[mdlug-discuss] OBD fuel monitor fine tuning

[Aaron Kulkis]

Drew wrote:
> 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,

Which is why there's unburned fuel, and if you're measuring
fuel and air DELIVERED to the cylinder, you have unburned
fuel, which is a "rich" mixture.

  especially with a cold engine, and (2) lean
> mixes produce NOX byproducts, which the EPA doesn't like.

...which further reinforces my argument -- that an
automobile engine running as designed is NOT running
a stoichiometric mix (by the air and fuel meters)