NOx emission is directly connected to combustion temperature, and as combustion temperatures increase, so do NOx emissions. Simple..... but, there is another benefit to the humble EGR valve as well.....
An engine relies on a cylinder, or multiple cylinders, with a piston moving up and down inside each cylinder, to fill with air and fuel at a specific ratio which, when ignited, causes expansion of the air, forcing the piston back down again. This is the basic operational principle of an internal combustion engine. Rotary (Wankel) engines don't have pistons but the principle remains the same.
Diesel burns much less cleanly than petrol and is inherently more polluting, but in either case, by burning a fossil fuel, whether it be petrol, diesel or LPG, carbon (soot) is produced as well as various gasses such as Carbon Monoxide, Nitrogen Oxide etc. The principle of an EGR valve is to recirculate some of the already burnt gasses back into the cylinders during times when you aren't demanding power (ie - cruising, light throttle or slowing down etc. The mechanical process I won't bore you with just yet), which results in less space for fresh, clean air to occupy. With less fresh air in the cylinder, the ratio of fuel needed becomes less and thus the combustion of the reduced fuel quantity produces both less carbon, but also less NOx gas as well, because the temperature of the combustion is lowered. Lower combustion temperature equates to lower NOx emissions.
As for the 'added benefit of an EGR valve, think logically for a moment about carbon and what happens when you burn it.
Soot is the inevitable consequence of burning a fossil fuel, so if we can burn it afterwards and convert it into ash, it becomes a less harmful substance if we breathe it in. In essence, that's what a DPF does, but that's a topic for another post..... Carbon will burn at temperatures above 400° celcius. It's the same principle used in a self-cleaning domestic oven.....
By sending already burnt exhaust gasses, containing soot, back into the engine and burning it within the combustion cycle of each cylinder, the process will burn the soot and convert it into ash, which will exit the exhaust as such. The overall effect being that it effectively reduces the carbon emissions of the engine.
This process is so efficient that the overall carbon emission of an engine fitted with an EGR valve is somewhere around 90% lower than one without, so it's not only to reduce NOx emissions. An EGR valve plays a massive role in reducing the harmful emissions from an engine, more-so in the case of a diesel!
Now, here's the caveat... In my opinion, an EGR valve causes so many issues we're all familiar with.....
Engines from the dawn of time have incorporated a 'crankcase breather' system. Often called a PCV or "Positive Crankcase Ventilation" system.
To explain......... As pistons move up and down within their cylinders, they create a draught, or wind, within the engine itself. That wind causes a lot of pressure within the engine, so it must be vented somewhere where it can't cause undue stresses on the engine internals, especially at higher rpm's. Because it naturally contains oil vapours from the engine oil itself, venting it to atmosphere is a big no-no, so instead it is vented into the inlet where it can be drawn into the cylinders and burnt through the engines natural combustion process. As a result, it's perfectly normal to find some oil residue in the engines inlet pipes, intercooler, inlet manifold etc. This is perfectly fine and has been done for decades without a problem..... Until the inclusion of the EGR valve...
I cannot fathom why manufacturers and engine developers continue to develop engines and emissions systems which mix exhaust gasses, containing soot/carbon via the EGR valve with oil vapour from the PCV system, and think it's a good idea! As has been proven time and time again, the combination of oil vapour and soot congeals to form an ever increasing layer of gunk in the inlet system which over time restricts airflow and causes the EGR valve to become so constricted that it either cannot allow enough air through into the engine, or it fails completely, leading to the reason many people chose to remove or blank the EGR as a more cost effective remedy which, not only has it's own legal implications but also, means the environmental benefits of the EGR are also removed causing the exact polution to our atmosphere it was designed to prevent!
The solution, in my opinion, is simple.... Fit an oil vapour catch-can and make draining it part of a standard service. A simple float sensor will indicate when the catch-can is becoming full, and a drain tap on the bottom will allow easy emptying of the oil it's accumulated. It's such a simple and effective concept! This way the soot can pass straight through the inlet and will never build up within the intake system, and the PCV system can also do its job at the same time without causing any issues to your engine at all.
For those who choose to remove or blank the EGR valve, I completely understand why. Manufacturers not only make the system self-failing, but in many cases extremely difficult to service. Utilising a catch-can system would make it fool-proof, but instead they continue to combine oil vapour and soot, often fitting the EGR in an inaccessible place, which means its often extremely difficult to remove the valve in order to clean and refit. The whole system is thus designed to fail!
If, however, you can forgive all its foibles, the environmental benefit of having an operational EGR valve system is huge! A fully functioning EGR valve has a profoundly beneficial effect of the emissions from an engine, specifically a diesel engine, and is probably the one, singlemost beneficial exhaust gas treatment devices ever developed!