Exhaust Gas Recirculation System
In recent days, the exhaust gas recirculation (EGR) technique is commonly used for in-cylinder NOx reduction for biofuels. Much research work has described that EGR is the most preferable potential NOx mitigation technique (Gill et al. 2012). This technology works by recirculating 10% to 40% of exhaust gas and bringing it back to the engine cylinder to reduce oxygen availability. About 15% of EGR reduced the NOx by about 80% (Agarw'al, Singh, and Agarwal 2011). The NOx controlling mechanism works based on two actions, w'hich are dilution and chemical effects. During the dilution effect, the portion of EGR in the suction air acts as an inert gas in the combustion and absorbs the in-cylinder heat to reduce the peak cylinder temperature. Also, the dilution effect increases the non-combustible mass portion (Song et al. 2012). During the chemical effect, dissociation occurs due to an increase in molecular complexity (Ladommatos, Abdelhalim, and Zhao 2000).
EGR is an effective method for NOx reduction even though there are some persistent disadvantages such as higher hydrocarbon, smoke, and CO emissions. Also, increased EGR decreased the BTE unless the quantity w'as optimized. Saleh (2009) reported that 12% of EGR was the optimized level for biodiesel combustion, which decreases the NOx by 36%. However, it increases the HC and CO emissions slightly and also increases the BSFC by 9%. Kass et al. (2009) identified that 27% of EGR effectively reduced the NOx emission by 87% at a 68 Nm load for raw soybean biodiesel. EGR technology increases the BSFC due to altering the air-fuel ratio, reducing the oxygen concentration, creating a dilution effect, and lowering the burn fall.
all of which make achieving stable combustion difficult (Qi et al. 20Ю). Further, EGR increases the inlet air temperature, which affects the volumetric efficiency due to a damnatory effect at a higher load (Yoon, Suh, and Lee 2009). At a rated load, the disassociation of CO, to CO can also cause higher CO formation without varying the HC emission (Tsolakis et al. 2007). Many research works suggest that EGR is the most suitable technology for combustion due to its low volume requirement and design simplicity. The oxygen content in the biofuel compensates for the oxygen deficiency during the biofuel combustion. Hence, EGR with less than 25% can effectively reduce the NOx without increasing the other emissions.
Water Injection
An important technique for controlling NOx emissions from the diesel engine is to inject the water into the cylinder. The water injection was carried out in two different ways: one is direct injection into the cylinder during combustion, and another one is injection (water fumigated) during the suction stroke through intake manifold (Bedford et al. 2000). A major benefit for injecting the water into the cylinder is the greater potential to decrease the NOx for the entire load range with a minimal detrimental effect on smoke emissions (Tauzia, Maiboom, and Shah 2010). The evaporation of injected water droplets absorbs the surrounding heat due to high latent heat, which significantly reduces the local adiabatic flame temperature, which results in low NOx emission (Park, Huh, and Park 2000). Fumigation is inducting or injecting the water to the upstream of the intake manifold. The water injection technology effectively decreases the NOx emission, although it has some disadvantages like incremental increase in HC and CO emission. Also, the BSFC value increased along with an increase in water injection rate. Water injection improves the low combustion temperature for the entire load range (Jiaqiang et al. 2017).
Tauzia, Maiboom, and Shah (2010) reported the effect of the water injection rate on the automotive direct injection diesel engine. They reported that an increase in the water flow rate extended the ignition delay period and lowered the NOx emissions, but with a penalty of higher CO and HC emissions. Tesfa et al. (2012) examined the influence of water injection on the 4-cylinder turbocharged diesel engine. They found water injection at 3 kg/h resulted in 50% lower NOx emissions without increasing specific fuel consumption. However, CO emissions increased by 40% compared to baseline. Overall, water injection effectively decreased the NOx emissions but with the penalty of higher CO and HC emissions. Hence, water injection combined with other HC and CO emission technology is a better option for biofuel combustion in the Cl engine.
Emulsion Technology
Fuel emulsion technology has been used to introduce water during the combustion of biofuels. Emulsion technology has some advantages compared to water injection, which improves combustion efficiency and simultaneously reduces the NOx, smoke, and other emissions (Lif and Holmberg 2006). A mixture of two or three immiscible fuels and liquids is called an emulsion. For example, water droplets are immersed in the oil phase emulsion, which helps distribute the water droplets uniformly in the combustion chamber. The fuel droplets are distributed uniformly everywhere using an adequate surfactant by mechanical or ultrasonic forces. Two main types of emulsion technology, namely two-phase emulsion and three-phase emulsion, have been used for biofuel combustion. Two-phase emulsion has been further divided into two types, namely water-oil emulsions and oil-water emulsions. Similarly, three-phase emulsion has been divided into water-oil-water and oil-water-oil. The important criteria for the selection of an emulsion are that it should possess characteristics similar to those of the fuel and that the presence of water does not overly influence the physicochemical characteristics of the biofuel (Matsumoto and Kang 1989).
