Influence of Exhaust Gas Recirculation and Fuel Injection Timings on a Diethyl Ether–Diesel Blend-Fueled Compression Ignition Engine Open Access

Compression ignition (CI) engines have wide applications in the automotive and industrial sectors due to their higher efficiency, durability, and reliability. However, they have been criticized for emissions. Currently, fossil fuels are used as the main energy source for CI engines, emitting harmful emissions. Nitrogen oxides (NO_x) and particulate matter (PM) emitted by CI engines harm the atmosphere and human health. To solve these emission issues, emission regulatory norms have been tightened globally over the past decade. Several solutions have been proposed. One of the solutions is to optimize the fuel injection strategy and parameters such as fuel injection pressure (FIP), number of injections, fuel injection timings, etc. Engine combustion and emission benefits can be obtained with optimization of fuel injection strategy. Higher FIP showed a shorter combustion duration, resulting in higher peak pressure and heat release rate (HRR) [1]. Higher FIP increased the brake-specific fuel consumption (BSFC), exhaust gas temperature (EGT), cylinder pressure, maximum rate of pressure rise (MPRR), carbon dioxide (CO₂), and NO_x emissions while lowering ignition delay (ID), smoke emission, brake thermal efficiency (BTE), and indicated thermal efficiency [2]. Fuel injection timing is a crucial parameter influencing the combustion and emissions of a CI engine. The atmosphere into which the fuel is introduced changes with injection timing, thus varying the ID. When fuel injection starts later (towards the end of compression stroke), the temperature and pressure are initially slightly higher, reducing the ID. Hence, injection timing significantly influence the engine performance and emissions, especially on BSFC, BTE, and NO_x emissions [3]. Fuel injection timing (both advanced and retarded) have an adverse influence on BTE and BSFC at all engine operating conditions. Sayin and Canakci [4] found that the retarded injection timings produce more nitric oxide (NO) and CO₂ emissions while emitting lower hydrocarbon (HC) and carbon monoxide (CO) compared to the original injection timing of 27 deg before top dead center (bTDC). Moreover, retarded and advanced injection timings compared to the original injection timing in all fuel blends gave negative results for BSFC and BTE.

The engine performance and emission characteristics are affected by fuel injection strategies and test fuel properties [5]. Various fuel additives such as nanoparticles, water, ethers, and alcohols can be introduced in biodiesel for improving the findings. The main determining factors for combustion characteristics are the physicochemical properties such as cetane number (CN), inherent oxygen, and evaporation rate. Due to the importance of sustainability, interest in renewable alternative fuels has grown over time. The feasibility of generating these fuels from carbon-neutral sources and their higher hydrogen-to-carbon ratio allows for reduced CO₂, making them interesting for lowering the lobal warming impact. Using alternative fuels contributes to improved performance, combustion, and emission characteristics. Global demand for diesel was the motivation for fuel compatibility studies with varying injection strategies for engine development. Turner et al. [6] claimed that oxygenated fuels benefit by a multiple fuel injection strategy. Diethyl ether (DEE) is an oxygenated alternative fuel with higher oxygen content and volatility. Ether can be generated from raw sources such as crude oil, natural gas, coal, biomass, and waste. Blending multiple fuels is an effective way to improve test fuel properties and derive enefit from reduced dependency on fossil fuels [7]. DEE with biodiesel–diesel blend reduces smoke and NO_x emissions at high engine loads [8]. The higher blend of DEE in biodiesel (15%, v/v) improves NO_x, CO, HC emissions, and thermal efficiency [9]. DEE, having higher CN, acts as an ignition improver, shortening the combustion duration (CD), leading to a greater combustion rate and lower CO emissions. The lower boiling point of DEE starts an earlier combustion, increasing the peak combustion gas temperature, allowing it to achieve the activation temperature of carbon combustion quickly. Thus, it enhances hydrogen oxidation and combustion, reducing the HC emissions [10]. DEE addition lowers the test fuel density and improves fuel-air mixture formation, resulting in 50% lower smoke emissions. A 15% v/v blending of DEE lowers the BSFC and EGT because of higher ID and lower calorific value (CV).

