Recent advances in the combustion of renewable biofuel diethyl ether: A review

Abstract

Diethyl ether (DEE, C₄H₁₀O) has emerged as a promising renewable alternative to conventional diesel fuels, offering potential solutions for sustainable energy development. This review systematically examines the fundamental combustion characteristics of DEE, including pyrolysis and oxidation behaviors, kinetic modeling, and actual combustion characteristics. It comprehensively summarized the key research progress and main findings in this field. Research has indicated that DEE demonstrates excellent ignition performance, whether used alone or as an additive, and significantly reduces soot formation during combustion by limiting the discharge of C₃–C₄ hydrocarbon species. However, a complete mechanistic understanding of DEE combustion still remains limited by the lack of key coupling reaction pathways, which directly restricted the accuracy of the reaction kinetic model. At the actual combustion level in devices, the effects of DEE on engine performance, combustion behavior, and emissions has been investigated. Although a large number of experiments have confirmed that DEE has a significant improvement effect in the above aspects, certain performance degradation phenomena and their internal mechanism still require further elucidation. Based on these insights, this review also analyzes the key challenges facing DEE in practical applications and discusses possible solutions, aiming to build a complete research framework spanning from fundamental studies to engineering application future development.

Graphical abstract

Keywords

diethyl ether (DEE / C₄H₁₀O) / fundamental combustion / pyrolysis / oxidation / actual combustion

1 Introduction

Due to the depletion of oil and fossil fuel reserves and the significant increase in carbon dioxide (CO₂) emissions and atmospheric pollution, there is growing concern and urgency to explore alternative energy sources and address environmental challenges. Consequently, interest in bio-derived fuels has grown considerably [1–3]. Biofuels can be produced from biodegradable household, industrial, and agricultural waste, as well as from energy crops [3] including biodiesel, bioethanol, biomethanol, and bioliquefied gas.

Diethyl ether (DEE, C₄H₁₀O) is considered to be a promising biofuel that can be synthesized from ethanol with a dehydration process [4,5]. DEE exhibits a high cetane number (> 125) [6,7], which is an indicator of its superior ignition quality. A higher cetane number means better combustion characteristics, leading to improved engine performance. Additionally, DEE has a high energy density (33.9 MJ/kg) [6,7], enabling it to provide more energy per unit mass compared to other bio-derived fuels. This makes it a potentially efficient source of energy. It is noteworthy that DEE has a high oxygen content as a biofuel [6] of 21.6% in the DEE molecule [8], which facilitates more complete combustion and contributes to reduced pollutant emissions.

Furthermore, DEE has a relatively low ignition temperature, which facilitates easy and quick ignition [9]. This property can lead to more reliable cold-start of engines, particularly in low-temperature and high-altitude environments. It also contributes to improved combustion efficiency and reduced emissions during engine operation. In particular, compared to dimethyl ether (DME), DEE has several advantages. With a high cetane number (> 125) and energy density (33.9 MJ/kg) [6, 7], DEE demonstrates superior combustion characteristics compared to DME (28.6 MJ/kg) [10]. Like DME, which is derived from methanol [11], DEE is a renewable biofuel produced through ethanol dehydration [12], offering a sustainable alternative. A key practical benefit of DEE lies in its liquid state under ambient conditions, enabling direct use as a diesel additive without engine modifications [13,14]. This property, combined with full compatibility with existing fuel infrastructure and vehicle technologies, significantly reduces implementation costs and operational barriers.

DEE was proposed as a potential alternative fuel for engines [15] and was usually blended with diesel or biodiesel to improve combustion characteristics, engine performance, and reduce pollutant emissions [11,16]. Moreover, DEE was also utilized as an engine cold-start ignition improver. For instance, it was used to address the cold-start issues resulting from the poor cold-flow properties of biodiesel [12,17] and enhance the cold-start performance of diesel engines under low-temperature and high-altitude conditions [18]. In addition to engine research, fundamental combustion research was also essential, as it could facilitate a molecular-level understanding of the combustion process of DEE or fuel mixtures in the engine, thereby enabling finer combustion control.

To demonstrate the significant development potential of DEE as a renewable fuel in a more comprehensive and detailed manner, this review summarizes recent advances in DEE combustion research. It covers the main research topics and conclusions in the field, including fundamental characteristics and combustion behavior of DEE in laminar flames, pyrolysis, and oxidation properties, ignition analysis in shock tube (ST) and jet-stirred reactor (JSR), development of kinetic models, and practical combustion performance of DEE in direct injection (DI) and compression ignition (CI) diesel engines. An overview of the main research area is presented in Fig. 1. Additionally, key challenges associated with the practical application of DEE are analyzed with a discussion of potential solutions to guide future research and development.

2 Fundamental combustion of DEE

The fundamental combustion research of DEE aims to reveal its combustion characteristics, reaction pathways, and product formation rules under combustion conditions and aids in deeply understanding its combustion mechanism and kinetic characteristics. By optimizing combustion processes, it is expected to improve the fuel utilization efficiency of DEE while reducing energy waste and environmental pollution. Furthermore, this research aids in assessing the potential of DEE as a clean energy source, investigating its application as a replacement for traditional fossil fuels, and fostering the advancement and adoption of clean energy technologies.

The combustion characteristics of DEE include ignition behavior, laminar burning velocity [10,19–24], combustion products, and reaction mechanisms [10,25–31].

2.1 Laminar burning velocity

Laminar burning velocity is a key parameter that influences ignition behavior and is a fundamental parameter in the study of fuels and combustion, drawing significant research interest. It plays a crucial role in combustion modeling and engine simulations, directly influencing the performance and emissions of various combustion systems. Additionally, it serves as a fundamental property of premixed combustible gases, aiding in the validation of kinetic models.

Various flame configurations have been employed to measure the laminar burning velocity of DEE, such as outwardly propagating spherical flames [19–21], the heat flux method in flat flame [22], and the stagnation plane flame method [10]. Among these, the outwardly propagating spherical flames method is the most widely used in the measurement of laminar burning velocity.

