The major global challenges today are global warming, climate change, price volatility and availability of energy resources because of the main focus on petroleum-based energy sources. Environmental concerns increased once the crucial carbon dioxide (CO2) level was surpassed. That’s why some countries are focusing on establishing CO2 capture and storage facilities. It became necessary to increase the proportion of renewable and clean energy sources while balancing the energy demand. In this case, alcohols can be a good alternative to gasoline and diesel, especially concerning the degree of pollution they cause.

Finding a reliable way to use alcohol as an internal combustion engine fuel source has been the focus of study in recent years. The use of fusel oil in internal combustion engines has drawn interest during the past 10 years. Fusel oil is obtained as a by-product during the fermentation process that contains higher alcohols.

1 Why is the Use of Renewable and Clean Fuel in Internal Combustion Engines Important?

It is well established that an internal combustion engine cannot theoretically operate with stoichiometric combustion, even in a case of an atmospheric flame [1,2]. An internal combustion engine has difficulty providing the right mixture, uniform temperature distribution, and adequate time inside the cylinder. The highest combustion efficiency for compression ignition (CI) and spark ignition (SI) engines is typically less than 96-97%. Unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions are hence unavoidable. This harm is significant when considering oxides of nitrogen (NOx) and carbon dioxide (CO2) emissions in the environment when fossil fuels are burned [3,4]. CO2 has escalated environmental concerns as the primary cause of climate change among these hazardous gases [5,6].

Several millions of healthy life might be lost globally due to air pollution alone, with low and middle income countries bearing the biggest disease burden. The health effects of air pollution increase with exposure, especially for older people, infants, pregnant women, and those with chronic diseases (including asthma, heart disease, obstructive pulmonary disease, etc).

The search for renewable and sustainable fuel has been ongoing for decades due to the destructive characteristics of fossil fuels and the dangers of depletion [7,8]. In addition, research on innovative vehicle technologies such as fuel cells, solar, and hybrid vehicles has increased during the past 30 years [9-11]. However, most of those are still in development, and due to some problems, it does not appear that a broad usage area could be found in the near future.

As a result, internal combustion engines will continue to power vehicles for a very long time. Using a relatively green fuel is very important since we are using a large portion of fossil fuel production in transportation.

2 Why are Oxygenated Fuels Important?

The most widely used alternative fuels are oxygenated fuels because they accelerate cylinder combustion and reduce emissions[12].

The fact that oxygenated fuels are made from bio-based renewable sources, including sugarcane, a variety of seed oils, used cooking oil, etc., is another attractive feature [13,14] of these fuels. Currently, alcohols are good oxygenated fuel sources for transportation areas.

Among the several forms of alcohol, ethanol is unquestionably the most frequently used in passenger vehicles. Its widespread use is its non-toxic structure and extensive bio-based manufacturing capability. In particular for SI engines, its high octane number, hydrogen/carbon ratio, low volatile emissions, flame propagation tendency, safe transportation and storage properties make it stable and reliable fuel [15-17].

Figure 1: Production route of ethanol and higher alcohols

3 Ethanol as an Internal Combustion Engine Fuel

In the last decade, ethanol has grown popularity as a biofuel for internal combustion engines. The usage of ethanol also reduces the quantity of CO2 in the exhaust gas; therefore, the more ethanol is present in the fuel-air combination, the less CO2 is present in the exhaust gas. When the engine operates at low and medium loads, the reduction in CO2 is the most noticeable. NOx emissions greatly decrease when ethanol is added to an engine cylinder through a pump.

3.1 Ethanol as Spark Ignition Engine Fuel

There are two types of spark ignition engines: two-stroke and four-stroke. The ethanol fuel use mechanisms vary depending on how the engines operate.

3.1.1 Ethanol as Four-Stroke Engine Fuel

Two important factors when using ethanol in a SI engine are important. First, ethanol has a far lower calorific value (CV) than gasoline. Second, compared to gasoline, substantially less air is needed to generate a stoichiometric air-fuel combination (1/9 vs. 14.7). As a result, the engine consumes more ethanol, but the engine’s maximum power can be enhanced due to the lower air-fuel ratio. E85 (85% ethanol and 15% n-heptane), E100 (100% ethanol), and low-grade ethanol have all been used in numerous engine testing worldwide. According to the findings, engine torque rises while using E100 and low-grade ethanol and various gasoline and ethanol mixtures. Additionally, using ethanol and its mixes increases engine power. Torque is an important index for determining engine power; hence an increase in torque will also increase engine power. Additionally, engine thermal efficiency rises to a certain level while using various ethanol fuels and their mixtures. Typically, this growth stays at 20%.

