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Energies 2019, 12, 809 26 of 41 a constant load for 75 min. The result of the test is reported as a wear scar diameter and hence a high diameter value is an indication of a fuel with low lubricity [219]. Diesel fuels that have low concentration of heteroatom molecules such as the Ultra-low Sulfur Content (ULSD) fuels used in modern engines, the FT diesel and the green diesel produced through hydro-processing generally have low lubricities with typical wear scar diameters above 700 μm. This is lower than the minimum lubricity required by all European and US standards and therefore a lubricity enhancer is usually added as in the cases of petroleum diesel. The typical chemical structure of a lubricity additive consists of a polar functional group attached to an oleophilic part Ri. Typical functional groups are carboxylic acid, carboxylic acid ester and carboxylic acid amide [218]. Then, the oxidation stability and the thermal stability of a diesel fuel are evaluated through the ASTM D2274 and ASTM D6468 tests, respectively. Both these tests have been designed for diesel fuels containing more than 500 ppm of sulfur and since the sulfur content of green diesel is extremely low (below 5 mg/Kg) may not be accurate in its case. Smagala et al. [217] have tested various green diesel samples and found that the oxidative and thermal stability of green diesel are comparable to petroleum diesel. Finally, conductivity is important for the safe handling of the fuel during transport. This can be adjusted with antistatic additives. 7.2. Green Diesel Performance in CI Engines Compression ignition (CI) engines are heat engines that convert the chemical energy of a fuel into mechanical work. The fuel is injected as a liquid spray inside the cylinder (or in an appropriate pre-chamber) under pressure through a fuel injector and after evaporation and mixing with air it autoignites due to the excessive compression produced by a moving piston. The piston reciprocates inside the cylinder between two zero-velocity positions called bottom dead center (BTC) and top dead center (TDC), which define the maximum piston travel distance known as a stroke [221,222]. Depending on the engine type, four or two strokes are necessary to complete one engine operation cycle which may be separated on the phases of air induction, air and fuel compression, combustion product expansion and combustion product exhaust. Fuel injection and autoignition sequentially take place at the final stages of the compression phase just before the piston reach at the TDC. After autoignition the combustion process begins, the chemical energy of the fuel is released as heat, and since the available volume inside the cylinder is small the combustion products are formed at high pressure and high temperature. These will push the piston from the TDC to the BTC during the expansion phase of the engine cycle and will produce positive mechanical work which is higher than the negative energy needs during intake, compression and exhaust. The reciprocating movement of the piston is finally converted into rotational shaft work by means of a piston - crank rod - crankshaft kinematic assembly. Each instant of the engine cycle may be signified by the angular position θ of the cranskshaft which is measured relative to the θ = 0◦ value assigned for the piston position at the TDC [221,222]. Figure 6 illustrates a typical variation of the cylinder pressure versus the instant angular position θ of the crankshaft. Starting from the left of the diagram the compression stroke take place as the piston approaches the TDC (θ = 0◦) and the air pressure increases gradually as the available volume decreases. At a crankshaft angle equal to about 20◦ before the TDC the fuel injection starts and the droplets of the fuel spray start to evaporate and mix with the cylinder air by diffusion and turbulent air motion. The pressure and the temperature inside the cylinder continues to increase and at a certain instant the autoignition of the fuel occurs and combustion begins. The time period or the crankshaft angle interval between the start of the injection and the instant of autoignition is the ignition delay period which depends on the cetane number of the fuel and other parameters such as the fuel volatility and viscosity, the pressure and angle of the injection, the piston and cylinder geometry etc. The exothermic combustion then takes place with the propagation of a flame front which travels inside the cylinder with a subsonic velocity and eventually reaches the cylinder walls and quenches.PDF Image | Green Diesel: Biomass Feedstocks, Production Technologies
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