I think you have been mislead by the anti-emissions control lobby. It is often said the EGR does not reduce emissions or may increase them. This is not true but is based on "tests" with normal garage emissions testers. These do not measure the NOx that EGR systems reduce. The air injection valves on early emissions controlled cars did not "dilute" the exhaust gases (it would take a huge volume to do that) but provided the additional air that the early CAT systems required to operate.
Selective Catalytic Reduction and Urea (adBlue) is a better solution to NOx than EGR but costs more. Another common myth is that DPFs just store particulates and then "dump" them. They don't dump them they "burn" them catalytically using extra fuel injected during the regeneration cycle.
Robert G8RPI.
The air injection systems I am thinking about were fitted to motorbikes before any of those bikes had catalysts. (e.g. the 1997 Yamaha XJ900 Diversion had them. That bike never had an exhaust catalyst during any of its production. Yamaha's 600cc and 1000cc sports bikes also had the air valves but with better control gear to reduce the losses. Things no doubt changed after they went to catalyser exhausts.
The valves really were all about diluting the exhaust gas and as stated the exhaust ports drop well below ambient pressure so they suck air in surprisingly well.
My point is that these add on devices are so often sold as giving and emissions benefit the in reality the user and the environment loses out in other ways. EGRs are a case in point. They look fine with a new clean engine but as the miles go by they cause more harm than good in terms of both engine life and emissions control.
There is one very good way to reduce internal combustion NOx - reduce the combustion chamber peak pressures and ensure a good supply of excess air.
Direct injected 2 strokes offer that chance. Air can be dumped right through either by missing fuel injection strokes or by leaving the ports open for longer so clean air gets pulled down the exhaust. There are speed and power ranges when this makes good sense. Two strokes generate lower peak pressure than fours strokes so generate less NOx directly. They also don't have hot valves sitting over the combustion bowl waiting to trigger NOx generation.
Fuel premixed with air is injected after all ports have closed just before the spark is fired. The fuel wont have spread throughout the chamber so burns rather than exploding. The spark can be advanced to the mechanically ideal point without preignition because until a moment before the spark there was no fuel to pre ignite. The flame front never reaches the metal wall so the flames don't get quenched.
We don't have 2 strokes today because they were labelled as filthy and dirty back in the bad old days (when they were - filthy and dirty) but have never really been looked at ever since. On the plus side Rotax are doing great things with their Skidoo and outboard engines. Low emissions, good fuel consumption, great power output and good mechanical life.
2 strokes also work well as diesels, but as with petrol engines the older mechanical only units were dirty. Check out a Deltic or Detroit going at full chat. Exciting but not exactly clean. These had normal oiled sumps so had reduced pumping losses but they were invariably over injected and there was no control system to inject fuel only when it could be properly burned.
