A review study on using diethyl ether in diesel engines: Effects on fuel properties,injection,and combustion characteristics

Abstract

This study was compiled from the results of various researches performed on using diethyl ether as a fuel or fuel additive in diesel engines. Three different techniques are used, the reduction of the harmful exhaust emissions of diesel engines. The first technique for the reduction of harmful emissions has improved the combustion by modification of engine design and fuel injection system, but this process is expensive and time-consuming. The second technique is the use of various exhaust gas devices like catalytic converter and diesel particulate filter. However, the use of these devices affects negatively diesel engine performance. The final technique to reduce emissions and also improve diesel engine performance is the use of various alternative fuels or fuel additives. The major pollutants of diesel engines are nitrogen oxides and particulate matter. It is very difficult to reduce nitrogen oxides and particulate matter emissions simultaneously in practice. Most researches declare that the best way to reduce these emissions is the use of various alternative fuels i.e. natural gas, biogas, biodiesel, or the use of fuel additives with these alternative fuels or conventional diesel fuel. Therefore, it is very important that the results of various studies on alternative fuels or fuel additives are evaluated together for practice applications. Especially, this study focuses on the use of diethyl ether in diesel engines as fuel or fuel additive in various diesel engine fuels. This review study investigates the effects of diethyl ether on the fuel properties, injection, and combustion characteristics.

Keywords

Diesel engine performance exhaust emissions fuel additives diethyl ether

Introduction

Diesel engines are widely used in both light and heavy-duty vehicles.¹ They are reliable, robust, and the most efficient internal combustion engines.² However, diesel engines suffer from their high emissions and drawbacks like particulate matters (PM), total gaseous hydrocarbons (THCs), nitrogen oxides (NOx), sulfur oxides (SOx), and smoke.^3,4 It seems that the most suitable way to reduce these emissions is the use of alternative fuels, especially made from renewable sources instead of the conventional fuels.⁵ However, complete replacement of fossil fuels with the renewable alternative fuels will require a comprehensive modification of the engine hardware and their combustion in the engine results in operational and technical limitations.⁶ The fuel side modification techniques such as blending, emulsification, and oxygenation are the easy ways for emission reduction without any modification on the engine hardware. Modification of diesel fuel to reduce exhaust emission can be performed by increasing cetane number, reducing sulfur content, reducing aromatic content, increasing fuel volatility, and decreasing fuel density to have the compromise between engine performance and engine-out emissions. One such change is the possibility of using diesel fuels with oxygenates.⁷ Among different alternative fuels, oxygenated fuel is a good alternative. Diethylene glycol dimethyl ether (DGM), dimethoxy methane (DMM), dimethyl ether (DME), methyl tertiary butyl ether (MTBE), dibutyl ether (DBE), dimethyl carbonate (DMC), methanol, ethanol, and diethyl ether (DEE) have played their role to reduce diesel emissions.^7–9 These fuels can be used as an additive with conventional diesel fuel or pure. These additives can also be used with biodiesel (BD) fuels.¹⁰ Among these oxygenates, DEE is a suitable fuel for diesel engines because it is a cetane improver besides an oxygenated fuel.¹¹ The presence of oxygen in the fuel molecular structure plays an important role to reduce PM and other harmful emissions from diesel engines by improving the combustion characteristics. However, NOx emissions can be reduced in some cases and it can be increased depending on the engine-operating conditions.^12,13 It is very important that the results of various studies on DEE are evaluated together for practice applications, while each study in the literature evaluates the only own results. Therefore, this review study aims to investigate the effects of using DEE in diesel engines in detail. The effects of DEE on the fuel properties, injection, and combustion characteristics are examined, because the engine performance and exhaust emissions strongly depend on the fuel properties, injection, and combustion characteristics. Thus, it can be possible for the development of new technologies by the enrichment of existing literature.

Properties of DEE

As shown in Figure 1, DEE is the simplest ether expressed by its chemical formula CH3CH2–O–CH2CH3, consisting of two ethyl groups bonded to a central oxygen atom.

Figure 1.

Diethyl ether chemical composition.³

DEE is regarded as one of the promising alternative fuels or an oxygen additive for diesel engines with its advantages of a high cetane number and oxygen content. DEE is liquid at the ambient conditions, which makes it attractive for fuel storage and handling. As seen in Figure 2, DEE is produced from ethanol by dehydration process, so it is a renewable fuel.¹⁴

Figure 2.

Production of diethyl ether from ethanol.¹⁴

As shown in Table 1, DEE has several favorable properties, including exceptional cetane number, reasonable energy density, high oxygen content, low autoignition temperature, and high volatility. Therefore, it can assist to the improvement of engine performance and reducing the cold starting problem and emissions when using as a pure or an additive in diesel engines.^14,15 There are some challenges with DEE such as storage stability, flammability limits, and lower lubricity. Storage stability of DEE and DEE blends is of concern because of a tendency to oxidize, forming peroxides in storage. It is suggested that antioxidant additives may be available to prevent storage oxidation. Flammability limits for DEE as seen in Table 1 are broader than those of many fuels, but the rich flammability limit of DEE is in question.¹⁴

Table 1.

The main fuel properties of diesel fuel and diethyl ether (DEE).¹⁵

Property	Diesel	DEE
Chemical formula	C_xH_y	C₄H₁₀O
Molecular weight	190–220	74
Density of liquid at NTP^a (kg/L)	∼0.84	0.71
Viscosity at NTP^a (cP)	2.6	0.23
Oxygen content (wt %)	–	21
Sulfur content (ppm)	∼250	–
Boiling temperature (°C)	180–360	34.6
Autoignition temperature in air (°C)	315	160
Flammability limit in air (vol %)	0.6–6.5	1.9–9.5
Stoichiometric air–fuel ratio (AFR_s)	14.6	11.1
Heat of vaporization at NTP^a (kJ/kg)	250	356
Lower heating value (MJ/kg)	42.5	33.9
Cetane number (CN)	40-55	125

NTP: normal temperature and pressure.

Studies on DEE in literature

There are a number of studies in the literature on the use of DEE in diesel engines as a fuel or a fuel additive. For example, as pure,¹⁶ with diesel fuel,^17–32 with diesel–ethanol blends,^33–40 with diesel–ferric chloride blends,⁴¹ with diesel–kerosene blends,⁴² with diesel–acetylene gas dual fuel,⁴³ with biogas (BG),⁴⁴ with liquefied petroleum gas (LPG),⁴⁵ with diesel–natural gas dual fuel,⁴⁶ with ethanol,^47,48 with various BD fuels,^49–68 with BG–BD blends,⁶⁹ with water–biodiesel emulsion (WBE) fuel,⁷⁰ with various BD–diesel blends,^71–109 with ethanol–biodiesel–diesel (EBD) blends,^110–113 and methanol–biodiesel–diesel (MBD) blends.¹¹³

Effects of DEE on fuel properties

The fuel properties significantly affect the injection and combustion characteristics and these are very effective on engine performance and exhaust emissions. Therefore, investigation of the effects of DEE on fuel properties is very important. Numerical values about DEE addition on the fuel properties for various fuels and blends fuels are tabulated in Table 2. The fuel properties i.e. density, viscosity, flash point, fire point, cloud point, pour point, and heating value generally decreases, but cetane number increases with DEE addition as seen in Table 2. Rakopoulos et al.¹⁷ performed the preliminary evaluation tests on the solubility of DEE in the diesel fuel with blending ratios up to 30% proved. They declared that the mixing was excellent with no phase separation for a period of many days, so emulsifying agent or solvent was not necessary. Rakopoulos et al.¹⁸ declared that addition of a low viscosity fuel such as DEE or ethanol to diesel fuel can reduce lubricity and create potential wear problems in sensitive fuel pump designs. Therefore, low percentage ratios can be applied in the blends due to the reduction of lubricity and the effect of reduced viscosity on the spray. Patil and Thipse¹⁹ stated that DEE was completely miscible with diesel in any proportion. The oxygen content and cetane number of the blends increased, while the density, kinematic viscosity, and calorific value of the blends decreased with the increasing DEE concentration. Miscibility tests were also carried out by Lee and Kim²³ to check the solubility of DEE in diesel, and no phase separation was observed in the blends for a long time.

Table 2.

Effects of diethyl ether on fuel properties.

