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Crank angle resolved results Figure 5 shows selected results from the 75% load case with an applied mixing rate of 3.29. The calculated NO concentration (B) is for the trapped cylinder gas only. Diluting with additional air, used in the scavenging process, results in a NO concentration of 1266 ppm which is equal to the measured value (after the exhaust gas turbine). As seen from Figure 5 (A) the SO2 concentration increase rapidly around TDC and the increase follows the rate of combustion very closely. Both SO3 and H2SO4 are at very low levels during the combustion period. This is due to the fact that the equilibrium conversion of SO2 (cf. Figure 5 (B)) is low at the very high temperatures (cf. Figure 5 (C))
as well as the time required for a significant conversion into SO3 and H2 SO4 has not elapsed. The mean combustion temperature in the parcels in Figure 5 is a result of both the expansion of the cylinder content as well as the rate at which the much colder fresh air is mixed into the parcels. After the combustion has ended the temperature has dropped sufficiently in order to favour a significant conversion fraction of SO2 into SO3 and H2SO4. As seen from Figure 5 (C) the reaction rates controlling the oxidation of SO2 appear to be very fast at least until up to 80 CAD ATDC. From this point the temperature has dropped enough in order to cause a slow-down in the rate of SO2 oxidation and the result is a deviation from equilibrium. It appears as though the oxidation of SO2 is not
frozen at the end of the simulation (corresponding to the opening of the exhaust valve). However during blow-down the hot combustion gases are expanded rapidly causing a further temperature drop, and once the scavenge ports are uncovered cold intake is mixed with the combustion gases causing a further cooling. Thus, it may seem reasonable to assume that the further oxidation of SO2 will be on a much smaller scale.
Comparison with equilibrium assumptions. In a previous study, related to cylinder liner corrosive wear due to sulfuric acid condensation, Teetz [7] applied the assumption that reactions responsible for SO2 oxidation are very fast i.e. Equilibrated above 1000oC, while below this temperature the reactions become frozen i.e. no further reactions take place. This assumption is valid according to [1] and references within. In the following we will apply the above assumption from now on denoted the frozen equilibrium assumption. The assumption is applied assuming a single zone combustion i.e. combustion takes place
in the same zone containing the entire amount of fresh air. This is in contrast to the previous multizone approach1. The result is shown in Figure 6 (A) where a comparison with the equilibrium conversion fraction as found in the burned zones using the multi-zone approach is made. The temperature in the single burned zone is depicted in Figure 6 (B).
The most important observation is the fact that the mean temperature using the single-zone approach is much lower than in the multi-zone approach,
coupled with the high pressure near TDC, this results in a very high equilibrium conversion fraction. Furthermore the resulting ε is approx. 18% at the end of simulation, which is almost four times larger than that found in Figure 5 with a mixing rate
of 3.3 and more than twice than assuming full equilibrium in the parcels. Although we have found evidence supporting the use of the frozen equilibrium assumption for premixed combustion2, it is clearly insufficient for modelling emission formation during the mixing controlled combustion in diesel engines.
Figure 5 — Results from multi-zone approach for the 75% load case. (A) Calculated concentrations of SO2,
SO3, and H2 SO4 (B) Calculated NO concentration (C) Conversion fraction of SO2, equilibrium conversion
fraction of SO2, and oxygen concentration in the burned zones (D) Temperature of the fresh cylinder air and
the average temperature in the burned zones.