Exhaust piping for a catalytic exhaust system

Details Exhaust piping for a catalytic exhaust system Exhaust piping for a catalytic exhaust system

1995-12-01

1997-12-16

Hart, Douglas

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

60272, 60322, F01N 714

Patent

active

056972156

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to exhaust piping for a catalytic exhaust system for a combustion engine, as well as to a composite tube especially intended, although not exclusively, to be associated with said exhaust piping.
As a result of the setting-up of new anti-pollution standards, exhaust systems of vehicles with combustion engines have to be equipped with catalyzers, the aim of which is to play an active part in the reduction of emissions into the atmosphere of more or less toxic combustion gases, so as best to preserve and respect the environment.
For that purpose, the catalyzer or catalytic converter of exhaust systems is connected to the outlet of the exhaust piping, the inlet of which is fixed to the manifold for combustion gases leaving the engine. Moreover, a muffler terminates the systems, being connected to the catalyzer by an exhaust pipe. Structurally, a catalyzer is composed of a rigid casing, inside which there is arranged a ceramic block or monolith with cells coated with alumina and precious metals (platinum, rhodium, etc.) which act, through catalysis, especially on carbon monoxide, the oxides of nitrogen, and unburnt hydrocarbons, in order to eliminate their harmful constituents and convert them into non-harmful emissions.
Furthermore, it is known that catalyzers are effective only when they reach a certain temperature (several hundreds of degrees), that is to say when the engine has been running for at least a few minutes, so that the monolith is sufficiently heated by the gases to initiate the catalysis reactions. As a consequence, as long as the monolith has not reached a given temperature, the gases leaving the engine are not processed, even though they do pass through the catalyzer. In addition, since the catalyzer is often remote from the manifold owing to the design of vehicles and safety criteria, its rise in temperature which is brought about by the gases traveling through the piping is all the more lengthy.
For this reason, in order to overcome these drawbacks, it has already been proposed to surround the exhaust piping on the outside by a composite thermal-insulation tube consisting of an internal tube, of an external tube and of thermally insulating substance provided between the internal and external tubes. In actual fact, it turns out that the catalyzer is not effective any more rapidly for doing so, because these composite tubes exhibit the following behavior: piping which has a high heat capacity owing to its wall thickness of the order of 2 to 3 millimeters, so that the temperature rise time of the catalyzer is therefore too great, when the vehicle is being started from cold; composite tube surrounds said piping so that the calorific energy of the gases when the engine is hot is not removed sufficiently, with the risk of the catalyzer being overheated if the temperature of the gases reaches about 1000.degree. C.
The object of the present invention is to overcome these drawbacks, and it relates to exhaust piping for a catalytic gas exhaust system, the piping being equipped with a composite tube whose design greatly favors the rise in temperature of the catalyzer at the time when the engine is started, while not hampering heat exchange when the engine is hot.
To this end, the exhaust piping for a catalytic gas exhaust system including a gas manifold and a catalyzer, said piping being situated between said manifold and said catalyzer and provided with a composite tube which consists of an internal tube and of an external tube defining between them a substantially annular space, is noteworthy, according to the invention, in that said composite tube is housed inside said exhaust piping, the external tube coming substantially into contact with said piping, and in that said internal tube and external tube exhibit thin walls, the thickness of which is less than 0.3 millimeter.
Thus, as the composite tube is situated inside the piping, this arrangement allows rapid temperature rise of the catalyzer when the vehicle is started from cold and after each time it has been stopped,

REFERENCES:
patent: 3209856 (1965-10-01), Saunders
patent: 3850453 (1974-11-01), Bentley et al.
patent: 4345430 (1982-08-01), Pallo
patent: 5331810 (1994-07-01), Ingermann

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