Technical Feasibility of Dimethyl Ether-Fueled Mechanical Fuel Injection System-Equipped Multicylinder Compression Ignition Engine

Mechanical fuel injection system-based internal combustion engines are robust, cost-effective, and easily adaptable to harsh conditions, making them ideal for road transport, off-road vehicles, marine engines, generators, mining and construction machinery, etc. However, these spark ignition and compression ignition (CI) engines are fueled by fossil-based fuels, which are expensive and emit harmful particulate matter and oxides of nitrogen [1]. Alternative and cleaner biofuels such as dimethyl ether (DME) can be produced indigenously from biomass, municipal solid waste, methanol, low-grade coal, and natural gas, and they can replace conventional fossil-based fuels [2]. DME's higher cetane number (55–70) than diesel (40–55) indicates its suitability for CI engines [3]. DME also has a much simpler molecular structure (CH₃-O-CH₃). It is an oxygenated (34.8% w/w oxygen) fuel without a C–C bond. Soot-forming Cn (n ≥ 2) radicals are absent in DME molecules. Therefore, a DME-fueled engine emits negligible soot and lower unburnt hydrocarbons [4]. DME possesses physicochemical properties similar to liquified petroleum gas. It is a non-hazardous, colorless, and odorless gas under atmospheric conditions. To be liquified, DME must be pressurized above 5 bar at room temperature (20 °C). DME has a very low boiling point temperature (−24.8 °C), lower kinematic viscosity (<0.1 cSt), and lower surface tension (0.012 N/m), enabling quicker spray droplet atomization and vaporization. Its fuel–air mixing in the combustion chamber is enhanced, leading to efficient combustion and higher brake thermal efficiency (BTE). Efficient combustion is the key to higher engine performance and lower tailpipe emissions. In addition, the freezing point of DME is also lower (−141 °C), making it an ideal fuel for use in subzero climatic conditions. The physicochemical properties of DME and diesel are given in Table 1.

However, DME also has certain limitations, which must be addressed when developing a dedicated DME-fueled engine and its fuel injection system. The presence of an oxygen atom in its molecular structure significantly reduces the DME's calorific value to 28 MJ/kg compared to baseline diesel (44 MJ/kg). Consequently, a DME-fueled engine requires ∼1.5 times higher fuel mass than baseline diesel to generate diesel equivalent power [9]. Generally, DME's fuel injection equipment (FIE) experiences some leakage and wear issues due to poor lubricity and lower viscosity of DME, which demands the addition of suitable lubricity additive in appropriate quantity in the fuel. DME is incompatible with elastomers; hence, DME-compatible materials, such as stainless steel, teflon, polytetrafluoroethylene, etc., should be used in the FIE. DME has a significantly higher vapor pressure, resulting in vapor lock issues in the fuel lines and high-pressure pump. Therefore, DME must be pressurized well above its saturation pressure throughout the fuel supply/return lines and the FIE to prevent vapor lock formation. DME has higher compressibility than diesel, requiring higher compression work by the high-pressure pump. Arcoumanis et al. [10] reported 10% higher compression work for DME than baseline diesel in an open system. Several researchers developed customized FIE for DME-fueled engines and performed experiments [10–14].

