Effect of alcohol additives on diesel engine performance a review (ảnh hưởng của các chất phụ gia alcohol lên hiệu suất của động cơ diesel một bài tổng quan)

With properties favourable to conventional fuels such as high oxygen content, high latent heat, low viscosity and density, alcohol-based chemicals are greatly interested in researchers

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Effect of alcohol additives on diesel engine

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Effect of alcohol additives on diesel engine performance: a review

Tan Trung Truong a, Xuan Phuong Nguyenb, Van Viet Pham b, Van Vang Lec, Anh Tuan Led, and Van Tam Buie

a Institute of Research and Applied Technological Science (IRATS), Dong Nai Technology University, Dong Nai, Vietnam;

b PATET Research Group, Ho Chi Minh City University of Transport, Ho Chi Minh city, Vietnam; c Institute of Maritime, Ho Chi Minh City University of Transport, Ho Chi Minh, Vietnam; d School of Mechanical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam; e Institute of Engineering, HUTECH University, Ho Chi Minh city, Vietnam

ABSTRACT

Environmental hazards are peaking as carbon dioxide emissions have

exceeded critical levels Therefore, finding and applying renewable and

clean energy sources to compensate for energy needs is extremely urgent

Alcohol compounds such as methanol, ethanol, butanol, etc emerging as

renewable fuel sources with the potential to replace diesel fuel used in

internal combustion engines With properties favourable to conventional

fuels such as high oxygen content, high latent heat, low viscosity and

density, alcohol-based chemicals are greatly interested in researchers on

internal combustion engines The use of alcohol-based biofuels with low

blending ratios in pure diesel or/and biodiesel fuel is seen as the alcohol

additive used in compression ignition engines In this work, a wide

assess-ment was made of the properties of alcohol additives as well as their effect

on their ability to improve combustion characteristics and engine

perfor-mance when alcohol additives are used in diesel engines By in-depth

discussion and analysis of recently surveyed results on the potential to

improve and double impact on brake thermal efficiency, specific fuel

con-sumption, and exhaust gas temperature, some prospects on the application

of alcohol-based additives in CI engines were also given in the conclusion.

ARTICLE HISTORY

Received 1 July 2021 Revised 17 November 2021 Accepted 20 November 2021

KEYWORDS

Alcohol additives; diesel engine; renewable energy; specific fuel consumption; brake thermal efficiency; exhaust gas temperature

Introduction

Rapid exhaustion of fossil fuels and contamination of the environment caused by emission become serious troubles globally It is difficult to carry out simultaneous improvement of engine performance and emission management (Hoang, Nižetić, and Pham 2020)(Nguyen et al 2021) Combining additives in different concentrations may enhance the properties of diesel fuel substantially to reduce emission to a standard level without deterioration of the engine performance (Atarod et al 2021) (Pourhoseini and Ghodrat 2021)(Hoang and Pham 2021) However, the trend of applying bio-based alternative fuels is still a test of time Indeed, fossil fuels including gasoline and diesel accounted for over 92% of the total fuel supply for the global road transport sector (Zhang, Lin, and Qiu 2021)(Yạci and Longo 2021), while alternative fuels contributed nearly 8% in 2018 (Figure 1a) (Daphné and Anne

2020)(Vinayagam et al 2021) The growth rates of biofuels such as Hydrotreated Vegetable Oil (HVO), Fatty Acid Methyl Ester (FAME) biodiesel and ethanol are depicted in Figure 1b

Indeed, fossil fuels can be conserved by renewable fuels such as biodiesels (ElKelawy et al 2019) (Hoang et al 2021d), vegetable oils (Hoang 2019), biogas (Bui et al 2021), hydrogen (Sok and Kusaka

2021)(Murugesan et al 2021), producer gas originated from biomass waste (Le et al 2022), and CNG