Base fuel + Additive	Density (variation%)	Kinematic viscosity (variation%)	Flash point (variation%)	Fire point (variation%)	Cloud point (variation%)	Pour point (variation%)	Cetane number (variation%)	LHV (variation%)	Ref.
D + 24% DEE	↓ 3.2	–	–	–	–	–	–	–	[¹⁸]
D + 15% DEE	↓ 2.3	↓ 31.8	–	–	–	–	↑ 21.1	↓ 4.7	[¹⁹]
D + 5-10% DEE	↓ 2.8–2.9	–	–	–	–	–	–	↑0.4–0.5	[²⁴]
D + 5-15% DEE	↓ 0.8–2.5	–	–	–	–	–	–	↓ 0.8–2.4	[²⁵]
D + 5-10% DEE	↓ 2.2–6.7	↓ 33.3–62.6	↓ 17.4–23.9	↓ 19.2–28.8	–	–	↑ 8.2–12.6	↑ 0.4–0.7	[²⁶]
D + 10-20% DEE	↓ 0.7–2.2	↓ 31.4–53.5	–	–	–	–	–	↓ 6.1–7.6	[²⁷]
D + 10% E + 5-15% DEE	↓ 1.3–2.3	↓ 21.2–36.9	–	–	–	–	–	↓ 6.5–11.4	[³³]
D + 5-10% DEE	↓ 0.9–1.7	–	–	–	–	–	↑ 9.7–17.3	↓ 0.9–1.8	[³⁷]
D + 5% E + 5-10% DEE	↓ 1.3–2.2	–	–	–	–	–	↑ 5.4–13.1	↓ 2.6–3.5	[³⁷]
D + 10% E + 5-10% DEE	↓ 1.7–2.6	–	–	–	–	–	↑ 8.8–13.9	↓ 5.0–5.3	[³⁷]
D + 15% DEE	↓ 2.5	–	↓ 26.0	–	–	–	↑ 32.0	↓ 2.9	[⁴¹]
D + 2-25% DEE	↓ 0.3–3.7	↓ 14.3–48.9	–	–	–	–	↑ 2.8–35.1	↓ 0.6–7.9	[⁴²]
D + 5-15% K + 15% DEE	↓ 2.4–3.0	↓ 34.7–47.3	–	–	–	–	↑ 18.0–20.0	↓ 4.9–5.2	[⁴²]
30-50% E + 70-50% DEE	↑ 3.2–5.3	–	–	–	–	–	–	↓ 6.5–10.7	[⁴⁷]
COBD + 20% DEE	↓ 3.9	–	–	–	–	–	–	↓ 1.6	[⁵¹]
KaOBD + 5-15% DEE	↓ 2.3–5.7	–	–	–	–	–	–	↑ 2.5–7.5	[⁵⁴]
KuOBD + 5-15% DEE	↓ 2.0–3.4	↓ 5.4–15.2	↓ 6.4–19.3	–	–	–	–	↓ 0.5–1.4	[⁵⁵]
NOBD + 5-15% DEE	↓ 0.9–2.6	↓ 6.7–20.6	↓ 10.3–38.2	–	–	–	↑ 5.9–19.6	↓ 0.9–2.5	[⁵⁹]
PaOBD + 2-8% DEE	↓ 0.4–1.2	↓ 6.5–24.9	–	–	↓ 6.2–43.7	–	–	↓ 0.9–4.8	[⁶⁰]
WPOBD + 5-10% DEE	↓ 0.5–5.5	–	↑ 2.4–3.6	↑ 4.4–6.7	–	–	–	↓ 1.2–2.5	[⁶⁷]
WPOBD + 5-15% DEE	↓ 0.3–1.1	↓ 4.5–10.2	–	–	–	–	–	↓ 2.3–3.8	[⁶⁸]
BD20 + 5-15% DEE	↓ 1.2–2.4	↓ 17.9–27.9	–	–	↓ 6.6	–	–	↓ 0.8–2.6	[⁷²]
BD50 + 5-15% DEE	↓ 1.2–2.3	↓ 21.5–33.7	–	–	↓ 4.5–5.3	–	–	↓ 0.7–2.1	[⁷²]
BD20 + 5-10% DEE	↓ 0.7–1.8	↓ 2.5–2.8	↓ 6.9–18.8	–	–	–	–	↓ 1.0–1.3	[⁷⁴]
BD20 + 15% DEE	↑ 0.3	↑ 2.3	↑ 5.1	–	–	–	–	↓ 0.8	[⁷⁴]
BD10 + 5-20% DEE	↓ 0.8–3.1	↓ 4.6–18.6	↓ 5.4–27.7	↓ 7.1–28.6	↓50.9–203.5	↓51.5–196.9	–	↓ 1.1–4.5	[⁷⁵]
BD20 + 5-15% DEE	↓ 1.2–1.8	↓ 7.1–10.5	–	–	–	–	–	↓ 0.8–2.4	[⁷⁸]
BD20 + 5-15% DEE	↓ 0.6–1.2	–	–	–	–	–	–	↓ 0.8–2.4	[⁸¹]
BD40 + 5-15% DEE	↓ 0.5–1.2	–	–	–	–	–	–	↓ 0.8–2.2	[⁸¹]
BD20 + 5-10% DEE	↓ 0.7–1.5	–	–	–	–	–	–	↓ 0.9–1.8	[⁸²]
BD15 + 5% DEE	↓ 0.6	–	–	–	–	–	–	↓ 0.7	[⁸⁵]
BD15-35 + 5% DEE	↓ 0.01–0.3	–	↓ 38.4–40.8	↓ 31.1–36.9	–	–	–	↓ 0.4–0.5	[⁸⁶]
BD25 + 5% DEE	↓ 2.2	↓ 0.5	↓ 10.3	–	–	–	–	↑ 0.5	[⁹⁰]
BD15 + 5% DEE	↓ 0.8	↓ 10.5	↓ 13.5	–	–	–	–	↓ 0.7	[⁹¹]
BD20 + 5-15% DEE	↓ 0.6–2.3	–	–	–	–	–	–	↓ 4.4–5.2	[⁹⁴]
BD30 + 2-8% DEE	↓ 0.3–1.2	↓ 2.2–26.6	↓ 3.5–27.9	–	↓ 100–300	↓33.3–166.7	↑ 1.9–3.9	↓ 0.7–4.2	[⁹⁵]
BD15 + 5% DEE	↓ 0.6	↓ 9.7	↓ 12.8	–	–	–	↑ 8.3	↓ 0.7	[⁹⁸]
BD15 + 5-10% DEE	↓ 0.8–1.4	↓ 11.2–16.9	↓ 14.9–24.6	–	–	–	–	↓ 0.7–1.3	[⁹⁹]
BD60 + 15% DEE	↓ 16.0	↑ 28.9	↓ 151.3	↓ 150.0	–	–	–	↓ 2.8	[¹⁰²]
BD20 + 10-20% DEE	↓ 2.2–3.7	↓ 4.6–6.9	↓ 1.4–4.2	–	–	–	↑ 11.5–25.0	↓ 0.7–0.9	[¹⁰⁵]
BD40 + 1-4% DEE	↓ 0.1–0.4	↓ 2.7–5.5	↓ 0.2–0.9	↓ 10.8–11.5	–	–	↑ 1.7–7.3	↓ 0.2–0.9	[¹⁰⁶]
BD15-35 + 5% DEE	↑ 0.02 ↓ 0.2	–	↓ 40.5–41.5	↓ 36.5–37.5	–	–	–	↓ 0.5 ↑ 0.9	[¹⁰⁸]
BD25 + 5% DEE	↓ 0.9	–	–	–	–	–	–	↓ 0.1	[¹¹²]

DEE: diethyl ether; LHV: lower heating value.