Numerous independent experimental and computational studies have been conducted to understand DME-fueled engines with respect to the base engine for combustion, performance, and emissions. Kim et al. [15] reported higher indicated mean effective pressure for DME than mineral diesel. Zhang et al. [16] reported lower maximum in-cylinder pressure (P_max), heat release rate (HRR), rate of pressure rise (RoPR), and exhaust gas temperature (EGT) for DME-fueled CI engines than mineral diesel. DME-fueled engines experienced prolonged injection duration but shorter ignition delays at varying engine load conditions. When the engine load was increased, the HRR varied from two peaks to one. Also, DME-fueled engines exhibited higher torque/power than mineral diesel-fueled engines, especially at lower engine speeds. Longbao et al. [17] indicated a longer injection delay for DME, attributed to the lower acoustic velocity and a lower pressure wave propagation velocity (Diesel: 1330 and DME: 980 m/s). However, they reported a shorter ignition delay and lower P_max, maximum heat release rate (HRR_max), and RoPR for DME. They also reported higher thermal efficiency for DME than baseline diesel, except at high load conditions. Sorenson and Mikkelsen [18] reduced the nozzle opening pressure (NOP) to 80 bar for DME and reported superior engine performance, equivalent thermal efficiency, and quieter combustion. Kajitani et al. [19] reported higher P_max with an NOP of 88.2 bar for DME than 201 bar for baseline diesel and lower brake-specific energy consumption (BSEC) and EGT. The combustion duration was smaller despite DME's longer injection duration. Gill et al. [20] reported that DME-fueled engines produced diesel-equivalent engine power. Loganathan et al. [21] reported a lower and higher HRR_max for DME than baseline diesel. These simulation results matched well with the experimental results of Kajitani et al. [19]. Sato et al. [22] reported a lower HRR_max for DME in premixed combustion while a higher HRR_max for diffusion combustion than diesel-fueled engines. DME combustion experienced a longer late combustion phase than mineral diesel. Park et al. [23] reported an increased P_max with an advanced injection timing for DME due to superior charge premixing. DME spray was uniformly distributed in the constant volume combustion chamber, with higher oxygen utilization efficiency. Kim et al. [24] reported lower P_max for DME than baseline diesel, owing to its lower calorific value when the same DME and diesel quantities were injected. They also reported that DME ignition started earlier than baseline diesel, although the fuel injection timing was identical. Benajes et al. [25] reported a faster HRR leading to higher P_max and net indicated efficiency for DME than baseline diesel. They cited shorter ignition delay and superior fuel–air mixing for this trend. Youn and Jeon [26] reported higher P_max and advancement in HRR for DME due to its shorter ignition delay. However, DME's combustion duration (CD) was longer due to a longer diffusion combustion. An et al. [27] observed a higher in-cylinder pressure for DME upon increasing the fuel injection pressure (FIP). There was no change in the ignition delay because of increasing FIP, but the CD and injection duration were reduced. Youn et al. [28] reported higher HRR and in-cylinder pressure for DME than mineral diesel. The peak in-cylinder pressure increased upon advancing the injection timing. Tripathi et al. [6] developed a DME-fueled genset engine for agricultural applications. Since the objective was to use an existing high-pressure pump (HPP) and perform minimal hardware modifications, the fuel injection timing was changed with respect to the baseline diesel engine. The fuel injection timing was advanced by 2°CA and 8°CA at lower loads (1.25 and 2.50 bar BMEP) and higher loads (3.75, 5.00, and 6.25 bar BMEP), respectively. DME-fueled engines exhibited lower BTE, EGT, HRR_max, RoPR_max, and a maximum in-cylinder pressure than baseline diesel. CO was ∼20% higher and ∼76% lower for DME-fueled engines than diesel engines at lower and higher loads, respectively. Agarwal et al. [29] assessed the technical feasibility of the existing engine for DME fueling without performing hardware modifications and by modifying FIE. The existing engine generated 46% lower torque and 57% lower power output upon DME fueling without performing any modifications in FIE. The DME-fueled engine produced diesel-equivalent power/torque using modified FIE. Also, combustion noise and EGT were lower for DME than baseline diesel. BTE was also higher for DME-fueled engines than baseline diesel. Agarwal et al. [30] also reported that DME-fueled engines emitted only nanoparticles and accumulation mode particles from the tailpipe. Accumulation mode particles from the tailpipe were negligible compared to baseline diesel. Another study [31] found that DME-fueled engines exhibited lower cyclic variations than baseline engines, especially at higher engine loads. The overall research gap was: (1) studies reported in the open literature were mostly for common rail direct injection systems using electronic control unit (ECU) and (2) studies on DME engines with mechanical fuel injection systems were mostly for single-cylinder engines at fixed engine speed. However, to test the reliability and stability of DME engines, testing a multicylinder engine at variable engine speeds/loads is important. Also, the distinguishing features of this study with respect to other studies performed by authors are: (1) this study evaluated the capability of DME for operating the engine under part throttle part load conditions and (2) the speed variations with respect to different loads are assessed for DME fueled engine. Therefore, this study aimed to evaluate the DME-fueled multicylinder engine's performance and combustion characteristics using a mechanical fuel injection system under a complete engine operating envelope. The findings of this study can be easily adapted to any ECU-based engine with appropriate calibration [32].

