	diesel/iso-butanol (DB) blends, diesel/pentanol (DP) blends, diesel/n-octanol (DO) blends (30% vol. alcohol)	increased: ID, pmax, HRRmax, and PPRmax; decreased: CD	-	-	for DB and DP, increased: HC; reduced: NOx, CO, and soot; for DO, reduced: NOx, HC, CO, and soot
[44] Nour et al.	One -cylinder, naturally aspirated, air-cooled, CR –17, speed of 1500 rpm, high load (18 bar IMEP)	diesel/butanol blends, diesel/heptanol blends, diesel/octanol blends, (10, 20% vol. alcohol)	increased: ID; degreased: CD	decreased: SFC; increased: TE	-	reduced: NOx, soot, and CO; increased: HC
[31] Jamrozik	One -cylinder, naturally aspirated, air-cooled, CR–17, speed of 1500 rpm, full load	diesel/ethanol blends (10, 20, 30, 40%, vol. alcohol)	increased: ID, pmax, HRRmax, and PPRmax; decreased: CD	increased: TE	increased: COVimep	increased: HC, NOx; reduced: CO, CO2
[40] Zhao et al.	One -cylinder, naturally aspirated, liquid-cooled, CR–17.1, speed of 4500 rpm, high load (55 MPa IMEP)	diesel/propanol blends, diesel/pentanol blends (20, 40% vol alcohol)	increased: ID; degreased: CD	increased: SFC; degreased: TE	-	increased: NOx; reduced: soot, CO, and HC
[49] Nour et al.	One -cylinder, naturally aspirated, air-cooled, CR–17, speed of 1500 rpm, 100% load	diesel/1-heptanol blends (10, 20, 30, 40, and 50% vol. alcohol)	increased: ID, CD, HRRmax, PPRmax	decreased: TE; increased: SFC	increased: COVimep	reduced: soot and NOx
[54] Ahn et al.	One -cylinder, naturally aspirated, air-cooled, CR–21, speed of 1700 rpm, partial load (0.49 MPa bmep)	diesel/n-octanol blends (10, 30, and 50% vol. alcohol)	-	increased: TE, SFC	-	reduced: CO, soot, HC, and NOx
[32] Rakopoulos et al.	One -cylinder, naturally aspirated, liquid-cooled, CR–19.8, speed of 2000 rpm, high load (5.37 bar bmep)	diesel/n-butanol blends (8, 16, and 24% vol. alcohol)	-	constant: TE	-	reduced: soot, NOx, CO; increased: HC
[29] Dogan et al.	One -cylinder, naturally aspirated, liquid-cooled, CR–17.5, speed of 1500 rpm, 100% load	diesel/1-hexanol blends (5, 10, 20% vol. alcohol)	-	decreased: TE; increased: SFC	-	reduced: CO2 and NOx; increased: HC
[64] Bhumula and Kumar	One -cylinder, naturally aspirated, liquid-cooled, CR–18, speed of 1500 rpm, 100% load	diesel/1-heptanol blends (30% vol. alcohol)	increased: ID, CD, pmax, HRRmax	decreased: TE; increased: SFC	-	reduced: CO, soot, HC, and NOx; increased: CO2
[47] Bhumula end Kumar	One cylinder, naturally aspirated, liquid-cooled, CR–18, speed od 1500 rpm, high load (4.44 kW)	diesel/1-heptanol blends (10, 20, and 30% vol. alcohol)	-	decreased: TE; increased: SFC	-	reduced: soot and NOx; increased: HC, CO, and CO2
[65] Duraisamy et al.	One -cylinder, liquid-cooled, CR–17.5, speed of 1500 rpm, high load (4.44 kW)	diesel/n-hexanol blends (10 and 20% vol. alcohol)	-	increased: TE;		reduced: CO, CO2, NOx; increased: HC
[23] Soni and Gupta	One -cylinder, liquid cooled, CR–17.5, speed of 1500 rpm, 100% load	diesel/methanol blends (10, 20, 30%, vol. alcohol)	increased: pmax, HRRmax	increased: SFC; decreased: TE	-	reduced: soot, HC, and CO; increased: NOx
[66] Guo et al.	One -cylinder, liquid-cooled, speed of 2000 rpm, full load	diesel/methanol blends (10, 20, 30%, vol. alcohol)	-	increased: SFC	-	increased: NOx and HC; reduced: CO
[24] Muthaiyan and Gomathinayagam	One -cylinder, liquid-cooled, CR—16.5, speed of 1500 rpm; full load	diesel/1-propanol blends (10, 15, 20%, 25% vol. alcohol)	increased: HRRmax, pmax, and ID	degreased: TE		reduced: NOx, CO, and soot
[33] Sundar and Saravanan	One -cylinder, liquid-cooled, CR—17.5, speed of 1500 rpm, peak load (5.2 kW)	diesel/hexanol blends (10, 20, 30, 40, and 50% vol. alcohol)	increased: pmax, HRRmax	increased: SFC, TE	-	reduced: NOx, soot
[67] Atmanli and Yilmaz	Four-cylinder, naturally aspirated, air-cooled, CR—19, speed of 1800 rpm, high load (9 kW)	diesel/1-pentanol blends, (5, 10, 20, 30, 35% vol. alcohol)	-	decreased: TE; increased: SFC	-	reduced: NOx; increased: HC and CO
[25] Turkcan and Çanakçı	Four-cylinder, naturally aspirated, liquid cooled, CR—21.47, speed of 1400 rpm, partial load (40 Nm)	diesel/methanol blends, diesel/ethanol blends (5, 10% vol. alcohol)	increased: ID, pmax, HRRmax, and PPRmax; decreased: CD	-	-	-
[68] Yilmaz et al.	Four-cylinder, naturally aspirated, liquid-cooled, CR—19, speed of 1800 rpm 75% max load (9 kW),	diesel/ n-propanol blends, diesel/n-propanol (5, 20, 35% vol. alcohol)	-	-	-	reduced: NOx, CO; increased: HC
[62] Pinzi et al.	Four-cylinder, turbocharged, liquid-cooled, CR—18, speed of 2400 rpm, high load (110 Nm)	diesel/ethanol blends, diesel/1-propanol blends (10, 20, 30% vol. alcohol)	-	-	-	reduced: NOx and soot; increased: CO and THC
[69] Chen et al.	Four-cylinder, turbocharged, liquid-cooled, CR—18, speed of 2200 rpm, high load (350 Nm)	diesel/iso-propanol and diesel/n-pentanol blends (20% vol. alcohol)	increased: ID; degreased: CD	-	-	increased: NOx
[63] Cheng et al.	Four-cylinder, turbocharged, liquid-cooled, CR—17.5, speed of 1500 rpm, medium load (0.86 MPa bmep)	diesel/n-butanol blends (10 and 30% vol. alcohol)	increased: ID	increased: TE, SFC	-	reduced: soot; increased: NOx
[45] Han et al.	Six-cylinder, turbocharged, liquid-cooled, CR—15.85, high load (18 bar IMEP)	diesel/n-butanol (DB) blends, diesel/ethanol (DE) blends (80% vol. alcohol)	increased: pmax, HRRmax, PPRmax	decreased: TE	increased: COVimep	for DB, reduced: CO and soot; increased: HC and NOx; for DE, reduced: NOx and soot; increased: HC and CO
[34] Sahin et al.	Four-cylinder, turbocharged, liquid-cooled, CR—18.25, speed of 2000 rpm, high load (145 Nm)	diesel/n-butanol blends (2, 4, 6% vol. alcohol)	decreased: HRRmax	decreased: TE; increased: IMEP, SFC	-	reduced: NOx and soot
[50] Zhou et al.	Four-cylinder, turbocharged, liquid-cooled, CR—16.5, speed of 1400 rpm, load (0.8 MPa)	diesel/n-butanol blends (10, 20, 30% vol. alcohol)	increased: ID, pmax, HRRmax	decreased: TE; increased: IMEP, SFC	-	reduced: CO and soot; increased: HC and NOx
[56] Pan et al.	Four-cylinder, turbocharged, liquid-cooled, CR—16.5, speed of 1400 rpm, high load (1.2 MPa bmep)	diesel/n-butanol blends (50% vol. alcohol)	increased: ID, HRRmax	increased: TE, SFC	-	reduced: soot; increased: HC and NOx
[26] Yusaf et al.	Four-cylinder, air-cooled, CR—15.5, speed of 2000 rpm, full load	diesel/methanol blends, diesel/ethanol blends (10, 20, 30% vol. alcohol)	-	increased: TE and SFC	-	-
[70] Labeckas et al.	Four-cylinder, liquid-cooled, CR—16, speed of 2200 rpm speed, full load	diesel/ethanol blends (5, 10, 15% vol. alcohol)	increased: ID, HRRmax	increased: SEC; decreased: TE	-	increased: CO and soot; reduced: NOx

