Performance and emission characteristics of a CI engine fuelled with carbon nanotubes and diesel-biodiesel blends

Highlights

• Engine performance fueled by CNTs as an additive to biodiesel blends were examined.

• The added CNTs reduced the viscosity and increased the density of blended fuels.

• Specific fuel consumption reduced due to better injection and atomization of fuels.

• CO, UHC, and soot emissions were dropped by 65.7, 44.98, and 29.41%, respectively.

• NO_x and exhaust gas temperature enhanced by 27.49% and 5.57%, respectively.

Abstract

Numerous studies have demonstrated the diesel fuel characteristics vary by adding the biodiesel. Furthermore, the nanoparticles or catalysts can mix with these fuels to improve their performance and exhaust emission characteristics. In this study, the Carbon Nanotubes (CNTs) as an additive were mixed with the B5 and B10 fuel blends to evaluate the performance and emission of a CI single-cylinder engine. The CNTs with the concentrations of 30, 60, and 90 ppm were used for each fuel blend. Tested characteristics were power, brake thermal efficiency (BTE), specific fuel consumption (SFC), exhaust gas temperature (EGT), and emissions of CO, CO₂, unburned hydrocarbons (UHC), NO_x, and soot for full load engine at speeds of 1800, 2300, and 2800 rpm. It was observed the CNTs added to fuel blends exhibit considerable enhancement in power (3.67%), BTE (8.12%), and EGT (5.57%) at all engine speed. The results also showed a significant reduction in SFC due to influence of added CNTs in diesel-biodiesel blend. The results of emissions characteristics showed the CO, UHC, and soot exhaust emissions decreased, and NO_x emissions increased. There was a negative statistical correlation between the CO and CO₂ emissions. Moreover, enhancing the EGT had an important role in NO_x emission.

Introduction

Increased demand for energy, fluctuations in oil prices, global warming due to emission of greenhouse gases, environmental pollution, and fast depletion of fossil fuel reserves are the key factors underpinning the search for alternative sources of renewable energy, particularly biofuels derived from agricultural products [1], [2], [3]. Biodiesel is a class of renewable clean bioenergy as it can be produced from vegetable oils, animal fats and micro-algal oil. The properties of biodiesel are almost similar to those of diesel fuel; thus it is becoming a promising alternative to diesel fuel. Biodiesel consists of mono alkyl esters formed by a catalyzed reaction of triglycerides in the oil or fat with a simple monohydric alcohol [2], [3]. Biodiesel is similar to diesel fuel; however, it does not have unpleasant ingredients such as sulfur and polycyclic aromatics. It can be applied instead of diesel fuel without the need to make any special modifications to the components of combustion engines [3].

Many strategies have been adopted to reduce NO_x and soot emissions simultaneously to achieve the emission targets in common diesel engines [4], [5]. Recently, research shows that using biodiesel in engines reduces the amount of unburned hydrocarbons (UHC), carbon dioxide (CO₂), carbon monoxide (CO), sulfur oxides, and solid particles emitted from the exhaust. There is only an increase in nitrogen oxide emissions that can be reduced by adjusting the fuel-injection timing [1], [6], [7].

Fuel economy and emission characteristics in internal combustion engines are controlled by physical and chemical properties of the fuel. Various additives as catalyst are used for fuels in order to improve fuel quality, achieve better combustion and reduce exhaust emissions. During combustion, catalysts accelerate fuel instability reactions and improve engine performance [8]. In recent years, nano-catalysts or nano-additives in fuels have improved the thermo-physical properties such as high surface area-to-volume ratio, thermal conductivity, and mass diffusivity. Based on the literature review, it has been found that nano-additives along with diesel, biodiesel, and their blends enhance the flash point, fire point, kinematic viscosity, and other properties [9]. Most of the nanoparticles are made of ceramic, metallic, and polymeric materials. The most common nanoparticles are ceramics and mostly metal oxides such as titanium, carbon, aluminum, and iron [10]. Large surface and high energy level increase the catalytic performance of metal nanoparticles [11].