Since the ethanol air-fuel combination needs more energy to evaporate than gasoline does, the high heat of ethanol evaporation results in a gain in BTE [18,19].

The engine’s volumetric efficiency is another factor that rises while using ethanol as fuel. According to many researchers, volumetric engine efficiency rises as ethanol percentage increases in fuel blends [20]. This is common for engines with intake manifold injection systems, in which the intake manifold is injected with ethanol to cool the air before it goes into the engine. Regardless of the fuel blend, engine cylinder effective pressure rises when using ethanol fuels in contrast to using gasoline [20].

3.1.2 Ethanol as Two-Stroke Engine Fuel

Ethanol usage in two-stroke engines is important since it reduces the amounts of CO and UBH in the exhaust from the engine. The fuel’s lubricating qualities remain the principal drawback of employing ethanol as a fuel in these engines. A typical two-stroke engine uses a mixture of lubricating oil and gasoline. The air-fuel combination must guarantee that the engine’s working surfaces are lubricated. Hence adding oil to the mixture is imperative. Because of the polarity of lubricating oil and ethanol, most two-stroke lubricating oils are often immiscible in ethanol. This is why the effects of pure ethanol fuel on a two-stroke engine’s output characteristics, exhaust gas emission, etc., have been extensively researched. On the other hand, the effects of ethanol and oil mixtures in the engine have received less attention.

The amounts of CO and other hydrocarbons in the exhaust gas are frequently increased when the lubricating oil is added to the fuel because heavier fuel fractions, such as oil, need high temperatures and prolonged combustion times.

According to studies, when ethanol and two-stroke engine oil blends are burned, the UHC quantity in the exhaust is somewhat lower than when gasoline and lubricant oil blends are burned. Simultaneously, CO2, CO, and NOx amount rise [21]. The endurance tests on the working surfaces of two-stroke engines employing ethanol and oil mixes have not revealed significantly faster wear of these surfaces. Corrosion is still the primary problem. So, it confirms that ethanol is a viable fuel for two-stroke engines.

3.2 Ethanol as Diesel Engine Fuel

Ethanol low CV mitigates the power loss of an unmodified engine. Hence, engine adjustment is generally needed while ethanol is utilized as a part of fuel blend in diesel engines.

Regardless of the technique of using bioethanol, the exhaust gas constituents CO2, CO, NOx, and soot (black powdery or flaky substance/carbon black) drop using ethanol as diesel engine fuel [22]. In case of ethanol injection into the engine as supplementary fuel rather than blended with gasoline, the engine’s thermodynamic performance becomes higher with reduced exhaust gas emission [23]. It is obvious by comparing the exhaust gas constituents of engines with various fuel delivery methods (common rail system / an injection pump) that CO% rises in common rail (CR) engines. The underlying cause is the increased vaporization heat of ethanol, which causes an ignition delay and hinders combustion of air-fuel combinations.

Furthermore, the amount of NOx produced is greatly reduced while using ethanol in an engine with a mechanical injection pump. The fraction of NOx in the exhaust gas emission from engines with a CR fuel delivery system falls by w50% for light and medium engine loads. This fraction either stays the same or exceeds pure diesel fuel under high engine load [24]. The greater ethanol vaporization heat causes the lower combustion temperature in the cylinder, which in turn causes a drop in the percentage NOx in exhaust gas emission. Additionally, using ethanol reduces the amount of soot in exhaust gas emissions. This relates to the engine’s design, setup, and fuel delivery system. The amount of soot in exhaust gas emissions drops by 80% under high engine loads [22].

It’s also crucial to note that as ethanol’s water content rises, so does the amount of soot in the exhaust gas [24]. As a result, ethanol’s combustion in the engine is hampered by increased water content. When ethanol is utilized, most of the hazardous chemicals in exhaust gas emissions decrease; nonetheless, the fraction of hydrocarbons increases.

