Propane/water/alcohol injection and O2 sensors

[Eck, Joel]

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? 
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?


-----Original Message-----
From: Garfield Willis [mailto:garwillis at msn.com]
Sent: Sunday, April 07, 2002 3:11 PM
To: diy_efi at diy-efi.org
Subject: Re: Propane/water/alcohol injection and O2 sensors


On Fri, 29 Mar 2002 11:37:27 -0800, "Geddes, Brian J" <brian.j.geddes at intel.com>
wrote:

>I'm contemplating a propane injection system for my car, but don't know how
>O2 sensors (Wideband and standard) will react to the presence of propane in
>the exhaust mixture.  Will a gasoline/propane mix influency the
>accuracy/lifespan of O2 sensors?  I have the same concerns about
>water/alcohol injection and the O2 sensors.  Any experience or theories
>here? 

No problem with either propane or alcohol, you will just have to understand the
difference in AFRs you see, since the stoich AFR value of these fuels is of
course different from petrol. Indeed, the NTK WBO2 sensor currently in vogue is
also widely used in natural gas furnace combustion monitoring. So no, these
fuels inNof themselves don't pose a hazard to lifespan; they are both
non-contaminating to the sensor. Also, I doubt if running either of them
continuously on the rich side is any more wearing on them than the same done
with petrol. Long-term rich running does *age* these sensors faster than
near-stoich running, which is why in racing & proformance tuning (unlike the
original OEM usage), you DO need a way to track this aging.

But another thing about your question puzzles me; you ask "will a petrol/propane
mix influence ...?". It's the word "mix" I'm wondering about; I'm going to
assume you just mean alternating between these fuels? I trust you don't mean
actual mixing of petrol & propane as a compound fuel? Or do you? Inquiring minds
wanna know.

Water injection is a bit of a different animal (in a way), since altho it's not
a fuel that's going to change your stoich AFR values because of different
combustion properties, it WILL affect the off-stoich AFR values simply because
now significantly increased amounts of water vapor in the exhaust affects the
partial pressures of all the other combustion byproducts. And it's the partial
pressures of mostly O2, CO, & HC that these sensors are actually sensing. In
fact, whenever pro people do gas calibrations, they will either create a "wet"
testgas mixture (using a thing called a "bubbler"), OR use a "dry" testgas
mixture & adjust for the absence of normal exhaust gas water vapor (a normal
product of combustion but often not included in testgas formulation, for obvious
reasons). This is the same sort of issue, except that NOW you're really adding
prodigious amounts of water vapor beyond just the normal products of combustion.
BTW, it's not water *vapor* that's a hazard for the WBO2s, but *liquid-phase*
water, which can shock-cool the sensor elements or heater, and cause them to
crack. 

The stoichiometric calcs to include the effect of added moles of water vapor in
the overall products of combustion, to correct your measured AFRs, isn't all
that involved, but "beyond the scope of this post", sotaspeak. You may even be
able to find a chemistry exercise or two on the net that shows these calcs. Just
do a search on "stoichiometry" and "vapor" or some such. There's actually a ton
of good stuff online regarding stoichiometry, if you're really curious about all
this (ask me how I know :). I'm hoping to get a whitepaper written up for our
website that goes thru these exercises, including alternate fuels, but judging
by how I'm doing so far on the rest of the site, I wouldn't want to suggest
anyone hold their breath. :)

All that said, I have to interject (ahem :), I'd personally be VERY interested
in monitoring the EGTs when it came to water injection. Increasing the thermal
capacity of your engine is primarily what you're after with H2O injection, so
seeing these effects first-hand via EGT measurement would be *must-have*
sensing, IMO.

HTH,
Gar Willis
Principal Engineer (actual engineer, not make-believe/fairy-tale engineer)
EGOR Techno
www.egortech.com

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