presents a synthetic overview of selected and available literature studies on compression ignition engines fueled by blends of diesel and various alcohols, mixed in volumetric proportions.

Based on the studies available in the literature, Table 3, Table 4,

Table 5. The relative percentage changes of selected parameters of CI engines fueled with diesel/methanol blends compared to the reference case, determined based on data from the literature.

% vol.	5	10	10	10	10	10	15	20	20	20	20	25	30	30	30	30	35	40
ID	8		17		14		21			36		43		57			79	79
pmax	1		1		4	6	5	12		7		9	17	9			−20	−38
HRRmax	18		64		21	10	40	17		48		40	28	10			−43	−75
PPRmax	8		42		18		29			30		27		1			−74	−87
CD	−3		−7		−6		−17			−22		−25		−34			−44	−50
TE		5			6	−3	12	−4	16	15		15	−6	12	12		−6
SFC		67		3	−20	8	−23	15	56	−27	3	−20	23	−13	39	10	0	60
COVimep					0		0			20		60					900	990
NOx					88	5	95	10		105			16			2	105	88
HC					−25	−10	−38	−20		−25		−13	−27	25		10
CO					−18	−15	−36	−31		−41		−50	−38	−59		−10	−64	−59
CO2					−8		−8			−17		−8		0			50
Soot						−5		−10					−14
	[23]	Soni and Gupta
	[25]	Turkcan and Çanakçı
	[26]	Yusaf et al.
	[27]	Jamrozik et al.
	[66]	Guo et al.

,

Table 6. The relative percentage changes of selected parameters of CI engines fueled with diesel/ethanol blends compared to the reference case, determined based on data from the literature.

% vol.	5	10	10	10	20	20	30	30	40	80
ID	7	5	8	8	15	5	38	15	69
pmax	0		4	1	5		11			6
HRRmax	18	13	40	45	87	10	67	13	7
PPRmax	8		12	33	55		32		−49
CD	−2		−30	−4	−33		−35		−49
TE		−1	3		6	−1	9	−3		−4
SFC		2				44		4
COVimep			50						900	22
NOx		−3	100	−4		−3		−13		−50
HC			7	1.5	15		19		31
CO		21	−11	12	−22	21	−33	21	−39
CO2			−17	3	−24		−9		−2
Soot		12		−2		12		15		−69
	[25]	Turkcan and Çanakçı
	[31]	Jamrozik
	[45]	Han et al.
	[62]	Pinzi et al.
	[70]	Labeckas et al.

,

Table 7. The relative percentage changes of selected parameters of CI engines fueled with diesel/propanol blends compared to the reference case, determined based on data from the literature.

% vol.	5	10	10	15	16	20	20	20	20	25	30	35	40
ID		10		12		17		12	14	22			43
pmax		6		10				9		1
HRRmax		28		42				38		28
PPRmax		63		69				54		13
CD						−5			−11				−50
TE		−4		15				−7	−7	−4			−4
SFC									15				18
NOx	−42	−6	7.5	−6	−35	11	−14	−8	23	−9	−22	−32	49
HC	63		−1		108		6		−29		15		−90
CO	−34	−53	−2	−62	12		11	−72	−20	−44	27	0	−20
CO2			−2				−3				6
Soot		−21	−1	−51			−1	−59	−25	−52	−8		−94
	[24]	Muthaiyan and Gomathinayagam
	[40]	Zhao et al.
	[62]	Pinzi et al.
	[68]	Yilmaz et al.
	[69]	Chen et al.

,

Table 8. The relative percentage changes of selected parameters of CI engines fueled with diesel/butanol blends compared to the reference case, determined based on data from the literature.

