Exhaust composition

In addition to predicting the exhaust composition of both gases and soHds, the abiHty of these chemical equiHbrium programs to do adiabatic calculations makes them useful for computing supplemental fuel requirements and the effect of excess oxidant on temperature. 

The detailed composition, referring to classes of compounds, is shown for C6 in Figure 9.3 with and without precolumn hydrogenation. In addition to paraffins, there are olefins—mainly with terminal double bond—and small amounts of alcohols (and aldehydes). The low detection limit of gas chromatography (GC) analysis allows precise determination even of minor compounds and provides exhaustive composition data also for use in kinetic modeling. Because of the short sampling duration of ca. 0.1 s,8 time-resolved selectivity data are obtained. 

Three-way catalysts are able to reduce NO as well as oxidize CO and hydrocarbons when the exhaust composition is held near the stoichiometrically balanced composition, or "stoichiometric point." This control of exhaust composition is accomplished, after the initial warmup period, through the use of the feedback control system illustrated in Figure 1 (2, 3, 4). ... 

A microcomputer periodically reads the sensor signal and adjusts the fuel control signal to bring the air-fuel ratio, and thus the exhaust composition, closer to the stoichiometric point. 

Figure 3. Oxygen sensor signal showing oscillation of the exhaust composition around the stoichiometric point during feedback control of the engine.

Although time-averaged conversion measurements provide valuable information, a detailed analysis of the dynamic behavior of three-way catalysts requires direct observation of catalyst responses to rapid changes in exhaust composition. 

Figure 8. CO concentrations measured at the inlet and outlet of a three-way catalytic converter during feedback control of the exhaust composition (15).

At the maximum time shown in Figure 9B, the outlet CO level had only risen to about 60% of the rich steady-state outlet level, which can be seen on the left side of Figure 9A. Approximately 25 s were required for the outlet CO to reach the new steady-state level after the lean-to-rich step. This time is much shorter than that mentioned above for catalysts in S02 free simulated exhaust, but is still long with respect to the periods of the exhaust composition oscillations observed during actual automotive operation. 

Because of this type of behavior, a sharp transition at stoichiometry but low sensitivity and temperature effects either rich or lean of this point, the oxygen sensor is most useful in controlling at the stoichiometric point. It is of limited usefulness at other exhaust compositions. However, as shown in Figure 5, this is exactly the point at which a three-way or dual bed catalytic converter is most efficient. Only when the exhaust composition is near the stoichiometric point will both the oxidation of the HC and CO and the reduction of the NO occur satisfactorily. 

Zirkind reviews investigations of radiation emission of rocket exhausts since 1940 (Ref 11). Emission characteristics of the exhaust plume are strongly dependent on rocket engine parameters the propellant system. If these motor operating conditions are stipulated, machine computations can give exhaust composition sometimes temperatures... 

However, there are several major drawbacks that hinder practical application of this NOx reduction method in automobile exhaust aftertreatment (i) The NO reduction activity is typically limited to a certain temperature window, for NM-based catalysts it is around the light-off—cf. Fig. 14 and Ansell et al. (1996), Jirat et al. (1999b), Burch et al. (2002) and Joubert et al. (2006). (ii) With low HC concentrations and the exhaust composition met in modern diesel engines, the achieved NOx conversions in real driving cycles are quite low (typically around 5-10%, cf., e.g., Kryl et al, 2005). (iii) The selectivity of NOx reduction is problematic, N20 may form up to 50% of the product (Burch et al., 2002 Joubert et al., 2006). Alternative (Cu-, Co-, Ag-, etc., based) catalysts may provide a wider temperature window or better selectivity for... 

To study the influence of N02 on SCR efficiency, ESC and ETC test cycles with 0% and 50% N02/NOx ratio in front of SCR catalyst were simulated. However, during transient test cycle operating conditions, a constant supply of optimum 50% N02/NOx ratio will be difficult to achieve. NO to N02 conversion over a DOC is dependent on exhaust temperature, space velocity and exhaust composition. Because of transient operating conditions, it becomes furthermore a function of time. Exploiting the fast SCR reaction is thus limited by the realistically achievable N02/NOx ratio in front of the SCR catalyst. To investigate this, ESC and ETC test cycles were also simulated for a combined system of DOC and SCR, where the N02/NOx ratio in front of the SCR is defined by the N02 conversion over the DOC (Fig. 52). 

Unless carried out very carefully, data from flow reactors may be influenced by experimental uncertainties. Potential problems with the flow reactor technique include imperfect mixing of reactants, radial gradients of concentration and temperature, and catalytic effects on reactor walls. Uncertainties in induction times, introduced by finite rate mixing of reactants, presence of impurities, or catalytic effects, may require interpretation of the data in terms of concentration gradients, rather than just exhaust composition [442]. 

Sensor. The control of the exhaust composition was essential to maintain the air-to-fuel ratio close to stoichiometric for simultaneous conversion of all three pollutants. This control came about with the invention of the 02 sensor.21,22 The sensor head of this device was installed in the exhaust immediately at the inlet to the catalyst and was able to measure the 02 content instantly and precisely. It generates a voltage consistent with the Nemst equation in which the partial pressure of 02 (P02)exhaust in the exhaust develops a voltage (E) relative to a reference. The exhaust electrode was Pt deposited on a solid oxygen ion conductor of yttrium-stabilized zirconia (Zr02). The reference electrode, also Pt, was deposited on the opposite side of the electrolyte but was physically mounted outside the exhaust and sensed the partial pressure (P02)ref in the atmosphere. E0 is the standard state or thermodynamic voltage. R is the universal gas constant, T the absolute temperature, n the number of electrons transferred in the process, and F the Faraday constant. 

Figure 8.1. The effect of air-fuel ratio on exhaust composition (not to scale only approximate ranges are given). (Adapted from Nishikawa et al. 2000 and Stem, 1976).

The light exposure test matrix includes six core experiments. L-1 is a baseline experiment for the light exposure series it examines the effeets of photolysis reaetions on the exhaust composition. Experiment L-2 examines the effeet of HO reactions on exhaust composition at relatively high NOx. Experiment L-3 is proposed to examine the effect of the... 

D-1 Diesel Exhaust Only Determine changes in exhaust composition due to aging in chamber. 

D-4 N2O5 + Diesel Exhaust Investigate effects of NO3 on diesel exhaust composition. N2O5 decomposes to form NO2 and NO3. 

L-1 Diesel Exhaust Only Examine effects of photolysis reactions on exhaust composition. 

Williamson, W.B, et. al, Durability of Palladium Automotive Catalysts Effects of Trace Lead Levels, Exhaust Composition, and Misfueling , Industrial Engineering Chemistry, Prod. Res. Dev. 23, 1984, pp531-536. 

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