Attempts for powering the existing engine with 100% DME yielded two critical findings: (1) the nozzle hole diameter of injectors must be changed such that a higher DME quantity could be injected and (2) the fuel injection pump would need certain modifications so that it injects DME at a higher FIP for a longer duration. Therefore, this experimental investigation aimed to use 100% DME in an existing CI engine by modifying FIE. Fuel injectors and HPP were modified to produce power output similar to baseline diesel fueling using 100% DME.

The engine was coupled to a regenerative alternating current (AC) dynamometer, as shown in Fig. 1. The specifications of the engine are given in Table 2.

DME was filled in a specially designed customized tank of 28.2-L capacity at 5-bar pressure. A lubricity additive (1000 ppm) was mixed with DME to compensate for DME's lower lubricity using a dosing pump while filling the tank. After filling DME, the cylinder was pressurized to 20 bar using compressed nitrogen. Further, DME pressurization was achieved up to 70 bar using a compressed air-driven pneumatic pump. Then, DME was supplied as input to the HPP's inlet. The pump pressurized DME to inject it into the engine cylinder via three injectors, one for each engine cylinder, connected by a high-pressure fuel line. Engine operating conditions selected for this experiment are listed in Table 3.

The calorific value of fuel is in kJ/kg, power is in Watts (W), and the fuel flowrate is in kg/h in this equation. All data were measured once the engine achieved thermal stabilization. The error bars in the plots represent the standard deviations of these measurements. The instruments used for this experiment are listed in Table 4.

The fuel injection timing settings for diesel and DME were identical in this study. The actual injection timing of the fuels might change slightly due to differences in their physicochemical properties, such as lower bulk modulus and density of DME compared to baseline diesel. This section discusses the engine performance results at varying engine loads and speeds in the entire engine operating envelope. The effects of engine load variations at constant speed (medium speed: 1600 rpm) and speed variations at constant load (medium load: 3.88 bar BMEP) are analyzed in subsequent sections.

Figure 2 shows the variations in the high-pressure fuel line's pressure corresponding to crank angle degrees (CAD) at varying engine loads at 1600 rpm. The values of the fuel line pressure were non-dimensionalized and represented in the arbitrary unit (a.u.). DME's higher compressibility and low density change the pressure buildup in high-pressure fuel lines. The peak of DME fuel line pressure was lower than mineral diesel. Due to DME's higher compressibility, the high-pressure pump had to do more compression work, adversely affecting the pressure buildup characteristics [7].

Higher compressibility and higher injected DME mass increased the injection duration. The injection of 50% higher DME quantity than baseline diesel was suitably assisted by the modified DME injectors having larger nozzle diameters. The peak fuel line pressure increased with increasing engine load. Once the fuel was injected, more residual pressure oscillations were observed for DME than baseline diesel. Sorenson et al. [14] and Teng et al. [34] also reported fuel line pressure buildup characteristics similar to this study. DME's high compressibility leads to higher compression energy stored in the high-pressure DME during its injection, which is the main cause of these oscillations [10].

Figure 3 shows that BTE was higher for DME by an average of ∼8.59% than mineral diesel. Other researchers also reported higher thermal efficiency for DME-fueled engines [15,17,21,25].

