Propane/water/alcohol injection and O2 sensors

[Garfield Willis]

On Tue, 9 Apr 2002 10:23:59 -0500, "Eck, Joel" <Joel.Eck at COMPAQ.com> wrote:

>I understand the desire to inject water in an attempt to stave off
detonation, especially under high boost, advanced timing, etc. However,
something bothers me about it. 

>1. If the water vapor cools the exhaust gas temps, then how
bad (overall bad, that is) will that be for the whole combination? By cooling
the exhaust, you are inhibiting the O2 sensor from reaching it needed
temperature (unless you're using a heated one, and then, does the cooling effect
of the water vapor then pose any problem to the actual heating of the sensor?),
so will the O2 sensor read right?

----------
This isn't really answering your question (see below), but the above involves a
common misconception about at least WBO2 sensors. If the heater circuitry is
done correctly, these sensors are easily able to maintain correct sensing temps
EVEN at the very end of the *tailpipe*. The actual amount of exhaust gas they
have to heat is VERY small compared to the mass-flow of the entire exhaust
stream.
----------

>2. Also, the cooling of the exhaust gases will keep the cat
from reaching its proper temps also, so how will that be affected?

>3. On top of all that, I've also been told that for optimal performance
of the exhaust system, you want to keep the temperature of the exhaust gases as
high as possible; otherwise, the gases (somewhat) condense, thereby flowing
through the tubing slower and creating more backpressure. By cooling the gases
indirectly with the water vapor's presence, does this mean that one would end up
with more backpressure by virtue of using the water injection so they could
tweak the system a little beyond what they would normally feel safe with, and,
does the benefit of the added power from not backing off the timing or boost
just a little outweigh the detriment of the added backpressure?

----------
So Joel, are you prone to worry alot? :)

Seriously, I think you might be missing the fundamental motive for water
injection. It is NOT to simply "lower temps" at all power levels, but to keep
them low enough to stay within the thermal capacity of the engine while making
highest power. So instead of thinking of it as potentially lowering temps
*below* optimal, which is the predicate of all three of your questions above,
think of it as being used to HOLD temps *within* optimal (or at least workable)
ranges.

If that's too vague, here's an example counter-point to your scenarios. Spose
that without h2o injection, your EGTs would show 1900degF at the dragon's belch.
Not so good. But further spose that *with* h2o injection, you can lower those to
around 1600degF, at the same power level. Now quite workable.

I suspect that's not going to satisfy your innate curiosity about "magnitudes of
effects", but it really is the basic answer to your 3 questions above.

HTH,
Gar

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