Settle an argument-Is it better to leave good cat converter in?

Roger Shoaf wrote:
> > wrote in message
> oups.com...

> > It does quite a bit, even if everything is working properly because it
> > can be much more efficient to clean the exhaust than to burn cleanly in
> > the first place.
>
> OK give me an explanation of this. Efficiency would be getting power out of
> the fuel you burn in the combustion chamber. Burning it in the tail pipe or
> letting it escape the combustion chamber unburned does nothing to move the
> machine down the road.

That's what I would think, but the matter is more complex than tha, and
I hope some expert will interject.

1. Why is maximum power obtained with a rich mixture rather than a
stoichiometric one? Shouldn't the presense of unburned fuel that
contributes nothing to the burn actually reduce energy output?

2. Similarly, why is fuel economy maximized with a lean mixture, where
some oxygen contributes nothing to the burn?

3. And why do engines produce less HC and CO (before any cat) when
tuned to burn more fuel rather than less fuel?

4. How much of a modern car's pollution output is reduced by the cat?

In article
.com>,
wrote:

> Roger Shoaf wrote:
> > > wrote in message
> > oups.com...
>
> > > It does quite a bit, even if everything is working properly because it
> > > can be much more efficient to clean the exhaust than to burn cleanly in
> > > the first place.
> >
> > OK give me an explanation of this. Efficiency would be getting power out of
> > the fuel you burn in the combustion chamber. Burning it in the tail pipe or
> > letting it escape the combustion chamber unburned does nothing to move the
> > machine down the road.

It's not like letting it down the tailpipe is intentional.
There will always be some unburned HC molecules due to the fact
that the HC will be quenched by the relatively colder cylinder
walls and that some HC will be trapped above the top piston ring
between the piston and the cylinder wall.

> That's what I would think, but the matter is more complex than tha, and
> I hope some expert will interject.
>
> 1. Why is maximum power obtained with a rich mixture rather than a
> stoichiometric one?

Maximum heat is at .95 lambda, which is 5% richer than
stoichiometric.

> Shouldn't the presense of unburned fuel that
> contributes nothing to the burn actually reduce energy output?

Indeed it does, but that is not necessarily the same as above.

> 2. Similarly, why is fuel economy maximized with a lean mixture, where
> some oxygen contributes nothing to the burn?

Maximum power occurs when there are no left over O2 molecules
(all O2 is utilized), maximum economy occurs when there are no
left over HC molecules (all fuel is utilized).

> 3. And why do engines produce less HC and CO (before any cat) when
> tuned to burn more fuel rather than less fuel?

Chemical balance.

> 4. How much of a modern car's pollution output is reduced by the cat?

Lots of variables, so it depends.
A 0.020 inch diameter evaporative leak can be a bigger HC loss
than what is coming out of the exhaust.