Using this method, Di et al. [19] and Zhang et al. [20,21] studied the effects of initial temperature, equivalence ratio, and pressure on the laminar burning velocity, as shown in Fig. 2. Their findings indicated that with the increase in initial temperature, the flame propagation velocity, and laminar burning velocity increased, while the Markstein length decreased. A positive Markstein length indicates increased sensitivity of flame speed to temperature or concentration gradients, while a negative value suggests reduced sensitivity [19,20]. The flame propagation velocity, laminar burning velocity, and Markstein length decreased with the increase of the initial pressure [19–21].

Gillespie et al. [22] introduced a new adiabatic laminar burning velocity for DEE in air and conducted measurements using a flat flame burner with the heat flux method. The adiabatic combustion velocity was found to increase with rising temperatures. Tran et al. [10] employed the stagnation plane flame method to investigate the impact of pressure on laminar burning velocity and found that the laminar burning velocity of DEE decreased with increasing pressure.

DEE was also added to NH₃ flames to improve the reactivity of NH₃. In NH₃ flames, the laminar burning velocity increased with the addition of DEE [23,24]. The higher DEE content in NH₃/DEE blends led to greater flame reactivity and a shorter ignition delay time (IDT) [24], suggesting DEE’s potential as a reactive promoter in low-reactivity fuel systems.

2.2 Combustion species data

The earliest speciation study on DEE was reported in the 1960s by Agnew W G and Agnew J T [25] in a stabilized flat flame for rich DEE/air mixtures. They identified and quantified a number of species including cyclic ether 2-methyl-1,3-dioxolane. Subsequently, Barnard and Cullis [26] conducted the first measurement of species concentration distributions in DEE diffusion flames.

Their analytical results showed that the initial pyrolysis products derived from DEE were acetaldehyde (CH₃CHO), ethane (C₂H₆), ethanol (C₂H₅OH), and ethylene (C₂H₄), which were identified as the initial products of DEE pyrolysis, as identified in R1 and R2. CH₃CHO and C₂H₆ were observed in a narrow region near the center of the wick, whereas C₂H₅OH and C₂H₄ were found in appreciable amounts in all regions of the flame that were examined. The two-dimensional image conducted by Barnard and Cullis [26] provided a clear depiction of the species distributions, clearly indicating that pyrolysis is the dominant process within the inner region of the DEE diffusion flame.

However, these results primarily emphasize that pyrolysis is the dominant process within the internal region of the DEE diffusion flame. Due to the limited availability of species data, the combustion characteristics of DEE were not given much attention. More recently, Tran et al. [10] identified and quantified more than 40 substances such as reactants, products, stable intermediates, and free radicals in premixed flames of DEE/oxygen/argon. They concluded that DEE consumption was mainly controlled by H extraction of H and OH radicals, followed by subsequent fuel radical decomposition of the resulting fuel radicals. They also conducted a detailed reaction pathway analysis, as shown in Fig. 3.

The progression of research on the fundamental combustion characteristics of DEE has evolved from initial species identification to detailed reaction pathway analysis. These studies provided a critical foundation for understanding combustion mechanisms of DEE and highlighted the need for further integration of experimental and simulation approaches to fully elucidate key intermediates and reaction pathways in DEE combustion.

(R1)\begin{array}{c}{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}\mathrm{C}}_2{\mathrm{H}}_5={\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+{\mathrm{C}}_2{\mathrm{H}}_6\end{array}

(R2)\begin{array}{c}{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}\mathrm{C}}_2{\mathrm{H}}_5={\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+{\mathrm{C}}_2{\mathrm{H}}_4\end{array}

By investigating DEE as a single fuel, fundamental insights into its decomposition mechanism during combustion have been gained. However, in practical applications, particularly in internal combustion engines, DEE is commonly used as an additive blended with other fuels. There remains a gap in comprehensive research examining the pros and cons of this fuel blend compared to using DEE as a sole fuel, as well as exploring the co-combustion reaction pathways involved. Addressing this gap is essential for optimizing fuel formulations and advancing the practical deployment of DEE in real-world combustion systems.

2.3 Species formation and soot transition as an additional fuel

Given the favorable combustion properties of n-butane as a hydrocarbon-based fuel, both DEE and its isomer, n-butanol were incorporated into n-butane flames for experimental investigation. Tran et al. [27] added these two oxygenated fuels to n-butane, and found that blending n-butane with DEE significantly reduced the formation of soot precursors (all intermediate products involved in soot formation process, including small molecular hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and reactive free radicals). This reduction was attributed to the primary decomposition pathway of DEE, which primarily released C₁–C₂ hydrocarbons into the system, while n-butane predominantly produced larger C₃ hydrocarbons during decomposition.

Additionally, when these two oxygenated fuels were added to n-butane, there was a notable increase in the formation of toxic carbonyl compounds like formaldehyde (CH₂O) and CH₃CHO. For DEE, the formation of CH₂O and CH₃CHO in DEE-containing mixtures was consistent with previous findings by Tran et al. [10]. Hashimoto et al. [28] discovered variations in the distribution of small species between DEE and n-butanol flames. In particular, higher concentrations of C₃ and C₄ compounds was higher in the n-butanol flame compared to the DEE flame. Conversely, the concentration of C₂ species and H₂ was higher in the DEE flame than in the n-butanol flame. These findings further support the notion that DEE is more proficient at suppressing the formation of soot precursors.

Iso-octane, a key reference fuel used in the determination of gasoline’s octane number and susceptibility to knocking [32], is specifically chosen as the base fuel for these assessments. Zeng et al. [29] conducted an analysis of flame structure and species composition changes resulting from the blending of iso-octane with DEE. Their study revealed that the mole fraction of C₁ and C₂ hydrocarbons increased upon mixing with DEE. The addition of DEE led to an augmented production of CH₂O, concurrently decreasing soot-related species for PAH formation such as acetylene (C₂H₂), propargyl (C₃H₃), and benzene (A1), and soot. The addition of DEE as a reference fuel to iso-octane demonstrated the potential to decrease soot formation. Zhao et al. [30] studied the sooting transition in iso-octane/DEE blends using counterflow diffusion flames. As the blending ratio of DEE increased, the onset of soot transition was consistently delayed. The addition of DEE conferred two significant benefits in soot reduction: directly suppression of PAHs and soot formation through a dilution effect; promotion of C₂H₂ generation, which played a direct role in C₃H₃ formation and further promoted the generation of subsequent PAHs, ultimately facilitating the conversion process of soot under controlled combustion conditions.

