No escape route for exhaust f&es Mechanical failure of the supports of catalytic converters can be a costly problem for automotive manufacturers, especially if it leads to a recall and replacement programme. In this article Raymond Scoboria of the Automotive Sales Office of ACS Industries, explains why the company’s end seal designs can reduce the occurrence of this type of failure.

The two designs are called V and Z seals and are

manufactured from 30309 stainless steel wire at

ACS Industries’ plants at Monterrey, Mexico.

These facilities perform all of the wire drawing

and annealing operations to reduce 5.5 mm

stainless steel redraw rod to the fine wire

required for manufacture. This process involves

tandem carbide roller wire flatteners, each with

two sets of rollers, which flatten the wire at

speeds of 460 to 600 mlmin.

The company uses approximately 6.8 million

kilograms of stainless steel wire per year,

including several different stainless steel alloys.

Currently, its capacity is for 9.1 million

kilograms per year, but with the addition of a

third rod breaker next year, capacity will increase

to 13.6 million kilograms per year.

The V seal The V seal is an exhaust-gas blocking seal used

with stainless steel knitted wire support mesh to

prevent exhaust from bypassing the ceramic

monolith of catalytic converters. The seal

contains 50% round wire on the inside and 50%

flattened wire on the outer sealing surface.

During converter assembly the V seal is pressed

onto the monolith, deflecting the inner seal wall

outwards and locking the seal on the monolith.

The support mesh, unlike the classic mat-type

support, does not require a high temperature

curing cycle. This makes it well suited to the

“cold hold” environment of downstream,

underbody catalysts or catalysts used on diesel

powered vehicles. In addition, because the mesh

does not contain either ceramic fibre or

vermiculite, no fibres can be expelled into the

exhaust through erosion.

Converters can be produced with either a

single V seal on the front of the monolith or with

seals on both the front and rear of the monolith

for high temperature or high vibration

applications.

The seal and support mesh system is effective The leak rate fixture free flow should not

with monolith skin temperatures up to 76O”C, exceed 80 000 cc/min at 0.134 bar, measured at

which relate to a gas inlet temperature of 899°C standard temperature and pressure. The leak rate

at the centre of the inlet cone. measurement is performed at a pressure

V seal leak rate measurements are performed differential of 0.134 bar. With a 80 x 120.7 mm

in fixtures simulating converters (Figure 1). monolith cross section and a seal circumference

These are “clam shell” designs using stamped of 341.1 mm, the leak rate should not exceed

! upper and lower metal shells welded together. i 55 600 cclmin. Measured values on production

; A simulated monolith, with the test seal j samples, fitted with the V seal, ranged from

; installed, is then placed inside the fixture. The i 45 000 to 51 000 cc/min.

shells and test fixture have a 3.2 mm radius at

the seam to facilitate assembly. The majority of

the leakage that occurred during the leak rate test

was air passing through the triangular opening

formed by the two 3.2 mm radii and the seal

edge.

LEAK RATE FIXTURE

Test seal

I 2 inch piece of fine

7 emory cloth on each end to simulate monolith surface

/ I

Thin rubber gaskets used in , these areas to prevent leakage

from gas source L 1 I

;I

T1

>

II ;I I___ --. ____il_=

Side view

Direction of gas flow -

’ Front view ’ Front view (sides only)

----Y 0.125 inch 4 places

@ Sealing Technology No. 61

V seal compression characteristic orce in Kg force per cm of seal circumference

I-

l-

l-

)-

)-

)-

)- Ill,

122.6

n 4.572 4.064 3.556 3.046 2.540 2.159

inches (0.160) (0.160) (0.140) (0.120) (0.100) (0.085)

I The V seal must be in the following force ranges at the 3 spectfted qaps I

I 4.572.mm 3.046.mm 2.159.mm / Max 5.11 Kg/cm 21.95 Kg/cm 122.6 Kg/cm

Min 0.50 Kg/cm 5.11 Kg/cm 13.29 Kg/cm

During normal operations, the overall erosion in converters used on high flow engines

pressure drop across the monolith was between with turbo chargers, superchargers or four valves

0.02 to 0.27 bar. When pressure gradients are per cylinder. Excellent long-term durability

taken into account, the V seals actually block performances have been achieved in converters

between 95% to 99% of the exhaust fumes that with inlet gas temperatures of 10 10°C. However,

would have normally bypassed the monolith in to be effective this seal should be mounted on

the absence of the seal. both front and rear of the ceramic monolith.

Figure 2 demonstrates the compression The Z seal is manufactured with the cross

characteristics of the V seal. The crosshatched section rotated outward by 30” so the leading-

area indicates the 4 0 statistical compression edge circumference is larger than the monolith

range of the V seal, while the two outer lines and the trailing edge smaller. During

represent the force limits on the seal installation, the trailing edge stretches, which

specification. A converter shell to monolith gap rotates the seal 30” so it sits square on the

of less than 2.159 mm is not permitted with the monolith. Thus, the stretched trailing edge grips

V seal, as the forces on the monolith will be the monolith tightly holding the seal in place

excessive. during converter assembly.

Figure 3 demonstrates the compression char-

acteristics of the Z seal. The crosshatched area

represents the 4 0 statistical compression range

of the Z seal, while the outer lines indicate the

force limits on the Z seal specification. A con-

verter shell to monolith gap of less than 3.302 mm

is not permitted with this Z seal design because

the forces on the monolith will be excessive.

The Z seal The Z seal is in fact an erosion barrier made

from 50% round and 50% flat wire in alternate

layers. This creates a torturous path for the

exhaust gas flow, so no defined gas velocity

occurs at the mat surface to cause erosion.

It was found to perform well in high Since it is an erosion barrier rather than a gas

temperature applications located close to or seal, the Z seal is always used in series with mat

attached to the exhaust manifold. Z seals prevent type supports, which provide excellent sealing if

Z seal compression characteristic force in Kg force per cm of seal circumference

mm 5.644 5.334 4.626 4.316 3.610 3.302 inches 0.230 0.210 0.190 0.170 0.150 0.130

I The Z seal must be in the following force ranges at the 3 soecified aaos I

I 5.586.mm 4.191.mm 3.302.mm 1 Max No limit 26.93 Kg/cm 85.92 Kg/cm

Min 1.64 Kg/cm 7.50 Kg/cm No limit

protected from erosion. In addition, as the

exhaust gas velocity increases so does the

effectiveness of the Z seal. This is due to the

incorporation of the flat wire throughout the

seal.

Currently, both seal designs are available in

approximately 50 different sizes, to

accommodate different monolith cross sections

and monolith to shell mounting gaps. And

tooling for new cross sections and mounting

gaps can be produced.

Decreased exhaust emissions These two end seal designs effectively reduce the

level of exhaust emissions through two different

mechanisms. The V seal acts to prevent exhaust

fumes escaping from the ceramic monolith,

while the Z seal acts to protect the mat supports

of the catalytic converter from erosion, which

seals the system.

Contact: Raymond Scoboria, ACS Automotive Incorporated, 33200 Schoolcraft, Suite 208, PO Box 51845, Livonia, MI 48151-5845, USA; Tel: +l 734 522 4750; Fax: + 1 734 522 4994

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