CONTACT Xuan Phuong Nguyen phuong@ut.edu.vn Institute of Research and Applied Technological Science (IRATS),

edu.vn Institute of Engineering, HUTECH University, Ho Chi Minh city, Vietnam

(Le et al 2020) The application of biodiesel can improve the efficiency of the engine but it is impossible to reduce significantly exhaust emission, especially, emission of nitrogen oxide (NOx) can sometimes increase to some extent (Huang et al 2016)(Ong et al 2021) Indeed, oxygen content and cetane number of fuel can affect higher combustion temperature that is mainly responsible for the growth rate of nitrogen oxide emissions (Hoang 2021) In this context, from an economic and environmental perspective, biodiesel is seen as the most promising alternative in the coming decades

to diesel fuel The use of biodiesel for vehicles can shorten the life cycle of carbon dioxide that contributes to effective control of global warming However, the benefits of biofuels still do not outweigh the disadvantages of their properties The first is that higher values of viscosity and density compared to diesel fuel can cause many problems with injection characteristics (Gülüm and Bilgin

2017)(Hoang 2018), combustion chamber deposit formation (Liaquat et al 2014)(Pham, Le, and Hoang 2019), and lubricating oil degradation (Hoang and Pham 2019) In addition, the stubborn disadvantage of biodiesel, in compression ignition (CI) engines, are out of control for NOx emissions (Ganesan et al., 2021)(Nayaka et al 2021) Meanwhile, economic treatment solutions to reduce NOx emissions are still limited when applied commercially (Żółtowski and Żółtowski 2015)(Pham 2019) Therefore, it is vital to search for chemical additives to blend with biodiesel to improve properties as well as engine performance Being components of fuel, very small concentrations of additives are mixed with automotive fuel, which is the aim of improving, maintaining, or imparting advantaged based-fuel properties The classification of additives can be broadly divided into inorganic and organic Organic additives consisting of the hydrogen-carbon chain have been used popularly by

Figure 1 Global energy (a) and biofuel (b) consumption in the road transport sector (Daphné and Anne 2020 ).

various researchers (P Berg, Berg, and Berg 2019)(Soudagar et al 2020b) Ethanol and methanol are the most widely studied lower alcohols in IC engines In the decade of 1980s, alcohol-based additives were used in initial researches in this field (Hansen, Zhang, and Lyne 2005)(Labeckas et al 2018) and conclusions obtained from these researches showed that the use of ethanol-diesel blends in diesel engines was accepted technically (Qi et al 2020) Alcohol-based additives are impressive additives in terms of higher oxygen content addition and stronger volatility which promote a cooling effect to lower the combustion temperature as well as cleaner-burning (Yilmaz and Atmanli 2017)(Kumar et al

2020)(Kowalski 2015) However, in the last decade, a considerable amount of research on higher alcohols has been recorded along with the advancement of higher alcohol production by modern microbial-based fermentation technologies (Saggi and Dey 2019) Furthermore, a trend of producing alcohol-based second or third-generation biofuels from biomass feedstocks by sustainable pathways is very promising It is apparent that most of the researches were published used alcohol-based additives and among them, the highest contribution belongs to n-butanol, which is about 22% (Sharma 2021) More importantly, in comparison with lower alcohols, higher alcohols have higher energy density, higher cetane number, better mixing stability, less hygroscopic nature and longer carbon chain which

is contributes to the improvement of the ignition quality of the alcohol molecules (Koivisto, Ladommatos, and Gold 2015) In the context, higher alcohol-based additives are attracting much interest from researchers because their production and use meet new environmental and specification standards However, in recent years, reviews of alcohol-based additives in CI engines have often focused on the first homologous serries of saturated alcohols (methanol and ethanol) (Zaharin et al

2017) or only on higher alcohols (propanol and butanol) (Kumar and Saravanan 2016) Therefore,

a comprehensive review of the relevant literature of production and application on CI engines of the entire range of saturated alcohol homologues from methanol to phytol can add illuminating insights into the remaining gap

This work is carried out on the effect assessments of all alcohol additives (methanol-to-phytol) blending with diesel, biodiesel and their blends on combustion characteristics and engine perfor-mance Also, this review briefly discusses the production process and the physicochemical and combustion properties of the variety of additives to answer the question of whether additives are suitable for diesel and/or biodiesel or not Last but not least, future research direction and enhance-ment in this field will be drawn based on a comprehensive review in which pros and cons and opportunities of the alcohol additive application concerning to performance of engine and combus-tion were discussed

Production process of alcohol additives

Methanol is an important chemical that can be used as a hydrogen carrier, fuel, or feedstock in organic synthesis processes Furthermore, methanol is used in fuel cells that run electric cars (Zhen and Wang

