peejay wrote: ↑
Mon Jan 13, 2020 3:44 pm
NOx is still a bad thing.
An engine burning at stoichiometry will produce nothing but water and carbon dioxide with perfect combustion. With imperfect combustion you get enough free oxygen and NOx to break down into free oxygen to mix with CO and HC in the catalyst. Either way the combustion has to start with stoichiometry.
Chemistry guy here...
The IDEAL combustion of any hydrocarbon is:
HC + O2 --> CO2 + H2O
However, this is a fantasy. The atmosphere is 80% nitrogen and only 20% oxygen.
Therefore... Since Nitrogen is present, it WILL get fused with oxygen into NOx. Heat and cylinder pressure are what cause this. In the old days before converters could split NOx into N2 and O2, they used low compression ratios and retarded cam timing to kill cylinder pressure to minimize NOx production.
Since oxygen is only 20% of the atmosphere, we encounter incomplete combustion... so instead of producing all CO2, we get CO formed. We also get the incomplete combustion of hydrocarbons (HC), so those exist in the exhaust as well.
The first stage of modern cats breaks down NOx... NOx ---> N2 + O2
The oxygen produced is then used to convert CO into CO2... CO + O2 ---> CO2
That oxygen is also used to finish burning any HC that makes it through.... HC + O2 ---> CO2 + H2O
NOx is a GOOD thing in modern cats. It breaks down to form the oxygen required for the other processes. It was bad before they came up with that first stage that breaks them down. Back then, they had to add air pumps to blow air into the exhaust so the old catalytic converters could react the CO and unburned hydrocarbons. You won't see limited compression ratios and cylinder pressure anymore. Higher compression, turbos, superchargers, etc are all very common nowadays.