Automotive catalytic converter exhaust system

Disclosed is a vehicle exhaust system containing a three-way catalytic converter. An insulated flexible duct connects the outlet of the engine exhaust manifold with the inlet of the three-way catalytic converter to retain the exhaust engine gases at a temperature greater than the ignition temperature of the three-way catalyst in the catalytic converter. The insulated flexible duct is preferably made of an inner flexible metal conduit surrounded by fibrous insulation which in turn is surrounded on the outside by a second flexible metal conduit. The system is economical, contains no active elements which would require maintenance or could get out of order, is adaptable to motor vehicles of all types (especially automobiles, light trucks and vans) including those powered by either gasoline and diesel engines and entirely eliminates the need for expensive and complicated light-off catalysts in the system.

TECHNICAL FIELD 
The invention herein relates to vehicle exhaust systems. More particularly 
it relates to exhaust systems containing "three-way" catalytic converters. 
BACKGROUND OF PRIOR ART 
For several years U.S.-made automobiles have been equipped with "two-way" 
catalytic converters to reduce the level of unburned hydrocarbons and 
carbon monoxide in the exhaust from automotive gasoline engines. These 
"two-way" catalytic converters have, however, not had the capability of 
making significant reductions in the amount of the third major component 
of automotive exhausts, nitrogen oxides. Under the air pollution 
regulations which are to come into effect in the near future, automotive 
exhaust systems must have the capability of reducing the level of nitrogen 
oxides in the exhaust as well as the unburned hydrocarbons and carbon 
monoxide levels. To accomplish this reduction of nitrogen oxide level, the 
automotive industry plans to use catalytic converters containing what are 
known as "three-way" catalysts, which have the capability of reducing the 
levels of all three major components in the automotive exhaust systems. 
The three-way catalysts, however, have the limitation that they must 
operate at significantly higher temperatures than the present two-way 
catalysts. In order to attain this high temperature of the exhaust gases 
at the inlet to the three-way catalytic converter, the automotive industry 
has been investigating the use of "light-off" catalysts placed in a 
container between the outlet of the engine exhaust manifold and the inlet 
of the three-way catalytic converter. The function of the light-off 
catalyst is to create an exothermic reaction with the unburned 
hydrocarbons in the exhaust system to raise the exhaust gases to a 
temperature at which the three-way catalyst will operate effectively to 
reduce the level of all three pollutants. 
This system including the light-off catalyst is, however, quite expensive, 
and requires the presence of an additional element--the light-off 
catalyst--in the system, which adds to the system complexity and expense. 
It also represents an active element in the exhaust system which can fail 
and which will at least require periodic maintenance and replenishment of 
the catalyst. It would therefore be quite advantageous to have an exhaust 
system which would provide for the needed high inlet temperatures of the 
exhaust gas at the inlet of the three-way catalytic converter but yet 
would contain no active components. It would also be advantageous for such 
a system to be simple in structure and relatively inexpensive. 
BRIEF SUMMARY OF THE INVENTION 
The invention herein is an improvement in a vehicular internal combustion 
engine exhaust system, wherein gases at elevated temperature are exhausted 
from the engine through an exhaust manifold following combustion and 
conveyed by the exhaust system from the manifold to the atmosphere, the 
exhaust system containing a catalytic converter having therein a three-way 
catalyst through which the gases pass for reduction of their content of 
unburned hydrocarbons, nitrogen oxides and carbon monoxide. The 
improvement of this invention comprises conduit means joining the exhaust 
manifold and the converter and through which the gases are conveyed from 
the exhaust manifold to the converter, the conduit means retaining 
sufficient heat in the gases such that the gases upon reaching the inlet 
of the converter are at a temperature above the minimum operating 
temperature of the three-way catalyst. In varius embodiments the conduit 
means comprises a flexible insulated duct, preferably a semi-flexible 
insulated duct. The duct may be formed of an inner flexible metal conduit 
and an outer flexible metal conduit with thermal insulation filling the 
annulus formed therebetween. The thermal insulation may be granular or 
preferably fibrous and is resistant to temperatures in the range of 
600.degree. F. to 1600.degree. F. (315.degree. C. to 870.degree. C.).