Likhitha et al. declared that diesel was thoroughly mixed with DEE due to lower density difference, blending was not difficult and also no phase separation observed. The blends had higher cetane value and lower density, lower flash, and fire point temperatures.²⁶ Madhu et al. stated that the addition of DEE improved the physical and chemical properties of diesel fuel, decreased heating value, and improved cold starting property.²⁹ Iranmanesh declared that with the addition of DEE to diesel–ethanol (E10) blend, the oxygen content of the blends improved and the properties such as calorific value, density, and viscosity reduced.³³ Patnaik et al. stated that the density of DEE15 blend decreased compared to diesel fuel. Cetane number and latent heat of vaporization of DEE15 blend were higher than diesel.⁴¹ Patil and Thipse mentioned that increase of DEE content in the blends decreased the density and calorific value and increased the oxygen content and cetane number of the blend, while considerable decrease occurred in kinematic viscosity.⁴² Polat determined that density increased and heating value decreased with the increase of ethanol in the blends to compare with pure DEE.⁴⁷ Sivalakshmi and Balusamy declared that increasing the DEE concentration in the blends increased the oxygen content and cetane number of the fuels and decreases were obtained in kinematic viscosity, density, and heating value of the blended fuel.⁵⁹ Ali et al. determined that density, viscosity, and heating value of palm oil BD–DEE blends decreased with the increase of DEE concentration and acid value slightly improved with increasing DEE additive. Increasing DEE content resulted in a significant improvement in low-temperature performance i.e. pour point. However, there was no significant difference in the cloud point of the BD with DEE.^60,61 Roy et al. declared that BD20 blends exhibited cloud points below −20°C when DEE was added.⁷² Kumar et al. determined that DEE decreased the density and the viscosity of the BD fuel blends.^73,74 Ganesha and Chethan determined that the density, viscosity, flash point, fire point, cloud point, pour point, and heating value of BD–diesel (BD10) blend decreased with increasing DEE concentration.⁷⁵ Srihari et al.⁷⁶ declared that the fuel properties i.e. viscosity, density, cetane number, and autoignition temperature improved with the addition of DEE to BD blends. Sudhakar and Sivaprakasam declared that blending of DEE with BD20 blend up to 30% showed good stability. More than 30% of DEE with BD20 was not stable for a long time.⁸⁸ Imtenan et al. stated that incremental addition DEE reduced the density and viscosity of the diesel–BD blend chronologically. In spite of lower calorific value of DEE, modified blends showed insignificant differences in calorific values than diesel fuel.⁹¹ Annamalai et al. determined that the density and calorific value of BD20 were decreased with the addition of DEE.⁹⁴ Ali et al. declared that the blended fuel viscosity was reduced with an increasing DEE additive because DEE additive has considerably lower kinematic viscosity than diesel and BD fuel. The density was also reduced with increasing additive ratios because DEE has remarkably lower density than diesel and BD fuel. DEE additive to BD30 lead to a further reduction in heating value with the increasing additive fractions due to the lower heating value (LHV) of the DEE. The addition of DEE resulted in reductions in pour point of the BD30 blend with the increasing DEE ratio. The addition of DEE to the blends further improved the cetane number of the blend because it has a high cetane number compared with mineral diesel and BD fuels. A drastic reduction in the flash point of the BD30 blend was observed by increasing the percentage of DEE additive.⁹⁵ Ali et al. determined that BD30 blended fuel density and acid value were significantly improved with increasing DEE ratio. A significant improvement was also observed in the kinematic viscosity of BD30 blended fuel with increasing DEE ratio. The cold flow properties of BD30 blended fuel improved with increasing DEE content. The pour point and cloud point decreased by 2°C and 3°C at 8% DEE ratio compared to BD30. The fuel heating value of BD30 was reduced significantly with increasing DEE ratio.⁹⁶ Ali et al. performed another study on DEE addition to palm oil methyl ester BD–diesel namely BD30 and BD40 blends. They determined that density and kinematic viscosity of the blends significantly decreased with the increase of DEE concentration. The maximum effect occurred with 6% DEE additive to the BD30 blend where the density reduced closed to that of diesel fuel and the viscosity reduced below that of petroleum diesel. The acid value of blend slightly improved with increasing DEE content and a significant difference was obtained in low temperature performance, with minimum pour point −7°C when adding 6% DEE with BD30 blend. The heating value of the BD blend decreased slightly with increasing of DEE portion and adding 6% DEE to the BD30 blend resulted in about 1% reduction in heating value.⁹⁷ Imtenan et al. determined that the addition of DEE into the palm oil BD–diesel blend reduced the viscosity and density on average 13% and 1.1% compared to BD20 blend. Flash points of the modified blends were also lower compared to BD20 blend. However, calorific values decreased for the DEE blends, it was only 1% lower than BD20.⁹⁹ Navaneethakrishnan and Vasudevan determined that increasing concentration of DEE in BD20 blend resulted in the corresponding remarkable decrease in the kinematic viscosity and flash point with DEE addition due to the lower flash point value of DEE. A decreasing trend was also observed for the calorific value in the BD20 blend with increasing DEE concentration.¹⁰⁵ Krishnamoorthi and Natarajan determined that the addition of DEE into BD–diesel blends improves the physical and chemical properties of blended fuels.¹⁰⁸ Shinde and Yadav declared that according to ASTM D975 standard 35% diesel, 20% DEE and 45% BD gave optimum blending ratio. Cetane number increased by 27.21% for this optimum blending ratio as compared to pure diesel and oxygen content increased to 14.57% by mass which was advantageous from better combustion point of view. The decrease occurred in density, lower calorific value, and kinematic viscosity of the blend by 1.43%, 7.48%, and 15.91% respectively for this optimum blend ratio as compared to pure diesel. The reduced viscosity was evaluated as advantageous from atomization point of view.¹¹⁴ As mentioned in above explanations, the fuel properties such as density, viscosity, flash point, fire point, cloud point, pour point, and heating value generally decreases, but cetane number increases with DEE addition. Diesel fuel is well mixed with DEE due to lower density difference and no phase separation occurs, but low viscosity of DEE can reduce lubricity of diesel fuel and create potential wear problems in the fuel pump. Therefore, low percentage ratios are most suitable for DEE–diesel blends. On the other hand, low density and viscosity of DEE can improve the spray characteristics when it blends with BD fuels or BD–diesel blends. Oxygen content and high cetane number of DEE can also improve the combustion characteristics and reduce the exhaust emissions. However, LHV of DEE can increase the engine fuel consumption.

Effects of DEE on injection characteristics

The injection characteristics have important effects on combustion, performance, and exhaust emissions of diesel engines. Therefore, the effects of DEE on injection characteristics were investigated by several researchers. The spray characteristics of DEE were numerically examined by Mohan et al. using KIVA-4 Computational Fluid Dynamics (CFD) code. They declared that the DEE showed high level of cavitation compared to conventional diesel fuel due to low viscosity. The spray characteristics i.e. spray tip penetration and Sauter mean diameter for DEE had been found to be shorter and smaller compared to diesel fuel. However, it was concluded that DEE exhibited excellent atomization behavior compared to diesel fuel because DEE was characterized by high Reynolds number and low Ohnesorge number.¹⁶ Rakopoulos et al. observed that the fuel pressure diagrams were distorted with increasing percentage of DEE as shown in Figure 3(a). Specifically, its uprising leg acquired a lower gradient, which was translated into a delay of the dynamic injection timing and furthermore its maximum value was slightly reduced and its final falling leg delayed. The different densities and bulk modulus of elasticity of DEE blends influenced the whole injection process. Certainly, the important new finding was that with increasing percentage of DEE in the blend, the dynamic injection timing decreased and the fuel injection process was prolonged.^17,18 Patil and Thipse determined that addition of DEE to diesel fuel caused to retard dynamic injection timing and the start of heat release. To compensate for this late injection and combustion, the injection timing needed to be advanced. If injection was delayed, the temperature and pressure would be slightly higher initially, but decreased with longer delay, resulted in incomplete combustion, reduced power output, and poor fuel conversion efficiency.¹⁹

Figure 3.

Effect of diethyl ether additive on fuel pressure of diesel fuel¹⁷ and cottonseed oil biodiesel fuel.⁵⁰

Saravanan et al. determined that when DEE composition was further increased beyond 10%, the engine became unstable and heavier smoke was observed. This could be due to the phase separation of the blend, which resulted in cavitation in the injector nozzle and it led to poor injection of the blend. This cavitation also resulted in reduction in injection pressure. This pressure reduction resulted in large droplets and nonuniformity of atomization. The large droplets of fuel starved of oxygen necessary for burning. This resulted in high smoky exhaust and poor efficiency at higher blends.²⁴ Balamurugan and Nalini declared that addition of DEE into diesel fuel increased the cetane number of the blended fuel which in turn increased the surface tension of the fuel. The increase in surface tension increased the fineness of the fuel particles injected into the cylinder, which resulted in high combustion rate and high combustion efficiency.²⁸ Madhu et al. determined that increasing composition of DEE to diesel increased brake-specific fuel consumption (BSFC) as injection pressure increased. This was due to the reduction in fuel particle size at higher injection pressure, which led to the increase in ignition delay period.²⁹ Iranmanesh declared that as the quantity of DEE in the blends increased, dynamic injection timing was altered due to the problems encountered with the fuel pump and lowering the density, viscosity, and bulk modulus of the blends, which led to late injection and consequently ignition retard. Then the start of combustion (SOC) and heat release was delayed.³³ Rakopoulos determined that the fuel injection pressure diagram for cottonseed oil BD–DEE (BD80DEE20) blend was delayed with its uprising leg acquiring a lower gradient as shown in Figure 3(b). This was translated into correspondingly higher delays of the dynamic injection timing. The different density and bulk modulus of elasticity of blend influenced the whole injection process. For a jerk pump when its plunger began to compress the fluid, a pressure wave was propagated down the connecting pipe which eventually reached the injector needle to open it, so that depending on the values of these properties, dynamic injection timing was affected despite the fact that the pump spill timing was kept constant.⁵⁰ Although DEE has good atomization behavior, it causes a delay in dynamic injection timing and prolongation of fuel injection process due to its lower density and bulk module of elasticity. Therefore, the injection timing needs to be advanced when using DEE to balance the late injection and SOC. On the other hand, DEE has high level of cavitation especially at high blending ratios due to low viscosity and this leads to poor injection. Hence, it can be more suitable for the percentages up to 10%.

Effects of DEE on combustion characteristics

Heat release, cylinder pressure, and pressure rise

Rakopoulos et al. declared that when DEE blended with diesel increased the ignition delay due to decreasing dynamic injection timing and higher latent heat of evaporation, despite its much higher cetane number. It was observed that premixed combustion seemed to decline with increasing percentage of DEE, thus leading to lower pressures and temperatures during initial part of the combustion. They determined that the SOC occurred later with increasing percentage of DEE, while the maximum pressure is fallen and occurred later as shown in Figure 4(a).^17,18 Lee and Kim determined that the higher DEE content resulted in smaller amounts of heat release during premixed combustion phase because of the shorter ignition delay due to the higher cetane number of DEE. The increase in heat release during the diffusion flame phase for the blended fuels with higher DEE content could be attributed to increased oxidation due to the oxygen present in DEE. They also determined that ignition delay became shorter as the DEE content increased in diesel fuel as seen in Figure 4(b), which enhanced the auto-ignitability of the blends. The shorter ignition delays for the DEE blends were mainly attributed to its high cetane number.²³

Figure 4.