% vol.	2	4	5	6	8	10	10	16	20	20	20	24	30	30	30	35	50	80
ID						6	3		18	13			13	25	10		40
pmax	1	1		−1			3			6				8	16			13
HRRmax	−2	−3		−6			5			24				43	88		20
CD						−12			−8						−9
TE	0	−1		−3	0	4	−1	4	0	−2		6	5	−2			4	−5
SFC	0	2		4	2	−1	4	0	−1	6		0	5	10			9
COVimep																		11
NOx	−6	−8	−41	−7	−8	−2	0	−12	−8	3	−36	−16	2	8	−8	−36	9	67
HC			75		13	−5	24	20	5	69	104	27		114	67		11
CO			−40		−3	−3	2	−6	18	−2	−5	−8	0	−10	−83	9		−50
Soot	−7	−8		−2	−21	−30	−18	−43	−27	−29		−50	−44	−54	−54		−81	−92
	[28]	Kumar et al.
	[32]	Rakopoulos et al.
	[34]	Sahin et al.
	[44]	Nour et al.
	[45]	Han et al.
	[50]	Zhou et al.
	[56]	Pan et al.
	[63]	Cheng et al.
	[68]	Yilmaz et al.

,

Table 9. The relative percentage changes of selected parameters of CI engines fueled with diesel/pentanol blends compared to the reference case, determined based on data from the literature.

% vol.	5	5	10	20	20	20	20	25	30	35	35	40
ID						9	8		−5			17
pmax									15
HRRmax									35
CD						−14	−2		−5			−17
TE		−6	−17	−1		−1		−4		−2		−2
SFC		8	24	5		5		11		11		21
NOx	−44	−45	−38	−39	−35	−31	13	−35	4	−28	−28	−52
HC	38	41				−17			33			−90
CO	−31	−33	0	33	38	−20		17	−67	−17	−14	−68
Soot						−31			−43			−52
	[28]	Kumar et al.
	[40]	Zhao et al.
	[67]	Atmanli and Yilmaz
	[68]	Yilmaz et al.
	[69]	Chen et al.

and

Table 10. The relative percentage changes of selected parameters of CI engines fueled with diesel/hexanol blends compared to the reference case, determined based on data from the literature.

% vol.	5	10	10	10	20	20	20	30	40	50
pmax		2			3			5	6	8
HRRmax		4			8			33	46	58
TE	−4	8	−5	4	7	5	−7	5	5	2
SFC	2	0	5		4		7	4	6	24
NOx	−3	−9	5	2	−9	−10	−38	−9	−11	−14
HC	80		103	13		20	25
CO				−50		−50
CO2	7		8	−15		−15	−17
Soot		−6			−6			−12	−18	−29
	[29]	Dogan et al.
	[33]	Sundar and Saravanan
	[65]	Duraisamy et al.

were prepared, which present the percentage changes of selected parameters of CI engines fueled with diesel/alcohol blends. For the sample studies, percentage changes were determined for indicators characterizing the combustion process (ID, CD, p_max, HRR_max, PPR_max), performance (TE, SFC), operational stability (COV_imep), and exhaust emissions (NO_x, HC, CO, CO₂, soot) of engines operating in diesel/alcohol blend mode.

• Diesel/methanol blends

The simplest alcohol fuel analyzed for blending with diesel fuel was methanol (CH₃OH), which is a short-chain aliphatic alcohol containing one carbon atom. It is more oxidized compared to other alcohol fuels due to its molecular structure and higher oxygen-to-carbon ratio. In Table 5, percentage changes of selected parameters of CI engines fueled with diesel/methanol blends are presented. Based on five literature sources, it is evident that the maximum volumetric methanol content in the blend with diesel fuel was 40%. Jamrozik et al. [27] studied the combustion, performance, stability, and emissions of CI engines for methanol content ranging from 10 to 40%. However, blends with 35% and 40% methanol did not allow acceptable engine operation and thus cannot be comparable to other studies. Lower methanol contents were investigated by Ali et al. [25], focusing solely on the combustion process analysis.

• Diesel/ethanol blends

Ethanol (C₂H₅OH) is a simple alcohol with two carbon atoms in its molecule. Similar to methanol, it belongs to the group of highly oxidized alcohols [71]. Ethanol has been extensively researched and debated as a motor fuel in the transportation sector, primarily for road transport. Researchers have explored not only the technical aspects of its application but also the understanding of its entire fuel life cycle and associated economic, environmental, and social issues. There are two main objectives for implementing ethanol: the potential to replace part of fossil fuels with renewable fuel and the reduction of greenhouse gases. From selected sources in the literature, Table 6 presents the percentage changes of selected parameters of CI engines fueled with diesel/ethanol blends. It is evident that the maximum ethanol content co-fired with diesel fuel reached up to 80%. Jamrozik [31] found that a 40% methanol content inhibited acceptable engine operation and cannot be compared with other studies. Han et al. [45] investigated combustion process, performance, stability, and emissions of an engine burning a blend with 80% ethanol content. Turkcan and Çanakçı [25] studied the lowest ethanol contents in the blend with diesel fuel. They focused on the analysis of in-cylinder combustion process for 5% and 10% ethanol content in CI engines.

• Diesel/propanol blends

Propyl alcohol (propanol) (C₃H₇OH) is an organic chemical compound from the alcohol group with three carbon atoms in its molecule. Propanol is a three-carbon alcohol with a simple chain structure, existing in two isomeric forms: 1-propanol (also known as n-propanol) and 2-propanol (also known as isopropanol or isopropyl alcohol) [18]. Table 5 presents the percentage changes of selected parameters of CI engines fueled with a diesel/propanol blend. Zhao et al. [40] investigated the combustion process, performance, and emissions of a CI engine fueled with a diesel/propanol mixture with a 20% and the highest 40% alcohol content. Lower shares of C₃H₇OH were studied by Yilmaz et al. [68], who analyzed engine emissions starting from a 5% alcohol content. Based on the data contained in Table 7, it is evident that none of the authors analyzed the stability of the CI engine operation when burning a diesel/propanol mixture.