DETAILED DESCRIPTION OF THE INVENTION 
In order to understand the simplicity and effectiveness of the present 
invention, it is necessary first to consider the prior art three-way 
catalytic converter exhaust systems presently designed and being 
considered for use in forthcoming automobile models to obtain compliance 
with the more stringent anti-pollution standards. These prior art systems 
are illustrated in FIG. 1. In an automobile 2 is an internal combustion 
engine 4 (here illustrated as a conventional V-8 gasoline engine). The 
exhaust gases resulting from the combustion in each cylinder are exhausted 
from the cylinders through exhaust manifold 6 and into a header pipe 8. In 
the prior art systems this header pipe 8 leads to a light-off catalytic 
converter 10 in which the cooled exhaust gases are reignited and heated to 
a temperature greater than the minimum inlet temperature required for 
proper operation of the three-way catalyst. The reheated exhaust gases are 
passed through first exhaust pipe 12 to the three-way catalytic converter 
unit 14 where the level of nitrogen oxides, unburned hydrocarbons and 
carbon monoxide are significantly reduced. The purified exhaust is then 
passed through second exhaust pipe 16, conventional muffler 18, third 
exhaust pipe 20 and resonator 22 (the last being an optional unit) to 
exhaust to the atmosphere through fourth exhaust pipe 24. We have 
determined that this system is unnecessarily complex and expensive and our 
invention described herein significantly simplifies the exhaust system 
while yet retaining the effectiveness of the three-way catalytic 
converter. 
In the system of this invention, as illustrated in FIG. 2, the automobile 
and engine are designated 2' and 4' respectively. The exhaust gases 
containing the undesirable pollutants again are withdrawn from the engine 
4' through exhaust manifold 6' to the outlet 34 of which is connected a 
critical component of the present invention, insulated flexible duct 26. 
This insulated flexible duct, the structure of which will be described 
below, retains sufficient heat in the exhaust gases so that when they 
reach the inlet of the catalytic converter, they are still at a 
temperature high enough to activate the three-way catalyst and allow it to 
function with full efficiency. We have found that the combustion gases 
exhausting from the manifold contain a quantity of heat which, if 
conserved, is sufficient to produce more than the minimum "light-off" 
temperature in the three-way catalytic converter, thus entirely 
eliminating the need for the separate light-off converter. The three-way 
catalyst then functions as designed to reduce the level of nitrogen 
oxides, unburned hydrocarbons and carbon monoxide to the desired low 
level. The purified gases are then exhausted to the atmosphere through the 
conventional exhaust system components pipes 16', 20' and 24', muffler 18' 
and resonator 22'. The present system is therefore considerably simpler 
and less expensive than the prior art systems which require the separate 
light-off catalyst unit and an expensive catalyst material in that unit. 
Further, the insulated flexible conduit 26 is effectively a passive 
component in the system, in that it has no moving parts nor active 
elements which require periodic replacement and/or maintenance. Further, 
as will be described below, the insulated flexible duct 26 is adaptable to 
all types of automobiles and other internal combustion engine powered 
motor vehicles without need for special designs and configurations for 
different engines and vehicle configurations. 
The insulated flexible duct 26 is illustrated in detail in FIG. 3. (For the 
purposes of this invention the structural properties defined by the term 
"flexible," which has its normal meaning, are deemed to include as one 
embodiment thereof the structural properties defined by the term 
"semi-flexible," which is herein defined to mean that while the duct may 
be readily bent or flexed to a desired extent of curvature, it has a 
sufficient degree of rigidity to retain that curvature even though 
subsequently subject to the vibration normally associated with vehicle 
operation. The latter embodiment is preferred.) The duct consists of an 
inner flexible metal conduit 28 which has an inner diameter of a size 
predetermined as being optimum for the particular engine to which the 
system is to be attached. Typically the interior diameter of the duct will 
be on the order of about 1 or 11/2 to 3 or 4 inches (2.5 or 3.8 to 7.6 or 
10.2 cm). Surrounding the inner conduit 28 and aligned essentially 
coaxially therewith but spaced apart therefrom is outer flexible metal 
conduit 30, which is conveniently made of the same type of flexible metal 
as the inner conduit 28 but is obviously made with a greater diameter. 