The above research shows that DEE as an oxygen-containing fuel has a significant advantage in inhibiting the formation of soot precursors. Future fundamental research should continue exploring the mixed use of DEE and other fuels to further optimize combustion efficiency and reduce pollutant emissions.

Moreover, investigations into the pyrolysis and oxidation behavior of DEE holds significant importance. Such studies improve the understanding of the decomposition and oxidation tendencies of DEE, as well as the molecular structure and characteristics of the fuel and diverse chemical byproducts generated during combustion, thus establishing a foundation for future fuel applications and technological improvements. Furthermore, delving into the pyrolysis and oxidation reactions of DEE is expected to elucidate its combustion mechanism, encompassing reaction pathways, product formation, and kinetic attributes, thereby facilitating a deeper understanding of the fuel conversion dynamics during combustion.

3 Pyrolysis species data

The pyrolysis of DEE is the initial phase of its combustion process and is essential to understanding its overall reactivity and decomposition behavior. The experimental and simulation conditions used in previous DEE pyrolysis studies, including reactor type, pyrolysis pressure and temperature, and research objectives, are summarized in Table 1.

Davoud and Hinshelwood [33] investigated the thermal decomposition of DEE, with a specific focus on quantifying the concentration of acetaldehyde at each phase of the reaction. Based on this, Freeman et al. [34–38] explored several critical aspects, including rate-pressure relationships, the influence of reaction conditions, the use of nitric oxide (NO) as an inhibitor, the formation of cyanides, and the production of ethanol. Laidler and Mckenney [39,40] examined the pyrolysis of DEE, as well as the inhibition of pyrolysis by nitric acid across various temperature and pressure ranges. Seres and Huhn [41] studied the thermal decomposition process of DEE under conditions of low conversion and calculated the decomposition rate constant of DEE at multiple temperatures.

Vin et al. [42] observed the complete breakdown of the reactant at 1080 K with a residence time of 2 s in a jet-stirred reactor (JSR) and identified carbon monoxide (CO), methane (CH₄), C₂H₄, and acetaldehyde (CH₃CHO) as the primary products of the process. The DEE reaction models from Yasunaga et al. [43] and Tran et al. [10] were used for calculation. Both experiments and simulations show satisfactory consistency in fuel conversion and product formation.

Sela et al. [44] measured the rate constant for the monomolecular decomposition of DEE in a shock tube (ST), employing the mechanism developed by Sakai et al. [45] for simulations. They extracted a global rate constant for the single-molecule DEE decomposition in the temperature range of 1185–1310 K and pressure range of 1.2–2.4 bar. The major stable products resulting from DEE pyrolysis included CH₄, C₂H₄, C₂H₆, CH₃CHO, and ethanol (C₂H₅OH).

In addition, Serinyel et al. [46] highlighted the significance of bimolecular reactions in DEE pyrolysis, especially hydrogen absorption reaction processes involving hydrogen atoms and methyl (CH₃) radical. These reactions played a leading role in fuel decomposition and intermediate product decomposition in the JSR environment. A representative reaction pathway analysis for DEE pyrolysis at 1000 K is shown in Fig. 4, showcasing the key decomposition routes and intermediates.

4 Oxidation properties and ignition analysis of DEE

The oxidation data of DEE indicate that the primary focus is on the low-temperature oxidation process of DEE, investigated using various experimental setups such as IDT measured in ST and rapid compression machines (RCMs), as well as the speciation data collected during the oxidation process in JSR.

4.1 IDT

IDT is a fundamental characteristic parameter in combustion studies, providing critical support for both fundamental research and actual combustion research. For fundamental research of the oxidation process of DEE, STs and RCMs offer well-controlled experimental environments where key boundary conditions such as temperature and pressure can be precisely regulated. These IDT data obtained in an ideal environment can be used to verify complex chemical kinetic models, isolating reaction pathways, and decouple complex combustion processes in actual combustion devices.

Currently, there are relatively limited studies that have examined the IDT of DEE using STs [43,47–50] and RCMs [51]. Hence, it is imperative to gather precise ignition data across a diverse range of conditions. Temperature, pressure, and equivalence ratio are important factors affecting IDT. For example, Inomata et al. [47] investigated the IDT of DEE in air and discovered that the mixture exhibited high reactivity, with a consistent decrease in IDT as temperature increased. Yasunaga et al. [43] studied the pyrolysis and oxidation reaction of DEE in ST, observing that, when the DEE concentration was constant, increasing oxygen concentration and pressure both decreased IDT. In addition, their developed chemical kinetic model showed good agreement with experimental data. Werler et al. [48] found that IDT had a strong dependence on temperature and pressure, while Zhang et al. [49] concluded that IDT prolonged with the increase of equivalence ratios and the decrease of pressure.

The key reactions in the ignition process of DEE under high pressure and low pressure were found using the mechanism of Yasunaga et al. [43] and the reaction pathway analysis showed that the consumption of DEE was dominated by H extraction reaction. Uygun [50] investigated the ignition behavior of undiluted DEE/air mixtures using the kinetic model developed by Sakai et al. [45]. The results indicated that the oxidation process of DEE after the incident shock wave could be significant, and even lead to weak and strong distal wall ignition.

DEE as an additive has a significant effect on improving the ignition performance of the fuel. The addition of DEE facilitated an earlier and more uniform combustion process by decreasing IDT, thereby enhancing the combustion efficiency of the mixture. For instance, Drost et al. [51] studied the IDT of CH₄/DEE/air mixture on RCM. They observed that the IDT decreased significantly with the addition of DEE and identified the most sensitive reaction at different temperatures. Similarly, Fikria et al. [52] suggested that the reactivity in PRF95 (95% iso-octane and 5% n-heptane, representative of gasoline) was improved by adding DEE.

