Chapter 2. LITERATURE REVIEW
2.5 Experimental studies on engine performance and emission level
The performance of a compression ignition engine is generally characterized by several key parameters including the brake specific fuel consumption (BSFC), brake thermal efficiency (BTE) and the mechanical efficiency.
By knowing the cylinder pressure and rate of heat release, the typical combustion characteristics including the rate of increase of cylinder pressure, ignition delay and total combustion duration can be determined. The injection timing, which is the start of fuel injection, was determined at the crank angle where the injector needle raised abruptly. The ignition timing, which is the start
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of combustion, was determined at a point when the heat was released. The time lag between ignition timing and injection timing is referred as the ignition delay. The end of a combustion process in a cycle was taken as a point where 90% of the cumulative heat release was completed.
The time lag between end of the combustion and the ignition timing was taken as the total combustion duration.
Kao et al. [2008] reported that an addition of aqueous aluminum nanofluid in diesel increased the total combustion heat in addition to the reduction of smoke and nitrogen oxides in the exhaust emission. Tyagi et al. [2008] presented the ignition characteristics of alumina and aluminum nanoparticles dispersed diesel without any surfactant. It was concluded that adding nanoparticle to the fuel caused significant improvements in the radiative and heat / mass transfer characteristics and hence the droplet was ignited at a much lower temperature, which has the potential of reducing the evaporation and ignition time of droplets in a diesel engine.
Huang et al. [2009] reported that the melting temperature of a 1 nm aluminum particle could be as low as 127 °C, one fifth of its bulk material, which had a significant influence on the ignition and combustion mechanisms. Tyagi et al. [2008] reported that the high specific surface area to volume ratio of the nanoparticles allowed more fuel in contact with the oxidizer. Because of the small interparticle distance, the time scale of the chemical reactions was very much different than the reactions associated with large sized particles and thus, the ignition delay time for nanosized particles would be reduced.
Gan et al. [2011b] compared the burning behavior of nano and micron suspension of aluminum in the fuel. The results showed the characteristics and structure of particle agglomeration formed during the droplet evaporation and the combustion behavior. It was concluded that the burning behavior of nano suspension was better than the micron suspension.
Kao et al. [2011] dispersed aqueous aluminum nanoparticles in diesel and the results showed the decrease in BSFC and increase in BTE compared to that of diesel. Zhu et al. [2011] used ferrous picrate catalyst in diesel and reported the reduction of smoke by 26.2% at full load condition.
Shafii et al. [2011] carried out the performance study of a four-stroke diesel engine using water based ferrofluid mixed diesel. Water based ferrofluid has two advantages: the diesel can be diluted, which can reap the benefits of water diesel emulsion; the magnetic nanoparticles can be
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collected at the exhaust of an engine. By adding a 0.4% ferrofluid to diesel, the BSFC was decreased by 3.2 - 6.5% and BTE was increased by 3.3 - 6.9%. The emission of NOx and CO was decreased by 9 - 15 ppm and 3.33 - 6.89%, respectively. Zhu et al. [2012] used ferrous picrate catalyst for diesel engines, where the reduction in BSFC and the increase in BTE were observed with an addition of the catalyst. At the catalyst dosing ratio of 1:10000, the BSFC was reduced by 3.3 - 4.2% at light engine load of 0.12 MPa and 2.0 - 2.4% at heavy engine load of 0.4 MPa. From the in-cylinder pressure and heat release rate analysis, it was found that the catalyst reduced the ignition delay and the combustion duration resulting the higher peak cylinder pressure and faster rate of heat release.
Ganesh et al. [2011] observed the reduction of BSFC and increase of BTE, when magnalium and cobalt oxide were dispersed in biodiesel. Sajith et al. [2010] reported that the cerium oxide supplied the oxygen for oxidizing the hydrocarbon and soot and it got converted into cerous oxide. As cerium oxide acts as an oxidation catalyst, it lowered the activation temperature of carbon combustion and enhanced the hydrocarbon oxidation promoting the complete combustion of fuel. An average reduction of 25 to 40% in the hydrocarbon emission was obtained for the catalyst dosing level ranging from 40 to 80 ppm. They have obtained a 30% reduction in NOx
and 25 to 40% reduction in hydrocarbon emission, when cerium oxide nanoparticles were dispersed with Jatropha based biodiesel. Ganesh et al. [2011] also reported that the fuel evaporation time was reduced by metal oxide nanoparticles leading to the reduction of physical delay. In addition to that a 70% reduction in hydrocarbon, 41% reduction in CO and 30%
reduction of NOx emission at 75% load were also observed.
From the above literature, it was clearly understood that the nanoparticles dispersed diesel enhanced the performance of an internal combustion engine in terms of BSFC and BTE. It helped to reduce the smoke, NOx and unburned hydrocarbon, which ensured its complete combustion. In general, the ignition delay and ignition temperature are the two critical parameters that influence the performance of a diesel engine. Both thermal efficiency and the reduction of emission level of an engine are improved by optimizing those parameters. Though there are many advantages of dispersing the nanoparticles in fuel including reduced ignition delay, increased energy density, high burning rate and reduced emission, a few works has been reported till date on the effect of dispersing the nanoparticles in diesel.
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