Filling the annulus between the two flexible metal conduits 28 and 30 is 
thermal insulation 32. This will be a thermal insulating material capable 
of withstanding the elevated temperatures of the exhaust gases while yet 
being flexible enough to be bent to follow the desired curves of the 
flexible duct without losing any significant degree of insulating 
efficiency. The thermal insulation 32 will completely fill the annulus 
between ducts 28 and 30. It will normally have a thickness of from 1/4 to 
3/4 inches (0.6 to 1.9 cm). It has been found that thicknesses of 
insulation of this magnitude are adequate to minimize the heat loss of the 
exhaust gases such that when the gases reach the end of the flexible duct 
26 and the inlet of the catalytic converter 14', sufficient heat has been 
retained to maintain an adequate light-off temperature for the three-way 
catalyst in the catalytic converter 14'. These thicknesses of insulation 
thus result in an outside diameter of the flexible duct 26 on the order of 
about 1.5 to 5.5 inches (3.8 to 14.0 cm). 
The inner flexible metal conduit 28 may be formed of any convenient metal 
which can be rolled or otherwise formed into a flexible conduit and which 
will withstand temperatures on the order of 600.degree. F. to 1600.degree. 
F. (315.degree. C. to 870.degree. C.), the conventional temperatures of 
automotive engine exhaust gases. Most conveniently the metals used for the 
inner flexible conduit 28 will be one of the stainless steels, which have 
the advantage of being relatively inexpensive and readily available. The 
stainless steels are also commonly quite conveyance of hot automotive 
exhaust gases. Other metals an/or metal alloys may be utilized, however. 
The flexible conduit 28 may be formed in any convenient manner of forming 
flexible metal duct. Preferably, the system will be formed from flexible 
metal conduit constructed in accordance with U.S. Pat. Nos. 3,621,884; 
3,708,867; 3,753,363; 3,758,139; 3,794,364 and/or 3,865,146. Flexible 
metal conduit of this type is available commercially from the 
Johns-Manville Corporation under the trademark FLEX-MET. The thickness of 
the sheet metal in the flexible conduit 28 will be selected on the basis 
of degree of flexibilty desired as well as the desired degree of erosion 
and vibration resistance. Commonly the sheet metal will be on the order of 
about 2 to 6 mils (0.05 to 0.15 mm) thick, although thinner or thicker 
gauge sheet metal may be utilized where desirable. 
The outer flexible metal conduit 30 is preferably made of the same type and 
gauge of sheet metal as the inner conduit 28. Since the outer conduit 30 
is not exposed directly to the exhaust gases, and since it is not exposed 
to temperatures nearly as high as those to which the inner conduit 28 is 
exposed, the outer conduit 30, if desired, may be formed of a type and 
gauge of metal which has much lower temperature and erosion and corrosion 
resistance, such as sheet aluminum. However, it should be recognized that 
the outer conduit 30 will be directly exposed to physical abuse from such 
sources as rocks, gravel or other debris being flung up under the vehicle 
by the tires as well as the corrosion caused by oil or other engine fluids 
leaking onto the duct 26 or rain water and salt water from rain, snow, icy 
highways which have been salted or sanded, as well as other adverse 
environmental conditions. Consequently, the use of stainless steel of a 
gauge consistent with the desired degree of duct flexibility is preferable 
for the outer conduit 30 as well as the inner conduit 28. 
