[mdlug-discuss] OBD fuel monitor fine tuning

[Drew]

At 06:26 PM 10/20/07, Aaron Kulkis wrote:
>If I remember correctly, a perfect stoichiometric mix is
>actually TOO LEAN -- it will burn out your engine.  The
>reason is that unburned fuel inside the cylinder absorbs
>some of the heat, and thus COOLS the engine.

     This is true of liquid fuel rocket engines. The Saturn V engines
ran at 5 to 1, instead of 8 to 1, because of the temperature limitations
of the engine. Excess hydrogen was chosen because of its low
molecular mass (2 kg/kmol versus 32 kg/kmol for oxygen), which
allowed both greater heat absorption per degree of cooling, and higher
exhaust speed, allowing the specific impulse to be most, though
not quite all, of what a stoichiometric mix would provide. Newer
engines run at 6 to 1, due to some tricks played with the exhaust
stream.

     However, rocket engines use pure oxygen as their oxidizer, and
the air used by automobile combustion drives is mostly nitrogen. So
there should be less of a need to dilute a stoich mix. Besides which,
excess air should do just as well as excess fuel; air has an average
molecular mass of 29 kg/kmol, while typical fuel hydrocarbons have
molecular masses ranging from around 60 to well over 100, allowing
excess air to carry away more heat than excess fuel.

     No, car engines run rich because (1) it's hard to get liquid fuels to
mix thoroughly with air, especially with a cold engine, and (2) lean
mixes produce NOX byproducts, which the EPA doesn't like.

     Diesel drives run very lean most of the time, since they are controlled
by fuel metering and have no air throttles.


>This solved the riddle of why old manuals (back in the
>carburetor days) always warned that leaning out the
>fuel mixture too much would cause engine overheating,
>and eventually engine damage.
>
>And this is why we have catalytic converters...because
>an automobile engine functioning as designed WILL have
>unburned fuel in the exhaust.

     If fuel *must* exit the cylinders unburned, can't it be recovered
instead of burned the rest of the way to no usefulness? What
percentage of the fuel does make it to the cat anyway?

----

- Drew.