4.2 Oxidation species data

JSRs are mainly used to study the concentration distribution of reaction species, reaction mechanism, and kinetic characteristics, exploring the formation and consumption of different species in the reaction process to reveal the complexity and dynamic changes of chemical reactions [53–57]. Serinyel et al. [53] explored the oxidation of DEE in JSR. The analysis of DEE mole fraction profiles, reaction intermediates, and products suggested the presence of robust low-temperature chemistry under high-pressure conditions. Tran et al. [54] conducted a comprehensive analysis of the oxidation product spectrum of DEE and developed a new low-temperature oxidation sub-mechanism of DEE, which provided valuable data for analyzing and understanding the complexity of the reaction mechanism in the important temperature range of this technology. Figure 5 illustrates the reaction pathway analysis for the consumption of DEE using this model.

After that, Tran et al. [55] and Belhadj et al. [56] continued to verify the mechanism [54] in JSR. Belhadj et al. [56] compared the experimental data with the simulation results from Tran’s model [54] (Fig. 6). It can be seen that the model well reflects the mole fraction distribution of fuel, O_2, H₂O, CH₂O, and CO₂. However, the model tended to underestimate the mole fraction of products such as CO, acetic acid, and CH₃CHO. Additionally, the comparison of the simulation results with the experimental results shows that the calculated fuel consumption is too fast in the temperature range of 480–540 K, and the same is true for O₂.

Tran et al. [55] further demonstrated that the rise in pressure significantly enhanced the overall fuel reactivity and changed the formation of species. After adding DEE to n-pentane, the reactivity of n-pentane in the fuel mixture was improved, and the formation of the products was changed. Despite these successes, Belhadj et al. [56] suggested that the model proposed by Tran et al. [54] overestimated the oxidation rate of DEE below 560 K.

The experimental and simulated conditions for DEE oxidation generally involve higher temperatures and pressures compared with those for DEE pyrolysis, as summarized in Table 2. Given the significance of investigating the intermediates formed during DEE oxidation, Demireva et al. [57] proposed a methodology to explore the fundamental photophysics of elusive radicals and unstable closed-shell compounds. This approach enables directly probing and quantification of these intermediates in complex reaction networks.

Liu et al. [58] conducted a comparative study involving five different ethers, including DEE, and proposed a three-stage oxidation reaction pathway for ethers: promotion of oxygen and peroxide absorption by the ether molecule; generation of free radicals by thermal decomposition; and complex oxidation reactions driven by these free radicals.

In addition to investigating the low-temperature reaction of DEE, Di Tommaso et al. [59] introduced density functional theory (DFT) analysis of DEE oxidation to identify the presence of hazardous intermediates, particularly peroxide species.

5 DEE kinetic models

To gain a comprehensive understanding of the chemical kinetics of DEE and to simulate its ignition behavior, researchers have developed various chemical kinetic models. The relevant parameters of these models, including the number of species, reactions, and the highest carbon number of species, are summarized in Table 3. There is a substantial expansion in both the reaction pathways and the number of chemical species involved in these kinetic models. This marked increase demonstrates significant progress in the fundamental understanding of DEE combustion chemistry. Notably, although the largest species of the current kinetic models generally remain limited to C4 species, the considerable augmentation of the reaction network’s complexity and scope provides a more comprehensive and refined representation of DEE combustion processes.

Yasunaga et al. [43] conducted DEE oxidation and pyrolysis experiments in ST facilities under specific conditions. They developed a detailed chemical reaction mechanism for DEE to elucidate its oxidation and pyrolysis kinetics. They believed that the most important reactions of DEE, including the four-center elimination reaction DEE → C₂H₅OH + C₂H₄, C–O bond fission (DEE → C₂H₅O + C₂H₅), and hydrogen atom abstraction by hydrogen atom mainly involving the secondary radical C₂H₅OCHCH₃, which in turn eliminated an ethyl radical to form acetaldehyde. Tang et al. [60] further improved Yasunaga’s model [43] by mostly incorporating critical low-temperature reaction pathways.

Eble et al. [61] proposed a low-temperature mechanism to describe the ignition behavior of DEE, including negative temperature coefficient behavior and two-stage ignition. Sakai et al. [45,62] calculated the single-molecule reaction kinetics of ethylethylperoxide (ROO) radicals, the high-pressure limiting rates for DEE radical single-molecule reaction, and estimated rate constants for DEE hydrogen absorption reaction and OOQOOH single-molecule reactions. Building upon these computations and estimations, they developed a DEE kinetic model that exhibited favorable ignition predictions within the temperature range relevant to engine operation. Based on the DEE mechanism proposed by Sakai et al. [45], vom Lehn et al. [63] conducted a detailed sensitivity analysis and uncertainty quantification of DEE kinetic and thermochemical parameters. They found that the latter had a more significant impact on the predictive accuracy of the model.

Tran et al. [10,54] studied the combustion and oxidation behavior of DEE under both high- and low-temperature conditions. Under high temperature combustion conditions, their study focused on the chemical reaction pathways and intermediate species formation mechanism in the flame, while under low temperature oxidation conditions, their research primarily focused on the reactivity of the fuel and the formation mechanisms of fuel-specific products, particularly highlighting the pathways involved in the generation of peroxides and intermediate species. The DEE module of the model used by Zeng et al. [29] was derived from Tran et al. [10]. Danilack et al. [64] employed electronic structure calculations and transition state theory to determine the rate coefficients of low-temperature ether oxidation reactions and assess the impact of non-Boltzmann reactions on DEE IDT. Serinyel et al. [53] proposed a kinetic mechanism to characterize both low- and high-temperature chemistry of DEE, which effectively captured the experimental data, including IDT and laminar flame speed from literature. Their reaction pathway analysis obtained by this model showed that H extraction is the main process of DEE consumption.