The addition of DEE enhances spray atomization and combustion characteristics due to lower test fuel viscosity. BTE improves (by 6.5%) with increased DEE fraction in test fuel blend. The higher peak pressure and advanced start of combustion (SoC) were found for DEE blends. This is because of improved fuel droplet vaporization, higher premixed combustion, and lower autoignition temperature of DEE. Pillai et al. [11] reported higher BTE, BSFC, and decreased CO₂ and NO_x emissions using 5% DEE with biodiesel blends. They concluded that the DEE-biodiesel blend improved test fuel viability. The main parameter may be the higher volatility of DEE, improving the air/fuel mixture and combustion. Jena et al. [12] characterized the engine performance, combustion, and emissions in partially premixed combustion mode for diesel and diesel–DEE blends. BTE improved up to 1.5 times for DEE40 compared to baseline diesel. They suggested optimization of exhaust gas recirculation (EGR) which significantly reduced the NO_x emissions. In another study, Sonawane et al. [13] proposed fueling the CI engine with DEE40 at 500 bar FIP because of its superior atomization and evaporation characteristics. This indicated that a low-cost fuel injection system could be used for the maximum diesel replacement by DEE. The rich zones in the combustion chamber reduced, lowering the CO emissions due to improved spray atomization and fuel-air mixing.

It is found that most experimental studies investigated conventional diesel combustion (CDC) mode of engine operation. However, there is a gap in the literature on the recalibration potential of a light-duty CI engine fueled with oxygenated alternative fuels like primary alcohols and DEE. The present study experimentally investigates the possibility of partial diesel replacement by DEE in CI engines with an objective of attaining superior engine performance and emission characteristics. The first part of this study shows the influence of main injection and pilot injection timings on engine performance and emissions for various EGR rates. The reason for selecting these techniques is a requirement of no additional hardware modifications in the engine. The preliminary experiments were therefore performed to optimize the fuel injection timings. The second part of this study included an experimental investigation of a CI engine fueled with diesel and DEE40 at optimized fuel injection parameters. Different engine load conditions were investigated at a constant engine speed of 2200 rpm. This study demonstrated that by careful recalibration of the main and pilot injection timings, lower NO_x emissions can be obtained without compromising performance with DEE40.

The literature has a gap in the recalibration potential of a light-duty CI engine fueled with oxygenated alternative fuel like DEE. This study showed that careful recalibration of the main and pilot injection timings lowered the NO_x emissions without compromising the engine performance. The present study experimentally investigated the possibility of partial replacement of conventional diesel by DEE in CI engines. This study recommended blending DEE (up to 40% v/v) in diesel for obtaining higher BTE without compromising the engine exhaust emission characteristics.

A production-grade four-stroke, 2-cylinder, water-cooled, naturally aspirated diesel engine was chosen for the experiments. The test engine had a common rail direct-injection fuel injection system. The fuel injection parameters were controlled by using an open ECU (MoTec GPR Diesel, M-142). An eddy current dynamometer was controlled using a closed-loop control through the dynamometer control panel. This closed-loop control can provide accuracy of ±5 rpm. The engine was maintained at a constant speed of 2200 rpm by varying the engine loads to 15 Nm (low), 30 Nm (mid), and 45 Nm (high). The measurements were taken after the thermally stabilized steady-state conditions were achieved. The schematic of the engine test setup is presented in Fig. 1. The specifications of the test engine and the dynamometer are given in Table 1.

The CI engine was fueled with diesel and a blend of DEE–diesel (DEE40, 40% v/v DEE blended with diesel). The properties of DEE and diesel are given in Table 2. It was observed that DEE can be easily miscible with diesel without phase separation. The engine load was achieved for both fuels by varying the amount of fuel injected in every engine cycle. Since the heating value of DEE40 was lower than the baseline diesel, a slightly higher amount of DEE fuel was injected to achieve the same engine operating point.

