Microscopic Spray Characteristics of Dimethyl Ether and Diesel in Ambient Conditions

Increasing global energy demands and limited fossil fuel reserves have led to sky-rocketing crude oil prices. There is a need to develop green and clean e-fueled engines that meet the most stringent emission norms and are also cost-effective. Dimethyl ether (DME) is a biofuel that substitutes mineral diesel completely and emits lower soot and NO_x emissions than baseline diesel. Its physical and chemical properties are ideal for engine applications. DME has superior properties, making it a viable solution as a green and clean oxyfuel for sustainable transport. It has a simple chemical structure, CH₃-O-CH₃. An oxygen atom in each molecule contributes to an oxygen percentage of 34.8% (w/w) in DME compared to 0% in conventional diesel. In addition to the oxygenated structure of DME, it also does not have a C–C bond, unlike diesel. The combined effect of these properties leads to efficient combustion with near-zero soot emissions and low unburnt hydrocarbon emissions [1–4]. DME's combustion generally shows a lower peak of the in-cylinder pressure and temperature, and the lower initial rate of pressure rise leads to quieter combustion than diesel [5]. DME has a significantly higher cetane number (68) than diesel (46). DME also has the additional advantage of lower fuel cost over conventional diesel since it can be produced from both renewable (biomass, methanol, municipal solid waste) and conventional (coal and natural gas) feedstocks [1].

The physicochemical properties of DME are shown in Table 1. The physical properties of DME are similar to that of liquified petroleum gas (LPG). It is an odorless and colorless gas at normal ambient conditions (298 K and 0.1 MPa) and is liquified when pressurized above 0.51 MPa at 298 K temperature [6]. DME's boiling point is very low (−24.8 °C), which makes it highly volatile, contributing to its excellent atomization and vaporization characteristics. Due to its superior cold working characteristics, DME's freezing point is also very low (−141 °C), which makes it a highly suitable fuel at subzero temperature conditions. DME shows no adverse effects on human health upon exposure except mild narcotic effects. However, continued exposure to DME can lead to frostbite [7].

The oxygen content in DME reduces its calorific value to 28.4 MJ/kg compared to 42.5 MJ/kg for diesel. This necessitates the DME supply rate of ∼1.5 times diesel to achieve the same engine power output. Also, DME has lower fuel viscosity and lubricity. Therefore, proprietary additives are needed in DME for its use in CI engines [12]. DME also has a lower bulk modulus, leading to its higher compressibility. Therefore, the work done by the superior capacity pump is generally higher [1]. This leads to a low-pressure growth in the high-pressure fuel line before injection. This leads to a lower spray momentum and, consequently, lower spray speed after the injection, affecting the air–fuel mixing in the engine combustion chamber. DME has a higher vapor pressure, posing an occasional vapor lock problem in the fuel line. Other limitations of DME include its incompatibility with elastomers, requiring the use of DME-compatible materials such as Teflon and polytetrafluoroethylene in the fuel injection system [7,8].