• Diesel/butanol blends

Butyl alcohol (C₄H₉OH), also known as butanol, is one of the alcohols containing four carbon atoms in its molecule. Butanol is an example of an alcohol with a longer carbon chain and is a more complex alcohol than ethanol. 1-butanol, also known as n-butanol (normal butanol), has a straight-chain structure with a hydroxyl group (–OH). There are four types of butanol isomers: normal butanol (n-butanol), secondary butanol (2-butanol), isobutanol (i-butanol), and tert-butanol (t-butanol) [18]. Table 8 presents the percentage changes of selected parameters of CI engines fueled with a diesel/butanol blend. It is evident that butanol garnered significant interest among researchers seeking alternative fuels for diesel engines. The studies encompass a wide range of butanol proportions in blends with diesel fuel. Han et al. [45] conducted co-combustion studies of C₄H₉OH with diesel fuel even at an 80% alcohol content, analyzing the combustion process, performance, stability, and emissions of the CI engine. Sahin et al. [34] studied the combustion process, performance, and emissions of the engine with only a 2% butanol content co-combusted with diesel fuel.

• Diesel/pentanol blends

Pentanol, also known as amyl alcohol (C₅H₁₁OH), belongs to the group of organic chemical compounds known as alkanols containing five carbon atoms. Pentanol has eight positional isomers. The most common is 1-pentanol, also known as n-pentanol or n-amyl alcohol. Table 9 presents the percentage changes of selected parameters of CI engines fueled with a diesel/pentanol blend. The highest volumetric proportion of pentanol co-combusted with diesel fuel was 40%. The studies conducted by Zhao et al. [40] included an analysis of combustion, performance, stability, and emissions of the CI engine fueled with pentanol and diesel fuel with 20% and 40% alcohol content. The widest range of pentanol proportions, from 5% to 35%, was investigated by Yilmaz et al. [68] and Atmanli and Yilmaz [67]. Their research focused on the performance, stability, and emissions of the engine fueled with a diesel and C₅H₁₁OH blend.

• Diesel/hexanol blends

Hexanol, also known as hexyl alcohol (C₆H₁₃OH), is an organic chemical compound belonging to the alcohol group, containing six carbon atoms in its molecule. There are several isomers of hexanol, with the most important being n-hexanol (1-hexanol) and 2-hexanol. Table 10 presents the percentage changes in selected parameters of CI engines fueled with a diesel/hexanol blend. Extensive research on compression ignition engines powered by a diesel and hexanol mixture was conducted by Sundar and Saravanan [33]. They analyzed the combustion process, performance, and exhaust emissions. The research covered a wide range of alcohol content from 10% to 50%. Lower percentages of C₆H₁₃OH were the subject of studies by Dogan et al. [29] and Duraisamy et al. [65]. In this case, engine performance and emissions were examined with alcohol content ranging from 5% to 20%.

• Diesel/heptanol blends

Heptyl alcohol (C₇H₁₅OH), also known as n-heptanol or 1-heptanol, is an organic chemical compound from the alcohol group. It belongs to the family of long-chain alcohols, containing seven carbon atoms in its molecular structure.

Table 11. The relative percentage changes of selected parameters of CI engines fueled with diesel/heptanol blends compared to the reference case, determined based on data from the literature.

% vol.	10	10	10	20	20	20	30	30	30	40	50
ID	3		5	12	10		15	19		20	20
CD	−8		15	−4	26		24	14		22	17
pmax								10
HRRmax	−3		−1	−3	3		8	4		8	16
PPRmax			1		3		3			4	4
TE		−8	0		2	−3	−7	−3	−11	−7	−9
SFC		14	4		0	26	14	5	37	14	18
COVimep			33		27		13			13	13
NOx	−9	−20	−6	−9	−6	−14	−15	−5	−7	−15	−26
HC	27	13		27		16		−24	24
CO	−25	93		−38		40		−38	7
CO2		6				9		17	14
Soot	−27	−13	−29	−35	−25	−21	−25	−23	−24	−43	−50
	[44]	Nour et al.
	[47]	Bhumula and Kumar
	[49]	Nour et al.
	[64]	Bhumula and Kumar

presents the percentage changes in selected parameters of CI engines fueled with a diesel/heptanol blend. Bhumula and Kumar [47,64] conducted research on the combustion, performance, and emissions of a CI engine powered by a diesel mixture with 10%, 20%, and 30% C₇H₁₅OH content. A broader range of studies, for heptanol content from 10% to 50%, was carried out by Nour et al. [44,49]. In addition to combustion, performance, and emissions, they also focused on engine stability.

• Diesel/octanol blends

Octanol (C₈H₁₇OH), also known as octyl alcohol, is an organic chemical compound from the alcohol group, containing eight carbon atoms. The most popular isomers of octanol include n-octanol (1-octanol) [72]. The properties of 1-octanol are similar to those of conventional diesel fuel and it represents a variety of medium-chain alcohols [73].

Table 12. The relative percentage changes of selected parameters of CI engines fueled with diesel/octanol blends compared to the reference case, determined based on data from the literature.

% vol.	10	10	20	30	50	50
ID	9		15		3
pmax					0
HRRmax	−3		−3		6
CD	−8		0		−2
TE		0		1		3
SFC		1		4		4
NOx	−11	−12	−7	−19	−2	−23
HC	27	−38	27	−50	−50	−60
CO	−13	−48	−25	−55	−56	−70
Soot	−36	−50	−32	−90	−32	−98
	[28]	Kumar et al.
	[44]	Nour et al.
	[54]	Ahn et al.

presents the percentage changes of selected parameters of CI engines fueled with a diesel/octanol blend. Similar to hexanol, studies on the use of octanol as a fuel in compression ignition engines are relatively rare. Co-combustion studies of diesel fuel with a 50% octanol blend were conducted by Kumar et al. [28] and Ahn et al. [54]. The study [54] examined 10%, 30%, and 50% alcohol proportions in the diesel blend, focusing on engine performance and emissions. The study [28] focused on the analysis of the combustion process and exhaust emissions. A similar range of studies was carried out by Nour et al. [44], who conducted experiments with 10% and 20% C₈H₁₇OH.