Finally, Duan et al. [65] evaluated four representative DEE chemical kinetic models [45,53,54,63] by incorporating computed high-pressure limiting rate constants and thermochemical data to evaluate their effectiveness in predicting IDT and oxidation characteristics.

6 Actual combustion of DEE in engines

Several studies have investigated the use of DEE in diesel engines, either as a standalone fuel or as a fuel additive. These studies involve the incorporation of DEE into various types of fuels, including diesel [11,66–70], biodiesel [71–76], natural gas [8], liquefied petroleum gas [77,78], the introduction of DEE into binary and even multivariate blends like diesel-ethanol mixture [79,80], diesel-kerosene mixture [81], diesel-biodiesel mixture [71,82,83], and ethanol-biodiesel-diesel (EBD) blends [6]. The biodiesel used in these experiments is derived from a range of feedstocks, including vegetable oils (such as edible oil, soybean oil, cottonseed oil, orange oil), animal fats (like fish oil), and certain waste materials such as waste plastics and tire pyrolysis oil. The primary aim of blending DEE with different fuels is to assess the effects of these DEE-blended fuels on engine performance, combustion characteristics, and emission profiles, while determining whether these blends maintain combustion characteristics comparable to conventional diesel but with low emissions.

6.1 Engine performance

The engine performance is comprehensively evaluated through three key macro-output parameters: brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), and brake specific energy consumption (BSEC). BTE measures the efficiency of the engine to convert chemical energy into useful work [84]. BSFC represents the ratio of fuel consumption to brake power (BP) output [84], while BSEC is a tool for comparing the performance of fuels with different calorific values, calculated as the product of BSFC and heating value of the fuel [84].

A comparative analysis of the engine performance with DEE addition is summarized in Tables 4 and 5. Studies demonstrate that DEE, as a fuel additive, has a significant yet inconsistent optimization effect on engine performance. These effects are mainly regulated by the blending ratio, the basic fuel type, and operating conditions such as engine load. Overall, the addition of DEE tends to increase BTE and reduce BSFC. However, for different fuels, especially biodiesel blends, the degree of improvement after DEE addition varies significantly. These performance discrepancies likely arise from the key role of DEE and the physicochemical characteristics of the base fuels.

For most diesel fuels, after adding DEE, the BTE of the engine increased, while the BSFC and BSEC decreased. The presence of oxygen in DEE has been found to enhance combustion efficiency and ensure more complete fuel burning, thereby reducing the energy consumption [11,69,85]. Additionally, the high volatility of DEE promotes better fuel-air mixing prior to combustion, further enhancing combustion efficiency. However, this improvement is not guaranteed for arbitrary DEE concentrations. Rakopoulos et al. [14] found that when 8% to 24% of DEE was added to diesel fuel, the BTE of the engine was almost unchanged at constant load compared with the corresponding use of pure diesel. Paul et al. [79] also observed that the BTE of the engine increased with 5% DEE addition, but decreased when the concentration was raised to 10% (Fig. 7). Therefore, several studies have proposed the optimal DEE addition in diesel engines, as summarized in Table 4.

In these diesel-DEE blends, the DEE added generally does not exceed 25%, though the content of DEE added by Lee and Kim [86] could be as high as 50%. Their results showed that indicated specific energy consumption and fuel conversion efficiency of diesel and blended fuels remained nearly identical, and DEE did not significantly improve engine performance under both low- and high-content conditions. Unfortunately, they did not conduct further analysis on the blends with such a high DEE content, although the effect of the high content of DEE on engine performance is very desirable.

Research on biodiesel fuels [12,87–89] derived from sources such as vegetable oil, animal fats, and waste pyrolysis oil is summarized in Table 5. Most studies [8,12,16,90,91] have shown that adding DEE to various biofuels could increase BTE and reduce BSFC and BSEC as expected, attributed to the combined effects of physical and chemical properties of DEE. After the addition of DEE, on the one hand, the viscosity, density, and calorific value of biofuel-DEE decreased, thereby improving the atomization of the fuel to enhance effective combustion. On the other hand, due to the increase of oxygen content in DEE, the fuel is more completely burned in the fuel-rich zone. However, Babu and Rao [92] reported contradictory results: after adding 3%–15% of DEE to Mahuva methyl ester, the BTE of engine decreased and the BSFC increased in most equivalent cases.

When DEE is added to binary or even multi-component fuels, it is expected that the engine performance improves, consistent with the findings discussed earlier regarding the addition of DEE to diesel and biodiesel. However, studies conducted by Yesilyurt and Aydin [5] and Venu and Madhavan [6] obtained contradictory results. With 3%–15% DEE addition to Mahuva methyl ester, Yesilyurt and Aydin [5] observed a decrease in BTE and an increase in BSFC. They suggested that the addition of DEE had a significantly negative impact on the brake specific consumption value. Venu and Madhavan [6] found that the BSFC of the engine increased by adding 5%–10% DEE to the EBD fuel, attributed to the ignition delay period decreased due to the higher cetane number of the blend, which greatly reduced the reaction time with air. The resulting heterogeneous mixture reduced the flammability and increased the fuel demand to maintain a constant engine speed.

Notably, the effects of DEE addition depend on both engine load and the DEE concentration. At high loads, particularly full load, DEE blending significantly improves BTE. However, engine performance does not increase linearly with DEE content, and a universally optimal DEE blending ratio remains undefined. Basha et al. [70] discovered that the BTE of all blends increased at higher loads after adding 2%–4% DEE. The addition of DEE enhanced BTE due to the presence of oxygen in DEE, which facilitated complete fuel combustion. At lower loads, the BTE of all diesel-DEE blends was nearly identical and only slightly increased with the increase in DEE concentration in diesel. Mallikarjun et al. [93] observed that the maximum BTE reached approximately 32.24% at 5% DEE addition which was higher than the case with no DEE addition. As the DEE addition increased beyond this concentration, the BTE decreased. At 20% DEE addition, the BTE was even lower than that of pure diesel. These results suggest that there may be an optimal DEE addition level, beyond which engine performance began to deteriorate.