All these issues necessitate upgrading a dedicated DME injection system for existing engines. The first step in ensuring the applicability of fuel in an IC engine is to study its spray characteristics. The spray characteristics determine the air–fuel mixing in the engine cylinder, an essential phenomenon for efficient combustion. Many researchers studied the spray characteristics of DME. Youn et al. [13] investigated the spray characteristics of DME. The tip penetration length of the spray was shorter, and the spray cone angle was larger for DME than for diesel. Park et al. [14] explored the spray characteristics of DME under different ambient conditions. They reported that when the constant volume spray chamber (CVSC) temperature increased, the fuel spray's overall Sauter mean diameter (SMD) showed an increasing trend for DME. They attributed this to decreased evaporation and flash boiling of DME under high-temperature conditions at the spray cone boundary. This was true for droplets of smaller sizes. In addition, the SMD values increased at higher ambient pressures and temperatures at the spray periphery. They cited the KH breakup mechanism for this trend, leading to smaller droplets generated when the spray atomization started. The total number of DME droplets decreased at higher ambient temperatures. They also investigated the DME spray behavior in the combustion chamber of a high-speed diesel engine. They reported that the spray targeting point of the re-entrained piston shape influenced DME's spray behavior. DME spray was uniformly distributed in the engine cylinder when DME was injected at 30° bTDC, targeting the piston lip. However, Kim et al. [15] conducted DME spray studies and reported that most of the fuel spray was distributed in the crevice region with advanced injection timings. Yu and Bae [16] studied the spray characteristics of DME in a common rail direct injection (CRDI) system and reported that the DME spray tip formed a mushroom-like structure at ambient pressure conditions. When the ambient pressure was increased, the DME spray became like diesel. DME vapor was ejected in atmospheric pressure conditions before the liquid discharge. However, only liquid DME was observed upon increasing the chamber pressure to 3 MPa. The DME spray dominated the spray edge region, promoting better ignitability upon induction to the combustion chamber of the CI engine. They also found that the vapourising part of the DME spray increased upon increasing the fuel injection pressure, irrespective of the chamber pressure. At 3-MPa chamber pressure, DME evaporated by phase change. Suh et al. [17] compared DME and diesel spray characteristics using a CRDI system in a CI engine. They reported that diesel exhibited a greater spray speed because of higher momentum due to larger diesel droplets. Higher fuel viscosity and surface tension of diesel also promoted the same. DME vaporization characteristics resulted in the same spray arrival time based on the fuel injection pressure. They concluded that higher injection pressure boosted the spray velocity, momentum, and atomization near the nozzle. SMD of the spray droplets for DME was lower than diesel's due to diesel's higher kinematic viscosity, resulting in inferior spray atomization. Park et al. [18] assumed the DME droplets to be spherical in their calculations of SMD in particle measurement and analysis system (PMAS) and concluded that the DME droplets were indeed not spherical. They cited DME's higher vapor pressure as responsible for irregular droplet shapes. Also, the atomization characteristics of DME were superior to diesel due to the lower kinematic viscosity of DME. Also, they reported lower SMD for DME spray droplets than diesel. Mukherjee et al. [19] reported a 25.6% higher maximum velocity for diesel droplets than DME. They inferred this behavior to the spray's higher turbulent kinetic energy (TKE). They also indicated a large amount of DME induction to the combustion chamber, responsible for reduced angular velocity. Mohan et al. [20] studied the spray properties of DME and diethyl ether (DEE) and explained them using their thermophysical properties. They reported cavitation to flow inside the nozzle due to its lower viscosity and vapor pressure. They also reported that DME and DEE spray penetrations were lower than diesel because of their lower fuel viscosities and densities. The high Reynolds number (Re) and low Ohnesorge number (Oh) of ethers showed superior atomization of ethers than diesel. Li et al. [21] reported that because of the consequences of flash boiling of DME, the DME–diesel blend had a more uniform droplet distribution in the spray. As the DME mass concentration increased in the blend, the SMD decreased, and the peak of the distribution curve moved toward the smaller droplet sizes. Also, the droplet distribution's relative span factor increased as the DME mass concentration increased. They suggested that the droplets became finer due to the collision and coalescence of the spray droplets. Kim et al. [22] found a reduction in SMD as the distance of the spray investigation point from the nozzle tip was increased. In addition, they reported a smaller SMD range of DME between 10–40 µm compared to the 30–70 µm SMD range for biodiesel and diesel. They concluded that DME had superior spray atomization characteristics due to its higher volatility and lower density than diesel. In another study, Kim et al. [23] reported that the fuel injection pressure (FIP) of 50 MPa was enough for DME because of the superior spray atomization and evaporation characteristics of DME, and they were akin to the ones observed for diesel at FIP of 50–200 MPa. This was because of DME's favorable physicochemical properties, such as lower boiling point, lower viscosity, lower surface tension, and higher vapor pressure. They also reported DME's lower droplet break-up time and breakup distance at lower FIPs.