Effect of diethyl ether additive on cylinder pressure of diesel fuel.^17,23

Saravanan et al. determined that there was a sharp decline in heat release rate (HRR) with the addition of DEE. The heat release starting time was also compensated by ignition delay period. Further, DEE addition to diesel fuel decreased the HRR. Thus, the cylinder temperature was decreased as DEE addition was increased. The high cetane number and high latent heat of vaporization of DEE tended to decrease in ignition delay. The peak pressure reduced with the addition of DEE into diesel fuel due to lower calorific value of DEE blends and reduced ignition delay period and retarded injection. They stated that further addition of DEE reduced more the peak pressure and there was a sharp decrease in peak cylinder pressure for DEE10 blend than that of DEE5 blend as shown in Figure 5(a).²⁴ Cinar et al. changed the premixed ratio of DEE from 0% to 40% in a homogenous charge compression ignition (HCCI)–direct injection (DI) diesel engine and they compared the results were with neat diesel. They stated that the cylinder pressure of mixed fuel was lower than neat diesel during the compression stroke due to higher latent heat of vaporization of DEE as seen in Figure 5(b). The higher ratio of the premixed DEE and direct injected diesel fuel during the ignition delay caused rapid combustion. This resulted in the increase of cylinder peak pressure and rate of heat release in the initial stage of combustion. When the premixed fuel ratio was 40%, excessive heat release was observed in a very short time which caused instantaneous pressure rise, peak pressure, and engine knocking. The cylinder pressure was 60.82 bar with diesel fuel; however, it was increased to 67 bar and 70.56 bar with the premixed ratio of 30% and 40%, respectively. While the rate of cylinder pressure rise was 2.98 bar/°CA with diesel fuel, it was 4.95 bar/°CA and 4.19 bar/°CA with 30% and 40% premixed DEE, respectively.³²

Figure 5.

Effect of diethyl ether additive on cylinder pressure of diesel fuel.^24,32

Iranmanesh investigated the various percentage of DEE addition to the optimum selected ethanol–diesel blend (E10) and optimization of its blending ratio to overcome the poor ignition quality of ethanol. They determined that combustion started with delay for all DEE blends according to cylinder pressure profiles as shown in Figure 6(a) and it became more prominent with higher DEE blends. Dynamic injection timing was altered as DEE quantity increased, due to lowering density, viscosity, and bulk modulus of DEE blends, which led to late injection and consequently ignition retard. Then, the SOC was postponed and the peak pressure also suppressed due to high latent heat of DEE.³³ Sudhakar and Sivaprakasam performed a study on DEE fumigation at ratios of 10%, 20%, and 30% in case of E15 blend operation. They declared that the increased DEE injection increased HRR inside the combustion chamber led to increase in cylinder pressure as shown in Figure 6(b). The maximum HRR for each test fuels was 117, 97, 112, 121, and 132 kJ/m³° respectively for pure diesel, E15, E15DEE10, E15DEE20, and E15DEE30. It was determined that cylinder pressure was increased 3 bar for 30% injection of DEE. When injected DEE percentage increased more than 30%, engine knock increased abnormally. Thus, the study was limited to 30% injection of DEE in inlet manifold due to audible knocking occurred. On the other hand, the blending ethanol with diesel, cylinder pressure was reduced by 13.8% than diesel. This was due to the decreased calorific value of E15, which leads to poor diffusion combustion phase.³⁴

Figure 6.

Effect of diethyl ether additive on cylinder pressure of ethanol–diesel blends.^33,34

Paul et al. determined that HRR for the DEE10 blend was quite low. The reduced HRR was attributed to the advanced ignition of the charge. The ignition delay was shorter due to higher cetane number, which reduced the amount of fuel injected into the combustion chamber. This reduced fuel flow and subsequently reduced the HRR. The injection period for DEE5 blend was shorter than DEE10 and hence the heat release was better than DEE10. Thus, DEE10 blend produced the least amount of cylinder pressure among the blends tested. The higher cetane of DEE10 blend might have ignited the charge at the crank angle prior to top dead center (TDC). As a result, a part of the fuel energy might have been wasted in countering the compression work, which caused in the lower cylinder pressures due to a reduction in effective work during power stroke. DEE5 blend produced noticeably a higher cylinder pressure than diesel, which indicated that 5% DEE blending to diesel was favorable for the CI engine combustion.³⁸ Patnaik et al. stated that SOC for DEE15 blend occurred later and obtained reduced combustion duration than that of diesel with FeCl₃ combination. The maximum cylinder pressure for DEE15 blend was decreased by 25% and 23% respectively for 60% and 80% load conditions as seen in Figure 7(a). It was explained that this was due to decrease in ignition delay with DEE blend in which the ignition started at earlier crank angles, leading to decrease in peak pressure. Additionally, low boiling point of DEE started earlier combustion and its higher latent heat of vaporization led to decrease in combustion temperature creating a lower peak pressure.⁴¹ Patil and Thipse declared that the dynamic injection timing decreased with increasing percentage of DEE and the fuel injection process was prolonged, which led to increase in ignition delay. It might be due to the lower density, kinematic viscosity, and bulk modulus and the higher latent heat of evaporation of DEE. As seen in Figure 7(b), they determined that the peak pressure of DEE15 blend was almost the same as diesel. The peak pressures of DEE–kerosene–diesel were less than neat diesel and reduced with increasing percentage of kerosene in DEE15 blends. It is stated that it might be due to an increase in ignition delay, which started combustion later, while the maximum pressure is fallen and occurred later.⁴²

Figure 7.

Effect of diethyl ether additive on cylinder pressure of diesel fuel.^41,42

Sudheesh and Mallikarjuna investigated the use of BG–DEE in HCCI engine mode at various excess air ratios and under different load conditions. They determined that the occurrence of peak HRR and the start of main combustion advanced as the DEE excess air ratio reduced. This was because of higher energy release by DEE. Overall, for the entire load range, the maximum HRR was about 80 J/°CA. They observed that the occurrence of peak pressure advanced with respect to the TDC with increasing load. Also, occurrence of peak pressure retarded with increasing DEE excess air ratio. At lower DEE excess air ratios, energy liberated from DEE was higher and thus autoignition occurred at an early stage. In general, peak pressure varied from about 51 to 72 bar for all load ranges. It was also observed that at lower DEE excess air ratio, occurrence of peak pressure advanced and peak cylinder pressure increased due to excess amount of DEE energy supply. This led to an increased rate of pressure rise and engine noise, whereas, at high DEE excess air ratio, the cylinder pressure reduced and the occurrence of peak pressure retarded due to a low DEE flow rate.⁴⁴ Jothi et al. studied the utilization of LPG as a primary fuel with DEE as an ignition enhancer in an HCCI engine. They observed that in the case of diesel operation, the maximum cycle pressure obtained was about 68 bar, whereas, for LPG operation, it was 52 bar. LPG fuel operation exhibited lower cycle pressure as compared to diesel operation as seen in Figure 8(a). It was stated that the reduction was attributed to the decrease in heat release that occurred after TDC as a result of lower cylinder gas temperature that led to the reduction of peak pressure. The maximum cycle pressures and rate of pressure rise in diesel fuel were also higher than those for LPG operation. This might be due to DEE that cooled the intake charge and thereby reduced the hotter environment in the engine cylinder as a result of lower cylinder–gas temperature that led to a reduction in peak pressure as well as rate of pressure rise.⁴⁵

Figure 8.

Effect of DEE additive on cylinder pressure of LPG⁴⁵ and E-DTPB blend.⁴⁸

Polat performed a study on the effects of DEE–ethanol blends on HCCI combustion, engine performance, and exhaust emissions. He observed that HCCI combustion was achieved with leaner mixtures with the decreasing amount of ethanol in the blends. The high octane number of ethanol has prevented in achieving HCCI combustion at leaner mixtures. In addition, lower ignition temperature and higher energy density of DEE allowed the autoignition even though the test engine was run with very leaner mixtures. It was stated that cylinder pressure and HRR were increased with decrease of lambda. The combustion was also advanced with the decreasing of lambda. The properties of DEE such as higher cetane number and lower ignition temperature had led to rapid HRR and knocking, especially at richer charge mixtures. When using the E30/DEE70 blend, undesirable pressure oscillations were also observed at the lambda value of 1.5. HCCI combustion was achieved for E30/DEE70 between the lambda values of 1.5 and 3, whereas for E50/DEE50, it was between 0.8 and 1.6.⁴⁷ Mack et al. investigated the influence of the additive di-tertiary butyl peroxide (DTBP) to ethanol (E) and ethanol–DEE blend on combustion of HCCI engines. They determined that pressure traces for blends of 1% DTBP in E-DEE25 blend and 3% DTBP in ethanol at a constant equivalence ratio were approximately equal as seen in Figure 8(b). They concluded that blends of DTBP and ethanol could generally behave like blends of DTBP, DEE, and ethanol.⁴⁸ Rakopoulos observed that the SOC for DEE20 blend occurred later because of lower dynamic injection timing and increased ignition delay. Thus, the pressure rise started later, the maximum pressure decreased and occurred later as seen in Figure 9(a).⁵⁰ Sivalakshmi and Balusamy determined that peak cylinder pressure of DEE blends was higher than diesel and BD as seen in Figure 9(b). DEE addition to BD increased the cetane number, which resulted in shorter ignition delay and strong premixed burning phase and gave rise in cylinder pressure. The less air–fuel mixture with increase of DEE in the blend was formed due to reduction in ignition delay and subsequently decreased peak HRR and the peak pressure.⁵⁹

Figure 9.