The highest BTE (40.9%) was observed for DME-fueled engines at a high load (5.18 bar BMEP) and medium (1600 rpm) engine speed. DME's higher BTE is attributed to its oxygenated molecule, leading to superior combustion. Also, favorable physicochemical properties such as lower boiling point and lower auto-ignition enhance its combustion characteristics. DME evaporates quickly in the hot in-cylinder environment and undergoes excellent fuel–air mixing, forming a homogeneous charge, leading to superior combustion than baseline diesel. BTE increased because of higher in-cylinder temperature and efficient combustion for both fuels at higher engine loads. Even though the load was varied, the friction power (FP) possibly remained constant for a particular engine speed. However, as the load increased, the brake power (BP) and indicated power increased [35], increasing the BTE. Figure 3 shows the trends for BSEC, which were opposite to those for BTE. BSEC indicates energy consumption per unit of useful power generated by the engine. DME's lower BSEC indicated that the DME-fueled engine efficiently converted fuel chemical energy into mechanical energy compared to baseline diesel. BSEC reduced upon increasing the engine load since, at lower loads, BTE was lower.

DME-fueled engines exhibited higher (~45%) BSFC than baseline diesel engines, as depicted in Fig. 4. Also, it reduced once the engine operated from low-to-high loads. The in-cylinder pressure increased while the engine load increased as the supplied fuel mass increased.

EGT is a qualitative measure of the combustion characteristics of any engine. It represents the average temperature of the combustion products. Figure 5 shows that the EGT increased for both test fuels as the load increased at all engine speeds. DME-fueled engines exhibited lower EGT than baseline diesel-fueled engines, which aligns with previous studies by other researchers [16,19,24,36,37]. The highest EGT was observed for diesel at a high load (6.47 bar BMEP) at 2000 rpm. In contrast, DME's EGT was 15 °C lower than baseline diesel.

EGT increased upon increasing the engine speed for both baseline diesel and DME. The residence time for heat transfer to cylinder walls was generally longer at lower engine speeds, leading to higher heat losses. This also led to higher specific fuel consumption.

In the in-cylinder pressure data, qualitative information about what is happening in the engine cylinder can be detected. A comprehensive analysis of in-cylinder pressure data was performed for baseline diesel and DME. The ensemble average of the in-cylinder pressure data corresponding to the CAD for 250 cycles at various engine loads was used to compare the DME and baseline diesel combustion characteristics.

An increase in the peak in-cylinder pressure was observed for baseline diesel and DME upon increasing the engine load, as shown in Fig. 6. At 1.29 bar BMEP, a DME-fueled engine produced higher in-cylinder pressure than baseline diesel. DME's superior spray atomization and vaporization characteristics possibly dominated over its higher latent heat of vaporization. DME mixed instantly with the in-cylinder air at a lower engine load, leading to superior combustion and higher peak in-cylinder pressure than baseline diesel. However, an opposite trend was observed at medium and high loads. At 3.88 and 6.47 bar BMEP, the P_max was lower by 11.12% and 11.87%, respectively, for DME than baseline diesel. This was mainly attributed to the higher latent heat of vaporization of DME. Several researchers [3,16–18,21,24,38,39] reported in-cylinder pressure trends for both test fuels, in line with the trends seen in the present investigation. A higher calorific value of baseline diesel (44 MJ/kg) than DME (28 MJ/kg) demanded a higher DME quantity for producing diesel equivalent power. Upon increasing the engine load, the peak in-cylinder pressure retarded since more fuel was injected at higher loads, taking more time for evaporation and combustion.