2015) People produce methanol from natural gas, coal, biomass, or CO2 (in the exhaust from cement plants, fossil fuel power plants, or from the atmosphere) Currently, 75% of the world’s methanol is produced from natural gas (Li et al 2018) The process of producing methanol from natural gas includes the following steps: natural gas reforming to produce syngas, converting syngas to crude methanol, then distillation of crude methanol to obtain the required purity of methanol In addition, the process of producing methanol from coal or biomass also includes gasification to produce syngas, synthesis of crude methanol, and refining of crude methanol In industry, methanol is produced mainly from syngas (a mixture of CO, H2, and a small amount of CO2) (Blumberg, Morosuk, and Tsatsaronis 2017) However, with fossil fuel resources increasingly depleted, the option of producing methanol from natural gas and coal needs to be gradually replaced in the future In addition to the problem of raw materials, the production of methanol from fossil fuels also releases a large amount of

CO2 emissions into the environment, causing climate change Therefore, the direction of methanol synthesis directly from CO2 and H2 is of particular interest In which, the reaction to generate CH3OH

is exothermic and reduces the volume, so reducing the temperature and increasing the pressure of the

reaction will shift the equilibrium to form CH3OH (Li and Tsang 2018) The technology to convert

CO2/H2 into methanol has the advantage of reducing the cost of methanol production by 28% compared with the traditional technology from syngas Besides, the application of membrane reactor technology can also reduce production costs by about 20% (Giuliano, Freda, and Catizzone 2020) In addition to the economic benefits, this research direction also has great environmental implications.Ethanol is produced by hydrolysis and fermentation of lignocellulose-containing agricultural wastes (such as rice straw, corn stalks, etc.), from energy grasses or other energy crops (Hoang et al

2021c)(Hoang et al 2019)(Chen et al 2021) The end product is the same as conventional bioethanol, for blending with gasoline and diesel Cellulose ethanol has the same properties as corn ethanol, but because it is created from the residue left on the ground after corn is harvested, this production cycle reduces carbon dioxide emissions by about 210,000 tons annually (Liu et al 2019) Moreover, the second generation of ethanol has escaped competition with the food industry because it uses only agricultural waste and huge sources of wood waste (Bharj, Singh, and Kumar 2020)(Pham, Tran, and Hoang 2018) Indeed, there are three types of ethanol fermentation from lignocellulose including separate hydrolysis and fermentation, simultaneous hydrolysis and fermentation, and consolidated bioprocessing, shown in Figure 2 Separated fermentation and hydrolysis was the first type of fermentation in ethanol production from lignocellulose (Tavva et al 2016) In which hydrolysis and fermentation are operated under favourable conditions, the efficiency of both processes is high On the other hand, for the second method, hydrolysis and fermentation are carried out at the same time in the same apparatus Therefore, the hydrolysis here is usually performed by commercial enzymes, and enzymes are introduced into the apparatus when microbial culture for fermentation (Harris et al

2014) However, because enzymatic hydrolysis and both hydrolysis and enzymatic processes do not take place under optimal conditions, the yield is lower and more time-consuming than hydrolysis and separate fermentation Finally, the process of ethanol production from lignocellulose that has attracted

Figure 2 Pathway for ethanol and butanol production from lignocellulose biomass (Hoang et al 2021b )(Satari, Karimi, and Kumar

2019 ).

much recent interest is consolidated bioprocessing It is a low-cost, environmentally friendly process because the processes take place in the same device and only microorganisms are used to carry out these processes However, because of its multi-tasking nature, almost no microorganism can effec-tively fulfill the tasks of an integrated biological process (Jin et al 2016)(Gełesz et al 2017) Even so, this is a promising trend of the future.

Production of butanol can take advantage of the existing infrastructure of ethanol production The oxo petrochemical process is the most feasible for the cheaper production of butanol from different biomass sources (Nanda et al 2017) The fermentation to produce butanol uses bacteria, while the fermentation to produce ethanol is mainly yeast Butanol fermentation requires less energy, but the product separation scheme is more complex Currently, in the world, there is a lot of infrastructure and many ethanol production plants from cane sugar and grass It would be political and economic to abandon all previous fundamental investments to recreate a new system for new fuel production In the first half of the 20th century, the production of butanol as a solvent, and other chemical applications, mainly used the microbial fermentation of acetone-butanol-ethanol (ABE bacterial fermentation process) In 2015, a joint venture project between BP and Dupont on bio-butanol production by ABE process in China was implemented, which has brought many prospects to promote this new fuel consumption market (Jiang et al 2015)

