Here's a close-up of the sensors...
Here's a close-up of the sensors used by the PEMS unit. They are attached to a tube coming off of the headers.
In the conversion of a hydrocarbon fuel into heat via the internal combustion process, there are multiple byproducts that constitute the "exhaust gas." Of them, unburned hydrocarbons (un-spent fuel or HC), carbon monoxide (CO), free oxygen (O2), and oxides of nitrogen (NOx) are particularly notable, not just for emissions reduction purposes but as data related to optimizing combustion efficiency. If you've been following Project G.R.E.E.N., you've likely seen we've begun including references to, and data from, the measurement of these emissions during initial engine dyno testing at Mast Motorsports. On-track emissions information is forthcoming.
Here's just one example of how such "emissions" can relate to the optimization of combustion efficiency in a racing engine. Oxides of nitrogen (NOx) are combustion heat related. At the risk of oversimplification, speed up the combustion process (by whatever means) and exhaust gas temperatures stand to be decreased.
In other words, if the burn rate is accelerated, more heat will be used to do work on the pistons, resulting in reduced e.g.t. when the exhaust valve opens. We know this often provides an opportunity to slightly reduce spark timing for additional gains in IMEP (net work on the pistons). Further (as you're "tuning" a given combination of engine parts), if you happen to be measuring several other components in the exhaust gas that includes CO (this relates directly to air/fuel ratio), you can begin quantifying the relative importance of enrichment and burn rate to net power.
Here's another perspective. If you are primarily focused on what's going into an engine and are only including exhaust gas temperature or a few emissions components, there's a chance you could overlook both subtle and significant changes affect-ing optimum power. It's generally believed that around 30 percent of the energy content in gasoline (even in a racing engine) is converted into useable heat (power), possibly less. Even if it was slightly more than this, it makes sense to be evaluating the impact that specific engine modifications (including tuning) are having on combustion efficiency, beyond the point of using time-honored brake-specific fuel consumption and exhaust gas temperature data as analysis tools. And while adding O2 sampling to the process of evaluating exhaust gas, would it not make sense to be measuring other combustion byproducts as well . . . notably oxides of nitrogen (NOx) and carbon monoxide (CO)?
And so that you don't get the notion that all this information is just a lofty academic perspective of what might be done through proper evaluation of exhaust emissions, here's a quote from Dr. Andrew Reading of Sensors, Inc. (a recognized information resource on internal combustion engine chemistry). "If we're talking about a hydrocarbon fuel, in a perfect world, we don't expect to see anything but carbon dioxide and water in the exhaust. Anything other than this indicates inefficiency in the combustion process. Analyzing combustion byproducts can be a key to optimizing engine output, which should be of interest to racers seeking to obtain the most power from their engine."
With that, we'll rest our case.