Original articleEffect of multi-walled carbon nanotubes and alumina nano-additives in a light duty diesel engine fuelled with schleichera oleosa biodiesel blends
Graphical abstract
Introduction
Energy is a key pillar for economic and sustainable growth of any nation. The world energy demand is rising by 1.3% every year and will witness significant rise by the year 2040 [1]. The present energy systems are heavily reliant on fossil fuels (coal, oil and gas), which are leading to the global issue of climate change. Diesel engines contribute significantly to environmental pollution; however, it still remains the technology of choice for running heavy-duty vehicles and agricultural farm machinery [2]. These engines provide 2/3rd of the energy for American farm machinery and a similar situation is prevalent in India where millions of agricultural engines are running on diesel fuel [3]. The higher efficiency, reliability and durability, together with their lower-operating cost make them the backbone of the global economy. Therefore, the world needs alternatives to petroleum diesel that can bridge-up the challenges between economic development and the environment.
The researchers are working on a variety of alternative technologies, which could result in cleaner combustion [4]. The introduction of electric vehicles technology in road transport has resulted in the reduction of crude oil demand. Electric cars are expected to occupy 50% of world transportation in the next two decades. However, the sustainability of electric vehicles in the world market is a big challenge due to the inability to recycle millions of lithium-ion batteries dumped into the open environment [5], [6]. Moreover, the execution of new technologies requires replacement of existing diesel engines which might affect the global economy adversely. Biodiesel is considered a promising drop-in fuel in the transport sector and for off-road vehicles. Moreover, the governments are also enforcing mandates for blending biofuels in petroleum-derived fuels [7], [8]. As per the United States Renewable Fuel Standard program, at least at least 36 billion gallons of alternative fuel should be amalgamated with transportation fuel per year by 2022 [9]. The research on biofuels - biodiesel in particular- suggest that a large number of feedstocks have been utilized for its production. However, debates on food versus fuel, especially in densely populated countries, mandates the use of non-edible oil as biodiesel feedstocks [10].
With the immense availability of seeds along with higher oil content (35–40%); schleichera oleosa (Kusum oil) is considered to be a favourable feedstock for producing biodiesel. It is a member of the sapindaceae family and is mainly found in sub-continents regions of India and other Asian countries such as Myanmar, Sri Lanka and East Timor. The presence of 0.03–0.05% of Cyanolipids (in form of hydrogen cyanide) makes schleichera oleosa (S. oleosa) highly poisonous for human consumption and a strong candidate for the producing energy [11]. The fatty acid profile of S. oleosa methyl ester also reveals its inedible nature. More than 40% of unsaturated fatty acids compositions such as Palmitoleic, Palmitelaidic, Oleic and Elaidic etc. and also iodine value ranging between 215 and 220 mg I2/100gm oil proves its incompetency to be used as an edible oil [12]. The tropical states of India having the largest production capacity include Punjab, Haryana, Western Uttar Pradesh, Madhya Pradesh, Rajasthan, and Gujarat. With the production potential of 25,000 tonnes of schleichera oleosa oil in India currently; only 4000–5000 tonnes is harnessed, and remaining is used for fuel wood or charcoal production [13]. This suggests that unutilized schleichera oleosa oil can be utilized for biodiesel production in Indian context. Several studies pertaining to use of schleichera oleosa methyl ester and its blend with diesel have been done in CI engines [14], [15], [16].