The heat generation from the fuel combustion in the engine combustion chamber is called the HRR. Figure 6 shows the HRR trends on the secondary Y-axis for both test fuels. It is observed that (HRR_max)_Diesel > (HRR_max)_DME at all loads. The higher calorific value of baseline diesel might be one of the reasons for higher HRR [40]. It can also be seen that the start of the sharp rise of HRR shifted toward the top dead center (TDC) with increasing engine load. It could be due to increased in-cylinder temperature and reduced ignition delay at higher loads. The actual injection timing of DME retarded more than mineral diesel because of DME's higher compressibility. The higher cetane number of DME shortened the ignition delay, leading to lesser DME accumulation before ignition, thereby a lower HRR in the premixed combustion phase. Other researchers also reported a lower HRR for DME than baseline diesel [16,17,19,21,22,39,41,42], which aligns with the trend observed in this study. Fuel pressure buildup in the high-pressure line for DME was lower than baseline diesel, as shown in Fig. 2. The pressure difference between the injector nozzle and combustion chamber influences the fuel injection velocity, affecting all downstream processes in the engine cylinder. This helps spray atomization, rapid evaporation, and flame traverse to better utilize the air [43]. Despite DME having superior spray atomization and vaporization characteristics, it can be inferred that these low-pressure differences lead to DME's lower spray velocity and, hence, a lower utilization of intake air, reducing the HRR. DME combustion shifted from dominantly premixed combustion to dominantly mixing-control combustion because of the shorter ignition delay of DME at higher engine loads, as shown in Fig. 6. At 1.29 bar BMEP, combustion primarily occurred in the premixed combustion phase as fuel consumption was also lower at the low-load conditions. However, most heat was released dominantly at higher loads in the mixing-controlled phase for DME. The wider flammability of DME than baseline diesel improved its combustion efficiency [40]. As shown in Fig. 6, a steep rise in HRR with increasing engine load was observed for baseline diesel and DME. At 1.29 bar BMEP, DME showed an earlier HRR than mineral diesel. The lower in-cylinder pressure and temperature are the reasons for the lower HRR at low loads than at high loads. This effect was more pronounced in baseline diesel because of the lower cetane number of diesel than DME.

Cumulative heat release (CHR) was initially slightly negative for both test fuels because the injected fuel evaporated by absorbing heat from the hot gases in the combustion chamber, as shown in Fig. 7. DME used more heat from the hot gases in the combustion chamber to evaporate due to its high latent heat of vaporization; hence, a higher negative CHR was observed for DME. The boiling point of DME is much lower than baseline diesel, making it more volatile, speeding up the vaporization, and generating a dominant charge cooling effect.

Results showed that at 1.29 bar BMEP, a sharp rise in the CHR was more advanced for DME than baseline diesel. However, CHR was lower for DME due to its longer injection delay caused by its higher compressibility. Higher CHR of DME than baseline diesel represents superior mixing-controlled combustion of DME due to superior atomization and vaporization characteristics, which enhance DME–air mixing for charge formation.

Combustion parameters such as start of combustion (SoC), combustion phasing (CP), and end of combustion (EoC) were derived from the CHR. In this study, SoC and EoC represent the crank angles at which 10% and 90% CHR occurred. The crank angle at which 50% CHR occurred is considered “CP”. The difference between SoC and EoC is represented as CD. Superior DME combustion characteristics lead to simultaneous fuel vaporization and auto-ignition in the combustion chamber, resulting in shorter CD than baseline diesel [19,21]. Variations in other combustion parameters of DME than baseline diesel at 1600 rpm engine speed are shown in Fig. 8.

It can also be seen in Fig. 8(a) that with an increase in engine load, late burning occurred since the CP retarded. This led to less time for cooling the engine at higher speeds than lower ones. CP retarded upon increasing engine load for both test fuels. The CP was highly influenced by the engine loads, i.e., (CP)_DME < (CP)_Diesel at no load and 1.29 bar BMEP and (CP)_DME > (CP)_Diesel at 6.47 bar BMEP. CD increased upon increasing engine load for DME and diesel. The CD was shorter for DME, representing faster combustion, as shown in Fig. 8(a). Faster combustion of DME leads to higher heat losses to the surroundings during the expansion stroke, further lowering the EGT. Also, the CD increased with increasing engine load since the fuel–air mixture became proportionately richer, taking longer to burn.

The SoC is affected by the ignition delay. SoC was more advanced (closer to TDC) for DME than baseline diesel at lower loads, and its value became less retarded upon increasing the engine load, as shown in Fig. 8(a). At 3.88 and 6.47 bar BMEP, SoC was comparable for DME and diesel, while the CD was lower for DME, as shown in Fig. 8(a). The SoC of DME was slightly advanced at 1.29 bar BMEP and slightly retarded at 6.47 bar BMEP and showed a mixed trend. The EoC of baseline diesel and DME retarded with increasing engine load. The EoC of DME occurred earlier than baseline diesel due to fuel oxygen and higher cetane number, which promoted faster combustion. DME's HRR_max was lower than mineral diesel, as shown in Fig. 8(a). At lower loads, the CAD of HRR_max advanced for DME than baseline diesel. Beyond 2.59 bar BMEP, DME exhibited almost the same crank angle position for HRR_max as baseline diesel.