The above-mentioned literature concluded that many researchers have investigated DME and diesel spray characteristics in different ambient conditions and FIPs. However, there is a general lack of comparative three-dimensional spray studies of DME and diesel and the spray characteristics of DME using mechanical fuel injection systems, that are typically used in the agriculture sector. There is a lack of studies on the three-dimensional spray droplet velocity distributions. Researchers have also investigated flame speed to understand various alternative fuels [24,25]. To address this gap in the literature, this study investigated three-dimensional spray droplet velocities and other microscopic spray characteristics such as mean droplet diameters, droplet counts, and DME and diesel droplet diameter distribution. Diesel and DME spray experiments were performed in a CVSC in atmospheric conditions using a mechanical fuel injection system. Three-dimensional droplet velocities (V₁, V₂, and V₃) components were measured using the PDI system's three channels (1, 2, 3) and then converted to orthogonal velocities, namely V_x (radial velocity), V_y (axial velocities), and V_z (tangential velocity) and reported.

Diesel and DME spray investigations were performed using a CVSC consisting of a cubic glass chamber having transparent windows and a mechanical injector mounted at the top of the chamber (Fig. 1). Figure 1 shows the experimental setup for the microscopic spray investigations. This investigation used a PDI system to measure the microscopic spray characteristics of DME and conventional diesel [26]. PDI has two transmitters, one receiver, and three advanced signal analyzers. It emits six solid-state laser beams which use three pairs of wavelengths: 491 nm (2 blue), 532 nm (2 green), and 561 nm (2 yellow). Transmitter-01 transmits four laser beams, two blue, and two green, so all these laser beams converge at a single point, creating the “Probe Volume.” Transmitter-02 emits two yellow laser beams, which converge within the probe volume. Thus, these six laser beams create a probe volume for the three-dimensional (3-D) spray parameter measurements. The 3-D spray characterization works on the generation of an interference fringe pattern. Interference fringes evolve when two laser beams of equal wavelength with unequal phase angles converge at a point. When the spherical droplets pass through the probe volume, light is scattered, which is detected by the receiver of the PDI for changes in frequency and phase difference, generating a fringe pattern (bright and dark). This information is passed to the photomultiplier tubes, which additionally gives information about the Doppler burst signals and frequency changes and provides data about the velocity distribution of the spray droplets passing through the probe volume. The change in the phase of signals contains information about the droplet size distribution.

The experimental setup consisted of an alternating current (AC) motor, a mechanical fuel injection pump, an optical sensor, a pneumatic feed pump, diesel and DME tanks, a function generator, and an oscilloscope (Fig. 1). A function generator and an oscilloscope were used to synchronize the PDI with the optical sensor signals. The diesel and DME sprays were injected into the CVSC at atmospheric conditions (1.0132 bar and room temperature) using a mechanical injector via the mechanical fuel injection pump. The pump was driven by a motor connected to it using a belt and a pulley. The motor was operated at 1400 rpm. The PDI system synchronised with the sensor (optical) when the pump initiated pressuring the fuel. The injection timing (static) was noted using an optical sensor fitted to the pump. The fuel's static injection timing was when the pump initiated pressuring the fuel.

The spray was characterized by pulse time, which correlated with fuel injection timing (static). The spray characteristics were measured 60 mm downstream of the nozzle exit at three radial distances: central position, 5 mm, and 10 mm radial distances with a fixed pulse time. Figure 2 shows the droplet velocities' orthogonal and non-orthogonal coordinate systems for understanding various spray characteristics. The velocity distribution of spray droplets in non-orthogonal coordinates was evaluated and denoted as V₁, V₂, and V₃. These components of velocities were converted in the direction of orthogonal coordinates radial velocity (V_x), axial velocity (V_x), and tangential velocity (V_z) to better describe the outcome of DME and diesel spray. Simultaneously, spray droplet size, mean diameters, and droplet number distributions were measured.

The spray experiments were performed for diesel and DME, and velocities of spray droplets, number of spray droplets, and spray droplet size distributions were measured and compared. This section includes the distribution of spray droplet velocity components variations, distribution of droplet numbers along with droplet size, distribution of droplet velocity along with droplet size, and droplet sizes for various mean diameters, as discussed in the following sections.

