Fig.15 Effects of excess air ratio of the burned zone and the amount of fuel injected on number of soot particle
4.2 Relationship between Soot Size and Spray Penetration
Figure 12 shows a simplified model of spray flame referring the spray flame indicated by Kazakov & Foster[7]. It is considered conceptually in the model that the fuel droplet size becomes smaller toward the spray tip by evaporation of the surface of fuel droplet during ignition delay period, therefore, the droplet size at the spray tip becomes smaller as the penetration length becomes longer. This model is not always sufficiently formed in the combustion chamber of an actual small diesel engine because the fuel spray is not conical due to swirl and squish. In addition, the fuel spray sometimes impinges on the piston cavity wall and the injection duration is not long enough to attain the penetration length.
As shown in Fig.13, the measured ignition delay decreases as the orifice size decreases. Therefore, a large decrease in droplet size at the spray tip estimated by the simple analogy seems to be diminished or suppressed by the reduction of ignition delay due to orifice size. Figure 14 shows a correlation between the penetration length and the soot size normalized by the orifice size. The abscissa "(Δtig・d)0.5" is the value proportional to the penetration length for the ignition delay period, which is reduced from the momentum theory suggested by Wakuri et al.[13]. In the present analysis, it is assumed that soot formation occurs in the burned zone alone, and the proposed spray flame model is very rough and hypothetical, then, some scattering can not be inevitable in the data based on experiment which were obtained under various engine operation conditions of brake mean effective pressure, injection timing and nozzle orifice size. Nevertheless, the figure shows a good correlation between them. This figure convinces us that the soot size increases proportionally to the orifice size, and deceases linearly with the penetration length for the ignition delay period.
4.3 Relationship between Number of Soot Particle and Excess Air Ratio
It seems to be a reasonable hypothesis that the number of soot particles is dependent on that of fuel droplets which is proportional to the amount of fuel injected as the primary factor. Therefore, the number of soot particles increases with fuel droplets if the droplet size is unchanged by an increase in fuel amount injected. On the other hand, if the size of fuel droplets becomes smaller, the probability of soot formation becomes small due to a faster evaporation of fuel droplets in the probability sense. Then, the number soot particles must be reduced if a soot size is small enough, if ignition delay is long enough for evaporation of fuel droplet, and if the oxygen content in the burned zone is high enough resulting in an increase in the premixed combustion without smoke. Thus, it is considered in the present analysis that the number of soot particles is dependent firstly on that of fuel droplets, and is secondarily dependent on the ignition delay and oxygen content. According to the two-zone model analysis, it was suggested that the excess air ratio of the burned zone "λb0" increases with the increase in ignition delay. According to Eq. (2), a larger soot-oxidation occurs if the partial pressure of oxygen increases due to an increase in excess air ratio.
Figure 15 shows a good correlation, showing some scattering in the data, between the number of soot particles "Ns", the amount of fuel injected "Gf(g/cycle/cylinder)" and the excess air ratio "λb0" of the burned zone. A part of data in Fig.15 might be overestimated in the number of soot particles according to Eq. (4) due to underestimating the soot size. This figure, however, convinces us that the number of soot particles increases almost in proportion to the amount of fuel injected and decreases almost linearly with the excess air ratio of the burned zone.
It is generally said that the premixed combustion in diesel engines is free from smoke and smoke increases in proportion to the amount of diffusion combustion. This increase in smoke seems to be due to increase in the amount of fuel injected, in other words, an increase in the number of fuel droplets results in an increase in that of soot particles. In the case of turbo-charged engine as used in the present experiment, the larger mean excess air ratio was attained at the high load in comparison with that of naturally aspirated engine, especially in the cases that the combustion duration became longer as the orifice size decreased. The large excess air ratio of the burned zone was also obtained even in the high load, which was nearly equal to the low load case. As a result, both data at the low and high loads could be evaluated equally in the same scales as shown in Fig.15.
5. CONCLUSIONS
The effect of nozzle orifice size on smoke emission was examined experimentally in a turbocharged DI diesel engine. And the size of soot particle and the number of soot particles were estimated by analyzing the measured combustion history theoretically using the two-zone model. The concluding remarks of the present study are summarized as follows.
(1) The size of soot particle decreases almost linearly with the nozzle orifice size which is the primary factor, and it is also decreased linearly with the increase in the spray penetration during ignition delay, the secondary but more important factor which indicates the effect of orifice size reduction.
