Full Length ArticleDi-ethyl ether-diesel blends fuelled off-road tractor engine: Part-II: Unregulated and particulate emission characteristics
Graphical abstract
Introduction
The world’s energy consumption is increasing rapidly due to increasing population, enhanced socio-economic development, and speedy urbanization. Global energy is currently (∼85%) extracted from fossil-based non-renewable resources such as petroleum, coal, and natural gas [1]. Irrespective of the sector consuming energy, energy conversion systems release harmful pollutants, which lead to global warming, eventually resulting in serious human health hazards and irreversible climate changes. The transport sector is regarded as one of the major sectors responsible for environmental deterioration since transport vehicles are powered mainly by petroleum product-fueled Internal Combustion engines (ICEs). Diesel-powered vehicles are known for their durability and excellent fuel economy, particularly for heavy-duty applications; hence they are the workhorse for surface transport worldwide. However, diesel engines emit significant amounts of oxides of nitrogen (NOx) and particulate matter (PM), which contribute extensively to environmental pollution and associated health risks [2], [3]. Worldwide ever-stringent emission norms have been adopted to control harmful emissions from vehicles/engines. Therefore, researchers and OEMs are developing new emission control technologies and emphasizing cleaner alternative fuels to comply with stringent emission regulations. Biodiesels [4], [5], ethanol [6], [7], methanol [8], [9], diethyl ether (DEE), and dimethyl ether (DME) [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] have emerged as cleaner alternatives to mineral diesel. These fuels can significantly reduce emissions without compromising engine performance.
Biodiesels are made from sulfur-free, oxygenated fatty acids, or triglycerides, present in edible vegetable oils such as Sunflower, Coconut, Soybean [21], [22], [23], Rapeseed [24], Canola [25], and non-edible vegetable oils such as Jatropha [23], Pongamia [26], Karanja [27], Waste Cooking Oils [28], [29] and Animal Fats. The transesterification process is used to convert these vegetable oils/ animal fats into diesel-like fuels. Promoting biodiesel production policies from edible sources faced the debate of the “food versus fuel” conflict; therefore, this drive of various governmental bodies has subdued in the recent past considerably. Nevertheless, biodiesel has been recognized as one of the most effective strategies to significantly contribute to cater to the rising global energy demand in an environmentally friendly manner [30]. On the other hand, ethanol and methanol are preferred as alternative fuels for gasoline engines due to their relatively higher octane number. Alcohol is being used as an oxygenated fuel additive for diesel engines as well. Ethanol/methanol and diesel are immiscible beyond 10% (v/v) concentration, making the blend unstable, leading to erratic engine operation [31]. Ethanol-diesel blends are hygroscopic, leading to clogging of the fuel injectors. Researchers have also tried to exploit the benefits of inherent fuel oxygen of ethanol/ methanol in CI engines via ternary blends, i.e., methanol-biodiesel-diesel, ethanol-biodiesel-diesel blends [32]. However, ternary mixtures face the challenge of phase separation with a higher concentration of alcohols and significant changes in desirable physicochemical properties, which adversely affect the engine performance. Also, from a storage and transport point of view, usage of ternary blends remains erratic and unreliable [33].
DME (CH3-O-CH3) has properties similar to liquefied petroleum gas (LPG), and its usage can simultaneously reduce PM and NOx emissions. However, DME cannot be used in an unmodified diesel engine in higher proportions. Additionally, its production costs are also high because of expensive feedstocks such as natural gas. Hence, its commercial usage is limited to countries like China and South Korea [34], where DME production costs are relatively lower. Japan has also started developing DME-fuelled prototype engines [35]. Recently, DEE (C4H10O, C2H5OC2H5) has gained attention due to its identical physicochemical properties as mineral diesel. DEE has been an excellent fuel since it exists in a liquid state under normal temperature and pressure conditions. It can be blended with mineral diesel and biodiesel effectively [36], [37], [38]. The automotive industry has started using DEE as an additive/ fuel due to its unproblematic operation in existing diesel engines with minor hardware modifications. However, if DEE is used for 100% replacement of mineral diesel, it would be essential to recalibrate the existing fuel injection system. DEE, an isomer of butanol (C4H10O) [39], has four carbon molecules, with an oxygen molecule attached to two ethyl groups. Therefore, it comes under the category of oxygenated fuels (oxygen content is 21.6% w/w) [40] which reduces CO and soot emissions simultaneously due to its shorter carbon chain and excellent ignition quality [41], [44]. Compared to mineral diesel, DEE's most remarkable properties include higher cetane number (>125), lower auto-ignition temperature, acceptable energy density, and wider flammability range. DEE has long been used as a cold-start performance improver for diesel engines [45], [46] by automotive industries. It is commonly used as an ignition improver for diesel engines since it thoroughly mixes with diesel [45], [47], [48]. DEE is a renewable fuel if synthesized from ethanol, and is produced from low-value biomass feedstock through the dehydration process [49], [50], [51]. However, its higher volatility increases the chances of vapor lock in the fuel injection system, complicating its engine usage.
