Exhaust gas purificating arrangement for an internal combustion engine having a gas adsorber and a catalyzer downstream from the gas adsorber
BACKGROUND OF THE INVENTION 
     This invention relates to exhaust gas purification arrangements containing both a catalyzer and a gas adsorber. 
     German Offenlegungsschrift No. 40 33 827 and European Patent Application No. 0 532 803 describe exhaust gas purification arrangements having both a catalyst and a gas adsorber which are especially suited for motor vehicle internal combustion engines. These documents also disclose information relating to the choice of a gas adsorber, which is generally a zeolite. However, the term &#34;adsorber&#34; as used herein, means any arrangement having a gas adsorbent action and, at higher temperatures, a desorbent action. 
     Providing an adsorber for the temporary retention of hydrocarbons contained in exhaust gases takes into account the fact that a catalyzer initiates its exhaust gas-purifying action only after it has reached its start-up temperature, which, depending on the age of the catalyzer material, is in the range of 250°C. to 350°C. Consequently, following cold-start of an internal combustion engine, the exhaust gases leave the exhaust system in an unpurified state, resulting in environmental problems, unless corrective action is taken. 
     It is known, for example, from German Offenlegungsschrift Nos. 1 299 005 and 42 08 624 and from German Engineering Association News (VDI-Nachrichten), No.38 of Sep. 24, 1993, page 11, to heat a catalyzer, either electrically or with the aid of a combustion chamber, so that it reaches its operating temperature more rapidly. As a rule, however, such arrangements do not permanently provide more rapid heating to a desired temperature within an acceptable time period. 
     In conventional arrangements for preheating catalysts within the required time period by heating the catalyzer to its operating temperature with hot exhaust gases, it has been assumed that the catalyzer will have reached its operating temperature, at least in significant regions, before a gas adsorber preceding the catalyst has been heated by the hot exhaust gases to its gas desorption temperature. At the gas desorption temperature, the adsorber releases the stored exhaust gases for catalytic treatment in the catalyzer. The gas-release requirement, however, which must be met under all age conditions of the catalyzer, is further complicated by another problem, particularly important in the case of motor vehicle internal combustion engines which, as is known, must be operated over a very wide range of ambient temperatures, down to -20°C. or less. This problem is that, in the case of very low ambient temperatures and short trips by the motor vehicle, the adsorber may not release the stored exhaust gases during such short operating times because cause it does not reach its desorption temperature. Consequently, the adsorber will not be capable of adsorbing and retaining newly-generated quantities of exhaust gas components. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an exhaust gas purification arrangement which not only ensures rapid heating of the catalyzer to its operating temperature while obviating the need for the high electrical-energy consumption required for electrical catalyzer heaters, but also avoids overloading of the adsorber during short-term operation at low ambient temperatures. 
     These and other objects of the invention are attained by providing an exhaust gas purifier having a burner arrangement for accelerating the heating of the catalyzer at low operating temperatures while avoiding direct heat exchange between the hot burner gases and the adsorber, at least when the catalyzer has not yet reached an operating temperature. 
     A significant advantage of the invention is that the invention requires only one heating arrangement, which is of a conventional type, not only for rapid heating of the catalyzer, but also for heating the adsorber under low ambient temperature conditions. The heating arrangement is a conventional burner supplied with a fuel/air mixture and an ignition device. 
     Thus, the invention not only provides a burner which is conventionally used for heating a catalyzer in an arrangement in which the burner pipe traverses the gas adsorber, it also assures that, in fact, the catalyzer is heated to its start-up temperature by the hot burner gases as quickly as possible under all external temperature conditions. The invention also assures that heat transfer from the burner gases to the gas adsorber is inhibited during the start-up time period so that the catalyzer reaches its operating temperature before the adsorber has been heated to its desorption temperature. 