Total fuel quantity injected was varied using ECU to cater to various engine load conditions at a constant engine speed of 2200 rpm. However, the pilot fuel quantity of 2 mg was kept the same for all engine operating conditions. The FIP was maintained at 1000 and 700 bar for diesel and DEE40, respectively. Lower FIP was set for the DEE40 due to its superior atomization and vaporisation characteristics [13], reducing the cost of a high-pressure fuel injection system.

The volumetric airflow rate was measured using a laminar flow element (Meriam; 50MC2-2F) installed upstream of the engine air inlet. The fuel consumption was measured by volumetric analysis using a burette mounted upstream of the fuel feed pump. The regulated emissions were measured using an exhaust gas emission analyzer (AVL; DiTEST MDS 450). This analyzer measures the concentrations of regulated species in the exhaust namely CO₂, CO, and HC. The present study compared the engine performance and emission characteristics of DEE40 vis-a-vis baseline diesel.

The injection timing significantly affects the engine performance and emissions from CI engines. Retarded or advanced injection timing diminished BTE due to incomplete combustion. Therefore, this study examined the individual effects of main injection and pilot injection timing for diesel. The optimized fuel injection timings of the main and pilot can be suggested from the research findings. The measured parameters were engine torque, HC, and NO emissions, as shown in Fig. 2. The engine was fueled with diesel and operated at a constant engine speed (2200 rpm) and medium load (30 Nm) with 0% and 20% EGR. Figure 2 shows the measured parameter variation with respect to the start of the main injection (SoMI) at a constant start of pilot injection (SoPI) of 35 deg bTDC. The torque, HC, and NO emissions reduced upon 20% EGR addition. The influence of EGR comprises of the chemical effect (CO₂ and H₂O in the exhaust gas lowers the rate of combustion), thermal effect (lower peak combustion temperature because of higher heat capacity of CO₂ and H₂O), and dilution effect (lower O₂ concentration), which reduce the peak in-cylinder temperature. Engine torque reduced significantly by advancing the SoMI timing for 0 and 20% EGR. Earlier injection timing resulted in a longer ID, lower maximum in-cylinder pressure, and engine output power [4].

A major portion of HC emissions forms due to inferior combustion because of formation of zones having over-lean charge [15]. This is due to inefficient oxidation because of the lower charge reactivity of lean mixtures at lower localised in-cylinder temperatures. Earlier injection timing from TDC to 10 deg bTDC reduces the HC emissions drastically. Advancing the SoMI timings causes an earlier SoC relative to the TDC. This causes higher temperatures as the in-cylinder charge is compressed with the piston movment towards TDC, reducing the HC emissions. Also, advanced fuel injection timings gives adequate time for superior mixture formation. However, a too-advanced injection timing may lead to wall wetting in piston crevice zones at lower ambient temperature and pressure, leading to inferior mixture formation and higher HC emissions.

Nitrogen is an inert gas; however, it oxidizes via the thermal oxidation mechanism at higher temperatures in the existence of oxygen and by the prompt mechanism in the presence of free HC radicals [16]. NO_x primarily comprises of NO, with minor concentration of nitrogen dioxide (NO₂) in the CI engine emissions. NO_x emission increases because of oxygen, temperature, and residence time at extreme conditions. NO emission increased with advancing SoMI timing for both EGRs [7]. At earlier SoMI timing, and higher peak HRR resulted in higher flame temperature, forming higher NO_x emissions [17]. Late injection timing reduced the peak in-cylinder pressure since more fuel burned after the TDC. Reduced peak cylinder pressures lowered the peak temperatures and reduced the NO_x emissions [4].

It can be concluded that the SoMI timing of 2 deg bTDC and 20% EGR was optimal for superior engine performance and lower NO_x emissions, but with a penalty of higher HC emissions. Hence, further investigations on the effect of SoPI timings on measured parameters was done at SoMI timing of 2 deg bTDC and 20% EGR. HC emissions and engine torque reduced as SoPI timing advanced with increased NO_x emissions. It was found that the effect of variations of SoPI timing was not as prominent as SoMI timing. It was found that the SoPI timing of 20 deg bTDC was better for controlling the HC and NO emissions.