P_max increased for DME and baseline diesel upon increasing engine load, as shown in Fig. 8(b). At 1.29 bar BMEP, P_max was higher for DME, while at all other loads, it was higher for baseline diesel. °CA P_max advanced for DME at all loads, except at 3.88 bar BMEP, where it was the same for both. The maximum rate of pressure rise (RoPR_max) was higher for baseline diesel than DME at all loads, except no load and 1.29 bar BMEP. A higher RoPR_max was observed for DME than mineral diesel, as shown in Fig 8(b). The °CA RoPR_max of DME was more advanced than baseline diesel, and the numerical value decreased with load for both test fuels.

Figure 9 shows a reduction in BTE for both DME and baseline diesel upon increasing engine speed. This might be because of higher frictional losses at higher engine speeds [35]. BTE was higher for DME than baseline diesel at all medium-load engine speeds.

BSEC followed an opposite trend of BTE and was lower for DME than baseline diesel at all speeds at medium engine load. As the engine speed increases, the friction power (FP) increases rapidly, reducing the rate of increase of BP than the rate of increase in fuel consumption, resulting in higher BSFC at higher engine speeds [39]. At higher engine speeds, the time for heat transfer per cycle through the combustion chamber is reduced. Therefore, it is seen that the EGT increased as the engine speed increased. However, at 2000 rpm, the EGT reduced for DME.

At 1600 rpm, the in-cylinder pressure decreased for both DME and diesel, as shown in Fig. 10. This is attributed to lower premixed combustion heat release at higher engine speeds. P_max was lower for DME than mineral diesel at 3.88 bar BMEP at all engine speeds. HRR_max was the highest at 1600 rpm for baseline diesel and DME and the lowest at 2000 rpm. At all engine speeds, lower HRR was observed for DME than baseline diesel, attributed to the lower fuel mass of DME burned in the premixed combustion phase.

While the engine speed increased, the ignition delay increased because of the higher fuel mass injected. However, it reduced and became almost identical to baseline diesel at 1600 rpm. The in-cylinder pressure was reduced at higher engine speeds due to shorter heat release duration in the premixed combustion phase. At 1600 rpm, the peak HRR increased slightly for DME than baseline diesel. Increased in-cylinder turbulence and fuel–air mixing improved with increasing engine speed, which improved the HRR.

On the contrary, the heat loss and expansion cooling reduce the HRR, and the net effect is reflected in the HRR curve. HRR during the mixing-controlled combustion phase was controlled by fuel atomization, vaporization, and fuel–air mixing processes [33]. Excellent vaporization and atomization characteristics of DME led to superior charge mixing, justifying the domination of mixing-controlled combustion. The engine speed strongly influenced the fuel–air mixture burning rate [44]. The CD increased slowly with increasing engine speed [45]. Therefore, adequate time was unavailable for increased injected fuel quantity to burn, leading to a lower HRR_max.

Figures 11(a) and 11(b) show variations in different combustion characteristics for different engine speeds at 3.88 bar BMEP. HRR_max increased when the engine speed increased from 1200 to 1600 rpm for baseline diesel and DME and decreased when the engine speed increased from 1600 to 2000 rpm. At all engine speeds, HRR_max was lower for DME than mineral diesel. The °CA HRR_max retarded for both baseline diesel and DME when the engine speed increased from 1200 to 2000 rpm. °CA HRR_max was retarded for DME compared to baseline diesel. This was because of the retarded SoC at 2000 rpm. At 1600 rpm, the SoC was almost similar for both test fuels, but it was retarded for DME at 2000 rpm than mineral diesel because of higher ignition delay. Higher compressibility of DME contributed to retarded SoC of DME than mineral diesel. A longer ignition delay was observed for DME than baseline diesel at 1200 and 2000 rpm. However, it reduced and became almost equal at 1600 rpm. EoC was earlier for baseline diesel than DME at 1200 rpm; however, at 2000 rpm, this trend reversed. At 1600 rpm, EoC was nearly the same for both test fuels. Diesel's CP retarded, but DME's CP advanced as engine speed increased. CP for DME advanced at 2000 rpm; however, it retarded at 1200 and 1600 rpm than baseline diesel. CD decreased at 1200 and 1600 rpm engine speeds and showed similar values for both test fuels. However, CD for DME was shorter than mineral diesel at 2000 rpm.