Many studies have been conducted on fuel additives and their effect on engine combustion, performance, and emission characteristics [12], [13], [14]. For example, Guru et al. empirically examined the effects of adding the synthesized magnesium, manganese, calcium, and copper to diesel fuel on emissions, efficiency, and quality of fuel ignition. They also studied the levels of pollutants like SO₂, CO₂, and CO and cetane number of fuels for diesel fuel with additives and pure diesel. It was observed that the organic based manganese drops the viscosity and flash point and improves the contents of the exhaust gases [15]. Gumus et al. examined the effects of alumina (Al₂O₃) and cupric oxide (CuO) nanoparticles added to diesel fuels on the engine performance and exhaust emissions. The storage and combustion characteristics were improved by adding nanoparticles. The engine torque and brake power output were slightly increased by the addition of alumina and cupric oxide to pure diesel. Since, added nanoparticles promote an increase in oxidation rate and a decrease in ignition temperature [16]. Shaafi and Velraj [13] investigated the combustion, performance, and emission characteristics of an engine running on two modified fuel blends, B20 (Diesel-soybean biodiesel) and diesel-soybean biodiesel-ethanol blends, with alumina as a nano additive (D80SBD15E4S1 + alumina), and their results were compared with those of pure diesel. The results showed that the cylinder pressure and the heat release rate during combustion were higher than those of the diesel fuel, which is due to the higher surface area exposure of the alumina nanoparticle supported by the inherent oxygen present in the soybean biodiesel, that helps in rapid combustion. The presence of oxygen in the soybean biodiesel and the better mixing capabilities of the nanoparticles reduce CO and UHC levels appreciably although there is a small increase in NO_x under full load condition.

Sajith et al. [17] studied the effects of cerium oxide (CeO₂) nanoparticles as additive to biodiesel fuel on engine performance, physicochemical properties of the fuel, and emissions resulting from the combustion. Different doses of cerium oxide with concentration of 20, 40, 60, and 80 ppm were used to achieve the optimal blend. The results indicated an increment of flash point and viscosity of biodiesel by adding cerium oxide nanoparticles. The values of NO_x and UHC emissions are greatly reduced. It could be due to a complex interaction among factors such as the combustion temperature, reaction time, and the oxygen content [17]. Yetter et al. and Dreizin have critically reviewed the reports on the metal nanoparticle combustion, and observed that the nano-scale metallic powders possess high specific surface area and can cause high reactivity. They have also revealed that adding nano-additives to the hydrocarbon fuels (such as diesel) will reduce ignition delay and soot emissions [18], [19]. Ganesh et al. examined the effects of cerium oxide nanoparticles (the size ranging from 10 to 20 nm) as additives in Jatropha biodiesel, and found that there was an appreciable increase in the flash point, volatility, and viscosity of biodiesel [20].

On the applications of CNTs, Selvan et al. studied the performance, emission and combustion characteristics of a diesel engine using cerium oxide nanoparticles (CERIA) and CNTs as fuel-borne nanoparticles (combined) additives in diesterol (diesel–biodiesel–ethanol) blends. The addition of CERIA and CNT to diesterol blend was increased the cylinder gas pressure as comparing with the pure diesterol blends. The combination of CERIA and CNT as additives to the diesterol fuel blend was led to cleaner combustion and significantly was reduced the harmful exhaust gas emissions [21].

Recently, several studies have been conducted on the effects of fuels on performance and emission characteristics of engines. With a focus on nano-fuels, many studies have been carried out on the engine performance and emissions. However, there are still many shortcomings about the effects of CNTs and diesel-biodiesel blends on these characteristics of engines. The literature reviews show that no research have been reported on performance and emission characteristics of diesel engines using CNTs, biodiesel, and its blends with pure diesel. Due to the potential properties of CNTs and the lack of studies focusing on diesel-biodiesel blended fuels, the present work is aimed to establish the effects on performance and emissions of a single-cylinder diesel engine. So, both advantages of combustion accelerator additives (CNTs) and oxygenate fuel (biodiesel) are used.