[peppercorn (OP)]

A friend of mine bought a Chevrolet Chevette when he came to the US.

D'ya mind! I learnt to drive in a 'vette! To be fair this was a Vauxhall Chevette and I was 16, so not that official!
On the other hand my brother was 13 and had to sit on an old chair cushion to see over the steering wheel []

Luckily there was a handy bit of waste ground the other side of the railway where we lived to, erm, practice on []

'Toto, I've a feeling we're not in Kansas any more' - That's how off topic it's becoming []

[CliffordK]

Note, you have a fine balance on tuning, especially with the NOx. ...

So, is more mechanical work made available from N₂'s reacting with O₂'s to make NOx's ? .... blah blah blah

I don't think making the NOx actually improves the performance. 

For Nitric oxide (NO), you start with dimers and end with a dimer, so you end up with no net change in moles.

N₂ + O₂ --> 2NO

But, if instead you are making Nitrogen Dioxide (NO₂), then you end up with fewer moles.

N₂ + 2O₂ --> 2NO₂         (3 moles --> 2 moles).

And, thus the formation of NOx would actually reduce the energy.

Assuming it is mildly endothermic, that means lower temperatures, and less energy too.

Breaking down the NOx, would produce more moles of product, and more kinetic energy.

[peppercorn (OP)]

For Nitric oxide (NO), you start with dimers and end with a dimer, so you end up with no net change in moles.
N₂ + O₂ --> 2NO
But, if instead you are making Nitrogen Dioxide (NO₂), then you end up with fewer moles.
N₂ + 2O₂ --> 2NO₂         (3 moles --> 2 moles).
And, thus the formation of NOx would actually reduce the energy.
Assuming it is mildly endothermic, that means lower temperatures, and less energy too.
Breaking down the NOx, would produce more moles of product, and more kinetic energy.

I'm sure your analysis is correct but is it purely a matter of counting moles for working out if there is an increase in volume? (I mean for a steady temperature that this is almost certainly not).

Can you also explain why, as mentioned earlier, advanced timing (in relation to a 'traditional' advance) will give a better, more complete burn?

[CliffordK]

Uhhh....
It has been too many years since I've studied "Science"   [xx(]

But, from the Ideal Gas Law:

pV = nRT

pressure * volume = moles * Constant * Temperature

R = 8.314472 ^J/_K·mol

So...
I think the pressure is independent of WHAT it is, but only dependent on how much you have (in moles) and what the temperature is.

So, anything that would reduce the temperature (endothermic reaction) or would reduce the moles, would also reduce the pressure.

----------

I'm not an expert on things like Engine Timing.
I need to get one of those exhaust gas gizmos for playing with 

I think advancing the timing increases the time for combustion...  and thus I would expect a more complete burn.
But, perhaps there are other influences.

I'm sure there is more technical info on the web somewhere.

[Geezer]

I think advancing the timing increases the time for combustion...  and thus I would expect a more complete burn.

I believe that's it. The ignition timing is not fixed for that reason. It varies with engine speed. In ye olden days we had mechanical distributors that had a centripetal advance mechanism. Presumably these have been superceded by fiendishly complicated electronics (and firmware even!) to discourage people like me from doing my own repairs.

I changed the spark plugs on my car recently, and I was slightly surprised to discover that the entire operation took well over two hours! The actual removal and replacement of the plugs only took about fifteen minutes. The rest of the time was spent disassembling and reassembling a load of kit that looked like it belonged more properly in a CRT type TV set  []

[peppercorn (OP)]

I think advancing the timing increases the time for combustion...  and thus I would expect a more complete burn.

Maybe I was unclear in my inquiry. Perhaps if I were to ask why you think that, in traditionally set-up engine, having the timing set for a burn that ends later than the ideal - will reduce NOx formation.
Or is it more to do with purposely leaving some CO/HC to oxidize at the Cat and drive the thermal dissociation of NOx's?

In ye olden days we had mechanical distributors that had a centripetal advance mechanism. Presumably these have been superceded by fiendishly complicated electronics

Those old points systems do wonder pretty horribly around the timing though. Anyone who's ever used a timing-light on an old car, esp. with a few decades ware in the dissy will have seen this.  As I'm sure you know really, electronic ignition has much to be said for it.

The rest of the time was spent disassembling and reassembling a load of kit that looked like it belonged more properly in a CRT type TV set

I'm sure there is an element of keeping the buyer from fixing their motor too easily (or at all!), but having coil-packs, etc is a really neat advance in autos IMO.

[CliffordK]

Remember that NOx is a combustion byproduct that is essentially opposite from Carbon Monoxide (CO), Hydrocarbons (HC), and Particulate Matter (PM).

Think of NOx as too much oxygen, and too much combustion.
CO, HC, PM, are essentially too little Oxygen, and too little combustion.

Thus graphs of NOx often go opposite the others.

A quick web search indicates suggestions of advanced timing increases NOx, and retarded timing decreases it.

--------------------------------

A little aside,
NOx can also be used as an oxidizer.

So, if you took: Dinitrogen tetroxide, (N₂O₄)

N₂O₄ --> N₂ + 2O₂ (1 mole --> 3 moles).  Also, one gets a higher oxygen/nitrogen mix than one would normally have in air.  Being Exothermic, it would also increase the combustion temperatures (and also increase cylinder pressures).

So, while producing NOx in the engine and releasing it into the exhaust takes away power.  Adding excess NOx to the intake increases power (and likely also causes a significant amount to be released in the exhaust).  But, if your goal is maximum acceleration, adding some NOx into your intake might just give you the edge you need.

(all based on the Ideal Gas Law which should more or less apply).
pV = nRT

_{(Note for the "topic police".  Sorry if we are rambling a bit.  But a rough understanding of emissions is critical to any discussion of decreasing them, or otherwise managing the emissions.)}

[peppercorn (OP)]

Remember that NOx is a combustion byproduct that is essentially opposite from Carbon Monoxide (CO), Hydrocarbons (HC), and Particulate Matter (PM).

Think of NOx as too much oxygen, and too much combustion.
CO, HC, PM, are essentially too little Oxygen, and too little combustion.

Thus graphs of NOx often go opposite the others.

But too much oxygen is also a prerequisite of 'lean-burn'. Unfortunately localised high temperatures can be difficult to avoid with lower fuel vaporisation cooling of lean-burn and may lead to NOx formation.

A quick web search indicates suggestions of advanced timing increases NOx, and retarded timing decreases it.

I suggested the reason for this: 'Is it more to do with purposely leaving some CO/HC to oxidise at the Cat and drive the thermal dissociation of NOx's?'
So it is an indirect effect....