4 Importance of Higher Alcohol

Researchers are interested in fusel oil (or higher alcohol) because of its ethanol-like characteristics. The fermentation process produces fusel oil, which is an oxygenated compound with a higher research octane number than ethanol [25]. It appears to be a suitable alternative fuel, especially for spark-ignition engines. After extracting the alcohol from the fermented mixes, which can include some water, fusel oil is added as an oily coating, and this second layer was originally named fousel (dark-brown color / bad odor). Biodiesel, isoamyl acetate, and bio-lubricants are the main products made from fusel oil [26]. Until 2012 [27], no research has been done on the use of fusel oil in internal combustion engines.

Any fermenting combination can be distilled to generate fusel oil. Higher alcohols like i-propanol, i-butanol, and i-amyl alcohol are typically found in fusel oils; depending on the type of fermented product, its structure changes [28].

Based on its chemical composition, fusel oil contains more alcohol. Higher alcohols ranging in carbon number from 3 to 20 can also be considered for internal combustion engine fuel. According to the investigation, the greater alcohol content of the diesel-biodiesel blends enhanced the cold filter plugging point [29]. However, density, kinematic viscosity, cetane number, and lower heating value were marginally decreased by increasing alcohol content. With all three higher alcohols, NOx emissions fall while CO emissions rise.

Experimental investigations into the emission characteristics and performance of a CR direct injection diesel engine utilizing a blend of diesel fuel and 1-pentanol were conducted in research [30]. In the research, exhaust gas recirculation was also used. It was discovered that 1-pentanol decreased NOx emissions while adversely affecting specific fuel consumption and thermal braking efficiency. Additionally, it was claimed that combining 1-pentanol and diesel fuel mixes caused the ignition delay to rise due to the 1-pentanol’s lower cetane rating.

According to research [31], isoamyl alcohol positively affects thermal efficiency, torque, and emissions. The improved engine performance is due to the oxygenated structure of isoamyl alcohol and more latent heat, which improve volumetric efficiency.

Fusel oil can potentially be utilized in CI engines and spark ignition engines. Major research on the use of fusel oil in internal combustion engines has been performed during the past ten years, and it can be said that this increased alcohol presents a potential source for internal combustion engines.

Since many years ago, MTBE has been included as a gasoline additive. The emission of MTBE has significantly increased due to the rise in fossil fuel usage. MTBE poses a significant long-term environmental threat since it is a hazardous chemical molecule [32]. The discontinuity of MTBE in some countries is anticipated to enhance the use of fusel oil as an oxygenate addition to solve this problem. Fusel oil in internal combustion engines is a viable approach to rebalance and partially replace fossil fuel in current CI and SI engines without substantially modifying the engines [33].

5 Fusel Oil Production Potential Globally

Traditional renewable fuels include alcohol-based fuels like methanol, ethanol, and butanol. In the United States, Brazil, and South Africa, ethanol is among the most recognized alternatives to fossil-based fuels due to its renewability, better energy density, and lower toxicity [34].

A high amount of energy used globally for transportation—about 3%—comes from bioethanol, which helps promote green transportation. The two countries that generate the most ethanol globally, making up 85% of it, are the United States and Brazil. However, these two countries’ fermentation processes are different, resulting in different characteristics of fusel oil.

Some governments have declared an ethanol-gasoline blend to raise the demand for utilizing oxygenates as a fuel additive. For example, the Chinese government has started a campaign to promote bio-ethanol as a renewable fuel that can be added 10% to gasoline [35].

Table 1 highlights how diverse feedstocks are fermented in different countries to generate bioethanol. The quantity of fusel oil produced as a by-product has significantly increased as ethanol production has increased, with concerns about environmental effects.

1000 kg of raw sugar from sugarcane/sugarbeet molasses yields approximately 523.8 L ethanol [36]. Additionally, distilling 1000 kg of ethyl alcohol yields 6.4 L of fusel oil [37]. This conversion yields a global estimate of 550 million liters of fusel oil output, which is 347 million liters of gasoline and 325 million liters of diesel fuel. Calculations show that the spatial concentration of fusel oil production is quite high. The United States synthesizes around half of the global fusel oil (54%), with Brazil coming in second (29%).