5.1. Combustion

5.1.1. Maximum In-Cylinder Pressure (p_max)

A fundamental measurement during the studies of combustion process in piston engines is determining the pressure characteristics in the cylinder as a function of the crankshaft angle. Based on the in-cylinder pressure and changes in cylinder volume, several significant combustion parameters are determined, including the heat release rate during combustion. Figure 3 shows the percentage changes in maximum pressure (p_max) during the combustion of diesel/alcohol blends relative to the combustion of pure diesel fuel for eight analyzed alcohol fuels. From Figure 3, it is evident that the combustion of a mixture containing alcohol fuel in the vast majority of studies caused an increase in p_max, up to approximately 16–17%, compared to the combustion of pure diesel fuel [23,28]. Higher p_max values were obtained with increasing volumetric proportions of alcohol. The highest increases were obtained with short- and medium-chain alcohols such as methanol [23,25,27], propanol [24], butanol [28,34], and pentanol [28]. For more complex long-chain alcohols such as hexanol [33] and octanol [28], the maximum combustion pressure was lower. According to the literature, a high alcohol content in the diesel blend increases the heat of vaporization of the mixture, resulting in a longer ignition delay. A long ignition delay period increases the peak of the pressure in the cylinder. Simultaneously, a long ignition delay period shifts the peak pressure in the cylinder closer to the top dead center (TDC). Therefore, a long ignition delay period influences the increase in maximum cylinder pressure and the heat release rate.

5.1.2. Maximum Heat Release Rate (HRR_max)

The curves of heat release rate (HRR) indicate the character of the combustion process inside a piston engine cylinder. Determining the characteristics of heat release during combustion requires precise data acquisition of the cylinder pressure changes, instantaneous cylinder volumes, information of the quantity, composition, and properties of the working fluid, as well as a reliable model of heat exchange between the working fluid and the combustion chamber walls. In Figure 4, percentage changes in the maximum heat release rate (HRR_max) during combustion of diesel/alcohol blends are presented relative to combustion of pure diesel fuel, for eight analyzed alcohol fuels. According to the literature, combustion of diesel/alcohol blends in CI engines generally results in faster heat release compared to combustion of pure diesel fuel in most cases. Increasing the proportion of alcohol leads to an increase in HRR_max. Combustion of diesel fuel with the addition of methanol [25,27], ethanol [31], and butanol [28] up to 30% showed the highest values of HRR_max. Complex alcohols such as hexanol [33], heptanol [49,64], and octanol [28,44] at these proportions resulted in a slight increase in HRR_max. Above 30% alcohol content, the highest HRR was achieved for diesel/hexanol blends [33]. According to analyzed studies, the increase in HRR is attributed to the presence of oxygen provided by alcohol, which limits the formation of fuel-rich zones in the engine cylinder, thereby reducing incomplete combustion losses. Additionally, the longer ignition delay time of alcohol-containing blends increases the time for the formation of a premixed charge, which burns rapidly and causes an increase in HRR. Moreover, the additional oxygen content accelerates the combustion of partially burned gases such as HCs [50].

5.1.3. Maximum Peak Pressure Rise (PPR_max)

The peak pressure rise rate (PPR) is a critical parameter for internal combustion engines, especially for diesel engines. High PPR can lead to harmful consequences such as deteriorated engine performance and increased engine noise. A very high rate of heat release leads to excessively rapid pressure rise, which limits the engine’s operating range, especially under high loads. Combustion of fuel/alcohol blends can result in an increase in PPR_max, primarily due to premixed combustion [45]. According to the literature, the safe allowable limit for PPR_max is 1 MPa/deg [74]. Figure 5 shows the percentage changes in maximum pressure rise rate (PPR_max) during combustion of diesel/alcohol blends relative to combustion of pure diesel fuel, for eight analyzed alcohol fuels. From the literature data, it is evident that increasing the volumetric proportion of alcohol fuel co-fired with diesel up to 30% results in an increase in PPR_max. Above 30%, the PPR_max either remains unchanged or decreases. Combustion of diesel fuel with propane exhibits the highest values of pressure rise rate [24]. The lowest PPR_max was achieved during combustion of diesel/heptanol blends [49]. According to Muthaiyan and Gomathinayagam [24], due to longer ignition delay in the combustion chamber, a greater amount of fuel accumulates, increasing the amount of fuel burned during the premixed combustion phase, resulting in higher cylinder pressures.

5.1.4. Ignition Delay (ID)

One of the fundamental stages of combustion in a piston engine is ignition delay. Various methods for determining ignition delay in compression ignition engines are described in the literature. The most common methods uses heat release characteristics or optical recording techniques [75]. According to Dhole et al. [76], ignition delay (ID) in a diesel engine can be defined as the time elapsed between the injection of the first droplet into the combustion chamber and the appearance of the first signs of combustion. Other sources indicate that the delay can also be defined as the angle between the injection time and the time at which 1% of the fuel mass is burned. Hong et al. [77] define ignition delay as the time from the fuel injection to the release of 10% of the heat. Figure 6 shows the percentage changes in ignition delay during the combustion of diesel/alcohol blends relative to the combustion of pure diesel fuel for eight analyzed alcohol fuels. Based on the studies presented in the cited literature, it can be conclusively stated that the addition of alcohol fuel to diesel fuel in a CI engine increases the ignition delay, with the increase being greater with a higher volumetric proportion of alcohol. The longest ignition delay was obtained during the combustion of a diesel/methanol mixture [25,27]. Shorter IDs were typically achieved by co-combusting diesel with longer-chain alcohols, such as butanol [28,44,63], pentanol [28,40,69], heptanol [49,64], and octanol [28,44]. The increase in ignition delay for diesel/alcohol blends compared to pure diesel fuel results from the higher heat of vaporization of alcohols. A high value of the heat of vaporization adversely affects ignition initiation because it lowers the initial temperature in the engine cylinder [36].

5.1.5. Combustion Duration (CD)

Combustion duration (CD) is defined as the time for fuel burning from 10% to 90% of its total combustion. In other words, it marks the beginning and end of the combustion process inside the cylinder. To enhance engine performance, combustion duration needs to be short so that the entire charge undergoes complete combustion [78,79]. Figure 7 presents percentage changes in combustion duration during combustion of diesel/alcohol blends relative to combustion of pure diesel fuel, for eight analyzed alcohol fuels. It is evident that the influence of alcohol fuel on combustion duration in a compression ignition engine is not straightforward. Most studies indicate a reduction in combustion duration with increasing volumetric fraction of alcohol. The shortest CD is observed for diesel/methanol [25,27] and diesel/ethanol blends [25,31]. The longest combustion duration was found for diesel/heptanol blends [49]. According to Kumar et al. [28], longer ignition delays promote better fuel-air mixing, leading to a higher premixed combustion fraction. Combustion duration is shorter due to faster combustion of a larger amount of premixed charge after a long ignition delay. Therefore, fuels with longer ignition delays exhibit shorter CD. Jamrozik et al. [36] suggest that the presence of oxygen in the molecular structure of alcohol fuels enhances the combustion process. During the main combustion phase, oxygen in the molecular structure accelerates this process. The longer combustion duration of hexanol was attributed to its higher viscosity, which adversely affects the evaporation of larger fuel droplets, leading to improper fuel–air mixture and detrimental effects on premixed combustion [47].