At higher engine speeds, increased turbulence complemented DME properties, leading to simultaneous fuel vaporization and auto-ignition in the combustion chamber, leading to faster combustion [19,21,42]. P_max decreased with increasing engine speed for diesel and DME, as shown in Fig. 11(b). At all engine speeds, (P_max)_DME < (P_max)_Diesel. DME's °CA P_max was advanced, whereas, for mineral diesel, it retarded with increasing engine speed. At 1600 rpm, °CA P_max was nearly similar for baseline diesel and DME. For diesel, RoPR_max increased with increasing engine speed. However, it first increased and then decreased with increasing engine speed for DME. °CA RoPR_max retarded for both baseline diesel and DME, but retardation was higher for baseline diesel.

The combustion and performance characteristics of a 100% DME-fueled multicylinder compression ignition engine were evaluated using a customized mechanical FIE. The entire engine operating envelope covered varying loads (1.29 to 6.47 bar BMEP) and speeds (1200, 1600, and 2000 rpm). This operating envelope showed the DME engine's extensive and reliable operating range with consistent performance. Higher BTE was observed for DME than baseline diesel at test engine speeds and loads. BSEC for DME was lower than mineral diesel, indicating superior conversion of fuel's chemical energy to mechanical energy in this engine. The EGT was lower for DME than baseline diesel at 2000 rpm engine speed. For 1600 rpm, the P_max of DME was higher at 1.29 bar BMEP, whereas it was lower at 3.88 and 6.47 bar BMEP. HRR mainly occurred in the mixing-controlled combustion phase for DME at higher engine loads. The change in actual injection timing for DME at 1600 rpm positively affected the combustion characteristics. The fuel line pressure showed that with a slightly longer injection duration for DME, the injection of 50% more fuel mass than baseline diesel was suitably assisted by the modified FIE. DME exhibited superior BTE, lower BSEC, and similar EGT at varying engine speeds than mineral diesel. DME exhibited lower P_max and retarded heat release with increasing engine speeds. The combustion duration of DME and baseline diesel also decreased with increasing engine speed. The DME engine with modified FIE delivered diesel-equivalent performance and combustion characteristics in the entire engine operating envelope. The findings of this study can be easily adapted to any ECU-based engine with appropriate calibration, and the effect of different injection parameters such as fuel injection timing, pressure, mass, and dwell timing can be optimized for further improvement in engine performance.

The authors express gratitude for the J C Bose Fellowship by SERB, Government of India (Grant EMR/2019/000920), and the SBI endowed Chair Professorship by State Bank of India to Professor Avinash Kumar Agarwal.

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.

P_max =

maximum in-cylinder pressure

AC =

alternating current

BP =

brake power

BSEC =

brake-specific energy consumption

BSFC =

brake-specific fuel consumption

BTE =

brake thermal efficiency

CAD =

crank angle degree

CD =

combustion duration

CI =

compression ignition

CP =

combustion phasing

DAQ =

data acquisition system

DME =

dimethyl ether

ECU =

electronic control unit

EGT =

exhaust gas temperature

EoC =

end of combustion

FIE =

fuel injection equipment

FP =

frictional power

HPP =

high-pressure pump

HRR =

heat release rate

HRR_max =

maximum heat release rate

LFE =

laminar flow element

NOP =

nozzle opening pressure

RoPR =

rate of pressure rise

RoPR_max =

maximum rate of pressure rise

SoC =

start of combustion

TDC =

top dead center