Besides the current process like ABE or oxo synthesis for biomass, they all announced their pursuit

of improved diversification of biomass sources for biorefineries Compared with other pre-feasibility projects such as the production of ethanol from cellulose, hydrocarbons from biomass, and diesel from algae, bio-butanol production from different biomass sources is the most feasible (Nanda et al 2017) With the advantage of the inherently low-cost oxo synthesis (Kazemi Shariat Panahi et al 2019), when successful in commercializing butanol production from diverse biomass sources, the cost will be lower and will help the market have a more abundant fuel supply in the future Furthermore, alcohols with

a high carbon structure can be produced from coal-derived syngas They can be prepared at efineries with a combination of structural ethanol thanks to the ageing process For example, new biocatalyst-assisted conversion technologies have improved the yield of pentanol production from glucose or glycerol (King et al 2015)

bior-Properties of alcohol additives

The exhaustive information of physicochemical and combustion behaviour of additives has

a contribution to understanding the typical properties of blends It will be straightforward to stand the quality of mixture and characteristics of combustion during the burning process of fuel blend with supporting to outstanding properties of oxygenated chemicals The detailed physicochem-ical properties of different alcohol-based additives are given in Table 1 All properties of fuel additives not only have the role of great importance but also affect to consider the fuel blend combustion process Hence, the following suggestions are recommended in choosing alcohol-based additives for blending with pure diesel fuel or/and biodiesel (Heywood 2011)(Ganesan 2012)(Hadiyanto et al

under-2020)

Viscosity and density of alcohol additives are lower than that of diesel fuel, which preserves these blends in stable and soluble conditions as well as improves additive-fuel blends pumping In blending with diesel fuel, the boiling point of the blended additive plays an essential role in uniform combustion Cetane number and oxygen content of alcohol additives are desired as high as possible to reduce the ignition delays as well as minimize knock chance, resulting in complete combustion of fuel (Hoang et al

2021a) Additives with lower latent heat can support the combustion process of the blend to happen better and quickly Nevertheless, most alcohol additives have a higher value of latent heat compared to diesel fuel Some of the recommendations from engine manufactures reported that a lower heat value of additives is not lower than diesel fuel because specific fuel consumption can rise if the value of this parameter is low Therefore, cost parameters should be considered when selecting an additive Additives with a lower auto-ignition temperature should be kept in mind for proper fuel blend combustion

The oxygen content in alcohol additives plays a significant role in improving the oxygen content of the fuel The increase in the atomic oxygen content of an alcohol additive fuel enhances the combus-tion quality in the combustion chamber It is believed that the increase in ignition capacity and the decrease in ignition delay are a consequence of the decrease in ignition temperature caused by the presence of oxygenated additives in the fuel (Fayyazbakhsh and Pirouzfar 2017)(Wang and Yao 2020) The degree of influence of alcohol additives depends on the atomic oxygen concentration in the compound as well as in the overall blends It is clear that the lower alcohols have significantly higher oxygen content than the higher alcohols, while the low calorific value sees the inverse Moreover, the low calorific value of alcohol additives is lower than that of biodiesel fuels (Le et al 2022) That makes the energy value of the alcohol additive mixture lower in comparison to pure fuel As a result, more amount of fuel with alcohol additives is required to meet the energy balance More clearly, the properties of the fuel such as density, flash point, kinematic viscosity, surface tension, and acidity index, are significantly affected by the content of alcohol additives (Kumar et al 2018) For example, in a study on adding ethanol to the sesame oil-based biofuel, Khalife et al (Khalife et al

2017) noticed a decrease in density, viscosity, and flashpoint in the modified blends On the other hand, with the addition of more OH radicals, there was an increase in the acidity index of the fuel In addition, (Qi et al 2020) combined two main fuels including biodiesel based on castor oil and pure diesel with two saturated alcohol additives including ethanol and n-butanol, resulting in improving key biofuel properties such as density, viscosity, oxygen content and latent heat of tertiary and quaternary fuels Specifically, the oxygen content in the blended fuels was recorded as 6.86%, 8.08% and 12.9% for DC80B20, DC80B10E10 and DC60B20E20 respectively, while the latent heat of vaporization was significantly increased because alcohol additives had a higher latent heat of vapor-ization (585 kJ/kg for n-butanol and 840 kJ/kg for ethanol)