The major disadvantage in emulsion technology is that water and oil separate over time and become stable in the dispersed phase. To maintain equal composition in the dispersed phase in the emulsion, the proper surfactants are added in the emulsion. The surfactants encase the water droplets in the oil phase emulsion and distribute them equally in the dispersed phase. Also, the surfactants prevent the water droplets from coalescing. During combustion, the dispersed phase water droplets are dissociated and absorb the heat of evaporation. Also, the specific capacity of fuel gas increases during the water dissociation from the emulsion. The emulsification technology increases the momentum of injected fuel. This is due to higher water content, which enhances the atomization and air entrainment (Hountalas, Mavropoulos, and Zannis 2007). Additionally, premixed combustion increases due to the higher atomization rate, which significantly reduces smoke emission. Further, the formation of OH radicals due to dissociation of water decreases the smoke and NOx emission (Nazha, Rajakaruna, and Wagstaff 2001).
Davis (2012) investigated the influence of 10% water emulsified in diesel and biodiesel combustion on a single-cylinder variable-speed diesel engine. They found that NOx emission decreased by 10% and 25%, respectively, for the emulsified B20 and B100 fuels. Compared to raw biodiesel, emulsified biodiesel (Eu-BlOO) performed well without reducing the engine power. This is due to the micro explosion of water droplets, higher vapor pressure, increase in velocity, and atomization of the fuel jet. The exhaust gas temperature of the B100 and Eu-ВЮО remains the same for the entire operation. Hence, the NOx reduction was mainly based on OH radical formation during combustion. However, a 15% increase in specific fuel consumption and a 14% drop in BTE were found for the emulsified B20 fuel (Davis 2012). Basha and Anand (2011) investigated the effect of 15% water and 2% surfactant added in jatro- pha emulsified biodiesel combustion. They reported 22% and 15% drops in NOx and PM, respectively; however, HC increased by 45% compared to raw biodiesel. Also, BTE and BSFC were improved by 2.4% and 2.7%, respectively. The improvements in the performance parameters have differed, and the reason for this is not clear: several researchers have found an increase in engine power and a few have observed a reduction. Hence, further research work is needed for utilizing the emulsion technology for the low-temperature combustion application.
Combustion Geometry Modification
An alternative method for overcoming the problems of a diesel engine powered by biodiesel is to make the correct modification in the combustion chamber and fuel injection. The injection time modification and nozzle opening pressure (NOP) play an essential role in the combustion. The pressure and timing should be optimized depending on the combustion chamber configuration. The modification in the combustion chamber should not affect the combustion efficiency and emission characteristics. Improving the air movement and swirling effect in the combustion chamber is essential to increase the homogeneity in the air-fuel mixture (Jiaqiang et al. 2017). The combustion chamber geometry creates a swirling effect of air, which creates the turbulence with biodiesel. Increasing turbulence and swirling effect increases the BTE and reduces the BSFC. New rotating grooves have been provided at the top of the piston to improve the turbulence of the air-biodiesel mixture by improving rotational movement.
Jaichandar and Annamalai (2012) examined the impact of three types of combustion chamber such as shallow depth combustion chamber (SCC), toroidal combustion chamber (TCC), and hemispherical combustion chamber (HCC) for the Pongamia biodiesel blend (B20) combustion. They found BTE increased for TCC compared to the other two types. Emission characteristics like PM, HC, and CO significantly decreased for TCC compared to other combustion chambers. However, NOx emission was slightly increased due to higher swirl motion, which increases the heat release rate and combustion temperature. Isaac et al. investigated the effect of turbulence inducer piston and injection pressure on Adelfa biodiesel blend (A20) combustion. They reported that smoke, CO, and HC emission were decreased due to improvement in the swirling movement, which enhances the premixed combustion. However, premixed combustion significantly enhances the NOx emission due to the increase in combustion rate (Isaac JoshuaRamesh Lalvani et al. 2016). Hence, combustion chamber modification will effectively work when injection pattern modification is adopted for biodiesel combustion.
