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Energies 2019, 12, 809 28 of 41 and air. Finally, the combustion process continues also after the quench of the flame front into the expansion stroke in the so-called late combustion phase. This is due to the appearance of equilibrium thermal decomposition reactions and also due to mixing which promotes more complete combustion. Fuels with low cetane numbers have a long ignition delay that increases the phase of premixed combustion and give rise to higher pressure and heat release rates inside the cylinder [222]. Green diesel has been tested as a CI engine fuel in various studies [224–244]. In the majority of them it was reported that, when compared to petroleum diesel, green diesel decreases both the maximum cylinder pressure and the rate of pressure increase [224–230]. The lower maximum pressures and the lower rates of pressure increase are attributed to the higher cetane number of green diesel which shortens the ignition delay period and the premixed combustion phase, in agreement to the conclusions of the previous discussion. Also, a possible lower viscosity of green diesel allows a better mixing of the liquid fuel droplets with air and reduces the ignition delay [229]. Caton et al. [224] has reported that at almost all engine loads and speeds the maximum cylinder pressure for green diesel is about 2–6% lower than for petroleum diesel. Aatola et al. [231] have tested green diesel on a turbocharged 8.4 L common rail engine and observed that the heat release was lower due to the lower ignition delay, as it was discussed above, and also due to the lower volumetric heating value of the biofuel. The same observation was also made in various other reports [226,232–234]. Also, almost all of the above studies concluded that green diesel increases the crankshaft interval of the combustion process by approximately 0.5–2◦ [224,225,228,231–234]. This is reasonable since the higher cetane number and the shorter ignition delay reduce the phase of rapid premix combustion and lengthen the slower phase were the combustion is controlled by the mixing rate of air and fuel. In terms of torque and brake power it was observed that the utilization of green diesel is comparable to petroleum diesel. Kim et al [235] used green diesel in a 1.5 L direct injection common rail engine and observed that the brake power was marginally lower than in the petroleum diesel operation. Similarly, Sugiyama et al. [232], have used green diesel in a 2.2 L direct injection common rail engine and found that the brake power was practically unchanged. On the other hand, Ogunkoya et al. [226] has tested green diesel in a 418 cm3 single cylinder engine and observed that the indicated power in the case of green diesel was higher. The authors attributed this behavior to effects such as the fuel ignition delay, the combustion phasing relative to the TDC, injection fuel quality etc. The same authors also observed that green diesel reduces the brake specific fuel consumption (bsfc) of the engine when comprised by either normal or branched saturated hydrocarbons. This was also reported in numerous other investigations [224,230,231,233–238] and is especially important since bsfc is the ratio of the mass of the fuel consumed per brake power output. The lower bsfc values translate into higher fuel economy during engine operation and also to lower emissions, since less fuel is burned for the same power output. In the case of pure green diesel the reduction of the bsfc may be by 5–10% and it is attributed to the higher H2 content and the higher calorific value (LHV, in mass basis). Finally, of major importance in the analysis of the internal combustion engines is the brake thermal efficiency, defined as the ratio of the brake power output to the rate of fuel energy consumption of the engine. In most research reports green diesel was found able to increase the brake thermal efficiency of the engine by up to 10%. For example, Kumar et al. [230] have tested green diesel in a 661 cm3 single cylinder engine and observed that it increased the brake thermal efficiency from 21.76 to 23% at 20% load, from 25.8 to 34.24% at 40% load, from 31.74 to 35.9% at 80% load and from 31.6 to 31.66% at full load conditions at 1500 rpm. This is attributed to the higher calorific value and to the lower ignition delay which shifts the combustion process so that a larger fuel fraction to be oxidized near the TDC. These two factors are considered to supersede the effect of the lower fuel density and lower volumetric heating value. 7.3. Green Diesel Emissions in Internal Combustion Engines Neste [216] has reported results of exhaust emission tests that have been performed over 36 trucks and buses or their engines and nine passenger car vehicles or engines in real road conditions and artificial driving cycles in test beds. Three of the passenger car engines were utilizing neat greenPDF Image | Green Diesel: Biomass Feedstocks, Production Technologies
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