Higher alcohols with a high-carbon structure consist of 5 to 20 carbons, with a high cetane number and high density very close to that of diesel fuel Furthermore, the calorific value of the higher alcohols

is higher than that of methanol and ethanol With hexanol, alcohol is not as water-soluble as n-butanol, so it can be mixed with diesel by the splash method Besides, it can be safer to use hexanol because it is less volatile than n-butanol The oxygen content of hexanol is about 15.7% when mixed

Table 1 Physico-chemical and combustion properties various alcohol additives.

Additive

Molecular

formula

Density (kg/

Viscosity (cSt)

Latent heat (kJ/kg)

Boiling point (°C)

Cetane number

Oxygen content (%)

Lower heating value (MJ/

kg)

Auto- ignition temperature

2020 )

with diesel can promote combustion (Ramesh et al 2019) The disadvantage of hexanol and higher alcohols is the high viscosity Therefore, the choice of blending ratio of long-carbon alcohols up to 20% with diesel should be considered and studied more.

Combustion characteristics of blends with alcohol additives

The presence of alcohol additives in quadratic or tertiary or quaternary blends has changed their properties as well as the combustion characteristics in CI engines The addition of homologous saturating alcohols into diesel and/or biodiesel fuel has reduced the maximum cylinder temperature

In a study by Datta and Mandal (Datta and Mandal 2017), it was shown that methanol and ethanol with the latent heat of vaporization were 1110 kJ/kg and 920 kJ/kg, respectively, higher than pure diesel and biodiesel That created a cooling effect for the fuel-air mixture and resulted in a decrease in cylinder temperature Latent heat of vaporization has been recorded as 410 kJ/kg of PE15, 437 kJ/kg of PM15, and 320 kJ/kg of B100 The maximum temperatures found were 1637.5 K, 1601.6 K, and 1596.8 K for B100, PE15, and PM15 respectively The above results are also recorded and explained similarly by (Zhu et al 2011) and (Cheng et al 2008) The higher latent heat of vaporization of the saturated alcohol additives also explains the maximum pressure drop and the moving tendency for the peak pressure to the right of the top dead centre with PE15 and PM15 blends In addition, the lower cetane number of the blends resulted in a reduced ignition delay as well as a slower initiation of combustion (Hoang, Nižetić, and I.Ölçerc 2020)(Le et al 2021) These results were similarly reflected

in the study by Jamrozik (Jamrozik 2017), the maximum in-cylinder pressure with Diesel-Methanol (DM) and Diesel-Ethanol (DE) blends both revealed a slight reduction compared to pure diesel The reason given by them is the lower calorific value of both methanol and ethanol compared to diesel fuel, the corresponding values are 20 MJ/kg and 27 MJ/kg Moreover, when increasing the content of methanol involved in the DM blends beyond 30%, a disorder of the combustion process was recorded

as well as a sharp decrease in combustion pressure in the cylinder However, no decrease in peak pressure was observed when increasing ethanol contributed to diesel fuel This proved that the addition of ethanol at high rates still maintains a stable combustion quality The addition of alcohol lengthened the ignition delay and increased the heat release rate

Furthermore, the rate of cylinder pressure rise is also strongly influenced by saturated alcohol- based additives The pressure rise ratio is in the range of 3–8 bar/degree, it is noted that the engine is

“soft,” while the ratio of reaching or exceeding 10 bar/degree, the noise and vibration level of the engine is alarming The pressure rise rate of DM15, DM20, and DM25 has been higher than 10 bar/ degree, while DE20 has recorded 12 bar/degree, which was over the allowable limit (Jamrozik 2017) The resulting increase in knock in the engine is thought to stem from the energy density when ethanol

is added Furthermore, the heat release rate for the blends with alcohol-based additives was also similar

to the trend of change in maximum pressure When the ethanol content was higher than 15%, the heat release rate (HRR) of the DE blends was found to be higher than the blends of lower ethanol and pure diesel In terms of ignition delay and burn time in diesel engines, the addition of methanol and ethanol

to diesel fuel has resulted in an increase in ignition delay due to alcohol’s higher latent heat of vaporization with an increased cooling effect and lower ignition temperature Furthermore, saturated alcohols have very low cetane numbers, making them unsuitable for the use of diesel-alcohol blends with an alcohol content higher than 30–40% (Tutak et al 2015)