Effect of diethyl ether additive on cylinder pressure of biodiesel fuels.^50,59

Satyanarayanamurthy investigated the effects of the palm kernel oil BD as a base fuel with secondary injection of water–DEE solution through the inlet manifold. He determined that ignition delay of BD fuel was less than diesel due to it has higher cetane number. Thus, BD fuel achieved a steep rise in pressure and SOC was advanced. On the other hand, it was concluded that the water–DEE solution delayed the combustion and peak pressure occurrence as seen in Figure 10(a).⁶³ Rajan et al. determined that the premixed combustion phase was increased for BD with 15% DEE compared to BD due to shortening of ignition delay. The HRRs obtained for diesel and neat BD are 68 and 58 kJ/m³°, respectively, and for neat BD with 10% DEE and 15% DEE, these values are 62 and 65 kJ/m³° at full load. The increase in HRR was attributed to acting of DEE as an ignition improver and hence it increased the premixed combustion rates, thus resulting in increased HRR. They declared that similar pressure traces were obtained for DEE addition to BD as seen in Figure 10(b). The peak pressure increased with DEE addition to BD fuel. This might be due to higher cetane number and high flammability of DEE which increased the premixed combustion phase resulting in higher peak pressure. The peak pressure for DEE10 and DEE15 blends were 67.1 and 68.3 bar, while the values were 70 and 64.6 bar for diesel and BD fuels. On the other hand, the maximum rates of pressure rise obtained for diesel and neat BD were 4.8 and 4.2 bar/°CA, respectively, at full load, whereas for BD with 10% and 15% DEE, these values are 4.4 and 4.6 bar/°CA, respectively, at full load. Increments in pressure rise were attributed to more oxygen content of DEE and BD, resulting in increased premixed combustion phase and hence increased the maximum rate of pressure rise.⁶⁴

Figure 10.

Effect of diethyl ether additive on cylinder pressure of biodiesel fuels.^63,64

Devaraj et al. examined the effects of waste plastic pyrolysis oil (WPPO) mixed with 5% and 10% DEE. They stated that higher cetane number of DEE shortened ignition delay period. They determined that SOC occurred later with increasing of DEE, while the maximum pressure has fallen and occurred later as seen in Figure 11(a). The premixed combustion phase seemed to decline with increasing of DEE, thus leading to lower pressures during the initial part of the combustion.⁶⁷ Kaimal and Vijayabalan evaluated the effects of DEE additive in a diesel engine fuelled with waste plastic oil (WPO). They determined that higher peak pressure for WPO was 5.9% more than diesel due to high viscosity and low volatility, which delayed the fuel–air mixture preparation and caused a sudden rise in cylinder pressure during premixed combustion phase. The peak pressure of WPO was higher at all loads because of its higher ignition delay and higher calorific value as seen in Figure 11(b). They also concluded that peak pressure reduced with increase of DEE in the blends when compared to WPO. This was due to low heating value DEE blends compared to WPO. High latent heat of vaporization of DEE also reduced cylinder temperature. Thus, the ignition delay increased and SOC delayed. The delay increased with increase of DEE.⁶⁸

Figure 11.

Effect of diethyl ether additive on cylinder pressure of biodiesel fuels.^67,68

Barik and Murugan investigated the effects of DEE injection on combustion, performance, and emission characteristics of Karanja methyl ester (KME) BD–BG fueled dual fuel diesel engine. As seen in Figure 12(a), they determined that the peak cylinder pressure for diesel, KME, and KME–BG dual fuel operation were about 75.7 bar, 71.3 bar, and 72.9 bar, which occur at 7.4 °ATDC, 6.8 °ATDC, and 8.1 °ATDC (After Top Dead Center), respectively. KME-BG dual fuel operation gave a higher cylinder pressure of 78 bar than the other fuels, which occurred at 6.2 °ATDC. The blends of DEE2, DEE4, and DEE6 exhibited a gradual increase in cylinder pressure of 78.6 bar, 79.7 bar, and 82.5 bar, which occurred at 2.7°ATDC, 1.4 °ATDC, and 1.2 °ATDC, respectively. They commended that the injection of DEE advanced the early burning and increased the premixed combustion. Moreover, the availability of oxygen and high cetane number of DEE formed many ignition centers at different locations inside the combustion chamber and increased the rate of combustion of premixed BG.⁶⁹ Sachuthananthan and Jeyachandran examined the effects of DEE addition to water–WBE fuel. They determined that maximum cylinder pressure for 30% WBE was to be 77.6 bar which occurred at 11.6 °ATDC, whereas the maximum cylinder pressure was 66.7 bar for pure BD as seen in Figure 12(b). The maximum cylinder pressure for 30% WBE was due to the increase in ignition delay of the emulsified fuel which increased the fraction of fuel burned in the premixed burning zone. The cylinder pressure of DEE15 blend was more than 30% WBE due to the complete combustion of more quantity of DEE. The maximum HRR was found to occur only for 30% WBE due to increased ignition delay and more quantity of fuel burned in the premixed burning zone. The heat release was found to be less for all DEE blends except for DEE15.⁷⁰

Figure 12.

Effect of diethyl ether additive on cylinder pressure of biodiesel–biogas dual fuel⁶⁹ and water–biodiesel emulsion.⁷⁰

Krishnamoorthi and Malayalamurthi determined that the maximum pressure reduced when addition of DEE in the bael oil–diesel blends. A shorter ignition delay and wider spray pattern had formed because of lower viscosity of the DEE in the blended fuel. Optimum cylinder pressure rise was observed due to a reduction in the premixed fuel and the lower LHV of DEE blends. The peak pressure rate principally relied on the rate of combustion in the initial stage and fuel taking part in the uncontrolled heat release phase. In case of blend BD30DEE10, the combustion started nearly the same crank position, which might reflect the pressure rise like that neat diesel. For BD40DEE10 fuels, the rate of pressure rise was lower compared to neat diesel and BD fuels due to the LHV and inefficient burning of fuel.⁷¹ Kumar et al. performed the combustion and emission analysis for a diesel engine fuelled with cashew nut shell oil (CNSO)–diesel blends with DEE as an additive. It was observed that HRR during the ignition delay period in BD20, BD20DEE10, and diesel is 66.335, 70.615, and 70.732 KJ/m³° at full load condition. Thus, increase in HRR for BD20DEE10 was 6.22% with respect to BD20. Also, it was observed that HRR for BD20DEE10 was much closed to the diesel. This might be due to DEE which acted as an ignition improver and hence it increased the premixed combustion rates, thus resulting in increased HRR. It was determined that peak pressure increased with the addition of DEE to BD20 blend. The peak cylinder pressure for BD20DEE10 was considerably increased as compared to BD20 under full load condition. They commended that addition of DEE could promote the formation of combustible mixture for a better combustion. However, with further increase in DEE ratio, the volatility effect of DEE was traded off by the significant reduction in calorific value. As a result of this, the peak cylinder pressure reduced reliably. This might be due to the higher cetane number and high flammability of DEE which increased the premixed combustion phase resulting in higher peak pressure. They concluded that the peak pressure for CNSO with adding 10% and 15% DEE was 68.922 and 68.360 bars, while for diesel and BD20, the values were 69.479 and 67.879 bars at full load condition.⁷⁴ Ganesha and Chethan performed an experimental investigation for the performance and emission of diesel engine fueled with cashew shell oil methyl ester (CSOME) and its blend with DEE. They determined that diesel fuel had less pressure rise during all loads. They observed that the least pressure in diesel at crank angle 360° pressures was 30.249 bar, the CSOME blends had pressure rise BD10 pressure 53.197 bar at 348°. They also determined that CSOME blends with DEE had pressure rise BD10DEE5 pressure 51.995 bar at 349°, BD10DEE10 pressure 53.042 bar and 353°, BD10DEE15 pressure 52.256 bar and 353°, BD10DEE20 pressure 50.952 bar and 358°.⁷⁵ Srihari et al. performed an experimental study on the performance and emission characteristics of partial charge compression ignition (PCCI)–DI engine fuelled with DEE–BD–diesel blends. They determined that advancement in the SOC for different blends of DEE was observed. This was due to low autoignition temperature and high vaporization rate of DEE. An increase in peak pressure was observed for all DEE blends when compared to those of diesel and BD20 blend. A reduction in ignition delay was also observed for all DEE blends and that was the reason for the increase in the peak pressure for these blends. A slight knocking tendency in the cylinder was also seen and this could be due to the higher HRR. The highest peak pressure was 64.199 bar for BD20DEE15 blend as seen in Figure 13(a). This could be due to increase in peak temperature, higher vaporization rate, and higher premixed combustion HRR when compared to the other blends. The peak pressure obtained for the BD20DEE15 blend was found to be 6% more than that of BD20 blend and 0.16% more than that of diesel fuel.⁷⁶ Abraham and Thomas performed a study on the use of alternative fuel blend consisting of 80% diesel, 20% jatropha, and 5% DEE by volume for different compression ratios. They declared that peak cylinder pressure depended on the burned fuel fraction during the premixed burning phase i.e. the initial stage of combustion. In spite of the slightly higher viscosity and lower volatility of the BD, the ignition delay seemed to be lower than diesel. This might be because a complex and rapid preflame chemical reaction took place at high temperatures. The use of DEE has decreased the ignition delay of BD mixture.⁷⁹

Figure 13.