6.2 Combustion characteristics

Combustion characteristics describe intrinsic features of the in-cylinder combustion process. Two key parameters, IDT and combustion duration (CD), are commonly used to evaluate the combustion behavior of the fuel in engines. IDT is defined as the interval between the start of injection and the start of combustion (SOC) measured in crank angle (CA) degrees [6,80,89]. CD is defined as the time interval between the SOC and the end of combustion, also in CA degrees [6]. The impacts of DEE addition on IDT and CD in blended fuel engines are summarized in Table 6.

Although the IDT was discussed in Section 4.1, its behavior in practical combustion devices differs significantly from that observed in idealized oxidation environments. Notably, these two research domains complement each other. Investigations in actual applications can verify and improve these kinetic theories, thereby guiding combustion system optimization and engine parameter design. This synergistic interaction between fundamental research and engineering applications creates a feedback loop that advances IDT studies.

As shown in Table 6, most studies [4, 120, 121] show that the addition of DEE prolongs the IDT after blended with diesel. Based on this result, Clothier et al. [120] suggested that DEE might chemically interact with aromatics in diesel fuel, delaying the onset of ignition. While Cinar et al. [4] believed the high vaporization specific heat of DEE resulted in a cooling effect on the inlet temperature, which in turn increased the IDT. Clothier et al. [120] acknowledged the influence of physical factors but believed that physical constraints were less important than chemical factors in determining the effect of additives on the ignition delay period under their experimental conditions. On the contrary, Banapurmath et al. [69] revealed the IDT was shortened with the addition of DEE. Unfortunately, they did not explain the phenomenon. Compared with ethanol, DEE-ethanol blends accelerated combustion due to the improved flammability of DEE [122].

The addition of DEE to various renewable biodiesel types also exhibits mixed effects on IDT, as evidenced in Table 6, with both prolongation and reduction observed. Rakopoulos et al. [123,124] found that compared with pure cottonseed oil, when these cottonseed oil-DEE blended fuels were used, the fuel injection pressure diagram was delayed, the dynamic injection timing was reduced, and the IDT was increased. Given the reason for this phenomenon, Yesilyurt and Aydin [5] attributed this to the addition of DEE to increase the cetane number and latent heat of vaporization of the test fuel, while Jeevanantham et al. [75] concluded that this was a reflection of the cooling effect of ethers rather than the longer IDT period caused by the cetane index. On the other hand, several researchers [12,74,89,105,110] reported that adding DEE reduced the IDT of biofuel-DEE fuel due to the higher cetane number of DEE. In addition, Geo et al. [101] also believed that DEE had the characteristics of high cetane number, low spontaneous combustion temperature, good atomization, and ignition performance, which together led to the reduction of IDT.

The CD could also be controlled to a certain extent by adjusting the blending ratios of fuel. It is generally accepted that DEE addition reduces CD [6,12,89,101,122] as shown in Table 6. For example, Venu and Madhavan [6] explained that DEE improved the latent heat of vaporization and volatility of the mixture, and formed several ignition centers in the combustion chamber, thereby shortening the overall mixing and reaction time and reducing the CD (Fig. 8).

6.3 Emission characteristics

The emission characteristics of the engine refer to the exhaust emissions generated during operation, mainly including NO_x, CO, CO₂, hydrocarbons (HCs), smoke, and particulate matter (PM). Smoke result from incomplete combustion, while PM forms due to insufficient oxygen in fuel-rich zones during heterogeneous combustion [6]. After adding DEE to diesel or blended fuels, the engine emission of NO_x, CO, CO₂, HC, smoke, and PM exhibited notable variations. A comparative analysis of the emission characteristics of the engine running on DEE-blended fuels is presented in Tables 7 and 8. Studies indicate that a DEE blending ratio of 5%–15% produces optimal emission effects. However, like engine performance, emissions are also significantly affected by fuel types and operating conditions, such as engine loads and BP.

Most researchers agree that the adding of DEE reduces NO_x emissions. This reduction is attributed to DEE’s characteristics, such as lower calorific value [69,105] compared to diesel, lower combustion temperature [13], and high latent heat of vaporization [82,89], which together led to a decrease in flame temperature [14], thereby inhibiting NO_x formation. In addition, the high cetane number of DEE is also believed to contribute to NO_x reduction. Conversely, some studies [12,86] suggest that the increased availability of oxygen from DEE addition resulted in a more complete combustion of the fuel-air mixture and a larger volume of combustion gas, leading to higher levels of NO_x in the high-temperature region during combustion. Another explanation attributes increased NO_x emissions to the addition of DEE, which shortened the IDT [79,86].

Higher CO emissions is a signal of poor combustion efficiency. Most studies report that CO emissions are reduced after adding DEE compared to pure diesel [8,12,83,113,114]. Barik and Murugan [12] speculated that the injection of DEE improved the ignition timing, reduced the IDT, and allowed more time for fuel oxidation. In addition, the port injection of DEE contributed to its proper mixing with the air-biogas mixture, forming many ignition centers in the combustion chamber, and lowering CO emissions. Nevertheless, introducing DEE might also potentially worsen CO production [90,91,116]. Nanthagopal et al. [116] believed that this was attributed to the fact that the interaction between DEE and diesel aromatics delayed the spontaneous combustion of fuel particles, and due to the high latent heat of vaporization, the cooling effect caused by DEE affected the complete combustion, resulting in higher CO emissions.

Regarding CO₂ emissions, findings are mixed. Some researchers [5,75,116] found that CO₂ emissions decreased after adding DEE, while the results of others were the opposite [6,89]. Jeevanantham et al. [75] proposed that this decreasing phenomenon was attributed to the high evaporation heat of DEE, leading to the slow mixing of fuel components and incomplete combustion. Yesilyurt et al. [5] reported that the lower carbon atom content in DEE compared with that of pure biodiesel and diesel was the reason for the lower CO₂ emissions generated during the entire combustion process. However, it was worth noting that their experiments [5,75] maintained the total fuel volume unchanged, leading to a decrease in the total mass of the fuel mixture upon the addition of DEE. Consequently, a reduction in total mass may also result in a decrease in CO₂ emissions. On the contrary, Tudu et al. [89] and Venu and Madhavan [6] speculated that the presence of oxygen in DEE enhanced the combustion of the fuel mixture, leading to more CO₂ emissions.