Further advancement to SoPI timing of 25 deg bTDC showed a sudden reduction in torque and higher NO emissions. Therefore, the authors considered the SoPI timing of 25 deg bTDC for further investigations under CDC mode. The aim was to improve torque and reduce NO emission for diesel-fueled engines by adding DEE.

The in-cylinder pressure variations with crank angle can illustrate the advancement of combustion. Figure 3 shows the in-cylinder pressure versus crank angle diagram for the CI engine fueled with diesel and DEE40 at various engine loads. An excellent repeatability of each cycle was demonstrated by superimposing pressure fluctuations corresponding to the lowest gas excitation frequency in the combustion chamber over successive cycles. In-cylinder pressure and apparent HRR directly represents combustion and are the basis of emission characteristics. The in-cylinder pressure depends on the collective influence of the chemical HRR and the heat losses through cylinder walls. Therefore, the HRR analysis can be more practical for understanding the findings of in-cylinder pressure. The HRR curves were calculated using the measured in-cylinder pressure [3]. HRR curve demonstrates the progress of energy conversion in the combustion chamber. The HRR differs from the chemical HRR as it considers the heat losses via the combustion chamber walls. The HRR curve shows two peaks associated with the heat release from the pilot and the main combustion events.

Maximum in-cylinder pressure (P_max) increased; however, the maximum heat release rate (HRR_max) reduced with increasing engine loads. Lower fuel mass may induce inferior air–fuel mixture formation at lower brake mean effective pressures (BMEPs) in CDC mode. The width of the HRR curve also provides details about combustion. A short width indicates a shorter CD of the main fuel injection. Hence, the HRR was more spread along the CAD axis at higher loads due to higher fuel injection duration.

Moreover, DEE40 showed more HRR curve width due to a longer injection duration to maintain a particular engine load. DEE40 showed lower P_max and HRR_max than diesel because of lower CV and higher CN of DEE. The difference in P_max for both test fuels increased with increasing engine load. Rajan et al. [18] investigated the engine performance at various loads with 10% and 15% DEE as an additive and found that 15% DEE decreased the P_max. However, Raman and Kumar [19] found opposite trends as the amount of DEE in diesel increased because of the oxygenated blends, leading to superior combustion and higher P_max. The longer ID of pilot fuel blends could be one of the reasons for increasing P_max.

The amount of fuel injected was increased to cater to a higher engine load. In the present study, an engine load of 45 Nm was not achieved with 20% EGR for diesel. However, DEE40, with a 20% EGR, achieved this higher load. Fuel-bound oxygen contributed to combustion when air intake was reduced with increasing EGR. Therefore, using oxygenated alternative fuels like DEE is preferable with use of higher EGR. Figure 3(b) shows similar P_max and HRR_max for higher engine loads, which were reduced by adding EGR at 45 Nm. Late combustion phasing (CP) and inferior combustion because of the oxygen unavailability based on EGR were the causes for worsened engine performance for DEE40 at higher loads [20].

Figure 4 shows the mass fraction burned analysis of SoC, CP, and CD. The CAD corresponds to the signal sent to the injector is defined as the SoI. SoC is CAD, at which 10% of overall heat is released; CP and the end of combustion (EoC) are CAD, corresponding to 50% and 90% of overall heat released. SoC relies upon the physical as well as chemical delay. SoC indicates the start of the combustion. The combustion efficiency would be worse when CP is too retarded. This is due to lower in-cylinder temperature and oxygen when the piston is in an expansion stroke [14]. ID is the difference in crank angle between SoC and SoI. At the same time, the CD is measured as the crank angle between the SoC to EoC. The combustion of a CI engine is comprised of a premixed as well as a diffusion combustion. The premixed combustion is finished quickly because of the fast-burning speed [21]. Hence, the CD is predominately affected by diffusion combustion. ID affects the combustion and emission of CI engines by influencing atomization and air–fuel mixture formation. SoC is not changed considerably with the engine load variations [14]. The SoC in CDC mode was controlled by the dwell time between the pilot and the main injections, which remained fixed over the entire engine operating range [12]. Results showed retarded CP and longer CDs with an increasing engine load. This was predominately due to changes in fuel injected quantity at various engine loads. For example, more fuel was injected to produce greater power when the engine load increased. In their study, Zhao et al. [14] reported agreeable findings. They also mentioned that higher alcohol blends exhibited a longer CD at constant engine load.