Table 1: Renewable/bio-ethanol targets by different countries

6 Physiochemical Characteristics of Fusel Oil Relating to Engines

It’s critical to examine how the physiochemical characteristics of fusel oil change and impact the efficiency of internal combustion engines. The characteristics of fusel oil rely on several factors, including the decanting/distillation procedure, the fermentation conditions, and the type of bio-based material employed. Table 2 shows the fusel oil’s composition and properties.

Table 2: Physical characteristics and composition of fusel oil

The primary variables used to measure the quality of the blended fuel are the chemical and physical characteristics of fusel oil. Such qualities can directly impact internal combustion engine combustion and exhaust emission characteristics. Table 3 lists the primary physical and chemical characteristics of ethanol, fusel oil, and traditional fossil fuels (such as diesel and gasoline) [43]. Compared to gasoline and diesel, fusel oil has a higher oxygen content, which improves combustion efficiency. Compared to diesel and gasoline fuel, the oxygen concentration also results in a reduced air/fuel ratio demand. The low heating value caused by the high oxygen concentration is a serious drawback. Fuse oil possesses higher flashing point and auto-ignition temperature than diesel and gasoline fuel. It is, therefore, safer to utilize this fusel at high temperatures.

Fusel oil’s miscibility in simple diesel, particularly at low temperatures, which results in incomplete combustion, is the fundamental disadvantage of fusel oil/diesel mixtures. Blends of diesel fuel and fusel oil that contain biodiesel can be used to solve this issue. Fusel oil has a greater cetane rating comparing ethanol. High cetane fuel values generally improve engine braking performance and lower NOx generation. Furthermore, due to its higher viscosity than ethanol and diesel fuels, fusel oil can favor in-cylinder pressure and injection timing. Similar to ethanol, fusel oil is a higher-density fuel. Fuel efficiency is enhanced by the greater fuel density, which raises the mass flow rate and reduces fuel loss during the injection. Furthermore, a water content of 3.5% to 20% in fusel oil would result in a lower heat release rate and combustion temperature, resulting in a notable reduction in NOx [44].

Table 3: Physiochemical properties of gasoline, diesel, ethanol and fusel oil

7 Fusel Oil as an Internal Combustion Engine Fuel

7.1 Fusel Oil as Spark Ignition Engines Fuel

The longer structure of fusel oil is attractive because of its good influence on engine performance and exhaust pollutants. SI engines can run on fusel oil without requiring major engine modifications. Particularly in the last 10 years, several studies on the usage of fusel oil in SI engines have been conducted.

7.1.1 Engine Performance

Experimental evidence supports the possibility for improved torque/engine brake efficiency (BTE) when using fusel oil. At all tested engine speeds, which ranged from 2000 to 5000 rpm, the improvement in braking torque was measured at 3%. A fusel oil blend of 30% was claimed to produce the most engine torque.

Contrary to the results above, a study in 2015 [28] claimed that adding fusel oil to gasoline negatively impacted engine torque. It claimed that combustion of the 50% and 100% fusel oil blends resulted in 2% and 6% decreases in engine torque, respectively. The engine torque reduction was attributed mostly to the low heating value (LHV) and water content of fusel oil. Additionally, BTE variations followed a similar pattern and declined by around 6% when fusel oil content increased. Several reports have revealed the harmful effects of fusel oil’s water content on combustion characteristics and engine performance.

Research [37] examined the impact of removing fusel oil moisture content on the CV of blended fuel quality. They said that some enhancements in brake power, CV, and BTE could be seen as water content of fusel oil dropping from 13.5% to 6.5%. However, the combustion duration for fusel oil/gasoline mixtures was lower than that of gasoline. In addition, they discovered that the engine power had improved somewhat, by 0.7% on average, compared to the fuel mixes before the water was removed.

Another research [45] combined fusel oils with varying water contents with gasoline fusel. The tests were run at various engine speeds while maintaining a constant engine load of 45%. The combustion and engine performance parameters improved, and similar results were attained. The impact of water extraction on the properties of the fusel oil/gasoline combination was examined in intriguing research. They asserted that the examined emulsion increased brake power and BTE. Because the fuel blend burned completely compared to pure gasoline, lowering the water concentration also improved combustion temperature and performance.