     In a representative embodiment, this is accomplished by providing an intermediate layer between the burner pipe and the adsorber which diminishes or delays the transfer of heat from the burner pipe to the adsorber. A particularly effective embodiment includes a temperature-responsive bypass around the adsorber so that the hot burner gases are actually applied to the adsorber only under very low ambient temperature conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a view in longitudinal section schematically illustrating a representative exhaust gas purifier arranged in accordance with the invention; and 
     FIG. 2 is a fragmentary view showing an alternative intermediate layer arrangement. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the typical embodiment of the invention shown in FIG. 1, an exhaust gas purifier includes a gas adsorber 2 and a catalyzer 3 mounted in a common housing 1 with the catalyzer 3 downstream from the gas adsorber 2 in the direction of exhaust gas flow, as indicated by the arrow in the drawing. Both the catalyzer 3 and the gas adsorber 2 have a conventional structure which is known to those skilled in the art and therefore will not be explained in detail. Moreover, the invention is not limited to any specific design of catalyzer or gas adsorber. 
     A burner 4, which is schematically shown in the drawing, is equipped with supply lines for fuel and combustion air and with an ignition device (not shown) and is located ahead of the gas adsorber 2 in the purification arrangement. During cold start-up, the burner 4 is ignited to supply hot burner gases to the front surface of the catalyzer 3 through a burner pipe 5 extending through the gas adsorber 2. The operation of the burner is discontinued by means of a temperature sensor (not shown) no later than the time when the catalyzer 3 has reached its operating temperature. In the illustrated embodiment, a turbulence-generating arrangement 6 is disposed in the space between the gas adsorber 2 and the catalyzer 3. 
     The burner pipe 5 passes through the central part of the gas adsorber 2. Between the pipe 5 and the adsorber 2, there is an intermediate layer 7 made of a material which diminishes heat transfer from the burner pipe 5 to the adsorber 2 and is arranged so that, at very low ambient temperatures, the adsorber is at least partially unloaded (by release of some of the adsorbed gases) even during short trips while at the same time assuring that the catalyzer, having been subjected to the hot burner gases, will have essentially reached its operating temperature at the time that the adsorber has been heated to its desorption temperature. 
     If desired, the transfer of heat from the burner pipe 5 to the adsorber 2 through the intermediate layer 7 throughout the entire range of ambient temperatures can be avoided by passing the hot burner gases through the gas adsorber 2 only at very low ambient temperatures. For this purpose, a bypass 8 is provided which extends from the burner pipe 5 around the gas adsorber 2 outside the housing 1 and reenters the housing upstream from the catalyzer 3. 
     In this arrangement, an actuating device 9 containing an ambient-temperature sensor controls a flap 10 at the entrance to the bypass 8 so that the adsorber 2 is heated by the hot burner gases flowing through the burner pipe 5 only at low ambient temperatures. At all other ambient temperatures, the flap 10 opens the bypass 8 to circumvent the adsorber 2. With this arrangement, thermal shielding is provided for the adsorber, reducing its aging characteristics. 
     If desired, the intermediate layer 7 may be made with thermally-responsive elements 11, for example, of the bi-metal type, which are mounted between opposite walls of the layer 7 and change their shape as a function of the temperature in a manner such that they provide heat-conducting bridges between the adsorber 2 and the burner pipe 5 only when the adsorber is at low ambient temperatures. This intermediate layer arrangement is shown in FIG. 2. 
     When the thermally-responsive elements 11, such as bimetallic elements, are at a low temperature (e.g., below 0°), they are in a position 11a to provide heat bridges between the burner pipe 5 and the adsorber 2. At a higher temperature, the thermally-responsive elements 11 bend toward the adsorber 2 to a position 11b providing more heat insulation in the intermediate layer 7. Generally, to obtain the best temperature effects, the thermally-responsive elements 11 are mounted adjacent to the gas adsorber 2 and not on the side of the layer 7 toward the burner pipe 5 so that they are more responsive to the temperature of the adsorber 2 than to the temperature of the burner pipe 5. This results in a slower response to the burner heat in the pipe 5. 
     Thus, the invention provides an exhaust gas purification arrangement which, by making dual use of a conventional burner, assures both rapid heating of the catalyzer to its operating temperature and a reduction in the time required for heating the adsorber to its desorption temperature as a function of the ambient temperature. 
     Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.