Given its higher CN, DEE combustion initiated earlier than baseline diesel, showing earlier SoC and lower ID. Moreover, DEE formed a combustible mixture instantly due to its higher evaporation rate and lower boiling temperature than diesel. This implies that the blending of DEE markedly facilitated the auto-ignitability of the blended fuels at low-load conditions [10]. Moreover, higher in-cylinder temperature and pressure at high loads improved the evaporation of fuel blends. Hence, the minor influence of CN on ID at higher loads and variations between test fuels reduced [10]. Similar observations of minor differences in ID among all diesel–alcohol-blended fuels at higher engine loads were reported by Zhao et al. [14].

Diesel showed a shorter total CD and more advanced CP at given engine conditions than DEE40. This may be the cooling effect of the DEE blend, prolonging the combustion. EGR obstructs conventional combustion by adding non-reacting species. Lower oxygen presence in the exhaust caused ignitability issues, and the dissociation of CO₂ and H₂O reduced combustion speed and prolonged the CD (Fig. 4(b)).

Figure 5 shows the coefficient of variance (CoV) of indicated mean effective pressure (IMEP) and CP for optimized SoMI and SoPI timings at 2200 rpm. CoV indicates combustion stability and evaluates cycle-to-cycle variations and engine noise. CoV_IMEP indicates the combustion stability of each cycle. Generally, vehicle drivability problems arise for CoV (IMEP) of >10%, as suggested by Byttner et al. [22] and Heywood [3]. In contrast, a CoV of 3% is shown as the maximum threshold [23]. Combustion temperature increased with engine load, leading to lower CoV [20]. Results showed that DEE40 exhibited higher CoV than baseline diesel at low engine loads. This shows that the use of the DEE40 blend does affect the cyclic variability (irregularity). However, CoV reduced drastically as the engine load increased to 30 Nm. Lee and Kim [10] claimed stable engine operation for a wide range of loads up to 50% DEE by mass and comparable fuel conversion efficiency to baseline diesel. Diesel showed almost similar CoV for all engine loads, indicating repeatability of combustion and reliable engine operation. DEE40 showed improved CoV and combustion stability at higher loads, especially with EGR. At 45-Nm load, the D100 fueled engine was not able to operate with 20% EGR; however, the DEE40 blend showed stable operation with EGR due to the presence of fuel-bound oxygen. In addition, the DEE40-fueled engine showed stable working even at a higher load of 50 Nm but without the EGR. Higher amount of fuel injected at higher loads required more oxygen for stable combustion. At high engine loads, the presence of fuel-bound oxygen in DEE40 promotes superior combustion, even with EGR, which was not exhibited by baseline diesel. This showed the capability of DEE to improve high engine load performance.

MPRR is one of the origins of combustion noise that induces discontinuity in the cylinder pressure frequency spectrum and increases the extent of high-frequency zones [24]. Figure 5(c) shows the MPRR of diesel and DEE40. MPRR improved with increasing engine load. Results showed a lower MPRR for DEE40 than baseline diesel, indicating smoother working of the engine.

In the present study, two different test fuels were used; hence, it is unreliable to compare BSFC because of the lower heating value of DEE. Instead, it is more appropriate to use brake-specific energy consumption (BSEC). BTE indicates the amount of fuel energy converted to brake power output. The lowest BTE of ∼21% was noticed for diesel at a lower engine load of 15 Nm due to lower in-cylinder temperature, resulting in lower combustion efficiency. However, BTE increased slightly with engine load due to higher in-cylinder temperatures. Selvan [25] reported similar results of higher BTE at higher engine loads because of improved fuel droplet vaporization and proper fuel-air mixing, resulting in lower ID. BTE for the DEE40 blend was superior to diesel, attributed to DEE's lower density, viscosity, and surface tension, leading to superior atomization and combustion [13]. DEE40 blend required more heat to vaporize as DEE has higher latent heat of vaporization, which deteriorated the fuel conversion efficiency. However, the higher volatility of DEE improved fuel-air mixing and formed a homogenous charge, improving the fuel conversion efficiency [10]. In addition, fuel-bound oxygen assisted in the efficient combustion of locally fuel-rich zones.