5.2. Performance

5.2.1. Thermal Efficiency (TE)

The thermal efficiency of an internal combustion engine is the ratio of the total energy contained in the fuel supplied to the engine to the amount of energy used to perform useful work. Figure 8 illustrates the percentage changes in thermal efficiency of engines burning diesel/alcohol blends relative to burning pure diesel fuel, for eight analyzed alcohol fuels. A review of the literature reveals that alcohol added to diesel engines can either improve or worsen thermal efficiency. From the analysis of Figure 8, it can be observed that simple alcohols with short molecular chains generally lead to an increase in thermal efficiency, with the highest improvement observed for diesel/methanol blends [26,27]. Conversely, thermal efficiency decreased for complex alcohols, with the largest declines observed for blends of diesel with pentanol [67], heptanol [47], and hexanol [29]. According to Yusaf et al. [26], the improved thermal efficiency for blends with added methanol results from complete combustion of the lean mixture when alcohol is used. The increased combustion rate of methanol compared to diesel results not only in higher charge temperatures in the reaction zone during initial combustion but also in an increased number of ignition points for diesel fuel. The positive impact of alcohol evaporation and faster combustion significantly reduces heat losses from the combustion chamber [27]. Conversely, the decrease in thermal efficiency for pentanol, heptanol, and hexanol according to sources [29,49,67] is due to the loss of the fuel blend’s heating value and the higher heat of vaporization of the alcohol, as well as the higher viscosity of the blend (complex alcohols have higher viscosity) and the presence of molecular oxygen. Extended combustion of long-chain alcohols leads to heat loss within the engine compartment [47].

5.2.2. Specific Fuel Consumption (SFC)

The specific fuel consumption expressed in g/kWh indicates how much chemical energy contained in the fuel needs to be used to produce one kilowatt-hour by the engine. Figure 9 presents the percentage changes in specific fuel consumption for an engine burning diesel/alcohol blends relative to burning pure diesel fuel, for eight analyzed alcohol fuels. In most conducted studies, the inclusion of alcohol fuel resulted in a slight, few percent increase in SFC compared to pure diesel fuel. The most significant discrepancies in assessing the impact of alcohol on fuel consumption in CI engines occurred with methanol. Yusaf et al. [26] reported an increase in SFC by several percent, whereas in studies conducted by Jamrozik et al. [27], the addition of methanol to diesel fuel decreased fuel consumption by several percent. Fuel consumption depends on fuel properties such as cetane number, heating value, and heat of vaporization. Alcohols have a higher heat of vaporization, lower cetane number, and lower heating value, which negatively impact combustion and result in lower energy generation. Specific fuel consumption for diesel/alcohol blends increases due to their lower heating value and higher viscosity of blends containing complex alcohols [47].

5.3. Stability

Coefficient of Variation of Indicated Mean Effective Pressure (COV_imep)

COV_imep is an important measure of cyclic variability in internal combustion engines, derived from pressure data. It is the standard deviation of IMEP divided by the mean IMEP and is typically expressed as a percentage (%) [80,81]. Figure 10 illustrates the percentage changes in coefficient of variation of IMEP (COV_imep) determined for an engine burning diesel/alcohol blends, referenced to burning pure diesel fuel, for eight analyzed alcohol fuels. Figure 10 indicates that there is relatively little research focused on the stability of engines fueled by diesel/alcohol blends. Based on available data, it can be concluded that adding alcohol to diesel fuel slightly worsens the stability of CI engine operation. For blends of diesel fuel with methanol [27], ethanol [31,45], butanol [45], and heptanol [49], an increase in COV_imep is observed, indicating greater cycle-to-cycle variability and reduced engine stability. The increase in COV_imep signifies a deterioration in engine stability, likely due to the leaner air–fuel mixture resulting from the addition of alcohol fuel [45,57].

5.4. Emissions

5.4.1. NO_x Emissions

NO_x is considered one of the most significant air pollutants emitted from all types of internal combustion engines. Air pollution and NO_x emissions from diesel engines are a global concern due to their potential impact on human health. The study by O’Driscoll et al. [82] indicates that compression ignition engines emit significantly higher amounts of NO_x compared to spark ignition engines. Gasoline engines provide an 86–96% reduction in NO_x emissions compared to diesel engines. Figure 11 shows the percentage changes in NO_x emissions from an engine burning diesel/alcohol blends compared to pure diesel fuel, for eight analyzed alcohol fuels. It is evident that in the majority of studies, adding alcohol fuel to diesel fuel has a positive impact on NO_x emissions from compression ignition engines. In some studies, adding alcohol resulted in a slight increase in NO_x emissions. The largest increase in emissions was observed in studies conducted by Jamrozik et al., particularly with blends of diesel fuel with methanol [27] and ethanol [31]. According to [27,31], the increase in NO_x emissions is attributed to the higher combustion intensity and heat release rate in engines fueled with blends containing methanol or ethanol. Additionally, the additional oxygen provided by these highly oxygenated alcohols may enhance the combustion process, leading to a faster oxidation reaction that promotes NO_x formation. According to [40], delayed ignition timing with alcohol blends resulted in more premixed charge that kinetically burned in the subsequent stages, increasing cylinder temperature and exacerbating NO_x formation. Conversely, according to [24], the reduction in NO_x emissions is attributed to the high latent heat of vaporization of alcohols, which has a cooling effect on the charge and lowers combustion temperature.

