What's the purpose of having the cat right after the exhaust manifold

Question

In a lot of the cheap cars I have looked at their exhaust components, I always find the catalytic converter right after the exhaust manifold instead of being after the down/front pipe in a "typical" exhaust setup. Why do cheaply manufactured cars come like that, what kind of gain putting the cat right after the manifold can the manufacture make other than having shorter wires for the o2 sensors? Or am mistaken and those cars come with two cats like I have seen in some other cars?

Answer 1

The main purpose of putting it right after the manifold is heat. The cat functions best when hot. Placing it as close as possible to the manifold helps the cat heat faster and stay hotter, which produces better results in what it's designed to do: curb exhaust emissions.

Answer 2

On cheap models the cat is part of the exhaust manifold - i.e. the manifold and cat are a single unit. This is done to save cost.

The answer about heat retention is valid but this isn't the factor that really affects cheap models. On some vehicles, you may find the turbo is integrated into the manifold or the manifold is part of the head itself. Its all down to cost.

Cheap cars like this are designed with different serviceability ideas in mind. In some cases, it actually means the cat is easier to replace although I don't believe that the designers consider this for disposable models, perhaps they anticipate the vehicle would be scrapped long before the cat needs replacing.

Have a look at Toyota Aygo cat images.

Answer 3

Small catalytic converters mounted close to the exhaust manifold to warmup quickly and be active as soon as possible after engine start-up. The start-up mixture is usually rich and needs to be oxidized to reduce HC and CO emissions. An O2 sensor mounted ahead of the cc catalyst controls rich/lean mixture to operate the engine close to the chemically correct stoichiometric Air Fuel ratio for best cat operation. The second catalyst mounted under the seat is there to promote reduction in NOx after the HC and CO are eliminated upstream. The O2 sensor between cats monitors the exhaust mixture so that the fuel injection can be altered to be sure that very little O2 enters the second cat. The NOx needs to give up its Oxygen and the best way is for exhaust to be slightly rich. So. The cc cat needs a slightly lean mixture to oxidize the HC and CO. The UF cat needs a slightly rich mixture to reduce the NOx. The O2 sensors keep telling the engine control computer the status of the exhaust and thereby control the injected fuel. It is a delicate balancing act by the fuel management system to yield great results.