The role of ethanol and n-butanol additives in tertiary and quaternary blends in improving combustion behaviour in cylinders is prominently shown in the study of (Qi et al 2020) on

a 4-stroke, 6-cylinder CRDI engine with a dual injection strategy Results on the evolution of cylinder pressure and heat release rate revealed that in comparison with pure diesel fuel, the reduction in cetane number extended the ignition delay and shortened the burn duration (BD) with the greater presence

of alcohol-based additives in the blends, resulting in significantly rise peak in-cylinder pressure and HRR Figure 3 depicts the changes in-cylinder pressure and HRR for the four fuels tested Regarding the maximum pressure in the cylinder, at a speed of 1000 rpm and a load of 0.64 MPa, DC60B20E20

recorded the largest value (8.7 MPa) compared to the remaining fuels, while pure diesel was the lowest (8.15 MPa) The trend of HRR change in Figure 3 revealed the role of calorific value and cetane number in HRR behaviour Indeed, fuel components have an impact on atomization, vaporization and mixing through blend properties such as viscosity, density, vaporization temperature and physical- chemical ignition delay mechanism Therefore, the HRR of tertiary and quaternary blends revealed great growth in comparison with neat diesel fuel In addition, the contribution of fatty acid unsatu-rated substances in biodiesel has adversely affected evaporation The time required for vaporization was longer and caused the ignition delay to be prolonged More interestingly, ethanol and n-butanol have higher autoignition and latent heat of vaporization while the cetane number is much lower than that of diesel fuel, leading to further prolongation of the ignition delay However, alcohol-based chemicals have brought with them interesting properties of lower viscosity and density, which stimulate better atomization and mixing, resulting in faster combustion and HRR is higher The results and interpretation of HRR were also consistent with viewpoints in a study by (Labeckas et al

2018) and (Hurtado et al 2019) A study by (Imdadul et al 2016) carried out on TF120M engines with biofuel blends added 10%, 15% and 20% (vol.) pentanol revealed the lower viscosity and higher volatility of pentanol resulted in an increase in the fuel-air mixture in the premixed combustion phase

As a result, for modified blends, the start of combustion is delayed and the peak pressure in the cylinder is higher Thus, the combustion attributes were significantly improved with the participation

of pentanol in the modified blends

In summary, the significant positive effects of alcohol additives on combustion have shown that they can be a promising choice in the search for alternative fuels using CI engines As a result, an improvement in combustion performance can have beneficial effects on engine performance The impact of alcohol-based additives in diesel/biodiesel blends on combustion characteristics and engine performance are discussed in this review, these contents have been summarized and described in

Figure 4

Effect of alcohol additives on engine performance

Various alcohol additives with different physicochemical properties and combustion which able influences on the operating parameters of the engine and characteristics of emission Concerning the purpose of improving the engine performance, the recent studies incorporated diesel/biodiesel fuel with different types of additives at various fuel blending ratios (Subramanian, Chandrasekaran, and Rajesh 2009) Alcohol additives are mainly associated with diesel or biodiesel fuels (Kumar et al 2013)

consider-Figure 3 Change in in-cylinder pressure and HRR with different blends at 1000 rpm and 0.64 MPa (Qi et al 2020 ).

They are more especially attractive for purposes of optimization of engine performance, minimization

of emissions, mileage increase, combustion rate improvement, operating as anti-oxidants, allowing fuels to work under extreme operating conditions, environmental preservation, etc (Hoang, Ölçer, and Nižetić 2020) It is fact that alcohol additives have paid special attention because as they are compared to diesel fuel, they have better combustion behaviour as well as their molecular structure has high available oxygen content (Table 1) The improved combustion characteristics of the blend between additives and diesel lead to engine performance improvement (BTE, BSFC, etc.) and reduc-tion of emissions characteristics (Geng et al 2017)