Effect of diethyl ether additive on peak cylinder pressure⁷⁶ and maximum rate of pressure rise.⁸³

Manickam et al. determined that the premixed combustion phase is increased with 15% DEE compared to BD blend. This might be due to the shortening of ignition delay as compared to 20% BD. The HRR obtained for diesel and BD20 are 68 J/° and 66 J/° respectively, and for neat BD with 10% DEE and 15% DEE are 70 J/° and 72 J/° at full load. This increase in HRR might be due to DEE which acted as an ignition improver for BD; hence it increased the premixed combustion rates and resulted in increased HRR. They declared that peak pressure mainly depended upon the combustion rate in the initial stages, which was influenced by the fuel taking part in uncontrolled heat release phase. They observed that a similar kind of results was obtained for addition of DEE with BD. The peak pressure for neat BD with DEE blends increased compared to neat BD as seen in Figure 14(a). This might be due to the higher cetane number and high flammability of DEE which increased the premixed combustion phase resulted in higher peak pressure. They determined that peak pressures for BD with 10% and 15% DEE were 67.1 bar and 68.3 bar respectively at full load, whereas for the diesel and BD were 70 bar and 64.6 bar respectively at full load. They commended that peak pressure reduced at high power output with the introduction of DEE along with BD BD due to reduction in ignition delay compared to diesel fuel. The maximum rate of pressure rise obtained for diesel and 20% BD were 4.8 bar/°CA and 4.2 bar/°CA respectively, at full load, whereas for BD with 10% and 15% DEE are 4.4 bar/°CA and 4.6 bar/°CA respectively, as seen in Figure 13(b). The maximum rate of pressure rise was increased by 15% DEE with DEE at full load. This might be due to more oxygen content present in the DEE and BD blend, resulting in increased premixed combustion phase and hence increased maximum rate of pressure rise.⁸³ Sudhakar and Sivaprakasam declared that vaporization of fuel accumulated during ignition delay period led to negative HRR at the beginning of combustion. BD20 and DEE blends were produced as improved precombustion than diesel fuel. After the combustion initiated, the heat release became positive and combustion started earlier for BD20 blend. The peak HRR of BD20 blend was slightly higher than DEE blends and BD20DEE30 was the lowest. They commended that reason for this might be high latent heat of vaporization of DEE. In addition, the high cetane number of BD and DEE resulted in shortened ignition delay.⁸⁸

Figure 14.

Effect of diethyl ether additive on cylinder pressure of biodiesel fuel⁸³ and biodiesel–diesel blend.⁹¹

Sathiyamoorthi et al. investigated the combined effect of nanoemulsion and exhaust gas recirculation (EGR) on combustion and emission characteristics of neat lemongrass oil (LGO)–DEE–diesel blend fuelled diesel engine. The combustion improved with a reduction in ignition delay with the addition of DEE additive. The reason might be due to the controlled combustion, which depends on the availability of air–fuel mixture and mixing rate. The heat release showed the ability of the oxygenated fuel DEE to promote the diffusion combustion rate for nano-emulsified LGO25, which results in a complete combustion of the air–fuel mixture. They determined that addition of DEE additive with nano-emulsified fuel blend with EGR helped for a considerable increase in cylinder pressure. It was mainly due to the autoignition of the air–fuel mixture and the high volatile elements presented in the abrupt combustion of fuel blend which led to the development of rapid pressure rise during the premixed combustion in combination with the rapid pressure exerted by the combustion of DEE. It led to a better combustion that resulted in higher cylinder pressures for nano-emulsified LGO25 with DEE blend than other blends of LGO25. Overall, the cylinder pressure for nano-emulsified LGO25 with DEE and EGR mode increased by 4.4%, 0.4%, and 2.5% than LGO25, emulsified LGO25, and nano-emulsified LGO25 fuels respectively.⁹⁰ Imtenan et al. determined that 10% DEE helped to create significantly lower temperature during the vaporization of the fuel and delayed the SOC more. However, in the mixing controlled zone (area after the first sharp peak), both of the modified blends exhibited higher HRR than BD20, which actually indicated better atomization of fuel due to lower density and viscosity of DEE. They determined that addition of DEE reduced the maximum cylinder pressure as seen in Figure 14(b). Maximum cylinder pressures for BD15DEE5 and BD10DEE10 were observed 86.92 and 86.10 bar respectively at 10.1 °ATDC and 10.4 °ATDC. Slight late and lower maximum cylinder pressures for DEE blends were explained with its higher latent heat of evaporation of DEE.⁹¹ Tudu and Patel investigated the effect of DEE in a DI diesel engine run on a tyre-derived light fraction pyrolysis oil (LFPO) fuel–diesel blend. They determined that the ignition of LFPO40 blend at full load commenced a little later than diesel ignition as seen in Figure 15(a), which was about 2 °CA, because of its lower cetane number than diesel. By adding DEE to LFPO40 blend, the start of ignition was advanced closer to that of diesel by about 1 °CA at full load. The advancement was attributed to the increase in cetane number. As a result of the early start of ignition, the peak pressure of diesel attained was closer to the TDC. The peak pressure of the LFPO40 blend was attained later by about 11.7 °CA from the TDC. The LFPO40 had a lower cylinder peak pressure which was attributed to the poor mixture formation, and with DEE addition the peak pressure increased for the DEE1, DEE2, DEE3, and DEE4 operation. This was because DEE provides oxygen to LFPO40 blend, which led to more complete combustion. With 4% DEE addition, the peak pressure of DEE4 blend was found to be the highest among all the fuels. This might be mainly due to the higher HRR in the premixed combustion phase. The difference in the peak pressure value between diesel and DEE4 was about 0.6 bar at full load.¹⁰⁶ Senthil et al. investigated the effect of DEE on the performance and emission characteristics of a diesel engine using BD–Eucalyptus (Eu) oil blends. They determined that cylinder pressure values of BD20Eu70DEE10 were slightly higher than the values of BD30Eu60DEE10, BD40Eu50DEE10, and BD50Eu40DEE10 for almost all the load conditions as seen in Figure 15(b). But, the cylinder pressure of all the BD blends tend to be lower than diesel fuel. The reason for this was considered to be the higher cylinder temperatures, which resulted in higher cylinder pressures.¹⁰⁹

Figure 15.

Effect of diethyl ether additive on cylinder pressure of various biodiesel–diesel blends.^106,109

Venu and Madhavan investigated the effect of DEE and Al₂O₃ nano additives in diesel– BD–ethanol blends. The variation of cylinder pressure was given in Figure 16(a) at full load condition operating with BD40E20, BD40E20DEE5, and BD40E20DEE10. The peak pressures were 65.4 bar, 66.384 bar, and 58.03 bar, respectively. BD40E20DEE5 exhibited highest cylinder pressure of 66.384 bar followed by BD40E20. Earlier fuel combustion was caused by higher latent heat vaporization of DEE mixture. The presence of DEE and ethanol improved the latent heat properties of the mixture along with low viscosity and higher volatility, resulting in more fuel–air mixture formation during ignition delay followed by strong premixed combustion phase and higher peak pressure. However, higher DEE concentration in BD40E20DEE10 blend affected the strong premixed combustion phase and thereby causing lower peak pressure. Despite the higher cetane number of DEE, the higher concentrations of DEE prolonged the delay period. Hence, with BD40E20 blend higher volatile and low latent heat properties of DEE diminished, leading to longer ignition delay followed by lowered HRR and peak HRR occurring away from TDC.¹¹⁰ Qi et al. investigated that effect of DEE and ethanol additives on the combustion and emission characteristics of BD–diesel blend fuelled engine. They determined that the peak cylinder pressure of BD30 was similar to that of BD25DEE5 and higher than that of BD25E5 at 15% of full engine loads, and the peak cylinder pressure of BD30 was earlier than BD25DEE5 and BD25E5 as seen in Figure 16(b). At 90% of full engine loads, the peak cylinder pressure of BD30 was lower and the crank angle of peak cylinder pressure was almost same to that of BD25DEE5 and BD25E5.¹¹²

Figure 16.