The superior fuel properties of DEE40 compensated for the lower CV of DEE, showing higher BTE and lower BSEC than diesel at constant engine operating conditions [11]. Similar findings of improved BTE due to improved evaporation, homogenous mixture formation, and combustion were reported by Raman and Kumar [19]. However, they reported a subsequent increase in DEE from 10% to 20%, deteriorated BTE marginally compared to diesel owing to dominant charge cooling over the CN of the DEE/diesel blend. DEE/diesel blends bring a cooling effect, eventually lowering the peak combustion temperature, leading to inefficient combustion. As the engine load increased, BSEC decreased considerably. The underlying cause for the reduced BSEC with a higher load was superior fuel conversion and mechanical efficiency. DEE40 showed lower BSEC than diesel at all engine loads. DEE acted as an ignition enhancer in premixed charge, leading to more efficient combustion and reducing BSEC [19]. More amount of fuel energy was required for diesel compared to DEE40. This showed the DEE as a promising and efficient alternative fuel for partial diesel replacement. Figure 6(b) shows that BTE reduced by an absolute value of ∼4% with the addition of EGR at 45-Nm load for DEE40. BSEC is reciprocal of BTE and showed opposite trends at each test condition.

Figure 7 compares CO, HC, and NO emissions with respect to engine load for both test fuels. Conversion of raw emission concentration to “g/kWh” is done by using the methodology followed in our previous publication [26]. CO emissions from the CI engine were due to incomplete combustion, influenced by in-cylinder temperature and oxygen presence. The lower in-cylinder temperature at 15-Nm load conditions due to a leaner fuel-air mixture resulted in higher CO emission. Increasing the amount of fuel injected improved the CO emission at 30 Nm due to higher in-cylinder temperature, resulting in superior combustion [4]. However, further increase in fuel quantity for achieving 45 Nm increased the CO emission for baseline diesel. The oxygen availability in DEE40 expedited the CO oxidation and lowered CO emission than baseline diesel at a 45-Nm engine load without the EGR [4]. However, trends were opposite at lower engine loads with EGR. Higher CO emissions for DEE40 were due to its higher latent heat of vaporization, which cooled the in-cylinder combustion, reducing the CO oxidation rate. However, the influence of charge cooling was dominant at higher engine loads. Hence, CO emission rely on the dominance between in-cylinder cooling over the fuel-bound oxygen and CN for diesel–DEE blends. CO emission were higher with the addition of EGR for DEE40 as shown in Fig. 7(b).

The variations of HC emissions improved with increasing engine load [19]. DEE40 lowered the HC emissions because of the presence of oxygen. The higher CN advanced the combustion and improved HC emissions for diesel–DEE blends. Sonawane et al. [13] reported that the DEE–diesel spray exhibited higher vaporization and smaller droplets downstream of the spray than baseline diesel. This resulted in efficient air–fuel mixture formation and superior combustion. Higher HC emissions can be explained by the lower BTE resulting in inferior combustion. Our findings are in agreement with the results by Lee and Kim [10], Pillai et al. [11], and Ahlawat et al. [27], while they are contradictory to the results presented by Rakopoulos et al. [28]. Higher HC emissions with DEE addition were due to the charge-cooling effect, leading to poor fuel droplet evaporation and fuel-air mixing. Contradiction in findings may be because of the diverse experimental setups and test conditions.