Methanol is considered the most promising alcohol-based additive candidate because of its highest oxygen content at 50% by weight which promotes a cleaner and more stable combustion Furthermore, the production of methanol can come from renewable and inexpensive sources such

as biomass Therefore, methanol has been piloted on both SI and CI engines On CI engines, methanol can be injected into the cylinders through solutions such as dual injection, mixing, and emulsification (Valera and Agarwal 2019) However, the dissolution of methanol in diesel fuel has encountered many difficulties because the OH group has a large degree of polarity in solutions that do not use co-solvents such as diesel fuel In a study by Jamrozik (Jamrozik 2017), the ability to improve engine performance when increasing the addition of ethanol and methanol to diesel fuel was concluded The indicated thermal efficiency (ITE) of the methanol-diesel blend (DM25) has been increased by about 14.5% compared to pure diesel fuel However, if the increase in the amount of methanol continues, ITE has recorded a sharp decrease, with DM40, ITE has only reached 20%, which is 12% lower than diesel Meanwhile, the increase in ITE of the ethanol-diesel blend was proportional to the increase of ethanol The reason for the decrease in the ITE of the methanol-diesel blend was a significant reduction of the combustion pressure and heat release rate due to the very low cetane number and lower calorific value

in comparison with ethanol Thus, the addition of methanol of no more than 30% had very positive effects on the ITE of the CI engine

Another study by (Agarwal et al 2019) demonstrated a suitable combination of diesel-methanol blend with 1-Dodecanol additive to improve stability of blends as well as BTE, BSEC, and EGT The BTEs of the MD10 and MD15 blends have seen higher values than pure diesel fuels Obviously, the

Figure 4 Effects of alcohol-based additives’ physicochemical properties on combustion and engine performance.

lower cetane number caused to slowed down combustion, moreover, the higher oxygen content promoted more complete combustion In addition, the latent heat of methanol is reported to be higher than that of diesel, which reduces the out-work required in compression These factors have helped the BTE of MD10 and MD15 to be higher On the other hand, the lower calorific value of methanol causes the mixtures to be injected in larger quantities to achieve a power level Indeed, the BSECs of MD10 and MD15 have outperformed pure conventional fuels at different loading condi-tions In addition, the changing trend of EGT has been similar to that of BTE The higher latent heat of methanol has been implicated as the source of the reduction in EGT of MD10 and MD15 compared with diesel.

A numerical study using Diesel RK software was conducted to evaluate the engine characteristics when using biodiesel blend fuels with the addition of methanol and ethanol (Datta and Mandal 2017) The calculation results showed that the addition of ethanol and methanol to the PSME biodiesel fuel significantly improved BTE at higher loading conditions At full load, the BTE of PE15 and PM15 were 0.33% and 0.56% higher than that of PSME fuel The increase in BTE was reported to be due to the increase in oxygen-enriched fuel, the viscosity and density of the mixture decreased with the increase

of the addition ratio of the alcohol-based additive that improved the spray characteristics These factors contributed to favorable conditions for better atomic oxidation, leading to improved combus-tion quality and BTE In addition, BSFC has shown predictive data with similar trends to BTE The BSFC increased the same percentage of alcohol additions Compared with PSME fuel, at full load calculation condition, the increase in BSFC was 0.06 kg/kWh for PE15 and 0.04 kg/kWh for PM15

Figure 5 shows in detail the trend of changes in BSFC of fuels supplemented with methanol (a) and ethanol (b) with brake power The cause was found to be based on the calorific value of alcohol-based additives, with lower energy densities requiring more fuel supply to achieve power balance On the other hand, it can be observed that the calculated data for EGT of blended fuels were lower than pure fuels The reason may have come from the higher latent heat of methanol and ethanol so the cooling effect during the mixing reduced the combustion temperature in the cylinder

Ethanol has been confirmed by researchers and regulators that its application prospects are huge for CI engines However, its poor solubility in diesel fuel remains a major challenge In the study of (Wu et al 2020), a very small amount of Tetrahydrofuran (THF) was added to the ethanol-diesel blend fuel to improve the homogenous solubility of the blends BTE and BSFC were measured on

a heavy-duty, 4-stroke and 6-cylinder diesel engine supplied with four fuels including diesel and three blends (RE0, RE10, and RE30) with 5% THF and ethanol addition ratios of 0%, 10%, and 30% The addition of THF reduced the BE0 fuel viscosity, thus increasing the energy loss during fuel injection Therefore, the BTE of blends with the addition of THF are slightly reduced compared to pure diesel fuel However, the addition of ethanol resulted in a longer ignition delay, better volatility and miscibility, and higher combustion quality Experimental data have shown that there is a slight increase in BTE when blends are supplemented with THF and ethanol In full load mode, the gain of BE10 and BE30 was 0.64% and 0.7% higher compared to BE0 Furthermore, the effect of ethanol on engine torque and power in the study of (Ağbulut, Sarıdemir, and Albayrak 2019) also showed