In research [46], the effects of adding various amounts of fusel oil to gasoline (25, 50, 75, and 100%) at various engine load levels (i.e., 20, 40, 60, 80, and 100%) were examined. According to their research, adding fusel oil to pure gasoline reduced BTE and braking torque by 14% and 4%, respectively, since the cylinder’s combustion temperatures were lower.

7.1.2 Fuel Consumption

Many studies have looked into variations in fuel usage brought on by fusel oil blending with pure gasoline. Most prior studies have found that adding fusel oil to gasoline negatively impacts brake specific fuel consumption (BSFC), mostly because the CV of the fuel blends decreases. For instance, the study discovered that the specific fuel consumption (or consumption per unit mass) rose with increased fusel oil in the final fuel blend for all engine speeds. Utilizing a 30% fusel oil mix increased fuel consumption per unit mass up to 7.7% compared to mineral gasoline [27].

7.1.3 NOX Emissions

Although the content of oxygen in fusel oil can help reduce emissions, the amount of water in the fuel has the opposite effect. It is widely acknowledged that combustion physics and chemistry play a major role in generating NOx emissions [47]. Using fusel oil instead of pure gasoline greatly reduces NOx emissions. According to research that employed different mixes of fusel oil and gasoline, the water content in the fusel oil led to lower combustion temperatures, which resulted in lower NOx emissions (41% max) than gasoline. Fusel oil concentration of 50% resulted in the maximum NOx decrease (83.04%).

7.1.4 UHC and CO Emissions

Numerous factors, such as the qualities of the fusel oil, the kind of engine, and/or the operating circumstances of the engine, have been documented to be related to the generation of CO emissions. Additionally, incomplete combustion is the factor that has the biggest impact on the creation of CO emissions. Different investigations found that the combustion products’ temperatures significantly dropped in case of fusel oil blend with gasoline, which led to a striking rise in CO emissions. In other words, the energy produced by the combustion of the fusel oil/gasoline mixture was insufficient to provide enough heat to convert CO to CO2. According to a study [46], when fusel oil content and engine load increased, CO emissions rose by 22%.

Contrary to these assertions, another research [48] said that the fusel oil/gasoline mixes generated noticeably less carbon monoxide (CO) than clean gasoline fuel at all engine loads. As per the findings, compared to gasoline fuel, CO emissions for F10, F20, F30, and F50 dropped by 13%, 26%, 63%, and 82%, respectively. The portion of fusel oil and engine operating conditions also influenced overall UHC emissions.

Studies show that using fusel oil significantly reduced the combustion temperature. Incomplete combustion products are produced more often as a result of this reduction. One of the main issues with employing the fusel oil and gasoline blend is the higher CO and UHC emissions [37].

7.2 Fusel Oil in Compression Ignition Engines

7.2.1 Engine Performance

Fusel oil is taken in research as an environmentally friendly additive for CI engines compared to SI engines. Studies show that the brake power and torque will decrease when fusel oil concentrations in blend with diesel grow by up to 20% by volume. In comparison to pure diesel fuel, fusel oil’s moisture content and low cetane number cause a modest reduction in engine power and torque [49].

7.2.2 NOX Emissions

Several studies have focused on reducing NOx emissions from diesel engines. Fusel oil as a gasoline additive has been shown to significantly reduce NOx emissions by up to 25% [49].

For instance, research evaluating plain diesel fuel blended with fusel oil at various engine speeds and loads found that the NOx emissions were reduced on average by 22.5% compared to diesel fuel. Blending fusel oil with diesel fuel can lower the temperature of the gas inside the cylinder, lowering NOx generation since fusel oil has less thermal energy than diesel fuel. Additionally, the decrease in NOx generation using fusel oil is mostly correlated with the water concentration. The in-cylinder temperature, oxygen levels, and ignition timing significantly impact NOx generation [50].

In research, F0, F5, and F10 fusel oil blends were investigated in a direct diesel engine under a full-load operating situation [43]. It concluded that increasing the fusel oil/diesel combination from 5% to 10% caused a significant decrease in NOx emissions.