Interestingly, CO and CO₂ emissions often show opposite trends [89,97,105,107,110], though some studies indicate that CO and CO₂ emissions increased and simultaneously decreased following the addition of DEE [5,6,75,88,108]. Venu and Madhavan [6] delineated the impact of DEE on CO and CO₂ separately, attributing it to the alteration of fuel injection characteristics, oxygen content, oxidation rate, in-cylinder temperature, and ignition center formation, thereby influencing CO formation. Meanwhile, the generation of CO₂ is associated with the carbon element, hydrogen/carbon ratio, mixture density, total effective oxygen, and other factors during combustion. The debate on the role of DEE in the combustion process of blended fuels and how it affects the generation of CO and CO₂ requires further study and more in-depth discussion.

The origins of unburned HC within the engine cylinder are varied, and theoretical research still faced certain challenges [14]. Two conflicting views exist regarding the impact of adding DEE on HC emissions. Anand and Mahalakshmi [68] attributed increased HC emissions to the ignition delay caused by the addition of DEE, whereas Cinar et al. [4], Purushothaman and Nagarajan [99], and Imtenan et al. [84] considered the retention of fuel in the combustion chamber crevices as the primary cause. Meanwhile, Kannan and Marappan [103] and Sivalakshmi and Balusamy [104] proposed that the high latent heat of evaporation of the DEE mixture resulted in slower evaporation, leading to poor fuel-air mixing and lower combustion temperature. Additionally, Sivalakshmi and Balusamy [104] also claimed that this phenomenon might be attributed to the delayed entry of residual fuel trapped in the nozzle capsule volume into the cylinder, facilitated by the addition of ether, which enhances fuel evaporation and entry into the cylinder.

On the contrary, Basha et al. [70] believed that the presence of DEE promoted the oxidation of HC [126] and significantly reduced the combustion activation temperature of carbon. Due to the combined effect of secondary atomization and micro-explosion effect, DEE, as an ignition improver [77], forms multiple ignition cores [101] in the combustion chamber. In this case, the fuel-air mixture may contain significant DEE and tiny water droplets, allowing for more complete combustion. Dinesha et al. [118] also observed that emulsified fuels containing DEE exhibited reduced HC formation, attributed to the combined effects of DEE’s higher cetane number, increased volatility, and internal oxygen content.

The addition of DEE is expected to decrease smoke and PM emissions. Mohanan et al. [67] identified an optimal DEE addition level of 5%, where DEE acted as an ignition enhancer, improving diesel combustion. However, in mixtures with a high DEE concentration, the engine emitted more smoke due to incomplete diesel combustion. This issue might arise from insufficient diesel combustion caused by phase separation within the mixture. Similarly, studies by Subramanian and Ramesh [126] and Iranmanesh et al. [98] also indicated that incorporating DEE resulted in decreased smoke emissions. The oxygen in DEE facilitated the oxidation of generated PM, thereby reducing PM emissions when DEE was introduced to diesel engines. Mohebbi et al. [80] agreed that the higher cetane number of DEE and its superior overall fuel-air mixing capability contribute to improving the combustion and lowering emissions.

In addition to the previously discussed impact of DEE on fuel emissions, three key factors deserve emphasis: the characteristics of biofuels, the quantity of DEE added, and the influence of engine load on the effect of DEE. Typically, DEE is introduced into biofuels to compensate for their shortcomings such as high ignition temperature and low cetane number, enhancing biofuel performance while limiting carbon emissions. Consequently, numerous studies have compared the performance of biofuel-DEE blends with that of diesel [127]. Nevertheless, the emission characteristics of various biofuels may exhibit a different trend after being blended with DEE compared to before blending. Moreover, the spray and atomization characteristics of DEE blended fuel are important factors affecting the combustion process and emission performance of the engine. Zhan et al. [128] found that the evaporation characteristics of DEE/gasoline blends were significantly improved under subcritical and supercritical conditions, which helped to improve combustion efficiency. Yao et al. [129] further revealed the characteristics of DEE/iso-octane blended fuel at the near-field spray tip, indicating its potential as a gasoline compression ignition fuel. Mohan et al. [130] analyzed the spray characteristics of DEE and DME using numerical simulation, and found that the high volatility of DEE helped accelerate droplet breakup and evaporation. These findings have shown that the spray characteristics of DEE mixed with gasoline, diesel or biodiesel have significant differences under different conditions, which plays an important role in improving engine performance and reducing emissions. Therefore, it is crucial to conduct further research on the properties of different biofuels, rather than solely focusing on DEE.

The quantity of DEE added and the engine load have once again been demonstrated to be critical factors. Even with the same DEE content, the characteristics of blended fuel may vary. The underlying reasons for this outcome and whether there exists an optimal amount of DEE addition warrant further discussion. Varied engine loads can lead to contrasting fuel emission outcomes. Banapurmath et al. [69] found that under low load conditions, despite DEE possessing a higher cetane number, its evaporation latent heat was marginally higher than that of diesel, resulting in inadequate vaporization and insufficient time for complete fuel combustion, consequently leading to a notable increase in CO emissions. Conversely, at high loads, there was ample time for combustion, improved mixing, and inherent fuel oxygen content that promote complete combustion, reducing CO emissions. At full load, CO emissions from different fuels showed little difference.

7 Critical challenges in practical deployment

Although the fundamental combustion characteristics and engine performance of DEE have been systematically established in Sections 3 to 6, several technical and infrastructure challenges remain before transitioning from laboratory research to practical applications. This section discusses the key challenges related to fuel property limitations, production pathway sustainability, and emission control.