The thermal insulation material 32 used in the insulated semi-flexible duct 
26 will be a thermal insulation which is resistant to the aforementioned 
600.degree. F. to 1600.degree. F. (315.degree. C. to 870.degree.) exhaust 
gas temperatures and which will preferably be a fibrous material, although 
granular insulation materials could also be used in some cases. Although 
materials such as asbestos fiber, some glass and quartz fibers, carbon 
fibers and some metal fibers have service temperatures suitable for use in 
this invention, the preferred fibers based on economics, availability and 
known thermal performance are the "refractory" fibers which are commonly 
formed predominantly of silica and alumina, optionally with small amounts 
of other oxides present. Most preferred are the aluminosilicate fibers 
which are formed from melts containing 40 to 60 weight percent of silica, 
40 to 60 weight percent of alumina and 0 to 10 percent by weight of oxides 
such as chromia, iron oxide, calcia, magnesia, soda, potassia, titania, 
boria and/or mixtures of these oxides. Among the specific materials which 
can be used are fibers containing 54.0% silica, 45.5% alumina and 0.5% 
other oxides available commercially from the Johns-Manville Corporation 
under the trademark CERAFIBER, with a service temperature of 2400.degree. 
F. (1315.degree. C.); fibers composed of approximately 50% silica, 40% 
alumina and 10% burned dolomite (mixed calcia and magnesia) commercially 
available from the Johns-Manville Corporation under the trademark 
CERAWOOL, with a service temperature of 1600.degree. F. (870.degree. C.); 
and fibers containing approximately 55% silica, 40.5% alumina, 4% chromia 
and 0.5% other oxides commercially available from the Johns-Manville 
Corporation under the trademark CERACHROME with a service temperature of 
2600.degree. F. (1430.degree. C.). 
Conveniently the flexible duct of this invention can be formed by methods 
of the type described in the patents cited above. A continuous formation 
method would be as follows. The inner flexible duct 28 would be 
continuously formed as described for instance in U.S. Pat. No. 3,621,884. 
As the formed tube was discharged from the forming apparatus a layer of 
fibrous felt or blanket insulation would be spirally would around the 
inner duct to the thickness desired in the finished duct. Thereafter the 
insulated inner conduit would be passed to a second metal forming 
apparatus which would form the outer conduit spirally around the outside 
of the insulation layer again in a manner such as that described in the 
aforementioned U.S. Pat. No. 3,621,884. The completed duct upon discharge 
from the second metal forming apparatus could then be severed into 
convenient desired lengths for shipment or coiled into long lengths for 
shipment to a remote location where the desired lengths of insulated 
flexible duct would be cut for installation on the vehicles. Thus, for 
instance, a shipment of duct to an automotive assembly plant in which a 
small number of different vehicle models were produced might appropriately 
consist of precut lengths of duct, whereas a shipment to an assembly plant 
in which a wide variety of different types and models of vehicles were 
produced might well consist of coiled extended lengths of duct which the 
automobile assembler would cut to fit the appropriate vehicles at the time 
of assembly. 
FIGS. 4 and 5 illustrate typical means of attaching the flexible metal duct 
to the exhaust manifold and/or the catalytic converter inlet. In the 
Figures the exhaust manifold outlet pipe is designated 34 and the mounting 
flange is designated 36. Inserted into the inner conduit 28 of duct 26 is 
flanged end element 38. Element 38 consists of a neck portion 40 which is 
inserted into the inner conduit 28 for a distance sufficient to enable the 
two to be effectively coupled, as will be described below. Element 38 also 
contains flange 42 which corresponds in configuration to the flange 36 of 
the exhaust manifold 34. Normally gasket 44 is placed between flanges 36 
and 42 to provide for a positive seal. The opposed flanges are then joined 
in a conventional manner by bolts 46 and nuts 48. 
In the configuration shown in FIG. 4 the end portion 50 of conduit 26 is 
encircled by a strap-like clamp 52. This strap-like clamp 52 is 
constructed in a form similar to that of a conventional hose clamp but is 
made of significantly heavier metal in order to be able to compress the 
end portion 50 of duct 26 sufficiently to form a substantially gas-tight 
seal and to prevent the duct 26 from separating from the neck portion 40 
of element 38. 