Effect of diethyl ether additive on cylinder pressure of diesel–biodiesel–ethanol blend¹¹⁰ and diesel–biodiesel blend.¹¹²

Venu and Madhavan investigated the influence of DEE addition in EBD and MBD blends. They determined that at 25% load, with EBDDEE5 3.57% higher pressure was observed (55.4 bar), while EBDDEE10 did not enhance the pressure and it diminished the pressure attained to 46.8 bar which is 13.33% lower than EBD. This was perhaps, due to lower ignition delay of EBD in comparison with EBDDEE10 thereby causing higher cylinder pressures. With 10% DEE addition, as a result of higher latent heat of vaporization of the mixture, the peak pressure attained lower value and occurs away from TDC later during the expansion stroke. Also, with increasing engine load, the cylinder pressure of diesel approached to that of EBD and EBDDEE5. Overall, the presence of 5%DEE both at lower and higher loads improved the cylinder pressure, while 10% DEE addition in EBD did not make any impact due to prolonged ignition delay as a result of higher latent heat vaporization of mixture.¹¹³

Cylinder gas temperature

Rakopoulos et al. determined that temperatures reached up to their maximum values and a little beyond decreased and appear delayed with increasing percentage of DEE in the blend as seen in Figure 17(a), while later on during expansion they seemed to recover and even slightly switched over the diesel fuel ones. This is due to the delayed and prolonged part of diffusion combustion.^17,18 Patnaik et al. determined that mixture of diesel with DEE15 indicated a decrease in the mean combustion gas temperature as compared to standard diesel as seen in Figure 17(b). The reason might be due to high heat of vaporization of DEE which reduced the in-cylinder temperature. With the addition of DEE, the ignition started slightly earlier due to its lower boiling point, which improved the spray characteristics of diesel during combustion. This leads to a lower heat release in diffusion phase combustion as compared to diesel.⁴¹

Figure 17.

Effect of diethyl ether additive on cylinder gas temperature of diesel fuel.^17,41

Sudheesh and Mallikarjuna determined that cylinder gas temperature followed a similar trend as that of the cylinder gas pressure. For a given load condition, the cylinder gas temperature was higher for lower DEE excess air ratio due to advanced combustion phasing. Peak cylinder gas temperatures were obviously higher at higher loads. In general, with a reduction in DEE excess air ratio, peak cylinder gas temperature increased due to advanced combustion phasing.⁴⁴ Krishnamoorthi and Malayalamurthi determined that at 25% and 100% load, the maximum cylinder gas temperature was 3% lower for BD20DEE10 blend compared to neat diesel. In 100% engine load, the average maximum cylinder temperature was 2% lower for BD30DEE10 blend compared with neat diesel. In 75% engine load, the maximum cylinder temperature is 1% lower for BD30DEE10 fuel compared with neat diesel.⁷¹ The dynamic injection timing decreased with increasing percentage of DEE in diesel fuel and the fuel injection process is prolonged, which led to increase in ignition delay due to the lower density, kinematic viscosity, and bulk modulus and the higher latent heat of evaporation of DEE. The SOC occurs later and the premixed combustion declines with increasing of DEE ratio and thus lower pressures and temperatures occur during initial part of the combustion and the maximum pressure falls and occurs later due to increase of ignition delay. As another evaluation, the higher DEE content in diesel fuel results in smaller amounts of heat release during premixed combustion phase because of the shorter ignition delay due to the higher cetane number and the higher auto-ignitability of DEE. The peak pressure reduces with addition of DEE due to lower calorific value of DEE and reduced ignition delay period and retarded injection. However, an increase occurs in heat release during the diffusion flame phase with higher DEE ratios due to increased oxidation because of the oxygen content of DEE. When the DEE premixed fuel ratio is 40% in an HCCI engine, excessive heat release occurs in a very short time, which causes instantaneous pressure rise, increased peak pressure, and engine knocking. The properties of DEE i.e. higher cetane number and lower ignition temperature have led to rapid HRR and knocking in an HCCI engine, especially at rich charge mixtures. On the other hand, DEE eliminates the poor ignition quality of ethanol when it uses with ethanol–diesel blends. The dynamic injection timing is delayed as the DEE quantity increased due to lowering density, viscosity, and bulk modulus of DEE blends which leads to late injection and consequently ignition retard. Then, the SOC is delayed and the peak pressure also reduced due to high latent heat of DEE. DEE fumigation with ethanol–diesel blends increases HRR and raises the cylinder pressure. When injected DEE percentage increases more than 30%, engine knock abnormally increases. DEE addition to BD fuels increases cetane number, which results in shorter ignition delay and strong premixed burning phase and gives rise in cylinder pressure and HRR. The increase in HRR is attributed to acting of DEE as an ignition improver and hence it increases the premixed combustion rates, thus resulting in increased HRR. The peak pressure also increases with DEE addition to BD fuel. Increments in pressure rise are attributed to more oxygen content of DEE and BD, resulting in increased premixed combustion phase and hence increased the maximum rate of pressure rise. The availability of oxygen and high cetane number of DEE forms many ignition centers at different locations inside the combustion chamber and increased the rate of combustion. However, the volatility effect of DEE is changed by the significant reduction in calorific value with further increase of DEE. As a result of this, the peak cylinder pressure reduces reliably. As another evaluation, peak pressure reduces with increase of DEE in BD fuel because of the reduced cylinder temperature. This is due to low heating value and high latent heat of vaporization of DEE.

Ignition delay

Rakopoulos et al. determined that ignition delay increased with DEE addition to diesel fuel as seen in Figure 18(a). They declared that the SOC is delayed with DEE addition as a consequence of synergy of the lower dynamic injection timing and increased ignition delay.¹⁸ Saravanan et al. declared that as DEE ratio in the blend is increased, there is decrease in the ignition delay period; this was more obvious at higher engine loads as seen in Figure 18(b). This might be the higher cetane number of DEE and higher latent heat of vaporization. At the low load, less DEE was injected and in this condition, the high cetane number might become a dominant factor, leading to a short ignition delay and small premixed fuel during the ignition delay. However, at the high load, more DEE was injected so higher latent heat of vaporization might become a dominant factor.²⁴

Figure 18.

Effect of diethyl ether additive on ignition delay of diesel fuel.^18,24

Patil and Thipse declared that it would be expected that due to high cetane number of DEE, the ignition delay of DEE–diesel blends should decrease due to reduction in the premixed portion of the combustion process. They determined that an increase occurred in ignition delay with increasing of DEE in blends as seen in Figure 19(a). They commented that it might be due to retarded dynamic injection timing because of lower density, kinematic viscosity, and bulk modulus of DEE and the higher latent heat of evaporation of DEE.⁴² Rajan et al. determined that ignition delay obtained for diesel and neat BD was 6°CA and 8°CA, whereas, for BD with 10% and 15% DEE, the ignition delay was 7°CA and 6.5°CA, respectively at full load as seen in Figure 19(b). The decrease in ignition delay for BD with DEE might be due to faster combustion of the BD with low boiling point of DEE, which improved the fuel–air mixing rate before combustion. Also, high cetane number of DEE minimized the ignition delay period. The presence of oxygen in the DEE also took part in the combustion process. The DEE has a high cetane number, low autoignition temperature, and good atomization and ignition properties, resulting in a decrease in ignition delay.⁶⁴

Figure 19.

Effect of diethyl ether additive on ignition delay of diesel⁴² and biodiesel⁶⁴ fuels.

Devaraj et al. determined that there was a decrease in the ignition delay period as the percentage of DEE in the fuel blend was increased as seen in Figure 20(a). This might be the higher cetane number of DEE and higher latent heat of vaporization. The higher latent heat of vaporization might become the dominant factor. DEE evaporation will have a great influence on the increase in cylinder temperature and decreased ignition delay.⁶⁷

Figure 20.

Effect of diethyl ether additive on ignition delay of biodiesel⁶⁷ and biodiesel–biogas⁶⁹ fuels.

Barik and Murugan declared that KME–BD exhibited a shorter ignition delay than those of diesel and dual fuel operations throughout the load spectrum as seen in Figure 20(b). This was due to the higher cetane number of KME and the existence of oxygen in the fuel. The ignition delay of BD–BG dual fuel was longer than that of KME at full load. This was due to the induction of BG through the intake manifold replaced the fresh air supply, which reduced the oxygen concentration in the charge and effects the preignition of BD–BG mixture, leading to a delay in ignition. DEE injection in the dual fuel operation offered a reduction in the ignition delay by about 2°CA, at full load. This shorter ignition delay may be due to the higher cetane number of DEE. The autoignition temperature of DEE is lower than that of KME and BG. During compression of the charge, early burning started for DEE, BG, and KME absorbed the surrounding heat to autoignite. Hence, the ignition delay in the dual fuel operation decreased with the increase of DEE.⁶⁹ Manickam et al. determined that ignition delay for diesel and 20% BD blend are 6°CA and 8°CA respectively at full load, whereas for the BD20 with 10% and 15%DEE, are 7°CA and 6.5°CA respectively at full load as seen in Figure 21(a). The decrease in ignition delay for BD with DEE may be due to faster combustion of the BD with low boiling point of DEE, which improves the fuel–air mixing rate before combustion. Also, high cetane number DEE minimizes the ignition delay period. The presence of oxygen in the DEE also will take part in the combustion process. DEE has a high cetane number, low autoignition temperature, and good atomization and ignition properties results in decrease in ignition delay.⁸³

Figure 21.

