Inteligencia intrapersonal

Diesel engine particulate emissions consists mainly of solid carbonaceous material, called soot, generated during combustion, on to which some organic compounds become adsorbed. Although most of the PM comes from the fuel (0.2-0.5% of the fuel becomes soot), up to 25% of PM is formed from the lubricating oil [199]. For light-duty diesel vehicles the PM emission rates are typically 200-600 mg/km [1].

The Euro 6 limit for particulate emissions is 5 mg/km [198]. Particulate emissions can cause several health problems, including irregular heartbeat, aggravated asthma, decreased lung function, increased repertory systems, cancer, and even premature death in people with heart or lung disease [200].

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Particulates are formed in diesel engines at temperatures about 1000-2800K at pressures 50-100 atm. First, the soot precursors condense from the gas phase and form the first very small (diameter below 2 nm) observable soot particles. These precursors include typically unsaturated compounds, especially acetylene, and polycyclic aromatic hydrocarbons (PAH). At temperatures below 1700K only aromatics and highly unsaturated species form soot, whereas in diffusion flames at temperatures above 1800K all hydrocarbon fuels can lead to soot formation [1]. The soot formation propensity of hydrocarbons can be presented in a descending order as follows: aromatics > alkynes > alkenes > alkanes [58]. After the particulates are generated, they grow through surface growth, coagulation and aggregation, as shown for soot formation in Figure 6. Surface growth contributes to the initial particulate growth: hydrocarbon intermediates from the gas phase are deposited on the surface of the particulate nuclei in agglomerating collisions. At the early stages of the particle growth, two particulates may collide and coagulate into a single spheroid. Once surface growth ceases, the coalescence of particulates continues to form chain like structures in aggregation. Eventually large clusters of solid carbon spherules with diameters from 10 nm to 80 nm are formed. In diesel engines, the particulate concentration is highest in the fuel-rich fuel spray core. Efficient atomization and mixing reduce the soot levels by increasing air concentration in the fuel rich spray core, which enhances the oxidation of both soot precursors and the soot particles that have already formed. Therefore higher soot levels indicate lower ITE. The highest particulate concentration occurs when the engine is under full load and the overall engine equivalence ration is high, even approaching the stoichiometric value [1,219].

Figure 6. Schematic diagram of soot and particulate formation [219]

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Alternative fuels, such as alcohols, ethers and esters, generally produce lower levels of PM mass emissions compared to diesel fuel, as was stated in a review article by Westbrook [202]. Diesel fuel tends to produce higher PM levels, compared to alternative fuels, because of its higher content of aromatics and sulphur compounds, which can act as soot precursors. Additionally, the oxygen in the molecular structure of alternative fuels can reduce the locally fuel-rich regions and thus reduce PM formation compared to diesel fuel. Several articles, e.g. the review articles of Sharma et al. [218], Bergthorson and Thomson [220] and Shahir et al. [201], have reported that biodiesels produce less PM emissions compared to diesel fuel.

In the case of alcohols, Giakoumis et al. [209] concluded in their review article that blending alcohols to diesel fuel tends to reduce the PM emissions due to the better volatility of alcohols compared to diesel fuel. Reduction in PM emissions of a diesel engine with alcohol addition to diesel fuel has been reported by e.g. Merola et al.

[87], who observed a reduction of about 20 % in smoke with 20 v-% butanol addition in a turbocharged diesel engine with start of combustion occurring either 3 CAD before TDC of 5 CAD after TDC. Balamurugan and Nalini [188] reported an increase of 10% to 15% in smoke density with 4 and 8 v-% addition of either propanol or butanol under both medium and high load ranges. Interestingly, Guariero et al. [211]

observed that 6 v-% ethanol addition to a B5 soybean biodiesel resulted in an increase in the number of particulates from 9.6*10⁶ to 1.1*10⁷ particulates in cm³ due to a shift from larger particulates to smaller, lighter particulates. Furthermore, Zhang and Balasubramanian [221] reported that both 15 and 20 v-% butanol blended to diesel fuel reduced the total mass and number of particulates in diesel engine exhaust gas, but increased the number of particulates with diameter less than 15 nm. Additionally, they observed an increase in PAH emissions with butanol addition to diesel fuel.

When considering ethers, the review articles of both Arcoumanis et al. [41], Park and Lee [98] and Thomas et al. [216] concluded that DME produces significantly lower levels of PM, compared to diesel fuel, due to the high oxygen content. For example, Yoon et al. [222] reported that DME produced low PM emissions compared to fossil diesel fuel at advanced injection timing. Raopoulos et al. [45] reported a reduction in smoke opacity of a diesel engine of about 14% to 29% with 8, 16 and 24 v-% addition of DEE to the diesel fuel. Paul et al. [101] observed the particulate emissions of a diesel engine to decrease by about 2% to 90% with addition of 5 and 10 v-% of DEE

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to diesel, the decrease being more significant at higher loads. Cinar et al. [217]

observed with DEE that increasing the ratio of premixed combustion in a HCC engine from 0 to 40 v-% decreased the soot emissions up to 19.4%. They attributed the reduction in soot to the reduced number of fuel rich regions in the combustion chamber, the oxygen in the DEE and the lower C:H ratio of DEE compared to diesel fuel.