According to a study [33], water content in fusel oil and engine operating conditions are responsible for the reduction in NOx emissions caused by the injection of fusel oil and showed up to 20% reduction in NOx emissions.

Additionally, NOx emissions can be minimized at low engine loads. NOx generation increases significantly when the engine load increases because of increased exhaust gas temperatures in the cylinder chamber and declining fuel atomization [51].

7.2.3 UHC and CO Emissions

Lower NOx generation rates may come from increased fusel oil content, but UHC and CO formation rates can increase because of lower temperatures and worse combustion.

According to a study [44], the lower CV of fusel oil, which in turn caused a lower combustion cylinder temperature, was the main factor determining CO emissions from fusel oil combustion.

8 Summary and Recommendations

Alcohols offer an exciting new fuel source for internal combustion engines. In several countries today, including the United States, Brazil, and Sweden, ethanol is often utilized as an engine fuel. Ethanol can be used in a variety of ways as engine fuel. Ethanol is blended with gasoline and additives to provide a smooth engine start in cold weather, reduced deterioration in engine subsystems and fuel sustainability.

The following is a list of the key findings on the use of ethanol as an internal combustion engine fuel:

  1. Since ethanol has a lower calorific value and contains some water, the engine must be adjusted before it can be used in a diesel engine.
  2. Regardless of the technique of utilizing bioethanol, whether it be in a fuel mix or as an additional fuel while using ethanol, the volume of exhaust gas emission constituents CO2, CO, NOx, and soot decrease.
  3. Ethanol with a high water content causes more soot in exhaust gases. As a result, ethanol with more water inhibits engine combustion. Hydrocarbons rise when ethanol is used, although most dangerous compounds fall.
  4. Regardless of the fuel blend, ethanol fuels result in more effective pressure in the spark engine cylinder than gasoline. Additionally, it requires a lower fuel/air ratio.
  5. When ethanol and two-stroke engine oil (lubricant) mixes are burned, fewer hydrocarbons make up the exhaust gas than when gasoline and lubricant oil blends are burned. The ratios of CO, CO2, and NOx all rise at the same time. The common lubricant is incompatible with ethanol in a two-stroke engine, which is challenging.

This research has analyzed the key findings of using fusel oil or higher alcohols with a higher carbon number (3 or higher) in internal combustion engines. In general, fusel oil has not found a broad application area to be exploited despite its high production rate besides a few research projects on the synthesis of biodiesel and bio-lubricant. This article has reviewed the fusel oil utilization in SI and CI engines that utilize substantial volumes of fossil fuel on a daily basis throughout the world.

Important insights about viability of fusel oil as a fuel for internal combustion engines are as follows:

  1. The fusel oil’s high octane rating is good, especially for SI engines. Blends of gasoline and fusel oil enable the engine to run at greater compression ratios. This energy allows for an increase in conversion ratio and a decrease in overall CO2 output in practical applications.
  2. Except for a few experiments, fusel oil influenced engine torque favorably. The combination has nearly the same calorific value as gasoline while having a lower heating value. The fusel oil’s oxygenated composition also helps to cure combustion.
  3. Raw fusel oil contains around v5-20% water depending on the distillation method and storage circumstances. The water content of the fusel oil can be used to explain the decrease in engine torque.
  4. Fusel oil increased brake specific fuel intake for both SI and CI engines due to its lower CV. This will result in a reduction in the driving range of vehicle while retaining the same fuel storage capacity.
  5. Since alcohol is one of the main components of fusel oil, it seems to work well with spark ignition vehicles. Proper blending of fuel and fusel oil prevents phase separation. However, phase separation happens in diesel and fusel oil mixtures, particularly at higher fusel oil rates after 20%. It is possible due to the water and other types of alcohol presence that are available in the fusel oil.
  6. NOx emissions for CI and SI engines are lowered when fusel oil is used. Its water content significantly affects how much NOx emissions are reduced. However, the water content in fusel oil results in a high CO generation. The simultaneous reduction in NOx and soot is an incredible milestone because it is difficult to eliminate both pollutants at the same time. CI engines are responsible for the production of these pollutants.

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