The higher thermal efficiency (BTE) and upto 15% reduction in carbon monoxide (CO), unburnt hydrocarbon (HC) and smoke emissions were observed in single cylinder diesel engine fuelled with schleichera oleosa-diesel blends due to improved cetane index of blends. Also, lower rate of pressure rise and lower premixed combustion peak was observed in biodiesel blends as compared to diesel. However, the nitrogen oxide (NOx) and specific fuel consumption was significantly higher [17]. Pali et al. [18] blended biodiesel (10%, 20%, 30% and 40%) in varying blends of schleichera oleosa-diesel to run a stationary agricultural engine. The biodiesel blends showed lower fuel atomization due to poor viscosity and surface tension. At maximum load, the brake thermal efficiency was lower than diesel. Also, NO emissions were 4–20% higher. However, the CO, unburnt hydrocarbon and soot emissions were reduced by 7–42% as compared to neat diesel. Similarly, the engine characteristics performed on a variety of other feedstocks have also reported several long run operating issues such as wax formation, deposits formation and chocking of engine parts, lower calorific value and higher NOx formation [19], [20], [21].
Thus, the adaptation of biodiesel has some inherent drawbacks which could be overcome by using fuel reformulation techniques. Mixing of specific chemical compounds (also known as additives) with diesel, biodiesel and their blends can improve the working characteristics of CI engines significantly [22]. With the advancement in nanotechnology, the nanoparticle (measuring nanometres) can be efficiently prepared and added with diesel/biodiesel fuels. The unique physico-chemical features of nanoparticles such as higher surface to volume ratio, thermal conductivity, and catalytic activity can significantly improve the Ignition probability, fuel atomization, specific fuel consumption and harmful emissions from the diesel engines. The earlier investigations suggest that metal oxides, carbon nanotubes, ferromagnetic np’s have proven to be the most promising fuel additives for diesel–biodiesel blends [23].
The addition of 25 and 50 ppm dosage of alumina nanoparticles showed significant change in density, viscosity, calorific value and flash point of diesel-Ziziphus jujuba biodiesel blends. Also, the results showed about 20% rise in thermal efficiency at peak load [24]. Anchupogu et al. [25] used exhaust gas recirculation technique (EGR) along with alumina nanoparticles to study the combustion, performance and emission characteristics of Calophyllum inophyllum biodiesel blends. A single cylinder stationary agricultural engine was used for testing the blends. The dispersion of alumina np’s and exhaust gas recirculation has no net effect on the engine efficiency, CO, HC and smoke emissions. However, the NOx emissions were reduced by 35% with the use of 40 ppm alumina and 20% EGR blends. These days the manoeuvring of used cooking oil biodiesel is a smart initiative to valorise the waste oil generation. However, the presence of higher impurities in waste cooking oil biodiesel blends might result in poor engine performance and clogging of engine parts at later stages. With the objectives to address these issues, Roy et al. [26] added alumina nanoparticles to investigate the performance of waste cooking oil biodiesel and its blend in a single cylinder diesel engine. To ensure the effective dispersion of nanoparticles, isopropanol was used as a surfactant. The presence of oxygen content in metal oxide nanoparticles results in decrease of HC, CO and opacity as compared to diesel, biodiesel and its blends. However, NOx emissions were found to be slightly higher. Also due to increase in-cylinder pressure of nanoparticle blends, the brake specific fuel consumption (BSFC) was also reduced.
The higher mechanical strength and thermal properties of carbon nanostructure makes it a viable fuel additive for improving the diesel engine performance. El-Sessy et al. [27] reported 35%, 50%, and 60% reduction in NO, CO, and HC respectively with the addition of 20 mg/litre. dosage of MWCNT. Also the combustion parameters such as in-cylinder pressure peaks, the maximum rate of pressure rise and the heat release rate peaks were improved by 7%, 4%, and 4% as compared to diesel–biodiesel blends. The literature has also reported several issues of poor stability and agglomeration of carbon nanotube (CNT) particles in the blends. Therefore, Sadik and Basha [28] used Tween 80 and Span 80 as surfactant to improve the stability of CNT nanoparticles in biodiesel blends. The results showed no sign of phase separation and particle agglomeration after regular monitoring for 1 month. The study suggested that 3000 rpm is the optimum speed to form stable blends of nano-fuels. Also, the engine trial revealed upto 20% reduction in NO emissions at 100 ppm dosage of CNT nanoparticles. The past studies have also reported greater influence of CNT in reduction of NOx emissions from the diesel engines as compared to alumina nanoparticles [29], [30].