a similarity in experimental results Diesel and biodiesel blending with 10% vol ethanol such as D90E10, D70C20E10 have recorded a decrease in torque and power compared to pure diesel The calorific value and cetane number of THF and ethanol are lower than that of pure diesel, which has reduced the calorific value and cetane number of the blends That means a larger amount of fuel needs to be delivered through the high-pressure pump to ensure the same amount of work in the cylinder Indeed, the BSFC of BE0, BE10, and BE30 has been reported to be higher than that of pure diesel In addition, the value of BSFC increased sharply with more ethanol addition (Wu et al 2020) However, in a study on optimizing the engine performance parameters when changing the speed and blending content in diesel-biodiesel-ethanol fuel, it was concluded that the participation ratio of ethanol does not seem to affect BSFC (Mirbagheri, Ardebili, and Kiani 2020) In another study by (Wei, Cheung, and Ning 2018), tests were conducted on CI engines with variable load to evaluate BSFC and BTE when adding 5%, 10%, and 15% each of ethanol and butanol to biodiesel BE5, BE10,

and BE15 have more pronounced adverse effects on engine performance than Bu5, Bu10, and Bu15

In general, the BSFC of fuels with alcohol-based additives increased with the increase in the proportion of the additive The main reason has also been confirmed that their calorific value is lower while latent heat is higher Regarding BTE, compared with biofuels, both BE and Bu have shown slight reductions under different loading conditions An experimental study to optimize the parameters of brake specific fuel consumption and thermal efficiency (TE) for 4-stroke, 4-cylinder diesel engines using different fuels including pure diesel and 5 blended fuels of diesel, biodiesel, and ethanol (Krishna et al 2019) The blending ratios of ethanol were 5%, 6%, 7%, 8%, 9% and 10%, respectively, in the BDE fuels The BSFC at full load showed that BDE9 had the largest value, 13.4% higher than diesel The increase in BSFC was observed to be proportional to the increase in ethanol Meanwhile, in terms of TE, BDE6 saw a slight increase compared to BDEopt, however, the rise of ethanol from 7% to 9%, there was a decrease compared to both BDE6 and diesel (Tran, Le, and Hoang 2020)

The review of (Khalife et al 2017) indicated that values of BTE and BSFC are enhanced by adding alcohols (< 10% vol.) into diesel Adelman 1979 and (Wagner et al 1979) presented the reasons for benefit of blending alcohols into the pure fuel deal with the performance such as (1) Due to its viscosity

Figure 5 Change in BSFC of biodiesel blends with methanol (a) and ethanol (b) (Datta and Mandal 2017 ).

are lower than diesel fuel, it has better atomization, injection, and mixing; (2) Its laminar flame propagation velocity is around 0.45 to 0.55 m/s, which is faster than that of diesel (about 0.45 m/s) at

an equivalence ratio of 1.0, which results in the early combustion process and improvement of BTE (Chong and Hochgreb 2011); (3) Engine performance has been improved with conditioned reactions

of the compression ignition process, which was influenced by the cooling mechanism during ization and compression process caused by the higher latent heat of the oxygenated additives (Pan

vapor-et al 2017) Furthermore, a by-product of the fermentation and distillation of ethyl alcohol that has attracted a lot of interest in the search for an alternative fuel is fusel In the study of (Akcay and Ozer

2019), pure diesel fuel and four blends of diesel and fusel were fed to a DICI engine to investigate engine performance such as BSEC and EGT under variable load conditions BSEC data has revealed that it increased with the fusel addition ratio Compared to pure diesel fuel, the BSEC of DF5, DF10, DF15 and DF20 increased by 10.1%, 13.5%, 18.4% and 23.7%, respectively Suitable reasons for those results came from the calorific value and latent heat of the fusel Specifically, with a calorific value

Figure 6 Change in EGT (a) and BSFC (b) along with various loads (Bencheikh et al 2019 ).