The air–fuel mixing in the combustion chamber is governed by spray characteristics such as penetration length of spray, size of droplets, velocity of droplets, atomization, and vaporization. The spray characteristics also affect wall quenching inside the engine combustion chamber. FIP and thermophysical properties of fuel affect the spray droplet's velocity distribution [20,27–31]. This study mainly evaluates the influence of the thermophysical properties of DME and diesel, which affect the spray droplet's velocity. These fuel properties were surface tension, viscosity, density, boiling point, and vapor pressure [20]. Surface tension controls the break up and atomization of fuel sprays, which influences the spray droplet velocity distribution. The fuel density affects the breakup length of the fuel spray. Fuel viscosity affects the spray breakup, spray droplet speed, and size. Increasing the fuel viscosity delays the spray breakup, resulting in postponed development of the spray ligaments and droplets. The lower fuel viscosity improves the fuel spray atomization, generating smaller droplets and better air–fuel mixing. The lower surface tension of test fuels enhances fuel atomization, forming smaller droplets and promoting air–fuel mixture formation. The vapor pressure of fuel also influences the spray speed and atomization of the spray [13,29,30]. The vapor pressure of DME (5.17 bar) is higher than the ambient pressure (1.0132 bar), causing cavitation inside the nozzle, which causes a hydraulic flip [20,31]. The cavity generation over the outer periphery of the flow inside the injector nozzle is called a hydraulic flip. Hydraulic flips enhance spray droplet speed. Higher DME compressibility also affects the spray droplet's speed. DME has higher compressibility than diesel, requiring more compression work by the high-pressure fuel injection pump. This lowers the fuel injection pressure of DME in the high-pressure fuel line as some pump work is used for the compression of DME [32]; therefore, the spray droplets' initial momentum reduces. However, the density of DME is 1.3 times lower, and a smaller droplet size distribution than diesel causes lesser droplet deacceleration due to drag, resulting in higher spray droplet velocity distributions.

Three PDI channels, namely channel 1, channel 2, and channel 3, measured the V₁, V₂, and V₃ velocities, divided into three components in X, Y, and Z directions (Fig. 2). These velocities were named radial, axial, and tangential velocities. The contour plots in the following subsections show the droplet density of different droplet velocities.

X-direction droplet velocity was the same as the velocity measured by channel 1, represented by V₁, and called the radial velocity of spray droplets perpendicular to the spray's axial direction. Figure 3 shows variations in the radial velocity of spray droplets along the pulse duration for DME and diesel in the radial direction at three radial positions under ambient conditions. The radial direction is right angle around the axis of the spray. These velocities of DME and diesel spray droplets were presented in terms of the spray droplet's velocity density factor. These velocity density factors involved droplets passing through the probe volume at a particular time. The spray droplet density factor for DME decreased with increasing radial distance. However, the diesel droplet density factor was the maximum at 5-mm radial distance and the lowest at the central position. At the central part, the density factor for DME was higher than diesel because of the superior spray atomization of DME than diesel. The delay period for DME was slightly smaller than diesel because of the higher droplet velocity of DME, as seen in Fig. 3. The X-axis velocity was more scattered for DME than diesel. The maximum radial velocity magnitude for diesel and DME spray droplets were 20.9 m/s and 34.2 m/s, respectively, at 10-mm radial distance.

The thermophysical properties namely viscosity, vapor pressure, boiling point, density, and surface tension influence DME's radial spray droplet velocity distribution [20]. The spray droplet velocity distribution was superior for DME than diesel. The greater velocity of DME spray droplets was because of the hydraulic flip. Hydraulic flips occurred due to DME's higher vapor pressure, resulting in cavitation inside the injector nozzle, which progressed throughout the periphery of fuel flow [17]. Flash boiling due to extremely low boiling point and higher vapor pressure of DME was another reason for the greater velocity of DME spray droplets than diesel [28,33,34]. It caused faster atomization and vaporization of DME, which promoted smaller spray droplet formation. Smaller spray droplets were exposed to smaller decelerating drag forces, resulting in higher droplet velocity. Very low viscosity of DME (<0.1 cSt) compared to diesel (3 cSt) and the smaller droplet size distribution of DME caused higher spray droplet velocity distribution for DME. Lower surface tension of DME resulted in the secondary breakup of larger droplets, generating smaller droplets with higher radial velocity [27]. Since the surface tension of DME was lower than diesel, more number of spray droplets formed [35]. The droplet velocity reduced as time passed due to aerodynamic drag and evaporation under atmospheric conditions. Reduction in spray droplet velocity was observed earlier in diesel than DME because the distribution of diesel droplet size was higher, leading to greater drag. The overall concentration of the spray droplet's velocity in the radial direction was lower for DME, indicating that smaller droplets were present due to faster evaporation.