Combustion challenges due to DEE fuel characteristics: First, the flash point of DEE (−41 °C [119]) is significantly lower than that of diesel (49 °C [119]), and its high volatility leads to a sharp increase in the risk of fire and explosion during storage and transportation. Consequently, existing fuel infrastructure, including gas station storage tanks and vehicle fuel tank explosion-proof standards, requires upgrading. Moreover, although DEE has excellent low-temperature fluidity, its latent heat of vaporization (356 kJ/kg [125]) is higher than that of diesel (250 kJ/kg [125]), which may lead to a sudden drop in cylinder temperature during cold start and affect combustion stability. To mitigate this, adding chemical stabilizers or blending DEE with more stable fuels (such as ethanol, with a latent heat of 840 kJ/kg [112]) may be necessary to reduce combustion instability while maintaining its high energy density.

Sustainability challenges of DEE as a biofuel production pathway: The International Energy Agency classifies DEE as a biofuel because it is produced from biomass (e.g., corn or sugarcane) through ethanol conversion and subsequent dehydration, thus meeting synthetic biofuel standards [132]. However, if DEE is synthesized through fossil fuel-intensive agricultural practices or energy-demanding ethanol dehydration processes, its lifecycle greenhouse gas emissions may significantly offset its potential environmental benefits. This raises concerns about whether DEE can consistently deliver meaningful emission reductions compared to conventional fossil fuels or more sustainable biofuel alternatives.

Controversial impacts of DEE emission characteristics: As detailed in Section 6.3, research on the impact of DEE addition on engine emissions yields contradictory results. While some studies report reductions in NO_x and HC emissions, others observe increases in these pollutants. This apparent paradox suggests complex, nonlinear interactions between the combustion chemistry of DEE and engine operating conditions that remain poorly understood. Moreover, if emissions deteriorate in practical applications, exhaust aftertreatment systems may be required to meet emission standards.

In summary, the application of DEE as an alternative fuel still faces key challenges such as combustion stability, production sustainability, and emission control. To enable its transition from laboratory research to industrial application, efforts should focus on fuel blend optimization, life cycle carbon management, and coordinated post-processing technology advancement.

8 Conclusions

This study presents a systematic review of research progress on DEE as a promising alternative fuel in combustion applications, comprehensively examines its fundamental combustion characteristics, pyrolysis and oxidation characteristics via STs and JSRs, as well as its practical combustion performance in diesel engines.

The innovation of this study lies in constructing, for the first time, a multi-scale research framework system of DEE from molecular scale reaction kinetics to actual combustion in engine, thereby effectively integrating fundamental research with engineering applications. Simultaneously, it systematically identified the key challenges involved in transitioning DEE from laboratory research to engineering application. This research framework not only deepens theoretical understanding of the combustion characteristics of DEE, but also lays a scientific foundation for its practical development.

The main conclusions are as follows:

1) Fundamental combustion of DEE: Laminar burning velocity, an essential parameter affecting ignition, and the combustion species used to study the reaction mechanism of DEE have been thoroughly investigated. Results show that laminar burning velocity increases with temperature and decreases with pressure. Species analysis indicates that the decomposition of DEE mainly occurs at the α-C position, and the subsequent decomposition primarily generates C2 species. When blended with n-butane or iso-octane, DEE leads to a reduction in soot precursors, as it mainly releases C1–C2 species during combustion.

2) Pyrolysis and oxidation characteristics: Studies of the pyrolysis products and oxidation products of DEE at different temperatures and pressures, as well as IDTs under various temperatures, pressures, and equivalence ratios in oxidation conditions, have yielded extensive data critical for further research. It is found that the ignition delay time of DEE strongly depends on temperature and pressure, shortening with an increase in temperature and pressure, but lengthening with the increase of equivalence ratios. Adding DEE as a fuel additive promotes combustion efficiency and shortens the ignition delay time. A dynamic mechanism for DEE simulation has also been developed.

3) Diesel engine applications: DEE is often used as an additive in diesel engines to evaluate its impact diesel, biodiesel, and mixed fuels. Most studies show that DEE addition improves BTE and reduce BSFC and BSEC. It also shortens IDT and CD. Regarding emissions, DEE incorporation has been proven to lower NO_x, CO, smoke, and PM.

Therefore, the combustion characteristics of DEE deserve further investigation to support its widespread practical implementation in combustion systems. Future research could explore the potential of DEE in combustion from the following perspectives to yield significant advancements:

1) Knowledge gaps in DEE blended fuel combustion: Current fundamental research focuses primarily on the initial decomposition and the data of DEE decomposition species. However, as DEE is frequently utilized as an additive, there is a lack of studies regarding the common reaction pathways of DEE blended fuels, especially regarding how DEE impacts species formation and consequently influences the mechanism of soot formation. More detailed quantitative species analysis is needed. In addition, since DEE has a molecular structure similar to DME, further study is required to determine whether the addition of DEE in fuels (especially C₂H₄) consistently yields positive effects and further explore the optimal blending ratios.

2) Kinetic modeling: Existing DEE kinetic models typically cover species ranging from C4 to C8, and often require coupling the DEE mechanism with base fuel mechanisms. This restricts their use in studying reactions involving species with higher carbon numbers, leading to a lack of understanding regarding the impact of DEE addition on emissions generation, such as soot formation. Moreover, coupling models of DEE with that of a base fuel significantly complicates the simulation process and raises challenges in determining the reasonableness of the coupled model. Therefore, high-precision kinetic models are urgently needed to support experimental research, elucidate reaction mechanisms, and enable detailed analysis.

3) Fuel blend characterization: In practical applications of DEE in diesel engines, DEE is commonly blended with diesel and biodiesel. However, given the diversity in biodiesel compositions available, it is inappropriate to directly compare combustion characteristics of DEE and biodiesel blends with those of diesel. Therefore, before evaluating the effect of DEE on combustion characteristics, it is necessary to clearly define the specific properties of the biodiesel used (such as composition, oxidation stability) to ensure a comprehensive and accurate evaluation and comparative analysis of the combustion performance of the blended fuel.

4) Nonlinear combustion effects: Numerous studies indicate that the effect of DEE addition on the combustion process of the engine is nonlinear. The optimal blending ratios is primarily from 5% to 20%. Factors influencing such as engine load require further investigation, as does the specific mechanism underlying this nonlinear behavior.

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