Results and Discussion
5.2 DOC Modeling Results
Figure 5.16: Raw particle size distribution during experiments at 75% load
5.2 DOC Modeling Results
In this section, results from the calibration of the DOC model are presented.
The 20, 40, 60 and 75% load at rated speed (2100 rpm) data from the raw gaseous emission characterization experiments were used for the calibration of the model. In addition, data from a 25% load at rated speed experiment from the dilute emission characterization experiment of reference [46] were also used.
The DOC model kinetics can be calibrated by tuning the pre-exponential factors and activation energies for the oxidation of HCs, CO and NO. With the oxidation of the HCs, CO and NO (ppm levels) in the gas phase, there is a corresponding change in O2, CO2 and H2O (%vol. levels) concentrations across the DOC, but these are two low to be measured and their depletion is not considered by the model. The DOC model can also be used to predict the drop in pressure of the exhaust as it flows through the DOC.
The data describing the input data to the DOC model are shown in Appendix
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C. The DOC model takes the temperature, actual volumetric flow rate, pressure and concentrations of O2, H2O, CO2, N2, NO, NO2, HCs, and CO entering the DOC as input. The concentrations of HCs, CO, NO and NO2 and the pressure drop across the DOC are the model output data.
The results from the DOC pressure drop model are shown in Table 5.8. The model predicts the pressure drop within 0.5 kPa. The DOC pressure drop model calculates the pressure drop of the exhaust gas due to friction by laminar flow through the channels in the DOC (equation 3.4). However, this is only the major component of the total pressure loss across the DOC. This model does not include the losses due to inlet channel contraction and outlet channel expansion as the exhaust flows in and out of the DOC, losses in the DOC channels before the flow becomes laminar. This is the reason the DOC model always under predicts the pressure drop. Since these losses generally increase with exhaust flow rate, the error in prediction increases with load.
Table 5.8: DOC pressure drop model results
% Load Model predicted (kPa) Avg. expt. (kPa) Difference (kPa)
20 1.5 1.8 0.3
40 2.0 2.3 0.3
60 2.7 3.1 0.4
75 3.3 3.8 0.5
The results of the gaseous emission oxidation calibration along with a compar-ison with the experimentally measured concentrations of HCs, NO, NO2 and CO upstream and downstream of the DOC are shown in Table 5.9. All concentrations were predicted within 3 ppm of the experimentally measured values. Since CO con-centrations downstream of the DOC were too low to be measured by the Pierburg emissions analyzer, the model was calibrated to predict CO concentrations within 1 ppm downstream of the DOC.
To calibrate the model kinetics, activation energies for oxidation of HCs, CO and
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NO were adopted from the work of Triana [6, 7] which were also used previously by references [41, 51]. The activation temperatures and the pre-exponential factors used to calibrate the kinetics of the DOC model are shown in Table C.3 of Appendix C. The activation energies are assumed to be constant with exhaust temperature (load) and the pre-exponential factors were changed with load to calibrate the model kinetics.
Table 5.9: DOC model gaseous emission kinetics calibration results
% Load Location HC (ppmC) NO (ppm) NO2 (ppm) CO (ppm)
To unify the model kinetics, so that one apparent activation energy and pre-exponential factor can be used for each gaseous emission species, Arrhenius plots are used. The mathematical basis of the construction of Arrhenius plots is given in Appendix B. The activation temperatures and pre-exponential factors shown in Table C.3 are used to calculate the model reaction rates and then plotted versus the inverse of the absolute DOC channel wall temperature shown in Table C.1. A linear regression fit for each species then yields an ’apparent’ activation energy and pre-exponential factor for that species. These apparent kinetics are different from real parameters because of the influence of diffusion and mass transfer effects of this DOC on the apparent kinetics determined from Arrhenius plots. The results from
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such an analysis is shown in Figure 5.17. The HCs, CO and NO kinetics were fit
Figure 5.17: Arrhenius plots of HCs, CO and NO kinetic data for the DOC
with an R2 value of 0.99. For the entire temperature range of 280oC-460oC, the HCs and CO kinetics can be described by one apparent activation energy and pre-exponential factor, while the NO kinetics are best described by apparent activation energies and pre-exponential factors in two regimes as shown in Figure 5.17. The two regimes in the NO calibration result could possibly be the result of the transition from kinetically limited oxidation at low temperatures to thermodynamically limited oxidation at high temperatures (section 2.1). The fact that the the two regimes separate at a temperature of about 350oC lends credence to this interpretation. Such behavior of best fits in different temperature and flow rate windows was also reported by Triana [6, 7] in the calibration of a DOC in a CRT . A comparison of theR gaseous emission oxidation kinetics of the DOC used in this research with that used by Triana [6, 7] in a CRT is given in Appendix D.R
A summary of the ’apparent kinetic’ parameters is shown in Table 5.10. The apparent kinetic parameters determined from the DOC model, shown in Figure 5.17