Figure 4 shows diesel and DME's spray droplet velocity distribution along the Y-axis with pulse time at three radial distances (central position, 5 mm, and 10 mm). Y-direction was along the spray's direction and represented the spray droplets' axial velocity, as shown in Fig. 2. The axial velocity of the spray was measured at channel 2 of the PDI. The maximum spray droplet axial velocity was higher for DME than diesel. The higher spray droplet's axial velocity of DME was mainly because of its thermophysical properties [17]. DME properties such as boiling point, viscosity, surface tension, bulk modulus, and density were much lower than diesel; however, DME's vapor pressure was higher than diesel. Hence, these test fuel properties of DME dominated the diesel and supported higher droplet axial velocity for DME. The axial velocity of diesel and DME decreased when moving away from the central position of the spray in the radial direction. This may be due to larger interactions with the ambient air, resulting in higher momentum loss of droplets at the outer periphery of the spray. The axial velocity density factor for DME was much lower than diesel because of a significantly lower boiling point, resulting in the rapid evaporation of DME. This density factor decreased as the radial distance increased for diesel; however, for DME, it first increased and then decreased.

The density of droplets increased for DME and diesel with pulse time, which approached lower spray droplet's velocity due to a longer duration of ambient air drag forces on the droplets. The number of DME spray droplets possessing velocity in the Y-direction was much lower than diesel. At the central position, the maximum velocity of DME and diesel droplets along the fuel spray direction were 49.6 m/s and 55.5 m/s, respectively. These axial velocities of diesel and DME dropped to 27.8 m/s and 36.2 m/s, respectively, at a radial distance of 10 mm from the central position. The spray droplet's axial velocity of DME was more scattered than diesel because of the very low fuel viscosity and surface tension. This promoted the formation of smaller droplets in larger quantities. However, the droplet density for DME was lower due to a faster evaporation rate of DME, as smaller droplets showed more significant interactions with the environment. DME's very low boiling point caused its flash boiling, which promoted faster DME spray atomization and vaporization.

The velocity component at the right angle to the axial and radial velocity of the spray cone is called tangential velocity. At any point, it is in the tangential direction to the conical curvature (Fig. 2). This velocity component is measured at channel 3 and is also responsible for the formation of a spray plume of conical shape. Figure 5 shows the velocity distributions of diesel and DME spray droplets in the Z direction. This velocity is investigated at three radial positions, 60 mm downstream of the injector nozzle. The spray droplets' tangential velocity of DME and diesel reduced as the pulse time increased. Also, both test fuels' tangential velocity density factor increased as the pulse time increased. The density factor of DME was lesser than diesel because of quicker evaporation of DME caused by its physicochemical properties, which were lower than diesel. As the radial distance increased from the central position, the tangential velocity of the spray droplets decreased for DME and diesel. DME's tangential spray droplet velocity showed a higher maximum velocity than diesel. The top-most velocities of DME and diesel were 32.1 m/s and 28.6 m/s, respectively, at the central position, 60 mm away from the injector nozzle.

The density factor for diesel and DME was higher at 5-mm radial distance and lower at 10-mm radial distance. However, DME's overall velocity density factor was lower than diesel at all radial distances. DME's lower velocity density factors were due to faster vaporization of DME's droplets than diesel. The faster vaporization of DME was due to its much lower boiling point. Most diesel spray droplets' tangential velocities were smaller than DME.

DME's lower boiling point, surface tension, and viscosity boosted the formation of smaller spray droplets [35]. Figure 6 shows the number of spray droplet changes with corresponding droplet diameters at 60 mm axial distance at three radial positions. DME showed nearly thrice the diesel spray droplet counts at all investigation points in the spray chamber. The main reason for DME's higher spray droplet count was its lower boiling point, surface tension, and viscosity. The very low boiling point resulted in flash boiling of DME, generating many spray droplets. The droplet count of sizes between 2 and 7 µm for DME was higher than the diesel at all measurement positions. The largest number of DME spray droplets at all three positions was of 3 µm. However, the diesel spray droplet's full count was in the range of 3–7 µm droplet diameter, and peaks were at 5 µm, much lower than corresponding DME droplet peaks. DME had a maximum count of 9232 droplets corresponding to a 3 µm diameter. In comparison, diesel exhibited a maximum count of 2883 spray droplets corresponding to the 5-µm droplet diameter at the central position of the spray, as shown in Fig. 6.