There are many studies conducted in the past on nano-additives; however, they are mainly focused on performance and emission parameters. A very few literature have been reported in which the effect of nanoparticles on the ignition probability and fuel droplet atomization of diesel, biodiesel and their blends has been studied. Higher viscosity along with lower calorific value had an adverse effect on the biodiesel performance. An improvement in ignition characteristics were reported by adding carbon nano-particles with neem biodiesel due to improved calorific value and cetane number [31]. Kumar et al. [32] conducted a comparative study to examine the influence of nano-additives on ignition characteristics and spray droplet size of biodiesel. The alumina and MWCNT in concentration of 30 ppm were added as fuel additives. The results showed significant changes in physicochemical of the fuel, ignition probability, evaporation rate and spray characteristics with the doping of nanoparticles in neat biodiesel.
The fuel burning ability and droplet size, are the critical parameters that affect the combustion, performance and emission of a diesel engine. However, limited work is available on fuel droplet atomization which is mostly affected by flow properties such as density, viscosity surface tension, and also the breakup length. The dosage of nano-additives also plays a significant role, yet limited studies are available in which higher dosage of nano-additives i.e. above 50 ppm is dispersed in the fuel blends. This might be due to the agglomeration tendency of nano-particles in the base fluid.
Therefore, the current study investigates the effect of alumina and MWCNT np’s on the engine characteristics of diesel-schleichera oleosa biodiesel blend. The spray characteristics (SMD), engine combustion, performance and emission characteristics of nano-fuels were evaluated and compared with diesel and biodiesel blend. The droplet diameter of the different fuels was examined by undergoing a series of trials on a laser-guided Malvern spraytec setup. A light-duty diesel engine widely used for farming and other agriculture purposes was chosen for carrying out engine trials. Moreover, the study also highlights the widely used and cost-effective methods for preparing alumina and MWCNT nanoparticles. As highlighted in the preceding section, Schleichera oleosa is a promising biodiesel feedstock; still, there are few studies available on its utilization in CI engines. Hence, the main motive of the present research is to increase the sustainability of Schleichera oleosa biodiesel by improving its usability as a renewable fuel for farmland diesel engines.
Section snippets
Preparation of schleichera oleosa biodiesel
The schleichera oleosa oil was procured from Surajbala Export Private Limited, a local vendor of New Delhi, India. The primary step of biodiesel production involves the determination of free fatty acid (FFA) content present in the oil. Subsequently proceeding onto the esterification (FFA > 2 wt%) or direct transesterification reaction (FFA ≤ 2 wt%). The FFA content in S. Oleosa oil was found to be more than 2%. Therefore, directly proceeding onto the transesterification reaction might lead to
Results and discussions
The influence on physico-chemical properties of neat diesel with the addition of S. oleosa biodiesel and nano-additives of different sizes and concentration are discussed further in-detail. The results and trends of sauter mean diameter analysis of different fuel samples carried out using Malvern spraytec setup are also shown. In the end, engine trials were conducted at different loading conditions. The impact of nano-additives and their dosage on engine combustion, performance and emissions
Conclusions
In the present study, two different nano-additives (alumina and MWCNT) were tested in a light-duty CI engine with an attempt to improve the sustainability of schleichera oleosa oil biodiesel. Various nano-fuels containing alumina and MWCNT nanoparticles of two different concentrations were prepared and their physico-chemical properties were evaluated as per ASTM D6751. Further, the experiments were also conducted to study the spray and engine characteristics of nano-fuels and results were
CRediT authorship contribution statement
Mukul Tomar: Concept formulation, Experimentation, Data acquisition, Writing - original draft. Naveen Kumar: Methodology, Reviewing and Editing, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Executive summary
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