Effect of diethyl ether additive on ignition delay of biodiesel⁸³ and biodiesel–diesel blended¹⁰⁶ fuels.

Tudu et al. determined that ignition delay decreased with the increase of load which was due to increase in the cylinder temperature. The ignition delay of diesel was the lowest among all the fuels tested as seen in Figure 21(b) because of its higher cetane number, while the ignition delay of BD40 blend was the longest throughout the engine operation due to its lower cetane number. Adding DEE to the BD40 blend reduces the ignition delay. A maximum reduction of about 16.0 °CA is achieved with 4% DEE addition at no load and 13.5 °CA is achieved at full load. The values of ignition delay for DEE1, DEE2, DEE3, and DEE4 were about 14.4, 14.1, 13.8, and 13.5 °CA respectively at full load.¹⁰⁶

Venu and Madhavan declared that ignition delay reduced with increasing engine load due to higher cylinder temperature inside the cylinder. Ignition delay was increased with increasing DEE concentration as seen in Figure 22(a) due to lowered formation of ignition centers in the cylinder.¹¹⁰ Venu and Madhavan determined 5% DEE addition reduced the ignition delay while 10% DEE addition showed higher ignition delay as seen in Figure 22(b). When the concentration of DEE increased, in spite of higher cetane number of the blends, ignition delay increased due to reduced formation of ignition centers inside the combustion chamber.¹¹³ The ignition delay duration increases with DEE addition to diesel fuel due to the retarded dynamic injection timing because of lower density, kinematic viscosity, and bulk modulus of DEE and the higher latent heat of evaporation of DEE. As another evaluation, decreases become in the ignition delay period because of higher latent heat of vaporization of DEE. DEE addition to BD fuel results in a decrease in ignition delay because DEE has a high cetane number, low autoignition temperature, and good atomization and ignition properties. The presence of oxygen in DEE also makes extra contribution to reduction of ignition delay. On the other hand, the ignition delay reduces with increasing engine load due to higher cylinder temperature inside the cylinder.

Figure 22.

Effect of diethyl ether additive on ignition delay of ethanol–biodiesel–diesel blended fuels.^110,113

Combustion duration

Lee and Kim declared that combustion durations of diesel and the DEE blends were comparable except for the DEE50 blend. Since the LHV of the blended fuel required more fuel quantity than diesel, a longer injection period and consequently longer combustion duration would be expected. However, the blended fuel showed comparable or even shorter combustion duration compared to diesel as seen in Figure 23(a). This may be explained by the high volatility of DEE, which led to the mixing of fuel and air, and consequently, short combustion duration.²³

Figure 23.

Effect of diethyl ether additive on combustion duration of diesel²³ and biodiesel–biogas dual⁶⁹ fuel.

Barik and Murugan determined that combustion duration was about 37.4 °CA and 39.3 °CA for diesel and KME operations, at full load respectively. Combustion duration was about 40.8 °CA for BD–BG dual fuel operation at full load. The reason for increase in the combustion duration for dual fuel operation than that of KME–BD was due to the slower rate of burning of BG. The dual fuel operation with DEE gave a gradual decrease in the combustion duration as seen in Figure 23(b). This could be attributed to the faster ignition and increased ignition cornels in the charge, due to the high cetane number of DEE. Also, the burning velocity of DEE was very high, and it improved the flame speed of BD and BG and reduced the combustion duration.⁶⁹ Sathiyamoorthi et al. declared that combustion duration increased with an increase of fuel quantity. Combustion duration was higher for BD25 with water emulsion (WE) than standard diesel and BD25 as seen in Figure 24(a). The combustion durations were 38.5, 40.3, 38.5, and 37 °CA for diesel, BD25, BD25 + WE, and BD25 + WE + DEE with EGR respectively at full load. On the other hand, it became shorter at medium and higher loads. Furthermore, the addition of DEE and unburned fuel molecules in EGR contributed a considerable progress in the combustion process which led to shorter combustion duration and higher HRR than other modes of BD25 fuel blend.⁹⁰

Figure 24.

Effect of diethyl ether additive on combustion duration of biodiesel–diesel blended fuels.^90,106

Tudu et al. declared that combustion duration for diesel was the lowest among all tested fuels as seen in Figure 24(b), as a result of better mixture formation and faster burning. The slow combustion as a result of poor mixture formation of BD40 led to longer combustion duration. The combustion duration decreased with the increase of DEE addition from 1% to 4%. The combustion duration of diesel was 38.1 °CA and for the BD40, it is 42.5 °CA at full load. In the case of DEE1, DEE2, DEE3, and DEE4, the values of combustion duration were 40.3, 41.9, 39.6, and 40 °CA, respectively.¹⁰⁶ Venu and Madhavan determined that combustion duration increased due to more fuel fraction taking part in combustion with increasing engine load. Combustion duration reduced tremendously with DEE addition as seen in Figure 25(a) and 25(b), because DEE acted as an ignition enhancer. Combustion duration for DEE10 blend was lowered by 3.3%, 13.6%, 13.5%, and 4.4% at engine loads of 25%, 50%, 75%, and 100%, respectively. This could be attributed to latent heat and volatility properties of DEE which forms various ignition centers within the combustion chamber, which reduces the overall burn duration.^110,113

Figure 25.

Effect of diethyl ether additive on combustion duration of ethanol–biodiesel–diesel blended fuels.^110,113

The DEE–diesel blends have comparable or shorter combustion duration compared to diesel due to the high volatility of DEE, which guarantees the good mixing of fuel and air. The combustion duration of BD–BG dual fuel operation decreases with DEE addition due to the faster ignition and increased ignition cornels in the charge because of high cetane number of DEE. Additionally, the higher burning velocity of DEE increases the flame speed and reduces the combustion duration of BD and BG. The combustion duration of BD–diesel blend (BD40) decreases with DEE addition due to better mixture formation and faster burning. The volatility properties of DEE also create various ignition centers within the combustion chamber, which reduces the burn duration. The combustion duration of the BD blend (BD25) with WE also decreases with DEE additive.

Conclusions

The effect of DEE addition to various diesel engine fuels and fuel blends is investigated on the fuel properties, injection, and combustion characteristics in this review. The following conclusions can be summarized as results of the study.

The addition of DEE to various fuels and fuel blends generally results in the reduction in density, viscosity, flash point, fire point, cloud point, pour point, and heating value, while it improves cetane number of the fuels. Low viscosity of DEE can reduce lubricity of diesel fuel and create potential wear problems in fuel pump. Therefore, low percentage ratios are most suitable for DEE-diesel blends. However, low density and viscosity of DEE can improve the spray characteristics when it blends with BD fuels or BD–diesel blends.

The addition of DEE to various fuels and fuel blends generally causes to retard dynamic injection timing and injection process is prolonged. Therefore, the injection timing needed to be advanced to compensate for these behaviors. It is determined by some researchers that DEE shows high level of cavitation compared to that of diesel fuel due to low viscosity although it exhibits good atomization behavior.^16,24

The addition of DEE to various fuels and fuel blends generally increases the ignition delay due to decreasing dynamic injection timing and higher latent heat of evaporation. Therefore, the injection timing needs to be advanced when using DEE to balance the late injection and SOC. However, some researchers declared that shorter ignition delay is obtained due to higher cetane number of DEE.^{23,24,38,59,67,71,79,83}

The addition of DEE to various fuels and fuel blends generally reduces the HRR and peak pressure due to lower calorific value of DEE, reduced ignition delay period, and retarded injection. However, some researches declared that increase of DEE injection increases HRR and cylinder pressure especially in the HCCI engine,^32,44 the PCCI,⁷⁶ and in case of DEE fumigation.³⁴ Moreover, it is determined by some researches that the addition of DEE into various BD or BD–diesel blended fuels improves the combustion and increases the heat release and cylinder pressure.^{59,63,64,69,74,83,90,106,110}

Similar to cylinder pressure, the cylinder gas temperatures generally decrease with the addition of DEE to various fuels and blends due to LHV and higher heat of vaporization of DEE. Lower combustion temperature can decrease the emissions i.e. NOx.

The shorter combustion duration is generally obtained with DEE in addition to various fuels and blends because of high cetane number, high volatility, and oxygen content of DEE. Additionally, high burning velocity of DEE makes an extra contribution to the decrease of combustion duration as expected. The shorter combustion durations improve the engine performance and also exhaust emissions.

The effects of DEE utilization in diesel engines on the engine performance, exhaust emissions, and fuel cost need be investigated in detail for the development of new technologies. Additionally, different technologies such as adsorption and membrane processes need to be evaluated in detail.

Footnotes

Declaration of conflicting interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author received no financial support for the research, authorship, and/or publication of this article.

İsmet Sezer completed his undergraduation in 1997 from Karadeniz Karadeniz Technical University, Trabzon, Turkey. He obtained his MS degree in 2002 and PhD degree in 2008 from Karadeniz Technical University. He worked as a research assistant from 2002 to 2008 at Karadeniz Karadeniz Technical University. He was appointed as an Assistant Professor in 2010 and Associated Professor in 2012 at Mechanical Engineering Department, Gumushane University, Gumushane, Turkey. Currently, he is a Professor at Mechanical Engineering Department at Gumushane University.

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