Most DME spray droplets were less than 10 µm, compared to less than 20 µm in diesel. This trend was because of DME's lower viscosity and surface tension than diesel, which encouraged smaller spray droplet formation. DME's favorable thermophysical properties, such as lower boiling point, surface tension, and viscosity compared to diesel, were the reasons for its superior spray atomization and smaller droplets [35]. The number of spray droplet counts decreased when moving away from the central position for DME; however, for diesel, it first increased from the central position to the 5-mm radial distance and then reduced from 5-mm to 10-mm radial distance. The lower spray droplet count at the outer periphery of the spray plume was because of the faster evaporation of DME. From this graph, DME showed superior spray formation and better fuel atomization than diesel, promoting superior air–fuel mixing.

Mixing and evaporation of the spray droplets with air inside the combustion chamber mainly depends on the spray droplet's velocity and size distribution. Figure 7 shows the velocity distribution with spray droplet diameter in the X-direction at three different radial positions (0, 5, and 10 mm from the center line) under ambient conditions for diesel and DME. These distributions are presented in terms of density factors, and it was observed that the density factor for DME was much more significant than diesel at all radial positions. The density factor increased when moving away from the radial direction and from the central position. This trend was due to superior atomization of DME than diesel.

The velocity of DME spray droplets was higher for smaller droplets. The DME spray droplet sizes were less than 20 µm. However, diesel showed spray droplets up to 40 µm. The maximum droplet velocities for DME in the X-direction were 34.2 m/s at 10 mm radial distance and 14.1 m/s at 5 mm radial distance for diesel, and corresponding spray droplet diameters were 7.3 and 2.1 µm, respectively.

For diesel, the maximum droplet velocities in the X-direction were 17.4 m/s and 11.6 m/s, corresponding to the droplet diameters of 2.6 µm and 1.6 µm, respectively. Due to superior sprays and smaller droplet diameter distribution, DME droplets showed more scattered velocities than diesel. Smaller droplets mainly contributed to negligible droplet velocity for DME due to their lower boiling point, resulting in superior spray vaporization. Diesel showed a greater probability of droplet coalescence in the secondary spray breakup zone. In this, smaller diameter droplets agglomerated to generate bigger droplets. Hence, the spray droplet size increased. Most of the larger spray droplets have negligible velocity, but the count of these droplets is a smaller number. DME showed a negligible density of larger spray droplets (20–60 µm). In contrast, diesel showed a comparable number density of droplets up to 55 µm diameter.

Here, i = histogram bin number; n_c = number of samples in each bin: corrected size count of droplets, d = diameter of spherical particle, ρ = density of the fluid (kg/m³), v = velocity (m/s), L = characteristic length, typically the droplet diameter (m), and σ = surface tension (N/m).

Figure 8 shows all measured mean diameters (D₁₀, D₂₀, D₃₀, and D₃₂) for DME and diesel spray at three radial positions. D₁₀ is used to explore the length of spray droplets of different diameters. However, the surface area of the spray droplets is explored by D₂₀. D₃₀ explains the volumetric action of the spray droplets. The diametric parameter D₃₂ represents the mass transfer action of spray droplets. All these mean diameters of spray droplets were lower for DME than diesel due to the superior spray atomization. The main reason for DME's superior atomization was its thermophysical properties, as explained in the previous sections.

