In recent years, automotive
exhaust systems have been the subject of dramatic design changes. Nowhere is this
more apparent than the technology being aimed at the exhaust manifold. The drivers
for change and the reasons for the current diversity can be summarized into four
major areas: Exhaust temperature, start up emissions, aspiration, and refinement.
Cast iron is the material
of choice for 60 – 70 percent of today’s exhaust manifold market. While S.G
iron will perform quite satisfactorily at temperatures of up to 850 degrees
centigrade, beyond this temperature, the material’s reduced yield strength and
the associated thermal expansion problems make the material less attractive.
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Ni resist iron will maintain adequate strength at temperatures up to 1000 degrees
centigrade and has, as a result, largely replaced S.G iron as the choice of
material for cast manifolds. It is, however, more difficult to cast, more difficult
to machine, and more expensive.
Increasingly stringent regulations
on startup emissions have led to a fascinating diversity of manifold design
across the automotive spectrum. With current technologies, the catalyst light-up
time is a crucial factor and so, therefore, so is its location in respect to
the combustion chambers. Rear wheel drive cars like Mercedes Benz and B.M.W,
have their catalysts located in the under-floor position, quite remote from
the engine position. This has resulted in a need to insulate the gases passing
through the manifold using a double wall air gap design. A double wall design
can either be an inner and outer tube design or inner and outer fabricated steel
pressings. Both designs use stainless steel capable of withstanding temperatures
of 1000 degrees centigrade and above. However this rule does not apply to all
designs of car. Generally, front wheel drive cars use fabricated steel single
tube manifolds to accommodate exhaust temperatures, while being able to meet
emissions regulations with suitable catalyst positioning.
So how does a car’s aspiration
have an effect on the choice of manifold design and material? Well, naturally
aspirated engines still have a wide diversity of exhaust manifolds as described
in the foregoing, but with turbocharged and supercharged gasoline engines the
trend is toward fabricated steel manifolds, for temperature considerations.
More specifically though, turbo-charged engines are moving towards fabricated
steel pressings that permit the design of a manifold with the strength and rigidity
to support the turbocharger, within the available space envelope. We are now
seeing more and more implementation of steel manifolds, based purely on raising
quality within the overall package.
So far, the view we have
had of manifold development appears to follow a logical course concerning emissions,
structural strength, and cost. You might be surprised then, to find a double
wall air gap manifold, (using pressings rather than cheaper tubing), on a Toyota
Yaris. The explanation is of course, that a double wall manifold offers the
additional benefit of noise reduction, particularly in the car’s interior, where
engine sound quality is perceived.
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By GlobalDataThe move towards steel manifolds
is a progressive one, and CSM forecasts that within the next 4-6 years, this
steel will surpass cast iron as the primary exhaust manifold material.
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