In configuration shown in FIG. 5 element 38' is formed with an annular rib 
54 which is as long as neck 40', is coaxial therewith, and is spaced 
therefrom. The spacing between neck 40' and rib 54 is somewhat greater 
than the annular width of duct 26 so that the end of duct 26 can be 
slipped over neck 40 and inserted in the annular space 56 between neck 40' 
and rib 54 while leaving extra space 58 between the outer conduit 30 and 
the inner surface of rib 54. Following insertion of the duct 26 into the 
annular space 56 creating the smaller annular space 58, the smaller 
annular space 58 is packed with refractory cement 60. This cement 60 flows 
into the corrugations of the outer conduit 30 and effectively secures the 
duct 26 in position between the neck 40' and the rib 54. It may also be 
desirable to encircle the end of the rib 54 with a clamp 62 having an 
offset portion 64 which slightly compresses the duct 26 and which serves 
to retain cement in the annular space 58. Since the clamp 62 does not 
exert as much compressive force as the clamp 52 in FIG. 4, clamp 62 may be 
made of a lighter gauge material. 
While the above descriptions of the structures shown in FIGS. 4 and 5 have 
been described in terms of the joining of the flexible duct 26 to the 
exhaust manifold 6', it will be immediately apparent that exactly the same 
structures may be used to attach the outlet end of the flexible duct 26 to 
a flanged inlet of the catalytic converter 14'. In such case the flanged 
members 34/36 would then represent the inlet of the catalytic converter 
14' rather than the outlet of the exhaust manifold 6'. 
Alternatively either or both the catalytic converter 14' and the exhaust 
manifold 6' can be fitted with necked but unflanged devices having the 
same general configuration as the elements 38 and 38' but without the 
flanges 41. The flexible metal duct could then be connected directly to 
the exhaust manifold outlet and/or the catalytic converter inlet in the 
manner shown in either FIG. 4 or FIG. 5. 
The securing methods illustrated in FIGS. 4 and 5 are not meant to be 
exclusive. It is anticipated that any coupling method will be appropriate 
as long as it provides for adequate securement of the duct 26 to the 
exhaust manifold outlet and/or catalytic converter inlet, will withstand 
the vibration inherent in the vehicle operation and will provide a 
substantially gas-tight seal. 
In the actual design of an automotive exhaust system, the length of duct 26 
to be used between the exhaust manifold 6' and the catalytic converter 14' 
will be chosen on the basis of economics and engine and vehicle design. 
However, the length of duct must not be so great that the temperature of 
the gases at the outlet end of the duct is below 700.degree. F. 
(370.degree. C.) which is the minimum temperature required for proper 
functioning of the three-way catalyst in catalytic converter 14'. It will 
be recognized, of course, that with thicker insulation 32, longer runs of 
duct may be used without undue heat loss and temperature decline. The 
optimum combination of insulation thickness (and therefore duct diameter) 
and duct length will be determined by the particular vehicle and engine 
configuration, the cost of materials used in the duct, and the thermal 
resistance (insulating efficiency) of the fibrous insulating material. In 
most cases it will be desirable to determine the duct diameter and length 
on the basis of obtaining an exhaust gas temperature at the entrance to 
the catalytic converter considerably above the minimum 700.degree. F. 
(370.degree. C.) to compensate for possible heat loss through other 
components of the exhaust system such as the exhaust manifold. In 
addition, it might be advisable to insulate the exhaust manifold itself to 
aid in the retention of heat through the system to the catalytic converter 
14'. 
While the description above has been in terms of automobiles, which are 
normally equipped with gasoline or light diesel engines, it will 
immediately be apparent that the system herein is equally suitable for use 
with all types of internal combustion engines and motor vehicles where 
catalytic converters are utilized. This could include automobiles, light 
trucks, vans and the like vehicles equipped with both gasoline and light 
diesel engines. The system could also be extended to heavy vehicles such 
as heavy trucks and busses to the extent that these can be equipped with 
three-way catalytic converters. It will also be recognized that the 
exhaust gases from diesel engines are normally at considerably higher 
temperatures than the exhaust gases from gasoline engines, and the design 
of the duct 26 for a diesel vehicle will of course take this into 
consideration. In particular the type of material thus chosen for the 
interior conduit 28 as well as the type and thickness of the insulation 32 
will reflect the higher temperature environment. 
STATEMENT OF INDUSTRIAL APPLICATION 
The invention herein is intended for use with motor vehicles of all types 
which utilize internal combustion engines and three-way catalytic 
converters. This includes many types of conventional automobiles, light 
trucks and vans, as well as heavier vehicles of some types.