DME's lower kinematic viscosity and surface tension promoted the formation of smaller droplets. The flash boiling of DME also promoted the generation of lower mean-diameter spray droplets. The extremely low boiling point (−24.8 °C) of DME resulted in faster atomization and evaporation, which caused the conversion of larger-diameter droplets to smaller ones [38]. Also, the increased environmental exposure led to an enhanced evaporation rate and lower mean diameters of DME spray droplets. The spray droplet's mean diameters (D₁₀, D₂₀, D₃₀) decreased when moving outwards in the radial spray direction from the central position for DME. However, these mean dimeters were higher for diesel. The minimum values of (D₁₀, D₂₀, and D₃₀) for DME and diesel were (3.8, 5.4, and 8.5 µm) and (9.7, 12.7, and 15.6 µm), respectively. The maximum values of (D₁₀, D₂₀, and D₃₀) for DME and diesel were (4.5, 6.2, and 9.2 µm) and (9.9, 13.2, and 16.3 µm), respectively. D₃₂ shows arbitrary trends for diesel and DME; the minimum D₃₂ for diesel and DME was 23.6 µm and 18.9 µm, respectively. The superior evaporation of DME promoted homogeneous air–fuel mixture formation in the engine cylinder, resulting in more efficient engine combustion and lower emissions [39]. The momentum loss of smaller spray droplets was faster, resulting in lower spray penetration length, suppressing the wall quenching, and aiding in sootless combustion of DME.

Microscopic spray studies of DME and diesel were carried out at three radial positions to evaluate different spray confines in atmospheric conditions of a CVSC. The spray parameters evaluated were spray droplet velocity-size distributions and distinct mean diameters. These velocities were radial, axial, and tangential. The velocities of all these spray droplets for DME were greater than diesel at every point in the spray. The maximum spray droplet radial velocities at the central position, 5 and 10 mm radial distances for diesel were 12.5, 14.1, and 20.9 m/s, and the corresponding values for DME were 30.5, 30.3, and 34.2 m/s, respectively. The tangential velocities for diesel were 28.6, 28, and 16 m/s, and the corresponding values for DME were 32.1, 28, and 20.9 m/s, respectively. However, axial velocities showed the maximum values for the corresponding radial positions among these three velocity components. The axial velocity values at three radial positions for diesel were 49.6, 48.4, and 27.8 m/s, and the corresponding values for DME were 55.5, 48.4, and 32.1 m/s, respectively. Thus, the maximum velocity for DME and diesel spray droplets were 55.5 m/s and 49.6 m/s, respectively, in the axial direction, which showed the nature of the spray droplet interactions with the ambient environment. Most DME spray droplets formed were observed to be smaller (3 µm) than diesel because of their lower surface tension and viscosity. The topmost spray droplet count for DME was nearly thrice that of diesel. The higher number of spray droplets of DME was because of its lower boiling point, surface tension, and viscosity. DME spray's mean droplet diameters were lower than diesel's at all the measurement points. Hence, the overall spray characteristics of DME were superior to diesel and generated a homogeneous air–fuel mixture, helping in efficient and sootless combustion.

The authors acknowledge Sir J. C. Bose Fellowship by SERB, Government of India (Grant EMR/2019/000920) and SBI endowed Chair Professorship from State Bank of India to Professor Avinash Kumar Agarwal, which enabled this work. The contributions of Mr. Manojit Pal and Mr. Roshan Lal for their help in developing the experimental setup are gratefully acknowledged. The authors acknowledge the technical support and assistance during the experiments of Ms. Utkarsha Sonawane, Mr. Ashutosh Jena, and Mr. Ankur Kalwar.

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent is not applicable. This article does not include any research in which animal participants were involved.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

3-D =

Three-dimensional

AC =

Alternating current

CI =

Compression ignition

CN =

Cavitation number

CRDI =

Common rail direct injection

CVSC =

Constant volume spray chamber

D₁₀ =

Arithmetic mean diameter

D₂₀ =

Surface mean diameter

D₃₀ =

Volume mean diameter

D₃₂ =

Sauter mean diameter

DME =

Dimethyl ether

FIE =

Fuel injection equipment

IC =

Internal combustion

ICEs =

Internal combustion engines

LHV =

Latent heat of vaporization

NOP =

Nozzle opening pressure

Oh =

Ohnesorge number

PDI =

Phase Doppler interferometer

PTPE =

Polytetrafluoroethylene

SI =

Spark ignition

SMD =

Sauter mean diameter

SPL =

Spray penetration length