Several researchers have investigated DEE-diesel blends for their combustion, performance, and emission characteristics in a diesel engine. Subramanian and Ramesh [45] optimized engine performance using DEE5, DEE10, and DEE15 (% w/w) blends. They reported DEE10 as the optimum blend, which delivered the engine's highest brake thermal efficiency (BTE). With advanced injection timings, smoke and carbon monoxide (CO) emissions were reduced drastically without significant NOx emissions change. Mohanan et al. [46] reported that a relatively higher concentration of DEE (DEE20 and DEE25) in the blends resulted in lower BTE and increased CO and smoke emissions. These results were attributed to the phase separation issues, which lead to poor spray atomization and fuel droplet vaporization. Similar results of deteriorated engine performance were also reported by Górski and Przedlacki [47] for a 20% (v/v) DEE blend. The relatively lower viscosity of DEE creates difficulties in the engine starting and high-fuel leakage from the fuel injection system. In contrast, Rakopoulos et al. [48] achieved stable engine operation with higher concentration (up to 24% v/v) DEE-diesel blend. Experimental studies with higher DEE concentrations (up to 30% v/v) by Anand and Mahalakshmi [49] demonstrated that DEE20 combined with 5% EGR provided the most promising engine performance results.
Many previous studies demonstrated that DEE enhanced the engine performance when used with other alternative fuels in ternary blends such as biodiesel, ethanol, methanol, etc. DEE addition to biodiesel increased the oxygen content in the test fuel further, reducing tailpipe emissions. DEE has been reported to be the best additive for diesel–biodiesel blends for minimizing NOx emissions [50]. Swaminathan and Sarangan [51] demonstrated that 2% DEE was the most effective concentration to be used with fish oil biodiesel-diesel blend to reduce all emissions and EGR use. A study by Iranmanesh et al. [52] demonstrated that 5% DEE-diesel blend and 15% DEE-biodiesel-diesel ternary blends were potentially promising combinations for enhancing engine performance and reducing emissions. However, fluctuations and instability in the engine speed and power output were reported when DEE concentration exceeded 15%. Imtenan et al. [53] investigated DEE's influence as an additive on the engine performance of Jatropha biodiesel–diesel and palm biodiesel–diesel blends [54]. They recommended that 70% diesel/20% biodiesel/10% DEE was an optimum blend based on the engine performance, combustion, and emission characteristics. Other researchers also successfully used 5%, 10%, and 15% (v/v) DEE as an additive for rubber seed biodiesel-diesel blend [55], waste plastic pyrolysis oil-diesel blend [56], and Karanja oil methyl ester-diesel blend [57] in CI engines. Sivalakshmi and Balusamy [58] recommended an optimum concentration of 15% (v/v) DEE in Jatropha oil methyl ester-diesel blend based on engine performance and emissions. Pugazhvadivu and Rajagopan [59] observed that significant NOx emission reductions could be achieved with 20% (v/v) DEE addition to Pongamia biodiesel-diesel blend vis-à-vis other combinations. The addition of 20% (v/v) DEE was also recommended by Kannan and Marappan [60] for Thevetia Peruviana biodiesel-diesel blend based on engine performance and emission characteristics. Krishnamoorthi and Malayalamurthi [61] investigated the influence of 10% (v/v) DEE on diesel-aegle marmelos oil blends and reported that 60% diesel/30% oil/10% DEE improved the engine BTE along and reduced the CO, HC, and NOX emissions. Venu and Madhavan [62] performed an experimental study with 5 and 10% (v/v) DEE addition in diesel/biodiesel/methanol, and diesel/biodiesel/ethanol blends. In the former blend, DEE addition was not found to be an effective approach since it resulted in higher CO2, CO, PM, and smoke emissions. In addition, in-cylinder pressure, brake specific fuel consumption (BSFC), heat release rate (HRR) and combustion duration reduced compared to baseline mineral diesel. In the latter blend, NOX, PM, and smoke emissions reduced, while in-cylinder pressure, combustion duration and BSFC increased due to its higher ignition delay and higher latent heat of vaporization.