5.4.2. HC Emissions

Hydrocarbons, or organic emissions, result from incomplete combustion of hydrocarbon fuels [74]. Figure 12 shows the percentage changes in HC emissions from a CI engine burning diesel/alcohol blends compared to burning pure diesel fuel, for eight analyzed alcohol fuels. Based on the reviewed literature, the impact of adding alcohol fuel to diesel fuel on HC emissions from CI engines cannot be conclusively determined. The largest increases in hydrocarbon emissions were observed for propanol [68], butanol [28,50,68], and hexanol [29]. Conversely, blending diesel fuel with methanol [23,27] and octanol [28,54] resulted in the highest decreases in HC emissions. According to Kumar et al. [28], the increase in HC emissions may be due to the lower cetane number of blends with alcohol additives, which worsens the autoignition properties and promotes a quenching effect in the cylinder in the leaner mixture zone. Conversely, the higher oxygen content in diesel/alcohol blends may contribute to oxidizing unburned hydrocarbons at relatively high cylinder temperatures, leading to reduced HC emissions. Nour et al. [44] suggest that the increase in HC emissions, especially when burning blends containing long-chain alcohols, could be attributed to the negative impact of higher fuel viscosity (poorer atomization) and a higher number of carbon atoms per molecule [34].

5.4.3. CO Emissions

CO emissions depend on the combustion of the fuel–air mixture and the carbon content in the fuel. During combustion, carbon in the fuel oxidizes with oxygen from the air, forming CO and CO₂ depending on conditions [83]. Figure 13 shows the percentage changes in CO emissions from a diesel/alcohol blend engine compared to pure diesel combustion, for eight analyzed alcohol fuels. It can be observed that blending diesel with alcohols generally has a positive effect on CO emissions from compression ignition (CI) engines [24,27,28]. However, there are studies where the addition of alcohol adversely affects CO emissions [47,62,67]. The literature studies presented do not conclusively answer how alcohol fuels co-combusted with diesel affect the concentration of carbon oxides in CI engine exhausts. The decrease in CO emissions is attributed to the presence of oxygen in alcohol fuel, which promotes the oxidation of carbon monoxide leading to more complete combustion and significant reduction in CO emissions [24,28]. The increase in CO emissions, especially for complex alcohol blends characterized by high viscosity, may be due to poor atomization of the fuel mixture, resulting in ineffective oxidation and CO reduction.

5.4.4. CO₂ Emissions

CO₂ is the principal greenhouse gas. Although not directly harmful to our health, CO₂ emissions contribute to climate changes [84]. Figure 14 shows the percentage changes in CO₂ emissions from CI engines fueled by diesel/alcohol blends, referenced to conventional diesel engines, for eight analyzed alcohol fuels. It can be observed that methanol [27] and ethanol [31] added to diesel promote a reduction in CO₂ emissions from the engine. Conversely, heptanol, as its proportion increases in the blend with diesel, leads to an increase in carbon dioxide emissions [47,64]. According to [36], the reduction in CO₂ emissions may result from a decrease in the elemental carbon supplied from alcohol fuel into the engine cylinder. Conversely, the additional oxygen provided by alcohol contributes to an increase in CO₂ emissions.

5.4.5. Soot Emissions

Diesel soot, one of the major environmental pollutants, is the fine particle produced during the high temperature pyrolysis or combustion of diesel fuel [85]. Figure 15 presents the percentage changes in soot emissions from an engine burning diesel/alcohol blends compared to pure diesel, for eight analyzed alcohol fuels. One of the fundamental reasons and main expected benefits of using alcohol fuels in diesel engines is the reduction of soot content in engine exhaust. A comprehensive review of literature studies confirmed that adding alcohol fuel to diesel contributes to reducing soot emissions from CI engines. For seven out of the eight analyzed alcohol fuels (methanol [23], propanol [24,40,62], butanol [28,32,34,45,50,56], pentanol [28,40], hexanol [33], heptanol [44,47,49,64], octanol [28,44,54]), the effect of reducing soot emissions increased with higher alcohol volume fractions in the blend with diesel. Only the blend of diesel/ethanol showed a slight increase in soot emissions compared to pure diesel combustion [70]. According to Muthaiyan and Gomathinayagam [24], the presence of oxygen in alcohol fuel helps reduce the formation of soot precursors in fuel-rich zones, leading to reduced soot emissions. Nour et al. [44] suggest that the OH group in alcohol molecular structures promotes the oxidation of soot and reduces its emission. Additional OH radicals assist in combustion processes and accelerate soot oxidation within the flame zone. Moreover, the lower elemental carbon content introduced into the cylinder with alcohol fuel helps limit soot formation.

6. Discussion

Alcohol fuels belong to renewable fuels, mostly produced from biological sources, making them fully biodegradable. Replacing conventional fuels with alcohol fuels brings many benefits, including greenhouse gas emission reduction and improvement of crop rotation. Research has shown that using alcohols significantly reduces the emission of many toxic components from engine exhaust compared to fossil fuels. In addition to environmental benefits from the biofuels industry’s development, there are numerous economic advantages, such as sustainable development, diversification in the fuel sector, vehicle efficiency improvement, rural area activation through new job opportunities, agricultural market development, increased income tax revenue, and enhanced international competitiveness. Another significant aspect is reducing dependence on conventional raw material suppliers, thus increasing energy security. Alcohol fuels, however, come with several operational disadvantages, such as worsened engine cold-start characteristics, lower fuel durability, and restrictions on mixing certain biofuels with conventional fuels. Introducing them into circulation requires design and material changes in engines and new investments, including the establishment of more alternative fuel refueling stations.

One of the most important aspects of using renewable fuels, such as alcohols, in diesel engines is the long-term operation of the engine and the associated challenges. Long-term tests are crucial for assessing the impact of unconventional fuels on engine components, the formation of harmful deposits in the engine, and the wear and degradation of lubricating oil. The limited number of such tests suggests that biofuel blends may pose long-term issues, leading to decreased combustion efficiency and, over time, increased emissions [86]. The use of biofuels in diesel engines also results in increased corrosion due to their oxygen content and higher wear on engine components and the fuel system.

It is also important to consider the potential environmental impact of alcohol production. Although the goal of using biofuels is to reduce dependence on petroleum, the sourcing of raw materials for their production has been criticized. Due to emissions associated with ILUC (Indirect Land Use Change), the use of biofuels may have a worse impact on the climate than the fossil fuels they are intended to replace [87]. Biofuel production can indirectly contribute to deforestation and land conversion, including changes in the use of sensitive ecosystems.

