Cold start emission

J. K. Hockmuth and co-workers. Hydrocarbon Trapsfor Controlling Cold Start Emissions, SAE 930739, Society of Automotive Engineers, Warrendale,... 

In this section we will first discuss the mode of converter warmup/lightoff following a cold start, and then the results of cold-start emission calculations for two different vehicle emission control systems to illustrate some of the model applications mentioned above. 

Fig. 1. Effects of front brick volume on cold-start emissions at various poison penetration depths (underfloor converter location front brick with 225 g/fl Pd loading + 6 in. rear brick with 90 g/ft noble metal loading).

Additional simulations show that the cold-start emission performance of a close-coupled converter is also much less sensitive to poisoning than that of an underfloor converter (results not shown). These results suggest that optimizing the converter location provides an effective means of improving cold-start emission performance at lower noble metal loadings. 

In view of their effectiveness in reducing cold-start emissions, we are particularly interested in the behavior of small-volume electric heaters. For an uncatalyzed (inert) heater of sufficiently small size, the following explicit, analytical asymptotic solution can be obtained [10] ... 

To study the possibility of application in the reduction of cold start emission, Pitchon et al. tested the reaction of total oxidation of a mixture of light hydrocarbons... 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

Yamamoto, S., Masushita, K., Etoh, S. and Takaya, M., In-Line Hydrocarbon (HC) Adsorber System for Reducing Cold-Start Emissions, SAE, paper No. 2000-01-0892 (2000). 

However, diesel engines and some gasoline engines are operated under lean-burn conditions, where the oxygen is fed in excess, i.e., 10-20% more than is required to meet the stoichiometry for combustion of the fuel [132,489]. Gold catalysts have therefore been examined for their potential in low-temperature activity to combat cold-start emission problems and removal of NOj, from lean-burn engines [202]. 

In order to meet future legal requirements several concepts are proposed for reducing cold-start emissions. In this contribution their performance is determined by detailed simulation studies based on a one-dimensional model. The dynamic behavior during start-up and the influence of local catalyst deactivation on the efficiency of the different cold-start concepts is described. An irmovative concept is developed which shows lowest cold-start emissions for fresh and aged catalyst in the simulation study as well as in cold-start experiments. 

Three-way catalyses (TWC) require a minimum temperature of approx. 3500C for proper catalytic combustion. Due to the heat capacity of the exhaust system it takes about 1 min after engine start until this temperature level is reached if the catalyst is only heated by the exhaust gas. The amount of toxics produced during this cold-start period presents a considerable fraction of the total amount during one test cycle [1]. Due to more stringent legal purification requirements several concepts were developed to reduce the catalyst heat up time. Presently the main approaches to lower the cold-start emissions are the use of an electrically heated catalyst (EHC) [2], a burner heated catalyst (BHC) [3, 4] and hydrocarbon adsorber systems [5, 61. 

Within the scope of this study an irmovative concept is developed, where the light-off of the monolith is induced by an exothermic reaction at the catalytic surface (combustion heated catalyst, CHC). It will be shown that in comparison with the other concepts the cold-start emissions with the CHC-concept are lowest level and almost independent of local deactivation of the catalyst. 

In the simulations as well as in experiments the CHC-concepts shows the best performance for the reduction of cold-start emissions under the assumption of a fresh catalyst. For each start-up only a little amount of hydrogen ( s 0.5 g) is needed. Experience in production and... 

Figure 11 shows the quantitative influence of catalyst aging on the conversion behavior of automotive catalysts using different cold-start concepts. In this figure the cold-start emissions resulting from simulation study are given over the mean activity of the main catalyst. In order to... 

Figure 11 Cold-start emissions dependant on the catalytic activity...

Using only a fresh main catalyst the cold-start emissions of hydrocarbons are about three times higher compared to the future legal requirements (ULEV). With decreasing catalyst activity the cold-start behavior of the main catalyst gets worse which results in increasing HC-emissions. 

The BHC-concept reduces the cold-start emissions considerably but is most strongly influenced by catalyst deactivation. Due to the short front part of the catalyst which is heated to high temperature levels (Figure 4) the length of an inert front area of an aged catalyst has a strong influence on the HC-emissions. 

Future legal requirements can be fulfilled with the EHC-concept for fresh catalysts. As the main catalyst is only heated by the exhaust gas again a considerable increase of the cold-start emissions results from catalytic aging. Hence, the ULEV-limit can not be kept for lower activities than 40 %. 

The conversion of CO to CO2 and of unburned hydrocarbons to CO2 and H2O is very slow at normal temperatures, but it takes place quite rapidly at temperatures around 600 °F. This means that the removal of the CO and hydrocarbon pollutants does not take place efficiently when you first start your engine. The pollutants released when a car is first started are called cold-start emissions. Recent research has focused on developing a way to insulate the catalytic converter so it retains its heat even when the car is not running. Some of the prototypes can maintain their operating temperatures for almost a day while the engine is shut off. With these new, insulated catalytic converters installed in most cars, cold-start emissions could be reduced by at least 50%. This could have a significant effect on air pollution. 

The catalyst performance was evaluated by using the U.S. Federal Test Procedure. Since the key purpose of preconverter is to reduce the cold-start emissions, bag 1 of FTP cycle was split into two bags bag lA which represents 0 to 220 sec. and bag IB which represents 220-505 sec. However, at t = 220 sec. the temperature of the main converter is relatively stable and sufficiently high to ensure rapid reaction kinetics. 

Recent attention has been placed on the use of Pd-based catalyst formulations because of their ability to catalyze the oxidation of hydrocarbons and CO at relatively low temperatures as a strategy to improve cold-start emissions and thus comply with low emission vehicle regulations, and because of the lower cost of Pd metal relative to Pt and especially Rh [27-32]. Three-way catalysts using Pd or Pd and Pt were evaluated in the late 1970 s and early 1980 s, but were found to be easily poisoned by sulfur [33,34]. Indeed, recent studies... 

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