The recent literature review above provides an insight into the benefits of using DEE as a renewable additive in mineral diesel or biodiesel, which can potentially contribute to partial diesel replacement along with enhanced engine performance and emission characteristics, particularly at lower DEE concentrations. From the literature review, it is pretty apparent that a significant research gap exists in understanding the effect of DEE-diesel blends on unregulated and particulate emission characteristics. It is established that diesel particulates adversely affect human health and the environment. Diesel particulates are a complex mixture of toxic compounds (including various organic substances, polycyclic aromatic hydrocarbons (PAHs), and trace metals), which cause serious health hazards, including damage to the DNA and cancer [63]. Diesel particulates (<200 nm) readily deposit deeper in the lungs/ respiratory system. Exposure to diesel particulates can cause lung inflammation, which increases the risk of lung cancer [64]. Diesel particulates can get distributed in the entire body from the lungs, resulting in systemic inflammation, oxidative stress, and genotoxicity [64].
On the other hand, several harmful organic gaseous species are emitted as unregulated emissions. Even in very low concentrations, these species pose significant hazards to human health [65], [66], [67], [68], [69]. Species such as pentane, octane, butene, etc., affect the central nervous system [66], [69]. Aldehydes and organic acids irritate the respiratory organs and are carcinogenic [65], [67], [68]. Therefore, it is crucial to assess these pollutants emitted from the engine fuelled with DEE-diesel blends. The ultimate goal of this study is, therefore, to explore the effect of using DEE-diesel blends on particulate emission characteristics, including Particulate Number (PN) - Size Distribution, Particle Mass (PM) - Size Distribution, Total Particle Number (TPN), Total Particle Mass (TPM), Count Mean Diameter (CMD), and unregulated emissions including Individual Oxides of Nitrogen (NO, NOx, NO2), Saturated and Unsaturated Hydrocarbons (n-pentane (n-C5H12), n-octane (n-C8H18) and iso-butene (iso-C4H8)), and several inorganic and organic gaseous species (Sulphur dioxide (SO2), formic acid (HCOOH) and formaldehyde (HCHO)). Therefore, an extensive experimental study was conducted on an unmodified, naturally aspirated, mechanical fuel injection system equipped 3-cylinder, in-line, medium-duty tractor engine at the Engine Research Laboratory (ERL), IIT Kanpur, India.
Section snippets
Materials and methods
In this experimental investigation, BS-VI-compliant mineral diesel was purchased from the IOC Petrol Pump at IIT Kanpur. DEE was purchased from Chemicals and Instruments Pvt. Ltd., Kanpur. In this study, up to 45% v/v diesel displacement by DEE was targeted. Four test fuels used for experiments in an unmodified tractor diesel engine were: (i) DEE0 (100% Mineral Diesel), (ii) DEE15 (15% v/v DEE with diesel), (iii) DEE30 (30% v/v DEE with diesel) and (iv) DEE45 (45% v/v DEE with diesel).
Results and discussion
To study the effect of DEE-diesel blends (0–45% v/v DEE) on the unregulated and particulate emissions of the 3-cylinder tractor engine, exhaust gas sampling was done at a constant engine speed of 1500 rpm at varying engine loads. In each case, results were compared with baseline diesel. The results of unregulated emissions are discussed first, followed by particulate emissions. Table 4 shows the experimental test matrix for this study. Due to vapor lock challenges, data for DEE30 and DEE45
Conclusions
This study explored the effect of DEE-diesel blends on unregulated emissions, NOx, and particulate emission characteristics of a tractor engine. 0, 15, 30, and 45% (v/v) DEE blended with mineral diesel were used as test fuels. Experiments were performed at the constant engine speed of 1500 rpm at varying engine loads (no-load, 30, 60, 90, 120, and 150 Nm) without any hardware modifications in the engine. DEE-diesel blends emitted lower NOx and NO compared to baseline diesel. However, the trend
CRediT authorship contribution statement
Avinash Kumar Agarwal: Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Writing – review & editing. Prashumn: Experiments, Data curation, Formal analysis, Methodology, Writing – original draft. Hardikk Valera: Writing – review & editing. Nirendra Nath Mustafi: Writing – review & editing.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
The authors are grateful to Sh. Roshan Lal, Sh. Tushar Kakkar, Sh. Anshul Soni, Sh. Hemant Kumar and Sh. Ankur Kalwar of ERL, IIT Kanpur, for their assistance in test rig development, conducting the experiments and preparing the draft of this manuscript.
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