NO_x emissions depend on peak combustion temperature, and residence time, and local oxygen availability. A 15-Nm load exhibited higher NO emissions compared to 30 Nm. A higher load more often resulted in higher in-cylinder temperature and NO_x emissions. However, Fig. 7 shows a declining tendency with increasing load to 30 Nm. It is worth noting that the unit of NO is g/kWh, indicating that thebrake power impacted the brake specific NO emission. The lower NO with increasing load was because of the higher power that surpassed the higher NO formation [14]. Higher NO emission were found at 45 Nm without EGR, which use of EGR further reduced. The lower HRR of DEE40 reduced the NO formation by thermal oxidation and reduced free radicals available in combustion by prompt mechanism. In addition, P_max and HRR_max reduced because of the charge-cooling effect of DEE, reducing the NO emission in DEE blends [9]. Our results are in agreement with Rakopoulos et al. [28], Pillai et al. [11], and Sadhik Basha [29]; however, contradictory findings were presented by Ahlawat et al. [27] and Lee and Kim [10]. The opposite NO_x trends are explained by fuel-bound oxygen, resulting in superior combustion and greater formation of NO due to high-temperature combustion.

In Fig. 7(b), 20% EGR showed negligible influence on HC emissions, suggesting further investigations are required at higher EGR. However, HC emissions were higher at 50-Nm load. CO emission were adversely impacted by 20% EGR rate. However, NO emissions improved. Increased CO and HC emissions with load were because of lesser ambient air presence [30]. Similar results were obtained by Ozturk and Can [20], particularly at the mid and high loads. They reported that EGR introduction to biodiesel blend lowered the NO_x, and smoke emissions. The higher global equivalence ratio and lower oxidation temperatures because of charge dilution and thermal effects of EGR affected the combustion adversely. Higher combustion temperature and overall fuel-air ratio with increased engine loads led to higher NO_x formation [30]. However, DEE40 reduced the NO_x emission due to its higher latent heat of vaporization.

Three moderate changes in EGR, injection timings, and blending of DEE were explored in the CDC mode in a CI engine. The findings revealed that the 20% EGR improved the NO emissions; moreover, the retarded SoMI timing of 2 deg bTDC was the most effective method for improving the engine performance and NO emissions. SoPI timing of 20 deg bTDC was better for HC and NO emissions. Further advancement to SoPI timing to 25 deg bTDC showed a sudden reduction in torque and increased NO emissions. Therefore, the authors considered the SoPI timing of 25 deg bTDC for further investigations in the CDC mode.

P_max was the same for increasing engine loads. However, HRR_max reduced. HRR curve had wider spread along the CAD axis at higher loads due to higher injection duration. Moreover, DEE40 showed a wider HRR curve as more fuel was injected to maintain a particular engine load. DEE40 exhibited higher CoV than baseline diesel at a low engine loads, which improved drastically as the engine load increased. BTE of DEE40 blend was superior to baseline diesel due to its lower density, viscosity, and surface tension, resulting in enhanced spray atomization and superior combustion. DEE40 showed a reduction in NO_x emissions due to its charge-cooling effect. HC emissions were lower for DEE40 with a slight penalty in CO emission. It can be concluded that diesel–DEE blend (DEE40) could be used as an alternate fuel for diesel engines without any hardware modifications.

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent was obtained for all individuals. Documentation is provided upon request. 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

BSEC =

Brake-specific energy consumption

BSFC =

Brake-specific fuel consumption

bTDC =

Before TDC

BTE =

Brake thermal efficiency

CAD =

Crank angle degree

CD =

Combustion duration

CI =

Compression ignition

CN =

Cetane number

CO =

Carbon monoxide

CO₂ =

Carbon dioxide

CoV =

Coefficient of variance

CP =

Combustion phasing

CV =

Calorific value

ECU =

Engine control unit

EGR =

Exhaust gas recirculation

EGT =

Exhaust gas temperature

EoC =

End of combustion

FIP =

Fuel injection pressure

HC =

Hydrocarbons

HRR =

Heat release rate

HRR_max =

Maximum heat release rate

ID =

Ignition delay

IMEP =

Indicated mean effective pressure

MPRR =

Maximum pressure rise rate

NO =

Nitric oxide

NO₂ =

Nitrogen dioxide

NO_x =

Nitrogen oxides

SoC =

Start of combustion

SoI =

Start of injection

SoMI =

Start of the main injection

SoPI =

Start of pilot injection

TDC =

Top dead center