I'll start w/ a disclaimer that my place of underemployment advertises in
the magazine below, other than that, no associations w/ this post.

Figaro makes a variety of sensors to detect various chemicals and gases.
Some are highly selective, responding only to certain gases and ignoring
most everything else, others are more general in what they detect. They
have sensors for gasoline & diesel exhaust, carbon monoxide, R-12 & R-134a,
combustible gases such as propane & hydrogen, and many others:
http://www.figarosensors.com/gaslist.html
Prices range from about $15 to $57 USD.

The Sept. '04 issue of Nuts & Volts magazine has an article on building an
"electronic nose" using a Figaro sensor. Basically the sensor is configured
as one leg in a Wheatstone Bridge, and a PIC microcontroller is used
to measure the Bridge's imbalance and drive a speaker to give an audible
indication of gas concentration. The schematic is in the article, but
unfortunately the parts list was left out, it can be downloaded here:
http://www.nutsvolts.com/toc_Pages/T...04_sniffer.htm
Back issues of the magazine can be purchased.

The author cleverly laid out the PCB in a round disk the size of a MagLite
flashlight lens. A 4 D-size cell model of the flashlight provides the
case and power supply. No permanent modifications are made to the MagLite
so it can be reverted back to a flashlight. The author has made the PCB
and programmed uC available, sensors are available from Figaro, and the
rest of the electronic components are from DigiKey & Radio Shack. Smoldering
it all together is done by you.

rgds,
philh

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Any idea how well this would work in an automotive tuning application?

The article makes a point of it's circuit being qualitative rather than
quantitative, i.e., it could detect in your breath whether you've been
drinking, but not whether your blood alcohol was over the legal limit.

It also mentions the sensors are affected by temperature and humidity, and
the output is logarithmic, so you'd have quite a challenge calibrating
things, but it still might be quasi-useful in giving you a trending
indication, such as NOx reached a minimum and is now increasing again.
The gasoline & diesel exhaust sensor is actually dual sensors in one package,
the diesel side sensitive to NOx.