Emissions is a broad term.
The typical list of emissions is:
CO -- Carbon Monoxide
CO2 -- Carbon Dioxide
HC -- Hydrocarbons
NOx -- Oxides of Nitrogen
H20 -- Water
Perfect combustion has a hydrocarbon fuel and produces H20 and CO2.
But we know perfect combustion doesn't exist. So you get things such as unburnt fuel, HC, NOx because the atmosphere isn't simply oxygen, but about 78% Nitrogen, IIRC, and CO.
If you tune to have few if any HC, you'll get better fuel economy because you are not sending unburnt fuel out the exhaust to be handled by the catalyst.
The other side of that coin is if you have a lean fuel mixture, while it provides good fuel economy, you create NOx at higher combustion chamber temperatures.
So if you are running a lean mixture to minimize unburnt HC (and soot for that matter) you are producing more NOx.
It was NOx where VW was not meeting the standard. So perhaps they were fattening up the fuel mixture and manipulating the EGR to pass the emissions test, but would lean it out and introduce less EGR to give better drivability and fuel economy.
That's one way you can have more of some emissions and still have better fuel economy.
The key is to realize that emissions is not one monolithic substance, but rather a collection of compounds that each have individual federal standards.
Originally Posted By: ZeeOSix
So I don't quite understand how the fuel mileage would go down in "cheat mode" if that is supposed to give lower emissions. I would think that lower emissions would mean the engine is running leaner and more efficient, and therefore give better fuel mileage ... ??