The use of diesel/alcohol blends as engine fuels requires an economic evaluation from several perspectives. This is a complex issue that, in addition to considering the price of the alcohol product itself, should also encompass infrastructure, environmental, and market costs. The cost of alcohol production, which can be derived from various feedstocks, depends on whether the raw material is sourced from crops or is a by-product of another process. For example, the cost of producing ethanol from corn is lower but subject to market fluctuations of this raw material. An alternative could be obtaining ethanol from cellulose processing, but due to the technology involved, it is more expensive. When preparing a blend with diesel fuel, the costs of equipment and phase separation prevention measures must be considered. For the application of alcohol-rich fuel, costs related to adapting the engine’s fuel system should be taken into account, and the fuel infrastructure must be resistant to corrosion caused by alcohol. It is important to note that alcohol has a lower LHV (Lower Heating Value) than diesel fuel, which may result in reduced engine power and increased operating costs. However, this can be mitigated by making appropriate modifications to the engine’s fuel system.

Regarding soot emissions, alcohols significantly reduce their emission, which can positively impact the cost of soot removal systems. However, for users of such fuels, there must be a clear incentive in the form of cost-effectiveness. Government subsidies or tax incentives for biofuels could make diesel/alcohol blends more competitive. Policies promoting renewable energy sources or carbon taxes on fossil fuels could further enhance the economic viability of alcohol/diesel blends. A detailed economic analysis is necessary, taking into account current and forecasted prices of diesel fuel, alcohol, and raw materials. This analysis should consider both direct costs and external factors, such as environmental and health benefits. Factors such as oil price fluctuations, technological advancements in alcohol production, and changes in the regulatory environment should be considered. Sensitivity analysis may help determine the conditions under which alcohol/diesel blends become economically viable.

7. Conclusions and Recommendations

In this paper, a literature review was conducted on CI engines powered by diesel/alcohol blends. The impact of eight types of alcohol fuels—methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, and octanol—on combustion, performance, stability, and emissions of diesel engines was analyzed. The literature review indicates that:

• the addition of alcohol fuel to diesel in CI engines increases ignition delay, with a greater increase corresponding to a higher volumetric share of alcohol. The highest ignition delay was observed with the diesel/methanol blend (57%);
• combustion of a blend containing alcohol generally resulted in an increase in p_max compared to pure diesel combustion (by approximately 15–20%). Higher volumetric shares of alcohol resulted in progressively higher p_max values. The highest p_max was achieved with short- and medium-chain alcohols, such as methanol, propanol, butanol, and pentanol;
• an increase in the volumetric share of alcohol in the diesel blend, up to 30%, is associated with an increase in PPR_max, with the highest pressure rise rates observed with diesel and propanol blends (approximately 70%). Unfortunately, the increase in p_max and PPR_max also leads to higher mechanical loads on the engine, which may reduce its durability;
• higher alcohol content leads to an increase in HRR_max. Diesel combustion with the addition of methanol, ethanol, and butanol up to 30% showed the highest HRR_max values, even up to approximately 80% higher compared to diesel alone;
• the effect of alcohol fuel on the combustion duration of diesel/alcohol blends in CI engines is not straightforward. Most studies indicate a reduction in combustion duration with an increase in alcohol volumetric share, with the shortest CD observed in diesel with methanol and ethanol (approximately 50%). Increased heat release rates and shorter combustion durations may lead to higher instantaneous thermal loads on the engine cylinder, potentially worsening the lubricating properties of the engine oil;
• the addition of alcohol to diesel in CI engines can either improve or worsen thermal efficiency. For simple, short-chain alcohols, thermal efficiency (TE) increased, with the highest TE for the diesel/methanol blend (approximately 15%). For complex alcohols, TE decreased (by approximately 10%). This inconsistency in results may be due to the lack of optimization of the engine’s thermal cycle (CA50), which should be conducted for each adapted engine;
• the addition of alcohol to diesel slightly deteriorates the stability of CI engine operation, as defined by the COV_imep coefficient (approximately 60%);
• most studies indicate a positive impact of alcohol fuel addition to diesel on NOx emissions in CI engines, with the greatest reductions in emissions reaching approximately 50%;
• based on the literature review, it is not possible to definitively determine the impact of alcohol fuel addition on HC emissions from CI engines. The largest increase in hydrocarbon emissions was observed with propanol, butanol, and hexanol (approximately 100%). However, diesel blends with methanol and octanol resulted in the greatest reduction in HC emissions (approximately 60%);
• the reviewed studies do not conclusively answer how alcohol fuels co-combusted with diesel affect CO and CO₂ concentrations in CI engine exhaust. Unburned hydrocarbon and CO emissions are closely related to the so-called gap effect, which is a consequence of engine design. The literature does not specify the volumetric share of gaps relative to the combustion chamber volume, which could provide insight into emission evaluation;
• the addition of alcohol fuel to diesel contributes to a reduction in soot emissions from CI engines. For all analyzed alcohols (except ethanol), increasing the volumetric share of alcohol in the diesel blend resulted in a greater reduction in soot emissions (up to approximately 90%). Authors often limit their evaluation to soot emissions, but a comprehensive assessment should also include PM emissions divided into different fractions.

Despite the extensive research dedicated to fueling CI engines with alcohol blends, our literature analysis indicates that this topic remains relevant and current. Research efforts should be directed towards several technical aspects:

• optimization of alcohol blends for optimal engine performance: due to the variety of research setups, results are inconsistent, making it difficult to definitively determine the most beneficial ratio of individual alcohols in diesel blends for engine performance. Research should focus on identifying the optimal blend and exploring the potential for additives that enhance and stabilize these mixtures;
• engine design aspects: adaptation of engines to operate on alcohol blends involves modifications to injection systems and engine control strategies. Research should investigate the best practices for integrating these adaptations to improve engine performance and reliability;
• durability testing: there appears to be the most work needed in this area. The literature reveals a gap in studies on the long-term effects of high alcohol content on engine durability, particularly for naturally aspirated and turbocharged engines under various loads;
• exhaust emissions: development work is recommended on exhaust cleaning systems, particularly for NOx and particulate matter (PM). Research should focus on advancing these systems to meet emission standards effectively;
• economic analysis (LCA—Life Cycle Assessment): a comprehensive approach is proposed, including an economic analysis covering the entire life cycle of alcohol fuels. This should include the production process, the economic impact of engine operation, exhaust emissions, recycling of engine components, and the economics of fuel distribution. Additionally, the feasibility of producing alcohols from agricultural waste and lignocellulosic biomass should be evaluated.