Automotive exhaust composition

Why does an automotive exhaust catalyst have a control system to regulate the composition of the air-fuel mixture How is this control performed ... 

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. 

The composition, temperature and flow amount of automotive exhaust gases vary continuously with driving conditions. The circumstance of the exhaust catalysts exposed is far more different from that in chemical factories. The automotive catalysts are requested to purify infinitesimally small amount of the... 

The most popular use of CSZ is as a gas sensor (2 sensor) for automotive exhaust, the structure of which is shown in Fig. 3.2. The sensor with the standard electrode (air) is inserted in exhaust pipes. The relation between the ratio of air to fuel (RAF) and the EMF is depicted in Fig. 3.3. The EMF shows a sharp drop at RAF = 15 (equivalent composition), left of which is called the rich burn region and right of which is called the lean burn region. Because the bad gases, such as CO and NO , , in exhausted gas are minimized at the point of equivalent composition, the value of RAF is controlled by the feedback of EMF. 

Another important application of heterogeneous catalysts is in automobile catalytic converters. Despite much work on engine design and fuel composition, automotive exhaust emissions contain air pollutants such as unburned hydrocarbons (CxHy), carbon monoxide, and nitric oxide. Carbon monoxide results from incomplete combustion of hydrocarbon fuels, and nitric oxide is produced when atmospheric nitrogen and oxygen combine at the high temperatures present in an... 

LEIS has been applied to study the surface composition of Co-Mo and Ni-Mo hydrodesulfurization catalysts [58-60], Fe-based Fischer-Tropsch [61], automotive exhaust catalysts [62], ammonia synthesis catalysts [63], and of model systems such as Pt evaporated on Ti02 [64] and Au and Pd on thin Si02 layers [65]. In this respect, the review of Horrell and Cocke [66] describes several applications. 

FIGURE 3.1 Plot of the approximate effect of various air to fuel ratios on the composition of automotive exhaust gas. Compiled from the data of Acres [I] and Stoker and Seager [7]. 

The Nernst glower is one of the oldest sensors and is used to measure the composition of automotive exhaust. 

The laboratory reaction system used was a conventional flow system with a tubular fixed-bed reactor as described elsewhere(18). The characteristic feature of this system is its ability to simulate various air to fuel ratios (A/F) of automotive exhaust gases using eight mass flow controllers. In this study, catalytic activity on the catalysts in simulated automotive exhaust gases was measured as a function of X, which is a normalized value of A/F by a stoichiometric one in the simulated exhaust gas, at 300°C and 420,000 h space velocity. The compositions of the simulated exhaust gases for each X are shown in Table 1. Catalytic activity was expressed as percent conversions of NOx(NO+N02), CO, and HC. 

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. 

The experiments are complicated by the fact that the activity for a certain experimental condition depends on the conditions applied prior to the experiment. Chemieal changes of the catalyst, such as oxidation and reduction of ceria or rhodium and the appearance and disappearance of crystal surfaces, are thought responsible for the latter observation. The importance of the dependence of the catalyst structure and composition on the experimental conditions was reported previously for automotive exhaust catalysts [16] and is in the present paper referred to as extrinsic relaxation. In order to obtain reproducible experimental data, a so-called standard condition was applied in between two subsequent experiments. The conditions corresponding to a given experiment were applied for only 10 min. Next, the standard condition was applied for at least 30 min. In such a way, the catalyst changes due to... 

In this paper, the effect of sintering atmosphere and temperature on the activity of a typical three-way automotive exhaust catalyst system (Pt/Rh/y-Al O /CeO ) and the implications of correlating fundamental catalyst characterization data such as average noble metal crystallite size and composition with catalytic activity are discussed. 

Because automotive exhaust gas has a cyclic fluctuation of lean-rich composition, a component that can store oxygen and that readily undergoes redox cycles, can provide oxygen for oxidation of CO and hydrocarbons in the fuel-rich region. In the reduced state the component can remove oxygen from the gas phase when the exhaust gas cycles into the lean region. Thus, cerium oxide not only promotes the oxidation activity of the catalyst, but also widens the air-foel ratio composition range where all three major pollutants, CO, hydrocarbons, and NO can be removed. 

A typical example of surface coverage optimization is the reaction between CO and O2. A periodic change in the inlet composition has a particularly important application, namely the catalytic automotive exhaust cleaning. The feed to the catalytic monolith changes periodically, both in composition and in temperature. In this way, the highest possible reaction rate according to Equation 9.34 is achieved. 

These are carbon monoxide, CO, unburned hydrocarbons (HC), and the nitrogen oxides, NO. In the U.S.A., a program called Auto/Oil (Burns et al., 1992), conducted by automotive manufacturers and petroleum companies, examined the effect of overall parameters of fuel composition on evaporative emissions and in the exhaust gases. The variables examined were the aromatics content between 20 and 45%, the olefins content between 5 and 20%, the MTBE content between 0 and 15% and finally the distillation end point between 138 and 182°C (more exactly, the 95% distilled point). 

Fig. 2. Effect of mixture strength on exhaust gas composition (dry basis) and brake specific fuel consumption (BSFC) for an unsupercharged automotive-type engine usiag iadolene fuel, H/C = 1.86, where the ignition is tuned to achieve maximum best torque (MBT), the brake mean effective...

---