I had the similar thoughts, Pat. Their claim is it helps keep the engine clean, but I haven't seen any details about it. Thought someone here might have some info.
Don't completely discount enhanced gasoline detergency. Look up Top Tier gas- there are differences between fuels sold at retail, and I respect Shell for consistently trying to differentiate it's products in language that laypersons can understand.
There are even differences in detergents. While all that I am aware of are N based, some older technologies may clean injectors at the expense of increasing combustion chamber deposits. Some more advanced chemistries may claim to remove CCDs, but from the data I've seen the effects are modest. Secondary claims on mileage- or power improvement are sometimes made for detergents by inferring IF a car has very dirty injectors then cleaning them will produce slight but measurable improvements in performance. In practice I suspect most of the folks who are interested in performance hop-ups from fuel additives don't have dirty injectors that the products would be effective on. But heck, everybody has to make a living somehow . . .
In any case, gasoline detergents are effective at very low treat rates (ca. 10-50 ppm Nitrogen if memory serves). So treat rates can be boosted significantly without drastically affecting the other fuel properties.
NOx formation primarily comes from the molecular nitrogen in the air. So I don't see how increasing trace amounts in the fuel could do much of anything. The ratio of nitrogen in the air to that in the fuel is very large.
As black2003cobra says. Interesting case in point- one big supplier in the early 90's had a detergent they believed could reduce NOx from automobiles. They worked with the EPA to set up emissions trading, idea being to get the power industry to pay for credits, allowing the supplier to offset or eliminate charges for the detergent added to gasoline at the fuel depot. Validating and valuing the effects were problematic, so the concept never made it to commercial. Intriguing business model, though perhaps a bit ahead of it's time . . .
Nitrogen-based compounds work well as combustion accelerants, but that's not a desired property in SI engines due to detonation and pre-ignition. Nitrates like amyl nitrate and 2-EHN (2-ethyl-hexyl-nitrate) are often used as additives to improve the auto-ignition quality or cetane index of Diesel fuels. But what's good for a compression ignition fuel has the inverse desirability for spark engine fuel.
There's different 'flavors' of nitrogen used in fuel. Recall Chem 101 (or maybe 201)- Nitrogen gas in the air is N:::N and very inert. Gasoline detergents are usually polymers (for oil solubility) containing amine -NH2 or amide -N(C=O)R2 groups for polarity to dissolve oxygenated varnishes. Quarternary ammonium compounds are salts, which makes them water soluble and generally more common in water-based cleaners than fuel additives. Nitrates -NO2 are very reactive and as noted find application in both diesel engines (2-EHN) as cetane improvers and also in spark engines (methyl nitrate, aka nitromethane) as a liquid oxygen carrier for crazy acceleration.
Air is about 780,000 ppm N2. Cetane improvers are used at about 7500 ppm. Gasoline detergents are used at around 20-150 ppm.
